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Functionalization of carbonaceous materials
for photovoltaic devices
By
Messai Adenew Mamo, MSc.
A dissertation submitted in fulfillment of the requirements for the degree of
Doctor of Philosophy in the Faculty of Science
School of Chemistry
University of the Witwatersrand
Private Bag X03
WITS
2050
Supervisors: Professor Neil J. Coville
Professor Willem A. L. van Otterlo
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Declaration
I declare that ?Functionalization of carbonaceous materials for photovoltaic devices? is my own,
unaided work submitted for the degree of Doctor of Philosophy at the University of
Witwatersrand, Johannesburg. It has not been submitted for any degree or examination in any
other University, and all sources I have used or quoted have been indicated and acknowledged by
means of complete references.
Name: Messai Adenew Mamo (Mr.) Date?????..2010
Signature:................................................................
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Acknowledgements
First and foremost I would like to give special thanks to my supervisors, Professor Neil J. Coville
and Professor Willem A. L. van Otterlo, for their outstanding help and guidance during the
course of this research project.
I would also like to thank Miss Zikhona N. Tetana, Mr. Roy P. Forbes, Mr. Robert Black, Miss
Ellen Kwenda, Miss Manoko Maubane, whom I co-supervised in their research projects, Dr.
Sabelo D. Mhlanga, and the rest of the CATOMAT group for providing a great working
environment.
I also thank Professor Michael J. Witcomb, Mr. Rudolph M. Erasmus, Mr. Richard Mampa, Mr.
T. A. van der Merwe, and Dr. Manuel A. Fernandes for their assistance with TEM, Raman
spectroscopy, nuclear magnetic resonance spectroscopy, mass spectroscopy and X-ray
diffraction analysis, respectively.
I would also like to acknowledge the assistance of the following people during my research visit
at Instituto de Qu?mica, Universidade Estadual de Campinas, SP, Brazil: Professor Ana Fl?via
for her kind advice and discussions, and members of LNES Lab Jo?o, Agnaldo, C?sar, Lucas
especially Fl?vio, Giovanni and Jilian who helped me a lot as well as others occupants of the
laboratory.
I also thank Mr. Basil Chassoulas, Mr. Steve Gannon, Mr. Barry Fairbrother, Mr. David Moloto
and Mr. Elias Valoyi, for their technical support. I also thank the School of Chemistry,
University of the Witwatersrand, for giving me the opportunity to conduct this research on their
premises.
The financial support for this work by the DST/NRF Centre of Excellence in Strong Materials
for a scholarship and India-Brazil-South Africa (IBSA), South Africa and Brazil for a research
visit to Brazil, are gratefully acknowledged.
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List of publications
The following publications originated from different parts the work presented in this thesis:
M. A. Mamo, F. S. Freitas, J. N. de Freitas, W. A. L. van Otterlo, A. F. Nogueira, N. J. Coville,
?Poly(3-hexylthiophene) covalently linked to fullerene for use in hybrid solar cells? submitted to
New J. Chem.
M. A. Mamo, F. S. Freitas, J. N. de Freitas, W. A. L. van Otterlo, A. F. Nogueira, N. J. Coville,
?Application of 3-hexylthiophene functionalized CNTs in photovoltaic devices? to be submitted
M. A. Mamo, R. P. Forbes, N. J. Coville, W. A. L. van Otterlo, ?Ring-opening metathesis co-
polymerization of a C60-cyclopentadiene cycloadduct and N-(cycloheptyl)-endo-norbornene-5,6-
dicarboximide? to be submitted
M. A. Mamo, N. J. Coville, W. A. L. van Otterlo, ?Ring-opening Metathesis Co-polymerization
of a C60-cyclopentadiene Cycloadduct and Norbornene with the Grubbs Second-generation
Catalyst? Fullerenes, Nanotubes, and Carbon Nanostructures, 2007, 15, 341.
Research visit
From November 2008 to April 2009 research visit at Instituto de Qu?mica, Universidade
Estadual de Campinas, SP, Brazil.
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Presentation at conferences and seminars
Date Name and place Type of
presentation
July 2007 SACI, Inorganic, Cape town Poster
August 2007 CATOMAT seminar, Room C509 Humphrey Raikes
Building, Wits University.
Oral
February 2008 DST/NRF Centre of Excellence in Strong Materials?
Seminar, Room C6 Humphrey Raikes Building, Wits
University.
Oral
July 2008 CATOMAT seminar, Room C509 Humphrey Raikes
Building, Wits University.
Oral
January 2009 Instituto de Qu?mica, Universidade Estadual de
Campinas, seminar room, SP, Brazil
Oral
March 2009 Instituto de Qu?mica, Universidade Estadual de
Campinas, seminar room, SP, Brazil
Oral
May 2009 CATOMAT seminar, Room C509 Humphrey Raikes
Building, Wits University.
Oral
June 2009 DST/NRF Centre of Excellence in Strong Materials?
Seminar, Room C6 Humphrey Raikes Building, Wits
University.
Oral
September
2009
IBSA Meeting on Nanotechnology, Curitiba, Brazil. Oral
October 2009 SACI Young Chemists Symposium ?Gauteng Region,
Potchefstroom Campus of the North-west University,
Ou Senaatsaal (Building F4) Building, Wits University
Oral
October 2009 School of Chemistry departmental seminar, PhD thesis
- final, Room C6 Humphrey Raikes Building, Wits
University.
Oral
Note: CATOMAT = catalysis-organometallics-materials research group
IBSA India, Brazil and South Africa
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Abstract
A C60-cyclopentadiene cycloadduct was readily synthesized, by a Diels-Alder reaction between
C60 and freshly cracked cyclopentadiene. The functionalized C60 and norbornene were then pre-
mixed in varying molar ratios and co-polymerized using a ROMP approach with catalytic
amounts of Grubbs second generation catalyst. A series of C60-containing polymers were also
synthesized by the co-polymerization of a C60-cyclopentadiene cycloadduct and N-(cycloheptyl)-
exo-norbornene-5,6-dicarboximide in varying ratios using similar procedures. The
polymerization was facilitated by use of a catalytic amount of Grubbs second generation catalyst.
The C60 co-polymers formed were investigated by FT-IR, UV-visible,
1H NMR and 13C NMR
spectroscopy, mass spectrometry, differential scanning calorimetry (DSC) and thermal
gravimetric analysis (TGA).
Functionalization of C60 was achieved by using a 1,3-dipolar addition of azomethine ylides to
C60 that resulted in fulleropyrrolidines containing 2- and 3-thiophenecarboxaldehyde. Finally,
the two C60 derivatives, together with different ratios of either thiophene or 3-hexylthiophene,
were oxidatively copolymerized using FeCl3 as catalyst.
Either nitrogen-doped or undoped carbon nanotubes were synthesized from ferrocene, pyridine
and toluene and decomposition of acetylene over a catalyst respectively, were functionalized
using azomethine ylides from the thermal condensation of N-methylglycine and 5-norbornene-2-
carboxaldehyde. The polymerization from the side walls of the carbon nanotubes using
bicyclo[2.2.1]hept-2-ene as a monomer was achieved using the Grubbs? second generation
catalyst. The synthesised CNTs and polymer-attached carbon nanotubes were subsequently
characterised.
The attachment of different organic functional groups to the carbon nanotubes from the thermal
condensation of N-methylglycine and 2-thiophenecarboxaldehyde was achieved. The
functionalized carbon nanotubes, and either thiophene or 3-hexylthiophene were used in
copolymerization reactions by oxidative polymerization, using FeCl3 as catalyst. The copolymers
containing the nanotubes, were found to be more regioregular than pure poly(3-hexylthiophene).
The synthesised CNTs and polymer-attached carbon nanotubes were then characterised.
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The thiophene-based C60-copolymers and thiophene polymer-attached CNTs, with and without
TiO2, were deposited on the surface of TiO2 paste with, and without, dye impregnation. A
sandwich-type cell made from a TiO2 thin film electrode with, and without, dye impregnation,
ionic liquid electrolytes and a Pt-coated fluoride tin oxide (FTO) counter electrode was prepared.
Both organic and dye sensitized solar cells (DSSC) were subsequently assembled. The
efficiency, current densities, open circuit voltages and fill factors were found to decrease as the
concentration of C60 derivative in the copolymer decreased. Furthermore, pre-mixing the
copolymers with TiO2 nanoparticles improved the overall performance of the photo cell. In
addition, the polymer-attached N-doped CNTs performed better in the photo cells than polymer-
attached undoped CNTs.
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Table of contents
Title Page Number
Acknowledgements ...................................................................................................................... iii
List of Figures .............................................................................................................................. xv
Introduction ................................................................................................................................... 1
1.1 Background and rationale ......................................................................................................... 1
1.2 Objectives ................................................................................................................................. 4
1.3 Thesis outline ........................................................................................................................... 5
1.4 References .......................................................................................................................................... 7
Literature review ........................................................................................................................ 10
2.0 General ................................................................................................................................... 10
2.1 Carbon nanotubes, fullerene (C60) and other carbonaceous materials .................................... 10
2.2 Chemistry of fullerene (C60) ................................................................................................... 11
2.2.1 General introduction to fullerenes (C60) .................................................................................. 11
2.2.2 Reactions of fullerene (C60) .................................................................................................... 14
2.2.3 [2+2] Cycloadditions............................................................................................................... 14
2.2.4 [3+2] Cycloadditions............................................................................................................... 15
2.2.5 [4+2] Cycloadditions............................................................................................................... 16
2.2.6 Other types of reactions on Fullerenes (C60) ........................................................................... 17
2.2.7 Application in polymers .......................................................................................................... 19
2.3 Chemistry of carbon nanotubes (CNTs) ................................................................................. 21
2.3.1 General introduction ............................................................................................................... 21
2.3.2 Covalent functionalization of carbon nanotubes ..................................................................... 21
2.3.3 Sidewall halogenation of CNTs .............................................................................................. 21
2.3.4 Hydrogenation ......................................................................................................................... 23
2.3.5 Cycloadditions ........................................................................................................................ 23
2.3.6 Amidation/Esterification Reactions ........................................................................................ 26
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2.3.7 Grafting of Polymers ............................................................................................................... 27
2.3.8 Other reactions ........................................................................................................................ 29
2.3.9 Mechanochemical functionalization ....................................................................................... 30
2.4 The application of fullerene (C60) and carbon nanotubes in solar cells .................................. 30
2.4.1 General introduction ............................................................................................................... 30
2.4.2 C60 fullerene in photo cells ...................................................................................................... 35
2.4.3 Carbon nanotubes in solar cells............................................................................................... 40
2.5 References .............................................................................................................................. 42
Chapter 3.. ................................................................................................................................... 57
Ring-opening co-polymerization of a C60-cyclopentadiene cycloadduct and norbornene with
the Grubbs second generation catalyst* ................................................................................... 57
3.1 Introduction ............................................................................................................................ 57
3.2 Experimental .......................................................................................................................... 59
3.2.1 General procedure ................................................................................................................... 59
3.2.2 Synthesis of the C60-cyclopentadiene cycloadduct ................................................................. 59
3.2.3 Polymerization of Polynorbornene 3.4 ................................................................................... 59
3.2.4 Co-polymerization of 3.3 and norbornene 3.4 ........................................................................ 60
3.3 Results and Discussion ........................................................................................................... 60
3.3.1 Synthesis of copolymers ......................................................................................................... 60
3.3.2 Spectroscopic Studies of synthesized materials ...................................................................... 62
3.3.2.1 FTIR studies of synthesized materials .................................................................................... 62
3.3.2.2 UV-visible absorption spectroscopic studies of synthesized materials ................................... 63
3.3.3 Thermal Degradation Studies of synthesized materials .......................................................... 65
3.3.3.1 Differential scanning calorimetry (DSC) ................................................................................ 65
3.3.3.2 Thermogravimetric analysis (TGA) ........................................................................................ 67
3.5 References .............................................................................................................................. 69
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Chapter 4.. ................................................................................................................................... 71
Ring-opening metathesis co-polymerization of a C60-cyclopentadiene cycloadduct and N-
(cycloheptyl)-endo-norbornene-5,6-dicarboximide ................................................................. 71
4.1 Introduction ............................................................................................................................ 71
4.2 Experimental .......................................................................................................................... 72
4.2.1 General procedures ................................................................................................................. 72
4.2.2 Synthesis of C60-cyclopentadiene cycloadduct 4.7 ................................................................. 72
4.2.3 Synthesis of N-(cycloheptyl)-endo-norbornene-5,6-dicarboximide 4.4.................................. 72
4.2.4 Polymerization of N-(cycloheptyl)-endo-norbornene-5,6-dicarboximide 4.4 to afford 4.9A . 73
4.2.5 Co-polymerization of C60-cyclopentadiene 4.7 and N-(cycloheptyl)-endo-norbornene-5,6-
dicarboximide 4.4 ................................................................................................................... 73
4.3 Results and Discussion ........................................................................................................... 74
4.3.1 Synthesis of 4.4 and copolymers ............................................................................................. 74
4.3.2 Spectroscopic Studies of the synthesized materials ................................................................ 77
4.3.2.1 FTIR studies of synthesized materials .................................................................................... 77
4.3.2.2 UV-visible absorption spectroscopic studies of synthesized materials ................................... 78
4.3.3 Thermal Degradation Studies .................................................................................................. 80
4.3.3.1 Differential scanning calorimetry (DSC) ................................................................................ 80
4.3.3.2 Thermogravimetric analysis (TGA) ........................................................................................ 82
4.4 Conclusion .............................................................................................................................. 83
4.5 References .............................................................................................................................. 84
Chapter 5.. ................................................................................................................................... 87
Polymerization of a C60 derivative with thiophene in the presence of FeCl3 ......................... 87
5.1 Introduction ............................................................................................................................ 87
5.2 Experimental .......................................................................................................................... 89
5.2.1 General procedures ................................................................................................................. 89
5.2.2 Functionalization of C60 .......................................................................................................... 89
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5.2.3 Polymerization reactions ......................................................................................................... 90
5.2.3.1 Polymerization reactions to synthesize polythiophene 5.4 and poly(3-hexylthiophene) 5.5 []
................................................................................................................................................ 90
5.2.3.2 Copolymerization reactions .................................................................................................... 91
5.2.3.3 Copolymerization reactions to form polymers 5.6 and 5.7 ..................................................... 91
5.2.3.4 Copolymerization reactions to form polymers 5.8a, 5.8b and 5.8c ........................................ 91
5.2.3.5 Copolymerization reaction to form polymer 5.9 ..................................................................... 92
5.3 Results and discussion ............................................................................................................ 92
5.3.1 Synthesis of 5.2, 5.3 and copolymers ...................................................................................... 92
5.3.2 Polymerization reactions of the synthesised materials ............................................................ 93
5.3.3 Characterization of the synthesized copolymer ...................................................................... 95
5.3.3.1 1H NMR spectroscopy of the synthesized copolymers ........................................................... 95
5.3.3.2 FTIR spectroscopy of the synthesized copolymers ................................................................. 97
5.3.3.3 UV-visible spectroscopy of the synthesized copolymers ...................................................... 102
5.3.3.4 Thermogravimetric analysis (TGA) ...................................................................................... 103
5.4 Conclusion ............................................................................................................................ 106
5.5 References ............................................................................................................................ 107
Chapter 6.. ................................................................................................................................. 112
Functionalization of nitrogen doped and undoped carbon nanotubes using ring opening
metathesis polymerization with norbornene .......................................................................... 112
6.1 Introduction .......................................................................................................................... 112
6.2 Experimental ........................................................................................................................ 115
6.2.1 General procedures ............................................................................................................... 115
6.2.2 Synthesis of carbon nanotubes ............................................................................................. 115
6.2.2.1 Catalyst preparation for CNT synthesis ............................................................................... 115
6.2.2.2 Carbon nanotube synthesis .................................................................................................... 116
6.2.2.3 Nitrogen doped multiwall carbon nanotubes ........................................................................ 116
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6.2.3 Purification of carbon nanotubes .......................................................................................... 117
6.2.4 Functionalisation and polymerization of carbon nanotubes ................................................. 117
6.2.4.1 Functionalisation of carbon nanotubes.................................................................................. 117
6.2.4.2 Ring opening metathesis and functionalization of CNTs with norbornene .......................... 117
6.3 Results and discussion .......................................................................................................... 118
6.3.1 Synthesis of CNTs ................................................................................................................ 118
6.3.2 Functionalization reactions of CNTs .................................................................................... 118
6.3.3 Polymerization reactions of functionalized CNTs ................................................................ 119
6.3.4 Characterization of the synthesised products ....................................................................... 120
6.3.4.1 Raman Spectroscopy of the synthesised products ................................................................ 120
6.3.4.2 Transmission electron microscopy (TEM) ............................................................................ 122
6.3.4.3 Spectroscopic studies on the copolymers synthesized .......................................................... 124
6.3.5.1 Thermogravimetric Analysis (TGA) of the synthesized materials ....................................... 129
6.3.5.2 Differential scanning calorimetry studies (DSC) of synthesized copolymers ...................... 131
6.4 Conclusion ............................................................................................................................ 132
6.5 References ............................................................................................................................ 133
Chapter 7.. ................................................................................................................................. 138
Synthesis of polythiophene covalently liked to nitrogen doped and undoped carbon
nanotubes.. ................................................................................................................................. 138
7.1 Introduction .......................................................................................................................... 138
7.2 Experimental ........................................................................................................................ 140
7.2.1 General procedures ............................................................................................................... 140
7.2.2 Synthesis and purification of carbon nanotubes .................................................................... 140
7.2.3 Functionalization of 7.1 and 7.3 [18] .................................................................................... 140
7.2.4 Polymerization reactions ....................................................................................................... 140
7.2.4.1 Preparation of copolymers 7.5 and 7.6 .................................................................................. 140
7.2.4.2 Preparation of copolymers 7.7 and 7.8 .................................................................................. 141
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7.2.4.3 Preparation of 7.11 ................................................................................................................ 141
7.2.4.4 Preparation of 7.9 and 7.10 [] ............................................................................................... 141
7.3 Results and discussion .......................................................................................................... 142
7.3.1 Functionalization reactions of CNTs .................................................................................... 142
7.3.2 Polymerization reactions ....................................................................................................... 143
7.3.3 Characterization of functionalized CNTs and copolymers ................................................... 143
7.3.3.1 Raman Spectroscopy of functionalized CNTs ...................................................................... 143
7.3.3.2 Transmission Electron Microscopy (TEM) .......................................................................... 145
7.3.3.2.1 Poly(3-hexylthiophene)/CNT composite ...................................................................... 146
7.3.3.2.2 Polythiophene/CNT composite ..................................................................................... 147
7.3.3.3 1H NMR spectroscopic studies of the synthesized materials ................................................ 149
7.3.3.4 FT-IR studies of the synthesized materials ........................................................................... 151
7.3.3.5 UV-visible and photoluminescence spectra of the synthesized materials ............................. 154
7.3.4.1 Thermogravimentric analysis (TGA) .................................................................................... 157
7.3.4.2 Differential scanning calorimetry studies ............................................................................. 159
7.4 Conclusions ........................................................................................................................... 160
7.5 References ............................................................................................................................ 162
Chapter 8.. ................................................................................................................................. 165
Application of 3-hexylthiophene functionalized C60 and carbon nanotubes in solar cells . 165
8.1 Introduction .......................................................................................................................... 165
8.2 Experimental ........................................................................................................................ 168
8.2.1 Assembly of the DSSC ......................................................................................................... 168
8.2.2 Assembly of the organic solar cell ........................................................................................ 168
8.3 Results and discussion .......................................................................................................... 169
8.3.2 Application of copolymers in dye-sensitized solar cells (DSSCs) ........................................ 171
8.3.2.1 Current-Voltage Characteristics ............................................................................................ 171
8.3.2.1.1 C60-copolymers 5.3a, 5.3b and 5.3c in DSSC ............................................................... 171
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8.3.2.1.2 Solar cell assembled with functionalized carbon nanotubes ......................................... 180
8.3.3 Organic solar cells ................................................................................................................. 184
8.4 Conclusions .......................................................................................................................... 187
8.5 References ............................................................................................................................ 188
Chapter 9.... ............................................................................................................................... 192
9.1 Conclusions and future work ................................................................................ 192
9.1.1 Conclusions ........................................................................................................................... 192
9.1.2 Future work and recommendations ....................................................................................... 194
9.1.2.1 Functionalization of carbonaceous materials ........................................................................ 194
9.1.2.2 Photovoltaic Device applications .......................................................................................... 195
9.2 Reference ................................................................................................................ 197
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List of Figures
Page
1. Figure 2.1 Diagrams of (a) metallic (5,5) SWNT, (b) pyramidalization angle (?P), and (c) the
?-orbital misalignment angles (?) along the C1-C4 in the (5,5) SWNT and its capping
fullerene, C60. ??????????????.??????????????.?...13
2. Figure 2.2 Polymers involving the C60 moiety. (Ia), Pendant on-chain (Ib) pendant on-
surface, (II) in-chain, (III)dendritic, (IV) cross-link and (V) end-chain ??.?????...20
3. Figure 2.3 Photoinduced generation of reactive nitrenes in the presence of
nanotubes????????????????????????????.?...??24
4. Figure 2.4 1,3-dipolar cycloaddition of azomethine ylides n the surface of CNTs. ???..25
5. Figure 2.5 Reaction pathway for obtaining water-soluble ammonium-modified
nanotubes????????????????????.?.. ???????.?..?.25
6. Figure 2.6 1,3-Dipolar cycloaddition of nitrile imines to nanotubes. ??????.??..26
7. Figure 2.7 Derivatization reactions of acid-cut nanotubes through the defect sites of the
graphitic surface. ...????????????????????????.??...?27
8. Figure 2.8 Direct thermal mixing of nanotubes and long chain amines. ???.????.27
9. Figure 2.9 Operation principle of a dye sensitized solar cell. ???????.???..?32
10. Figure 2.10 Current-voltage (I-V) curves of a solar cell?????????..????..33
11. Figure 2.11 Schematic representation of interfacial electron transfer following light
absorption for cis-[Ru(dcbH2)2LL?] with some ancillary igands??????.????..34
12. Figure 2.12 Representation of a donor?acceptor heterojunction. ?????????.?36
13. Figure 2.13 Theoretical principle of a donor?acceptor heterojunction????.????37
14. Figure 2.14 Molecular structures of some semiconducting conjugated polymers used in
fullerene-based solar cells. ?????????????????????..???..38
15. Figure 2.15 Schematic representation of a bulk-heterojunction solar cell with the
ITO/PEDOT-SS/P3HT:[60]PCBM/LiF/Al device. ???????..?...??.??.....?39
16. Figure 3.1 FT-IR spectra of the samples in KBr pellet form. (A) Polynorbornene 3.4, Co-
polymers 3.6B-E: (B) mole ratio 50:1, (C) mole ratio 100:1, (D) mole ratio 500:1, (E) mole
ratio 1000:1, (F) C60-cyclopentadiene cycloadduct 3.3, (G) C60 3.1????????..?63
17. Figure 3.2 UV-visible spectra in toluene of co-polymers 3.6B-E and polynorbornene 3.6A:
(A) Polynorbornene, Copolymers 3.6B-E: (B) 50:1 mole ratio, (C) 100:1 mole ratio, (D)
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500:1 mole ratio, (E) 1000:1 mole ratio, (F) C60 3.1, (G) C60-Cyclopentadiene cycloadduct
3.3??????????????????????????????????...65
18. Figure 3.3 Differential Scanning Calorimetry Tg analyses (scan rate 5oC/min): Co-polymers
3.3A-E: (A) Polynorbornene, (B) 50:1 mole ratio, (C) 100:1 mole ratio, (D) 500:1 mole
ratio, (E) 1000:1 mole ratio???????...???..??????...............................67
19. Figure 3.4 TGA curves of the polymers (rate 10oC/ min) under a nitrogen atmosphere: (A)
Polynorbornene, Co-polymers 3.6B-E: (B) 50:1 mole ratio, (C) 100:1 mole ratio, (D) 500:1
mole ratio, (E) 1000:1 mole ratio?????????????.??..?????...?.69
20. Figure 4.1 FT-IR spectra of the samples in KBr pellet form (mole ratio 4.4:4.7): 4.9A (1:0),
4.9B (1000:1), 4.9C (700:1), 4.9D (500:1), 4.9E (300:1), 4.9F (100:1), 4.9G (50:1), 4.9H
C60-cyclopentadiene cycloadduct 4.7, 4.9I C60 4.5..????.??????????.?78
21. Figure 4.2 UV-visible spectra of co-polymers 4.9A-I (in toluene, mole ratio 4.4:4.7): 4.9A
(1:0), 4.9B (1000:1), 4.9C (700:1), 4.9D (500:1), 4.9E (300:1), 4.9F (100:1), 4.9G (50:1),
4.9H C60-cyclopentadiene cycloadduct 4.7, 4.9I C60 4.5????????.?.???.....79
22. Figure 4.3 Differential scanning calorimetry Tg analyses (scan rate 5
oC/min) under nitrogen
atmosphere: Co-polymers 4.9A-G (mole ratio 4.4:4.7): 4.9A (1:0), 4.9B (1000:1), 4.9C
(700:1), 4.9D (500:1), 4.9E (300:1), 4.9F (100:1), 4.9G (50:1)???..?.???...?..?82
23. Figure 4.4 TGA curves of the polymers (rate 10 oC/ min) under a nitrogen atmosphere (mole
ratio 4.4:4.7): 4.9A (1:0), 4.9B (1000:1), 4.9C (700:1), 4.9D (500:1), 4.9E (300:1), 4.9F
(100:1), 4.9G (50:1)???...??????????????????????...?..84
24. Figure 5.1 Different types of traid regioisomers of poly(3-hexylthiophene)........................95
25. Figure 5.2 1H NMR spectra of the monomer, polymer and copolymers of the synthesised
polymers (CDCl3, r.t). ...........................................................................................................97
26. Figure 5.3a. FT-IR spectra of the polythiophene derivatives in KBr pellet. ????..?...98
27. Figure 5.3b. FT-IR spectra of the polyhexyl derivatives in KBr pellet. ??........??.?...99
28. Figure 5.4 UV-visible absorption spectrum of poly(3-hexylthiophene) 5.5, compounds 5.2
and 5.3, and copolymers 5.8a, 5.8b, 5.8c and 5.9 in THF .................................................104
29. Figure 5.5a. TGA thermograms of copolymers 5.6, 5.7 and polythiophene (5.4)......??.105
30. Figure 5.5b. TGA thermogram of copolymers 5.8a, 5.8b, 5.8c and poly(3-hexylthiophene)
(5.5)?????????????????????????????????...106
31. Figure 6.1 Raman spectra of A) unfunctionalized N-CNTs 6.3; B) functionalized N-CNTs
6.4; C) unfunctionalized CNTs 6.1; D) functionalized CNTs 6.2........................................122
32. Figure 6.2 TEM image of (1) undoped CNT 6.1; (3) N-CNTs 6.3?.....??????.?123
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33. Figure 6.3 TEM images of A) polymer attached CNTs 6.5; and B) polymer attached N-
CNTs 6.6?.........................?????.??????????..???????..?.124
34. Figure 6.4 Schematic diagram of the cis and trans olefinic and allylic protons...................125
35. Figure 6.5 1H NMR spectra (in CDCl3) of the monomer norbornene; polynorbornene;
copolymer from undoped CNT-polynorbornene 6.5 and copolymer from N-doped-
polynorbornene copolymers 6.6????..?.?????????..???????.126
36. Figure 6.6a FT-IR of Pristine undoped CNTs 6.1; functionalized undoped CNTs 6.2; pristine
N-CNTs 6.3; functionalized N-CNTs 6.4?????.??????????.?.?.?.128
37. Figure 6.6b FT-IR of polynorbornene-attached CNT 6.5; polynorbornene-attached N-CNT
6.6; polynorbornene and norbornene monomer....................................................................129
38. Figure 6.7a TGA scan of 1) pristine CNT 6.1; 2) pristine N-CNT 6.2; 3) functionalized CNT
6.3; 4) functionalized N-CNT 6.4.........................................................................................130
39. Figure 6.7b TGA scan of polynorbornene-attached CNT 6.5; polynorbornene-attached N-
CNT 6.6; and polynorbornene..............................................................................................131
40. Figure 6.8 DSC scan of polynorbornene-attached CNT 6.5; polynorbornene-attached N-
CNT 6.6; and polynorbornene run under N2 at 5
oC/min heating rate.................................132
41. Figure 7.1 A) Poly(3-hexylthiophene) 7.9; B) undoped CNT-poly(3-hexylthiophene) 7.5;
C) N-doped CNT-poly(3-hexylthiophene) 7.6.....................................................................144
42. Figure 7.2 Raman spectra of A) unfunctionalized CNTs 7.1; B) functionalized CNTs 7.2; C)
unfunctionalized N-CNTs 7.3; D) functionalized N-CNTs 7.4............................................145
43. Figure 7.3 TEM images of A) CNTs 7.1 and B) N-CNTs 7.3..?????..???..??146
44. Figure 7.4 TEM images of functionalized N-CNTs 7.4 ?....???????..???...147
45. Figure 7.5a TEM images of A and B are for poly(3-hexylthiophene) attached undoped
CNTs 7.5 ; C and D for poly(3-hexylthiophene) attached N-CNTs 7.6...?????.....148
46. Figure 7.5 b TEM images of A and B, and C polythiophene attached N-CNTs 7.7 and
undoped CNTs 7.8, respectively????????????????????...........149
47. Figure 7.5c TEM images for unfunctionalized CNT and polythiophene mixed 7.11..........150
48. Figure 7.6 1H NMR spectra of the monomer and copolymers (CDCl3), Poly(3-
hexylthiophene) 7.9; undoped CNT poly(3-hexylthiophene) 7.5; N-doped poly(3-
hexylthiophene) 7.6???..???????????????????????.?151
49. Figure 7.7 FT-IR of unfunctionalized undoped CNTs 7.1; functionalized undoped CNTs 7.2;
unfunctionalized N-CNTs 7.3; functionalized N-CNTs 7.4????..?...????...?.153
xviii | P a g e
50. Figure 7.8a FT-IR of poly(3-hexylthiophene) 7.9; poly(3-hexylthiophene) attached
undoped CNTs 7.6; poly(3-hexylthiophene) attached N-CNTs 7.5 and 3-hexylthiophene
?????????????????????????????????.?.?154
51. Figure 7.8b FT-IR of polythiophene attached N-CNT 7.7; polythiophene attached undoped
CNT 7.8 and polythiophene 7.9..........................................................................................155
52. Figure 7.9 UV-vis absorption spectra in THF of A) poly(3-hexylthiophene) 7.9 ; B)
polymer attached undoped CNTs 7.5; C) polymer attached N-doped CNTs 7.6 ....??..156
53. Figure 7.10 Photoluminescence at 500 nm excitation wavelength in THF of A) poly(3-
hexylthiophene) 7.9; B) polymer attached undoped CNTs 7.5; C) polymer attached N-CNT
7.6????..????????????????????????..????.?157
54. Figure 7.11 TGA profile of functionalized CNTs 7.2 and 7.4?????....????.?158
55. Figure 7.12a TGA profile of A) poly(3-hexylthiophene) 7.9; B) polymer attached undoped
CNTs 7.5; C) polymer attached N-doped CNTs 7.6; D) unfunctionalized undoped CNTs 7.1;
E) unfunctionalized N-doped CNTs 7.3??????.?????.?.??.??...??159
56. Figure 7.12 b TGA profile of F) polymer attached N-CNTs 7.7; G) polymer attached
undoped CNTs 7.8; H) polythiophene 7.10???.????.?.???????..??.159
57. Figure 7.13 DSC scan of the synthesized of A) poly(3-hexylthiophene) 7.9; B) polymer
attached undoped CNTs 7.5; C) polymer attached N-CNTs 7.6..????..???.??.161
58. Figure 8.1. Schematic representation of: (a) a DSSC with the FTO/TiO2/C60-containing
copolymer +TiO2/ electrolyte/Pt; (b) microscopic representation of the TiO2 and C60-
copolymer interaction on FTO glass. (Note: the schematic representations are not drawn to
scale.)??????????????????????..??????.??.??173
59. Figure 8.2 DSSC current-voltage curves (100 mW cm-2 incident light) for the TiO2/C60-
copolymer + TiO2/electrolyte/Pt system: A) 5.3a; B) 5.3b; C) 5.3c. Power-voltage curves:
D) 5.3a; E) 5.3b; F) 5.3c..????????????????????.????....174
60. Figure 8.3 DSSC current density vs voltage (vs Pt counter electrode) curves obtained under
dark conditions for the cells assembled with A) 10:1 mole ratio (5.3a); B) 500:1 mole ratio
(5.3b) and C) 1000:1 mole ratio (5.3c) C60:hexylthiophene copolymer??????.?.175
61. Figure 8.4. Schematic representation of a DSSC with: a) the glass-FTO/TiO2/dye/5.3a
copolymer/electrolyte/Pt; b) the glass-FTO/TiO2/dye/5.3a copolymer + TiO2/electrolyte/Pt;
c) microscopic representation of the TiO2+dye and 5.3a copolymer + TiO2 interaction on
FTO glass. (Note: the schematic representations are not drawn to scale.).????.?....176
xix | P a g e
62. Figure 8.5. DSSC current-voltage curves (100 mW cm-2 incident light): A) 5.3a copolymer
without TiO2; B) 5.3a copolymer with TiO2; C) only with N719 dye????..?..?...177
63. Figure 8.6 Current density - voltage (I-V) curves obtained under dark conditions with
DSSCs based on 5.3a copolymer: A) mixed with TiO2; B) without TiO2????..??178
64. Figure 8.7. DSSC current-voltage curves (100 mW cm-2 incident light) for the TiO2/dye/C60-
copolymer + TiO2/electrolyte/Pt system: A) 5.3a; B) 5.3b; C) 5.3c. Power-voltage curves are
also shown: D) 5.3a; E) 5.3b; F) 5.3c. ??????..?????????..?.?..?.179
65. Figure 8.8. DSSC current-voltage curves under dark conditions for the cells made from
TiO2/dye/C60 copolymer + TiO2/electrolyte/Pt system: (A) 10:1 mole ratio (5.3a); (B) 500:1
mole ratio (5.3b); (C) 1000:1 mole ratio (5.3c)???..??????????.?..?...180
66. Figure 8.9. Schematic representation of the DSSC solar cell with the glass-FTO/TiO2/CNT-
copolymer + TiO2 / electrolyte/ Pt. (Note: the schematic representation is not drawn to
scale)???????????????.????????????????...?.181
67. Figure 8.10. DSSC current-voltage curves (100 mW cm-2 incident light) for the TiO2/CNT-
copolymer + TiO2/ electrolyte/ Pt system: A) N-CNT copolymer 7.6; B) CNT copolymer
7.5. Power-voltage curves are also shown: C) N-CNT copolymer 7.6; D) CNT-copolymer
7.5????????.???????????????????????.??...182
68. Figure 8.11 Current-voltage curves under condition dark for A) N-CNT copolymer 7.6; B)
CNT copolymer 7.5????..?????????????????????...?.182
69. Figure 8.12 Schematic representation of the DSSC made from glass-FTO/TiO2/dye/CNT
copolymer / electrolyte/ Pt. (the schematic representation is not to scale)????...?...183
70. Figure 8.13. DSSC current-voltage curves (100 mW cm-2 incident light) of the glass-FTO /
TiO2 / dye/CNT + TiO2/electrolyte/Pt system: A) CNT copolymer 7.5; B) N-CNT copolymer
7.6. Power-voltage curves are also shown: C) CNT copolymer 7.5; E) N-CNT copolymer
7.6????????????????.???????????????.?.....184
71. Figure 8.14 Schematic representation of the organic solar cell with the ITO/PEDOT-
PSS/P3HT:C60-copolymer/Al device. (Note: the schematic representation is not drawn to
scale)?????????????????...??????????.???.?..186
72. Figure 8.15 Current-voltage curves of organic solar cells based on: a) glass-ITO/PEDOT-
PSS/P3HT: 5.3a (10:1 mole ratio C60-copolymer)/Al; b) glass-ITO/PEDOT-PSS/5.3b
(500:1mole ratio C60-copolymer) /Al (both under dark conditions and 60 mW cm
-2
irradiation)???????????????.????????????..??.....187
xx | P a g e
List of Tables
1. Table 3.1 UV-visible ?max of the co-polymers 3.6B-E and polynorbornene 3.6A?....?..66
2. Table 3.2 Tg of C60-containing polymers 3.6B-E and 3.6A. ????????????.68
3. Table 4.1 Details of the synthesized polymer 4.9A and the co-polymers 4.9B-I...?.?.......78
4. Table 4.2 UV-visible ?max of the polymer 4.9A and the co-polymers 4.9B-I .?....???81
5. Table 4.3 Tg and thermal decomposition temperature of C60-containing polymers 4.9B-E and
4.9A. ????????????????????????????.???..??83
6. Table 5.1 Summary of FTIR of the copolymers, polymers and C60 derivatives (values in cm
-
1) ...........................................................................................................................................102
7. Table 5.2 UV visible spectra of poly(3-hexylthiophene) (5.5), 5.2, 5.3, copolymers 5.8a-c
and 5.9...................................................................................................................................103
8. Table 5.3 summary of thermal decomposition temperature vs mass losses of copolymers and
pure polymers.......................................................................................................................107
9. Table 6.1 Summary of cis and trans ratios of the polymer and functionalized
copolymers???????????????????????????????127
10. Table 7.1 intensity ratios of the D and G Raman bands ?.??????.???...?....146
11. Table 7.2 Summary of FT-IR of the monomer, poly(3-hexylthiophene) and CNT attached
polymers (values in cm-1) .????????????...????????????153
12. Table 7.3 Summary of the Photoluminescence and UV-visible spectra maximum in THF
???????????????????????????????????..157
13. Table 8.1 Photovoltaic performance of the DSSCs based on TiO2/C60-copolymer + TiO2
/electrolyte/Pt??????????????????????????????175
14. Table 8.2 Photovoltaic performance of the DSSCs based on glass-FTO/TiO2/dye/C60-
copolymer + TiO2 /electrolyte/Pt ??????????????????????.180
15. Table 8.3 Photovoltaic performance of the DSSCs made from glass-FTO/TiO2/CNT
copolymer + TiO2 /electrolyte/Pt ..????????????????????.?..183
xxi | P a g e
16. Table 8.4. Photovoltaic performance of the DSSCs based on glass-FTO/TiO2/dye/CNT-
copolymer + TiO2 /electrolyte/Pt ??????????????????????.184
17. Table 8.5 Photovoltaic performance of organic solar cell based on glass-ITO/PEDOT-
PSS/C60-copolymer /Al????..???????????..????????.??187
List of Abbreviations
a. u. Arbitrary units
CNT(s) Carbon nanotubes(s)
CVD Chemical vapor deposition
ROMP Ring opening metathesis polymerization
FAB Fast atom bombardment
FT-IR Fourier transform infrared
TGA Thermal gravimetric analysis
DSC Differential scanning calorimetry
TEM Transmission electron microscope
ID/IG ratio The intensity ratio of D to G band
Ih symmetry Icosahedral symmetry
LUMO Lowest unoccupied molecular orbital
HOMO Highest occupied molecular orbital
MS Mass spectrosopy
m/z Mass-to-charge ratio
r.t rom temprature
NMR Nuclear magnetic resonance
Tg Glass transition temperature
PEDOT poly(3,4-ethylenedioxythiophene)
PSS poly(styrenesulfonate)
ITO indium tin oxide
FTO Floride tin oxide
MEH-PPV poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene]
[60]PCBM [6,6]-Phenyl C61 butyric acid methyl ester
xxii | P a g e
P3HT Poly(3-hexylthiophene)
DMF N,N-Dimethylformamide
THF Tetrahydrofuran
TMS Tetramethylsilane
UV-VIS Ultra-violet visible
IR Infrared
DSSC Dye sensitized solar cells
Voc open circuit voltage
Isc short circuit current
FF fill factor
min minute
h hour
mA milliamperes
V Volt
AM1.5G Air mass 1.5 global (Part of the standard test conditions for PV cells. The intensity
of insolation equivalent to the Sun shining through the atmosphere to sea level, with
oxygen and nitrogen absorption, at an oblique angle 48.2deg from the zenith.)
Chapter One
1 | P a g e
Introduction
1.1 Background and rationale
Among the renewable energy sources, solar energy is of great importance. It is clean and
environmentally friendly. Sunlight can be transformed into electricity using solar cells. Solar
cells have applications in many different fields such as in calculators, solar lamps and can be
used on spacecraft and satellites.
Bulk heterojunction structures based on carbon materials have attracted a great deal of interest
for both fundamental scientific reasons and for potential applications in various new
optoelectronic devices, e.g. photovoltaic solar cells. Thus far, although many materials have been
reported for making a heterojunction-based solar cell, only silicon has found commercial use [1,
2]. Conventional solar cells hve been built from inorganic materials such as silicon and the
efficiency of such solar cells has reached 24 % [3]. However, the cost of these solar cells is too
high to to allow their extensive use in daily life since the materials are expensive and they
require energy intensive processing techniques to produce them. Moreover, silicon has also a
drawback that under illumination it degrades, which limits its lifetime and stability. Therefore, it
is crucial to find a new kind of clean and cheap material for solar cells. In the search for
alternative materials, carbon is highly attractive because it is expected to have similar properties
to silicon and would be highly stable. Carbon is a remarkable element existing in a variety of
stable forms ranging from insulator/semiconducting diamond (or diamond-like amorphous film
[4]) to metallic/semi-metallic graphite (or graphene [5, 6]), conducting/semiconducting
fullerenes (e.g. C60) [7] and carbon nanotubes (CNTs) [8, 9], all of which show many interesting
optoelectronic, physical and chemical properties. The various forms of carbon have attracted a
Chapter One
2 | P a g e
great deal of interest in recent years because of their unique structures, properties and potential
applications in energy storage and conversion [10-12].
A lot of effort is being put into the development of new fabrication techniques using organic,
[13], hybrid [14] and photoelectrochemical (dye sensitized) solar cells [15] which could act as
alternatives to conventional silicon solar cells. Organic solar cells mainly consist of two organic
materials; an electron-donating material and an electron-accepting material that make a
percolating structure with interpenetrating networks [16]. A hybrid solar cell on the other hand,
is a combination of both organic and inorganic materials and therefore combines the unique
properties of inorganic semiconductors with the film forming properties of conjugated polymers
[17]. Organic materials are generally inexpensive, easily processable and their functionality can
be tailored by molecular design and chemical synthesis. On the other hand, inorganic
semiconductors can be manufactured as nanoparticles. Inorganic semiconductor nanoparticles
offer the advantage of having high absorption coefficients and size tunability. By varying the
size of the nanoparticles the bandgap can be tuned and therefore the absorption range can be
tailored [18].
Since the first demonstration by O?Reagan and Gratzel in 1991 [19], dye sensitised or
photoelectrochemical solar cells (DSSCs or PECs) have attracted significant and sustained
interest [20-22] and moreover, these cells have been regarded as promising cells for next
generation photovoltaic devices due to their attractive features of high power conversion
efficiency (>10%) and low production cost [23-28]. DSSCs have been studied extensively using
wide band gap nano-crystalline TiO2 sensitized with ruthenium polypyridine complexes [29] or
with metal free organic dyes [30] as photo-electrodes.
Chapter One
3 | P a g e
Carbon nanotubes offer a wide range of band gaps [31-33] to match the solar spectrum, enhanced
optical absorption [34, 35] and reduced carrier scattering for hot carrier transport [36, 37]. The
latter may even result in a near-ballistic transport in nanotubes with submicron-meter lengths
[38]. Castrucci et al. [39] have demonstrated that MWNTs can generate a photocurrent in the
visible and ultraviolet spectral range. Recently, Cheong et al. [40] have investigated the
photoresponsive conductance switching of MWNTs-SPO (SPO = spironaphthoxazines) under a
365 nm UV irradiation. In their study, they reported that during the cyclic irradiation of
MWNTs-SPO by UV light the composites showed a reversible response, in which the change of
HOMO?LUMO band gap in SPO strongly affects the conductivity of the MWNTs.
Among donor?acceptor type organic solar cells, the most promising material combination is
poly(3-octylthiophene) (P3OT), poly(3-hexylthiophene) (P3HT) and the fullerene derivative
(6,6)- phenyl-C61-butyricacidmethylester (PCBM) [41]. These are reagents although expensive
though PCBM can form film-like structures with high electron mobility. Investigation of carbon-
based organic solar cells has been conducted towards developing alternative low-cost, light
weight, flexible devices. Two typical carbon materials, fullerenes (C60) and CNTs, are always
involved, particularly when combined with p-conjugated polymers and several have been found
to be photo-active materials. It is well known that C60 is a good electron acceptor and is efficient
in charge separation [42]. Semiconducting CNTs can be a suitable replacement for C60 by
forming ideal hetero-junctions. The work function (F) of CNTs is in the range of 4.5?5.1eV,
which is close to the valence band of P3OT/P3HT. Therefore, CNTs can help to improve exciton
dissociation at the CNT/polymer interface and provide efficient hole or electron transport.
Chapter One
4 | P a g e
Functionalization of carbonaceous material is a crucial step in making their solar cell devices. In
this regard, a [3+2] cycloaddtion method has been employed by research groups for the
functionalization of carbonaceous materials [43-45]. During the reaction a reactive intermediate,
azomethine ylide, is generated in situ by the decarboxylation of immonium salts derived from the
thermal condensation of amino acids and aldehydes (or ketones), or the thermal ring opening of
aziridines. It has been shown that this approach is one of the most flexible methods for the
functionalization of fullerenes and CNTs, and has been widely used [43-45]. The application of
azomethine ylides for the functionalization of CNTs was first reported by Georgakilas et al. [45].
The 1,3-dipolar cycloaddition of azomethine ylides with alkene or alkyne is a very effective
method for the construction of pyrrolidine- and pyrrole-rings in the synthesis of pyrrolidine- and
pyrrole-containing molecules. Different fragments with important electronic properties have
been covalently attached to the fullerene system using azomethine ylides and molecules such as
porphyrins [46], subphthalocyanines [47], dendrimers [48] and conjugated oligomers [49].
1.2 Objectives
i. To synthesize both nitrogen doped and undoped MWCNTs using well developed
methods.
ii. To functionalize C60 and both nitrogen doped and un-doped MWCNTs using azomethine
ylides (the Prato reaction) and subsequently to bind the C60 and MWCNT derivatives
covalently either to a polymer polythiophene or polynorbornene system.
iii. To study the photovoltaic behavior of the newly synthesized materials in DSSC and
organic solar cells.
iv. To characterise the synthesised materials using transmission electron microscopy (TEM),
thermogravimetric analysis (TGA), as well as NMR, Raman, FT-IR, UV visible and
photoluminescence spectroscopy.
Chapter One
5 | P a g e
1.3 Thesis outline
Chapter 1: gives a motivation of the work presented in the subsequent chapters. Although each
chapter briefly introduces itself, the information contained in this chapter is of a general nature
and gives a justification to the problems presented and briefly what measures were undertaken to
address them and how this was achieved.
Chapter 2: is a general literature review. The chapter gives a detailed description of the
chemistry of carbonaceous materials. Common methods of CNT/C60 functionalization, polymer
composite synthesis and their application in photovoltaic devices are also described.
Chapter 3: presents a report on the synthesis of a C60-cyclopentadiene cycloadduct and
polymerization of a co-monomer with norbornene, using the Grubbs second generation catalyst,
in order to obtain a new range of samples containing different amounts of C60.
Chapter 4: this chapter presents the investigation of the synthesis and properties of a series of
novel C60-containing polymeric materials based on the ROMP of a N-(cycloheptyl)-endo-
norbornene-5,6-dicarboximide monomer with a C60-cyclopentadiene cycloadduct. This reagent
was chosen in the hope that it would enhance the solubility of the fullerene containing polymers.
Chapter 5: this chapter describes the functionalization of C60 using azomethine ylides (the Prato
reaction) and its subsequent reaction to incorporate the C60 derivatives covalently to a polymer
polythiophene system. The electronic and thermal properties of the synthesized copolymers were
investigated.
Chapter 6: in this chapter a new approach to the functionalization of the N-doped and non-
doped CNTs based on ring opening metathesis (ROMP) using Grubbs? catalyst, after sidewall
functionalization of both N-doped and non-doped CNTs were studied is given. The thermal and
electronic properties of these new co-polymers were investigated.
Chapter One
6 | P a g e
Chapter 7: a new approach for the functionalization of N-doped and undoped CNTs using a 1,3-
dipolar cycloaddition of azomethine ylides and their subsequent in situ polymerization with
thiophene is discussed. The resulting polymers were characterized and their thermal and
electronic properties studied.
Chapter 8: in this chapter the photovoltaic behaviour of the synthesized materials were studied.
The use of polymer attached C60 and MWCNTs were investigated both in DSSC and organic
solar cells.
Chapter 9: presents a general summary and gives conclusions to the work presented in this
thesis. The chapter highlights the successes of the project and the usefulness of the outcomes.
Future studies described from the conclutions are given.
Chapter One
7 | P a g e
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W. K. Ge, X. Xiao, Phys. Rev. Lett., 2004, 93, 017402.
38 A. Javey, J. Guo, Q. Wang, H. Dai, Nature, 2003, 424, 654.
39 P. Castrucci, F. Tombolini, M. Scarselli, E. Speiser, S. Del Gobbo, W. Richter, M. De
Crescenzi, M. Diociaiuti, E. Gatto, M. Venanzi, Appl. Phys. Lett., 2006, 89, 253107.
Chapter One
9 | P a g e
40 I. W. Cheong, S. Wang, H. S. Ki, S.-H. Kim, Curr. Appl. Phys., 2009, 9, 1269.
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Funct. Mater., 2004, 14, 1005. (b) C. Waldauf, P. Schilinsky, J. Hauch, C. J. Brabec, Thin
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N. S. Sariciftci, J. Mater. Chem., 2006, 16, 45.
43 M. Maggini, G. Scorrano, M. Prato, J. Am. Chem. Soc., 1993, 115, 9798.
44 M. Prato, M. Maggini, Acc. Chem. Res., 1998, 31, 519.
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10 | P a g e
Literature review
2.0 General
2.1 Carbon nanotubes, fullerene (C60) and other carbonaceous materials
Carbon-based materials, clusters, and molecules are unique in many ways. One distinction
relates to the many possible configurations of the electronic states of a carbon atom, which is
known as the hybridization of atomic orbitals and relates to the bonding of a carbon atom to its
nearest neighbours. Carbon is the sixth element of the periodic table and has the lowest atomic
number of any element in column 14 of the periodic table. Each carbon atom has six electrons
which occupy the 1s2, 2s2, and 2p2 atomic orbitals. The 1s2 orbital contains two strongly bound
core electrons. Four more weakly bound electrons occupy the 2s22p2 valence orbitals. In the
crystalline phase, the valence electrons give rise to 2s, 2px, 2py, and 2pz orbitals which are
important in forming covalent bonds in carbon materials.
The various bonding states are connected with certain structural arrangements, so that sp bonding
gives rise to chain structures, sp2 bonding to planar structures and sp3 bonding to tetrahedral
structures. The sp2 bonded graphite is the ground state phase of carbon under ambient conditions
while at higher temperatures and pressures, sp3 bonded cubic diamond is more stable. Other
forms of stable carbons such as hexagonal diamond, hexagonal carbenes [1-3], and liquid carbon
[4] have been reported. It is believed that a variety of novel ?-electron carbon bulk phases remain
to be discovered and explored.
Recently, much attention has been focused on small carbon clusters [5], since the discovery of
fullerenes in 1985 by Kroto et al. [6] and of carbon nanotubes in 1991 by Iijima [7]. A carbon
nanotube is often considered as a rolled sheet of graphene cylinder [8]. Nanotubes can be rolled
from a graphene sheet in many ways [5,8]. There are many possible orientations of the hexagons
on the nanotubes, even though the basic shape of the carbon nanotube wall is a cylinder. The
electronic bonding in carbon nanotubes is sp2 which is characteristic of graphite and plays a
significant role in the properties of carbon nanotubes. The curvature of the nanotubes contains a
Chapter Two
11 | P a g e
small amount of sp3 bonding so that the force constants (bonding) in the circumferential direction
are slightly weaker than along the nanotube axis.
Nanotubes can be broken down into two broad categories. The first, called the multiwalled
carbon nanotubes (MWNT), were the first to be discovered. These are similar to hollow graphite
fibers [9], except that these have a much higher degree of structural perfection. They are made of
concentric cylinders placed around a common central hollow, with a spacing between the layers
close to that of the interlayer distance in graphite (0.34 nm). This interlayer spacing in MWNTs
is slightly larger than the single-crystal graphite value (0.335 nm) since in the nanotubes there is
a severe geometrical constraint when forming perfect cylinders while maintaining the graphite
spacing between them. The three-dimension structural correlation that prevails in single crystal
graphite (ABAB stacking) is lost in the nanotubes, and the layers are rotationally disordered with
respect to each other. The second variety is close to an ideal fullerene fiber; in size they are close
to fullerenes and have single-layer cylinders extending from end to end [10, 11]. These tubes are
called the single-walled nanotubes (SWNTs) and possess good uniformity in diameter (1-2 nm).
When SWNTs are produced in the vapor phase, they self-assemble into larger bundles (ropes)
that consist of several tens of nanotubes [12-14]. These tubes then assemble into a one-
dimensional triangular lattice structure with a lattice constant of 1.7 nm and a tube-tube
separation of 0.315 nm [14]. This organization of the nanotube units into a crystal structure was
theoretically predicted prior to their observation [15]. Both varieties of nanotubes can be
regarded as aggregates of nanotube units (cylinders), the MWNTs consisting of a concentric
assembly and the SWNTs made up of ropes of close packed nanotube units.
2.2 Chemistry of fullerene (C60)
2.2.1 General introduction to fullerenes (C60)
The existence of fullerenes in sooting flames was first revealed by mass spectrometry studies
[16] and then macroscopic quantities of fullerenes were first generated by resistive heating of
graphite [17]. This method was based on the technique for the production of amorphous carbon
films in a vacuum evaporator [18]. Since then, different methods have been used to maximize the
production of fullerenes [19-21]. For instance, fullerenes can also be obtained by the pyrolysis
Chapter Two
12 | P a g e
of hydrocarbons, preferably aromatics. The first example of this methodology was the pyrolysis
of naphthalene at 1000 ?C in an Ar stream [22].
Each fullerene contains 2(10 + M) carbon atoms, corresponding to exactly 12 pentagons and M
hexagons. This building principle is a simple consequence of Euler?s theorem. Starting at C20,
any even-membered carbon cluster, except C22, can form at least one fullerene structure. With
increasing M the number of possible fullerene isomers rises dramatically, from only 1 for M = 0
to over 20 000 for M = 29 [23, 24]. The C60 isomer [60-Ih] fullerene is the smallest stable
fullerene. The structure of [60-Ih] fullerene was determined theoretically [25-28] and also
experimentally [29-32]. These investigations confirm the icosahedral structure of [60-Ih]
fullerene. The two features of this C60 structure are that all twelve pentagons are isolated by
hexagons and that the bonds at the junctions of two hexagons ([6,6] bonds) are shorter than the
bonds at the junctions of a hexagon and a pentagon ([5,6] bonds). The pentagons within
fullerenes are needed to introduce curvature, since a network consisting of hexagons only, is
planar.
Electronically, C60 is described as having a closed-shell configuration consisting of 30 bonding
molecular orbitals with 60 p electrons [33], which gives rise to a completely full fivefold
degenerate hu highest occupied molecular orbital (HOMO) that is energetically located
approximately 1.5 to 2.0 eV lower than the corresponding anti-bonding lowest unoccupied
molecular orbital (LUMO) [34, 35]. The first electron, on reduction of C60, is added to a triply
degenerate t1u unoccupied molecular orbital that is highly delocalized [36]. This threefold-
degeneracy, together with the low-energy possession of the LUMO, makes C60 a fairly good
electron acceptor with the ability of reversibly gaining up to six electrons upon reduction [37,
38]. This high degree of symmetry in the arrangement of the molecular orbitals of C60 provides
the foundation for physicochemical, electronic, and magnetic properties. Although the
semiconducting [39], magnetic [40, 41], and superconducting [42] properties of unmodified C60
have been intensively investigated, the functionalized fullerenes have as yet not been fully
explored.
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13 | P a g e
The fullerenes are more reactive than CNTs. This is due to the fact that enormous strain is
engendered by their spherical geometry as reflected in the pyramidalization angles of the carbon
atoms [42a]. For an sp2-hybridized (trigonal) carbon atom with pyramidalization angle of ?P =
0?, planarity is strongly preferred, whereas an sp3-hybridized (tetrahedral) carbon atom requires
?P = 19.5? (Figure 1.1b). All of the carbon atoms in C60 have ?P = 11.6?, and hence their
geometry is more appropriate for tetrahedral than trigonal hybridization. Thus the chemical
conversion of any trivalent carbon atom in C60 to a tetravalent carbon atom relieves the strain at
the point of attachment and eases the strain at the 59 remaining carbon atoms [42a]. Hence,
stability is accelerated by strain relief, and this strongly favours fullerene addition chemistry
[42a-44]. Just as in the case of a fullerene, a perfect SWNT without functional groups is
chemically inert. However, curvature-induced pyramidalization and misalignment of the ?-
orbitals [42a, 45-52] of the carbon atoms induces a local strain (Figure 2.1), and carbon
nanotubes are therefore expected to be more reactive than a flat graphene sheet.
Figure 2.1 Diagrams of (a) metallic (5,5) SWNT, (b) pyramidalization angle (?P), and (c) the ?-
orbital misalignment angles (?) along the C1-C4 in the (5,5) SWNT and its capping fullerene,
C60 [42a].
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14 | P a g e
2.2.2 Reactions of fullerene (C60)
Chemical functionalization allows for the preparation of soluble C60 derivatives that maintain the
electronic properties of fullerenes. The chemistry of [60]fullerene is characteristic of electron-
deficient alkenes and, as a consequence, it reacts with nucleophiles and its [6,6] bonds are good
dienophiles. Cycloaddition reactions, such as [2+2], [3+2], and [4+2], play an important role in
the preparation of C60 derivatives and a wide variety of cycloadducts have been prepared [53].
In
the next sections representative examples of these types of cycloadditions will be described.
2.2.3 [2+2] Cycloadditions
Four-membered rings fused to 6,6-ring junctions are formed upon [2+2] cycloadditions. Hoke et
al. [54] and Tsuda et al. [55] were the first to report the [2+2] thermal addition of benzyne to C60.
In addition, Schuster and coworkers [56-59] reported photochemical [2+2] cycloadditions of
cyclic enones and 1,3-diones. Recently, the interest in the synthesis of fullerene dimers has
increased [60], as they can be used as building blocks for nanotechnological applications. The
first report was by Komatsu and coworkers [61, 62]; when the researchers reacted C60 (Scheme
2.1) in a high-speed vibration mill (HSVM), in the presence of potassium cyanide, it produced a
dimer, but when the reaction was conducted in solution only C60-cyanated derivatives were
obtained [63]. Similarly, using the same technique, Murata et al. [64], Forman et al. [65] and
Tagmatarchis et al. [66] have reported reactions between C60 and pentanocene, the dimerization
of C70, as well as the synthesis of C60?C70, respectively.
+
KCN (cat.)
High Speed
Vibration
Milling (HSVM)
Scheme 2.1 Synthesis of C120 by reaction with KCN under the ?high-speed vibration milling?
conditions.
Chapter Two
15 | P a g e
2.2.4 [3+2] Cycloadditions
In [3+2] cycloadditions, five-membered rings fused to 6,6-junctions can be formed. Azomethine
ylides are reactive intermediates that can be generated in situ by the decarboxylation of
immonium salts derived from the thermal condensation of amino acids and aldehydes (or
ketones) (Scheme 2.2), or the thermal ring opening of aziridines. It has been shown that this
approach is one of the most flexible methods for the functionalization of fullerenes and has been
widely used [67, 68]. For example, the 1,3-dipolar addition of azomethine ylides to C60 yields
fulleropyrrolidines that are formed across the 6,6-junction. The most important features of this
type of reaction are that the utilization of functionalized aldehydes leads to the formation of 2-
substituted fulleropyrrolidines, whereas utilization of N-substituted glycines affords N-
substituted fulleropyrrolidines (Scheme 2.2). Moreover, mono-fulleropyrrolidines are formed by
controlling the stoichiometry of the reagents and the reaction conditions. Therefore, through this
methodology, different fullerene derivatives can be obtained, either by using properly
functionalized azomethine ylides or by modifying a fulleropyrrolidine intermediate.
NR2
R1
C60
R1NHCH2COOH + R2CHO
Scheme 2.2 [3+2] Cycloaddition using azomethine ylides.
Functionalized fullerene skeletons combined with further substituents or reactions can lead to the
making of fullerene-based materials with unique properties. For instance, amphiphilic
fulleropyrrolidines bearing suitable hydrophilic addends have been found to form true
monolayers that transfer onto solid substrates, while the formation of Langmuir?Blodgett (LB)
films could not be formed from their corresponding hydrophobic analogs [69-75]. The
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16 | P a g e
functionalization of fullerene with polar side-chains [76] and with positively charged groups [77]
has been reported and the enhancement of the solubility of these adducts allowed sufficiently
high concentrations to be produced to allow study of the biological activity of the
fulleropyrrolidines [78]. In addition, the solubilization of fullerenes in water allows the
possibility to explore their properties in different fields such as medicinal chemistry and
biotechnology.
In further examples, Kang et al. [79] reported a fulleropyrrolidine?mercaptophenyl hybrid
material that was synthesized and self-assembled in two-dimensional arrays. In their report they
indicated that the material showed reversible electrochemistry and electronic properties suitable
for making well-ordered nanostructural morphologies and thin film functional materials.
Finally, using the 1,3-dipolar additions method, Holmes et al. [80]prepared large unnatural
amino acids containing the natural ?-amino acid proline condensed to a [6,6] ring junction of C60
[81]. Alternatively, fulleroprolines could be obtained via the thermal ring opening of aziridines
[82]. In addition, di- and tri-peptides can also be prepared by incorporating fulleroproline at their
N- or C-terminal [83-85].
2.2.5 [4+2] Cycloadditions
The [6,6] double bonds of [60]fullerene are excellent dienophiles (comparable to maleic
anhydride). Thus, C60 can react with different dienes by a Diels?Alder cycloaddition reaction.
Depending on the reactivity of the diene, heating the reaction under reflux in a high boiling
solvent may be required. In some instances the use of microwave energy afforded better results
[86].
The first example of a [4+2] cycloaddition reaction of C60 was carried out using cyclopentadiene
[59] as the 1,4-diene system (Scheme 2.3). Cyclopentadiene derivatives, such as
methylcyclopentadiene and cyclopentadienone, afforded the corresponding monoadducts [87,
88]. The dienophilic behaviour of C60 in [4+2] cycloaddition reactions was also demonstrated in
the reaction of C60 with anthracene (Scheme 2.3) and other anthracene derivatives [89].
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17 | P a g e
anthracene,
toluene,
reflux
cyclopentadiene,
toluene, R.T
Scheme 2.3 [4+2] cycloaddition reactions of C60.
2.2.6 Other types of reactions on Fullerenes (C60)
2.2.6.1 Electrochemical reactions
The possibility of electrochemical production of C60 anions in a defined oxidation state was
reported by applying a proper potential to synthesize fulleride salts [90-96]. The resulting
fulleride anions could also be used to synthesize covalent organofullerene derivatives by
quenching the anions with electrophiles. This was well demonstrated in the synthesis of
dimethyldihydro[60] fullerene [97]. Other methods that have used C60 anions as precursors for
the synthesis of fullerene derivatives usually involve chemical formation of the anion. Alkylation
of C60 has also been accomplished, for example, by reduction with propanethiol and potassium
carbonate in DMF [98, 99], sodium methanethiolate in acetonitrile [100], the naphthalene radical
anion in benzonitrile [101], potassium naphthalide [102] or simply with zinc [103].
2.2.6.2 Reduction with metals
Fullerenes can easily be chemically reduced by reaction with electropositive metals such as
alkali and alkaline earth metals [104]. The anions C60
n? (n = 1?5) can be generated in solution by
titrating a suspension of C60 in liquid ammonia with a solution of rubidium in liquid ammonia
[105]. Similarly alkaline earth metals can also be intercalated with C60 [104, 106, 107]. The
preparation of Ca5C60, Ba6C60 [108], BaxC60 (with x = 3?6) or Ba4C60 and Sr4C60 [108] have been
achieved by the direct reaction of C60 with the corresponding alkaline earth metal vapor. A bulk
Chapter Two
18 | P a g e
reduction of C60 in solution was reported with less electropositive metals such as mercury,
leading to C60
? or C60
2? [109].
2.2.6.3 Addition of carbon nucleophiles
C60 readily reacts with alkyl, phenyl or alkynyl organolithium and Grignard compounds to form
the anions RC60
? as primary intermediates [110]. Other Li acetylides, Li-C?C-R with R = hexyl
[111] or benzylether dendrons [112, 113], have been attached to C60. Besides protonation, alkyl-,
benzyl-, cycloheptatrienyl-, benzoyl- or vinylether-derivatives, formaldehyde and
dichloroacetylene have also been used as electrophiles [110f, 114].
2.2.6.4 Addition of amines
Owing to their high nucleophilicity, primary and secondary aliphatic amines undergo
nucleophilic additions with electron-deficient C60 [110, 115]. Seshadri et. al. [115b] have
reported that a reaction of C60 with excess methylamine in toluene solution at r.t. was nearly
instantaneous and gave a yellow product containing a mixture of adducts. Characterization of the
products using a mass spectroscopy further indicated that up to 14 amine units add to C60.
Furthermore, the addition of a secondary amine such as dimethylamine to C60 under similar
conditions also gives a yellow product mainly containing adducts corresponding to the addition
of 1, 2 and 6 amines units. Kampe et. al. [116] have revealed that diamines such as N,N'-
dimethylethylenediamine, piperazine, or homopiperazine in toluene between 0 and 100 oC, (both
reactants in low concentrations) togather with C60 gave mono- and bisadducts as the predominant
products.
2.2.6.5 Addition of phosphorus nucleophiles
Compared with the wide range of existing carbon or nitrogen nucleophiles that react with C60
there are few examples of reactions with phosphorus nucleophiles. Neutral trialkylphosphines are
less reactive with C60 even at elevated temperatures [117]. Lithiated phosphines and phosphites
however readily, add to the [6,6] double bond of C60 [117, 118].
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19 | P a g e
2.2.7 Application in polymers
Polymers involving C60 [119] are of considerable interest since: (a) the fullerene properties can
be combined with those of specific polymers, (b) suitable fullerene polymers should be spin
coatable, solvent-castable or melt-extrudable, and (c) fullerene-containing polymers as well as
surface-bound C60 layers are expected to have remarkable electronic, magnetic, mechanical,
optical or catalytic properties [120]. Several prototypes of polymers or solids containing the
covalently bound C60 moiety are possible (Figure 2.2) [119b, 121]. They include: fullerene
pendant systems Ia with C60 on the side chain of a polymer (on-chain type or ?charm bracelet?)
[122] or on the surface of a solid Ib [123], in-chain polymers II with the fullerene as a part of the
main chain (?pearl necklace?) [122], dendritic systems III, starburst or cross-link type IV or end-
chain type polymers V that are terminated by a fullerene unit. For III and IV, one-, two- and
three-dimensional variants can be considered. In addition, combinations of all of these types of
polymers are possible.
The addition of living polystyrene to C60 leads to the formation of star-shaped polymers with C60
in the center (Figure 2.2, III) [124]. These polymers are highly soluble and melt processable
[124]. Different ?living? anionic polymers such as polystyrene, the block copolymer
polystyrene-bpoly(phenylvinylsulfoxide) [125] (as a precursor for polyacetylene) or
polyisoprene [126] have been grafted onto C60. Star-shaped polymers with up to six branches are
also possible by using reactive carbanions such as styryl or isoprenyl [119b].
A similar approach was used in the grafting of C60 onto a pregenerated lithiated polyethylene
surface [123]. A polyethylene film with terminal diphenylmethyl groups was deprotonated with
BuLi to yield an anionic polyethylene surface that was treated with C60 and quenched with
methanol. This reaction also worked for polyisopropene, polybutadiene [123], poly(vinylbenzyl
chloride) or poly(N-vinylcarbazole) (PVK) [120] with BuLi or NaH as a base.
The facile addition of primary and secondary amines to C60 has also been used to synthesize
polymer-bound C60 [127-134]. Solutions of precursor polymers containing primary amino groups
in the side chain or secondary amino groups in the main chain [133] were allowed to react with
C60 in a ?buckyball? fishing process. Fullerene end capped polymers (type V) were accessible by
Chapter Two
20 | P a g e
reaction of amino terminated polystyrene [129], poly(ethylene glycol) or poly(propylene glycol)
with C60 [130].
Ia Ib
II
III
IV
V
Figure 2.2 Polymers involving the C60 moiety. (Ia), pendant on-chain (Ib) pendant on-surface,
(II) in-chain, (III) dendritic, (IV) cross-link and (V) end-chain [121-124].
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21 | P a g e
2.3 Chemistry of carbon nanotubes (CNTs)
2.3.1 General introduction
The insolubility of CNTs, practically in all solvents, makes then difficult to manipulate.
Although dispersion of CNTs in some solvents is possible by sonication, precipitation
immediately occurs when this process is interrupted. On the other hand, it has been demonstrated
that CNTs can interact with different classes of compounds to form composites [135-140].
The curvature-induced pyramidalization of ?-orbitals of carbon atoms also induces misalignment
of the ?-orbitals of carbon atoms in SWNTs [135]. As a result, the sidewall of a SWNT should
be more reactive than a flat graphene sheet; a SWNT of smaller diameter having larger ?-orbital
pyramidalization and misalignment angles suffers more severe curvature-induced weakening of
?-conjugation and, hence, is more reactive. However, due to the different bending patterns,
nanotubes have less curvature compared with fullerenes with comparable diameters (see Figure
2.1); accordingly nanotubes are generally less reactive than fullerenes, and harsher experimental
conditions are required to functionalize carbon nanotubes.
By utilization of the misalignment of the ?-orbitals of carbon atoms that is induced by the
curvature of the side wall CNTs, CNTs can undergo chemical modifications that make them
more soluble for integration into inorganic, organic, and biological systems [141]. The main
approaches for modification can be grouped into three categories: (1) the covalent attachment of
chemical groups through reactions with the ?-conjugated skeleton of CNT; (2) the noncovalent
adsorption or wrapping of various functional molecules around the CNTs; and (3) the endohedral
filling of the inner empty cavity of the CNTs.
2.3.2 Covalent functionalization of carbon nanotubes
2.3.3 Sidewall halogenation of CNTs
The side wall fluorination of CNTs has been achieved using elemental fluorine in the range
between room temprature and 600 ?C [142-146]. Nakajima et al. [142] studied the fluorination
of CNTs under a wide range of temperatures and found that covalently fluorinated CNTs could
be made between 250 ?C and 400 ?C. On other hand, Mickelson et al. [144] indicated that the
Chapter Two
22 | P a g e
best results for fluorination reactions on the sidewall of CNTs was between 150 and 400 ?C as at
higher temperatures the graphitic network decomposes. The highest degree of functionalization
was estimated to be about C2F by elemental analysis. However, when fluorination was applied to
small diameter single-walled CNTs, the nanotubes were cut to an average length of less than 50
nm [145].
Theoretical studies on the thermodynamics of side wall fluorination have been reported [146-
148]. In addition, the structures of fluorinated CNTs have been compared both experimentally
and theoretically. However there is controversy regarding the favorable pattern of F addition
onto the sidewalls of CNTs. Based on scanning tunneling microscopy (STM) images and
semiempirical calculations, Kelly et al. [149] proposed that two possible sidewall addition
reactions, 1,2-addition or 1,4-addition, were possible. From their studies they concluded that the
latter addition reaction gives a more stable product. However, using DFT calculations on a
fluorinated carbon nanotube it was predicted that a 1,2-addition pattern is energetically more
favorable than the 1,4-addition by about 16 kJ/mol [146e].
The fluorination reaction is a very useful reaction since further modification reactions can be
accomplished after F addition [150-152]. For example, alkyl groups could replace the fluorine
atoms, using Grignard [151b] or organolithium [152] reagents. As a result, the alkylated CNTs
can be well dispersed in common organic solvents such as THF. In addition, several diamines
[151a] or diols [153] were reported to react with fluoronanotubes via nucleophilic substitution
reactions.
Mickelson et al. [154] reported that a moderate degree of solvation was achieved after sonicating
fluorinated nanotubes in a variety of alcoholic solvents and further reactions were reported on
those CNTs by reacting them with hydrazine. On the other hand, recovery of pristine CNTs was
possible after fluorination by heating under an inert atmosphere [155, 156] the majority of the
fluorine atoms could be detached after suspension of CNTs in 2-propanol hydrazine solution
[147c, 157].
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23 | P a g e
2.3.4 Hydrogenation
Hydrogenation of CNTs has been reported by Pekker et al. [158] by reducing pristine CNTs with
Li metal and methanol dissolved in liquid ammonia (Birch reduction). The hydrogenated
material was found to be stable up to 400 ?C. Moreover, CNTs have been functionalized with
atomic hydrogen using a glow discharge [159-161] or by proton bombardment [162]. The
hydrogen storage capacity of CNTs (hydrogen to carbon atom ratio of 0.52) was reported by Liu
et al. [163] at r.t. under a modestly high pressure (about 10 mega Pascal) for a SWNT. Recently,
Yang et al. [164] have reported that the hydrogen storage capacity increased from 1.2 wt % to
1.52 wt % at 77 K and 1 bar and from 0.3 wt % to 0.61 wt % at 298 K and 95 bar using a hybrid
composite of acid-treated multiwalled carbon nanotubes (MWNTs) and MOF-5 [MOF-5 =
Zn4O(bdc)3; bdc = 1,4-benzenedicarbocylate].
2.3.5 Cycloadditions
Cycloadditions are reactions in which two or more unsaturated molecules (or parts of the same
molecule) combine with the formation of a cyclic adduct in which there is a net reduction of the
bond multiplicity.
The Haddon group was the first to apply carbene [2+1] cycloadditions to pristine CNTs [165-
169]. During the reaction, carbene was generated in situ using a chloroform/sodium hydroxide
mixture or a phenyl(bromodichloromethyl) mercury reagent. The [2+1] cycloadditions reactions
then took place on the side wall of the CNTs.
Hirsch et al. [170, 171] performed studies that showed that the nucleophilic addition of carbenes
to CNTs preferably produced zwitterionic 1:1 adducts rather than cyclopropane systems. Another
possible [2+1] cycloaddition reaction was the thermal functionalization of CNTs by nitrenes.
During this reaction, the first step was initiated by thermal decomposition of an azide which
produces alkoxycarbonylnitrene via nitrogen elimination, followed by the [2+1] cycloaddition of
the nitrene to the sidewalls of the CNTs affording alkoxycarbonylaziridino-CNT [170-174].
Chapter Two
24 | P a g e
Different types of organic functional groups, such as alkyl chains and crown ethers, were
successfully attached onto CNTs using this method. A subsequent reaction on the modified
CNTs containing chelating donor groups in the addends allowed complexation of metal ions,
such as Cu and Cd [172].
Another example involved the irradiation of a photoactive azidothymidine in the presence of
CNTs, which resulted in the formation of very reactive nitrene groups in the proximity of the
carbon lattice. In a cycloaddition reaction, these nitrene groups coupled to the CNTs and formed
aziridine adducts (Figure 1.3). Theoretical studies have supported the possibility of the reactions
of CNT with carbenes (or nitrenes) from a thermodynamic point of view [175, 176].
Figure 2.3 Photoinduced generations of reactive nitrenes in the presence of nanotubes.
The application of azomethine ylides was first reported by Georgakilas et al. [177] for the
functionalization of CNTs. The azomethine ylides were formed thermally in situ by
condensation of an R-amino acid and an aldehyde, and was successfully added to the graphitic
surface via a 1,3-dipolar cycloaddition reaction, forming pyrrolidine fused rings [177, 178]
(Figure 2.4). The 1,3-dipolar cycloaddition reactions of azomethine ylides involves planar
molecules composed of one nitrogen atom and two terminal sp2 carbons, and they have four ?
electrons spread over the three-atom C-N-C unit. The 1,3-dipolar cycloaddition of azomethine
ylides with alkene or alkyne is a very effective method for the construction of pyrrolidine- and
pyrrole-rings in the synthesis of pyrrolidine- and pyrrole-containing molecules. These molecules
are very important pharmaceuticals, natural alkaloids, organic catalysts, and building blocks in
organic synthesis [17].
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25 | P a g e
Figure 2.4 1,3-dipolar cycloaddition of azomethine ylides n the surface of CNTs.
The amino functionalized CNTs were particularly suitable for the covalent immobilization of
molecules or for the formation of complexes based on positive/negative charge interaction [179].
Various biomolecules have also been attached onto amino-CNTs, such as amino acids, peptides,
and nucleic acids (Figure 2.5) [179-184].
Figure 2.5 Reaction pathway for obtaining water-soluble ammonium- modified nanotubes.
Alvaro et al. [185] studied modified CNTs obtained by the thermal 1,3-dipolar cycloaddition of
nitrile imines. Similarly the reaction under microwave conditions afforded the functionalized
Chapter Two
26 | P a g e
material in 15 min (Figure 2.6) [185a]. Photochemical studies showed that, by photoexcitation of
the modified tubes, electron transfer took place from the substituents to the graphitic walls
[185a].
Figure 2.6 1,3-Dipolar cycloaddition of nitrile imines to nanotubes.
Hayden et.al. [186] have investigated the interactions between MWNTs and diaminotetrazine. In
their report they claim that a series of interactions occurred between tetrazines and carbon
nanotubes including ??? interactions, cycloaddition (Diels?Alder) and cross-linking reactions.
Finally, Delgado et al. [187] studied the reaction of a dienophile with the sidewalls of CNTs
under microwave irradiation.
2.3.6 Amidation/Esterification Reactions
A carboxylation reaction is an oxidation reaction by an oxidizing acid, or a combination of acids
such as a mixture of concentrated nitric and sulfuric acids [188]. This method is widely used for
the purification of the raw CNTs [189]. This is achieved by inducing the opening of the tube caps
as well as the formation of holes in the sidewalls. Liu et al. [190] demonstrated that carboxylate
groups generated by the acid-cut nanotubes could be derivatized chemically by thiolalkylamines
through an amidation reaction. In addition, Chen et al. [167] were the first to treat oxidized
nanotubes with long chain alkylamines via acylation with the result that the functionalized
material was soluble in organic solvents (Figure 2.7). Direct thermal mixing of oxidized CNTs
and alkylamines to produce functionalized material through the formation of zwitterions has also
been reported [191].
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27 | P a g e
Esterification reactions can produce soluble functionalized nanotubes (Figure 2.7) [190]. Sano et
al. reported that the condensation reaction of a carboxylate and other oxygenated functional
groups produced perfect rings at the ends of the oxidized SWNT [192]. Using similar
methodology Sun and coworkers [193-196] were able to attached lipophilic and hydrophilic
dendrimers to oxidized CNTs via amidation or esterification reactions (Figure 2.8). More
recently, Shi et al. [197] have studied multifunctional dendrimer modified MWNTs for in vitro
cancer cell targeting.
Figure 2.7 Derivatization reactions of acid-cut nanotubes through the defect sites of the graphitic
surface.
Figure 2.8 Direct thermal mixing of nanotubes and long chain amines.
2.3.7 Grafting of Polymers
The covalent reaction of CNTs with polymers is an important reaction since long polymer chains
could help to dissolve the tubes in a wide range of solvents, even with a low degree of
functionalization. There are two main methodologies for the covalent attachment of polymeric
substances to the surface of nanotubes, namely ?grafting to? and ?grafting from? methods.
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28 | P a g e
?Grafted to? is the synthesis of a polymer with a specific molecular weight, followed by end
group transformation. Subsequently, this polymer chain is attached to the graphitic surface of
CNTs. The ?grafting from? method is based on the covalent immobilization of the polymer
precursors on the surface of the CNTs and subsequent propagation of the polymer in the
presence of monomeric species. In the next section a number of representative examples will be
described.
Chemical reaction of CNTs and poly(methyl methacrylate) (PMMA) using the ultrasonication
method of ?grafting to? was reported by Koshio et al. [198] Similarly Wu et al. [199] have
studied the nucleophilic reaction of polymeric carbanions with CNTs. In their studies
organometallic reagents, such as sodium hydride or n-butyllithium, were mixed with
poly(vinylcarbazole) or poly-(butadiene), and the resulting polymeric anions were grafted to the
surface of CNTs. An alternative approach was reported by Blau et al. [200] using MWNTs
functionalized with n-butyllithium and subsequently coupled with halogenated polymers.
Finally, Qin et al. [201] reported a grafting of functionalized polystyrene to CNT via a
cycloaddition reaction.
Jia et al. [202] have reported the ?grafting from? of CNT-polymer composites by an in situ
radical polymerization process. They further claimed that the double bonds of the nanotube
surface were opened by initiator molecules and that the CNT surface played the role of grafting
agent. Qin et al. [203a] have studied the grafting of polystyrenesulfonate (PSS) by in situ radical
polymerization. In their study they reported that due to the negative charges of the polymer
chain the resulting material dispersed in aqueous media. In a subsequent study [203b] they have
reported polyvinyl pyridine (PVP)-grafted polymers from SWNTs by in situ polymerization.
MWNTs grafted with poly(methyl methacrylate) have been prepared by emulsion
polymerization of the monomer in the presence of a radical initiator [204a], or a cross-linking
agent [204b]. Petrov et al. [205] have studied the modification of MWNTs with polyacrylonitrile
chains by applying electrochemical polymerization of the monomer. More recently, Che et al.
[206] reported the ?grafting-from? approach to the synthesis of dendritic poly(amidoamine)
(PAMAM) on SWNTs and they demonstrated the good dispersion and high reinforcement
efficiency of these functionalized CNTs in an epoxy matrix. Finally, Yan et. al. [207] have
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29 | P a g e
recently reported a novel approach to graft polyamide 6 (PA6) onto the surface of MWNTs.
MWNTs were initially covalently functionalized with copoly(styrene-maleic anhydride) (SMA)
via free radical polymerization followed by a ring-opening polymerization of ?-caprolactam to
graft PA6 onto the surface of MWNTs. The resulting product had good dispensability in many
organic solvents such as formic acid and melted ?-caprolactam.
2.3.8 Other reactions
Radical addition is another form of covalent sidewall functionalization of CNTs. This type of
reaction has been intensively studied [170, 171, 208-211]. The formation of aryl radicals is
triggered by an electron transfer between CNTs and the aryl diazonium salts in a self-catalyzed
reaction [212-219]. A similar reaction was later described, utilizing water-soluble diazonium
salts, which have been shown to react selectively with metallic CNTs [215].
Electrophilic [220] and inorganic compound [221-226] additions has also been extensively
studied. Reaction of CNTs with trans-IrCl(CO)(PPh3)2 gave nanotube-metal complexes [227]. It
was found that coordination mainly occurred at defect sites [227b, c]. The development of this
chemistry was crucial for applications of SWNTs as a reusable catalyst support. Carbon
nanotubes-metal interconnections were obtained by covalent attachment of CNTs to an inorganic
metal complex treated in a ammonia atmosphere with [ruthenium-(4,4?-dicarboxy-2,2?-
bipyridine)(2,2?-bipyridyl)2](PF6)2 [228].
Studies have also shown that ozonolysis reactions of single-walled CNTs can occur at r.t. [229]
and at -78 ?C [230]. On the other hand, pristine CNTs have been subjected to cleavage by
chemical treatment with hydrogen peroxide or sodium borohydride [230a], yielding a high
proportion of carboxylic acid/ester, ketone/aldhyde, and alcohol groups on the CNT surface. This
substantially broadens the chemical reactivity of the carbon nanostructures. Furthermore, Cai et
al. [231] have demonstrated the attachment of ozonized CNTs to gold surfaces by using
conjugated oligo(phenylene ethynylenes).
Nucleophilic addition to CNTs has been studied by Basiuk et al. [232]. Their approach involved
the use of a solvent-free amination of the closed caps of MWNTs with octadecylamine. It was
Chapter Two
30 | P a g e
suggested that the addition took place only on the five membered rings of the graphitic network
of the nanotubes and that the benzene rings were inert to direct amination. Lastly, to covalently
modify CNTs with both alkyl and carboxylic groups, Chen et al. [232b] treated pristine material
with sec-BuLi and subsequently with carbon dioxide.
An alternative approach to the chemical modification of CNTs, involving radiofrequency glow-
discharge plasma activation, has been developed by Chen et al. [233]. In this study they treated
CNTs with aldehyde plasma, and subsequently amino dextran chains were immobilized through
the formation of Schiff-base linkages. The resulting material possessed a highly hydrophilic
surface due to the presence of the polysaccharide-type moieties.
2.3.9 Mechanochemical functionalization
The ball-milling of MWNTs in reactive atmospheres has also been shown to produce short tubes
containing different chemical functional groups such as amines, amides, thiols and mercaptans
[234]. In an analogous strategy, SWNTs have been reacted with potassium hydroxide through a
simple solid-phase milling technique [235]. Using the same approach, Li et al. [236] have
studied the attachment of C60 fullerene to the graphitic network of CNTs.
2.4 The application of fullerene (C60) and carbon nanotubes in solar cells
2.4.1 General introduction
Among the renewable energy sources, solar energy is of great importance. The Sun has always
been the most powerful energy source for earth and it provides energy that is clean and
environmentally friendly. Sunlight can be transformed into electricity using solar cells. Solar
cells have applications in many different fields such as in calculators, solar lamps and can be
used even on spacecraft and satellites. Historically, conventional solar cells were built from
inorganic materials such as silicon. The efficiency of such conventional solar cells made from
inorganic materials, for instance silicon crystals in solar cells, has reached 24% [237]. However,
solar cells made from inorganic materials are generally expensive and require energy intensive
processing techniques. For example, purification of silicon is difficult and much silicon is wasted
Chapter Two
31 | P a g e
during purification. In addition, since the performance of silicon cells degrades as the
temperature increases, the long lasting, concentrated operation of silicon cells requires a cooling
system.
A lot of effort is being put into the development of new fabrication techniques using organic,
[238], hybrid [239] and photoelectrochemical (dye sensitized) solar cells [240] which could act
as alternatives to conventional silicon solar cells.
Organic solar cells mainly consist of two organic materials, an electron-donating material and an
electron-accepting material that make a percolating structure with interpenetrating networks
[241]. The realization that photoinduced charge transfer can occur from a conjugated polymer to
fullerene derivatives has led the development of ?bulk heterojunction? organic solar cells [242].
A hybrid solar cell is a combination of both organic and inorganic materials and therefore
combines the unique properties of inorganic semiconductors with the film forming properties of
the conjugated polymers [243]. Organic materials are generally inexpensive, easily processable
and their functionality can be tailored by molecular design and chemical synthesis. On the other
hand, inorganic semiconductors can be manufactured as nanoparticles. Inorganic semiconductor
nanoparticles offer the advantage of having high absorption coefficients and size tunability. By
varying the size of the nanoparticles the bandgap can be tuned and therefore the absorption range
can be tailored [244].
In dye-sensitized solar cells (DSSC), combinations of several different materials, such as two
transparent conducting oxide (TCO) substrates, and fluorine doped tin oxide (FTO) on glass or
polymeric substrates, has been used [245, 246]. One TCO is a photo anode, composed of a
sensitizer adsorbed onto the surface of the nanocrystalline semiconductor electrode (typically
nanostructured TiO2), and the other a photo inert counter electrode with a thin layer of a catalyst
(for instance, platinum) sandwiching an electrolyte/relay medium (usually a solution containing
the I3
?/I? pair). n-TiO2 is the most employed semiconductor material in dye-sensitized
photoelectrochemical solar cells due to its favorable energetics, stability, low price and easy
process ability [247, 248]. Semiconductor colloids are typically obtained via a sol?gel process
from titanium isopropoxide or directly from commercial TiO2. The preparation of colloids from
Degussa TiO2 is easier and faster than from the isopropoxide hydrolysis method and results in
Chapter Two
32 | P a g e
transparent to translucent semiconductor films [249]. A schematic presentation of the operational
principles of a DSSC is given in Figure 2.9.
Figure 2.9 Operational principles of a dye sensitized solar cell [249b].
In the first step, the photoexcited sensitizer injects electrons into the conduction band of the
semiconductor. The oxidized sensitizer (S+) is then quickly reduced back to S by the redox
mediator couple, I3
?/I?, present in the electrolyte, it is regenerated at the counter electrode,
concluding the redox cycle. The reduction of I3
? is catalyzed in situ by Pt deposited on the
surface of the counter electrode [249].
The photovoltaic characteristics of a solar cell can be evaluated from an I?V curve. The I-V curve
is shifted down compared to the curve obtained in the dark (Figure 2.10). In the dark, there is
almost no current flowing, until the contacts start to inject heavily at a forward bias for voltages
larger than the open circuit voltage. Under illumination, the current flows in the opposite
direction compared to the injected current. At (a) in Figure 2.10 the maximum generated
photocurrent flows under a short circuit current, while at (b) in Figure 2.10 the photo generated
current is balanced at zero (flat band condition). Between (a) and (b), the device generates
power. At a certain point, denoted as the maximum power point (MPP), the product between
current and voltage, and hence the power output, is largest i.e. Impp X Vmpp= Pmax [249].
Chapter Two
33 | P a g e
Figure 2.10 Current-voltage (I-V) curves of a solar cell (dark: dashed; illuminated: full line). The
characteristic intersections with the abscissa and the ordinate are the open circuit voltage (Voc)
and the short circuit current (Isc), respectively. The largest power output (Pmax) is determined by
the point where the product of voltage and current maximized. Division of Pmax by the product of
Isc and Voc yields the fill factor (FF) [249].
To determine the efficiency of a solar cell, this power needs to be compared with the incident
light intensity. Typically, the fill factor is calculated as:
Where Vmpp = voltage at maximum power point
Impp = current at maximum power point
to denote the part of the product of Voc and Isc, that can be used. Using this equation, the power
conversion efficiency can be written as:
Where Pout = power output
Pin = power input
Chapter Two
34 | P a g e
The most efficient photosensitizers have an intense absorption in the visible region, strong
adsorption onto the semiconductor surface and efficient electron injection into the conduction
band of the semiconductor. Moreover, the photosensitizers must be rapidly regenerated by the
mediator layer in order to avoid electron recombination processes and be fairly stable, both in the
ground and excited states. Many different compounds have been investigated for semiconductor
sensitization, such as porphyrins [250, 251], phthalocyanines [252], coumarin 343 [253, 254],
and carboxylated derivatives of anthracene [254, 255]. Among the photosensitizers investigated,
transition metal complexes have been the best so far [256, 257].
Metal complex sensitizers usually have anchoring ligands and ancillary ligands. Anchoring
ligands are responsible for the complex adsorption onto the semiconductor surface and are also
often chromophoric groups (Figure 2.11).
Figure 2.11 Schematic representation of interfacial electron transfer following light absorption
for cis-[Ru(dcbH2)2LL?] with some ancillary ligands [258].
Polypyridinic complexes of d6 metal ions show intense metal to ligand charge transfer (MLCT)
bands in the visible region with potential interest for promoting charge injection processes to the
conduction band of wide band gap semiconductors, such as TiO2, SnO2 and ZnO. The energies
Chapter Two
35 | P a g e
of the MLCT states can be altered systematically by modifying the anchoring ligands as well as
by changing the ancillary ligands or its substituents. The wide possibilities to tune the MLCT
energy have led to the preparation of many different compounds that have been investigated for
semiconductor sensitization. Among them, the best light-to-electricity conversion efficiency has
been achieved by using ruthenium(II) polypyridyl complexes as TiO2 sensitizers in dye-
sensitized solar cells. Ruthenium polypyridinic complexes have been intensively employed as
sensitizers due to their appropriate redox, spectroscopic, and excited-state properties [259]. In
particular, ruthenium(II) complexes with carboxylic pyridine derivatives are able to react readily
with oxide surfaces to form the corresponding esters [257], presenting efficient adsorption onto
the semiconductor surface and improved light harvesting efficiency, leading to good results.
2.4.2 C60 fullerene in photo cells
Since the discovery of C60 fullerene [260] and its subsequent large scale production at the
beginning of the 1990s [261], this molecule has drawn the attention of researchers for its utilization
in materials science. The interaction of C60 with light has attracted considerable interest in the
exploration of applications related to photophysical, photochemical and photoinduced charge
transfer properties of [60]fullerene derivatives. Its unique electrochemical properties, with six
reversible single-electron reduction waves [262], and its photophysical properties [263] make C60 an
interesting molecule to study photo-driven redox phenomena. Photoinduced electron and energy
transfer processes are of great significance since they govern natural photosynthesis, and
considerable effort has been devoted to the construction of C60-based molecular structures as
artificial photosynthetic systems [264]. Moreover, Sariciftci et al. [265] demonstrated that a n-
conjugated polymer was able to efficiently transfer electrons to the C60 core giving rise to long-
lived charge separated states. Since then, intensive research programs have been focused on the
utilization of fullerene derivatives acting as electron acceptors in organic solar cells.
Organic solar cells are based on the photosynthesis process in plants, in which the absorption of
sunlight by the chlorophyll "dye" creates a charge separation, thus converting carbon dioxide,
water in the presence of minerals into organic compounds and oxygen. Typically, solid-state
heterojunctions are fabricated using p-type donor (D) and n-type acceptor (A) semiconductors.
Organic [60]fullerene-based solar cells are fabricated by inserting the p-type and n-type materials
between two different electrodes. One of the electrodes must be (semi-) transparent, often indium
Chapter Two
36 | P a g e
tin oxide (ITO), but a thin metal layer can also be used. The other electrode is very often
aluminium although calcium, magnesium, gold are also used (Figure 2.12).
Figure 2.12 Representation of a donor?acceptor heterojunction: (a) structure of the solar cell; (b)
under illumination, electron transfer from the donor to the acceptor and generation of excitons
followed by charge separation and transport of carriers to the electrodes inducing a photocurrent.
The principal feature of a p-n heterojunction is built-in potential at the interface between both
materials presenting a difference of electronegativities [266]. The promotion of an electron
from the highest occupied molecular orbital (HOMO) [or the valence band (VB)] of the donor
to the lowest unoccupied molecular orbital (LUMO) [or the conducting band (CB)] of the
acceptor induced by the absorption of light generates an exciton at the interface of the junction
[267]. The charge separation occurs at donor/accepter interfaces and free charge carriers are
transported through semiconducting materials with the electron reaching the cathode (Al) and
the hole reaching the anode (ITO) (Figure 2.13).
Chapter Two
37 | P a g e
Figure 2.13 Theoretical principle of a donor?acceptor heterojunction. It is usually assumed that
for a semiconductor the HOMO corresponds to the VB and the LUMO corresponds to the CB
[267].
Miller et al. [268] indicated that solvent-cast films of [60]fullerene showed a photovoltaic
response typical of n-type semiconductors. Similarly, Brabec et al. [269] reported in their
studies that, after photo-excitation an ultra-fast photoinduced electron transfer from a conjugated
polymer to C60 occurred in approximately 45 fs. This implies a large exchange integral of the
excited state orbitals of donor and accepter molecules. Sariciftci et al. [270] and Morita et al.
[271] independently reported the photophysical properties of blends composed of pristine C60
and MEH-PPV or poly(alkylthiophene) (PAT), respectively (Figure 2.14).
The first hetero junction with a conjugated polymer and C60 was reported by Sariciftci et al. [270].
In their work under monochromatic illumination, a relatively high fill factor (FF) of 0.48 and a energy
conversion efficiency of 0.04 % was reported. Moreover, it was demonstrated that the
photocurrent increased by a factor of twenty when a bilayer photo-diode polymer/C60 was used
instead of a single polymer layer device, indicating that [60]fullerene strongly assists the charge
separation. Significant improvement was reported by Roman et al. [272] with a photodiode in
which the aluminum electrode was used as electron collector and a PEDOT-PSS/ITO [(poly(3,4-
ethylenedioxythiophene)-poly(styrenesulfonate)/indium tin oxide] electrode was used as hole
collector.
Chapter Two
38 | P a g e
Figure 2.14 Molecular structures of some semiconducting conjugated polymers used in fullerene-
based solar cells.
The "bulk-heterojunction" concept, first realized by Yu et al. [273], was a crucial and major
breakthrough towards efficient-organic devices. It involved an interpenetrating network of a (p-
type) donor conjugated polymer and C60 or another fullerene derivative as (n-type) acceptor material.
The photoactive layers of those types of solar cells consist of blends of a conjugated polymer and
a fullerene derivative (Figure 2.12). The effective interaction between the donor and the acceptor
compounds within bulk-heterojunction solar cells can take place in the entire device volume.
Hence, the separated charge carriers are transported to the electrodes via an interpenetrating
network. However, this kind of device is associated with the tendency, especially for pristine C60,
to phase separate and then to crystallize. This aggregation phenomenon imposes important
consequences on the solubility of C60 within a conjugated polymer matrix.
The first example of this type of cell used a blend between MEH-PPV (poly[2-methoxy-5-(2'-
ethyl-hexyloxy)-1,4-phenylene vinylene]) and [60]PCBM and it exhibited a PCE of 2.9 % under
monochromatic low intensity light [273, 274]. However, there were concerns about a major
difference of relative-acceptor strength between C60 and [60]PCBM. Pristine C60 appears to be a
Chapter Two
39 | P a g e
poorer electron acceptor. The LUMO of C60 (-3.83 eV) is lower in energy than that of PCBM (-3.75
eV) [275, 315].
Figure 2.15 Schematic representation of a bulk-heterojunction solar cell with the ITO/PEDOT-
SS/P3HT:[60]PCBM/LiF/Al device. The use of a LiF/Al electrode is now commonly adopted
providing an ohmic contact between the metal and the organic layer [276].
In order to increase the efficiency of organic photovoltaic devices an alternative approach was
reported by Baffreau et al. [277] and Gomez et al. [278]. In both studies the researchers reported
C60-antenna dyads that could absorb strongly in the visible light range. In this case, the dye
could act as an antenna by absorption of sunlight with the aim of inducing an intramolecular
energy transfer to the fullerene. Dendrimer based light-harvesting structures have also attracted
attention, the peripheral chromophores being able to transfer the collected energy to the central
core of the dendrimer. A fullerene core and peripheral oligophenylenevinylene (OPV) subunits
appear as potentially interesting systems with light-harvesting properties [279].
Very recently, a visible light-absorbing nano-array integrating four C60 moieties and a single ?-
conjugated oligomer was synthesized [280]. It was demonstrated that pure oligomer-
tetrafullerene gave no significant photovoltaic effect. However, when P3HT was incorporated in
the device a photovoltic effect was noted; this phenomenon was explained by an energy transfer
occurring between C60 units and the n-conjugated oligomer.
On the other hand, double-cable molecular systems, fullerene-based substituents that are grafted
onto the conjugated polymer chain, were also explored in order to overcome some problems
Chapter Two
40 | P a g e
associated with bulk-heterojunction systems. Limitations, such as limited miscibility of both
donor and acceptor materials, especially because clusters of fullerenes can be formed within the
photoactive film [281], so that the transport of electrons is located in separated domains, are
avoided. In order to prevent such undesirable effects, a second molecular strategy was proposed
by simply chemically linking the hole-conducting moiety to the electron-conducting fullerene
subunit. This direct covalent bonding of different n-electron donors [282] and ?-conjugated
oligomeric systems [283] has emerged as a very active field of research to develop new organic
photovoltaic devices. This has given rise to the synthesis of a huge number of dyads or triads
involving donor groups covalently attached onto C60.
In such systems, the effective donor-acceptor interfacial contact is maximized and the phase
separation and clustering phenomena prevented as well. Basically, the realization of effective
double-cable polymers brought the p-n heterojunction to the molecular level [281]. Double-
cable materials could be consequently seen at the frontier with the molecular heterojunction.
[284] They can exhibit characteristic electronic and excited-state properties, which make them
promising candidates for the investigation of photoinduced electron transfer processes and long-
lived charge-separated states.
2.4.3 Carbon nanotubes in solar cells
Carbon nanotubes offer a wide range of band gaps [285-287] to match the solar spectrum,
enhanced optical absorption [288, 289] and reduced carrier scattering for hot carrier transport
[290, 291]. The latter may even result in a near-ballistic transport in nanotubes with submicron-
meter lengths [292]. Castrucci et al. [293] have demonstrated that MWNTs can generate
photocurrent in the visible and ultraviolet spectral range. Recently, Cheong et al. [294] have
investigated the photoresponsive conductance switching of MWNTs-SPO (SPO=
spironaphthoxazines) under a 365 nm UV irradiation. In their study, they reported that during the
cyclic irradiation of MWNTs-SPO by UV light the composites showed a reversible response, in
which the change of HOMO?LUMO band gap in SPO strongly affects the conductivity of the
MWNTs.
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41 | P a g e
Khaziji et al. [295] have investigated the photo-electrochemical properties of single wall carbon
nanotubes (SWNTs) and the photon-to-photocurrent efficiency (IPCE). However, the efficiency
has remained low (about 0.15 %) due to rapid exciton annihilation [296]. On other hand,
covalently linking of the desired molecular assemblies on the SWNTs walls was found to enhanc
the photoconversion efficiency [297]. Results were also obtained from the dispersion of
semiconducting quantum dots [298] and metallic nanoparticles [299] on SWNTs and MWNTs
sidewalls.
Blending CNTs with conjugated polymers may not only yield electron acceptors but also allow
the transferred electrons to be efficiently transported along their length, thus providing
percolation paths [300]. Indeed, the extremely high surface area, ~1600 m2/g, reported for
purified SWNT [301] offers a tremendous opportunity for exciton dissociation. Since SWNTs
have diameters of ~1 nm and lengths of ~1?10 ?m, these materials exhibit very large aspect
ratios (>103). Thus, percolation pathways could be established at low doping levels, providing
the means for high electron mobility. Electrical conductivity data has validated that SWNT-
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heterojunction configuration [313]. For the same purpose, Jin and Dai suggested that instead of
randomly mixing CNTs with polymers, a cell could contain a network of vertically aligned
CNTs separated by vertical polymer layers [314].
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Chapter Three
57 | P a g e
Chapter 3
Ring-opening co-polymerization of a C60-cyclopentadiene cycloadduct
and norbornene with the Grubbs second generation catalyst*
3.1 Introduction
Since methods for the mass production of fullerenes have been developed, there has been much
interest in the synthesis of fullerene-containing polymers [1]. The physical and chemical properties of
C60-attached polymer chains change significantly when compared to the properties of the parent
polymer [1]. The polymer properties affected include increased filming properties [2], improved
optical limiting properties [3], enhanced thermal stabilities [4] and photoconductivities [5]. In addition
to this, fullerene-containing polymers can potentially acquire many of the properties associated with
the fullerene [6]. It has also been noted that the incorporation of covalently-bound fullerene into
polymers significantly affects the mechanical behaviour, including the tensile strength and fracture
toughness, of the polymers [7]. Within the field of biological applications, the neuroprotective,
enzymatic, antiapoptotic, antibacterial, DNA photocleavage, nitric oxide synthase inhibition, and
chemotactic activities of fullerenes and their derivatives have been studied [8-10].
Due to their unique electronic and chemical properties, fullerenes have a tremendous potential as
building blocks for molecular engineering, new molecular materials and supramolecular chemistry
[11]. In this regard, heterogeneous mixing of fullerenes and fullerene derivatives with ?-conjugated
polymers has been used to produce excellent materials for photovoltaic devices [12]. Upon irradiation
of fullerene/polymer blends, charge transfer from the polymer to C60 occurs, resulting in efficient
photo conductivities.
On other hand, physical and thermal properties of fullerene containing natural rubber have been
studied by Jurkowska et al [13]. In the study, the addition of fullerene was shown to lead to increased
elastic modulus at different elongations, Schob elasticity, and hardness of cured rubber to a degree that
depends on fullerene content. Moreover it significantly reduced the effect of accelerated aging of
__________________________________________________
*Published: M. A. Mamo, N. J. Coville, W. A. L. van Otterlo, Fullerenes, Nanotubes, and Carbon Nanostructures, 2007, 15,
341.
Chapter Three
58 | P a g e
rubber and no visible influence of fullerene concentrations on Tg, tan ?, and G-modulus within a
temperature range -150 to -50?C when rubber is rigid was noted.
The synthesis of modified C60 scaffolds has seen tremendous activity over the last decade. Due to
C60?s electron-deficient ?poly-alkene? characteristics, fullerene addition reactions have therefore been
one of the main chemical transformation types investigated [14]. In particular, it has been observed
that C60 reacts as an electron deficient dienophile with a number of electron-rich dienes and
cycloaddition reactions have thus become a very useful method for the functionalization of C60 [14].
It
has been through this controlled modification of C60 that it has been possible to obtain different types
of C60-containing polymers such as side chain polymers, main chain polymers, star-shaped polymers,
etc, and this field has been extensively reviewed [1]. Amongst many approaches, the application of
ring opening metathesis polymerization (ROMP) to synthesise C60-containing polymers was first
reported in 1995 [15]. In this communication, by Prato and co-workers, the fullerene monomer was
initially prepared by the cycloaddition of quadricyclane to C60, and the C60-containing polymer was
prepared from the monomer by a Schrock catalyst-mediated ROMP reaction with norbornene [15].
While the study and exploitation of ROMP reactions have carried on unabated in the polymer
literature, little extension of this methodology to building carbon materials (C60, carbon nanotubes,
carbon spheres, etc.) into polymers by way of this approach, have since been reported [16].
We have now commenced a systematic study to exploit the ROMP methodology and to apply this
approach to the manufacture of carbon-containing materials incorporating fullerenes, carbon
nanotubes and carbon spheres. This should lead to the generation of novel materials based on covalent
linkages between polymers and carbon materials. To this end we have synthesized a C60-
cyclopentadiene cycloadduct and have polymerised this co-monomer with norbornene, using the
Grubbs second generation catalyst, in order to obtain a new range of samples containing different
amounts of C60. This work describes our investigation into the synthesis and properties of these new
C60-containing polymeric materials.
Chapter Three
59 | P a g e
3.2 Experimental
3.2.1 General procedure
Infrared spectra of the polymers were recorded using a Varian 800 FT-IR spectrometer using KBr
pellets. Transmittance values are reported on the wave number (cm-1) scale in the range of 400-3200
cm-1. Ultraviolet and visible spectra were recorded using a Varian 50 CONC UV-Visible
spectrophotometer. Mass spectra were collected using a VG70-SEQ instrument in a positive ion mode
using FAB ionization.
Differential scanning calorimetry (DSC) data was obtained on a DSC822e calorimeter from Mettler-
Toledo. Each pan was run in the temperature range 30-75 oC under a nitrogen atmosphere at a rate of 5
oC/min Thermal gravimetric analysis (TGA) was performed at a heating rate of 10 oC/min under
nitrogen with a Perkin Elmer, Pyris 1 TGA instrument.
3.2.2 Synthesis of the C60-cyclopentadiene cycloadduct 3.3
The synthesis of C60-cyclopentadiene cycloadduct 3.3 was performed according to a literature
procedure [12] in which cyclopentadiene 2 (0.060 g, 0.83 mmol), dissolved in toluene (15 mL), was
added drop-wise to C60 1 (0.50 g, 0.69 mmol) in toluene (15 mL) over 15 min at r.t. During this time
the initial purple color of the solution slowly changed to a brown color. After the reaction mixture had
been left to stir for a further 30 min, the solvent was removed under reduced pressure to afford a
brown solid. This solid was purified using column chromatography (SiO2, 5 % CS2: toluene) to afford
the desired cycloadduct (0.26 g, 52 % based on C60).
3.2.3 Polymerization of Polynorbornene 3.4
Norbornene (bicyclo[2.2.1]-2-heptene) (1.0 g, 10.6 mmol) 4 was dissolved in xylene (~35 mL) and
stirred for 15 min under dry N2, at r.t. Grubbs II catalyst 5 (10 mg, 11.8 mmol) was then added to the
solution, after which the reaction mixture was left stirring for a further 45 min The polymerization
process was then terminated by the addition of ethyl vinyl ether (~2?3 drops). The reaction mixture
was then poured into an excess of MeOH (100 mL) containing a few drops of 1 M HCl to precipitate
the crude polymer. This material was obtained after filtration and then purified by solubilization in
CHCl3. Subsequent precipitation by the addition of excess MeOH (~100 mL), followed by filtration
Chapter Three
60 | P a g e
then afforded the purified polymer 3.6A. This polymer was then dried in a vacuum oven at 358 oC to a
constant weight (0.88 g, yield: 88 % based on norbornene 3.4).
3.2.4 Co-polymerization of 3.3 and norbornene 3.4
All co-polymerization reactions were carried out under a dry N2 atmosphere in xylene (~35 mL) at r.t.
In all cases, a catalytic amount of metathesis co-polymerization of C60-cyclopentadiene Grubb?s (II)
catalyst (10 mg, 11.8 mmol) was added to the solution to initiate polymerization. The molar ratios of
norbornene 4 to C60-cyclopentadiene adduct 3 was varied in order to obtain 50:1, 100:1, 500:1 and
1000:1 molar ratios. The solutions containing the co-monomers were stirred for 15 min before the
catalyst was added, under N2, after which the reaction mixtures were left to stir for a further 45 min
The polymerization reactions were then inhibited by the addition of ethyl vinyl ether (a few drops).
The solutions were then poured into an excess of MeOH (~100 mL) containing a few drops of 1 M
HCl. The co-polymers were purified by solubilization in CHCl3 and then precipitated by the addition
of MeOH (~100 mL). The obtained co-polymers 3.6B?E were dried in a vacuum oven at 358 oC to
constant weight and were found to be dark to light brown in color depending on the amount of C60-
cyclopentadiene cycloadduct 3 used as co-monomer (polymer mass returns: 3.6B 93 %, 3.6C 84 %,
3.6D 86 %, 3.6E 87 %).
3.3 Results and Discussion
3.3.1 Synthesis of copolymers
The C60-cyclopentadiene cycloadduct 3.3 was readily synthesized, by way of a Diels-Alder reaction
between C60 3.1 and freshly cracked cyclopentadiene 3.2, as previously described in the literature
(Scheme 1) [17,18]. The functionalized C60 3.3 and norbornene 3.4 were then pre-mixed in varying
molar ratios (3.4:3.3: 3.6A 1:0, 3.6B 50:1, 3.6C 100:1, 2.6D 500: 1, 3.6E 1000:1) and co-polymerized
using a ROMP approach with catalytic amounts of Grubbs second generation catalyst 3.5 (Scheme
3.1). After 30 min of stirring the reaction mixtures became highly viscous. For the pure
polynorbornene reaction 3.6A a clear-coloured reaction solution was observed, but for the C60-
cycloadduct-norbornene co-polymers 3.6B-E the solution colours varied from a dark brown to light
brown colour with the colour proportional to the concentration of the C60-cyclopentadiene cycloadduct
Chapter Three
61 | P a g e
3.3. After a further 15 min of stirring the polymerization reactions were terminated and the rubbery,
purified C60-containing polymers 3.6B-E were obtained by precipitation. The yields obtained for the
dried polymers 3.6B-E were all good (84-93 %). From the polymeric material in hand it was evident
that the color of the polymers was indicative of C60 incorporation as the polymer color lightened in the
order 3.6B?3.6C?3.6D?3.6E. The solubility of the co-polymerized polymers 3.6B-E was good in
organic solvents such as toluene and CH2Cl2 but decreased noticeably as the content of C60 increased
[19]. Similar observations concerning the relationship between C60-content of the polymers and
solubility have also been reported in the literature [20].
N N
Ru
Cl
Cl
Ph
PCy3
MesMes
3.5
C60
n
3.1 3.2
3.3
3.4
3.6 (3.4 :3.3)
A (1:0)
B (50:1)
C (100:1)
D (500:1)
E (1000:1)
m
Scheme 3.1 Synthesis of C60-cyclopentadiene adduct-norbornene polymers 3.6B-E by ROMP
Chapter Three
62 | P a g e
3.3.2 Spectroscopic Studies of synthesized materials.
3.3.2.1 FTIR studies of synthesized materials
FTIR spectra were recorded for all the samples of the C60-containing polymers 3.6B-E and the data
confirmed both the presence of C60 and polynorbornene (Figure 3.1) in the polymers. The
characteristic peak for C60, ~528 cm
-1 (see inset Fig 3.1), was only observed in samples with relatively
low norbornene content, viz. 3.6B and 3.6C [21]. Medium and broad bands that appeared in the region
of 3013-2828 cm-1 were assigned to the C-H stretching mode. The intense, broad line centered at 1727
cm-1 was assigned to the -C=C- stretching mode of an alkene chain and the medium band at around
1455 cm-1 was assigned to the -CH2- bending frequency [15]. The strong band at 964 cm
-1 was
assigned to a trans _HC=CH_ linkage [14].
Chapter Three
63 | P a g e
3000 2500 2000 1500 1000 500
A
wave number (cm
-1
)
E
D
C
F
B
529.5 cm
-1G
Figure 3.1 FT-IR spectra of the samples in KBr pellet form. (A) Polynorbornene 3.4, Co-
polymers 3.6B-E: (B) mole ratio 50:1, (C) mole ratio 100:1, (D) mole ratio 500:1, (E) mole ratio
1000:1, (F) C60-cyclopentadiene cycloadduct 3.3, (G) C60 3.1.
3.3.2.2 UV-visible absorption spectroscopic studies of synthesized materials
Electronic absorption UV-visible spectra of the fullerene co-polymerized polymers 3.6B-E were also
informative (summarized in Table 3.1). The spectrum of the C60-containing polynorbornene 3.6C and
3.6D displayed two new absorption peaks at 256 and 326.5 nm which are due to the presence of C60
[2b, 21a,22 ] and were absent in polynorbornene 3.6A (Figure 3.2). In contrast the UV-visible
spectrum of C60 3.1 (G) showed a peak at 364.8 nm and for the C60-cyclopentadiene cycloadduct 3.3
(F) at 358.3 nm, respectively, shifting to shorter wavelengths relative to the 326.5 nm peak for the C60-
Chapter Three
64 | P a g e
containing polymers (see inset Fig 2.2). This indicates that the electronic structure of C60 has been
modified by incorporation into the norbornene polymer by the ROMP reactions [2, 3b].
250 300 350 400 450 500 550 600 650 700 750 800 850
0
2
400 500 600
0.00
0.05
0.10
0.15
0.20
D
E
A
432.8 nm
In
te
ns
ity
W avelength (nm)
432.8 nm
E
D
A
C B
G
F
In
te
ns
ity
Wavelength (nm)
326 nm
256nm
Figure 3.2 UV-visible spectra in toluene of co-polymers 3.6B-E and polynorbornene 3.6A: (A)
Polynorbornene, (B) 50:1 mole ratio, (C) 100:1 mole ratio, (D) 500:1 mole ratio, (E) 1000:1 mole
ratio, (F) C60 3.1, (G) C60-Cyclopentadiene cycloadduct 3.3.
Chapter Three
65 | P a g e
Table 3.1 UV-visible ?max of the co-polymers 3.6B-E and polynorbornene 3.6A
Polymer ?max (nm)
3.6A No ?max
3.6B 326.5, 432.8, 489.5
3.6C 256.0, 326.5, 432.8, 494.0,
3.6D 256.0, 326.5, 432.8
3.6E 256.0, 326.5, 432.8
3.1 F 364.8, 406.0, broad peak 460 ? 650
3.3 G 326.5, 434.3, 358.3, 496.3
The presence of a band centered at 326.5 and 432.8 (weak) nm for all co-polymers is regarded as
characteristic of a 6-6 ring fusion. In recent studies the bands at these wavelengths have been
considered indicative of a ?closed? 6-6 ring bridged methanofullerene derivatives [23]. A band
appeared at 434.3 nm for the case of C60-cyclopentadiene cycloadduct 3.3. This band is slightly blue
shifted to 432.8 nm for the fullerene-containing polymers and it is very weak in the case of the
products formed in the 1000:1 (3.6E) and 500:1 (3.6D) mole ratio polymerization reactions (inserted
in Figure 3.2).
3.3.3 Thermal Degradation Studies of synthesized materials
3.3.3.1 Differential scanning calorimetry (DSC)
The glass transition (Tg) temperatures for each of the polymers 3.6A-E synthesized were determined
using a heating rate of 5 oC/min under N2. It is clear from the results that the Tg of the C60-containing
Chapter Three
66 | P a g e
co-polymers was directly proportional to the C60 content (Figure 3.3). A similar trend has been
observed for the relationship between the C60 content and Tg in other C60-containing polymers
[3b,21c,24]. The Tg change with C60 content can be explained in terms of the modification of the
plasticizing and reinforcing abilities of the fullerene in the polymer [3c]. It has been postulated that
when the C60 content of the polymers is increased, many polymer chains or several part of a polymer
chain may be linked by the C60 molecules, thus increasing the Tg of the polymer (Table 3.2) [3].
30 40 50 60 70 80
-2.2
-2.0
-1.8
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
en
do
o
C
m
W
B
D
A
CE
Figure 3.3 Differential Scanning Calorimetry Tg analyses (scan rate 5oC/min): Co-polymers 3.6A-E:
(A) Polynorbornene, (B) 50:1 mole ratio, (C) 100:1 mole ratio, (D) 500:1 mole ratio, (E) 1000:1 mole
ratio.
Chapter Three
67 | P a g e
Table 3.2 Tg of C60-containing polymers 3.6B-E and 3.6A.
Polymer Mole ratio
3.4 : 3.3
C60 content
(mole %)
Yield
%
Tg
(oC)
Decomposition
Temp. (oC)
C.Pa
decomposition
Temp. (oC)
3.6B 50:1 2 93 59.6 377 327
3.6C 100:1 1 84 58.8 359 418
3.6D 500:1 0.2 86 57.1 373 395
3.6E 1000:1 0.1 87 50.3 385 359
3.6A Polynorbornene 0 88 49.9 346 283
a C.P= Cross polymerized
3.3.3.2 Thermogravimetric analysis (TGA)
Thermogravimetric analysis (TGA) results provides a measure of the C60 content in the polymers. The
TGA data clearly reveal that as the C60 content of the norbornene-C60 co-polymers 3.6B-E increases,
the thermal stability of the materials increases relative to polynorbornene 3.6A (see Figure. 3.4 and
Table 3.2). It is also to be noted that the residual material after reaction at To > 600oC provides a
qualitative measure of both the ruthenium and C60 content. The polymers which contain C60 appeared
to be thermally more stable than polynorbornene. This finding is contrary to an earlier report where the
thermogravimetric analysis showed that the polymer degraded at 470 oC, indicating that the C60-
containing polymer is a robust material, but of lower thermal stability than polynorbornene [15].
Significant mass losses were observed after ~350 oC for polynorbornene 3.6A and for the C60-
containing co-polymers 3.6B-E the mass loss occurred at about 360 oC in all cases. Interestingly the
residue after thermolysis increases with C60 content in the polymers, A = ~0%, B = 23%, C = 15%, D
= 11%, E = 7%. Similar observations, regarding the C60 content of a polymer and the residues after
thermolysis have also been reported [21c].
The synthesized co-polymers were also subjected to cross-polymerization [15] by heating at 80 oC for
72 h and their thermal stability compared with the parent polymer 3.6A (Table 3.2). Of interest was
Chapter Three
68 | P a g e
that the results were found not to be linearly related. In some cases the cross polymerized compound
was found to be more thermally stable than the parent polymer 3.6A, for instance, the polymers
containing 100:1 and 500:1 mole ratios, 3.6C and 3.6D respectively. However for the other C60-
containing polymers this was not the case.
100 200 300 400 500 600
0
20
40
60
80
100
A
E
D
C
B
m
as
s l
os
s (
%
)
Temperature
o
C
Figure 3.4 TGA curves of the polymers (rate 10oC/ min) under a nitrogen atmosphere: (A)
Polynorbornene, Co-polymers 3.6B-E: (B) 50:1 mole ratio, (C) 100:1 mole ratio, (D) 500:1 mole
ratio, (E) 1000:1 mole ratio.
3.4 Conclusion
We have successfully synthesized a set of C60-cyclopentadiene cycloadduct/ norbornene polymers
using ROMP with the Grubbs second-generation catalyst. By spectroscopic evaluation we were able to
show that incorporation of the fullerene into the polymers had occurred and that the relative amount of
C60 affected the polymers thermal properties by increasing both the decomposition and the glass
transition temperatures, relative to polynorbornene.
Chapter Three
69 | P a g e
3.5 References
1 (a) C. Wang, Z.-X. Guo, T. Yadav, S. Fu, W. Wu, D. Zhu, Prog. Polym. Sci. 2004, 29, 1079. (b)
M. Prato, J. Mater. Chem., 1997, 7, 1097.
2 See for example: A. Kraus, K. Mullen, Macromolecules, 1999, 32, 4214.
3 See for example: (a) R. Tong, H. Wu, B. Li, R. Zhu, G. You, S. Qian, Y. Lin, R. Cai, Physica B,
2005, 366, 192. (b) B. Z. Tang, S. M. Leung. H. Peng, N. T. Yu, K. C. Su, Macromolecules,
1997, 30, 2848. and references cited therein.
4 See, for example, (a) C. Mathis, B. Schmaltz, M. Brinkmann, C. R. Chimie, 2006, 9, 1075. (b) O.
F. Pozdnyakov, A. O. Pozdnyakov, B. Schmaltz, C. Mathis, Polymer, 2006, 47, 1028.
5 For recent reviews see: (a) H. Spanggaard, F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2004, 83,
125. (b) J.-F. Nierengarten, Sol. Energy Mater. Sol. Cells, 2004, 83, 187. (c) C. J. Brabec, Sol.
Energy Mater. Sol. Cells, 2004, 83, 273.
6 S. Shi, K. C. Khemani, Q. C. Li, F. Wudi, J. Am. Chem. Soc., 1992, 114, 10656
7 See for examples: (a) T. Song, S. H. Goh, S. Y. Lee, Polymer, 2003, 44, 2563. (b) C.-C. M. Ma,
S.-C. Sung, F.-Y. Wang, L. Y. Chiang, L. Y. Wang, C.-L. Chiang, J. Polym. Sci., Part B: Polym.
Phys., 2001, 39, 2436 and references cited therein.
8 T. Da Ros, M. Prato, Chem. Commun., 1999, 663.
9 A. W. Jensen, S. R. Wilson, D. I. Schuster, Bioorg. Med. Chem., 1996, 4, 767.
10 S. R. Wilson, Biological aspects of fullerenes. In Fullerenes: Chemistry, Physics, and
Technology, (Kadish K, Ruoff R (Ed). Wiley: New York, 2000, 437).
11 (a) F. Diederich, M. Gomez-Lopez, Chem. Soc. Rev., 1999, 28, 263. (b) F. Diederich, M.
Gomez-Lopez, Chimia, 1998, 52, 551.
12 A. Cravino, N. S. Sariciftci, J. Mater.Chem., 2002, 12, 1931.
13 B. Jurkowski, P. Kamrowski, S. S. Pesetskii, V. N. Koval, L. S. Pinchuk, Y. A. Olkhov, J. Appl.
Poly. Sci., 2006, 100, 390.
14 (a) M. A. Yurovskaya, I. V. Trushkov, Russ. Chem. Bull. Int Ed., 2002, 51, 367. (b) P.
Hudhomme, C. R. Chimie, 2006, 9, 881. (c) R. Taylor, C. R. Chimie, 2006, 9, 982. (d) J. Yli-
Kauhaluoma, Tetrahedron, 2001, 57, 7053.
Chapter Three
70 | P a g e
15 N. Zhang, S. R. Schricker, F. Wudl, M. Prato, M. Maggini, G. Scorrano, Chem. Mater., 1995, 7,
441.
16 (a) Z. T. Ball, K. Sivula, J. M. J. Fr?chet, Macromolecules, 2006, 39, 70. (b) K. Sivula, Z. T. Ball,
N. Watanabe, J. M. J. Fr?chet, Adv. Mater., 2006, 18, 206.
17 V. M. Rotello, J. B. Howard, T. Yadav, M. M. Conn, E. Viani, L. M. Giovane, A. L. Lafleur,
Tetrahedron Lett. 1993, 34, 1561.
18 For other examples, involving the reaction of substituted or unsubstituted cyclopentadienes to
C60 by way of a Diels-Alder reaction, see: (a) S. R. Wilson, M. E. Yurchenko, D. I. Shuster, A.
Khong, M. Saunders, J. Org. Chem., 2000, 65, 2619. (b) R. Schwenniger, T. Muller, B. Krautler,
J. Am. Chem. Soc., 1997, 119, 9317. (c) B. Nie, V. M. Rotillo, J. Org. Chem., 1996, 61, 1870.
(d) M. F. Meidine, A. G. Avent, A. D. Darwish, H. W. Kroto, O. Ohashi, R. Taylor, D. R. M.
Walton, J. Chem. Soc. Perkin Trans. 2, 1994, 1189. (e) K. I. Guhr, M. D. Greaves, V. M.
Rotello, J. Am. Chem. Soc., 1994, 116, 5997.
19 On formation the polymers 6B-E were generally very soluble. However after precipitation and
drying of the polymers, addition of organic solvents caused gel formation rather than facile
solubilization.
20 H. W. Goh, S. H. Goh, G. Q. Xu, J. Polym. Sci., Polym. Chem., 2002, 40, 1157.
21 (a) K. E. Geckeler, A. Hirsch, J. Am .Chem. Soc., 1993, 115, 3850. (b) T. Suzuki, Q. Li, K. C.
Khemani, F. Wudi, J. Am. Chem. Soc. 1992, 114, 7301. (c) X. Zhang, A. B. Sieval, J. C.
Hummelen, B. Hessen, Chem. Commun., 2005, 1616.
22 W. J. Li, W. J. Liang, Spectrochim. Acta Part A: Mol. Biomol. Spectrosc, 2007, 67, 1346.
23 (a) C.-C. Chu, T.-I. Ho, L. Wang, Macromolecules, 2006, 39, 5657. (b) S. Xiao, Y. Li, Y. Li, H.
Liu, H. Li, J. Zhuang, Y. Liu, F. Lu, D. Zhang, D. Zhu, Tetrahedron Lett., 2004, 45, 3975. (c) S.
Xiao, Y. Li, H. Fang, H. Li, H. Liu, Z. Shi, L. Jiang, D. Zhu, Org. Lett., 2002, 4, 3063. (d) F.
Giacalone, J. L. Segura, N. Mart?n, J. Org. Chem., 2002, 67, 3529. (e) M. W. J. Beulen, J. A.
Rivera, M. A. Herranz, B. Illescas, N. Mart?n, L. Echegoyen, J. Org. Chem., 2001, 66, 4393.
24 See for example: (a) A. G. Camp, A. Lary, W. T. Ford, Macromolecules, 1995, 28, 7959. (b) L.
Y. Chiang, L. Y. Wang, C. ?S. Kuo, Macromolecules, 1995, 28, 7574. (c) L. Dai, A. W. H. Mau,
H. J. Griesser, T. H. Spurling, J. Phys. Chem., 1995, 99, 17302. (d) C. J. Hawker,
Macromolecules, 1994, 27, 4836.
Chapter Four
71 | P a g e
Chapter 4
Ring-opening metathesis co-polymerization of a C60-cyclopentadiene
cycloadduct and N-(cycloheptyl)-endo-norbornene-5,6-
dicarboximide
4.1 Introduction
The discovery and consequent characterisation of fullerenes has generated particular interest in
the field of materials science. The incorporation of fullerenes into existing materials such as
polymers, electronic devices, thin films and liquid crystals have all been attempted, and in so
doing so, scientists have produced unique carbonaceous materials. In particular, the development
of new synthetic polymers incorporating fullerenes has seen keen interest [1]. It has also been
convincingly demonstrated that the physical and chemical properties of C60-attached polymer
chains change significantly when compared to the properties of the parent polymer and that the
fullerene-containing polymers can potentially acquire many of the properties usually associated
with the fullerene [1,2]. Examples of the properties affected by the incorporation of fullerene
include the polymeric tensile strength and fracture toughness [3].
Ring-opening metathesis polymerization (ROMP) is seeing increasing usage in the synthesis of
C60-containing polymers. This approach was first reported in 1995 by Prato and co-workers
using the molybdenum-based Schrock?s catalyst [4].While the study and exploitation of ROMP
reactions have carried on unabated in the polymer literature, the Prato approach to fullerene-
containing polymers is still under-utilized. Relatively few examples of this methodology, to
incorporate carbon materials (C60, carbon nanotubes, carbon spheres, etc.) into polymers, have
been reported [5]. Of interest is that the utilization of olefin metathesis on nanostructures has
been the focal point of a recent review [6], and two examples of the use of ROMP on derivative
nanotubes were discussed [7,8].
In the previous Chapter the copolymerization of a C60-cyclopentadiene cycloadduct with
norbornene [9] was described using the Grubbs second generation catalyst [10]. In the work
described in this Chapter we reported our investigation into the synthesis and properties of a
series of novel C60-containing polymeric materials based on the ROMP of a N-(cycloheptyl)-
Chapter Four
72 | P a g e
endo-norbornene-5,6-dicarboximide monomer with a C60-cyclopentadiene cycloadduct. This
reagent was chosen in its hope that it would enhance the solubility of the fullerene containing
polymers. Indeed the polymers thus synthesisied show better solubility in most common polar
organic solvents than the products described in Chapter 2.
4.2 Experimental
4.2.1 General procedures
Infrared spectra of the polymers were recorded with a Varian 800 FT-IR spectrometer using KBr
pellets. Transmittance values are reported on the wave number (cm-1) scale in the range of
400?3200 cm-1. Ultraviolet and visible spectra were recorded with a Varian 50 CONC UV-
Visible spectrophotometer and mass spectra were collected with a VG70-SEQ instrument in a
positive ion mode using FAB ionization. Differential scanning calorimetry (DSC) data was
obtained on a DSC822e calorimeter from Mettler-Toledo; each pan was run in the temperature
range 30-75 oC under a N2 atmosphere at a rate of 5
oC/min Finally, thermal gravimetric analysis
(TGA) was performed at a heating rate of 10 oC/min, under N2, with a Perkin Elmer Pyris 1 TGA
instrument.
4.2.2 Synthesis of C60-cyclopentadiene cycloadduct 4.7
The synthesis of C60-cyclopentadiene cycloadduct 4.7 was performed according to a literature
procedure [13] and as described by us previously [9].
4.2.3 Synthesis of N-(cycloheptyl)-endo-norbornene-5,6-dicarboximide 4.4
Cis-norbornene-endo-2,3-dicarboxylic anhydride 4.1 (5.0 g, 30 mmol) and cycloheptylamine 4.2
(3.8 mL, 30 mmol) were dissolved in distilled toluene (35 mL), and stirred at 90 ?C for 24 h. The
resulting mixture was then cooled to r.t. Ac2O (26.5 mL, 278 mmol) and NaOAc (2.42 g, 17.8
mmol) were added to the foregoing mixture, which was then stirred at 90 ?C for a further 24 h.
Upon cooling, a white crystalline matrix formed throughout the solution. The toluene solvent
was removed under reduced pressure and the resultant solid was repeatedly extracted with a
mixture of hexane and distilled H2O (3:1 ratio, ~250 mL total volume). The hexane layer was
subsequently collected and evaporated, resulting in a crystalline product 4.4. This material was
Chapter Four
73 | P a g e
dried under reduced pressure to a constant mass (5.5 g, 83 %). The structure of product 4.4 was
confirmed from its spectral data. 1H NMR: 6.09 (2H, br s, 2 ? C=CH), 3.97?3.90 (1H, m, NCH),
3.37 (2H, br s, 2 ? CH), 3.17?3.16 (2H, m, 2 ? CH), 2.11?2.01 (2H, m, CH2), 1.76?1.69 (4H, m,
2 ? CH2), 1.61?1.36 (8H, m, 4 ? CH2);
13C NMR: 177.6 (2 ? C=O), 134.2 (C=C), 53.1 (NCH),a
52.1 (CH2),
a 45.2 (2 ? CH), 45.0 (2 ? CH), 31.7 (2 ? CH2), 27.4 (2 ? CH2), 25.5 (2 ? CH2); FT-
IR: 2931, 2861, 1685, 1399, 1370, 1335, 1210, 1166, 843, 723; m/z (FAB): 260 (M+1, 100%),
194 (60), 164 (20).
4.2.4 Polymerization of N-(cycloheptyl)-endo-norbornene-5,6-dicarboximide 4.4 to afford
4.9A
Norbornene derivative 4.4 (0.500 g, 1.83 mmol) was dissolved in toluene (25 mL) and stirred for
30 min, under Ar at r.t. Grubbs? 2nd generation catalyst 4.8 (6 mg, 7.08 ?mol) was then added to
the solution, after which the reaction mixture was left stirring for a further 5 days. The
polymerization process was then terminated by the addition of ethyl vinyl ether (~2-3 drops).
The solvent was then removed under vacuum and the resultant material washed with hexane (100
mL). After filtration, the solid polymer was dissolved in CHCl3 (20 mL) and then poured into an
excess of MeOH (100 mL), containing a few drops of 1 M HCl to precipitate the crude polymer.
This was followed by filtration of the solid which afforded the purified polymer 4.9A. This
polymer was then dried in a vacuum oven at 35 oC to a constant weight (0.480 g, yield: 96 %
based on norbornene derivative 4.4).
4.2.5 Co-polymerization of C60-cyclopentadiene 4.7 and N-(cycloheptyl)-endo-
norbornene-5,6-dicarboximide 4.4
All co-polymerization reactions were carried out under Ar in toluene (25 mL) at r.t. In all cases a
catalytic amount of Grubbs 2nd generation catalyst 4.8 (6 mg, 7.08 ?mol) was then added to the
solution to initiate polymerization. The molar ratios of N-(cycloheptyl)-endo-norbornene-5,6-
dicarboximide 4.4 to C60-cyclopentadiene adduct 4.7 was varied in order to obtain 1000:1 4.9B,
700:1 4.9C, 500:1 4.9D, 300:1 4.9E, 100:1 4.9F and 50:1 4.9G molar ratios. The solutions
containing the co-monomers were stirred for 30 min before the catalyst was added, after which
the reaction mixtures were left to stir for a further 5 days under Ar. The polymerization reactions
were then inhibited by the addition of ethyl vinyl ether (a few drops). The solvent was then
Chapter Four
74 | P a g e
removed under vacuum and the resultant solid then washed with hexane (100 mL). After
filtration the solid polymer was dissolved in CHCl3 (20 mL), and then poured into an excess of
MeOH (100 mL), containing a few drops of 1 M HCl, to precipitate the crude polymer. After
filtration, the obtained co-polymers 4.9B-E were dried under vacuum and were found to be dark
to light brown in color, depending on the amount of C60-cyclopentadiene cycloadduct 4.7 used as
co-monomer (polymer mass returns: 4.9B 80 %, 4.9C 84 %, 4.9D 79 %, 4.9E 92 %, 4.9F 80 %
and 4.9G 96 %).
4.3 Results and Discussion
4.3.1 Synthesis of 4.4 and copolymers
The work started with the synthesis of N-(cycloheptyl)-endo-norbornene-5,6-dicarboximide 4.4
using commercially available norbornene-endo-5,6-dicarboxylic anhydride 4.1 as a precursor,
and used methodology developed by Riande (Scheme 4.1) [11]. We found that the condensation
reaction of compound 4.1 and heptyl amine 4.2 readily afforded the novel N-(cycloheptyl)-endo-
norbornene-5,6-dicarboximide 4.4 in 83 % yield, presumably by way of intermediate 4.3.
4.1
4.2
4.3
4.4
N
O
O
O
O
O
H2N
OH
N
O
O
Toluene, 90oC
Ac2O,
NaOAc, 90oC
H
Scheme 4.1 Synthesis of N-(cycloheptyl)-endo-norbornene-5,6-dicarboximide 4.4
Chapter Four
75 | P a g e
The C60-cyclopentadiene cycloadduct 4.7 was also readily synthesized, by way of a Diels-Alder
reaction between C60 4.5 and freshly cracked cyclopentadiene 4.6, as previously described in the
literature [13-15] and our previous paper (Scheme 4.2) [9]. C60-cyclopentadiene cycloadduct 4.7
and N-(cycloheptyl)-endo-norbornene-5,6-dicarboximide 3.4 were then pre-mixed in varying
molar ratios (4.4:4.7: 4.9A 1:0, 4.9B 1000:1, 4.9C 700:1, 4.9D 500: 1, 4.9E 300:1, 4.9F 100: 1,
4.9G 50:1) and co-polymerized using a ROMP approach with catalytic amounts of Grubbs?
second generation catalyst 4.8 (Scheme 4.2). After 30 min of stirring the reaction solutions
became viscous to afford reaction mixtures with colours that varied from a dark brown to a light
brown colour, with the colour proportional to the concentration of the C60-cyclopentadiene
cycloadduct 4.7. It was found that allowing the ROMP to proceed for 5 days gave the best
polymer returns, as less reaction time resulted in poor polymer yields. Thus after 5 days of
stirring, the polymerization reactions were terminated and the powdery C60-containing polymers
4.9B-H were obtained by precipitation (see experimental Section for details). In addition a
polymer containing no fullerene was also generated by the ROMP of monomer 4.4, which
resulted in polymer 4.9A. Use of the approach afforded products for which the yields obtained
for the dried polymers 4.9A-H were all good (84-96 %); the colours of the obtained polymers
(light to dark brown) was indicative of the amounts of C60 incorporated into them. The results of
the ROMP syntheses are summarized in Table 4.1. Of interest was that the solubilities of the co-
polymerized polymers 4.9B-H were very good in organic solvents such as toluene,
dichloromethane and chloroform but noticeably decreased as the content of C60 increased, as was
the case in our earlier investigation [9,16,17]. A comparative solubility test was conducted on
each of 4.9D and 3.6D (60 mg of copolymer in 15 mL toluene in each case) and after 24h stirring
at r.t. copolymers 4.9D showed better solubility than copolymers 3.6D.
Chapter Four
76 | P a g e
N N
Ru
Cl
Cl
Ph
PCy3
MesMes
4.8
C60
m n
4.5 4.6
4.7
4.4
N
O
O
NNNO
O OOOO
m
NNO
OOO
N
O O
N N
Ru
Cl
Cl
Ph
PCy3
MesMes
4.8
4.4
4.9
N
O
O
Scheme 4.2 Synthesis of C60-cyclopendadiene adduct?N-(cycloheptyl)-endo-norbornene-5,6-
dicarboximide polymers 4.9B-E by ROMP (mole ratio 4.4:4.7): 4.9A (1:0), 4.9B (1000:1), 4.9C
(700:1), 4.9D (500:1), 4.9E (300:1), 4.9F (100:1), 4.9G (50:1).
Chapter Four
77 | P a g e
Table 4.1 Details of the synthesized polymer 4.9A and the co-polymers 4.9B-G.
Polymer Mole ratio
4.4:4.7
C60 content
(mole %)
Yield
%
4.9A 1:0 0 96
4.9B 1000:1 0.092 80
4.9C 700:1 0.128 84
4.9D 500:1 0.183 79
4.9E 300:1 0.303 92
4.9F 100:1 0.916 80
4.9G 50:1 1.832 96
4.3.2 Spectroscopic Studies of the synthesized materials
4.3.2.1 FTIR studies of synthesized materials
FTIR spectra were recorded for all the synthesized samples. In the C60-containing polymers
4.9B-G the presence of C60 in some of the synthesized polymers (Figure 4.1) was confirmed by a
characteristic peak at ~526 cm-1 [18]. This peak was only observed in samples with relatively
high C60 content, viz. 4.9G and 4.9F. This effect was also seen in our previously published work
[9]. In addition, strong peaks at ~1761 cm-1and ~1694 cm-1 were evident for all the synthesized
polymers and these bands were assigned to the C=O stretches [11b]. The the C=O stretches peak
is absent in 4.9H and I since those are pure C60 and the functionalized C60 respectively with out
further copolymerization.
Chapter Four
78 | P a g e
3500 3000 2500 2000 1500 1000 500
4.9G
4.9
4.9E
4.9H
4.9I
4.9D
4.9C
4.9B
4.9A
Tr
an
sm
itt
an
ce
wave number cm
-1
523 cm
-1
Figure 4.1 FT-IR spectra of the samples in KBr pellet form (mole ratio 4.4:4.7): 4.9A (1:0), 4.9B
(1000:1), 4.9C (700:1), 4.9D (500:1), 4.9E (300:1), 4.9F (100:1), 4.9G (50:1), 4.9H C60-
cyclopentadiene cycloadduct 4.7, 4.9I C60 4.5.
4.3.2.2 UV-visible absorption spectroscopic studies of synthesized materials
Electronic absorption UV-visible spectra of the fullerene co-polymerized polymers 4.9B-H were
also indicative of the presence of C60 in the polymers (Figure 4.2, data summarized in Table 4.2).
The spectrum of the C60-containing poly[N-(cycloheptyl)-endo-norbornene-5,6-dicarboximide]
4.9B-G complexes displayed a new absorption peak at 329 nm which is presumably due to the
presence of C60 [18a,19]. However, an absorption band was recorded at 380 nm for both C60 4.5
Chapter Four
79 | P a g e
(4.9I) and C60-cyclopentadiene cycloadduct 4.7 (3.9H), but was absent in poly[N-(cycloheptyl)-
endo-norbornene-5,6-dicarboximide] 4.9A (Figure 4.2). The relative shift in the absorption band
indicates that the electronic structure of C60 has been modified by its incorporation into the
norbornene polymer by the ROMP reaction [20].
300 400 500 600 700
0
2
400 500 600 700
0.0
0.1
0.2
433nm
Absorb
an
ce
Wavelength (nm)
380.2 nm
406.3 nm
329.3 nm
4.9I
4.9H
4.9G
4.9F
4.9E
4.9D
4.9C
4.9B
4.9A
A
bs
or
ba
nc
e
Wavelength (nm)
4.9G
4.9E
4.9C
4.9A 4.9D
4.9F
4.9B
Figure 4.2 UV-visible spectra of co-polymers 4.9A-I (in toluene, mole ratio 4.4:4.7): 4.9A (1:0
mole ratio), 4.9B (1000:1 mole ratio), 4.9C (700:1 mole ratio), 4.9D (500:1 mole ratio), 4.9E
(300:1 mole ratio), 4.9F (100:1 mole ratio), 4.9G (50:1 mole ratio), 4.9H C60-cyclopentadiene
cycloadduct 4.7, 4.9I C60 4.5.
Chapter Four
80 | P a g e
Table 4.2 UV-visible ?max of the polymer 4.9A and the co-polymers 4.9B-I
Polymer mole ratio
4.4:4.7
?max (nm)
4.9A 1:0 No ?max
4.9B 1000:1 329, 406, 433
4.9C 700:1 329,406, 433
4.9D 500:1 329, 406, 433
4.9E 300:1 329, 406, 433
4.9F 100:1 329, 406, 433
4.6G 50:1 329, 406, 433
3.9H 0:1 380, 434, 496, 709
3.9I 4.5 only 380, 406, broad peak 460?650
In other published studies, the UV-visible absorption band of methanofullerene derivatives [21]
and C60 monoaddducts [22] were recorded at ~432 nm; this band being attributed to the 6-6 ring
fusion present in the compounds. Similarly, in this work, a band centered at 433 nm for all co-
polymers (weak, inset Figure 4.2) and the C60-cyclopentadiene cycloadduct 4.7 was considered
as characteristic of a 6-6 ring fusion.
4.3.3 Thermal Degradation Studies
4.3.3.1 Differential scanning calorimetry (DSC)
The glass transition temperatures for the synthesized polymer and copolymers were also
determined under a nitrogen atmosphere. According to the results, there is no clear correlation
Chapter Four
81 | P a g e
between the concentration of C60 in the polymers and the glass transition temperatures, unlike
that observed in our early work [9]. However, for 4.9D-G there is an observable trend in that as
the concentration of C60 increases in the polymers, the Tg decreases (see Figure 4.3). For pure
polymer 4.9A and the co-polymer 4.9G, two Tg values were observed at 77
oC and 80 oC and 72
oC and 62 oC respectively (see Figure 4.3). For the other polymers only a single Tg temperature
was observed. (See Table 4.3 for a summary of these results).
40 60 80 100 120
-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
2
4
4.9B
4.9C
4.9D
4.9G
4.9F
4.9A
4.9E
H
ea
t f
lo
w
[m
W
]
Temperature
o
C
Figure 4.3 Differential Scanning Calorimetry Tg analyses (scan rate 5
oC/min) under nitrogen
atmosphere: Co-polymers 4.9A-G (mole ratio 4.4:4.7): 4.9A (1:0), 4.9B (1000:1), 4.9C (700:1),
4.9D (500:1), 4.9E (300:1), 4.9F (100:1), 4.9G (50:1).
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82 | P a g e
Table 4.3 Tg and thermal decomposition temperature of C60-containing polymers 4.9B-E and
4.9A.
Polymer Mole ratio
4.4:4.7
Tg (
oC) Decomposition
Temp.a
4.9A 1:0 77, 80 192, 436
4.9B 1000:1 80 190, 430
4.9C 700:1 77 252, 433
4.9D 500:1 80 242, 428
4.9E 300:1 75 443
4.9F 100:1 74 193,440
4.9G 50:1 72, 62 432
a Measured by derivative method
4.3.3.2 Thermogravimetric analysis (TGA)
The thermogravimetric analysis (TGA) results revealed that incorporation of C60 in the polymer
did not significantly increase the thermal stability of the synthesised polymers. Two stages of
decompositions were observed for 4.9A and for the C60-containing co-polymers 4.9B-H at
approximately 200 and 440 oC; however significant mass losses were observed after ~430 oC
(Figure 4.4). During the first stage decomposition of co-polymers 4.9C and 4.9D mass losses
were 29 and 36 %, respectively, while for the others it ranged from 5 to 10 %. After thermal
decomposition of the polymers (700 oC) the mass of the residues obtained were proportional to
the content of C60 in the polymers, i.e. polymers which contained lesser amounts of fullerene
retained smaller amounts of residue [23].
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83 | P a g e
100 200 300 400 500 600 700
0
20
40
60
80
100
G
F
E
D
C
B
A
W
ei
gh
t %
Temperature (?C)
Figure 4.4 TGA curves of the polymers (rate 10 oC/ min) under a nitrogen atmosphere (mole
ratio 4.4:4.7): 4.9A (1:0), 4.9B (1000:1), 4.9C (700:1), 4.9D (500:1), 4.9E (300:1), 4.9F (100:1),
4.9G (50:1).
4.4 Conclusion
In this work we successfully synthesized a series of C60-containing polymers by the co-
polymerization of a C60-cyclopentadiene cycloadduct 4.7 and N-(cycloheptyl)-endo-norbornene-
5,6-dicarboximide 4.4 in varying ratios. This work demonstrated that the methodology we
developed for the production of covalently-linked, fullerene-containing polymers has been
amenable to the use of a different co-monomer (in place of norbornene). The polymerization was
readily facilitated by a catalytic amount of Grubbs? second generation catalyst 4.8. The co-
polymers thus formed were investigated by spectroscopic and thermal techniques and the results
of these studies confirmed that the relative amounts of C60 in the polymers affected the
polymer?s physical properties.
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84 | P a g e
4.5 References
1 (a) F. Giacalone, N. Mart?n, Chem. Rev., 2006, 106, 5136. (b) C. Wang, Z.-X. Guo, S. Fu,
W. Wu, D. Zhu, Prog. Polym. Sci., 2004, 29, 1079. (c) K. E. Geckeler, S. Samal, Polym.
Int. 1999, 48, 743. (d) M. Prato, J. Mater. Chem., 1997, 7, 1097.
2 See for example: S. Shi, K. C. Khemani, Q. C. Li, F. Wudl, J. Am. Chem. Soc., 1992,
114, 10656.
3 See for examples: (a) T. Song, S. H. Goh, S. Y. Lee, Polymer, 2003, 44, 2563. (b) C.-C.
M. Ma, S.-C. Sung, F.-Y. Wang, L. Y. Chiang, L. Y. Wang, C.-L. Chiang, J. Polym. Sci.,
Part B: Polym. Phys., 2001, 39, 2436 and references cited therein.
4 N. Zhang, S. R. Schricker, F. Wudl, M. Prato, M. Maggini, G. Scorrano, Chem. Mater.,
1995, 7, 441.
5 See for example: (a) Z. T. Ball, K. Sivula, J. M. J. Fr?chet, Macromolecules, 2006, 39,
70. (b) K. Sivula, Z. T. Ball, N. Watanabe, J. M. J. Fr?chet, Adv. Mater., 2006, 18, 206.
(c) A. de la Escosura, M. V. Mart?nez-D?az, T. Torres, R. H. Grubbs, D. M. Guldi, H.
Neugebauer, C. Winder, M. Drees, N. S. Sariciftci, Chem. Asian J., 2006, 1-2, 148.
6 X. Liu, A. Basu, J. Organomet. Chem., 2006, 691, 5148.
7 F. J. G?mez, R. J. Chen, D. Wang, R. M. Waymouth, H. Dai, Chem. Commun., 2003,
190.
8 Y. Liu, A. Andronov, Macromolecules, 2004, 37, 4755.
9 M. A. Mamo, N. J. Coville, W. A. L. van Otterlo, Fullerenes, Nanotubes and Carbon
Nanostructures, 2007, 15, 341.
10 For a paper describing the development of this catalyst see: (a) T. M. Trnka, R. H.
Grubbs, Acc. Chem. Res., 2001, 34, 18. For recent applications of this catalyst to our
work, see the following recent references and citations therein: (b) R. Pathak, J.-L.
Panayides, T. D. Jeftic, C. B. de Koning, W. A. L.van Otterlo, S. Afr. J. Chem. 2007, 60,
1. (c) E. M Coyanis, J.-L. Panayides, M. A. Fernandes, C. B. de Koning, W. A. L. van
Otterlo, J. Organomet. Chem., 2006, 691, 5222.
11 (a) A. P. Contreras, M. A. Tlenkopatchev, M. del Mar L?pez-Gonz?lez, E. Riande,
Macromolecules, 2002, 35, 4677. (b) B. Liu, X. Wanga, Y. Wang, W. Yan, H. Li, I.
Kim, Reactive & Functional Polymers, 2009, 69, 606.
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85 | P a g e
12 For related work concerning N-alkylated norbornene dicarboximides and their application
in ROMP see the following representative publications: (a) J. Vargas, A. A. Santiago, R.
Gavi?o, A. M. Cerda, M. A. Tlenkopatchev, Express Polym. Lett., 2007, 1, 274. (b) A. A.
Santiago, J. Vargas, R. Gavi?o, A. M. Cerda, M. A. Tlenkopatchev, Macromol. Chem.
Phys., 2007, 208, 1085. (c) S. Hilf, A. F. M. Kilbinger, Macromol. Rapid Commun.,
2007, 28, 1225. (d) C. C. Thomas, K. Ezat, R. H. Lian, Macromolecules, 2006, 39, 5639.
(e) T. C. Castle, E. Khosravi, L. R. Hutchings, Macromolecules, 2006, 39, 5639. (f) R.
Madan, R. C. Anand, I. K. Varma, J. Polym. Sci.: Part A: Polym. Chem., 1997, 35, 2917.
13 V. M. Rotello, J. B. Howard, T. Yadav, M. M. Conn, E. Viani, L. M. Giovane, A. L.
Lafleur, Tetrahedron Lett., 1993, 34, 1561.
14 For other examples, involving the reaction of substituted or unsubstituted
cyclopentadienes to C60 by way of a Diels-Alder reaction, see the following papers and
references cited therein: (a) S. R. Wilson, M. E. Yurchenko, D. I. Schuster, A. Khong, M.
Saunders, J. Org. Chem., 2000, 65, 2619. (b) R. Schwenninger, T. M?ller, B. Kr?utler, J.
Am. Chem. Soc., 1997, 119, 9317. (c) B. Nie, V. M. Rotello, J. Org. Chem., 1996, 61,
1870.
15 For reviews on cycloadditions to C60 see: (a) M. A. Yurovskaya, I. V. Trushkov, Russ.
Chem. Bull. Int. Ed., 2002, 51, 367. (b) P. Hudhomme, C.R. Chimie, 2006, 9, 881. (c) R.
Taylor, C.R. Chimie, 2006, 9, 982. (d) J. Yli-Kauhaluoma, Tetrahedron, 2001, 57, 7053.
16 H. W. Goh, S. H. Goh, G. Q. Xu, J. Polym. Sci. Part A: Polym. Chem., 2002, 40, 1157.
17 On formation, the polymers 3.9A-G were generally very soluble. However after
precipitation and drying of the polymers, addition of organic solvents often caused gel
formation rather than facile solubilization.
18 See for example: (a) K. E. Geckeler, A. Hirsch, J. Am. Chem. Soc., 1993, 115, 3850. (b)
T. Suzuki, Q. Li, K. C. Khemani, F. Wudl, J. Am. Chem. Soc., 1992, 114, 7301. (c) X.
Zhang, A. B. Sieval, J. C. Hummelen, B. Hessen, Chem. Commun., 2005, 1616.
19 W. J. Li, W. J. Liang, Spectrochim. Acta Part A, 2007, 67, 1346.
20 B. Z. Tang, S. M. Leung, H. Peng, N.-T. Yu, K. C. Su, Macromolecules, 1997, 30, 2848.
21 See examples in the following papers and references cited therein: (a) C. -C. Chu, T.-I.
Ho, L. Wang, Macromolecules, 2006, 39, 5657. (b) S. Xiao, Y. Li, Y. Li, H. Li, J.
Chapter Four
86 | P a g e
Zhuang, Y. Liu, F. Lu, D. Zhang, D. Zhu, Tetrahedron Lett., 2004, 45, 3975. (c) S. Xiao,
Y. Li, H. Fang, H. Li, H. Liu, Z. Shi, L. Jiang, D. Zhu, Org. Lett., 2002, 4, 3063.
22 A. Kraus, K. Mullen, Macromolecules, 1999, 32, 4214.
23 See for example: (a) A. G. Camp, A. Lary, W. T. Ford, Macromolecules, 1995, 28, 7959.
(b) L. Y. Chiang, L. Y. Wang, C.-S. Kuo, Macromolecules, 1995, 28, 7574. (c) L. Dai, A.
W. H. Mau, H. J. Griesser, T. H. Spurling, J. Phys. Chem., 1995, 99, 17302. (d) C. J.
Hawker, Macromolecules, 1994, 27, 4836.
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87 | P a g e
Chapter 5
Polymerization of a C60 derivative with thiophene in the presence of
FeCl3
5.1 Introduction
Since the discovery of C60 (fullerene) [1], its subsequent large scale production has attracted the
attention of researchers in terms of chemistry [2] and applications [3]. A large number of studies
have indicated that fullerenes possess interesting electrochemical [4], photophysical [5], optical [6],
semiconducting [7] and magnetic [8] properties. As result novel properties associated with
functionalized fullerenes have been reported by various researchers include superconductivity,
ferromagnetism, and optical nonlinearity [9].
The design of molecules bearing covalently linked electron donors to C60 has received increasing
attention in the past few years as these systems can be used in artificial photosynthesis and for
photoelectronic applications [10]. Compounds that combine C60 with ?-conjugated oligomers are
of particular interest in photoelectronics [11-14]. On one hand, they provide entry into
photoinduced intramolecular processes such as energy and electron transfer [12]. On the other
hand, such hybrid compounds can be used for the preparation of solar cells allowing a detailed
structure?activity exploration for a better understanding of the photovoltaic system [13].
It is well know that 1,3-dipolar addition of azomethine ylides to C60 yields fulleropyrrolidines
that form across the 6,6-junction [15]. The azomethine ylides are reactive intermediates that are
generated in situ by decarboxylation of immonium salts derived from thermal condensation of
either amino acids and aldehydes (or ketones), or the thermal ring opening of aziridines. It has
been shown to be one of the most flexible methods for the functionalization of fullerenes and has
been widely used [16]. Different functional groups with important electronic properties have
been attached to the fullerene system using azomethine ylides such as porphyrins [17],
subphthalocyanines [18], dendrimers [19] and conjugated oligomers [20].
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88 | P a g e
In donor-acceptor-linked fullerene molecule systems the donors have been constructed from a
variety of dyad molecules including aniline [21], carotenoid [22], porphyrin [23,24], pyrazine
[25], and tetrathiophene [26] moieties, making use of synthetic strategies which include the Prato
reaction. These systems have been designed to achieve efficient intramolecular energy transfer
[11f]. In addition, the particular problem of phase segregation caused by bi-component
composite (mixture) materials in these devices, is thought to be minimized [27, 28]. However, it
is important to note that in many triad and dyad systems using the Prato reaction, the ?-
conjugated repeating units are limited to below ten [11f, g].
The use of electron-accepting fullerenes in combination with ?-conjugated systems, as electron
donors, offers several attractive features. In particular, fullerene, due to its low reorganization
energy [12b] in electron-transfer reactions, accelerates charge separation and decelerates charge
recombination, compared to two dimensional, planar electron acceptors [27]. This is beneficial
for stabilizing the charge-separated state in C60-based materials as required in artificial electron
transfer systems [28]. The design of electron donors covalently linked to C60 has thus received
increasing attention in the past few years as these systems can be used in artificial photosynthesis
and for photoelectronic applications [24]. Compounds that combine C60 with ?-conjugated
oligomers are of particular interest in photoelectronics [11].
In solar cells, conductive organic polymers such as poly(3-alkylthiophenes) (P3ATs) [29] play a
crucial role as an electron donor and in charge transport [30]. They have also attracted much
attention in organic electronics, because of their chemically tunable electronic properties and
their processability from a variety of solvents [31].
Covalent bonding of polymers with C60 [32-34] is of considerable interest since the fullerene
properties can be combined with those of specific polymers. Suitable fullerene polymers should
be spin coatable, solvent-castable or melt-extrudable and the fullerene-containing polymers as
well as surface-bound C60 layers are expected to have remarkable electronic, magnetic,
mechanical, optical or catalytic properties [32]. Fullerenes can also be incorporated to and on the
side chain of a polymer (on-chain type or ?charm bracelet?) [35] or on the surface of a solid [36],
in chain polymers with the fullerene as a part of the main chain (?pearl necklace?) [35], dendritic
Chapter Five
89 | P a g e
systems, starburst or cross-link type material. End-chain type polymers that are terminated by a
fullerene unit have also been reported.
In this work we report the functionalization of C60 using azomethine ylides (the Prato reaction
[15]) and on subsequent reactions that incorporate the C60 derivatives covalently to a polymer
polythiophene system. The electronic and thermal properties of the synthesized copolymers were
then investigated and the results will be discussed in this Chapter.
5.2 Experimental
5.2.1 General procedures
The synthesised polymers, copolymers and C60 derivatives were characterised by thermo-
gravimetric analysis (TGA), differential scanning calorimetry (DSC), mass spectrometry, UV-
visible, FT-IR and NMR spectroscopy. The infrared spectra were recorded using a Varian 800
FT-IR spectrometer (KBr pellets) and transmittance values are reported in the wave number (cm-
1) scale in the range of 400?4000 cm-1. Ultraviolet and visible spectra were recorded with a
Varian 50 CONC UV-visible spectrophotometer and mass spectra were collected with a VG70-
SEQ instrument in a positive ion mode using FAB ionization. The NMR spectra (chemical shift
data in ppm) were recorded in CDCl3 at ambient probe temperature using a Bruker Avance 300
(1H, 300.13 MHz) spectrometer. Finally, TGA analyses were performed at a heating rate of 10
oC/min, under air using a Perkin Elmer Pyris 1 TGA instrument.
5.2.2 Functionalization of C60
5.2.2.1 Synthesis of 5.2 [15]
C60 5.1 (0.3 g, 0.42 mmol) was added to predried toluene (~50 mL) and the mixture was heated
to 115 oC to dissolve all the C60. After the solution was cooled to r.t., 2-thiophenecarboxaldehyde
(0.048 g, 0.42 mmol) was added to the toluene solution. N-methylglycine (0.08 g, 0.9 mmol) was
added to the above reaction mixture in small portions over 5 days (about 0.016 g per 24 h). The
reaction mixture was then stirred at reflux for a further five days during which the purple color of
the solution slowly changed to brown. The reaction mixture was then left to cool to r.t. and the
solvent was removed under reduced pressure to afford a brown solid. The brown product was
then purified using column chromatography (SiO2, toluene, then 1 % DMF in toluene). After
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90 | P a g e
removing the solvent under reduced pressure the product was dried under vacuum for 48 h to
afford the desired product 5.2 (0.306 g, 85 % based on C60).
5.2.2.2 Synthesis of 5.3 [15]
C60 5.1 (0.5 g, 0.69 mmol) was added to pre-dried toluene (~75 mL) and the mixture was heated
to 115 oC to dissolve all the C60. After the solution was cooled to r.t., 3-thiophenecarboxaldehyde
(0.077 g, 0.69 mmol) was added to the toluene solution. N-methylglycine (0.13 g, 1.426 mmol)
was then added to the reaction mixture in small portions over 5 days (about 25 mg per 24 h). The
reaction mixture was then left to heat at reflux under continuous stirring for a further five days
during this time the purple color of the solution slowly changed to brown. The reaction mixture
was then left to cool to r.t. and the solvent was removed under reduced pressure to afford a
brown solid. The brown product was then purified using column chromatography (SiO2, toluene,
then 1 % DMF in toluene). After solvent removal under reduced pressure the product was dried
under vacuum for 48 h to afford 5.3 (495 mg, 83 % based on C60).
5.2.3 Polymerization reactions
5.2.3.1 Polymerization reactions to synthesize polythiophene 5.4 and poly(3-
hexylthiophene) 5.5 [37]
FeCl3 (3.90 g, 23.6 mmol) was added to dry chloroform (~25 mL) in each of two round bottom
flasks and the mixtures were stirred for 10 min. Either thiophene (0.05 mg, 5.95 mmol) or 3-
hexylthiophene (1.0 g, 5.9 mmol), in dry chloroform (~25 mL), was then added drop-wise over
10 min to the flask that contained the FeCl3 mixtures and the reaction mixtures were stirred
overnight under Ar. The polymerization reactions were terminated by pouring the reaction
mixtures into an excess of MeOH (~50 mL) to precipitate the crude polymers. The synthesised
polymers were subsequently filtered using a membrane filter paper (1 ?m). The products were
then washed with ethanol (100 mL), several times with a 1:1 distilled water and acetone mixture
(500 mL) and finally several times with acetone (500 mL). The dark brown solid products were
dried under vacuum for 48 h and 0.326 g (65 % based on thiophene monomer) of a brick red
powder of pholythiophene and 0.855 g (86 % based on hexylthiophene monomer) of a dark
brown spongy solid of poly(3-hexylthiophene) was obtained, respectively.
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91 | P a g e
5.2.3.2 Copolymerization reactions
5.2.3.3 Copolymerization reactions to form polymers 5.6 and 5.7
FeCl3 (0.35 g, 2.15 mmol) was added to dry dichloromethane (~25 mL) in each of two round
bottom flasks and the mixtures were stirred under Ar for about 15 min. Thiophene (each 0.039
g, 0.46 mmol) and either 5.2 or 5.3 (each 0.040 g, ~0.046 mmol) were dissolved in dry
dichloromethane (~10 mL) and the mixture was then added drop-wise over 30 min to a flask that
contained the FeCl3. The reaction mixtures were then left to stir for 24 h. The polymerization
reactions were terminated by pouring the reaction mixture into an excess of MeOH (~50 mL) to
precipitate the crude products. The precipitates that formed from the two reactions were filtered
using a membrane filter (1 ?m). The filtrates were each washed with a 1:1 mixture of distilled
water and acetone (5x, total volume of 500 mL). The products were then dried under reduced
pressure for 48 h to afford 0.069 g (87 % yield) and 0.073 g (92 % yield) of the dark brown
products of 5.6 and 5.7, respectively.
5.2.3.4 Copolymerization reactions to form polymers 5.8a, 5.8b and 5.8c
FeCl3 (0.35 g, 2.15 mmol for 5.8a; 7.0 g, 40 mmol for 5.8b and 13.0 g, 80 mmol for 5.8c) was
added to dry dichloromethane (~25 mL) in each of three round bottom flasks and the mixtures
were stirred under argon for about 15 min A mixture of 3-hexylthiophene (0.078 g, 0.463 mmol
for 5.8a; 1.66 g, 9.85 mmol for 5.8b and 3.32 g, 19.7 mmol for 5.8c) and 5.3 (0.040 g, 0.046
mmol for 5.8a; 0.016 g, 0.0184 mmol for 5.8b and 5.8c) in different mole ratio (1000:1 for 5.8c,
500:1 for 5.8b, 10:1 for 5.8a) were dissolved in dry dichloromethane (~10 mL) and added to the
FeCl3 mixture, drop-wise over 25 min The reaction mixtures were then left to stir for 24 h.
Methanol (~50 mL) was added to the mixtures and the precipitates that formed were filtered
using a membrane filter (1 ?m). The filtrates were washed with a 1:1 mixture of distilled water
and acetone (5x, total volume of 500 mL). The products were then dried under reduced pressure
for 48 h to afford 0.103 g (87 % yield), 1.59 g (96 % yield) and 3.22 g (97 % yield) of dark
brown products of polymers 5.8a, 5.8b and 5.8c, respectively.
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92 | P a g e
5.2.3.5 Copolymerization reaction to form polymer 5.9
FeCl3 (0.350 g, 2.15 mmol) was added to dry dichloromethane (~ 25 mL) and the mixture was
then stirred under argon for 15 min A mixture of 5.2 (0.040 g, 0.046 mmol) and 3-
hexylthiophene (0.078 g, 0.463 mmol) was dissolved in dry dichloromethane (~10 mL) and was
then added to the FeCl3 mixture drop-wise over 25 min The reaction mixture then was left to stir
for 24 h. MeOH (50 mL) was then added to the mixture and the precipitate that formed was
filtered using a membrane filter (1 ?m). The filtrate was washed with a 1:1 mixture of distilled
water and acetone (5x, total volume of 500 mL). The product was then dried under reduced
pressure for 48 h to afford 0.105 g (89 % yield) of a dark brown product.
5.3 Results and discussion
5.3.1 Synthesis of 5.2, 5.3 and copolymers
Prato et al. [15] have developed a powerful procedure for the functionalization of C60. The Prato
reagent is made by adding an aldehyde and a glycine. The reagents form azomethine ylides at
high temperature that react via a 1,3-dipolar cycloaddtion with C60 to give functionalized
fullerenes. This procedure permits the synthesis of a wide range of substituted fullerenes by
variation of the aldehyde and glycine functional groups. In this study the use of 2 and 3-
thiophene aldehyde and N-methylglycine was utilized for the synthesis of the C60 complexes.
The products were isolated and then purified using column chromatography (SiO2, toluene, 1%
DMF in toluene) in good yields (> 83 %.). In an earlier report [11c], using 3-thiophene aldehyde,
the overall isolated yield of this type of reaction was 10-40 % after heating the reaction mixture
at reflux for 5-24 h. In our case, by heating the reaction mixture at reflux for longer times (five
days) the overall yield was much improved. Mass spectroscopic (m/z = 860 [M+1] with 2.6%)
and UV-visible results confirmed that indeed the reaction had offered the expected addition
products 5.2 and 5.3 (Scheme 5.1).
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93 | P a g e
S
O
H
CH 3
NH
CH 2
COO
H
,
tolu
ene
at 1
15
o C
S
CH
3 NHCH
2 COOH
,
toluene at 115 oC
O
H
S
R
S
R
FeCl3 dry Dichloromethane
5.6: R= H
5.9: R=hexyl
5.7: R= H
5.8: R=hexyl
5.8a= 10:1
5.8b= 500:1
5.8c= 1000:1
5.1
5.2
5.3
FeCl3 dry Dichloromethane
N
S
S
N
S
S
S
S
N
R
R
R
S
S
S
R
N
R
n
nm
5.a
5.b
5.c
5.c
5.d
5.d
Scheme 5.1 Synthetic route to new copolymers using the Prato reaction to functionalization C60
followed by oxidative polymerization of thiophene.
The two C60 derivatives (5.2 and 5.3) have similar chemical formulae but different reactiveties in
the polymerisation reactions with the thiophene used. Thiophene derivative 5.3 will generate
pearl type polymers since it has two reactive sites at which propagation can occur. However,
Thiophene derivative 5.2 can only result in end-cup polymers since only a single site is available
for a polymerization reaction (see scheme 5.1).
5.3.2 Polymerization reactions of the synthesised materials
3-Alkylthiophenes have been previously polymerized to the corresponding polyalkylthiophenes,
either by electrochemical [38] or chemical polymerization [39] methods. However, studies have
indicated that electrochemical polymerization reactions have several drawbacks when compared
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94 | P a g e
to chemical polymerisation reactions. These include lower yields and generally a higher degree
of regio-irregularities which results in decreased ?-electron delocalization that limits the
solubility of the polymer, as compared to polymers obtained by chemical methods [40]. Jen et
al. [39] and ?sterhom et al. [39b] have used Grignard coupling reactions to chemically prepare
polyalkylthiophenes. An earlier study by Sugimoto et al. [37] indicated that a direct one step
oxidation of 3-alkythiophene with excess FeCl3 (4 times stoichiometric proportion to the
monomer) in chloroform, at r.t., gave the polymerized product in high yield.
It is to be noted that during polymerization the 3-alkyl substituent in a thiophene ring can be
incorporated into a polymer chain with two different regioregularites, namely head-to-tail (HT)
and head-to-head (HH). This results in four traid regioisomers in the polymer chain: the HT-HT,
HT-HH, TT-HT and TT-HH traids (see Figure 5.1).
S
S
S
R
R
R
*
HT-HT
S
S
S
R
R
*
TT-HT
S
S
S
R
R
*
HT-HH
S
S
S
R
*
TT-HH
R
R
R
R= Hexyl
R
Figure 5.1 Different types of traid regioisomers of poly(3-hexylthiophene) [41].
The synthesised pure polymers 5.4 and 5.5 (Scheme 5.2), were obtained using the latter approach
as brick red powders and a spongy, dark brown coloured material in good yields of 66 % and 86
% for 5.4 and 5.5, respectively. However, polymer 5.4 was not soluble in any organic solvents,
Chapter Five
95 | P a g e
while polymer 5.5 was soluble in THF and 1,2-dichlorobenzene and sparingly soluble in
dichloromethane, chloroform and hexane.
FeCl3 dry chloroform
S
R
S
S
R
S
R
n
5.4: R= H
5.5: R=hexyl
R
Scheme 5.2 Synthetic route to the thiophene polymers.
The copolymers (5.6, 5.7, 5.8a, 5.8b, 5.8c, and 5.9) were subsequently obtained in good yields
(> 87 %) as brown powder or spongy, dark brown colour materials from the reaction of 5.2 and
5.3 with thiophene and 3-hexylthiophene, respectively (Scheme 5.1). The unsubstituted
polythiophene copolymers (5.6 and 5.7) found to be totally insoluble in all solvents. The hexyl
substituted copolymers (5.8a, 5.8b, 5.8c and 5.9) found to have similar solubilities to the pure
poly(3-hexylthiophene) (5.5); however the solubility decreased as the C60 concentration increase
in the series 5.8c < 5.8b < 5.8a .
5.3.3 Characterization of the synthesized copolymer
5.3.3.1 1H NMR spectroscopy of the synthesized copolymers
Solution NMR studies could not be performed on the insoluble, unsubstituted polythiophene
polymers and co-polymers. The copolymers that were soluble were analyzed by using NMR
spectroscopy in CDCl3. The
1H NMR spectra provided important information on the substitution
pattern in the polymer backbone [42]. The substitution patterns have been shown to play an
important role in the ?-electron delocalization and solubility of the resulting polymers [46].
The NMR spectrum of 3-hexythiophene (Figures 5.2) contained a narrow peak at ? = 0.9
assigned to the methyl protons f. Peaks at ? = 1.3 and 1.6 were assigned to the e and d methylene
protons of the monomer, respectively. Similarly a triplet peak at ? = 2.6 was assigned to the
methylene protons c. The aromatic hydrogen atoms of the thiophene rings were assigned at ? =
Chapter Five
96 | P a g e
6.9 and 7.2 to the a/a?, and b protons respectively [41]. This spectrum provides the references for
the analysis of the hexylthiophene polymers and co-polymers.
8 6 4 2
S
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
a
a'
b
c
c
d
d
d
d
e
e
e
e
f
7.20 7.15 7.10 7.05 7.00 6.95 6.90 6.85 6.80
7 .
04
5 7 .
02
6
7 .
00
1
6 .
98
4
5.5
b
a,a' c d
e
f
5.3
5.8a
5.8b
5.8c
hexylthiophene
chemical shift (ppm)
Figure 5.2. 1H NMR spectra of the monomer, polymer and copolymers for the synthesised
polymers (in CDCl3, 40 mg samples were used in each case, r.t., 300.13 MHz).
As expected, the resonance peaks found in the spectrum of poly(3-hexylthiophene) (5.5) and the
copolymers attached to C60 have similar chemical shifts to those observed for the monomer. A
broad peak at ? = 0.9 in both polymer and copolymers was assigned to the methyl protons f.
Similarly broad peaks between ? = 1.3-1.6 was assigned to the methylene protons e and d. A
triplet peak at ? = 2.6, was proposed to be due to the methylene proton c in the monomers, that
splits into two broad peaks at ? = 2.6 and 2.8 after polymerization. This is due to the existence of
HT and HH regioisomers, respectively [41]. The HT:HH ratio of the poly(3-hexylthiophene) was
Chapter Five
97 | P a g e
deduced from the 1H NMR spectrum by integration of the peaks at ? = 2.6 and 2.8 and was
estimated to be 60:40. Disappearance of the thiophene aromatic hydrogen atoms of a/a? at ? =
6.9 after polymerization is evidence that polymerization indeed has taken places. The proton in
the b position of the thiophene ring can occur in four different chemical environments in a
mixture of the four possible traid regioismers (see Figure 5.1). These four chemical shifts give
distinct protons that are uniquely distinguished in the 1H NMR spectra of poly(3-hexylthiophene)
(5.5) (see inseted Figure 5.2). The observed spectra were consistent with a new totally random
mixture of the four traid structures depicted (HT-HT at ? = 6.98, TT-HT at ? = 7.00, HT-HH at ?
= 7.03, TT-HH at ? = 7.05) [41].
Similarly, the HT:HH ratio of the copolymers 5.8b and 5.8c were deduced from the 1H NMR
spectrum by integration and found to be 70:30 and 65:35, respectively. Those incorporation of
the C60 thiophene derivative into the thiophene backbone appeared to slightly favor the HT
isomers. Due to poor resolution of 1H NMR for 5.8a it was not possible to deduce the HT:HH
ratio for this particular copolymer.
5.3.3.2 FTIR spectroscopy of the synthesized copolymers
The FTIR spectra of the polymers are shown in Figure 5.3a and 5.3b for the polythiophene and
poly(3-hexylthiophene) derivatives respectively. A summary of the FTIR band positions, and
their assignments for the polymers, is given in Table 5.1
A distinct peak for the fullerene derivatives (5.2 and 5.3) and copolymers in the range of 520-530
cm-1 is expected for a substituted C60 complexes
[43]. This strong and distinctive peak in the
spectra progressively disappeared as the concentration of C60 derivative was reduced in the
copolymers (see Fig 5.3a).
In the thiophene monomer, a strong band at 702 cm-1 was attributed to the =C-H out of plane
vibration. Unlike in the polymer and copolymers, bands at 1510 and 1457 cm-1 were absent in the
monomer. These bands are associated with 2,5-disubstituted thiophene in the polymers [44,45].
Finally, the aromatic C-H stretching band, at 3108 cm-1 appeared to be very weak for all the
samples investigated.
Chapter Five
98 | P a g e
4000 3500 3000 2500 2000 1500 1000 500
526 cm
-1
787 cm
-1
5.4
5.7
5.6
5.3
5.2
5.1
Tr
an
sm
itt
an
ce
(a
.u
)
wave number (cm
-1
)
702 cm
-1
Thiophene
Figure 5.3a. FT-IR spectra of the monomer and polythiophene derivatives KBr pellets.
The IR spectra for 3-hexylthiophene materials are shown in figure 5.3b. A strong and
characteristic band at 660 cm-1 was attributed to the =C-H out of plane vibration for the 3-
hexylthiophene monomer and a band at ~720 cm-1 was assigned to the C-S-C out of plane
deformation [46]. The aromatic C=C stretching vibration of the thiophene ring was recorded at
1654 cm-1. Furthermore, the aliphatic C-H stretching bands were recorded between 2823-2714
cm-1, and the aromatic C-H stretching appeared to be very weak at 3095 cm-1.
Chapter Five
99 | P a g e
4000 3500 3000 2500 2000 1500 1000 500
Tr
an
sm
itt
an
ce
(a
.u
)
wave number (cm
-1
)
526 cm
-1
823 cm
-1
5.2
5.3
5.5
Hexylthiophene
5.8c
5.8b
5.1
5.8a
5.9
Figure 5.3b. FT-IR spectra of the poly(3-hexylthiophene) derivatives KBr pellets.
Unlike in the monomers, some new and characteristic bands were recorded for both polymers
and copolymers after polymerization. A band at around 787 cm-1 and 823 cm-1 were assigned to
the =C-H out of plane vibration for the 2,5-disubstituted thiophene chains [44,45], in copolymers
5.6, 5.7 and polythiophene (5.4) and, 5.8b and 5.8c and poly(3-hexylthiophene) polymers,
respectively. However, in the case of 5.8a and 5.9 this particular band shifted to lower energy at
835 cm-1. Chen et al. [41] reported that regioregular HT poly(3-hexylthiophene) gave an
absorbtion band at 820-822 cm-1. On the other hand, regiorandom poly(3-hexylthiophene) gave
bands at 827-829 cm-1. Accordingly, our synthesized copolymers 5.8a and 5.9 are considered to
be regiorandom (due to the presence of the high concentration of C60), while the 5.8b and 5.8c
Chapter Five
100 | P a g e
and poly(3-hexylthiophene) (4.5) were more regioregular. A band at 666 cm-1, that probably
arises from 2-monosubstitution [44,45] at the end of the polymer chains, was noted in the two
C60 derivatives (5.2 and 5.3), and copolymers 5.8a-c, 5.9 and poly(3-hexylthiophene) (5.5).
However, in the case of 5.6, 5.7 and polythiophene (5.4) it was recorded at lower energy (< 690
cm-1). In the case of functionalized C60 derivatives 5.2 and 5.3 this stretching band was probably
due to the thiophene attachment that was associated with the Prato reagents. In all cases this band
was very weak. The stretching vibration bands of the 2,5-disubstituted thiophene rings were
recorded at around 1490 cm-1 and 1442 cm-1 for 5.6, 5.7 and polythiophene (5.4) and 1510 cm-1
and ~1457 cm-1 for polymers 5.8a-c, 5.9 and poly(3-hexylthiophene) (5.5) [44,45,47,48]. The
aliphatic C-H stretching band was recorded at more or less similar wave numbers from 2955-
2850 cm-1 and 2992-2720 cm-1 for all the copolymers [49] and C60 derivatives, respectively. As
before, the aromatic C-H stretching vibration at 3065 cm-1 also appeared to be weak.
Chapter Five
101 | P a g e
Table 5.1 Summary of FTIR of the copolymers, polymers and C60 derivatives (values in cm
-1)a
Sample Ar C-H
str.
Alipha C-H str. Ring str. Ar C-H out of plane C60 band
5.1 526 (s)
5.2 2992-2720 (br. s) 696 (vw) 526 (s)
5.3 2992-2720 (br. s) 696 (vw) 526 (s)
5.4 3065 (w) 1490 (vw) 1442 (m) 787 (m) 690 (s)
5.5 3065(w) 2955-2850 (br. s) 1510 (vw) 1457 (s) 823 (s) 666(m)
5.6 3065 (vw) 2955-2850 (w) 1490 (vw) 1442 (m) 787 (m) 690 (m) 520 (vw)
5.7 3065 (vw) 2955-2850 (br. s) 1510 (vw) 1457 (s) 823 (s) 666 (vw) 527 (vw)
5.8a 3065 (vw) 2955-2850 (w) 1490 (vw) 1442 (m) 787 (m) 690 (m) 520 (vw)
5.8b 3065(w) 2955-2850 (br. s) 1510 (vw) 1457 (s) 823 (s) 666(m)
5.8c 3065(w) 2955-2850 (br. s) 1510 (vw) 1457 (s) 823 (s) 666(m)
4.9 3065 (vw) 2955-2850 (br. s) 1510 (vw) 1457 (s) 829 (s) 666 (vw) 527 (vw)
thiophene 3108 (vw) 702 (s)
3-hexylthiophene 3095 (vw) 2823-2714 (br. m) 660 (s)
Ar = Aromatic; a vw = very weak, w = weak, m = medium, s = strong, br. s = broad and strong
Chapter Five
102 | P a g e
5.3.3.3 UV-visible spectroscopy of the synthesized copolymers
The UV-visible absorbance spectra of C60 derivatives (5.2 and 5.3) and the 5.8a-c, 5.9
copolymers were recorded in THF (see Table 5.2 and Figure 5.4). A characteristic peak at 431
nm was observed for both C60 derivatives (5.2 and 5.3) and copolymers 5.8a-c (see inset Fig.
5.4). In the case of copolymer 5.9, however, the peak shifted into the blue region, a region
largely dominated by the absorption band from poly(3-hexylthiophene). This absorption peak is
proposed to be due to 6-6 fusion following monoaddition of the ylide to C60 [11c].
Table 5.2 UV visible spectra of poly(3-hexylthiophene) (5.5), 5.2, 5.3, copolymers 5.8a-c and
5.9
Sample ? max centre in nm
5.2 298, 432
5.3 298, 432
5.5 276, 426
5.8a 278, 432,
5.8b 276, 429
5.8c 276, 429
5.9 278, 432
The absorption peak at 276 nm is associated with poly(3-hexylthiophene) (5.5). A strong
absorbance peak at 426 nm, due to poly(3-hexylthiophene), corresponds to the ?-?* transition of
its conjugated segments [50]. In case of 5.8b and 5.8c, this absorption peak was found to be
shifted to the red region (429 nm), relative to the pure poly(3-hexylthiophene) (5.5).
Chapter Five
103 | P a g e
300 400 500 600 700 800
0
2
300 400 500 600 700 800
0.0
0.1
0.2
0.3
0.4
0.5
0.6
ab
so
rb
an
ce
(a
.u)
Wavelength (nm)
5.2
5.3
5.8a
5.9
431.8 nm
ab
so
rb
an
ce
(a
.u
)
Wavelength (nm)
5.5
5.8b
5.8c
5.2
5.3
5.8a
5.9
Figure 5.4 UV-visible absorption spectrum of poly(3-hexylthiophene) (5.5), compounds 5.2 and
5.3, and copolymers 5.8a, 5.8b, 5.8c and 5.9 in THF.
5.3.3.4 Thermogravimetric analysis (TGA)
The thermal stability of the synthesized polymers were examined by TGA [51] under air, by
heating to 600 oC at a 10 oC/min heating rate (see Figure 5.5). The summary of the
decomposition stages and mass losses are given in Table 5.3.
Copolymer 5.7 shows two decomposition reactions. The first decomposition occurs at 279 oC,
and the second one occurs at 425 oC after 80 % mass loss. In contrast, copolymer 5.6 had three
Chapter Five
104 | P a g e
decomposition reactions stages, while polythiophene (5.4) underwent only a single stage of
decomposition at 214 oC (Figure 5.5a.). The first decomposition temperature of copolymer 5.6
was much higher at 260 oC, followed by the second decomposition at 371 oC. The second stage
of the decomposition reactions were accompanied by a 37 % mass loss for the copolymer 5.6.
This multiple decomposition reaction for copolymer 5.6 could be the end cuped polymer de-
attached from the fullerene. The final decomposition reaction was at about 472 oC. From the
TGA results copolymers 5.6 and 5.7 were more thermally stable than polythiophene 5.4.
Moreover, copolymer 5.6 was found to be thermally less stable than copolymer 5.7.
100 200 300 400 500 600
0
20
40
60
80
100
5.4
5.7
5.6
m
as
s l
os
s %
Temperature (?C)
Figure 5.5a. TGA thermograms of copolymers 5.6, 5.7 and polythiophene (5.4)
The copolymers 5.8a, 5.8b, 5.8c and poly(3-hexylthiophene) 5.5 all underwent one stage of
decomposition at 300 oC (Figure 5.5b). During this stage about > 99 % mass losses were
recorded for copolymers 5.8a, 5.8b, 5.8c and poly(3-hexylthiophene) (5.5). From the TGA
results, incorporation of C60 into the polymers did not seem to improve the thermal properties of
Chapter Five
105 | P a g e
the copolymers unlike that observed for the unsubstituted polythiophene derivatives of
copolymers.
100 200 300 400 500 600 700
0
20
40
60
80
100
m
as
s l
os
s %
Temperature (?C)
5.8a
5.5
5.8b
5.8c
Figure 5.5b. TGA thermogram of copolymers 5.8a, 5.8b, 5.8c and poly(3-hexylthiophene) (5.5).
Chapter Five
106 | P a g e
Table 5.3 summary of thermal decomposition temperature vs mass losses of copolymers and
pure polymers.
5.4 Conclusion
A successful covalent functionalization of the C60 with thiophene was achieved by using a Prato
approach. The synthesis of the two C60 derivatives (5.2 and 5.3) were confirmed by mass
spectroscopic (m/z = 860 [M+1] with 2.6 %) and UV-visible techniques. Furthermore, covalent
attachment of a poly(3-hexylthiophene) backbone were accomplished by FeCl3 oxidative
polymerization. According to the FT-IR results copolymers 5.8a and 5.9 were found to be
regiorandom (due to the presence of the high concentration of C60), while the copolymers 5.8b
and 5.8c, and poly(3-hexylthiophene) (5.5) were more regioregular. From the TGA results
copolymers 5.6 and 5.7 are more thermally stable than polythiophene (5.4). Moreover, end-
capped, copolymer 5.6, was found to be less stable than the pearl copolymer 5.7. However, in
case of copolymers 5.8a, 5.8b, 5.8c, and 5.9, incorporation of C60 derivatives into the thiophenes
backbone did not improve the thermal stability of the polymer.
Sample
1st decomposition 2nd decomposition
Temp.
(oC)
Mass loss
(%)
Temp.
(oC)
Mass loss
(%)
5.4 ~214 >97
5.5 ~300 >99
5.6 279 80 425 >98
5.7 260 55 371 87
5.8a ~300 >99
5.8b ~300 >99
4.8c ~300 >99
Chapter Five
107 | P a g e
5.5 References
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16 See the following representative examples: (a) F. Effenberger, G. Grube, Synthesis, 1998,
1372. (b) Y. Obara, K. Takimiya, Y. Aso, T. Otsubo, Tetrahedron Lett., 2001, 42, 6877. (c)
S. Campidelli, R. Deschenaux, J.-F. Eckert, D. Guillon, J.-F. Nierengarten, Chem.
Commun., 2002, 656. (d) T. Gu, J.-F. Nierengarten, Tetrahedron Lett., 2001, 42, 3175. (e)
N. Armaroli, F. Barigelletti, P. Ceroni, J.-F. Eckert, J.-F. Nicoud, J.-F. Nierengarten, Chem.
Commun., 2000, 599. (f) T. Yamasshiro, Y. Aso, T. Otsubo, H. Tang, Y. Harima, K.
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109 | P a g e
Chem. Commun., 2000, 701. (h) J. J. Apperloo, C. Martineau, P. A. van Hal, J. Roncali, R.
A. J. Janssen, J. Phys. Chem. A, 2002, 106, 21. (i) J.-F. Nierengarten, J.-F. Eckert, J.-F.
Nicoud, L. Ouali, V. Krasnikov, G. Hadziioannou, Chem. Commun., 1999, 617. (j) J.-F.
Eckert, J.-F. Nicoud, J.-F. Nierengarten, S.-G. Liu, L. Echegoyen, F. Barigelletti, N.
Armaroli, L. Ouali, V. Krasnikov, G. Hadziioannou, J. Am. Chem. Soc., 2000, 122, 7467.
(k) E. Peeters, P. A. van Hal, J. Knol, C. J. Brabec, N. S. Sariciftci, J. C. Hummelen, R. A.
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Chapter 6
Functionalization of nitrogen doped and undoped carbon nanotubes
using ring opening metathesis polymerization with norbornene
6.1 Introduction
Since the discovery of CNTs in 1991 [1], the interest in their utilization has been steadily
increasing. The prospect of developing novel carbon-based nanomaterials from those materials
has been explored because of their unique structure-dependent electronic and mechanical
properties [2-4]. For example, carbon-based nanoscale diodes or transistors have become one of
the main topics in CNT-based nanoelectronics [5].
Furthermore, doping of heteroatoms into CNTs may lead to the formation of electron-excess n-
type (e.g., N-CNTs) or electron-deficient p-type (e.g., B-CNTs) semiconducting nanotubes [6].
Doping with different elements, such as nitrogen, is thus a promising method to tailor the
electronic properties of CNTs [7] as the additional electrons contributed by the nitrogen atoms
provide electron carriers for the conduction band [8]. N-doped nanotubes have been found to be
either metallic or narrow energy gap semiconductors [9,10], thus offering the possibility of
greater electrical conductivity as compared to pure carbon nanotubes.
Different kinds of precursors, including carbon and nitrogen containing molecules such as
C5H5N
[11], CH3CN [12d], and HOCN(CH3)2 [13f] or mixtures of carbon/nitrogen containing
gaseous species such as CH4/N2 [14a-c] and C2H2/NH3 [14c,e,f], have been used for the synthesis
of N-CNTs. In most cases the growth temperature of N-CNTs is in the range of 700-1100 ?C,
and the average nitrogen content is between 1-10 % [11,13].
The mechanical properties of CNTs are thought to have important potential for future
developments in the areas of science and technology. In studies [15,16] the Young?s modulus
and tensile strength were found to be up to 1 TPa and 60 GPa respectively. However, to access
these extraordinary properties, the polymer-nanotube interfacial stress transfer must be
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maximized in order to achieve the best possible reinforcement. Cadek et al. [17] showed a
significant increased in Young?s modulus by up to a factor of two to 1.4 TPa.
In nanotube-based nanocomposites, solvent casting and melt mixing are two of the most
common methods to make composites. Qian et al. [18] studied a MWNT/polystyrene
(PS)/toluene nanocomposite and the data revealed a product with enhanced elastic modulus and
break stress. Benoit et al. [19] have obtained electrically conductive nanocomposites by
dispersing SWNTs and poly(methyl methacrylate) (PMMA) in toluene, followed by drop casting
of the mixture on substrates. Generally nanocomposites with a range of thermoplastic matrices
have been fabricated by solvent casting [20], but nanotubes tend to agglomerate during solvent
evaporation, which leads to inhomogeneous nanotube distribution in the polymer matrix. To
solve this problem, Haggenmueller et al. [21] used combined methods of solvent casting and
melt mixing with sonication, to make SWNT/PMMA composites. With this procedure they
found a considerable improvement in nanotube dispersion.
In addition to the solvent casting, melt mixing, and coagulation methods, which combine
nanotubes with high molecular weight polymers, in situ polymerization methods have also been
used to make nanotube-based nanocomposites starting with nanotubes and monomers. The most
common in situ polymerization methods involve epoxy in which the resins (monomers) and
hardeners are combined with SWNTs or MWNTs prior to curing (polymerization) [22,23].
Many of the methods described above require nanotubes to be well dispersed in solvents. The
nanotube dispersion in the polymer matrix largely depends on the state of nanotube dispersion in
a solvent. However, the chemical structure of CNTs makes dissolving long CNTs in common
solvents to form true solutions, virtually impossible.
Large fractions of individual nanotubes have thus only been achieved, either by functionalizing
the nanotubes or by surrounding the nanotubes with dispersing agents, such as surfactants and
polymers. The improved nanotube suspensions resulting from functionalization or dispersing
agents can be employed in many of the methods described above to make nanotube
nanocomposites with improved nanotube dispersion. Furthermore, functionalized CNTs might
also allow covalent bonds to form between the nanotubes and the polymer matrix, thereby
influencing the nanotube-polymer interaction.
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Recently, chemically modified and solubilised carbon nanotubes have emerged as an area of
research on nanotube-based materials and successful covalent functionalisation of both single-
walled (SWNTs) and multi-walled (MWNTs) carbon nanotubes have been reported [24-28].
Covalent functionalisation could be possible thorough two main reactions: the first category
involves direct attachment of functional groups to the CNT side wall [28,29]. The second
category can arise from intrinsic or induced carbon defects. The latter generally refers to the
creation of functionalized terminal carbons by the acidification of CNTs during oxidation
[30,31]. Although it is widely accepted that chemical functionalisation disrupts the extended ?-
conjugation of nanotubes and thereby reduces the electrical conductivity of isolated nanotubes,
recent reports show that covalent functionalisation can indeed improve the electrical properties
of the composites [32-34].
Georgakilas et al. [35] have observed the functionalisation of CNT sidewalls using a 1,3-dipolar
cycloaddition of azomethine ylides, generated by the condensation of an amino acid and an
aldehyde (the Prato reaction) . The main advantage of this type of reaction is the easy attachment
of substituted pyrrolidine rings to the sidewalls of nanotubes. After this, subsequent reactions to
afford products with customized properties could be achieved. This method of reaction has
previously resulted in functionalized CNTs that form stable solutions in organic solvents [36], or
have been used in polymer composites [36b], medical application [36c], drug delivery [37], or
energy conservation [38] studies.
In this work, we have investigated a new approach to the functionalization of N-doped and non-
doped CNTs based on ring opening metathesis (ROMP) using Grubbs? catalyst, after sidewall
functionalization of both N-doped and non-doped CNTs. The thermal and electronic properties
of these new co-polymers were then investigated.
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6.2 Experimental
6.2.1 General procedures
The synthesised CNTs were characterised by elemental analysis, thermogravimetric analysis
(TGA) transmission electron microscopy (TEM), as well as Raman, NMR and ATR-FTIR
spectroscopy. Elemental analysis was carried out by the Institute for Soil, Climate and Water,
Pretoria, South Africa. Infrared-spectra were recorded on an ATR-FTIR Bruker Tensor 27.
Absorption values are reported between 4000-550 cm-1. The NMR spectra (chemical shift data
in ppm) were recorded in CDCl3 at ambient probe temperature using a Bruker Avance 300 (
1H,
300.13 MHz) spectrometer. DSC data were obtained on a Mettler-Toledo DSC822e calorimeter;
each sample was recorded in the temperature range 100-250 oC under a N2 atmosphere at a rate
of 10oC/min Thermogravimetric analysis (TGA) was performed using a Perkin-Elmer Pyris 1 TG
analyzer at a heating rate 10 oC /min under either air or nitrogen atmospheres. TEM images were
recorded on a Jeol JEM 100s electron microscope operating at 80 keV. Samples for analysis
were prepared by spreading them on a holey carbon film supported on a copper grid. Raman
spectra were obtained with a Jobin-Yvon T64000 Raman spectrometer operated in single
spectrograph mode with either a 600 lines/mm grating or an 1800 lines/mm grating. The 514.5
nm line of an argon ion laser was used as the excitation source. Laser power at the sample was
kept at 1.2 mW or less to minimize local heating. Laser plasma lines were removed from the
incident beam using a narrow band pass interference filter and a Kaiser Optics holographic notch
filter was used to remove the Rayleigh scattered light from the backscattered beam. Spectra
accumulation time was 120 seconds and was collected using a liquid nitrogen-cooled CCD
detector. Data was ued to measure the intensity of the D and G bands of the CNTs.
6.2.2 Synthesis of carbon nanotubes
6.2.2.1 Catalyst preparation for CNT synthesis [39]
A catalyst was prepared using a mixture of Fe(NO3)3?9H2O and Co(NO3)2?6H2O, both purchased
from Sigma Aldrich, as sources of Fe and Co respectively. A calculated amount of the Fe and Co
salts were mixed, ground to a fine powder and dissolved in distilled water to make a 50:50 Fe-Co
precursor solution (0.3 M). This solution (28 mL) was then added to CaCO3 (10 g) and the
suspension was left to age for 30 min while stirring. The mixture was then filtered and allowed to
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dry at r.t., after which time it was further dried in an air oven at 120 ?C for 12 h. The resulting
course powder was then cooled to r.t., ground and finally screened through a 150 ?m sieve. The
fine catalyst powder was then calcined at 400 ?C for 16 h.
6.2.2.2 Carbon nanotube synthesis [39]
MWNTs were synthesized by the decomposition of acetylene (C2H2), (Afrox) in a tubular quartz
reactor (51 cm ? 1.9 cm i.d.) that was placed horizontally in a furnace. The furnace was
electronically controlled such that the heating rate, reaction temperature and gas flow rates could
be accurately maintained as desired. The catalyst (0.2 g) was loaded into a quartz boat (120 mm
? 15 mm) at r.t. and the boat was placed in the centre of the quartz tube. The furnace was then
heated at 10 ?C/min while N2 was passed over the catalyst at 40 mL/min Once the temperature
had reached 700 ?C, the N2 flow rate was set to 240 mL/min and C2H2 was introduced at a
constant flow rate of 90 mL/min After 60 min of reaction time, the C2H2 flow was stopped and
the furnace was left to cool to r.t. under a continuous flow of N2 (40 mL/min). The boat was then
removed from the reactor and the product (carbon deposit) that formed along with the catalyst
was weighed.
6.2.2.3 Nitrogen doped multiwall carbon nanotubes
Synthesis of N-CNTs was carried out at a temperature at 800 ?C, under 5 % H2 in Ar (v/v)
(AFROX) at atmospheric pressure. The flow rate of H2 in Ar was kept constant at 200 mL/min A
mixture of pyridine (2 g, 20 %) and ferrocene (1 g, 10 %) was dissolved in toluene (7 g, 70 %)
(Merck chemicals) and the solution was placed in a 10 mL syringe driven by a SAGE syringe
pump. The solution was injected at ~0.80 mL/min rate into a quartz tube reactor (800 ? 28 mm
i.d.) via a specially designed quartz reactor delivery tube, cooled by water. This designed tube
enabled the solution to be injected directly into the high temperature region of the large quartz
tube reactor. The carbon deposit that formed was scraped from the walls of the quartz tube and
weighed.
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6.2.3 Purification of carbon nanotubes
The synthesized nitrogen doped and undoped carbon nanotubes were purified by adding the
carbon nanotubes to hydrochloric acid (35 %) which was heated at 80 oC for 24 h. After filtering
and washing several times with distilled water and acetone (3 x 500 mL), the purified carbon
nanotubes were finally heated in an oven under an inert atmosphere at 400 oC for 30 min
6.2.4 Functionalisation and polymerization of carbon nanotubes
6.2.4.1 Functionalisation of carbon nanotubes
In each of two round bottomed flasks an equal amount of N-doped and undoped carbon
nanotubes (each 0.100 g) were suspended in 1,2-dichlorobenzene (50 mL) and sonicated for 30
min 5-Norbornene-2-carboxaldehyde (Sigma-Aldrchi) (0.12 g, 0.98 mmol) was then added to
each CNT suspension. N-methylglycine (0.158 g, 1.9 mmol) was then added to each reaction
mixture in small portions over 5 days (about 0.032 g per day) at 160 oC. The reaction mixtures
were then left to cool to r.t. and the solid material filtered using 1 ?m pore size membrane filter
papers. The products were then washed with DMF (100 mL), ethanol (300 mL), 1:1 distilled
water: acetone mixture (3 x 500 mL) and several times with acetone (500 mL). The black solid
products were then dried at 45 oC under vacuum for 48 h to yield 0.157 g and 0.148 g of dark
gray functionalized N-doped and undoped carbon nanotubes, respectively.
6.2.4.2 Ring opening metathesis and functionalization of CNTs with norbornene
In two round bottom flasks an equal amount of both functionalized N-doped and undoped CNTs
(each 50 mg) were suspended in dried chloroform (25 mL) and sonicated for 2 h. Grubbs? second
generation catalyst (30 mg, 40 ?mol) was then added to the two suspensions and sonication was
carried on for a further hour. Norbornene (bicyclo[2.2.1]hept-2-ene) (1.00 g, 10.4 mmol) was
dissolved in dry chloroform (25 mL) and then added dropwise over 60 min to each reaction
mixture. The reaction mixtures were then left to stir overnight at r.t. The polymerization
reactions were then terminated by adding a few drops of ethylvinylether (4-5 drops). The
mixtures were then poured into an excess of MeOH (50 mL), containing a few drops of 1 M HCl,
to precipitate the crude polymers. The synthesized polymers were filtered using a filter paper and
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dried under vacuum for 48 h to afford gray coloured polymers 0.977 g (93 % yield) and 0.875 g
(82 % yield) for the N-doped and undoped CNTs, respectively.
6.3 Results and discussion
6.3.1 Synthesis of CNTs
As described in the experimental section the undoped CNTs were synthesized by the
decomposition of acetylene at 700 oC for 1 h. No pre-reduction of the catalyst was necessary
since the catalyst was reduced in-situ by the H2 released from the decomposition of the feed
stock gas (C2H2). Fe(NO3)3.9H2O and Co(NO3)2.6H2O were used as sources of Fe and Co
respectively to give a Fe/Co on CaCO3 catalyst that has been shown to efficiently give high
yields of CNTs [39].
On the other hand, the N-CNTs were synthesized by the flotation catalyst methodology [40],
using ferrocene as a catalyst and toluene as the carbon source. Pyridine was a source of both
nitrogen and carbon for the formation of the N-CNTs [11]. The reaction temperature was carried
out at 800 ?C in a 5 % H2 in Ar gas mixture and the synthesized N-CNTs were finally purified
using concentrated hydrochloric acid under reflux for 24 h. After washing with distilled water
and drying under vacuum, TEM images revealed that the synthesized N-CNTs contained were
multiwalled (see section Figure 5.3.4.2).
6.3.2 Functionalization reactions of CNTs
Functionalization of carbon nanotubes using azomethine ylides was first reported by Georgakilas
et al. [35] The azomethine ylides were formed thermally in situ by the condensation of an R-
amino acid and an aldehyde. The reagent was successfully added to the graphitic surface via a
1,3-dipolar cycloaddition reaction, forming pyrrolidine fused rings on the side wall of the CNTs.
The 1,3-dipolar cycloaddition reaction of azomethine ylides with alkene or alkyne is a very
effective method for the construction of pyrrolidine- and pyrrole-rings in the synthesis of
pyrrolidine- and pyrrole-containing molecules. This procedure initially was developed for
organic modification of C60 fullerenes [41] and has been termed the ?Prato reaction?.
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In this study, the functionalisation of N-doped and undoped CNTs, by 5-norbornene-2-
carboxaldehyde and N-methylglycine by way of the Prato reaction, was investigated. The
norbornene-functionalized CNTs were obtained as a gray dark product for both the N-doped and
undoped CNTs (see Scheme 6.1). The increase in mass between 32-36 % after the reaction
indicated that the side walls of the CNTs had organic functional groups attached to the CNTs.
Furthermore, the TGA scan after functionalization indicated that the decomposition temperatures
were lower than the pristine CNTs (see Figure 6.7a).
1,2 Dichlorobebzene at 160 oC
Ru(II) catalyst
dry chloroform
=
n n
n
where 6.1 = CNT
6.2 = functionalized CNT
6.3 = N-CNT
6.4 = functionalized N-CNT
6.5 = undoped CNT- polynerbornene
6.6 = N-CNT-polynerbornene
6.1, 6.3 6.2, 6.4 6.5, 6.6
HN
O
OH
+
6.a 6.b
6.c
N NCHO
Scheme 6.1 Functionalization and polymerization methodology for the carbon nanotubes.
6.3.3 Polymerization reactions of functionalized CNTs
Ring-opening metathesis polymerization (ROMP) using ruthenium alkylidene catalysts has
attracted significant attention due to its versatility, effectiveness, and functional group tolerance,
allowing for a variety of monomers bearing polar and charged functional groups to be
successfully polymerized [42]. It has recently been shown that extremely narrow polydispersities
and excellent control over polymer molecular weight and architecture can be achieved if
appropriate ligands are utilized, where high ligand dissociation rates result in rapid initiation of
polymerization [43,44]. Recently, Gomez et al. [45] have reported a successful ROMP from the
surface of SWNTs, where the ruthenium alkylidene catalysts were physiosorbed on the nanotube
surface using pyrene molecules as anchors. Similarly, Liu and Adronov [46] showed that
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catalyst-functionalized nanotubes initiated effectively the ROMP of norbornene, resulting in
rapid polymerization from the catalyst sites on the nanotube.
During the polymerization reactions in our work the Grubbs? second generation catalyst was
used as a polymerization catalyst in chloroform under argon. The monomer, bicyclo[2.2.1]hept-
2-ene, was added dropwise to the mixture of the functionalized CNTs and Grubbs? second
generation catalyst at r.t. The reaction gave a dark gray spongy solid in good yield, which was
recovered by precipitation in methanol and then purified by dissolving in chloroform and re-
precipitation from methanol, followed by filtration. The polymers were found to be soluble in
dichloromethane and chloroform. Their characterization will be described in the next sections.
6.3.4 Characterization of the synthesised products
6.3.4.1 Raman Spectroscopy of the synthesised products
Raman spectroscopy is a very useful tool to characterize CNTs [47]. Raman spectra were
obtained with a Jobin-Yvon T64000 Raman spectrometer operated at 514.5 nm line of an argon
ion laser excitation source. The Raman spectra of N-doped and undoped CNTs are shown in
Figure 6.1. The spectrum of N-doped unfunctionalized CNTs 6.1 (Figure 6.1 spectrum A)
showed two bands. One band occurred at 1352 cm-1 (D band) which is attributed to disorder, or
sp3-hybridized carbons, in the hexagonal framework of the nanotube walls. The second band at
1585 cm-1(G band) is associated with tangential modes. The spectrum of the functionalized N-
doped material 6.2 gave the conventional bands at 1363 cm-1 and 1583 cm-1, respectively (Figure
6.1 spectrum B). After functionalization, the G band slightly shifts up field by ?2 cm?1, as
compared with that of N-doped unfunctionalized CNTs 6.1. This shift should be associated with
organic functional group covalently attached to the nanotube surface [48]. The spectrum of
undoped unfunctionalized CNTs 6.3 gave two bands at 1351 cm-1 and at 1583 cm-1 (Figure 6.1
spectrum C), however, the spectrum of the functionalized undoped 6.4 sample was dominated by
fluorescence, i.e. no D and G bands were observable (Figure 6.1 spectrum D).
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Figure 6.1 Raman spectra of A) unfunctionalized N-CNTs 6.3; B) functionalized N-CNTs 6.4;
C) unfunctionalized CNTs 5.1; D) functionalized CNTs 6.2.
A comparison of the Raman spectra of the unfunctionalized CNTs (Figure 6.1 spectrum A vs C)
reveals that the ID/IG value (0.78 for 6.1 and 0.75 for 6.3) increased due to the doping with
nitrogen. The ID/IG ratio appears to be even greater for the N doped material after
functionalization, 0.88 for 6.4 and 0.78 for 6.1 (see Figure 6.1). These values are distinctly
larger than 0.78 for 6.1 and 0.75 for 6.3, indicating the introduction of covalently bound moieties
to the nanotube framework wherein significant amounts of the sp2 carbons have been converted
to sp3 hybridization. Thus, both functionalized carbon nanotubes samples display similar
modifications, but to different degrees.
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6.3.4.2 Transmission electron microscopy (TEM)
Samples for transmission electron microscopy (TEM) analysis were prepared by sonication of
the copolymeric materials in methanol, followed by placing a few drops of the resulting
suspension onto a holey copper TEM grid for analysis. The results from such analyses offer the
most direct evidence for the presence of carbon nanotubes in the samples.
Figure 6.2 TEM image of A) undoped CNT 6.1; B) N-CNTs 6.3
The undoped CNTs 6.1, with a diameter range of 25-30 nm, were clearly observed from the
TEM analysis (Figure 6.2 image A). The outer diameter of the nanotubes ranged from 20-25
nm with the inner diameter ranging from 3-5 nm. Similarly, for the N-CNTs 6.3, the diameters
were measured to be in a range of 32-40 nm, with an overall wall thickness of 8 nm.
The CNTs grown from the mixture of pyridine and ferrocene in toluene, had a bamboo structure
(Figure 6.2 image B), and this bamboo structure is known to be a characteristic of nitrogen-
doped MWNTs [49]. The bamboo-like structure and distinctive compartment layers are probably
due to the incorporation of nitrogen atoms such as pyridinic nitrogen or graphitic nitrogen [50-
54] into the growing structure. Finally, elemental analysis indicated that indeed, nitrogen atoms
had been doped into the N-CNTs with the nitrogen content being 3.4 %.
After the polymerizations reaction, the functionalized carbon nanotubes (both undoped 6.5 and
N-doped 6.6) were found to have much larger diameters (65-130 nm) than the pristine N-CNTs
Chapter Six
123 | P a g e
(6.3) (32-40 nm) (see Figure 6.3). This is due to the polymer that formed on the surface of the
CNTs, i.e. the polynorbornene (see Scheme 6.3). This polymerization was further substantiated
by 1H NMR, FT-IR and TGA studies (see Figure 6.5, 6.6b, 6.7b).
Figure 6.3 TEM images of A) and B) polymer attached CNTs 6.5; C) and D) polymer attached
N-CNTs 6.6.
In Figure 6.3 TEM images of A, B and C show polymer formation on the surface of the
nanotubes. The underlying tube can readily be seen in Figure 6.3, image C in the circled region.
In some instances some carbon nanotubes were not visibly covered by the polymers (see the
arrow in Figure 6.3, image C). This could be due to the N-CNTs 6.4 only being partially
functionalized or inhomogeneous in the polymerization reaction. The polymerization is
inhomogeneous, but in general polymer formation gives tube covered by polymer (Figure 6.3
image B). This inhomogeneous is readily seen in Figure 6.3 image A, where polymer ?blobs? are
seen attached to the polymer on the tubes. This inhomogeneity is independent of doping (Figure
6.3, image C). The TEM in Figure 6.3, image D shows two functionalized N-CNTs that were
Chapter Six
124 | P a g e
zipped together during ROMP. This indicated that if two tubes are in closed proximity then a
covalently linkage between the tubes is possible. The implication is that if tubes could be aligned
after functionalization then a composite with high strength could be synthesised. In summary, the
data revealed that while polymer coverage of both doped and undoped CNTs is possible,
however the growth of the polymer still need to be optimized to ensure both total coverage of all
tubes as well as constant growth on the tubes.
6.3.4.3 Spectroscopic studies on the copolymers synthesized
6.3.4.3.1 1H NMR studies of synthesized copolymers
The lH NMR spectrum of the polymer-attached CNTs are shown in Figure 6.5. Both cis and
trans resonances for the olefinic and the allylic protons [55] were observed in all the polymers
obtained (see Figure 6.4 below). The cis and trans olefinic protons of the pure polynorbornene
resonate at 5.21 and 5.35 ppm, respectively. The cis:trans ratio for pure polynorbornene (Figure
6.5) was calculated by integration and found to be 59:41.
.
HaHa
Ha
Hc
Figure 5.4 Schematic diagram of the cis and trans olefinic and allylic protons.
The cis/trans proton resonances for the polynorbornene formed in the presence both 6.5 and 6.6
were broader than those found for polynorbornene (See Figure 6.5). The broadening of the
signals is caused by the NMR nuclei spin-lattice (T1) and spin-spin (T2) relaxation time; they are
associated with diamagnetic species of low mobility [56].
Chapter Six
125 | P a g e
8 6 4 2 0
cis
trans
TMS
CDCl
3
6.5
6.6
polynorbornene
norbornene
chemical shift (ppm)
Figure 6.5 1H NMR spectra (in CDCl3) of the monomer norbornene; polynorbornene; (6.5)
copolymer from undoped CNT-polynorbornene and (6.6) copolymer from N-doped-
polynorbornene copolymers.
In the case of the CNT-attached polymers, 6.5 and 6.6, the ratios of cis and trans proton were
estimated to be 52:48 and 44:56, respectively. From the spectra it is seen that both N-doped and
undoped CNTs attached to the polynorbornene favored the trans isomer relative to the pure
Chapter Six
126 | P a g e
polynorbornene. Further the N-doped CNT?s gives a reversal of the isomers amount. While this
might be an artifact of the broadened signals, it does suggest potential content of isomers ratio by
controlling doping or functionalization of CNTs.
Table 6.1 Summary of cis and trans ratios of the polymer and functionalized copolymers.
Polymer cis trans
Polynorborene 59 41
6.5 (CNTs) 52 48
6.6 (N-CNTs) 44 56
6.3.4.3.2 FT-IR studies of synthesized copolymers
FT-IR spectroscopy was used to further characterize the synthesized polymers. As shown in
figure 6.6a new absorption bands were recorded for both functionalized carbon nanotubes (6.2
and 6.4), relative to the pristine CNTs. Absorption bands between 3074-2800 cm-1 were assigned
to the C-H stretching bands. These arise from the norbornene and methylglycine functional
groups in the Prato reaction precursors. The presence of an absorption band at 1050 cm-1 is
indicative of the N-C stretching vibration [57] that is associated mainly with Prato products.
Those were not observed in the unfunctionalized CNTs.
Chapter Six
127 | P a g e
4000 3500 3000 2500 2000 1500 1000
1050 cm
-1
6.4
6.3
6.2
6.1
Tr
an
sm
itt
an
ce
(a
.u
)
Wave number (cm
-1
)
Figure 6.6a FT-IR of Pristine undoped CNTs 6.1; functionalized undoped CNTs 6.2; pristine N-
CNTs 6.3; functionalized N-CNTs 6.4.
After the ROMP, new absorption bands at 967 and 745 cm-1 were seen and these were associated
with the norbornene-based polymer chain (Figure 6.6b). In addition, the absorption bands
between 3074-2800 cm-1 were more intense due to the increased contribution from the
polynorbornene CH and CH2 groups. In summary, the IR
Chapter Six
128 | P a g e
4000 3500 3000 2500 2000 1500 1000
745cm
-1
967 cm
-1
norbornene
polynorbornene
6.6
6.5
Tr
an
sm
itt
an
ce
(a
.u
)
Wave number (cm
-1
)
Figure 6.6b FT-IR of polynorbornene-attached CNT 6.5; polynorbornene-attached N-CNT 6.6;
polynorbornene and norbornene monomer.
Absorption bands at 745 cm-1 and 967 cm-1 were assigned to the cis and trans double bonds of
the polymer, respectively; however, in the vinylcyclopentane units resulting from ROMP the cis
and trans positions have been reported at lower wavenumbers 730 cm-1 and 960 cm -1 for cis and
trans respectively [58].
Chapter Six
129 | P a g e
6.3.5 Thermal analysis
6.3.5.1 Thermogravimetric Analysis (TGA) of the synthesized materials
TGA measurements on the materials synthesised in this chapter were also performed under air
(see Figures 6.7a and b). Both pristine N-doped 6.2 and undoped CNTs 6.1 showed a typical
TGA profiles that indicated the graphitic nature of the CNTs; they decomposed at 519 and 508
oC, respectively. Functionalized N-CNTs 6.4 showed two stages of thermal decomposition at
~307 and 510 oC. Similarly, the functionalized CNTs 6.3 showed two stages of decomposition
recorded at 241 oC and 403 oC (see Figure 6.7a). During the first decomposition reactions 3 and
45 % mass losses were recorded for 6.3 and 6.4, respectively. The first decomposition stages
might be due to the de-attachment of organic functional groups from the side wall of CNTs and
the second decomposition was due to the oxidation of the CNTs. These results would indicate
that the N-CNTs were have highly functionalized than the non-doped CNTs.
200 400 600
0
20
40
60
80
100
3
4
2
1
M
as
s
lo
ss
%
Temperature (
o
C)
Figure 6.7a TGA scan of 1) pristine CNT 6.1; 2) pristine N-CNT 6.2; 3) functionalized CNT 6.3;
4) functionalized N-CNT 6.4.
Chapter Six
130 | P a g e
The TGA profile of the N-CNT containing polymer 6.6 was very similar to that of pure
polynorbornene. Polynorbornene had only one decomposition temperature at 398 oC. The
differentiate curve for 6.6 (not shown) revealed two decomposition reactions; at 387 oC (92 %
mass loss) and at 527 oC (see Figure 6.7a).
100 200 300 400 500 600 700
0
20
40
60
80
100
Polynorbornene
6.5
6.6
M
as
s
lo
ss
%
Temperature (
o
C)
Figure 6.7b TGA scan of polynorbornene-attached CNT 6.5; polynorbornene-attached N-CNT
6.6; and polynorbornene.
For 6.5 there was only one very visible decomposition temperature at 214 oC. From this result it
appears that the copolymer decomposed over a broader range. The incorporation of the CNTs
into polynorbornene did therefore not affect the thermal stability of the N-CNT/polymer contain.
Chapter Six
131 | P a g e
6.3.5.2 Differential scanning calorimetry studies (DSC) of synthesized copolymers
The thermal stabilities of the synthesised polymers were also examined. DSC profiles of the
samples were thus recorded under a nitrogen atmosphere using a 5 oC/min heating rate (Figure
6.8). The Tg (glass transition) temperatures were found to be significantly affected by the
incorporation of the carbon nanotubes. The two polymers attached to CNTs, 6.5 and 6.6, were
found to have approximately the same Tg at ca. 46 oC, while for pure polynorbornene polymer
the Tg was recorded at 52 oC (see inserted graph in Figure 6.8).
40 60 80 100 120 140
0
5
10
15
20
40 60
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
5
6
Polynorbornene
m
W
en
do
Temperature
o
C
6.5
6.6
Polynorbornene
m
W
en
do
Temperature
o
C
Figure 6.8 DSC scan of polynorbornene-attached CNT 6.5; polynorbornene-attached N-CNT
6.6; and polynorbornene run under N2 at 5
oC/min heating rate.
Gao et al. [59] showed that the Tg of composites gives information on the mobility of molecular
chain segments. In their report they furthermore indicated that the lower the Tg, the easier the
motion of chain segments and that the Tg continued to decrease as the concentration of CNT in
the polymer increased.
Chapter Six
132 | P a g e
6.4 Conclusion
Functionalization of carbon nanotubes was achieved by using azomethine ylides that were
generated in situ by the decarboxylation of immonium salts derived from thermal condensation
of N-methylglycine and 5-norbornene-2-carboxaldehyde (the Prato reaction). ROMP on the side
walls of the carbon nanotubes 6.2 and 6.4 using bicyclo[2.2.1]hept-2-ene as a co-monomer, was
achieved using Grubbs? second generation catalyst. From the 1H NMR evidence that trans
resonances was favored for the olefinic and the allylic protons when CNTs incorporated in the
copolymer. The TEM images revealed that the polymer-attached carbon nanotubes (both
undoped and N-doped CNTs, 6.5 and 6.6 respectively) were found to have relatively larger
diameters than the pristine CNTs; this is due to the polymers were attached on the side wall of
the carbon nanotubes. It is also revealed that two functionalized N-CNTs were zipped together
during ROMP. This indicated that if two tubes are in closed proximity then a covalently linkage
between the tubes is possible. The implication is that if tubes could be aligned after
functionalization then a composite with high strength could be synthesised. Generally, the
incorporation of the CNT into polynorbornene did not improve the Tg and thermal stability.
Chapter Six
133 | P a g e
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Saegusa, T. Tsujino, J. Furukawa, Makromol. Chem., 1964, 78, 231.
59 Y. Gao, Y. Wang, J. Shi, H. Bai, B. Song, Polymer Testing, 2008, 27, 179.
Chapter Seven
138 | P a g e
Chapter 7
Synthesis of polythiophene covalently liked to nitrogen doped and
undoped carbon nanotubes
7.1 Introduction
The discovery of carbon nanotubes (CNTs) has attracted the attention of a number of research
groups with the prospect of developing novel carbon-based nanomaterials due to their unique
structure-dependent electronic and mechanical properties [1-4]. Carbon nanotubes (CNTs) are
believed to a potential alternative to silicon for development of carbon-based nanoscale diodes or
transistors, and has therefore become one of the main topics of research for CNT-based
nanoelectronics [5]. CNTs can readily accept electrons, which can then be transported under
nearly ideal conditions along the tubular axis. The fact that CNTs appear in structurally defined
semiconductive or conductive forms turns the CNTs into ideal components for various electronic
applications [6]. On account of the large number of concentric cylindrical graphitic tubes present
in MWNTs, they are considered even more suitable in electron-donor?acceptor ensembles than
SWNTs [7]. CNTs can be doped with heteroatoms, such as nitrogen, which is a promising
method to tailor the electronic property of CNTs [8]. Doping with nitrogen and boron leads to
the formation of electron-excess n-type and electron-deficient p-type semiconducting nanotubes,
respectively [5,6 ,9,10]. The application of CNTs in devices has become a significant challenge
since CNTs tend to be insoluble in most common solvents. However, dispersion of CNTs into
polymers has been improved by either mixing the materials in a conical twin-screw extruder
[11], using surfactants as processing aids [12], by appropriate functionalization [13] or by in situ
polymerization [14]. Composites based on polymers and nanotubes have the potential to make an
impact on a variety of applications ranging from general low-cost circuits and displays to power
devices, micro electromechanical systems, super capacitors, solar call sensors, and displays
[15,16].
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139 | P a g e
Successful covalent functionalisation of both SWNTs and MWNTs has been reported by many
researchers [17]. For example, Georgakilas et al. [18] have demonstrated the functionalisation of
the sidewalls of SWNTs using a 1,3-dipolar cycloaddition of azomethine ylides, generated by
condensation of an amino acid with an aldehyde (i.e. the Prato reaction).
The combination of CNTs with electron donors has also signified an innovative concept to
harvest solar energy and to convert it into electricity, and like C60, CNTs have been introduced
into conjugated polymers to produce organic photovoltaic devices [19]. Electrical and
photoelectrical properties of CNT/conjugated polymer composites and interfaces have been
investigated since the mid 1990s [20-25]. For example, bulk heterojunction solar cells based on
conjugated polymers blended with MWNT [26] and SWNTs [27-29] have been reported. In
addition, Kumakis et al. [28] have reported a high value of 0.75 V for Voc by using 1 %
SWNT/poly-3-octyl-thiophene (P3OT) in bulk heterojunction solar cells. In addition, they have
claimed that this achievement was due to the HOMO?LUMO band gaps that could be associated
with the P3OT and SWNT interactions. Improvement of the light absorption has also been
achieved by using dye (naphthalocyanine, NaPc) coated CNTs blended with P3OT in a bulk
heterojunction configuration [30]. For the same purpose, Jin and Dai suggested a cell that,
instead of using CNTs randomly mixing with polymers, could contain a network of vertically
aligned CNTs separated by vertical polymer layers [31].
Pradhan et al. [32] have demonstrated that physically blended MWNTs with polyhexythiophene
(P3HT) provides extra dissociation sites and assists hole transportation in a P3HT-MWNT/C60
double layer device. However, power conversion efficiency under white light illumination was
low. Similarly, MWNT sheets have been used as the hole collecting electrode in polymer solar
cells with P3HT as the donor material and PCBM as the acceptor material [33]. Recently, Reyes
et al. [34] have studied and compared nitrogen doped to undoped carbon nanotubes in a
photovoltaic cell that was fabricated using a P3OT polymer blended with undoped CNTs and N-
doped CNTs. Their results indicated that N-doped CNTs enhanced the efficiency of the P3OT
solar cells in comparison with the undoped CNTs.
In this work, we report a new approach for the functionalization of N-doped and undoped CNTs
using a 1,3-dipolar cycloaddition of azomethine ylides and a subsequent in situ polymerization
Chapter Seven
140 | P a g e
with thiophene. The resulting polymers were characterized and their thermal and electronic
properties studied and the results described in this Chapter.
7.2 Experimental
7.2.1 General procedures
The general procedures used in this Chapter were the same as those used in Chapter 5.
7.2.2 Synthesis and purification of carbon nanotubes
The procedures used for the synthesis of both N-doped and undoped carbon nanotubes were the
same as those used in Chapter 6. Similarly, the purification methods used in this Chapter were
also the same as those stated in Chapter 6.
7.2.3 Functionalization of 7.1 and 7.3 [18]
The functionalization methodology used to prepare 7.2 and 7.4 was the same as that used in
Chapter 6, Section 6.2.3.1. However, 2-thiophenecarboxaldehyde was used instead of 5-
norbornene-2-carboxaldehyde in the Prato reaction. Purified dark brown solids of functionalized
CNTs 7.2 and 7.4 were obtained in amounts of 0.131 g and 0.150 g, respectively.
7.2.4 Polymerization reactions
7.2.4.1 Preparation of copolymers 7.5 and 7.6
The same procedure was followed as described in Chapter 5, Section, 5.2.3.3. In two separate
round bottomed flasks functionalized N-doped and undoped CNTs (each 0.050 g) were
suspended in pre-dried chloroform (~25 mL). The reaction mixtures were subsequently sonicated
for 2 h. FeCl3 (~5.2 g, 32 mmol) was then added to each CNT suspension and then the mixtures
were left to sonicate for a further 1 h. 3-Hexylthiophene (1.00 g, 5.9 mmol) in chloroform (~25
mL) was then added dropwise over 60 min, under constant stirring, to each reaction mixture. The
reactions mixtures were then left to stir overnight. The polymerization reactions were terminated
by pouring each of the reaction mixtures into an excess of MeOH (~50 mL) to precipitate the
crude polymers. The synthesised polymers were then obtained by filtration using membrane
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141 | P a g e
filter papers (1 ?m). Both the products were washed with ethanol (~100 mL), with a 1:1 distilled
water and acetone mixture (500 mL) and finally twice with acetone (500 mL). The resultant
brown solid products were then dried under vacuum for 48 h to give dark brown spongy solids of
7.5 (0.977 g, 93 % yield) and 7.6 (0.917 g, 87 % yield), respectively.
7.2.4.2 Preparation of copolymers 7.7 and 7.8
The same procedure was followed as above to synthesis copolymers 7.7 and 7.8. In this case
unsubstituted thiophene (1.00 g, 12 mmol) and either functionalized N-doped (7.4) or
functionalized undoped CNTs (7.2) (each 0.050 mg) were used. After the reactions were
complete the products were purified and dried as before to give brown solid products of
copolymer 7.7 (0.658 g, 86 % yield) and 7.8 (0.470 g, 58 % yield), respectively.
7.2.4.3 Preparation of 7.11
The same procedure was adopted as above (Section 7.2.4.4) to polymerize the composite. In this
case unfunctionalized N-CNTs (0.050 g) and thiophene (1.00 g, 12 mmol) were used to make the
polymer composite. At the end of the reaction purifying and drying gave (0.500 g, 48 % yield) a
brown solid product.
7.2.4.4 Preparation of 7.9 and 7.10 [35]
The polymerization reaction was done in the same manner as that reported in Chapter 5, Section
5.2.3.1. At the end of the reaction the products were purified and dried to give brick red solid
polythiophene (0.658 g, 86 % yield) and polymer 7.9 as a dark brown spongy solid (0.855 g, 86
% yield), respectively.
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142 | P a g e
7.3 Results and discussion
7.3.1 Functionalization reactions of CNTs
The functionalization of the N-doped and undoped CNTs was achieved when 2-
thiophenecarboxaldehyde 7.a and N-methylglycine 7.b were used as the precursors for the Prato
reaction. The functionalized CNTs were obtained as a dark gray product for both the N-doped
and undoped CNTs. The increase in mass, by 50 mg and 31 mg for N-doped and undoped CNTs
respectively, of the product indicated that the side walls of the CNTs contained organic
functional groups attached to the CNTs (see Scheme 7.1).
1,2 Dichlorobebzene at 180 oC
S R
FeCl3
dry chloroform
S
R
S
R
**
=
S
n
S
n
n
,
where 7.1 = CNT
7.2 = functionalized CNT
7.3 = N-CNT
7.4 = functionalized N-CNT
7.5 = undoped CNT- polyhexylthiophene
7.6 = N-CNT-polyhexylthiophene
7.7 = N-CNT-polythiophene
7.8 = undoped CNT- polythiophene
7.9 = polyhexylthiophene
10 = polythiophene
11 = CNT& polythiophene mixture
6.1, 6.3 7.2,7.4
R = H
R = hexyl
7.5, 7.6, 7.7 and 7.8
HN
O
OH
S CHO
+
7.a 7.b
7.c/d
N N
S
R n
when R= hexyl; 7.9
R= H; 10
FeCl3
7.c/7.d
*
*
S
FeCl3
dry chloroform
,
6.d
SS
S
R
R
R
6.3 11
Scheme 7.1 Functionalization and polymerization methodology used to make polymer carbon
nanotubes/polymer composite.
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143 | P a g e
7.3.2 Polymerization reactions
In this study, FeCl3 was used as a polymerization initiator to polymerize thiophene in chloroform
under argon. The monomers, either thiophene 7.c or 3-hexylthiophene 7.d, were added dropwise
to the FeCl3 mixture at r.t. [36-39]. The resulting brick red and dark brown spongy solids were
recovered by filtration in good yields from both monomers respectively. Figure 7.2 shows the
synthesized polymers dissolved in THF.
Figure 7.1 A) Poly(3-hexylthiophene) 7.9; B) undoped CNT-poly(3-hexylthiophene) 7.5; C) N-
doped CNT-poly(3-hexylthiophene) 7.6.
7.3.3 Characterization of functionalized CNTs and copolymers
7.3.3.1 Raman Spectroscopy of functionalized CNTs
Raman spectroscopy has been shown to be a useful technique to characterize the crystallinity of
CNTs [40]. The Raman spectrum of undoped 6.1 (Figure 6.3 spectrum A) and N-doped CNTs
6.3 (Figure 6.3 spectrum C) are shown. The spectrum of N-doped unfunctionalized CNTs
showed the D band at 1352 cm-1 and a G band at 1585 cm-1 (Figure 7.3 spectrum C), while the
functionalized N-doped 7.4 material displayed the conventional bands at 1358 cm-1 and 1573 cm-
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144 | P a g e
1, respectively. Similarly the undoped unfunctionalized CNTs 6.1 (Figure 7.3 spectrum A) gave
two bands at 1351 cm-1 and at 1583 cm-1 while the functionalized undoped CNTs 6.2 gave bands
at 1346 cm-1 and 1582 cm-1 (Figure 7.3 spectrum B).
Figure 7.2 Raman spectra of A) unfunctionalized CNTs 6.1; B) functionalized CNTs 7.2; C)
unfunctionalized N-CNTs 7.3; D) functionalized N-CNTs 7.4.
The relative intensities of the ID/IG values increased in pristine N-doped CNTs relative to the
undoped CNTs. Furthermore, the ID/IG ratios increased in the functionalized CNTs this is
indicates the introduction of covalently bound moieties to the nanotube framework wherein
significant amounts of the sp2 carbons have been converted to sp3 hybridization after the
functionalization process. The ID/IG ratios of 7.2 found to be larger than 7.4, this could be the
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145 | P a g e
side wall of the undoped CNTs after functionalization became highly disordered than N-doped
CNTs (the see Figure 7.3 and Table 7.1).
Table 7.1 intensity ratios of the D and G Raman bands
7.3.3.2 Transmission Electron Microscopy (TEM)
The polymer attached CNTs in solution could be readily deposited directly onto a surface for
various microscopy analyses and the TEM images offered the most direct evidence for the
presence of carbon nanotubes in the samples.
The synthesized CNTs (both undoped CNTs 7.1 and N-CNT 7.3) were purified by heating then
reflux in concentrated hydrochloric acid to remove impurities. The TEM images also confirmed
that multi-walled CNTs had been synthesized and that the CNTs were relatively pure. In the case
of the nitrogen doped CNTs, they were found to have defective bamboo-like structures and
distinctive compartment layers due to the incorporation of nitrogen atoms into the CNTs, either
as pyridinic nitrogen or graphitic nitrogen.
Figure 7.3 TEM images of A) CNTs 7.1 and B) N-CNTs 7.3.
Sample Pristine
ID/IG ratio
After functionalization
ID/IG ratio
Undoped CNTs 0.72 0.94
N-doped CNTs 0.78 0.88
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146 | P a g e
After functionalization of the CNTs 7.4, the side walls became rougher (shown in Figure 7.4)
than the pristine CNTs (as shown in Figure 7.3).
Figure 7.4 TEM images of functionalized N-CNTs 7.4
Figure 7.5a-c shows the TEM images of 7.5, 7.6, and 7.7 and 7.8. From the TEM images the
CNTs were seen to be covered with poly(3-hexylthiophene) and polythiophene respectively. This
was further substantiated by 1H NMR, FT-IR and UV-visible studies (Section 7.3.3.3-7.3.3.5).
The morphology of the two classes of thiophenes will be discussed separately below.
7.3.3.2.1 Poly(3-hexylthiophene)/CNT composite
The poly(3-hexylthiophene)-attached CNTs (both nitrogen doped 7.6 and undoped CNTs 7.5)
were covered with polymer. The CNTs can readily be seen embedded in the polymer. The TEM
images (Figure 7.5a), revealed that the N-CNTs were not covered uniformly with polymers. In
the case of the undoped CNTs, the CNTs were more or less uniformly covered by the polymer.
The average overall thickness of 7.5 (shown in Figure 7.5a, image A and B) was measured to be
about 85 nm. Similarly for N-CNTs 7.6 (shown in Figure 7.5a image C and D) the average
diameter was found to be about 130 nm.
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147 | P a g e
Figure 7.5a TEM images of A and B are for poly(3-hexylthiophene) attached undoped CNTs 7.5
; C and D for poly(3-hexylthiophene) attached N-CNTs 7.6.
7.3.3.2.2 Polythiophene/CNT composite
The morphology of the polythiophene polymer attached CNTs (both N-CNT and undoped) was
found to be different from that of poly(3-hexylthiophene) attached to CNTs. The TEM images
revealed that, when using polythiophene, the CNTs were covered by a rough and bumpy polymer
(Figure 7.5b). The average overall thicknes of the polymer covered N-CNTs 7.7 (as shown in
Figure 7.5b A and B) was about 126 nm. Similarly, for the undoped polymer attached CNTs 7.8
(as seen in Figure 7.5b C) was mesured about 69 nm.
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148 | P a g e
Figure 7.5 b TEM images of A and B, and C polythiophene attached N-CNTs 7.7 and
undoped CNTs 7.8, respectively.
On other hand, for comparison purpose, a composite of pristine N-CNTs (not reacted with a
Prato reagent) and polythiophene was synthesized using pristine N-CNTs and thiophene in the
presence of FeCl3 under inert atmosphere. The TEM images show a clear difference between the
product produced by polymerisation with functionalised N-CNTs (Figure 7.5b image A and B)
and the product produced by mixing pristine N-CNTs with the thiophene (Figure 7.5c). The new
product shows that the CNTs and the ?roses? polythiophene existed separately, i.e there was little
interaction between the two reactants. This substantiate that the two reactants had only physical
interactions, as a result the surface of the N-CNTs was smoothly coated with polymers. It is thus
clear that the functionalization process leads to a new type of compounds.
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149 | P a g e
Figure 7.5c TEM images for unfunctionalized CNT and polythiophene mixed 7.11.
7.3.3.3 1H NMR spectroscopic studies of the synthesized materials
In a study by Leclerc et al. [41] it was found that ?-electron delocalization and solubility of the
polymers was largely affected by the substitution pattern of the polymer chain. In addition,
Maior et al. [42] and Sato et al. [43] have reported that 1H NMR spectroscopy studies provides
important information on the substitution pattern in a polymer backbone. The NMR spectra of
7.5, 7.6 and 7.9 were thus measured to provide information of the polymer backbone geometry.
The relative chemical shifts and the peak assignments of the poly(3-hexylthiophene) and 3-
hexylthiophene materials were discussed in detailed in Chapter 5, Section 5.3.3.1. The peaks
found in the spectra of both 7.5 and 7.6, had the same chemical shifts at 7 ppm as those for the
pure poly(3-hexylthiophene). The peaks were broad in all those cases (see Figure 7.6). This
broadening effect was also observed in the case of C60 incorporation in to the poly(3-
hexylthiophene) (Chapter 5). Results from the determination of the NMR nuclei spin-lattice (T1)
and spin-spin (T2) relaxation time suggest that the broadened proton signals are associated with
diamagnetic species of low mobility [44], namely, the nanotube-attached poly(3-hexylthiophene)
moieties.
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150 | P a g e
8 7 6 5 4 3 2 1 0 -1
TMS
CDCl
3
fe
dc
a,a'
b
S
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
a
a'
b
c
c
d
d
d
d
e
e
e
e
f
7.6
7.5
7.9
hexylthiophene
Chemical shift (ppm)
Figure 7.6 1H NMR spectra of the monomer and copolymers (CDCl3), Poly(3-hexylthiophene)
7.9; undoped CNT poly(3-hexylthiophene) 7.5; N-doped poly(3-hexylthiophene) 7.6.
The HT:HH ratio of the pure poly(3-hexylthiophene) was found to be 60:40. The HT:HH ratio
deduced from the 1H NMR spectrum for copolymers 7.5 and 7.6 were 69:31 and 70:30,
respectively. The copolymers 7.5 and 7.6 were found to be more regioregular than poly(3-
hexylthiophene) 7.9. Similarly, at high concentrations of C60 derivative in poly(3-
hexylthiophene) the HT:HH ratio found to be more regioregular (see Chapter 5, Section 5.3.3.1).
Chapter Seven
151 | P a g e
The presence of CNT during a polymerization reaction influences the HT orientation less
preferential. Similar results were reported by Saini et al. [45].
7.3.3.4 FT-IR studies of the synthesized materials
The FT-IR spectra of the synthesized polymers are shown in Figure 7.7 and a summary of FT-IR
band positions and their assignments is given in Table 7.2. The IR bands and assignments of the
thiophene, 3-hexylthiophene, polythiophene and poly(3-hexylthiophene) were discussed in detail
in Chapter 5, Section 5.3.3.2 and will not be repeated here.
No significant peaks were noticed after functionalization of the CNTs (see Figure 7.7), since the
CNT carbon atoms were more dominant than the added organic functional groups (see TGA
profiles Figure 7.11). However, after polymerization of the functionalized CNTs, new and
distinctive peaks were recorded. The positions of those peaks were found to be similar to those
found for polythiophene and poly(3-hexylthiophene) (see Figure 7.8a and b). However, for
polymer 7.5, the band at 1507 cm-1 was difficult to identify as it was overshadowed by a very
broad band in the same region.
Chapter Seven
152 | P a g e
4000 3500 3000 2500 2000 1500 1000
7.4
7.2
7.1
7.3
Tr
an
sm
itt
an
ce
(a
.u
)
Wave number (cm
-1
)
Figure 7.7 FT-IR of unfunctionalized undoped CNTs 7.1; functionalized undoped CNTs 7.2;
unfunctionalized N-CNTs 7.3; functionalized N-CNTs 7.4.
Table 7.2 Summary of FT-IR of the monomer, poly(3-hexylthiophene) and CNT attached
polymers (values in cm-1). *
Sample Ar C-H str. C-H str. Ring str. Ar C-H out of plane
3-hexythiophene 3095 (vw) 2823 -2714 (br. m) 660 (s)
7.5 3053 (vw) 2955-2850 (br. s) 1456 (w) 817 (s), 670 (w)
7.6 3053 (vw) 2955-2855 (br. s) 1507 (vw), 1456 (m) 824 (s), 670 (w)
7.7, 7.8, Polythiophene 2927-2856 (br. s) 1438 (m) 795(m), 689(s)
7.9 3053 (vw) 2955-2855 (br. s) 1507 (vw), 1456 (m) 822 (s), 670 (w)
Ar = Aromatic; *vw = very weak, w = weak, m = medium, s = strong, br. s = broad and strong
Chapter Seven
153 | P a g e
4000 3500 3000 2500 2000 1500 1000
1513 cm
-1
822 cm
-1
1507 cm
-1
660 cm
-1
670 cm
-1
1456 cm
-1
3-hexylthiophene
7.6
7.5
7.9
Tr
an
sm
itt
an
ce
(a
.u
)
Wave number (cm
-1
)
Figure 7.8a FT-IR spectra of 3-hexylthiophene; poly(3-hexylthiophene) 7.9; poly(3-
hexylthiophene) attached to N-CNTs 7.5 and poly(3-hexylthiophene) attached to undoped CNTs
7.6.
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154 | P a g e
4000 3500 3000 2500 2000 1500 1000
689 cm
-1
1438 cm
-1
795 cm
-1
7.8
7.7
Polythiophene
T
ra
ns
m
itt
an
ce
(a
.u
)
Wave number (cm
-1
)
Figure 7.8b FT-IR spectra of polythiophene attached to N-CNT 7.7; polythiophene attached to
undoped CNT 7.8 and polythiophene 7.9.
7.3.3.5 UV-visible and photoluminescence spectra of the synthesized materials
UV-vis absorption spectra of 7.5, 7.6 and 7.9 are shown in Figure 7.9 and a summary of the
absorption bands are given in Table 7.2. Pure polymer 7.9 showed an absorption band at 428 nm
which corresponds to the ?-?* transition of its conjugated segments [46]. In polymer 7.5 and 7.6
Chapter Seven
155 | P a g e
this particular band shifted slightly to the red region. Similarly, the red shift was also noted in the
C60 copolymers (see Chapter 5 Section 5.3.3.3). Although the absorption band shift to the red
region has been observed in some instances [47], this is contrary to the usual blue shift observed
for poly(3-hexylthiophene)/clay mixtures [48].
300 400 500 600 700 800
0
2
600 700 800
0.0
0.1
0.2
C
B
A
A
bs
or
pt
io
n
(a
.u
)
Wavelength (nm)
C
B
A
A
bs
or
pt
io
n
(a
.u
)
Wavelength (nm)
Figure 7.9 UV-vis absorption spectra in THF of A) poly(3-hexylthiophene) 7.9 ; B) polymer
attached to undoped CNTs 7.5; C) polymer attached to N-doped CNTs 7.6.
The photoluminescence spectra of 7.5, 7.6 and 7.9 are shown in Figure 7.10. Studies have
indicated that poly(3-hexylthiophene) has photoluminescence properties [46]. The
photoluminescence of 7.5, 7.6 and 7.9 were recorded at 500 nm excitation wavelength and
showed emissions at 578, 577 and 574 nm, respectively (see Table 7.3). In addition a small red
shift relative to the poly(3-hexylthiophene) photoluminescence emissions were observed in both
CNT derivatives.
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156 | P a g e
500 600 700
0
50
100
150
200
250
300
350
C
A
BIn
te
ns
it
y
(a
.u
)
Wavelength (nm)
Figure 7.10 Photoluminescence at 500 nm excitation wavelength in THF of A) poly(3-
hexylthiophene) 7.9; B) polymer attached undoped CNTs 7.5; C) polymer attached N-
CNT 7.6.
Table 7.3 Summary of the Photoluminescence and UV-visible spectra maximum in
THF.
Sample Photoluminescence Max
centered (nm)
UV-visible max
centered (nm)
7.9 574 276, 428
7.5 577 276, 431, 585
7.6 578 276, 435, 603, 771
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157 | P a g e
7.3.4 Thermal analysis
7.3.4.1 Thermogravimentric analysis (TGA)
TGA of the samples was performed under air for the polymers prepared in this chapter (see
Figures 7.11 and 7.12a and b). Functionalized CNTs display two stages of thermal
decomposition at ~ 200 and 494, and 250 and 516 oC for 7.2 and 7.4, respectively (see Figure
7.11). During the first decomposition reaction 5 and 37 % mass losses were recorded for 7.2 and
7.4 respectively. These results indicated that N-CNTs were more functionalized than the
undoped CNTs. Similar results were recorded for functionalized N-CNTs and undoped CNTs in
Chapter 5 (see Chapter 6, Section 6.3.4.4).
The TGA profiles of 7.5, 7.6 and 7.9 were all similar (see Figure 6.12a). Polymer 7.9 underwent
a single stage of decomposition at 330 oC, while 7.5 and 7.6 decomposed at lower temperatures
of 252 and 262 oC.
100 200 300 400 500 600 700
0
20
40
60
80
100
7.4
7.2
m
as
s %
Temperature (?C)
Figure 7.11 TGA profile of functionalized CNTs 7.2 and 7.4.
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158 | P a g e
100 200 300 400 500 600 700
0
20
40
60
80
100
C
B
A
E
D
M
as
s
lo
ss
%
Temperature (?C)
Figure 7.12a TGA profile of A) poly(3-hexylthiophene) 7.9; B) polymer attached undoped CNTs
7.5; C) polymer attached N-doped CNTs 7.6; D) unfunctionalized undoped CNTs 7.1; E)
unfunctionalized N-doped CNTs 7.3.
100 200 300 400
0
20
40
60
80
100
G
F
H
M
as
s
lo
ss
%
Temperature (?C)
Figure 7.12 b TGA profile of F) polymer attached N-CNTs 7.7; G) polymer attached undoped
CNTs 7.8; H) polythiophene 7.10.
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159 | P a g e
For 7.5 the first decomposition was at 263 oC; however the second decomposition could not
precisely be identified. Similarly for 7.6, the first thermal decomposition was at 263 oC. The
second thermal decomposition was recorded at 350 oC (33 % of mass loss).
In the case of copolymers formed from unsubstituted thiophene monomer, the TGA profiles
revealed that the first decomposition reactions were found to be generally occurred at a lower
temperature than observed for the substituted thiophenes. The first decomposition temperature
for the copolymer 7.8 was approximately at ~ 190 oC, while for 7.7 and polythiophene the values
were 204 and 215 oC, respectively (Figure 7.12b). During the first stage of decomposition 66, 70,
and 100 % mass losses were recorded for 7.7, 7.8 and polythiophene (7.10), respectively. The
second decomposition temperatures were noted at ~ 356 and 383 oC for 7.7 and 7.8, respectively.
7.3.4.2 Differential scanning calorimetry studies
The glass transition (Tg) temperature measurements of the polymers synthesized were performed
under a dynamic nitrogen flow in all cases. From the Differential Scanning Calorimetry (DSC)
measurements, the Tg value were taken as the midpoint of the transition region (see Figure 6.13).
DSC scans for 7.7, 7.8 and 7.10 did not give results that correlated with their structure. All the
polymers synthesized from 3-hexylthiophene showed a single Tg, indicating the absence of a
mixture of homopolymers or the formation of a block copolymer [49]. The Tg of pure poly(3-
hexylthiophene) 7.9 was found to be at 137 oC and for the polymer attached to carbon nanotubes
the Tg occurred at 150 oC and 154 oC for 7.5 and 7.6, respectively. Increases in Tg due to the
incorporation of both N-doped and undoped CNTs indicates a decrease in the chain mobility of
the polymer. This result is contrary to those recently reported by Gao et al. [50].No explanation
for this founding can be suggested at present.
Chapter Seven
160 | P a g e
100 150 200 250
-5x10
-2
0
5x10
-2
1x10
-1
1x10
-1
C
A
B
he
at
f
lo
w
W
/g
Temperature
o
C
Figure 7.13 DSC scan of the synthesized of A) poly(3-hexylthiophene) 7.9; B) polymer attached
undoped CNTs 7.5; C) polymer attached N-CNTs 7.6.
7.4 Conclusions
Undoped CNTs were synthesized by the decomposition of acetylene at high temperature. The
nitrogen doped CNTs (3.4 %) were synthesized by the floating catalyst methodology using
ferrocene as a catalyst, pyridine as a source of nitrogen and toluene as the carbon sources.
Attachment of organic functional groups on the side wall of the carbon nanotubes was achieved
through a 1,3-dipolar cycloaddition reaction by forming pyrrolidine fused rings on the side wall
of the CNTs. TEM images and Raman spectra were used to confirm that the organic groups were
attached to the side wall of the carbon nanotubes. Further covalent attachment of the thiophene
backbone was then accomplished by a FeCl3 mediated oxidative polymerization. A high HT/HH
ratio was noted after incorporation of CNT into the copolymers and the copolymers 7.5 and 7.6
Chapter Seven
161 | P a g e
were found to be more regioregular than pure poly(3-hexylthiophene) 7.9. Finally an increase in
the Tg of the copolymers was observed as the CNTs were incorporated into the copolymers.
Chapter Seven
162 | P a g e
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Chapter Eight
165 | P a g e
Chapter 8
Application of 3-hexylthiophene functionalized C60 and carbon
nanotubes in solar cells
8.1 Introduction
Diamond and graphite have long been the only known forms of carbon, thus the unexpected
discovery of fullerene, a new carbon allotrope, has fascinated the scientific community. Indeed
since the discovery of C60, intensive research activities have been focused on the properties of
fullerenes [1]. As a result, novel properties associated with functionalized fullerenes that have
been reported by various researchers include superconductivity, ferromagnetism, and optical
nonlinearity [2]. The synthesis of fullerene containing polymers has also been of interest from
the viewpoint of both basic research and practical applications and these novel polymers could
contribute to the development of processable fullerene-based specialty materials [3].
The interaction of C60 with photons has also attracted considerable interest and numerous research
articles have reported on the exploration of applications related to photophysical, photochemical
and photoinduced charge transfer properties of C60-derivatives. The unique electrochemical
properties of fullerene with six reversible single-electron reduction waves [4], and its photophysical
properties [5], make C60 and its derivatives interesting complexes to study photo-driven redox phe-
nomena. Photoinduced electron and energy transfer processes are of great significance and a large
number of studies have reported on the construction of C60-based molecular structures as artificial
photosynthetic systems [6].
The use of electron-accepting fullerenes in combination with ?-conjugated systems, as sacrificial
electron donors, offers several attractive features. In particular, fullerene, due to its low
reorganization energy in electron-transfer reactions, accelerates charge separation and
decelerates charge recombination, compared to two dimensional, planar electron acceptors [7].
This is beneficial for stabilizing the charge-separated state in C60-based materials, as required in
Chapter Eight
166 | P a g e
artificial electron transfer systems. Sariciftci et al. [8] demonstrated that a n-conjugated polymer
was able to efficiently transfer electrons to a C60 core giving rise to long-lived charge-separated
states. Since then, intensive research programs have focused on the utilization of fullerene derivatives
acting as electron acceptors in both organic and dye sensitized solar cells.
In typical organic solar cells, the solid-state heterojunctions consist of p-type donor (D) and n-
type acceptor (A) semiconductors. Organic C60-based solar cells can be fabricated by inserting
(or sandwiching) the p-type and n-type materials between two different electrodes. One of the
electrodes must be (semi-) transparent; the electrode is often indium tin oxide (ITO), but a thin
metal layer can also be used.
In donor-acceptor-linked molecules, the fullerene acts as an electron acceptor and the donor can
be made of dyad molecules such as aniline [9], carotenoid [10], porphyrin [10b,11,12], pyrazine
[13], or tetrathiophene [14]. In these molecules, the quantum yields of the charge-separation
processes were close to unity and the lifetimes of the charge-separated states were on the order of
sub nanoseconds. Imahori et al. [12b] reported that the reorganization energy of the dyad
molecule, including the fullerene acceptor, is small compared with other reported electron
acceptors. This feature has been one of the advantages of the fullerene-containing dyad
molecules, the use of which is aimed at attaining a long-lived charge-separated state with high
quantum yield for application in energy-storage systems or other sensitized reactions. As for the
donor moiety of the dyad molecule, several candidates have been proposed, in addition to those
listed above, because many examples of photoinduced electron transfer reactions between
fullerene and donors have been reported [15-18].
The application of carbon nanotubes in both organic and dye sensitized solar cells have been
investigated. A bulk heterojunction solar cell based on conjugated polymers blended with multi-
walled carbon nanotubes (MWNTs) [19] and single-walled carbon nanotubes (SWNTs) [20-22]
has been reported. For 1 % SWNT/ poly-3-octylthiophene (P3OT) bulk heterojunction solar
cells, high values of Voc (0.75 V) were achieved and this was reasonably well explained in terms
of the HOMO?LUMO electronic structures of P3OT and SWNTs [21].
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167 | P a g e
MWNTs exhibit metallic or semiconducting properties, which depend solely on their outermost
shell. On account of the large number of concentric cylindrical graphitic tubes present in
MWNTs, they are considered even more suitable in electron-donor?acceptor ensembles than
SWCNTs [23].
In dye sensitized solar cells (DSSC), the maximum current density is determined by how well the
absorption window of the dye overlaps the solar spectrum. The poor absorption of low-energy
photons by many dyes is consistent with the low performance of the cells. As a result,
considerable effort has been made to develop dyes and dye mixtures that absorb better at long
wavelengths [24-26].
In this study we report on the photovoltaic performance of both DSSC and organic solar cells.
Cells with different concentrations of derivatized C60 in C60-poly(3-hexylthiophene) mixtures
were studied. The poly(3-hexylthiophene) was attached to both N-doped and undoped carbon
nanotubes with and without ruthenium dye impregnated TiO2, and those mixtures were then
investigated. In addition to this, the effects on the solar cells, of physically mixing the
copolymers with TiO2 prior to deposition on the dye impregnated TiO2 was also investigated.
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168 | P a g e
8.2 Experimental
8.2.1 Assembly of the DSSC
TiO2 nanoparticles powder (3.0 g, Degussa P25), acetylacetone (0.10 mL), polyethylene glycol
(1.2062 g, F.W. 20,000), distilled water (5 mL) and Triton X-100 (0.025 mL) were added into a
mortar and ground to form a slurry. The mixture was then transferred to a container with a tight
stopper and left to stir for 48 h at r.t. A TiO2 nanoporous film was prepared from the mixture by
a doctor blading technique, by spreading the slurry onto SnO2:F conducting glass (FTO, fluorine-
doped SnO2, sheet resistance 8-10 ?/cm
2, Hartford Glass Co.). After the substrate was allowed
to dry at r.t., it was heated to 450 oC for 30 min and cooled down to r.t. The TiO2 thin film
electrode was then immersed in a 1.5?10?3 mol L?1 solution of the sensitizer, cis-
bis(isothiocyanato)bis(2,2?-bipyridyl-4,4'-dicarboxylate)-ruthenium(II) (N719) in EtOH for 20 h
at r.t.
A liquid electrolyte was prepared from LiI (0.10 mol L?1), I2 (0.05 mol L
?1),
tetrabutylammonium iodide (0.80 mol L?1), and 4-tertbutylpyridine (0.50 mol L?1) in 50 %
acetonitrile?50 % 3-methoxypropionitrile.
The copolymers, with and without TiO2, were suspended in chlorobenzene and stirred at r.t. for
48 h. Two drops of the suspension were deposited on the surface, with and without dye
impregnated TiO2. A sandwich-type cell with active area 0.25 cm
2 made from a TiO2 thin films
electrode, with and without a dye was impregnated into an ionic liquid electrolyte. A Pt-coated
FTO counter electrode was also prepared.
8.2.2 Assembly of the organic solar cell
A film of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solution
(Bayer) was spin coated (1500 rpm) on indium-SnO2 (ITO), followed by heating for 10 min at
120 oC in air. A C60-copolymer:P3HT 1:1 mixture (25 mg mL
-1 in chlorobenzene) was stirred
for 3 days at r.t. The solution was then spincoated on PEDOT (acceleration 800 rpm/s, 40 s). The
solar cells were fabricated by thermally evaporating and deposited aluminum metal (10-6 Torr)
on top of the polymer film with 70 nm thickness.
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169 | P a g e
The cell performance characterization was carried out by measuring the current-voltage (I-V)
characteristics in the dark and under AM 1.5G spectral distribution, which was obtained using a
150 W Oriel solar simulator plus filters. The samples were illuminated through the glass
substrate, and the radiance was controlled using neutral filters. The radiance was determined
using a calibrated silicon photo-detector.
8.3 Results and discussion
8.3.1 Synthesis of the copolymers
Prato [27] has developed a powerful procedure for the functionalization of C60. The Prato reagent
is made by adding an aldehyde and a glycine to C60. The reagents form azomethine ylides at high
temperature that react by way of a 1,3 dipolar cycloaddition with C60, to give functionalized
fullerenes. The same method has also been used by Georgakilas et al. [28] to functionalise
CNTs. This procedure allows the synthesis of a wide range of substituted fullerenes and CNTs
by variation of the aldehyde and glycine functional groups.
This procedure was used to make variants of the Prato reagent by specifically mixing C60 with 3-
thiophenecarboxaldehyde and N-methylglycine. The reaction gave the product 5.3, a thiophene
containing moiety covalently bonded to C60. Complex 5.3 was reacted with 3-hexylthiophene by
using a varied 2:3-hexylthiophene ratio [1:10 (5.3a), 1:500 (5.3b) and 1:1000 (5.3c)] in the
presence of FeCl3. Similarly a reaction between either 7.1 or 7.3 and the Prato reagent gave
products 7.2 and 7.4. Polymerization was achieved by varying the ratio of either 7.2 or 7.4 to 3-
hexylthiophene (7.d) (1:20) in the presence of FeCl3 to give either 7.5 or 7.6 (Scheme 8.1 and
8.3; see details in Chapters 5 and 7 section 7.3.1).
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170 | P a g e
CH3NHCH2COOH
,
toluene at 115 oC
S
R
R = hexyl
FeCl3 dry Dichloromethane
m = 1, n = 10, 500, 1000
5.1
5.2
5.3a = 10:1
5.3b = 500:1
5.3c = 1000:1
S
S
S
S
S
NR
R
R
R
S
S
O
N m
n
Scheme 8.1 Functionalization and polymerization methodology used for the synthesis of C60-
copolymers.
1,2 Dichlorobebzene at 180 oC
S R
FeCl3
dry chloroform
S
R
S
R
**
=
S
n
S
n
n
,
where 6.1 = CNT
7.2 = functionalized CNT
6.3 = N-CNT
7.4 = functionalized N-CNT
7.5 = undoped CNT- polyhexylthiophene
7.6 = N-CNT-polyhexylthiophene
7.9 = polyhexylthiophene
6.1,6.3 7.2, 7.4
R= hexyl
7.5, 7.6
HN
O
OH
S
CHO+
7.a 7.b
7.d
N N
S
R n
7.9
FeCl3
7.d
SS
S
R
R
R
Scheme 8.2. Functionalization and polymerization methodology used for the synthesis of
polymer-carbon nanotube copolymers.
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171 | P a g e
8.3.2 Application of copolymers in dye-sensitized solar cells (DSSCs)
8.3.2.1 Current-Voltage Characteristics
8.3.2.1.1 C60-copolymers 5.3a, 5.3b and 5.3c in DSSC
The charge recombination between conduction band electrons of TiO2 and triiodide ions, I3
?, of
the electrolyte decreases the open-circuit voltage (Voc) of a dye-sensitized solar cell (DSSC). In
order to reduce the recombination, attempts have been made to introduce a metal oxide blocking
layer on a TiO2 surface [29] this was achieved by using a composite semiconductor oxide film of
TiO2 and SiO2, Al2O3, or ZrO2 [30] that form an insulating film of poly(methylsiloxane) on parts
of the TiO2 [31], or by attaching long alkyl chains to the bipyridine rings of ruthenium dyes [32].
Lim et al. [33] were the first to incorporate C60 into a DSSC with a ruthenium dye as the
sensitizer. In this particular study, C60 was covalently linked to the N3 dye [cis-bis(4,4?-
dicarboxy-2,2?-bipyridine)dithiocyanate ruthenium(II)] via diaminohydrocarbon linkers with
different carbon chain lengths. A device was prepared by dipping a TiO2 film into a solution of
the modified dye. The authors [33] made the claim that the short-circuit photocurrent density
(Jsc) of the dye-sensitized solar cells (DSSCs) using the fullerene-attached sensitizers varied
markedly with on the chain length of the linker this may be correlated with the amount of total
sensitizer adsorbed on the surface of the TiO2 film, which was determined by absorption spectra
for each TiO2 film with a relatively large area. For example, for the linker 1,6-diaminohexane,
the Jsc, Voc and conversion efficiency of the pertaining cell were 11.75 mA cm-2, 0.70 V and 4.5
%, respectively, as against the values of 10.55 mA cm-2, 0.68 V and 4.0 %, respectively, for a
DSSC containing ordinary N3 dye.
The photoelectrochemical cells used in this study were assembled using the previously prepared
C60-copolymers 5.3a-c and TiO2 nanoparticles mixtures. The copolymers 5.3a-c and TiO2 were
mixed in a 1:1 mass ratio in chlorobenzene and two drops of the suspension were deposited on
the surface of the TiO2-FTO glass. The architecture of this device is shown in Figure 8.1a. The
cartoon, Figure 8.1b, (not to scale) shows the mixture of the C60-copolymers mixed with TiO2
nanoparticles deposited on the surface of the TiO2 pasted FTO glass. The overall thickness on
top of the FTO glass was measured to be about 6 ?m.
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172 | P a g e
Figure 8.1. Schematic representation of: (a) a DSSC with the FTO/TiO2/C60-containing
copolymer +TiO2/ electrolyte/Pt; (b) microscopic representation of the TiO2 and C60-copolymer
interaction on FTO glass. (Note: the representations are not drawn to scale.)
Figure 8.2 shows the current voltage (I-V) curves for the devices assembled with 5.3a-c. The
current density appears to be dependent on the concentration of C60 in the copolymers, following
the order 5.3a (0.9 mA cm-2) > 5.3b (0.6 mA cm-2) > 5.3c (0.5 mA cm-2). The C60 in such
devices is thought to act as an electron acceptor for the system [8]. The open circuit voltage
values (Voc) were observed to give the same trend, i.e. the sample with more C60 moieties was
the one that gave rise to the higher photovoltage, with 5.3a (0.63 V) > 5.3b (0.57 V) > 5.3c (0.47
V). The decrease in efficiency (?) could be due to the decrease in the photocurrent density (Jsc),
the open circuit potential (Voc) or the fill factor (FF) [34]. According to the UV-visible spectra
in Figure 5.4, the copolymer 5.3c demonstrates higher light absorption than copolymer 5.3a and
it would be expected that copolymers can also absorb light, increasing light-harvesting, and this
should improve Jsc. However, the solar cells performed better with copolymer 5.3a. This might
result from a reduction in the recombination effect (from the C60 contribution) being more
effective than the ?dye effect?, possibly because the polyhexylthiophene polymer does not have
carboxylic groups to chemically attach to the TiO2 as the sensitizers usually do [35].
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173 | P a g e
0.0 0.1 0.2 0.3 0.4 0.5 0.6
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0
1
2
3
4
5
6
7
8
9
10
P
(W
m
-2
)
J
(m
A
cm
-2
)
V (volt)
A
B
C
D
E
F
Figure 8.2 DSSC current-voltage curves (100 mW cm-2 incident light) for the TiO2/C60-
copolymer + TiO2/electrolyte/Pt system: A) 5.3a; B) 5.3b; C) 5.3c. Power-voltage curves: D)
5.3a; E) 5.3b; F) 5.3c.
The dark current was measured at 1.9 and 2.6 ?A cm-2 for the photocells made from the 5.3b and
5.3c copolymers, respectively (Figure 8.3). This implies that, the back-electron-transfer process,
corresponding to the reaction between the conduction-band electrons in the TiO2 and I3
- ion in
the electrolyte under dark condition [36], occurs more easily in the cell based on 5.3c, than on
the 5.3a copolymers. A predominant back-electron-transfer process would directly act to lower
the Voc of cell made from 5.3c copolymers, compared to the 5.3a copolymers. The results also
confirm that with a higher concentration of C60 in the copolymer the photocell behaves like a
diode (see Figure 8.3). This is in agreement with the functionalized C60 in the polymer acting to
accelerate charge separation and decelerate charge recombination due to the low reorganization
energy in the electron-transfer reaction [8,37].
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174 | P a g e
0.0 0.1 0.2 0.3 0.4 0.5
-2.0x10
-2
-1.0x10
-2
0.0
C
B
A
J
(m
A
cm
-2
)
V (volt)
Figure 8.3 DSSC current density vs voltage (vs Pt counter electrode) curves obtained under dark
conditions for the cells assembled with A) 10:1 mole ratio (5.3a); B) 500:1 mole ratio (5.3b) and
C) 1000:1 mole ratio (5.3c) C60:hexylthiophene copolymer.
Table 8.1 Photovoltaic performance of the DSSCs based on TiO2/C60-copolymer + TiO2
/electrolyte/Pt.
Mole ratio of
polymer to C60
incident light
(Pin)
(mW cm-2)
? (%) FF Voc (Volt) J (mA cm
-2) output
P max
(W m-2)
1000:1 (5.3c) 100 0.18 0.63 0.59 0. 49 0.15
10 0.08 0.45 0.45 0.04
500:1 (5.3b) 100 0.21 0.59 0.57 0.63 2.1
10 0.06 0.46 0.40 0.03
10:1 (5.3a) 100 0.34 0.61 0.63 0.89 3.4
10 0.27 0.64 0.53 0.08
A further investigation was performed by incorporating the synthesized copolymers with the well
known ruthenium(II) dye N719, [cis-bis(isothiocyanato)bis(2,2?-bipyridyl-4,4'-dicarboxylate)-
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175 | P a g e
ruthenium(II)]. The effect of mixing C60-copolymer with TiO2 nanoparticles was also
investigated (Figure 8.4b).
The schematic diagrams below show devices that were assembled with the 10:1 mole ratio (5.3a)
copolymer without (in Figure 8.4a) and with (Figure 8.4b) TiO2 mixtures deposited on the
surface of dye impregnated TiO2. The cartoon, figure 8.4c, (not drawn to scale) shows the
mixture of the C60-copolymers with TiO2 nanoparticles deposited on the surface of dye
impregnated TiO2 pasted FTO glass. The total thickness of the component was measured to be
about 6 ?m.
c
Figure 8.4. Schematic representation of a DSSC with: a) the glass-FTO/TiO2/dye/5.3a
copolymer/electrolyte/Pt; b) the glass-FTO/TiO2/dye/5.3a copolymer + TiO2/electrolyte/Pt; c)
Chapter Eight
176 | P a g e
microscopic representation of the TiO2+dye and 5.3a copolymer + TiO2 interaction on FTO
glass. (Note: the schematic representations are not drawn to scale.)
The results from the current?voltage measurements showed that the current density and the Voc
of the TiO2 mixed sample (B) increased as compared to the sample without TiO2 (A) (see figure
8.5). This might be due to the mixing of the copolymers with the TiO2 nanoparticles creating
better contacts between the C60-copolymer molecules and the electron transporter TiO2
nanoparticles. In other words, electrons from an electron acceptor, C60, are transported to the
TiO2 more readily. In both cases, A and B, the addition of C60-copolymer to the dye impregnated
TiO2 enhanced the overall performance of the cell as compared to the solar cell that had been
assembled without the C60-copolymer (C).
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0
2
4
6
8
10
12
C
B
A
J
(m
A
cm
-2
)
V(volt)
Figure 8.5. DSSC current-voltage curves (100 mW cm-2 incident light): A) 5.3a copolymer
without TiO2; B) 5.3a copolymer with TiO2; C) only with N719 dye.
The current?voltage curves under dark conditions were also measured for photocells made from
a 5.3a copolymer with and without TiO2 nanoparticles. According to the results, the photo cell
made with TiO2 (A) showed more diodic behavior than the sample without TiO2 (B) (see Figure
8.6).
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177 | P a g e
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
-1.0x10
-6
-8.0x10
-7
-6.0x10
-7
-4.0x10
-7
-2.0x10
-7
0.0
B
A
J
(m
A
cm
-2
)
V (volt)
Figure 8.6 Current density - voltage (I-V) curves obtained under dark conditions with DSSCs
based on 5.3a copolymer: A) mixed with TiO2; B) without TiO2.
The concentration of the C60 derivatives in the copolymer mixed with TiO2 nanoparticles was
also investigated. As shown in Figure 8.7 and Table 8.2, the concentration of the C60-derivatives
in the copolymer has pronounced effects on the current densities and Voc. The increase in the
amount of functionalized C60 in the sample led to an increase in the efficiency of the cell; 5.2 %
for the 5.3a as compared to 2.1 % for the 5.3c (100 mW cm-2 light intensity). This would suggest
that a high concentration of C60 in the copolymer creates less chance of electron recombination.
The cell efficiency was found to increase at low incident light (Table 8.2). This is due to the fact
that the mechanism of charge transport in the cells includes the diffusion of ionic species in the
electrolyte inside the nanoporous TiO2. Under high incident light radiation a large number of
charge carriers would be generated as a result of electrons being injected from the excited dye to
the conduction bands of TiO2 nanoparticles. This process occurs faster than the regeneration of
the dye by the electrolyte. This leads to an increase in the recombination of the injected electrons
and as a result the short circuit current and efficiency of the cell is reduced [38].
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178 | P a g e
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0
2
4
6
8
10
12
14
P
(W
m
-2
)
J
(m
A
cm
-2
)
V (volt)
A
C
B
D
Figure 8.7. DSSC current-voltage curves (100 mW cm-2 incident light) for the TiO2/dye/C60-
copolymer + TiO2/electrolyte/Pt system: A) 5.3a; B) 5.3b; C) 5.3c. Power-voltage curves are
also shown: D) 5.3a; E) 5.3b; F) 5.3c.
At a low concentration of C60 in the copolymers the current density and Voc of the cell is
suppressed as compared to the results for the solar cell assembled only with N719 dye.
Decreases in the open-circuit voltage can be explained as being due to the recombination of
photo-injected electrons with I3
- [29-32].
Chapter Eight
179 | P a g e
Table 8.2 Photovoltaic performance of the DSSCs based on glass-FTO/TiO2/dye/C60-copolymer
+ TiO2 /electrolyte/Pt
Sample mole
ratio
incident light
(Pin)
(mWcm-2)
? (%) FF Voc (volt) J (mAcm
-2) Output Pmax
(Wm-2)
1000:1 (5.3c) 100 2.1 0.58 0.72 5,0 21.2
10 1.6 0.61 0.61 0.43
500:1 (5.3b) 100 2.7 0.56 0.74 6.5 27.6
10 2.1 0.66 0.64 0.5
10:1 (5.3a) 100 5.2 0.47 0.84 13.18 52.2
10 6.8 0.70 0.76 1.28
Only N719
dye
100 3.9 0.44 0.83 10.7 39.0
10 5.0 0.86 0.74 1.0
Finally, the current?voltage curves under dark conditions, indicate that the photocell made from
the 5.3a copolymer with TiO2 nanoparticles (A) exhibits a less pronounced dark current than the
5.3b (B) and 5.3c (C) copolymers (see Figure 8.8).
0.00 0.05 0.10 0.15 0.20 0.25 0.30
-8.0x10
-7
-6.0x10
-7
-4.0x10
-7
-2.0x10
-7
0.0
2.0x10
-7
C
B
A
J
(m
A
cm
-2
)
V (volt)
Figure 8.8. DSSC current-voltage curves generated under dark conditions for the cells made
from TiO2/dye/C60 copolymer + TiO2/electrolyte/Pt system: (A) 10:1 mole ratio (5.3a); (B) 500:1
mole ratio (5.3b); (C) 1000:1 mole ratio (5.3c).
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180 | P a g e
8.3.2.1.2 Solar cell assembled with functionalized carbon nanotubes
The incorporation of CNTs, either functionalized or pristine into conjugated polymers in
photocells has been reported by various groups [19-21] and the performance of the cells was
found to increase when functionalized CNTs were used [39,40]. Kumakis et al. [20] have
proposed that the photovoltaic response of their devices was based on the introduction of internal
polymer/nanotube junctions within the polymer matrix. This is due to a photoinduced electron
transfer from the polymer to the nanotube that contributed to enhanced charge separation and
collection.
The photocells assembled in this study were assembled with N-doped and undoped carbon
nanotube-copolymer and TiO2 nanoparticles mixtures. The N-doped and undoped carbon
nanotube-copolymers with TiO2 were mixed in chlorobenzene to give a 1:1 mass ratio
respectively, and two drops of the suspension were deposited on the surface of TiO2 pasted FTO
glass (see Figure 8.9). As shown in Figure 8.10 and Table 8.3 the current density and Voc of the
N-CNT copolymer was found to be higher than the (undoped) CNT copolymer. This might be
due to (i) the additional electrons contributed by the nitrogen atoms that can provide electron
carriers for the conduction band [23] and or (ii) the creation of a narrower energy gap [23,41].
Figure 8.9. Schematic representation of the DSSC solar cell with the glass-FTO/TiO2/CNT-
copolymer + TiO2 / electrolyte/ Pt. (Note: the schematic representation is not drawn to scale)
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181 | P a g e
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0
2
4
6
8
10
12
14
W
/m
2
J
m
A
/c
m
2
V (volt)
A
C
B
D
Figure 8.10. DSSC current-voltage curves (100 mW cm-2 incident light) for the TiO2/CNT-
copolymer + TiO2/ electrolyte/ Pt system: A) N-CNT copolymer 7.6; B) CNT copolymer 7.5.
Power-voltage curves are also shown: C) N-CNT copolymer 7.6; D) CNT-copolymer 7.5.
The current-voltage curves under dark condition showed that the photocell made from CNT-
copolymer 7.5 had more diodic behavior than the one made from the N-CNT copolymer 7.6 (see
Figure 8.11). This might be due to the N-doped CNTs showing more metallic behavior than the
undoped CNTs since they are narrow energy gap semiconductors [23,41].
0.0 0.1 0.2 0.3 0.4
-2.5x10
-6
-2.0x10
-6
-1.5x10
-6
-1.0x10
-6
-5.0x10
-7
0.0
5.0x10
-7
A
B
J
(m
A
cm
-2
)
V (volt)
Figure 8.11 Current-voltage curves under condition dark for A) N-CNT copolymer 7.6; B) CNT
copolymer 7.5.
Chapter Eight
182 | P a g e
Table 8.3 Photovoltaic performance of the DSSCs made from glass-FTO/TiO2/CNT copolymer +
TiO2 /electrolyte/Pt
Sample incident light (Pin)
(mW cm-2)
?
(%)
FF Voc (Volt) J (mA cm
-2) Output Pmax
(W m-2)
N-CNT copolymer
7.6
100 0.53 0.63 0.64 1.31 5.3
10 0.39 0.63 0.54 0.11
CNT copolymer
7.5
100 0.28 0.59 0.61 0.77 2.8
10 0.12 0.48 0.45 0.06
The N-CNT copolymer 7.6 and CNT copolymer 7.5 with TiO2 were mixed in chlorobenzene to
give a 1:1 mass ratio respectively, and two drops of the suspension were deposited on surface of
dye impregnated TiO2 FTO glass (see Figure 8.12 for architectural arrangements). Here, unlike
in the previous investigation, incorporation of both carbon nanotube-copolymers with ruthenium
dye revealed that the current density and Voc showed better performance for the CNT copolymer
(7.5) than for the N-CNT copolymer (7.6) (see Figure 8.13 and Table 8.4).
Figure 8.12. Schematic representation of the DSSC made from glass-FTO/TiO2/dye/CNT
copolymer / electrolyte/ Pt. (the schematic representation is not to scale)
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183 | P a g e
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0
5
10
15
20
25
30
35
P
(W
m
-2
)
J
(m
A
cm
-2
)
V (volt)
A
B
C
D
Figure 8.13. DSSC current-voltage curves (100 mW cm-2 incident light) of the glass-FTO / TiO2 /
dye/CNT + TiO2/electrolyte/Pt system: A) CNT copolymer 7.5; B) N-CNT copolymer 7.6.
Power-voltage curves are also shown: C) CNT copolymer 7.5; E) N-CNT copolymer 7.6.
Table 8.4. Photovoltaic performance of the DSSCs based on glass-FTO/TiO2/dye/CNT-
copolymer + TiO2 /electrolyte/Pt
Sample incident light (Pin)
(mW m-2)
? (%) FF Voc (Volt) J (mA cm
-2)
N-CNT copolymer
7.6
100 0.37 0.42 0.62 1.4
10 0.48 0.51 0.55 0.18
CNT copolymer 7.5 100 1.28 0.61 0.68 3.1
10 0.51 0.53 0.54 0.18
Dye N719 100 3.9 0.44 0.83 10.7
10 5.0 0.86 0.74 1.0
The Voc and Jsc were found to increase with increasing Pin at lower light intensities (from 10 mW
cm-2 to 100 mW cm-2) in all cases. This dependence of Jsc on the incident light power indicates
that the photocurrent production is not limited by the diffusion kinetics of I3
?/ I? ions. This is due
to the rapid regeneration of the photo-oxidized dye molecules. However, in case of N-CNT
Chapter Eight
184 | P a g e
copolymer 7.6 and Dye N719 at high Pin value (100 mW cm
-2) the diffusion of I3
?/I? is too slow
to efficiently regenerate the oxidized dye molecules resulting in a decrease in the photocurrent.
This effect coupled with possible ohmic losses in the TCO support leads to the observed
decrease in the fill factor at higher light intensities (100 mW cm-2) [42]. These loss processes
have the effect of modulating the power conversion efficiencies of the cells at higher Pin.
8.3.3 Organic solar cells
Photovoltaic devices have also been fabricated by blending C60 with conducting polymers. Yu et
al. [43] have developed new conducting polymer composites that contain an electron-donating
species and an electron accepting species in a bi-continuous network. These photovoltaic
systems were based on the mechanism of photoinduced charge separation. The electron donor
phase utilized a soluble poly(phenylene vinylene) (PPV) derivative and poly(2-methoxy-5-(2'-
ethylhexyloxy) l,4-phenylvinylene), more commonly known as MEH-PPV. The acceptor phase
used one of two soluble forms of C60 known as [6,6] PCBM and [5,6] PCBM.
In this study, C60 covalently attached to poly(3-hexylthiophene) in a pearl necklace manner on
the polymer back bone was utilized (see Scheme 8.1). In this donor-acceptor-linked molecule,
the fullerene acted as an electron acceptor and poly(3-hexylthiophene) as a donor.
A film of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solution was
spin coated (1500 rpm) onto indium-SnO2 (ITO), followed by heating for 10 min at 120
oC in air.
A C60-copolymer:P3HT 1:1 mixture (25 mg mL
-1 chlorbenzene) solution was spincoated onto
PEDOT (acceleration 800 rpm/s, 40 s). The solar cells were fabricated by thermally evaporating
and depositing aluminum metal (10-6 Torr) of 70 nm thickness on top of the polymer film (see
Figure 8.14 for architectural arrangements).
Chapter Eight
185 | P a g e
Figure 8.14 Schematic representation of the organic solar cell with the ITO/PEDOT-
PSS/P3HT:C60-copolymer/Al device. (Note: the schematic representation is not drawn to scale).
Attempt to get I-V measurements of the devices fabricated using the functionalized CNT-poly(3-
hexylthiophene) and the 5.3c (1000:1 mole ratio) C60 copolymer was not successful. However,
the devices fabricated using the 5.3a (10:1 mole ratio) and 5.3b (500:1 mole ratio) C60
copolymer will be discussed below.
Due to low solubility of the 5.3a in chlorobenzene, the initial attempt to get current voltage
measurement was unsuccessful. However a 1:1 mixture of poly(3-hexylthiophene) to 5.3a in
chlorobenzene, gave a better film during spincoating. The 5.3b C60 copolymer spincoated
without the addition of poly(3-hexylthiophene).
The efficiencies of the solar cells were much lower than that reported in the literature for [6,6]
PCBM devices [44]. For the 5.3a C60 copolymer the efficiency was only 0.001 %; a value much
lower than the 2.5 % previously reported for a PDOT/PSS:MDMO-PPV:PCBM system [44]. In
addition, the current densities obtained in this study were significantly lower (see Figure 8.14
and Table 8.5). The open circuit potential was about 0.91 V while the literature reported values
of 0.6 V for PDOT:MDMO-PPV:PCBM [45] and 0.8 V for PDOT/PSS:MDMO-PPV:PCBM
[44] containing photo cells.
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186 | P a g e
-0.2 0.0 0.2 0.4
-4
-3
-2
-1
0
1
2
3
4
5
J
(?
A
cm
-2
)
V (volt)
under dark
A
-1.0 -0.5 0.0 0.5 1.0
-6
-4
-2
0
2
J
(
?
A
m
cm
-2
)
V(volt)
under dark
B
Figure 8.15 Current-voltage curves of organic solar cells based on: a) glass-ITO/PEDOT-
PSS/P3HT: 5.3a (10:1 mole ratio C60-copolymer)/Al; b) glass-ITO/PEDOT-PSS/5.3b
(500:1mole ratio C60-copolymer) /Al (both under dark conditions and 60 mW cm
-2 irradiation.)
Table 8.5 Photovoltaic performance of organic solar cell based on glass-ITO/PEDOT-PSS/C60-
copolymer /Al.
Sample Voc
(Volt)
Jcs (? Am cm
-2) FF
(%)
P max (mW cm
-2) Efficiency x 10-3
? (%)
500:1mole ratio
(5.3b)
0.372 1.6 23 0.14 0.23
10:1 mole ratio
(5.3a)(1:1 P3HT)
0.91
3.78 19 0.66 1.1
The source of the lower efficiencies can be related to the relatively low concentration of C60 in
the composite; in the report using [6,6]PCBM system the C60 content in the polymer was about
63 % [44] while in our cell it is about 4 %. Moreover, our synthesized poly(3-hexylthiophene)
Chapter Eight
187 | P a g e
has regioregularites in a 60:40 ratio of HT:HH (see Chapter 4 section 4.3.3.1). Studies have
indicated that high regio-irregularitiy contributes to a decrease in ?-electron delocalization [46].
8.4 Conclusions
Dye sensitized solar cells (DSSC) were assembled in diverse configurations with varying
concentrations of C60 in the copolymer; 5.3a was found to be more efficient than cells made with
5.3c. Incorporation of a higher concentration of the functionalized C60 in the polymer, led to a
better charge separation and this prevented charge recombination due to a lower reorganization
energy in the electron-transfer reaction. As a result, the photocurrent density (J), the open circuit
potential (Voc) and the fill factor (FF) increased. As a consequence this effect led to an increase
in efficiency (?). In all cases the C60-copolymer showed photovoltaic activity. Prior mixing of
TiO2 nanoparticles with the C60-copolymer before deposition on the dye impregnated TiO2,
further improved the overall efficiency of the solar cells.
Similarly, the polymer attached CNTs (7.5 and 7.6) were found to be photovoltaic active. The I?
V curves under dark conditions revealed that the N-doped CNT copolymer 7.6 was found to be
less conducting than the CNT copolymer 7.5. This was suggested to be due to the incorporation
of nitrogen atoms into the CNT structures that contribute additional electrons to the structure.
Finally, the organic solar cells were found to be less efficient than that made with the
[6,6]PCBM. This might be due to a relatively low concentration (4 %) of C60 in the copolymer
used when compared to the C60 content (63 %) in the [6,6]PCBM. In addition to this, the
synthesized poly(3-hexylthiophene) has more regioregularites which also contributes to a
decrease to a ?-electron delocalization.
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188 | P a g e
8.5 References
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Kadish, R. S. Ruoff, Eds., Electrochemical Society, Pennington, NJ, 1997, Vol. 4. (b)
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Chapter 9
9.1 Conclusions and future work
9.1.1 Conclusions
Addition and reduction reactions are among the most important reactions for the
functionalization of C60 and carbon nanotubes. In this thesis we have explored cycloaddition
reactions to functionalize both C60 and carbon nanotubes. The products were further
copolymerized with appropriate monomers and in most cases used to make photovoltaic devices.
In chapter 3 and 4, C60-cyclopentadiene was synthesized by a Diels-Alder reaction between C60
and cyclopentadiene. The functionalized C60 was further copolymerized either with norbornene
or N-(cycloheptyl)-exo-norbornene-5,6-dicarboximide using ROMP in the presence of the
Grubbs second-generation catalyst. Various C60/monomer ratios were used. However the N-
(cycloheptyl)-exo-norbornene-5,6-dicarboximide was found to give more soluble polymers than
those produced from the norbornene. Spectroscopic evaluation revealed that incorporation of the
fullerene into the polymers had occurred and that the relative amount of C60 affected the polymer
thermal properties by increasing both the decomposition and the glass transition temperatures,
relative to the pure polynorbornenes.
In chapter 5, functionalization of C60 was achieved by using the Prato?s reagent. The structure of
the two C60 derivatives (5.2 and 5.3) were confirmed by mass spectrometric (m/z 860 [M+1] at
2.6 %) and UV-visible spectral analysis. In addition, the overall yield was significantly improved
when compared to literature reports, by increasing the reaction time to five days. Furthermore,
covalent attachment either with thiophene or 3-hexylthiophene in varying ratios was
accomplished by a FeCl3 oxidative polymerization reaction. Incorporation of C60 derivatives into
the thiophene backbone was confirmed by UV-visible and FT-IR spectra. Finally, the
copolymers were also characterized by 1H NMR spectroscopy and TGA.
In chapter 6, nitrogen-doped carbon nanotubes (N-CNTs) 6.3 were also synthesized by the
floating catalyst methodology using ferrocene as catalyst, pyridine as a source of nitrogen and
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toluene as the carbon source. Undoped CNTs 6.1 was synthesized by the decomposition of
acetylene. The doping of nitrogen into carbon nanotubes were confirmed by elemental analysis.
Functionalization of the N-doped 6.3 and undoped carbon nanotubes 6.1 was achieved by using
azomethine ylides that were generated in situ by decarboxylation of immonium salts derived
from thermal condensation of N-methylglycine and 5-norbornene-2-carboxaldehyde.
Polymerization from the side wall of functionalized carbon nanotubes (6.2 and 6.4) with
bicyclo[2.2.1]hept-2-ene 6.c as a monomer was then possible using the Grubb?s second
generation catalyst. After the polymerization reactions, TEM images showed that the polymer-
attached carbon nanotubes (6.5 and 6.6) were found to have enlarged diameters relative to the
pristine CNTs (6.1 and 6.3). In subsequent NMR studies the N-doped CNTs/polymer was found
to have more trans isomer than the undoped CNT/polymer material. Finally, the glass transition
(Tg) temperatures were found to have decreased by the incorporation of carbon nanotubes.
In chapter 7, different organic functional groups were attached to the side walls of carbon
nanotubes using the same methodology as that used in chapters 5 and 6. A reactive intermediate,
that was generated in situ by decarboxylation of immonium salts derived from thermal
condensation of N-methylglycine 7.a and 2-thiophenecarboxaldehyde 7.b gave the products. The
functionalized carbon nanotubes (7.2 and 7.4) together with either thiophene 7.c or 3-
hexylthiophene 7.d were then used in copolymerization reactions which were initiated by FeCl3.
TEM images clearly indicated that the polymers were attached to the side wall of the carbon
nanotubes. NMR spectroscopy revealed that 7.5 and 7.6 were more regioregular than that of pure
poly(3-hexylthiophene) 7.9. The thermal properties of the synthesized copolymers were also
examined. Copolymers 7.5, 7.6 and poly(3-hexylthiophene) 7.9 all revealed single Tg
temperatures. Furthermore, the synthesised CNTs and the CNT/polymer were characterised by
elemental analysis, thermogravimentric analysis (TGA), as well as Raman and FT-IR, UV
visible and photoluminescence spectroscopy.
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Potential applications of the synthesized C60-thiophene copolymer and the CNT-thiophene
copolymer were also studied.
Dye sensitized solar cells (DSSC) were assembled in diverse configurations with varying
concentrations of C60 in copolymers; when the mixture contained larger concentration of C60
(5.3a) the DSSC was more efficient than when low concentrations of C60 were used (5.3c).
Incorporation of more functionalized C60 into the polymer, led to a better charge separation as
this prevented charge recombination due to a lower reorganization energy in the electron-transfer
reaction. As a result, the photocurrent density (JSC), the open circuit potential (VOC) and the fill
factor (FF) increased. As a consequence this effect led to an increase in efficiency (?) that
reached 0.34 % for the configuration: glass?FTO/TiO2/C60-copolymer + TiO2/electrolyte/Pt
layers. Furthermore, prior mixing of the TiO2 nanoparticles with the C60-copolymer, before it
was deposited on the dye impregnated TiO2, improved the overall efficiency of the solar cells.
Similarly, the polymer-attached CNTs (7.5 and 7.6) were found to be photovoltaic active. The
polymer-attached N-doped CNTs 7.6 performed better than polymer attached undoped CNTs
7.5. However, the I?V curves under dark conditions revealed that the N-doped CNT copolymer
7.6 was found to be less of a semiconductor than the CNT copolymer 7.6. This is probably due to
the incorporation of nitrogen atoms into the CNT structures that contributes additional electrons
to the structure. Finally, the organic solar cells were found to be less efficient than [6,6]PCBM.
This might be due to a relatively low concentration of C60 in the copolymers used.
9.1.2 Future work and recommendations
9.1.2.1 Functionalization of carbonaceous materials
In this study, functionalization of carbonaceous materials was achieved using a well known
procedure, based on the Prato reagent. In the particular of CNTs, the Prato reaction has been
demonstrated to be an efficient method for functionalization of CNTs. The degree of
functionalization on the side wall of CNTs has been estimated from the TGA profile. In this
project however, we noted that not all the CNTs were equally functionalized; some CNTs were
only partially functionalized. This had an effect during the polymerization reactions leading to
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CNTs that were either not covered or only partially covered by polymer. Therefore, controlling
the degree of functionalization is a crucial step in making the composites. Inhomogeneous
reactivity and nucleation effects had thus occurred during the polymerization reaction. This
could be due to a combination of the high reactivity of the reactants and poor dispersibility of the
functionalized CNTs in a solvent leading to CNTs not uniformly covered by polymer. Control of
the thickness of the polymer on the side walls needs to investigated. Thus it is recommended that
control of the amount of functional groups on the side walls of the CNTs, and dispersibility after
functionalization be investigated in detailed. This could be studied by varying the degree of CNT
sidewall coverage and the use of shorter length CNTs.
9.1.2.2 Photovoltaic Device applications
In this study, the potential application of the synthesized C60-thiophene copolymers and the
CNT-thiophene copolymers were explored in DSSC and organic solar cells. As a result, the
photocurrent density (JSC), the open circuit potential (VOC) and efficiency (?) of the cell were
measured on the synthesized materials to determine if the synthesized materials were
photovoltaically active. Improving the efficiency of the devices is still possible by further
manipulating of the existing molecular structures.
The DSSC is regarded as the next generation photovoltaic device due to its attractive features of
high power conversion efficiency (>10 %) and low production cost [1-6]. Among the
photosensitizers that have been investigated and used in DSSC, Ru(II)-based complexes are the
most efficient. Since this type of photosensitizer has an intense absorption in the visible region,
as well as a strong adsorption onto the semiconductor surface and efficient electron injection into
the conduction band of the semiconductor. Moreover, complexes of Ru(II) with 2,2?-bipyridine
ligands have long-lived metal-to-ligand-charge-transfer (MLCT) excited states. Their potential in
supramolecular architectures has been widely exploited for future photoinduced energy- and
electron-transfer processes [7, 8]. In this regard, Ru(II) complexes have a marked reducing
character that makes them ideal partners for C60 and CNT oxidants in the construction of donor-
acceptor arrays for photoinduced electron transfer devices. For instance, the use of a covalently
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linked electron acceptor, either modified C60 or CNTs, which bonded to the Ru(II)-based dye
could lead to a new class of supramolecular molecules. A photocell made from such materials
could be expected to give enhanced activity. It is recommended that the electronic properties and
utilization of Ru(II)-based dyes covalently linked to carbonaceouses materials in photovoltaic
devices be explored in detailed.
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9.2 Reference
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