i 
 

Synthesis of zirconium disulphide nanomaterials and their 

nanocomposites with radially aligned nanorutile and 

polyaniline for room temperature sensing of volatile 

organic compounds 

 

By 

Paul Olawale Fadojutimi 

 

                             Student no: 2165101 

 

 

 

A thesis submitted to the Faculty of Science, University of the Witwatersrand, 

Johannesburg, in partial fulfilment of the requirements for the award of the 

degree of PhD in Chemistry. 

 

 

 

 Supervisor: Dr John Moma 

       Co-Supervisor: Dr Siziwe Gqoba 

              Co-Supervisor: Dr Zikhona N. Tetana 

 

 

University of the Witswatersrand, Johannesburg, August 2022 



ii 
 

Declaration 

I declare that this thesis is my own unaided work. It is being submitted for the degree of 

Doctor of Philosophy in Science at the University of the Witwatersrand, Johannesburg. It has 

not been submitted before for any degree or examination at any other university. 

 

 

 

 

 
  (Signature of candidate) 

 

On this 17th day of August 2022 

 

 



iii 
 

Abstract 

Integration of 2D nanomaterials with a polymer or semiconductor metal oxide could help in 

the development of low-cost sensors for rapid detection of volatile organic compounds (VOCs) 

at room temperature. 

This study focuses on the fabrication of robust room temperature sensors of pristine radially 

aligned nanorutile, and zirconium disulphide/polyaniline (PANI) nanocomposites for chemical 

sensing of VOCs. 

ZrS2 was fabricated using both bottom-up and top-down methods of synthesis. Heat up and hot 

injection methods were employed to fabricate arrays of morphologies of ZrS2 nanomaterials 

using the colloidal method. However, the nanomaterials showed high oxophilicity which was 

confirmed by both XRD and XPS. The XPS peak of S2p was conspicuously absent while the 

peak Zr3d was very noticeable in the XPS spectra. XPS and EDS measurements indicated 

replacement of sulphur atom by the O atom on the surface of the nanomaterials. The stability 

study showed the nanomaterials were not stable in ambient environment.  

Nanoparticles of 11 nm and few layered nanosheets were obtained when bulk crystal samples 

of ZrS2 were exfoliated in cyclohexyl-2-pyrrolidone and N-methyl (-2-) pyrrolidone. 

Isopropanol served as a green solvent for the exfoliation of few-layered ZrS2 from the bulk 

crystal sample compared to amide solvents which are not environmentally friendly. However, 

the pristine ZrS2 nanomaterials could not sense VOCs at room temperature, this could be as a 

result of low conductivity and number of layers of the nanomaterials obtained using 

nanomaterials exfoliated in isopropanol. The sensitivity of raw PANI was greatly enhanced 

with loading of ZrS2 nanomaterials. The sensor displayed responses of 0.43, 0.58, 1.04 and 

0.34% which correspond to methanol, ethanol, isopropanol, and acetone vapours respectively. 

The relatively better responses of the sensor were credited to the synergistic effect of 

ZrS2/PANI composite structure. The sensor showed good response to low concentrations (7.7 

ppm, 11 ppm, 5.8 ppm and 6.1 ppm) which correspond to methanol, ethanol, isopropanol, and 

acetone respectively. The sensor was more sensitive to isopropanol compared to other alcohols 

tested for in this work. The behaviour of the sensor changed from p-type to n-type on exposure 

to ethanol vapour at elevated relative humidity. The sensor displayed good sensitivity, 

reproducibility, rapid response and recovery times towards alcohols and stability over 60 days. 



iv 
 

The hierarchical morphology, high surface area, high porosity and humidity contributed 

immensely to the titania sensor in the sensing of VOCs at room temperature. The TiO2 sensor 

showed high sensitivity with responses of -38.27, -86.75, -9.83 and 1.24% which correspond 

to methanol, ethanol, isopropanol, and acetone respectively. The sensor is more sensitive to 

ethanol gas compared to other chemical vapours tested. The sensor displayed good sensitivity, 

reproducibility, rapid response and recovery times towards alcohols and stability over 45 days. 

The surface area of the nanorutile decreased by 35% on loading of ZrS2, this could the main 

reason there was no response observed when the nanocomposite of TiO2-ZrS2 was tested for 

chemical sensing at room temperature. The active sites for adsorption of the vapour were not 

available probably due to the covering of pores of the nanorutile as well as low conductivity of 

ZrS2 at room temperature. 

Both sensors of nanocomposite of zirconium disulphide and polyaniline as well as titania    

could find application in breath analysers since the least detection for a breath analyser is 

reported to be 200 ppm. 

 

  

 

 

 

 

 

 

 

 

 

 

 



v 
 

 Dedication 

This work is dedicated to my late parents Mr and Mrs Ebenezer Olakunle Fadojutimi who 

valued good education and ensured they gave me the best legacy a parent can give to a child. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



vi 
 

Acknowledgements  

I would like to convey my sincere appreciation to everyone who made this work a success: 

❖ Unto God be the glory, great things He has done. I would like to thank God Almighty, 

the Elshadai for provisions and giving me wisdom, good health, and vigour to complete 

this research. 

❖ I am indeed grateful to all my supervisors, Dr John Moma, Dr Siziwe Gqoba, Dr 

Zikhona Tetana and Prof Nosipho Moloto for their guidance, advice and support in this 

research. Thank you so much for your constructive criticism and for your painstaking 

effort going through my thesis. I am indeed privileged to learn from the best. 

❖ A big thanks to all my colleagues in CATMAT group and most especially everyone in 

labs 117 and 330 for their support and friendly environment provided so far.  I would 

like to thank the following colleagues especially: Dr Rudo Sithole, Dr Pumza Mente, 

Dr Tumelo Phaahlamohlaka, Dr Victor Mashindi, Mr Clinton Masemola, Miss Boipelo 

Mathe and Mr Themba Ntuli. 

❖ Special thanks to every staff member of the Microscopy and Microanalysis Unit 

(MMU) for the XRD, SEM and TEM training.  My special appreciation goes to Dr 

Rudolph Erasmus for his assistance in Raman spectroscopy analyses and Dr Sanele 

Nyembe from MINTEK who assisted with AFM analyses. 

❖ Many thanks to Wits University for funding this research work. 

❖ Lastly, kudos to my lovely wife Oluwayemisi and my wonderful children 

Oluwakoretimi and Irenitemi Fadojutimi for your support despite my absence from 

home. 

 

 

 

 

 

 

 



vii 
 

Presentations 

1. Colloidal synthesis of zirconium disulphide (oral presentation), Catalysis and Materials 

Science group (March 2021). 

2. Nanocomposite of rutile titania and zirconium disulphide for room temperature sensing of 

acetone vapour (Poster presentation) National Young Chemist’s symposium (July 2021). 

3. Synthesis of zirconium disulphide nanostructures for room temperature sensing of acetone 

vapours (poster presentation), Wits Nanoscience Young researcher’s symposium (October 

2021). 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



viii 
 

Publications 

1. Paul Fadojutimi, Siziwe Gqoba, Zikhona Tetana and John Moma. Transition metal 

dichalcogenides in photocatalytic water splitting. (2022) Journal of Catalysts. Manuscript 

published. 

2. Siziwe S. Gqoba, Rafael Rodrigues, Sharon Lerato Mphahlele , Zakhele Ndala  , Mildred 

Airo  , Paul Olawale Fadojutimi , Ivo A. Hümmelgen  , Ella C. Linganiso , Makwena J. Moloto 

and Nosipho Moloto. Hierarchical Nanoflowers of Colloidal WS2 and Their Potential Gas 

Sensing Properties for Room Temperature. (2021). J. of Processes, 9, 1491. 

3. Paul Fadojutimi, Zikhona Tetana, John Moma, Nosipho Moloto and Siziwe Gqoba Colloidal 

synthesis of zirconium disulphide nanostructures and its stability against oxidation. (2022). 

Journal of Chemistry Select. Manuscript published. 

 

Manuscripts to be submitted. 

1. Paul Fadojutimi, Siziwe Gqoba, Zikhona Tetana, and John Moma.  Hierarchical dandelion 

rutile titania and their potential gas sensing properties for room temperature sensing methanol 

and ethanol.  

2. Paul Fadojutimi, Siziwe Gqoba, Zikhona Tetana, John Moma and Nosipho Moloto. 

Nanocomposite of polyaniline and zirconium disulphide for room temperature sensing of 

primary alcohol.  

3. Paul Fadojutimi, Siziwe Gqoba, Zikhona Tetana, Nosipho Moloto, and John Moma. 

Nanocomposite of polyanile and titania for room temperature sensing of primary alcohols. 

4. Paul Fadojutimi, Siziwe Gqoba, Zikhona Tetana, John Moma and Nosipho Moloto. Few 

layered synthesis and characterization of zirconium disulphide for optoelectronic application. 

 

 

 

 



ix 
 

Table of Contents 

Declaration…………………………………………………………………………………….ii 

Abstract……………………………………………………………………………………….iii 

Dedication…………………………………………………………………………………….v 

Acknowledgements…………………………………………………………………………..vi 

Presentations…………………………………………………………………………………vii 

Publication…………………………………………………………………………………..viii 

Table of contents…………………………………………………………………………….ix 

List of figures………………………………………………………………………………..xv 

List of 

tables………………………………………………………………………………………...xxi 

Abbreviation 

Description………………………………………………………………………………….xxii 

Chapter 1: Introduction and motivation……………………………………………………...1 

1.1 Background and motivation …………………………………………………………….1 

1.2 Problem statement……………………………………………………………………….3 

1.3 Aim and objectives………………………………………………………………………4 

1.4 Thesis outline……………………………………………………………………………4 

1.5 References……………………………………………………………………………….6 

Chapter 2: Literature review………………………………………………………………….8 

2.1 Transition metal dichalcogenides for chemical sensing………………………………….8 

2.2. Exfoliation……………………………………………………………………………….9 

2.3 Gas-Phase method of metal powder production…………………………………………12 

2.3.1 Chemical vapour deposition……………………………………………………………12 

2.3.2 Chemical vapour transport …………………………………………………………….13 



x 
 

2.4 Wet chemical synthesis…………………………………………………………………...14 

2.4.1 Colloidal synthesis …………………………………………………………………….14 

2.4.2 Hydrothermal or solvothermal synthesis……………………………………………….19 

2.4.3 Sol-gel method………………………………………………………………………….20 

2.5 Gas sensing mechanism in TMDCs………………………………………………………20 

2.6 Attributes of a gas sensor…………………………………………………………………21 

2.6.1 Response ……………………………………………………………………………….21 

2.6.2 Response and Recovery Time …………….. .. …………………………………………22  

2.6.3 Selectivity/Specificity   ………………………………………………………………...22 

2.6.4 Detection limits…………………………………………………………………………22 

2.6.5 Stability ………………………………………………………………………………...23 

2.6.6 Sensitivity and grain size……………………………………………………………….23 

2.7 Factors controlling sensitivity…………………………………………………………….24 

2.7.1 Receptor………………………………………………………………………………...24 

2.7.2 Tranducer……………………………………………………………………………….24 

2.7.3 Utility factor…………………………………………………………………………….25 

2.8 Gas sensing properties of 2D TMDCs and other 2D materials…………………………..26 

2.8.1 Metal doping……………………………………………………………………………30 

2.8.2 Metal Oxide for sensing VOCs …………………………………………………………30 

2.8.3 TMDCs with metal oxide composite …………………………………………………..32 

2.8.4 PANI for chemical sensing……………………………………………………………..34 

2.9 References………………………………………………………………………………..35 



xi 
 

CHAPTER 3: colloidal synthesis of zirconium disulphide and its stability against 

oxidation……………………………………………………………………………………...47 

3.1 Introduction………………………………………………………………………………48 

3.2 Experimental……………………………………………………………………………..50 

3.2 Chemicals and materials…………………………………………………………………50 

3.2.2 Synthesis……………………………………………………………………………….50 

3.2.3 Heat up Method………………………………………………………………………..50 

3.2.4 Hot injection method…………………………………………………………………..50 

3.3 Material Characterization………………………………………………………………..51 

3.4 Results and Discussion…………………………………………………………………..51 

3.5 Conclusion……………………………………………………………………………….63 

3.6 References……………………………………………………………………………….64 

CHAPTER 4: synthesis of radially aligned nanorutile and its application for chemical sensing 

of alcohols and acetone……………………………………………………………………...67 

4.1 Introduction……………………………………………………………………………...67 

4.2 Materials and methods…………………………………………………………………..69 

4.2.1 Chemicals……………………………………………………………………………..69 

4.2.2 Synthesis of radially aligned nanorutile (RANR)…………………………………….69 

4.2.3 Sensor fabrication ……………………………………………………………………70 

4.2.4 Gas sensing measurement …………………………………………………………....70 

4.3 Characterization of RANR……………………………………………………………...71 

4.4 Results…………………………………………………………………………………..72 

4.4.1 PXRD OF RANR……………………………………………………………………..72 



xii 
 

4.4.2 SEM analysis………………………………………………………………………….73 

4.4.3 TEM analysis………………………………………………………………………….74 

4.4.4 UV-Vis Spectra analysis……………………………………………………………….76 

4.4.5 Thermal analysis of RANR……………………………………………………………78 

4.4.6 BET analysis…………………………………………………………………………...78 

4.5 Gas sensing performance ………………………………………………………………..80 

4.6 Methanol vapour sensing………………………………………………………………...81 

4.7 Ethanol sensing…………………………………………………………………………..85 

4.8 Isopropanol………………………………………………………………………………94 

4.9 Acetone…………………………………………………………………………………...96 

4.10 Conclusion……………………………………………………………………………. 100 

4.11 

References…………………………………………………………………………………..101 

Chapter 5: Synthesis and characterization of few-layered zirconium disulphide via liquid 

exfoliation…………………………………………………………………………………...106 

5.1 Introduction ……………………………………………………………………………..106 

5.2 Chemicals and materials………………………………………………………………...107 

5.3 Exfoliation of bulk ZrS2…………………………………………………………………107 

5.4 Characterization…………………………………………………………………………108 

5.5 Results and Discussion………………………………………………………………….109 

5.5.1 XRD analysis……………………………………………………………………….....109 

5.5.2   UV-Vis spectra analysis…………………………………………………………......110 

5.5.3 Raman analysis………………………………………………………………………..113 



xiii 
 

5.5.4 AFM analysis…………………………………………………………………………114 

5.5.5 SEM analysis………………………………………………………………………….116 

5.5.6 TEM analysis………………………………………………………………………….117 

5.5.7 Energy dispersive x-ray spectroscopy (EDS) analysis………………………………..118 

5.5.8 Thermogravimetry analysis…………………………………………………………...119 

5.5.9. XPS analysis………………………………………………………………………….119 

5.6 Conclusion……………………………………………………………………………....123 

5.7 References………………………………………………………………………………124 

Chapter 6: Polyaniline and zirconium disulphide nanocomposite  and its application for 

chemical sensing of alcohols and acetone…………………………………………………..127 

6.1 Introduction……………………………………………………………………………..127 

6.2 Chemicals and materials………………………………………………………………...128 

6.3 Methods………………………………………………………………………………....129 

6.3.1 Synthesis of PANI…………………………………………………………………......129 

6.3.2 Synthesis of a nanocomposite of zirconium disulphide and polyaniline………………130 

6.3.3 Sensor fabrication……………………………………………………………………..130 

6.3.4 Gas sensing measurement…………………………………………………………......130 

6.4 Characterization…………………………………………………………………………131 

6.5 Results and discussions………………………………………………………………….131 

6.5.1 XRD analysis………………………………………………………………………….129 

6.5.2 TEM analysis………………………………………………………………………….130 

6.5.3 BET analysis…………………………………………………………………………..132 

6.6 Gas sensing of PANI-ZrS2 nanocomposite………………………………………………136 



xiv 
 

6.6.1 Primary alcohols and acetone sensing properties……………………………………..135 

6.7 Relative Humidity……………………………………………………………………….150 

6.8 Conclusion………………………………………………………………………………151 

6.9 References ………………………………………………………………………………152 

Chapter 7: Titania-zirconium disulphide nanocomposite for sensing acetone vapour……..155 

7.1 Introduction:…………………………………………………………………………….155 

7.2 Chemicals and materials………………………………………………………………...157 

7.3 Methods…………………………………………………………………………………157 

7.3.1 Synthesis of nanocomposite of titania with zirconium disulphide ……………………157 

7.3.2 Sensor fabrication……………………………………………………………………..157 

7.3.3 Gas sensing measurement……………………………………………………………..157 

7.4 Characterization…………………………………………………………………………158 

7.5 Results and discussions…………………………………………………………………158 

7.5.1 XRD analysis…………………………………………………………………………158 

7.5.2 Raman analysis……………………………………………………………………….159  

7.5.3 TEM analysis…………………………………………………………………………160 

7.5.4 UV-Vis spectroscopy…………………………………………………………………161 

7.5.5 Energy dispersive x-ray spectroscopy (EDS) analysis……………………………….162 

7.5.6 BET analysis………………………………………………………………………….163 

7.5.7 TGA analysis…………………………………………………………………………164 

7.6 Gas sensing performance of TiO2-ZrS2………………………………………………...165 

7.7 Conclusion………………………………………………………………………………167 

7.8 References………………………………………………………………………………168 



xv 
 

Chapter 8: General conclusions and recommendations for future studies………………….171 

8.1 General conclusions…………………………………………………………………….171 

8.2 Recommendations………………………………………………………………………172 

Appendix A…………………………………………………………………………………174 

List of Figures 

Figure 2.1: Schematic illustration of sensing mechanism of TMDCs……………………….21 

Figure 2.2: Schematic illustration highlighting the importance of grain size……………….24                    

Figure 2.3: Three factors determining the response of a semiconductor gas sensor………...25  

Figure 3.1: PXRD pattern of ZrS2 synthesized via the heat up and injection methods……..52 

Figure 3.2:  SEM micrographs of ZrS2 nanomaterials synthesized by the heat up method…54 

Figure 3.3: SEM micrographs of ZrS2 nanomaterials synthesized by the hot injection 

method……………………………………………………………………………………….54  

Figure 3.4: TEM images of ZrS2 synthesized by the heat up method……………………….55 

 Figure 3.5: TEM images of ZrS2 synthesized by the hot injection method………………....55 

Figure 3 6: (a) UV-Vis  spectrum and (b) band gap of  as-synthesized ZrS2 by the heat up 

method……………………………………………………………………………………….56 

Figure 3.7: (a)  UV-Vis  spectrum and (b) band gap of  as-synthesized ZrS2 by the injection 

method……………………………………………………………………………………….57 

Figure 3.8: FT1R  spectra of  (a) raw OA and (b) as-synthesized ZrS2 by the heat up 

method……………………………………………………………………………………….58 

Figure 3.9: TGA and DTG thermograms of ZrS2 synthesized by the heat up method……...59 

Figure 3.10: (a) and (b) EDS spectra of ZrS2 synthesized by the heat up method…………..60 

Figure 3.11:  High resolution spectra with focus on core levels C1s, O1s, N1s, valence, Zr3d  

and XPS survey spectrum of ZrS2 synthesized by heat up method…………………………61 



xvi 
 

Figure 3.12: Time-dependent study of ZrS2 nanomaterials with XRD (a) ZrS2 synthesized by 

heat up method and (b) synthesized by hot injection method……………………………….62 

Figure 4.1: Schematic diagram depicting the synthesis of RANR…………………………..70 

Figure 4.2: Gas sensing set up……………………………………………………………….71 

Figure 4.3: PXRD of radially aligned nanorutile……………………………………………73 

Figure 4.4: (a) and (b) show the SEM micrographs of the synthesized RANR………… ….74  

Figure 4.5: (a) and (b) TEM images of RANR ……………………………………………..75 

Figure 4.6: Mechanism of the synthesis of RANR………………………………………….75 

Figure 4.7: Particle size distribution of as-synthesized RANR……………………………..76 

Figure 4.8: The solid-state UV-Vis DRS plot of RANR……………………………………77 

Figure 4.9: The band-gap energy (hv) of RANR……………………………………………77 

Figure 4.10:   The thermogravimetric and derivative thermogravimetry analysis curves of 

RANR……………………………………………………………………………………….78 

Figure 4.11: Nitrogen adsorption/desorption isotherms of RANR…………………………79 

Figure 4.12:  BJH pore size distributions of the RANR…………………………………….79 

Figure 4.13: Response and recovery curves of RANR to methanol at different 

concentrations……………………………………………………………………………….82 

Figure 4.14: Normalized response of the RANR sensor as a function of methanol vapour 

concentrations……………………………………………………………………………….83 

Figure 4.15: Repeatability response curves for methanol…………………………………..84 

Figure 4.16: Static response characteristic of RANR based sensor towards methanol...85 

Figure 4.17: Static response and recovery curves of RANR to ethanol at different 

concentrations………………………………………………………………………………86 

Figure 4.18: Static responses curves of RANR sensor measured under series of ethanol 

vapour concentration………………………………………………………………………..87 



xvii 
 

Figure 4.19: Normalized response of the RANR sensor as a function of methanol vapour 

concentrations ………………………………………………………………………………87 

Figure 4.20: Repeatability response curves for ethanol…………………………………….88 

Figure 4.21: Transient response characteristic of RANR based sensor toward 385 ppm of 

ethanol vapour………………………………………………………………………………..89 

Figure 4.22: Response of RANR towards 77 ppm of ethanol vapour under different humidity 

conditions at 10, 33,51, 55, 64, 76 and 96%............................................................................90 

Figure 4.23: Stability of RANR sensor towards 77 ppm of ethanol vapour within 45 

days…………………………………………………………………………………………...94 

Figure 4.24: Static responses curves of RANR sensor measured under series of isopropanol 

vapour concentrations………………………………………………………………………...95 

Figure 4.25: Normalized response of the RANR sensor as a function of isopropanol vapour 

concentrations………………………………………………………………………………...96 

Figure 4.26: Static response and recovery curves of RANR toward acetone………………….97 

Figure 4.27: Normalized response of the RANR sensor as a function of acetone vapour 

concentrations………………………………………………………………………………...98 

Figure 4.28: Static responses curves of RANR sensor measured under series of acetone vapour 

concentrations………………………………………………………………………..............99 

Figure 4.29: Repeatability response curves towards acetone vapour………………………..100  

Figure 5.1: Exfoliation of bulk crystal of zirconium disulphide…………………………….108 

Figure 5.2: XRD of exfoliated ZrS2 nanomaterials…………………………………………110 

Figure 5.3: UV-Vis absorption of ZrS2……………………………………………………...111 

Figure 5.4: UV-Vis absorption of ZrS2 on exposure to ambient environment for five days…112 

Figure 5.5: Tauc plot of ZrS2 (a) Bulk (b) IPA exfol. (c) NMP exfol. (d) CHP exfol………113 

Figure 5.6: Raman spectra of bulk and exfoliated nanosheets of ZrS2 with different 

thickness…………………………………………………………………………………….114 



xviii 
 

Figure 5.7: AFM micrographs, height and spectrum of exfoliated ZrS2 nanosheets (a) ethanol 

(b) IPA (c) methanol and (d) water…………………………………………………………..115 

Figure 5.8:  SEM images of ZrS2 (a) and (b) bulk sample (c) IPA exfoliated (d) H2O exfoliated 

(e) MeOH exfoliated (f) EtOH exfoliated (g) CHP exfoliated (h) NMP exfoliated………..116 

Figure 5.9: TEM and HRTEM images of ZrS2 (Ai) and (Aii) CHP exfoliated samples (Bi) 

and (Bii) NMP exfoliated samples (Ci) and (Cii) IPA exfoliated samples (di) and (Dii) H2O 

exfoliated samples (Ei) and (Eii) MeOH exfoliated samples F (i) and F (ii) EtOH exfoliated 

sample………………………………………………………………………………………117 

Figure 5.10:  EDS of ZrS2 (a) bulk sample (b) NMP sample (c) CHP sample (d) MeOH 

sample (e) EtOH sample (f) IPA sample (g) H2O sample…………………………………..118 

Figure 5.11: Thermogravimetry and derivative thermogravimetry curves of bulk, CHP and IPA 

exfoliated sample……………………………………………………………………………119 

Figure 5.12: High resolution core level spectra of NMP exfoliated ZrS2 nanosheets with focus 

on O1S, C1S, S2P+Zr3d, N1s,valence and survey spectrum……………………………….121 

Figure 5.13: High resolution core level spectra of CHP exfoliated ZrS2 nanosheets with focus 

on O1S, C1S, S2P+Zr3d, N1s,valence and survey spectrum………………………………..122 

Figure 5.14: High resolution core level spectra of IPA exfoliated ZrS2 nanosheets with focus 

on O1S, C1S, S2P+Zr3d, N1s,valence and survey spectrum……………………………….123 

Figure 6.1: Schematic diagram of synthesis of PANI………………………………………..129 

Figure 6.2: XRD pattern of PANI-ZrS2 nanocomposite…………………………………….132 

Figure 6.3: TEM image of PANI…………………………………………………………….133 

Figure 6.4: TEM images of PANI-ZrS2 nanocomposite……………………………………..134 

Figure 6.5: (A) Nitrogen adsorption-desorption isotherm for PANI-ZrS2 (B): BJH pore size 

distributions of PANI-ZrS2………………………………………………………………….135 

Figure 6.6: (A) Nitrogen adsorption-desorption isotherm for PANI- ZrS2 (B): BJH pore size 

distributions of PANI……………………………………………………………………….135 

Figure 6.7: Schematic illustration of sensing mechanism of PANI-ZrS2…………………..137 



xix 
 

Figure 6.8: Dynamic response of PANI-ZrS2 towards methanol vapour at RT…………….138 

Figure 6.9: (A) Dynamic response curves of PANI-ZrS2 towards methanol vapour at RT (B) 

Response curves of the PANI-ZrS2 sensor taken under various methanol vapour 

concentration………………………………………………………………………………..139 

Figure 6.10: Normalized response of the PANI-ZrS2 sensor towards methanol, ethanol, and 

isopropanol…………………………………………………………………………………140 

Figure 6.11:  Typical response and recovery curves of PANI-ZrS2 sensor to ethanol at 77 ppm 

and 385 ppm respectively………………………………………………………………….141 

Figure 6.12: Dynamic response and recovery curves of PANI-ZrS2 towards ethanol vapour at 

RT…………………………………………………………………………………………..143 

Figure 6.13 Response and recovery characteristic curves towards methanol ……………..144 

Figure 6.14: Repeatability of the PANI-ZrS2 sensor to 385 ppm of ethanol vapour at 

RT…………………………………………………………………………………………..145 

Figure 6.15: Dynamic response and recovery curves of PANI-ZrS2 towards isopropanol vapour 

at RT……………………………………………………………………………………….146 

Figure 6.16: Comparison of PANI-ZrS2 sensor towards methanol, ethanol and isopropanol 

vapour at RT……………………………………………………………………………….147 

Figure 6.17: Static response and recovery curves of PANI-ZrS2 towards acetone vapour at 

RT………………………………………………………………………………………… 148 

Figure 6.18 :Normalized response of the PANI-ZrS2 sensor towards acetone vapour…….149 

Figure 6.19 :Response of PANI-ZrS2 towards (A) 111 ppm of methanol vapour; (B) 77 ppm of 

ethanol vapour under different humidity conditions at 33, 46,58, 64 and 76% for methanol and 

at 33, 51, 58, 75 and 96% for ethanol……………………………………………………...151 

Figure 7.1: PXRD of (a) nanocomposite of TiO2-ZrS2 and (b) TiO2 ……………………..159 

Figure 7.2: Raman of RANR and nanocomposite of ZrS2 and with ZrS2-TiO2…………..160 

Figure 7.3:  TEM of TiO2-ZrS2 nanocomposite…………………………………………...161 



xx 
 

Figure 7.4:  HRTEM of TiO2-ZrS2 nanocomposite…………………………………………161 

Figure 7.5: DRS-UV-Vis of (a) TiO2-ZrS2 and (b) TiO2……………………………….......162 

Figure 7.6: EDS of TiO2-ZrS2 nanocomposite……………………………………………...163 

Figure 7.7: (a) Nitrogen adsorption/desorption isotherms of TiO2-ZrS2 nanocomposite; and 

(b) BJH pore size distribution of the TiO2-ZrS2 nanocomposite……………………………164  

Figure 7.8: TGA of pristine RANR and TiO2-ZrS2 nanocomposite………………………...165 

Figure 7.9: Static response of pristine RANR towards 122 ppm acetone vapour at 

RT…………………………………………………………………………………………...166 

Figure 7.10: Static response of TiO2-ZrS2 towards 610 ppm acetone vapour at RT……167 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



xxi 
 

 

List of Tables 

Table 1.1: Classification of volatile organic pollutants ……………………………………….1 

Table 2.1: TMDCs, metal oxide and its nanocomposite for chemical sensing………………..28 

Table 2.2: Comparison of 2D TMDCs with Metal oxide-based sensors………………………33 

Table 4:1 Comparison of BET of hierarchical titania…………………………………………80 

Table 4.2: Comparison of different MOXs towards VOCs sensing………………………….92 

Table 6.1: Group IVB TMDCs and TMDCs nanocomposite with polymers for VOC 

sensing………………………………………………………………………………………149 

Table 7.1: The BET comparison between pristine RANR and nanocomposite of TiO2-

ZrS2…………………………………………………………………………………………163 

 

 

 

 

 

 

 

 

 

 

 

 



xxii 
 

 

Abbreviation Description 

Ammonium persulphate APS 

Polyanilime PANI 

Transition metal dichalcogenides TMDCs 

Interdigitated electrodes IDEs 

Metal oxide semiconductors MOXs 

Room temperature RT 

N,N dimethylformamide DMF 

N-methyl polypyrrolidone NMF 

Part per million ppm 

Relative humidity RH 

Volatile organic compounds VOCs 

Quantum dots QDs 

Polyvinylpyrrolidone PVP 

Printed-circuit board PCB 

World Health Organisation WHO 

Occupational Safety Health and 

Administration  

OSHA 

Two-dimensional 2D 

Radially aligned nanorutile RANR 

TEM Transmission electron microscopy 



xxiii 
 

SEM Scanning electron microscopy 

PXRD Powdered X-ray diffraction 

FTIR Fourier transform infrared spectroscopy 

XPS X-ray photoelectron spectroscopy 

DRS Diffuse reflectance spectroscopy 

UV-Vis Ultraviolet-visible spectroscopy 

BET Branauer-Emmett-Teller (BET) 

HAL Hole accumulation layer 

BJH Barrtt Joyner Halenda 



1 
 

CHAPTER 1: INTRODUCTION 

1.1 Background and motivation 

Volatile organic compounds (VOCs) can be simply defined as chemicals that have the ability 

to vapourize in ambient environment. These substances are omnipresent in both indoor and 

outdoor premises. The impact of indoor environmental contamination and pollution arising 

from VOCs has not been taken very seriously until very recently. The need for monitoring 

VOCs is very paramount since most people carry out their daily activities in enclosed premises. 

Statistics from World Health Organization (WHO) shows that close to 4 million of people die 

yearly due to indoor pollution yearly 1. Chemicals like acetone, ethanol and formaldehyde have 

been identified as common indoor pollutants. In a country like the United States of America, 

the environmental protection agency (EPA) regulates VOCs outdoor due to the possibility of 

formation of photochemical smog under precise conditions. According to WHO, organic 

pollutants are classified under three categories: very volatile organic compounds, volatile 

organic compounds, and semi volatile organic compounds. The classification is shown in 

Table 1.1. 

Table 1.1: Classification of volatile organic pollutants (adapted from WHO) 2. 

Description Abbreviation Boiling point range 

(⁰C) 

Examples of 

compounds 

Very volatile      

organic  compounds 

VVOC <0 to 50-100 Propane, butane, 

methyl chloride 

Volatile organic 

compounds 

VOC 50-100 to 240-260 Formaldehyde, d-

limonene, 

toluene, acetone, 

ethanol, 

isopropanol, 

hexanol 

Semi volatile organic 

compounds 

SVOC 240-260 to 380-400 Pesticides, 

plasticizers, and 

retardants 

 



2 
 

VOCs are generally available at home and work; it is inevitable not to be exposed to airborne 

VOCs. Common sources of VOCs include oil and gas field sites, industrial chemicals, fuel 

combustion such as petrol, wood, coal, natural gas, printing presses, pharmaceutical plants, and 

from solvents such as glues and paints. Industrial processes and emission from automobile 

vehicles have been reported as the main sources of outdoors VOCs. 

The era of nanotechnology has paved way for a rapid detection of volatile chemical compounds 

which have great advantages over the conventional methods of detection of chemical 

compounds such as gas chromatography (GC), ion mobility spectrometry (IMS) and reaction 

mass spectrometry. The development of nanotechnology has led to production of nanosensors 

such as breath analysers, E-nose and wearable nanosensors which provide a quick, affordable, 

harmless and a portable device for measuring VOCs with high sensitivity, good selectivity, 

ultra-fast response-recovery times and good repeatability. Chemical sensors find application in 

the following fields: healthcare-genetics, diagnostics, drug discovery, food processing, 

environmental and industrial monitoring, quality control, defence and security 1,3,4. 

Since the discovery of graphene, a two-dimensional (2D) material with great exotic properties 

and wide applications. But its application is not extended to optoelectronic applications due to 

lack of band gap. This have resulted into avalanche of other 2D nanomaterials among them are 

transition metal dichalcogenides (TMDCs) 5. Fascinating chemical and physical characteristics 

of 2D materials have made them to be explored in sundry applications such as thermoelectric 

device, field effect transistors (FET), photodetectors, solar cells, fibre lasers, optics, tribology, 

electrode materials, catalysis and sensors in the last two decades 5,6,7,8,9.  TMDCs materials 

represent a new set of sophisticated class of materials that possess a layered structure just like 

the clay structure, it may have single or few atoms depending on the thickness of the 

nanomaterial 10,11,12,13. A TMDC has a structure which is quite different from graphene.  A 

monolayer of TMDC is a three atoms stick which is made  of a layer of transition metal atoms 

such as Zr, Hf, Ti, Ta, Mo sandwiched between two planes of chalcogen atoms such as S, Se 

and Te 13,14. These materials have some fantastic characteristics that they exhibit when they 

exist as mono or few layered materials. 2D nanomaterials have been deployed as sensors 

because of their layered structures. They possess high surface area and special semiconducting 

features arising from the manipulation of their band gap makes them suitable in sensor 

application 15,16. 

 



3 
 

Unlike the group VIB members such as MoS2, WS2, MoSe2 and WS2 which have been 

investigated widely, few works are available on group IVB despite theoretical projections 

showcased about these nanomaterials as possessing astonishing properties. Theoretical 

speculation has proved TiS2 has a good potential for chemical sensing. It is believed that TiS2 

has more active sites for gas absorption than MoS2 owing to crystal structure and bonding that 

existed between the metal and the sulphur atoms 17. It was also predicted that monolayer ZrS2 

could find applications in chemical sensing, however ZrS2 with a defect such as S-vacancies 

will behave as a better sensor compared to ZrS2 with no defect 18. Presently, the application of 

ZrS2 been demonstrated as an optical coupled plasmon waveguide resonance (CPWR) sensor 

but not as a chemirestive sensor 19.  

 Decoration of polymers or semiconductor metal oxides with TMDCs could greatly enhance 

the sensor attributes arising from synergistic effects. Shokouh et al fabricated a nanohybrid of 

TiS2 and polyvinylpyrrolidone (PVP) for sensing ethanol vapour, the sensor displayed an ultra-

fast response-recovery times of 2 s and 60 s respectively. The delayed recovery time that is 

often associated with TMDCs has been eradicated by forming close interjections between the 

nanocomposite 17.  Qin et al made a nanohybrid of TiO2-WS2 by loading quantum dots of TiO2 

on WS2 nanosheets for sensing NH3 gas. The sensor displayed better response (17 folds) and 

recovery times, selectivity and stability compared to bare WS2 
20. 

1.2 Problem statement 

The poor or no sensitivity of metal oxide-based semiconductor sensors to chemical vapours or 

gases at room temperature has prevented their usage in detection of flammable and explosive 

gases; this has caused a great shift to two-dimensional (2D) nanomaterials.  Researchers are 

now investigating other 2D materials since graphene has properties that make it applicable in 

gas sensing.  The layered structure and large surface-to-volume ratio of 2D nanomaterials make 

them suitable for gas sensing, with a fascinating good sensor characteristic towards volatile 

organic compounds. Therefore, these nanomaterials have been explored recently for their gas 

sensing attributes. However, they still have drawbacks like poor sensitivity and recoverability. 

Several studies have shown formation of hybrid nanomaterials could greatly improve the 

performance of transition metal dichalcogenides in gas sensing. The construction of proper 

close interface between the composite will enhance the electrical and chemical functionalities 

of the sensor. Integration of TMDCs with metal oxides, polymers and carbon materials will 



4 
 

boost the sensor characteristics of TMDC-based sensors with a geometric advantage of the 

nanocomposite material. 

1.3 Aim and objectives 

The aim of this research is to synthesize nanocomposite catalysts of the transition metal 

dichalcogenide, ZrS2 with dandelion shaped TiO2 and polyaniline; and to evaluate their 

performance in the chemical sensing of volatile organic compounds at room temperature. 

To achieve the above-mentioned aims, the following were identified: 

1. Colloidal synthesis of ZrS2 nanomaterials using hot- injection and heat-up methods. 

2. Determine the optimal conditions for the synthesis of ZrS2 nanomaterials by varying the 

reaction time, coordinating solvents, temperature of reaction and concentration of precursors. 

3. Fabrication of few layered ZrS2 from the bulk crystal sample via liquid exfoliation with 

green solvents. 

4. Synthesis of dandelion shaped TiO2 by facile hydrothermal method. 

5. Synthesis of polyaniline (PANI) using facile chemical polymerization method. 

6. Functionalization of both the titania and polyaniline by decorating them with ZrS2 via 

ultrasonication. 

7. Characterization of the synthesized nanomaterials and heterostructures using techniques 

such as SEM, TEM, HRTEM, UV-Vis, AFM, FTIR, BET, TGA, Raman spectroscopy, XRD 

and XPS.  

8. Application of pristine and heterostructure nanomaterials in sensing of VOCs at room 

temperature. 

1.4 Thesis outline 

Chapter 1: Introduction 

This chapter enumerates the details of this research in topics. It provides insight into the 

background, problem statement, motivation, aim and objectives. 

 

Chapter 2: Literature review 



5 
 

This chapter discusses the current state of research with respect to advances in the synthesis 

routes of group IVB transition metal dichalcogenides and their nanocomposites with metal 

oxide or polymer for chemical sensing of volatile organic compounds at room temperature. 

Chapter 3: Colloidal synthesis of ZrS2 

The chapter is a journal article submitted to the Journal of Crystal Growth and is currently 

under review. The article reports on the novel synthesis of ZrS2 via the colloidal approach. The 

effects of using different sulphur precursors, different ligand and co-ligands were also 

investigated. The stability of ZrS2 was also probed. 

Chapter 4: Synthesis of rutile titania 

This chapter entails facile fabrication, characterization of radially aligned nanorutile titania and 

their applications for sensing primary alcohol and acetone gas. 

Chapter 5: Synthesis of ZrS2 nanomaterials 

This chapter demonstrates a facile top-down approach of obtaining few-layered ZrS2 via 

ultrasonication and its characterization. 

Chapter 6: Nanocomposite of PANI-ZrS2 and its applications in chemical sensing 

This chapter describes the fabrication of nanocomposite of ZrS2 and PANI, characterization 

and its application for sensing selected primary alcohol and acetone at room temperature. 

Chapter 7: Nanocomposite of TiO2-ZrS2 

In this chapter, the synthesis of ZrS2 and TiO2, characterization and its sensing potential towards 

volatile organic compounds is outlined. 

Chapter 8: This chapter presents a general conclusion from the research, as well as 

recommendation for future work. 

 

 

 

 



6 
 

1.5 References 

1.  Conti, P.P., Andre, R.S., Mercante, L.A., Fugikawa-Santos, L. and Correa, D.S., 2021 

Discriminative detection of volatile organic compounds using an electronic nose based 

on TiO2 hybrid nanostructures. Sensors Actuators, B Chem., 344, pp.130124. 

2.  McAughey, J.J.,  Pritchard,  J.N. and  Black, A., 1990. Risk assessment of exposure to 

indoor air pollutants. Environ Technol., 1990;11(4), pp.295-302.  

3.  Sharma, S. and Madou, Marc., 2012. A new approach to gas sensing with 

nanotechnology. Royal society, 370, 2448-2473.  

4.  Munawar, A., Ong, Y., Schirhagl, R., and  Tahir M.A., 2019.  Nanosensors for 

diagnosis with optical, electric and mechanical transducers. RSC Adv., 9, pp.6793-

6803. 

5.  Anichini C., Czepa, W., Pakulski, D., Aliprandi, A., Ciesielski, A., Samorì, P., 2018. 

Chemical sensing with 2D materials. Chem Soc Rev., 47(13), pp.4860-4908.  

6.  Pandit, A. and Hamad, B., 2021. The effect of finite-temperature and anharmonic 

lattice dynamics on the thermal conductivity of ZrS2 monolayer: Self-consistent 

phonon calculations. J Phys Condens Matter., 33 (42):1-23.  

7.  Mattinen, M., Popov, G. and Vehkamäki, M., 2019.  Atomic Layer Deposition of 

Emerging 2D Semiconductors, HfS2 and ZrS2, for Optoelectronics. Chem Mater., 15, 

pp.5713-5724. 

8.  Wen, Y., Zhu, Y. and Zhang, S. Low temperature synthesis of ZrS2 nanoflakes and 

their catalytic activity. RSC Adv., 5(81), pp.66082-66085.  

9.  Li, L., Lv, R. and Wang., 2019. Optical nonlinearity of ZrS2 and applications in fiber 

laser. Nanomaterials., 9(3), pp.315. 

10.  Choi, W., Choudhary, N., Han, G.H,, Park, J., Akinwande, D. and  Lee, Y.H., 2017. 

Recent development of two-dimensional transition metal dichalcogenides and their 

applications. Mater Today, 20, pp.116-129. 

11.  Tedstone, A. A., Lewis, D.J. and O’Brien, P., 2016. Synthesis, Properties, and 

Applications of Transition Metal-Doped Layered Transition Metal Dichalcogenides. 

Chem Mater., 28(7), pp.1965-1974.  



7 
 

12.  Shi, Y.,  and Li, L.J., 2017.  Synthesis of transition metal dichalcogenides. In: 2D 

Materials: Properties and Devices. Cambridge University Press, pp.344-358.  

13.  Yan, C., Gan, L., Zhou, X., et al., 2017. Space-Confined Chemical Vapor Deposition 

Synthesis of Ultrathin HfS, Flakes for Optoelectronic Application. Adv Funct Mater., 

27(39), pp.1-9.  

14.  Singh, A.K., Kumar, P., Late, D.J., Kumar, A., Patel, S.,  and Singh., J., 2018. 2D 

layered transition metal dichalcogenides (MoS2): Synthesis, applications and 

theoretical aspects. Appl Mater Today., 13, pp.242-270.  

15.  Kumar, R., Goel, N., Hojamberdiev, M., and Kumar, M., 2020. Transition metal 

dichalcogenides-based flexible gas sensors. Sensors Actuators, A Phys. 303, 

pp.111875.-111892. 

16.  Lee, E., Yoon, Y.S. and  Kim, D.J., 2018. Two-Dimensional Transition Metal 

Dichalcogenides and Metal Oxide Hybrids for Gas Sensing. ACS Sensors., 3(10), pp. 

2045-2060. 

17.  Hosseini-Shokouh S.H., Fardindoost, S.,  and Zad, A.I., 2019. A High-Performance 

and Low-Cost Ethanol Vapor Sensor Based on a TiS2/PVP Composite. 

ChemistrySelect, 2 4 (21), pp.6662-6666. 

18.  Nguyen, H.T.T. Hoang, D.Q., Dao, T.P. Nguyen, C.V., Phuc, H.V., Hiew, N.N., Hoat, 

D.M. Long, H.L., Tong, H.D. and Pham, K.D., 2020. The characteristics of defective 

ZrS2 monolayers adsorbed various gases on S-vacancies: A first-principles study. 

Superlattices Microstruct.,140, pp.88-104. 

19. Ma, J., Liu, K., Jiang, J., Xu, T., Wang, S. Chang, P., Zhang, Z.,Zhang, J., and Liu, T., 

2020. All optic-fiber coupled plasmon waveguide resonance sensor using ZrS2 based 

dielectric layer.Optica, 28(8), pp.11280-11289. 

20.  Qin Z., Ouyang, C., and Zhang, J., 2017. 2D WS2 nanosheets with TiO2 quantum dots 

decoration for high-performance ammonia gas sensing at room temperature. Sensors 

Actuators, B Chem., 253, pp.1034-1042.  

 

 



8 
 

CHAPTER 2: LITERATURE REVIEW 

2.1 Transition metal dichalcogenides for chemical sensing 

Among the new materials for sensor application, much attention is being given to two 

dimensional (2D) chalcogenide materials which have diverted a great research to explore them 

in the last few decades  1,2,3,4,5,6 . Transition metal dichalcogenides (TMDCs) materials represent 

a new set of sophisticated class of materials that possess a layered structure just like clay 

structure, it may have single or few atoms depending on the thickness of the nanomaterial 6,7,8,9.  

The shift to TMDCs emanated as a result of the exfoliation of graphene about two decades ago, 

a single-layered carbon with outstanding properties such as conductivity, mechanical, thermal, 

and electrical properties. Unlike graphene, a TMDC monolayer is three atoms thick consisting 

of a layer of transition metal atoms such as Zr, Hf, Ti, Ta, Mo sandwiched between two planes 

of a chalcogen atom such as S, Se and Te 9,10. There are weak van der Waals forces that exist 

in between the layers and covalent bonds within the materials. These materials have some 

fantastic characteristics that they exhibit when they exist as mono or few layered materials. 2D 

nanomaterials have been deployed as sensors as a result of their layered structures. They 

possess a high surface area and special semiconducting features arising from the manipulation 

of their band gap making them suitable in sensor application 2,3. TMDCs have some drawbacks 

which limit their application in gas sensing such as: (i) cross selectivity-the sensor may respond 

to many gases at a time, without being able to distinguishing one gas from the other (ii) TMDCs 

sensor response time is very slacking and the recovery time is often partial (iii) oxophilicity of 

TMDCs which refers to the tendency of the sensor surface to be somewhat topped by oxygen 

or moisture in ambient environment. This may slowly devalue the activity and stability of the 

sensor over time. 

Numerous methods have been used over the years to eradicate these drawbacks, which include 

novel metal doping and forming a junction with metal oxide semiconductors 3. In relation with 

TMDC materials, the monolayers and few-layers of hafnium dichalcogenides have astonishing 

physical properties speculatively, but  are rarely investigated 9,12,13,14. Results have 

demonstrated that, the calculated room temperature mobility for HfS2 and HfSe2 monolayers 

is about 5.3 and 10.3 times respectively; greater than properly investigated MoS2 
9,15. They also 

exhibited greater sheet current densities much higher than that of MoS2, this remarkable quality 

makes them valuable materials in FETs application 15. Notwithstanding these eminence in 

theoretical projection; correspondingly few experimental results are available presumably due 



9 
 

to the complexity in syntheses methods or as a result of oxophilicity of the nanomaterials 16,17. 

In the few published works, most of the studies use exfoliation method or chemical vapour 

deposition (CVD) method for their fabrication 14,18,19,20,21. The thickness and lateral dimension 

of the nanosheets cannot be manoeuvred with the use of  the mechanical exfoliation method 

15,18,19.  CVD is one the favourite methods of producing the group IVB TMDCs except for TiS2 

21,22. The method is economical, easy to carry out and produces a product with high quality; 

however, the CVD method still has some demerits. Ultrathin 2D TMDCs are mostly 

synthesized on substrates like mica, sapphire, SiO2 and hexagonal boron nitride (hBN) which 

require them to be conveyed to other substrates for further application 6,11,13,23. For application 

purposes, it is better that these nanosheets are in  discrete form than to be produced  on 

substrates 24. Hence, the solution method is still preferred for the synthesis of 2D TMDCs to 

CVD method.  

There are three common ways of preparing mono or few-layers TMDCs: (a) exfoliation, (b) 

chemical vapour deposition and (c) wet chemical/solution -based.  

2.2. Exfoliation 

This method of synthesis can be subdivided into two - mechanical and liquid exfoliation. It is 

employed in the fabrication of mono and few-layers of 2D layered materials. Mechanical 

exfoliation is a physical process which requires the use of the scotch-tape technique to separate 

off from the bulk of the material. The adhesive force of the tape helps to break the van der 

Waals forces that exist within the layers of the material. The isolated layer can undergo 

successive peeling to give mono and few-layers sample which is then deposited onto a 

substrate. This produces high quality single-layered nanosheets though with low output while 

liquid exfoliation is a solution-based method with great output. The first report on exfoliation 

of TMDCs from bulk samples was published in 2005 by Novoselov et al where mono layers 

of MoS2 and NbSe2 were isolated from the bulk of the material using scotch-tape 25. Group 

IVB TMDCs have been prepared using mechanical exfoliation of the bulk. The group IVB 

TMDC nanomaterials are easily affected in ambient environment and thus get oxidized 

26,27,28,29. To prevent their oxidation, exfoliation was carried out in a glove box and vacuum 

transfer chambers or immediate passivation with protective encapsulation layer was used in the 

synthesis 18,30,31. Chae et al fabricated few layers of HfS2 field effect transistor on a SiO2/Si 

substrate inside an integrated vacuum cluster system to prevent ambient oxidation. They 

realized uniform ambient oxidation of the HfS2 material, preferentially at defect sites which 



10 
 

resulted to thickness enlargement. The oxidized HfS2 performed poorly as a field effect 

transistor compared to the unoxidized sample 30. In a similar report Kanazawa et al synthesized 

few layer nanosheets from the bulk using scotch tape on single crystal HfS2, the small piece 

that was obtained on the tape, was then cleaved numerous times to obtain thin film of average 

size of thickness on SiO2/Si/Al2O3 substrate. The Al2O3 substrate surface was passivated with 

hexamethyldisilane (HMDS) before exfoliated HfS2 was transferred on it 32. In another similar 

work by the same group, HfS2 flakes were exfoliated on AlO3 substrate and the exfoliation 

method used suffered from the difficulty in manipulating the size and thickness of the 

fabricated TMDCs, as well as not being able to be scaled up for large batch production 26.  

Liquid exfoliation can be further subdivided into two main types. The one type is simple and 

does not involve intercalation. The bulk sample is dispersed into the appropriate solvent or 

surfactant followed by exfoliation through a sonication process. The second type is a two-step 

process with intercalation preceding the exfoliation process in a solvent 19,20,33,34. This method 

entails insertion of alkali metals into the bulk material with compounds like LiBH4, n-butyl 

lithium or organolithium compounds in solvent for 7-14 days at room temperature or at 100 ˚C 

for 3-4 days, succeeded by dispersing in an appropriate solvent. Care must be taken to ensure 

complete exfoliation, if not there is the tendency of generation of metal nanoparticles and Li2S 

being precipitated during the process. The lithiated layered material is recovered through a 

filtration technique and thorough rinsing with solvent such as hexane to eliminate lingering 

impurities of organic residues and the alkali metal. The intrinsic properties of the dispersing 

solvent or surfactants is  crucial which is to break the cohesive energy that exist in  the layered 

material and also determines the exfoliation output 33,35. The surface tension of the solvent 

needs to match the surface energy of the dissolved bulk sample. It is necessary to use the right 

solvent for dispersion of the bulk powder as this prevents re-stacking and aggregation of 

nanosheets in the solvent. Coleman et al reported that experimental parameters such as the 

starting mass of the bulk material, sonication period, centrifugation conditions and nature of 

solvent determines the concentration, thickness, lateral and broad size of the exfoliated 

nanosheets produced. An increase in concentration was favoured by increasing the sonication 

time (200 h) for MoS2, moreover, the nanomaterial had small lateral sizes and broad size 

distribution 20. The solvents that are commonly used are isopropanol (IP), dimethylformamide 

(DMF), hexane, N-methyl-cyclohexyl-2-pyrolidone (NCHP) and N-methyl-2-pyrrolidone 

(NMP) 35. NMP is the most suitable solvent for the fabrication of MoS2 but due to its toxicity 

and difficulty in obtaining free standing nanomaterial after sonication, its application is limited. 



11 
 

This has led to the use of other solvents such as aqueous solution or volatile solvents. Coleman 

et al used this method to prepare few-layers of some TMDCs and metal chalcogenides. For a 

solvent to be suitable for isolation of MoS2, it was observed its surface tension should be about 

40 mJm-2 20. However, when water is used as a dispersing agent for a material that hydrolyses 

in water, the nanomaterial gets oxidized. Traces of water have a tendency to oxidize group IVB 

TMDCs through hydrolysis, resulting  into the production of metal oxides (MOXs) 26,27,30,36. 

Sherrell et al gave an insight to the oxidation of TiS2. The group synthesized TiS2 nanosheets 

by insertion of alkali ions into TiS2 powder exfoliated in deionized water thereafter. It was 

observed that the exfoliated nanosheets were quickly destabilized by oxygen. The TiS2 

nanosheet suspension in water was oxidized to oxide of titanium. It first generated TiSO species 

as the intermediate product, at the same time H2S gas was liberated to the environment. This 

was evident, as a colour change was observed in the suspension, it first turned to grey and then 

to white within 7 days of observation 36.  This method is so popular in the synthesis of  single 

layers of MoSe2, WS2, MoS2, WS2, TaS2, TiS2 and ZrS2 
35. The obtained MoS2 and WS2 

nanosheets using this method were observed to undergo phase transition, it translated from 

semiconducting (2H) phase to non-semiconducting (1T) phase. A thermal annealing process 

was needed to reverse it to the semiconducting phase 37. The use of co-solvents has been 

developed to enhance the exfoliation process. For this, the use of Hansen solubility parameters 

(HSP) theory must come into play. Using HSP theory, a variety of a mixture of solvents have 

been explored to fabricate MoS2. Zhang et al exfoliated MoS2 nanosheets using a co-solvent 

of water and ethanol. In a similar way, MoS2 was also effectively exfoliated using a mixture of 

chloroform and acetonitrile 38.  Kaur et al synthesized few-layers of HfS2 by dispersing the 

bulk HfS2 in NCHP and ultrasonicated the solution. For the exfoliation in NCHP, the sheet 

formed was more stable in air as the solvent shielded the nanomaterial against ambient oxygen 

for a few days compared to using NMP and DMF as solvents 39.  Li et al synthesized ZrS2 

nanosheets by dispersing the bulk of the powder in IP, the suspension was then sonicated to 

give a few layers of ZrS2 
27.   

Zeng’s group were the first to report on simplified lithiation using electrochemical lithium 

intercalation method to produce single layers of MoS2, WS2, TaS2 and TiS2 by proper 

adjustment of the amount of lithium intercalated, which was followed by exfoliation in ethanol 

or water 40. In another related experiment, the same group executed a systematic study by 

manipulating circuit parameters such as voltage and current for the for production of few-



12 
 

layered inorganic compounds, such as TiS2, TaS2, WSe2, ZrS2, NbSe2, BiTe3, Sb2Se2 and BN. 

They optimized the parameters and produced high quality NbSe2 and BN nanosheets 40,41. 

Tandem molecular intercalation (TMI) is an improved method of exfoliation which is a facile 

singe step process that does not involve sonication, operated under safe and mild conditions. 

This method makes use of Lewis bases (short and long alkylamines or alkoxides) intercalates, 

in which short initiator molecules will be the first to intercalate, followed by long primary 

molecules. Group IV and V TMDCs are better synthesized using weak Lewis bases such as 

alkylamines while alkoxides are more appropriate for the synthesis of group IV TMDCs. 

Single-layered WSe2 has been exfoliated by intercalation of an alkali ethoxide and alkali 

hexanoate in dimethyl sulfoxide (DMSO) with agitation lasting for several hours at room 

temperature. This process is well accepted for the synthesis of TiS2, ZrS2, NbS2 and MoS2 
42. 

The use of surfactants or polymers has not been well explored like the use of solvents 

presumably due to the high cost or toxicity of some of these surfactants. Sodium chlorate is the 

common surfactant that is being used, it helps to coat the sheets in dispersion thus preventing 

agglomeration 43. 

2.3 Gas-Phase method of metal powder production 

Gas-phase method of synthesis is a bottom-up method of production of materials from unit 

atoms. Chemical vapour deposition and chemical vapour are gas-phase methods that are 

commonly being deployed in TMDC synthesis in particular group IVB TMDCs 17.  

2.3.1 Chemical vapour deposition 

Chemical vapour deposition (CVD) is a methodology that involves decomposition or chemical 

reactions of gaseous precursors through thermally induced means. Depending on the nature of 

material or the application, sometimes the use of subtrate may be introduced into the furnace 

onto which the product is formed. CVD is generally deployed as a bottom–up method for 

synthesis of various 2D materials in the last decade, especially the group IVB TMDCs 

6,11,13,44,45,46,47,48. In the synthesis, an inert gas (e.g., Ar) and H2 gas are introduced, which help 

to eliminate oxidation of the material, at the same time reduce formation of impurities 

9,13,17,23,49,50. 

 

 



13 
 

2.3.2 Chemical vapour transport  

 There are more reports on the use of chemical vapour transport (CVT) on the synthesis of 

group IVB TMDCs than mere CVD 11,51. CVT requires the use of halogens such as I2 as the 

transport gas and is used for bulk single crystal, which takes days for the synthesis to be 

completed. The synthesized materials are then exfoliated into single or few layered sheets 52.  

CVT is also used to synthesize transition metal trichalcogenides (MX3) which are then 

subjected to pyrolysis to give MX2 
27,53. Wen et al synthesized ZrS2 nanoflakes by reacting a 

stoichiometric ratio of elemental sulphur (S) and ZrCl4; S was added in excess due to its ease 

of evaporation at high temperatures. Various temperatures were evaluated, and in each case, 

the reaction was held for 1 h. ZrS3 was formed and later decomposed to ZrS2 on further heating. 

The optimal temperature was observed at 800 ˚C with no traces of impurities. 50. In a similar 

manner with little adjustments, Fu et al synthesized HfS2 by using HfCl4 powder and S powder. 

Both the metal precursor and S powder were placed upstream of the quartz tubes and heated at 

a temperature of less than 200 ˚C while the substrate was inserted in the hot zone at a 

temperature of about 930 ˚C. The heating was done simultaneously for a few minutes and then 

stopped. The unreacted precursors were immediately eliminated with the aid of magnets, after 

which the furnace was cooled down naturally 13. Zheng et al similarly synthesized HfS2 

nanoforests by placing the substrate in the hot zone; and the metal precursor and chalcogen 

precursors upstream. The heating was done at the same time, the temperature was set at 950 ˚C 

and 160 ˚C respectively, operated for 10 minutes and then terminated. At the end of the 

reaction, the furnace was rapidly cooled down 54.  Zhang et al comparably reported on synthesis 

of both ZrS2 and ZrSe2 by CVD at elevated temperatures above 800 ˚C synthesis. The reaction 

temperature was sustained for about 20 minutes and nanostructures of ZrS2 and ZrSe2 were 

grown on substrates 23.  Yan et al synthesized HfS2 nanoflakes using S powder and HfCl4 or 

HfO2 powder as precursors. The precursors were placed upstream and the substrate at 

downstream and the reaction was operated at 900 ˚C for just 10 minutes followed by cooling 

the furnace 9 . Shimazu et al synthesized a single crystal of ZrS2 by heating Zr, S8 and I2 in a 

sealed evacuated quartz operated for 3 days at 800 - 900 ˚C 11. TiS2 was also prepared by Gao 

et al using three temperature zones in which Ti/NH4Cl and S powder were placed at upstream 

where low temperature was applied, and the substrate was placed at downstream operated at 

450 ˚C 55. CVD is not commonly used for titanium dichalcogenide synthesis, they are often 

produced through solid-state reaction method. A mixture of titanium powder and sulphur 

powder are blended together before being transferred and sealed into an ampoule under a 



14 
 

vacuum. It is then calcined using muffle furnace for 12 h at 500 ̊ C. Afterwards, the temperature 

is  elevated to 800˚C and maintained for another 24 h after which the reaction is stopped and 

allowed to cool to room temperature 56,57,58. Fabrication of high quality 2D ultrathin TMDCs is 

very difficult as this method does not allow easy manipulation of reaction parameters. 

2.4 Wet chemical synthesis 

This method involves the use of surfactants or polymers in the solution during the synthesis 

process.  It is much easier for the fabrication of nanomaterials at low temperatures such as 130 

˚C and offers better control of kinetic parameters; unlike CVD which requires high 

temperatures of at least 400 ˚C and may run for several hours before the reaction gets to 

completion 59. The wet chemical synthesis method has four main variations which include 

colloidal, hydrothermal, sol gel and liquid exfoliation techniques. 

2.4.1 Colloidal synthesis 

Colloidal synthesis provides favourable merits, such as easy to direct and proper grip of the 

crystallinity, monodispersity and control over the edges of TMDCs 60,61,62. The method has 

been deployed for the production of quantum dots, metal nanoparticles, nanomaterials, etc. 

When applied for the synthesis of TMDCs, reaction variables which include reaction time, 

temperature, nature of the metal precursor, chalcogen precursor and the type of ligand used are 

crucial in determining the shape and size of the nanomaterial that is formed 59,63. This synthesis 

technique can be further subdivided into two: hot injection and heat-up (one-pot synthesis) 

59,64,65. Injection synthesis is often used when either or both the reactant(s) is/are solvent(s). 

The precursor that is in solid form is first dissolved in an organic solvent (ligand), then purged 

for about 20 minutes. The temperature of the system is raised at a controlled rate to a 

predetermined temperature, at this point the injection of the other precursor is introduced into 

the hot system by means of a syringe under vigorous stirring. The injection may be rapid or 

slow depending on the nature of the product required. One-pot synthesis is a non-injection 

technique which is commonly used when both metal and chalcogen precursors are solids. In 

this case the reactants are first mixed with the surfactant at room temperature before heating is 

introduced under inert conditions. Ligands that are commonly used in the colloidal synthesis 

of TMDCs include oleylamine (OLA), 1-hexadecylamine (HDA), oleic acid (OA), 

oleylalcohol (OYA), trioctylphosphine oxide (TOPO), dodecylamine (DDA), squalene, 1-

dodecanethiol (1-DDT), stearic acid, octadecylamine (ODA) and 1-octadecene (ODE) 64,65,66. 

The use of ligands in colloidal synthesis helps in controlling the morphology during the 



15 
 

synthesis of semiconductor nano-/micro- crystals by coordinating to the surface of the growing 

nanoparticles 66. This method of synthesis is quite popular because of its simplicity. The use of 

ligands has eased the synthesis of hierarchical structure based morphologies such as comb–

like, disk–like,  dendrite-like, snow flake-like, rod-like, flower–like and urchin–like structures, 

which show unique properties by combining the features of micrometer and nanometer 

building blocks in one crystal 65,67. 

The colloidal method has been employed in the synthesis of single or few-layered TMDCs. 

Caution is needed in the synthesis of group IVB TMDCs due to the oxiphilicity nature of single 

or few-layered nanomaterials to prevent contamination of the nanomaterials with metal oxides. 

There is a need to strictly avoid water or oxygen in the preparation of precursor solution. The 

chlorides of group IVB (TiCl4, ZrCl4 and HfCl4) are commonly used in the synthesis of these 

nanomaterials. These metal precursors are very hygroscopic; thus, they get hydrolysed while 

weighing in ambient air. Hence, weighing should be done in a glove box. Prabakar et al 

reported on simple colloidal synthesis of hierarchical structures of TiS2 using hot injection 

method by dissolving elemental S in non-coordinating ODE and at 300 ˚C,  TiCl4 was injected 

and heated up for 15 minutes. The metal precursor was injected into the system at a lower 

temperature of 150 ˚C to produce a flake-like structure unlike the flower-like structure that was 

observed at the higher temperature 68. In 2008, Park et al synthesized mono layers of nanodisks 

TiS2 using OLA in dried form; S powder was dissolved in OLA at 110 ˚C, the mixture was 

brought to room temperature. Thereafter, titanium was injected, and the temperature slowly 

raised to 215 ̊ C under argon gas, the reaction lasted for 12 h. The sample obtained was vacuum 

dried. On exposure of the nanomaterial to air, the colour changed from black to brown; 

however, when it was kept refrigerated under nitrogen, the colour remained unchanged. The 

thickness of the TiS2 nano-disk was about 0.6 nm and a lateral size of 50 nm. Increasing the 

concentration of the chalcogen atom with a rapid rate of temperature increase helped control 

the lateral size of the nano-disks. When the chalcogen concentration was increased by 100%, 

a decrease in lateral size was observed from 50 nm to 34 nm. At the same time, when the 

concentration of sulphur was increased by a double fold, there was reduction in the lateral size. 

However, the synthesized TiS2 single layer nano-disks were easily destabilised at room 

temperature, the S atom was displaced by oxygen atom in air. The authors were able to prove 

this using time independent energy dispersive spectroscopy (EDS) and powder XRD 69. In 

related work, Cheon’s group synthesized TiS2 by dissolving TiCl4 in OLA followed by 

injection of CS2 into the reaction mixture. The reaction was maintained for 15 min at 300 ˚C. 



16 
 

Increasing the concentrations of both metal and chalcogen precursors was so pivotal in 

controlling the lateral size of nanomaterial formed. When the concentrations were increased to 

2.4 times, it produced TiS2 of lateral size of 40 nm. The group changed the S source to S 

powder; one pot synthesis was deployed to produce TiS2 nanocrystals. The authours preferred 

the use of CS2 over elemental S based on production of highly reactive radicals by S powder 

which resulted in structural degradation of the nanocrystals produced. Presumably, it is better 

to use a chalcogen that contains carbon for the synthesis of group IVB TMDCs as the carbon 

can react with any traces of metal oxides formed during synthesis. The method was also 

employed in the synthesis of TiSe2 by using elemental Se as source of chalcogen. Not much 

has been reported in open literature on group IVB, in particular on hafnium dichalcogenides. 

Few reports are available on colloidal synthesis of HfS2. Cheon’s group reported on its 

synthesis, in which the reactants HfCl4, 1-DDT and OLA were heated up in a reaction flask in 

an inert atmosphere at 245 ˚C for the duration of 10 h 70. In a similar work, the group also used 

CS2 as a S precursor at higher temperature of 320 ̊ C 60. The same group also used a resembling 

synthesis procedure to synthesize sulphides and selenides nanosheets of the IVB and VB 

groups. To date, the studies by the Cheon’s group are the only ones available on ZrS2 synthesis 

70,71,72,73. They distilled and degassed OLA prior to use in order to purify it. However, they did 

not mention the oxiphilicity and the ease of oxidation of group IVB TMDCs. In 2014, Cheon’s 

group demonstrated the use of slow decomposing chalcogen precursors (1-DDT) and H2S gas 

in the generation of single–layers nanosheets of Group IVB metal sulphides. At high 

temperatures (over 150 ˚C) 1-DDT releases S atoms gradually. In the synthesis of HfS2 

nanosheet a combination of HfCl4, OLA and 1-DDT were heated up in a reaction flask in an 

inert atmosphere at 245 ˚C for 10 h.  The 1-DDT slowly decomposed to H2S during the entire 

synthesis time. Using the same procedure while differing the temperature and time of synthesis, 

ZrS2 and TiS2 nanostructures were also produced. The group also used H2S gas in a similar set-

up; the H2S was released slowly over 6 h and monolayers of TiS2, ZrS2 and HfS2 were obtained. 

A high level of precaution needs to be exercised when using H2S due to its toxicity which can 

be fatal. The nanocrystals obtained were of a poorer quality compared with those obtained with 

the use of 1-DDT 74. Different S precursors can be used, such as elemental S, 1-DDT, CS2, 

diphenylurea, thiourea and thioacetamide. CS2 and 1-DDT can be systematically introduced 

into the heating system using hot injection protocol while other precursors that are in solid form 

can be dissolved in the solvent prior to heating up with the metal precursor. CS2 is often used 

for hot injection as a result of its in-situ hydrogen sulphide generation. A metal chloride is 



17 
 

commonly used as the metal precursor and there is a need to wash the synthesized 

nanomaterials thoroughly due to lingering impurities such as chloride ions. 

Similar to group IVB, not much work has been reported for group VB.  Sekar et al was the first 

to report on synthesis of group VB TMDCs. The group synthesized NbSe2 nanowires using 

one- pot synthesis. Into a three-neck reaction flask, a mixture of OLA, DDA and the precursors 

(NbCl4 and Se) were introduced. Temperature of the reaction was increased to 280 ˚C under 

N2 atmosphere and sustained for 4 h. The reaction vessel was not cooled down and 

nanomaterials obtained were rinsed with hexane. It was then subjected to heating under N2 

atmosphere at 450 ˚C for a period of 3 h. The resulting NbSe2 wires had diameters varying 

from 2 - 25 nm. When the reaction vessel was cooled to room temperature before washing, 

nanoplates of NbSe2 were obtained. The nanoplates had lateral dimensions that range from 

500-1000 nm and thickness between 10-70 nm. The authors also varied the reaction parameters 

in order to achieve different sizes or morphologies, but no changes were observed 75.  Mansouri 

and Semagina reported on the synthesis of NbS2 using a mixture of ligands to produce different 

morphologies varying from nanosheets, nanospheres, nanohexagons and nanorods. Increased 

amounts of OA in OLA led to more production of the nanosheets, but better morphology and 

laterally confined 2D nanostructures were obtained with minimal use of OA. Also, of great 

importance is the timing of the reaction, with 0.25 h of reaction, monolayer of NbS2 nanosheets 

were formed with OLA as the sole solvent and as the reaction  time was increased few-layers 

of nanosheets were formed 61.  Han et al synthesized NbS2 by a one- pot synthesis method in 

which stoichiometric ratios of NbCl5 and 1-DDT were introduced into a three-neck reaction 

flask containing OLA in a glove box. The use of glove box indicated that the nanomaterial is 

sensitive to impurities (O2/H2O). The reaction was operated for 30 minutes at an elevated 

temperature of 280 ˚C after which it was cooled down. The authors do not mention if the 

reaction was conducted in an inert atmosphere. The synthesized ultrathin triangular NbS2 

nanosheets had a thickness of 3.9 nm which was ascribed to represent five layers 37.  

A great number of research has been reported on group VIB TMDCs most especially on MoS2 

and WS2 which have been demonstrated in sensor application. Mahler et al synthesized both 

prismatic 2H-WS2 and distorted octahedral 1T-WS2 structures 76. Hot injection reaction method 

was deployed, the metal precursor was added to OA in a vial, and the injection was made into 

a reaction flask containing OLA at 320 ˚C dropwise for about 0.5 h. Prior to attaining this 

temperature, CS2 was injected into the system. This resulted in controlled monolayers by the 

slow release of the precursors. The addition of HMDS after degassing helped in tuning the 



18 
 

crystal structure of the nanosheets from prismatic to distorted octahedral structure which was 

flowerlike in shape. In a similar report, Geisenhoff et al synthesized WSe2 using the hot 

injection method whereby tungsten hexacarbonyl was dissolved in  combination  with TOPO 

and OA  and selenide precursor 77. The mixture was injected at 330 ˚C and the process was 

completed in half an hour. The ligand mixture helped to adjust the precursor reactivity and an 

increased amount of OA limited the metal precursor reactivity, resulting in fewer nucleation 

and thus bigger nanocrystals were formed. Lin et al  prepared MoS2 quantum dots by dissolving 

the single-source precursors, ammonium tetrathiomolybdate ((NH4)2MoS4) in three different 

capping agents, OLA, OA and ODE 78. The reaction mixture was purged at 120 ˚C under 

vacuum for 2.5 h with stirring. Afterwards the reaction was sustained for 3 h at a temperature 

of 250 ˚C under N2 atmosphere before the reaction was quenched.  In a related experiment, 

Altavilla et al prepared both MoS2 and WS2 nanosheets using thio-salts of Mo and W by a one-

pot synthesis method. The single-source precursor and OLA was first degassed, and then the 

temperature was raised to 360 ˚C under N2 flow for 0.5 to 15 h. Interestingly, as the reaction 

time increased so did the number of layers produced 78. Antunez et al, in a similar experiment 

produced WSe2 nanosheets by injecting di-tertbutyl diselenide (tBu2Se2) into a reaction vessel 

already containing WCl4 in DDA at a temperature above 100 ˚C under N2 gas. The reaction 

lasted for 6 h under strong magnetic stirring at 225 ˚C. WCl4 is not a suitable metal precursor 

for the synthesis of group VI TMDCs, due to its ease to hydrolyse in air, thus the sample was 

introduced into a three-neck round-bottom flask in a glove box. It is well established that WCl4 

is easily reduced in the presence of organics if overheated, hence the temperature during 

heating must be well controlled. The organic solvent (DDA) was not used as supplied but was 

deoxygenated and distilled before use 79. Jung et al, used a one- pot method to synthesize WSe2 

monolayer nanosheets. W(CO)6, diphenyl diselenide were dissolved in OA, the system was 

degassed, and the temperature was raised to 330 ˚C, the reaction was operated for 12 h. 

Monolayer nanosheets with a lateral dimension of several nanometres were produced. The 

group also worked with other surfactants such as OYA and OLA. With OYA, few-layers (2-3) 

with lateral size of few nanometers were obtained while with OLA multi-layers with smaller 

lateral size were formed 80 . Zhou et al working with mixed surfactants, explored the influence 

of mixed solvents to produce different layers of the nanostructures of MoS2 and WS2 using 

injection protocols. The authors used Mo(AC)2 and W(CO)6, the surfactants stearic acid, 

TOPO, ODA and squalene under N2 atmosphere with stirring. The duration of heating was just 

1 h before being quenched. In a similar reaction, MoS2 monolayers were also generated but 

with a change of surfactants (OA, stearic acid and TOPO). By increasing the concentration of 



19 
 

the chalcogen source and decreasing the concentration of the metal as well as increasing the 

temperature of the reaction multi-layers (3-5) of WS2, nanosheets were formed. With little 

modification, both thioacetamide and Mo precursor were injected into the reaction system at 

different times to produce few and multi-layers MoS2 nanosheets 81. 

The mechanism for the formation of single-layered nanosheets of TMDCs is still complex. 

Three approaches variables have been employed to optimise their formation: firstly, the use of 

chalcogen precursor that gradually decomposes over a long-time during synthesis or delay 

injection of reactant for a long period, secondly, the nature of the organic solvent; and lastly 

the duration of the reaction have been varied. 

2.4.2 Hydrothermal or solvothermal synthesis 

Hydrothermal or solvothermal method is a versatile and effective synthetic route to produce 

nanomaterials with a different array of morphologies. Hydrothermal synthesis is one of the 

most important methods for producing fine powders of metal oxides. The process entails the 

reactants being dissolved in a solvent, which is then introduced into an autoclave. If the solvent 

for the reaction is non-aqueous, it is referred as solvothermal; whereas, if the solvent for the 

reaction media is water, it is termed hydrothermal 82. Teflon-lined autoclaves are used in this 

process; they are preferred over glass and quartz autoclaves, since they can tolerate high 

temperatures and pressures. Furthermore, they support alkaline solutions as well as being 

resistant to hydrofluoric acid. The flexibility of the method makes it easier to manipulate 

reaction parameters to produce nanomaterials with desired properties and quality. It has been 

extensively used for preparing metal oxide nanoparticles, chalcogenide, and phosphide 

nanomaterials 83,84,85. Chen and Fan synthesized NiSe2, NiS2, CoS2, FeS2, MoSe2 and MoS2 

using hydrothermal synthesis at a low temperature while varying the synthesis parameters. 

They found that adjusting the reaction variables could extend the method of synthesis to other 

products 83. The method is commonly used for the synthesis of group VIB TMDCs and their 

nano composites 86,87,88,89,90. Huang et al synthesized MoS2 nanosheets with a net-like 

morphology of well linked nanoflakes via the hydrothermal route; the nanoflower material had 

a thickness of a few nanometres. The prepared material had a large surface area and good 

conductivity giving it potential for application in high-performance supercapacitors 88.  Jang et 

al also prepared CdS nano particles using hydrothermal method as well as CdS nano wire/TiO2 

nano particle composite using solvothermal method to produce effective photocatalysts 91.  

 



20 
 

2.4.3 Sol-gel method 

The sol-gel method is a popular avenue to fabricate metal oxide catalysts such as oxides of Ti 

and Si composites. A variety of materials which includes nanostructures, nanomaterial 

nanoparticles, glass, ceramics, and nanocomposite are generally fabricated using this method. 

This process generally takes place in three steps viz: hydrolysis, condensation, and drying.  Sol-

gel can be sub-divided into two types: aqueous sol-gel and non-aqueous sol-gel method 82,92. 

To synthesizie these colloids, the common precursors are made of metal alkoxides and 

alkoxysilanes. The use of tetramethyoxysilane (TMOS), and tetraethylorthosilicate (TEOS) is 

most common. There is a need to first make a homogeneous solution of the alkoxides to be 

used92. This method is very suitable for production of group VIB TMDCs and their 

nanocomposites 93,94,95,96,97,97. The method offers some merits such as low synthesis 

temperatures, high reproducibility, cost effectiveness and products with high purity, porosity, 

and large surface 98. A one-step direct sol-gel synthesis method was employed to prepare p-

type few-layer MoS2 films in a large volume via deployment of Mo-containing sol-gel 

including 1% tungsten 94. It is very functional and good for large production using spin coating 

deposition method on a variety of substrates to produce 2H-MoS2 thin film having uniform 

surface area at moderate temperatures (300-400 ˚C) followed by annealing. The thin films 

MoS2 produced was of good quality and has great electronic properties with a narrow energy 

band gap of 1.35 eV which is consistent with the material. The product is of n-type 

semiconductor which finds application in electronic devices 95. 

2.5 Gas sensing mechanism in TMDCs 

 Semiconductor metal oxide sensors are popularly known for sensing of gases in the last 

decades especially toxic and dangerous gases, however the need of elevated temperature for 

their operation has inhibited their application in detection of flammable and explosives gases. 

The discovery of graphene, a 2D material for gas sensing has led to the emergence of TMDCs 

for gas sensing. TMDCs provide good selectivity, high sensitivity at room temperature due to 

ultrathin nature, high surface areas and adjustable bandgap which can find application in the 

detection of these gases. The key principle in gas sensing in TMDCs is principally based on 

charge-transfer activity connecting gas molecules and the immediate environment of the sensor 

which leads to a change in the electrical conductivity of the sensor. Electrostatic force is 

responsible for adsorption of gas molecules upon injection of the analyte gas on the 2D 

material. When air is flushed in the test chamber, desorption of the gas molecules takes place; 



21 
 

in this case there is restoration of the sensor to its original state. Figure 2.1 shows the 

mechanism of gas sensing of a typical TMDC with NH3 gas. The kind of interaction that exists 

between a gas analyte and sensing material will decide the level of the resistance adjustment.  

Based on the charge carrier of a semiconductor, it can be categorized as p-type or n-type. With 

a p-type semiconductor there is an increase in the resistance of the sensor upon interaction with 

a reducing gas whereas there is reduction in the resistance of the sensor with n-type 

semiconductor. The charge carriers on a p-type semiconductor are principally holes while it is 

electrons with n-type semiconductor. Due to high electron mobilities, TMDCs are proficient in 

sensing at room temperature and do not suffer from constant drift of signal which is typical of 

metal oxide occasioned by forwards and backwards distortion of oxygen vacancies since there 

is no oxygen atom present in the matrix of the sensor 2,3,5. 

 

Fig 2.1: Schematic illustration of sensing mechanism of TMDCs. The illustration is adapted 

from Gqoba et a l 99. 

2.6 Attributes of a gas sensor  

Several parameters are very important in determining the performance of a sensor which 

include response and recovery time, the 3S (sensitivity, selectivity, and stability of the sensor), 

operating temperature, concentration of analyte, humidity, and detection limit. 

2.6.1 Response  

The response is obtained by calculating the ratio of difference in conductance change of the 

analyte and in air and conductance in air. Mathematically this can be expressed as  

e-

e-



22 
 

R=
𝑅𝑔−𝑅𝑎

𝑅𝑎  
=

𝛥𝑅

𝑅𝑎
                                                                                         (2.1) 

R=
𝑅𝑔

𝑅𝑎
                                                                                           (2.2) 

R=
𝑅𝑔−𝑅𝑎

𝑅𝑎  
x 100 =

𝛥𝑅

𝑅𝑎
x 100                                                                                                     (2.3) 

R=
𝐼𝑜−𝐼𝑔

𝐼𝑔  

x 100 =
 𝛥𝐼

𝐼𝑔
x 100                                                                                                         (2.4) 

Reports have shown that the response and humidity have direct correlation. At extremely high 

humidity level, water vapour is adsorbed on the surface of the sensor by physisorption which 

may affect the electronic and ionic conducting properties of the sensor. The adsorbed water 

layer could serve as a barricade between the sensor surface and the analyte. Thermal treatment 

can be deployed to eliminate the humidity; however, the heating effect may result into 

agglomeration of the nanostructures. Reports have shown that acetone sensors perform poorly 

in terms of response at high temperature arising from thermal ripening of the sensor 100.    

2.6.2 Response and Recovery Times        

The response time can be defined as the time it a takes a sensor to attain 90% saturation upon 

injection of analyte gas. The recovery time is the time taken to bounce back to 90% of no load 

upon withdrawal of the analyte gas and consequently blowing dry air to the sensor.  A good 

sensor is expected to have rapid response and recovery times 100.  

2.6.3 Selectivity / Specificity    

The pragmatic use of a sensor lies on this key parameter, the selectivity of a sensor estimates 

the preferential ability of a sensor to detect analyte gas of interest in the presence of unwanted 

gases. The value ranges from 0 to 1, the closer the value to 1 is an indication of high selectivity 

of the sensor 99. 

2.6.4 Detection limits 

This can be defined as minimum concentration that can generate a signal response from a 

sensor. Two methods are commonly deployed in determination of LOD. Firstly, this is the most 

widely used method in which repetitive measurements are conducted without any introduction 

of any analyte gas and with introduction of minimum concentration of the analyte gas. 



23 
 

LOD = LOB + SD low concentration of analyte                                                                        (2.5) 

Where LOD = Limit of detection 

           LOB = Limit of blank 

           SD = Standard deviation 

The second method entails obtaining LOD by extrapolation from a linear calibration curve 101. 

A good sensor should be able to detect a very low concentration of gas analyte. This is often 

reported as part per million (ppm) or part per billion (ppb). 

2.6.5 Stability  

This is another key factor which determines the reliability and dependability of a sensor. It 

indicates the effectiveness and consistency of the sensor over a period. A good sensor should 

have a good stability close to 2-3 years and be recyclable. 

2.6.6 Sensitivity and grain size 

The size of sensing material plays an important role in its sensing capability.  A reduction in 

the grain size contributes to considerable enlargement in surface area and hence more surface 

sites for adsorption of analyte to react. This will therefore boost the sensing properties of the 

sensor 102.  No model has been proposed for TMDCs however, three different models have 

been developed for a metal oxide sensor. Firstly, if the grain size (D) also known as crystallite 

size is much bigger than space-charge layer (L) D >> L, the large grain does not contribute to 

improved sensitivity. The electrical conductivity is governed by grain boundaries which control 

the inner mobile charge carrier and the gas mechanism. The second model suggested that when 

the grain size reduces, the L moves closer into each of the grain. When grain size is very close 

to but still higher than 2L, the depletion region around each neck produces conduction channels 

as shown in Figure 2.2. Both the cross-sectional area and grain boundary barriers determine 

the electrical conductivity. The sensitivity of the gas sensor depends on the grain size when D 

≥ 2L. Thirdly, when the grain size undergoes reduction to 2 folds then D < 2L, the entire grain 

is depleted by the electrons, hence there is reduced conductivity as shown in Figure 2c. 

According to Dmorch et al the third model (D < 2L) is the most suitable system to improve gas 

sensing capability. Seal and Shukla reported using SnO2 as a case study; when the grain size is 

less than ⁓10 nm, this leads to better sensitivity for SnO2 sensor 102. Henceforth, a nanoparticle 

material is expected to display higher sensitivity compared to coarse-grained peer. Both the 



24 
 

receptor and transducer functions are influenced by grains. Therefore when the crystallite 

decreases, the higher the sensitivity of the sensor 103,100. 

 

 

Figure 2.2: Schematic highlighting the importance of grain size. Adapted from Dmorsh et al 1 

00. 

2.7 Factors controlling sensitivity 

Three important separate characteristics determines the response of a chemical sensor which 

are receptor, transducer, and utility 104.   

2.7.1 Receptor 

This entails how individual grains of a sensor interact with immediate atmosphere 

accommodating oxygen and target gas, this influences its rate, selectivity, and reversibility. 

Adsorption only occurs as a result of high reactivity of the sensor surface. The absorption can 

either be by physisorption and chemisorption 104,105. 

2.7.2 Tranducer 



25 
 

A transducer is a device that processes energy from one form to a readable signal. A tranducer 

function shows the capability of the sensor to translate the signal arising the interactivity 

occurring on the sensor surface into an electrical signal. A sensing material possesses a receptor 

and transducer functions, the boundary that exists within the grains plays this role of transducer 

function 104. 

2.7.3 Utility factor 

This is related to the diminishing of the response of a sensor arising from factors of diffusion 

and penetration of the target gas across the pores of the bulk. This explains how the target gas 

migrates into the inner grains of the sensor material. When the reaction rate is higher compared 

to diffusion this leads to decrease in response, this results from inability of the target gas to 

reach inner sites of the grains of the sensor material 104. Figure 2.3 illustrates the relationship 

that exists among these three factors. 

 

 

 

 

Figure 2.3: Three factors determining the response of semicomductor gas sensor. Adapted 

from Yamazoe and Shimande 104. 

                                                            

Utility factor (assembly issue)Transducer  function 

(interparticle issue)

Receptor function 

(interparticle issue)

O- O-

O-O-

Space charge layer

eH
2
O

H
2

Double Schottky barrier (?)

Gas diffusion and reaction



26 
 

                                                                                                                                                                                                                                                                              

2.8 Gas sensing properties of 2D TMDCs and other 2D materials  

The group VI TMDCs are recently being applied in chemical sensing, they have the advantage 

of being able to respond to chemical vapour at room temperature. More studies have been 

reported on MoS2 compared to WS2. MoS2 has been demonstrated to sense NO2, NO, O2, NH3 

and H2 at room temperature. Most of the early studies on the use of MoS2 as chemo-resistive 

gas sensor were conducted above room temperature. 

He et al reported on how adjustment of the number of layers can play a key factor in their 

sensing capabilities. It was observed that the thickness (layer) of the TMDCs plays a key role 

in determining the response to NO2 gas. The response of the sensor decreases on exposure to 

the gas with increasing thickness of MoS2, this is because of decrease in surface -to-volume 

ratio of the MoS2. The authors also reported better response of MoS2 over rGO based sensor. 

Integration of Pt nanoparticles with MoS2 nanosheets improved the sensitivity by a multiple of 

three 105 Late et al provided a contrary view in their studies where the five layered MoS2 activity 

was better than fewer layers (2-4). The insight of the mechanism responsible was not properly 

expantiated; it was merely attributed to electronic structures 106.  Cho et al vertically grew MoS2 

on SiO2 substrate using the CVD method. Three different arrangements were fabricated; 

vertical, horizontal, and mixed alignments. The vertical orientation’s performance was better 

than other alignments; this is ascribed to both the basal plane and edge sites providing more 

adsorption sites thus, better response 107. Kim et al produced microflakes and nanoparticles of 

MoS2 using mechanical and liquid exfoliation respectively. The nanoparticles, due to larger 

surface area and more edge sites than microflakes, produced better response to various O2 

concentrations. The n-type behaviour of the MoS2 sensor is akin to metal oxide semiconductor 

in which pre-adsorbed O2 molecules is very paramount in the sensing mechanism. The 

sensitivity of the MoS2 sensor (liquid) for ethanol was high because of enormous physisorption 

of O2 molecules on the MoS2 
108.  

Qin et al   exfoliated bulk WS2 into few layers; the number of the layers plays a key role in the 

determination of the sensitivity of the sensor. With high number of layers, the response 

increases but poor recovery. This delay in recovery was expantiated using first principle 

calculations. With increasing number of layers, the binding energy that exists between the gas 

molecules and the layer(s) increases hence it leads to longer recovery times (desorption of gas 

molecules) 110. Cha et al functionalized surface edges of WS2 using multiple tubular carbon 



27 
 

nano-fibres (WS2@MTCNFs) which resulted into greater specific surface area. The responses 

increase with increasing concentration of the gas molecules. The sensor sensitivity was quite 

good toward NO2 than both NH3 and C7H8 
111

.  Several publications have shown WS2 sensors 

are very sensitive to NH3 vapour: Gqoba et al synthesized hierarchical nanoflowers of WS2. 

The selectivity of the sensor toward increasing concentration of NH3 vapours was good. The 

response of the sensor towards acetone, ethanol, toluene, and chloroform was quite rapid as 

well, the hierarchical nanoflowers’ morphology provided large surface area for the adsorption 

of the gas molecule. However, the sensor displayed poor recovery due to delayed desorption 

of the NH3 molecules after strong adsorption on the sensor 99.  

Late et al demonstrated single layer MoSe2 for NH3 sensing at room temperature with quick 

response (150 s) with increasing concentration of NH3 vapours but poor recovery (540 s). 

Despite poor recovery from the sensor, studies showed the performance is far better than some 

reported work on graphene and MoS2 sensors for sensing NH3 and NO2 at room temperature 

112. In a related experiment, Guo et al exfoliated few layers WSe2; the fabricated sensor 

exhibited fantabulous sensitivity as low as detecting 0.05 ppm of NO2 gas with a rapid response 

time and recovery 113. 

SnS2 is a marvellous 2D material which is not part of the TMDCs, however it has a hexagonal 

structure akin to TMDCs with a layered structure. The material has a sizeable band gap when 

in few layers and large electronegativity that seems suitable for gas sensing application. In 

2006 Shi et al was the first  to demonstrate flower-like SnS2 synthesized using the hydrothermal 

process; the  sensor had good sensitivity and selectivity for NH3 gas at room temperature with 

an average response and recovery time of 45 and 110 s respectively 114. Xion et al  produced 

SnS2 nanoflowers using the solvothermal method. The hierarchical three-dimensional 

nanoflowers on exposure to 100 ppm NH3 produced  optimum sensitivity at 150 ˚C, though 

with poor response/recovery time, however optimum response/recovery time was gotten at 200 

˚C. Oxygen catalysed the performance of the sensor; with increasing oxygen content (0-40%) 

on the sensor, this improved the performance of the sensor in about 3.5 folds. The DFT 

calculations were used to corroborate on the effect of the increasing oxygen content on the 

sensor performance. The sensor was able to determine concentrations of NH3 as low as 0.5ppm 

and the best selectivity was toward NH3 over CO2, CH4, H2, C2H5OH and CH3COCH3 
115

. Wang 

et al demonstrated single layer SnS2 obtained via intercalation liquid exfoliation. The 

monolayer SnS2 sensor displayed a quick response (16 s) and good sensitivity towards 



28 
 

ammonia vapour. The response time was excellent, which is quite better compared to other 

NH3 gas sensors reported so far however, the author did not report on the recovery time 116. 

Guidi et al were able to demonstrate the of use of CdS and SnS2 and their metal oxide 

counterparts for sensing alcohols, acetone, and other chemicals. The CdS and SnS2 sensors 

were operated at room temperature with the aid of UV light as well as optimum temperature of 

300 ˚C. The performance of the metal sulphide and SnS2 sensors outweigh the metal oxide 

sensors, the CdS sensor displayed great selectivity for alcohols while no selectivity from CdO. 

The highest selectivity was to butanol, followed by ethanol and methanol was the least. The 

SnS2 showed good selectivity to acetone vapour. The operating temperature was maintained at 

300 ˚C. If exceeded, the S atoms and sulphur vacancies in SnS2 will be displaced with oxygen 

atoms 117. 

Only a few reports are available on chemical sensing of the novel nanomaterials of group IVB 

TMDCs.  Shokouh et al reported a fascinating recovery with an ultra-fast response time of 2 s 

and recovery time of 60 s by fabricating nanohybrids of TiS2 and polyvinylpolymer (PVP) for 

ethanol sensing 1. Table 2.1 shows some of the TMDCs that have been demonstrated for 

chemical sensing. 

Table 2.1: TMDCs, metal oxides and their nanocomposites for chemical sensing 

Material T ˚C Gas (ppm) Sensitivity 

(%) 

Response 

time (s) 

Recovery 

time (s)       

References 

WS2@MTCNFs 25 NO2             15 - - 111 

WS2 (single 

layer) 

25 NH3 - 200 271.5 110 

MoS2 400 O2                         - 5700 - 109 

MoS2 25 NO2 (0.1) - - - 108 

MoS2 (Five 

layers) 

25 NO2/NH3 1372/86 - - 107 

MoS2 (single 

layer 

25 NO2 (1.2) - >1800       >1800       106 



29 
 

WS2 25 NH3 (240) - 28 42 99 

MoSe2 25 NH3 - 150 540 112 

WSe2 25 NO2 (0.05) 5.06 50 105 113 

SnS2 25 NH3 (5) 21.6 40-50 100-120 114 

SnS2 200 NH3 (100) 7.4 40.6 624 115 

SnS2 25 NH3 (50) 2.4 - - 116 

SnS2 and CdS 300 CH3COCH3 

and ROH 

- - - 117 

Ag-TiO2 25 C2H5OH 

(5) 

4.35 52 63 118 

GaN-SnO2 25 ROH - ⁓100 ⁓100 119 

Chitosan 25 C2H5OH 19.35 - - 120 

WO3 300 C2H5OH 70 <15 <15 121 

NiO-Sn 340 C2H5OH 15.6 17 15 122 

MoS2-SnO2 25 NO2 (5) 18.70 - - 4 

MoS2-SnO2 280 C2H5OH - - - 123 

MoS2-ZnO 25 NH3 46.2 - - 124 

MoS2-TiO2 150 C2H5OH 14.20 - - 125 

WS2-TiO2 25 NH3 43.72 - 174.43 126 

TaS2-PANI 25 H2O 97.00 36 49 127 

TiS2-PVP 25 C2H5OH(2) 6800 60 2 1 

 

 



30 
 

2.8.1 Metal doping 

Transition metals which include Fe, Cr, Al, Ni, Sn, and Ce and precious metals such as Pt, Pd 

and Au can help  boost the sensing properties of a semiconductor by enhancing the activity and 

adjusting the resistance of the sensing material. This improves the overall attributes of the 

sensor such as 3S (sensitivity, selectivity and stability), response and recovery time; and 

working temperature especially metal oxide sensors 103. Extensive reports have shown that 

doped ZnO sensors are more effective in sensing acetone vapour over undoped ZnO. Kaur et 

al in their review also showed doped 1- dimensional (D) titanium dioxide with precious metals 

are very effective in sensing ethanol and acetone compared to undoped titania 99. In a similar 

report, doped SnO2 sensor showed enhanced sensing capability to VOCs especially acetone 

unlike the undoped SnO2. More studies are yet to be available on functionalization of TMDCs 

with metals and precious metals; however, 2-D materials like graphene sensors doped with 

precious metals showed better sensing performance over pristine graphene 99. 

2.8.2 Metal oxides for sensing VOCs  

Vast information is available on the sensing mechanism of the metal