Synthesis and characterization of solid, hollow, core-shell and worm-like carbon nanostructures for applications in organic photovoltaic devices and chemical sensors
Date
2016
Authors
Mutuma, Bridget Kanini
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Abstract
The synthesis of carbon spheres (solid and hollow) for application in organic photovoltaics
and chemical sensors is a means of using inexpensive and readily processable carbons to
eliminate global warming and to monitor harmful gases. The synthesis conditions used to
make solid carbon spheres can also be used to tailor their structural, paramagnetic and
thermal properties. More so, the ability to tailor the morphology, surface, structural and
electronic properties of the hollow carbon spheres by a templating method is an added
advantage to their applicability in electronic devices.
Solid carbon spheres were synthesized by a vertically oriented chemical vapor deposition
(CVD) reactor using acetylene as a carbon source and argon or hydrogen as the carrier gas.
The flow rates of the acetylene or carrier gases determined the particle sizes of the carbon
spheres. Annealing of carbon spheres in hydrogen resulted in an increase in thermal stability,
fewer defects and narrower paramagnetic signals relative to the carbon spheres annealed in
argon gas. In contrast, carbon spheres annealed in argon exhibited an increase in the number
of defects, a decrease in thermal stability and broader paramagnetic signals. Doped carbon
spheres portrayed an increase in ID/IG ratios, a decrease in thermal stability and stronger
paramagnetic signals due to the presence of defects induced by nitrogen. The N doped carbon
spheres synthesized in H2 comprised of 48% pyridinic-N, 22% pyrrolic-N and 24%
quaternary -N while the N doped spheres obtained in the presence of Ar had 17% pyridinic-
N, 20% pyrrolic-N and 49% quaternary-N. The presence of a higher percentage of pyridinic-
N confirms the presence of more edge defects in carbon spheres synthesized under H2 gas
corroborating with the stronger paramagnetic signal observed from the ESR spectra.
Consequently, a higher N/C ratio was exhibited in the N doped CSs obtained in the presence
of H2 (4.96) than in the presence of Ar (3.68). This could be attributed to the presence of edge
defects in carbon spheres synthesized in the presence of H2 gas. The induction of edge defects
in carbon spheres in the presence of H2 gas without the aid of a metal catalyst opens a
platform for regulating surface and catalytic reactions using H2 gas.
Pristine and mesoporous SiO2 spheres were synthesized using a modified Stober method.
Carbonization of the pristine SiO2, pristine SiO2@PVP, mesoporous SiO2 and mesoporous
SiO2@PVP spheres was carried out using a bubbling method with toluene as the carbon
source and argon as the carrier gas in a CVD reactor for 1 h. Upon SiO2 removal, hollow
carbon nanostructures of varying morphologies were obtained. The polyvinylpyrrolidone
(PVP) adsorption time, PVP concentration, SiO2 mesoporosity, SiO2 particle size dispersion,
and carbonization time played a role in the formation of unique hollow carbon
nanostructures; complete HCSs, broken HCSs, deformed HCSs, edge connected, open ended,
wormlike and bubble-like HCSs. The mesoporous broken HCSs and open ended HCSs
portrayed a hierarchical structure with a bimodal pore size distribution. The surface area
properties of these materials and the ease of control of the carbon morphology gives an
insight into the application of these materials as dye adsorbents. The effect of the size
dispersion of Au@SiO2 sphere templates for the synthesis of hollow carbon structures was
evaluated using a CVD nanocasting method. The diameter of the template, the presence of
the gold nanoparticles and the amount of PVP determined the size, thickness and shape of the
synthesized carbon nanostructures. Carbonization (and SiO2 removal) of Au@polydispersed
silica spheres for 1 h gave a graphene-like HCS layer while longer times (2-4 h) gave
nanotube like (or worm like) HCSs. These results highlight the potential use of Au@carbon
core shell structures for the generation of few layered graphene-like unusual nanostructures.
As a proof of concept, the wormlike carbon structures were incorporated in organic solar
cells and found to give a measurable photovoltaic response.
The incorporation of Au nanospheres and nanorods in a hole transport layer (PEDOT:PSS) of
a solar cell device increased the current density and the photo-conversion efficiency of the
device due to the local surface plasmon resonance and enhanced light scattering effects of
gold. However, high series resistance and leakage currents were obtained due to barrier
centres created by uneven dispersion of Au nanaorods within the polymer matrix. The
performance of bulk heterojunction organic photovoltaic cells based on poly(3-hexylthiophene-
2,5-diyl) (P3HT) and 6,6-phenyl-C61-butyric acid methyl ester (PCBM)
processed from chlorobenzene solution can be enhanced by solution heat treatment of the
blend. The morphology of films spin coated from the heat treated blend solution reveals a
more favourable diffusion of PCBM into the P3HT matrix than heating of the individual
solutions separately. The films obtained from heat treated P3HT and PCBM solutions had a
more homogeneous dispersion and enhanced light absorption than those obtained from
solutions heat treated separately. There was a significant improvement in the performance for
devices made from a solution heat treated blends relative to the non-treated blend; a
maximum power conversion efficiency of 3.5% and a fill factor up to 43% was achieved
under Air Mass 1.5 at 100 mW/cm2 illumination.
This study also reports on the sensing characteristics of ammonia in humid environment by
hollow carbon spheres, hollow carbon spheres-polyvinylpyrrolidone composite and annealed
hollow carbon spheres, at 20°C and 40°C. For device fabrication, a surfactant assisted
method was used to homogeneously disperse the hollow carbon spheres, allowing their
deposition onto an interdigitated electrode by casting. An enhanced response and recovery
time of the devices was observed at the higher working temperature. Annealing of the hollow
carbon spheres resulted in a tremendous decrease in the humidity dependent ammonia
sensing due to a decrease in the number of the oxygenated groups and defects in their
structure. The presence of hydroxyl groups on the pristine hollow carbon sphere surface
resulted in an enhanced proton conductivity. However, the ammonia sensitivity at high
relative humidity in the pristine hollow carbon spheres is negligible due to the inhibition of
ammonia adsorption sites by the high concentration of water molecules. The sensor response
was investigated by varying both ammonia concentration and relative humidity, determining
the topology of the response as a function of these two variables, and applying a tristimulus
analysis in an attempt to determine the ammonia concentration independently of the relative
humidity. This study demonstrates the critical role played by humidity and surface chemistry
in the ammonia sensing properties of hollow carbon spheres. The studies reveal the day to
day application of ammonia sensors, with temperature and humidity playing a critical role in
the carbon based sensor response and recovery of the materials. These carbon based sensors
that simultaneously measure ammonia and relative humidity could be applied in agricultural
industries to monitor ammonia concentration in soils, fishponds and in food industries to
monitor meat spoilage.
Description
A Thesis submitted for the faculty of Science at the University of Witwatersrand Johannesburg, in the fulfilment for the degree of Doctor of Philosophy in Chemistry. Johannesburg, November 2016.
Keywords
Citation
Mutuma, Bridget Kanini (2016) Synthesis and characterization of solid, hollow, core-shell and worm-like carbon nanostructures for applications in organic photovoltaic devices and chemical sensors, University of Witwatersrand, Johannesburg, <http://wiredspace.wits.ac.za/handle/10539/21652>