A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2014. / The synthesis of good crystalline nanomaterials by green methods is one of the means
to preventing global warming. Application of microwave thermal methods and the use of
green solvents to synthesize nanomaterials contribute to this goal. Further, the low cost
synthesis of nanomaterials contributes to their ease of availability in the market at
affordable costs.
In this study, different NiSx phases and GaN nanomaterials were obtained by
microwave-assisted solution phase synthesis. NiS2, GaN, -NiS, ( & ) NiS and Ni3S2
phases were obtained by using different reagents and applying different reaction
parameters. These materials were characterized by X-ray diffraction, transmission
electron microscopy, scanning electron microscopy and photoluminescence, to evaluate
their crystalline phases, morphologies, particle size distribution and optical properties
respectively.
Hierarchical structures of cubic phase NiS2 and spherical HDA capped
nanostructures were synthesized by a MW-assisted hydrothermal technique. The
product phase purity was optimized and the effect of precursor concentration and
capping agents were discussed. Further, optical properties of bare and HDA capped
NiS2 materials are reported. Detailed analysis of the PL properties shown by these
materials in the UV-vis range has been given by considering their calculated DOE
energy band diagrams.
Single phase -NiS nanostructures with uniformly distributed hierarchical
networks were synthesized for the first time in this study. The materials were evaluated
for thermal stability under an oxidative environment and at temperatures between 150
oC and 600 oC. NiS materials showed stability at 300 oC and NiO formation was
observed from 350 oC to 600 oC. The annealing effect on the crystalline size and IR
absorption of the annealed samples is reported by XRD and FTIR studied. The EPR
properties of the annealed materials were studied and compared to the oxidized
materials. The transition temperature of the -NiS was further confirmed by performing
electrical measurements on the as-synthesized material. Further, hydrostatic pressure
sensing properties, ethanol and tomato VOCs sensing properties of the -NiS/PVA
composite based devices were carried out and the results are reported. The ethanol gas
sensing properties of the devices prepared showed the highest response when
compared to hydrostatic pressure sensing and tomato VOCs gas sensing.
UV-blue emitting GaN nanostructures were obtained for the first time by a onestep
MW-assisted solvothermal technique. Sensor devices based on the hexagonal
wurtzite structures obtained and their PVA composites (GaN/PVA) were prepared with
different GaN NPs concentrations. A very high response to hydrostatic pressure was
achieved for the devices prepared. The sensitivity of a GaN/PVA composite based
device was analyzed for tomato VOCs detection and the results are presented.
Binary phase ( & ) synthesis of NiS materials is commonly reported for the
synthesis of Ni:S ratio of 1:1 stoichiometry. This is due to the formation of both phases
at temperatures lower than 200 oC. Here, the effect of NaOH and the S source was
investigated as reaction parameters. It was found that the concentration of OH- ions in
solution plays a huge role in the formation of binary phase NiS as well as its morphology
distribution in the nanostructures. Hexagonal nanoplatelets, nanorods and nanorodbased
flower-like structures were obtained when different reaction parameters were
varied in the presence of NaOH. Further, the solubility of different S precursors in the
solvent used was studied and found to affect both the morphology and crystalline phase
distribution of the products.
Preliminary work on the synthesis of Ni3S2 and Se and Te doped Ni3S2 is
presented in the last chapter. The crystallite sizes of the materials were determined by
use of the Scherrer equation and the elemental composition was confirmed by EDS
analysis. The relative humidity gas sensing of the samples materials was determined
and sensitivity response of the material to humidity was obtained for the first time.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/16871 |
| Date | 04 February 2015 |
| Creators | Linganiso, Ella Cebisa |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf, application/pdf |
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