Microwave assisted techniques for the synthesis of NiSx and GaN semiconductor nanostructures for applications in sensors

Linganiso, Ella Cebisa
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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.
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.