Group III-nitrides: synthesis and sensor applications

Submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in the Faculty of science Department of Chemistry University of the Witwatersrand. November 2016.An overview of the evolution of synthesis and applications of indium nitride and gallium nitride in modern science and technology is provided. The working principles and parameters of chemical vapour deposition (CVD) synthesis technique are explored in this study. In this study indium oxide, indium phosphate, indium nitride and gallium nitride materials are prepared by CVD. The versatility of CVD on the fabrication of one-dimensional (1D) structures is portrayed. Both change in dimensionality and change in size are achieved by a CVD technique. 1D indium oxide (In2O3) nanowires, nanonails and nanotrees are synthesised from vapour deposition of three-dimensional In2O3 microparticles. While 1D structures of the novel indium phosphate known as triindium bisphosphate In3(PO4)2 were obtained from reactions of In2O3 with ammonium phosphate. The effect of temperature, activated carbon and the type of indium precursor on dimensionality of the synthesized materials is studied. The inter-dependency between temperature and precursors is observed. The presence of activated carbon at high temperatures encouraged growth of secondary structures via production of excess indium droplets that act as catalysts. The combination of activated carbon and high temperature was found responsible for the novel necklace, nanonail, nanotree and nanocomb structures of In2O3. Indium nitride (InN) has for the first time been made by a combined thermal/UV photoassisted process. In2O3 was reacted with ammonia using two different procedures in which either the ammonia was photolysed or both In2O3 and ammonia were photolysed. A wide range of InN structures were made that was determined by the reaction conditions (time, temperature). Thus, the reaction of In2O3 with photolysed NH3 gave InN rod like structures that were made of cones (6 h/ 750 oC) or discs (6 h/ 800 oC) and that contained some In2O3 residue. Photolysis of In2O3 and NH3 by contrast gave InN nanobelts, InN tubes and pure InN tubes filled with In metal (> 60 %). The transformation of the 3D In2O3 particles to the tubular 1D InN was monitored as a function of time (1-6 h) and temperature (700-800 oC); the product formed was very sensitive to temperature. The band gap of the InN tubes was found to be 2.19 eV and of the In filled InN tubes to be 1.89 eV. Gallium nitride (GaN) and indium gallium nitride (InGaN) nanostructures were synthesized from thermal ammonification of gallium oxide (Ga2O3) as well ammonification of a mixture of In2O3 and Ga2O3 respectively. The effect of temperature on preparation of high purity GaN was studied. The GaN materials synthesized at 800 °C showed a mixture of the gallium oxide and the gallium nitride phases from the XRD analysis. However at temperatures ≥ 900 °C high quality GaN nanorods were obtained. The band-to-band ultraviolet optical emission value of 3.21 eV was observed from the GaN nanorods. However, the preparation of InGaN was complicated by the thermally stable In2O3. At lower temperatures inhomogeneous materials consisting of GaN nanorods and In2O3 were obtained. While at high temperatures (≥ 1050 °C) InGaN was obtained. However because indium has a high vapour pressure and a low melting point only a minute amount of it was incorporated in the crystal lattice. Hexagonally shaped nanoplates of In0.01Ga0.99N were successfully obtained. A shift in optical emission to longer wavelengths was observed for the InGaN alloy. A blue optical emission with the energy value of 2.86 eV was observed for the InGaN nanoplates. The two n-type group III-nitrides (InN, GaN) prepared in this study were used for the detection of CO, NH3, CH4 and NO2 gases in the temperature range between 250 and 350 °C. The InN sensor and GaN sensor responses were compared to the response of the wellestablished n-type SnO2 sensor under the same conditions. All the three sensors responded to all the four gases. However, InN and GaN were much more selective in comparison to SnO2. InN sensitivity to CO at 250 °C surpassed its sensitivity to any other gas at the studied temperature range. Its response towards CO at 250 °C was about five times more than that of SnO2 towards CO at the same temperature. While, GaN was the best CH4 sensor at 300 °C in comparison to InN and SnO2 sensors at all temperatures. Meanwhile SnO2 responded remarkably to both NH3 and CO across the studied temperature range with its performance improving with increasing temperature. The ability for InN to respond to both NH3 and NO2 at 250 °C opens up the possibility for an application of InN as an ammonia sensor in diesel engines. InN and SnO2 sensors were found susceptible to humidity interference in a real environmental situation. On the contrary, GaN sensor presented itself as an ideal candidate for indoor and outdoor environments as well as in bio-sensors because it showed robustness and inertness towards humidity. InN and GaN by showing activity at high temperatures only, presented themselves as good candidates for in-situ high temperate gas sensing applications. Response and recovery times for all sensors showed improvement with increasing temperature.MT201

Chemical vapour deposition synthesis of carbons from halogen and silicon sources

In this study carbonaceous materials were produced using the floating catalyst horizontal CVD method by injection of a hydrocarbon solution into the high temperature zone of the reaction chamber. Various hydrocarbon reagents were used. The reagents included toluene, chlorobenzene (ClBz), bromobenzene (BrBz), chloroform, tertraethylorthosilicate (TEOS), and triethylchlorosilane (TESCl). Catalytic and noncatalytic reactions were investigated. Ferrocene was used as a source of catalyst during catalytic reactions. The reactions were carried out under a flowing Ar/H2 (5 %) (v/v) gas mixture at atmospheric pressure. Carbon materials synthesized in this study were characterized by Raman spectroscopy, TEM analysis, SEM analysis, Infra red spectroscopy, TGA, EDS and XRD analyses. Catalytic pyrolysis of toluene, ClBz, and BrBz resulted in production of multi-walled carbon nanotubes (MWNTs). It was found that the percentage content of the haloarene in the feed impacts on the morphology of carbonaceous materials produced during catalytic pyrolysis. A change in morphology from spheres to a mixture of spheres and tubes and finally to tubes was observed. The influence of the gas flow rate on the type of carbons prepared from catalytic pyrolysis of ClBz has been studied. At high gas flow rates MWNTs were the sole product but at low flow rates carbon spheres resulted. Carbon spheres were also prepared from non-catalytic reactions. It was established that non-catalytic pyrolysis of ClBz or chlorinated species and toluene enhance product yields. It was established that the size distribution of spheres can be controlled by using hydrogen as a carrier gas. The impact of additives such as thiophene and ammonia on the pyrolysis of toluene and ClBz was investigated. They were found to inhibit formation of the graphitic carbon from toluene but not from ClBz. TEOS and TESCl were used as sources of silicon. CNTs were prepared from a TEOS/toluene feed. It was found that the preparation of CNTs from a feed containing low concentrations of TEOS enables control of the diameter distribution and also results in a remarkable improvement in CNT length as compared to CNTs prepared from toluene. However, it was found that high concentrations of TEOS in toluene are destructive. A drastic deterioration of lengths and yield was observed at high concentrations of TEOS in toluene. Non-catalytic decomposition of TEOS produced core/shell structures formed from codeposition of carbon and silica. Subsequent thermal treatment of the carbon/silica core/shell structures in air resulted in formation of hollow core silica structures. Decomposition of TESCl in the absence of a catalyst produced high yields of carbon spheres with a wide diameter distribution. EDS analysis revealed that the spheres prepared from TESCl are composed of carbon, silicon and chlorine. This suggests production of chlorinated silicon/carbon composites. Thermal treatment in air, at 1000 °C produced core/shell structures