ETD Collection
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Item Group III-nitrides: synthesis and sensor applications(2017) Kao, MahalieoAn 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.Item The effect of microwave heating on manganese promoted iron based Fischer-Tropsch catalysts(2012-01-18) Mohiuddin, EbrahimA study was performed in order to investigate the effect of preparation method and the effect of microwave heating on a manganese promoted iron based Fischer-Tropsch catalyst. The effects of preparation method and microwave heating on the structure and morphology of the catalyst, its surface area and reduction behavior were investigated using various techniques such as Transmission electron microscopy (TEM), Powder x-ray diffraction (PXRD), surface area measurements (BET) and temperature programmed reduction (TPR). The FTS performance of the catalysts were also studied using a fixed bed reactor with Fischer-Tropsch Synthesis conditions (270 C, flow rate of 30 ml/min, H2/CO ratio = 2, pressure of 10 bar). Characterization of the catalysts calcined at 350 C revealed that manganese enriched the surface of impregnated Mn/Fe catalysts and suppressed the reduction of the iron catalyst. However, the Mn acted as a structural promoter in the co-precipitated catalysts and also promoted the reduction of Fe2O3 as the manganese content increased. The co-precipitated catalyst calcined at 650 C suppressed the reduction of iron. The impregnated catalysts showed similar conversion (~ 70%) for catalysts with Mn loadings 5%, 10% and 20%. This suggests Mn promotes the activity of the iron catalyst since less iron is present in the catalyst as the manganese loading is increased. The co-precipitated catalysts showed a 10 wt% Mn loading to be the optimum amount for increased activity and selectivity to C2 – C4 hydrocarbons, lower molecular weight olefins and a lower selectivity to heavier molecular weight hydrocarbons relative to Mn loadings of 5, 20 and 50 wt%. Mn loadings in excess of 10 wt% showed a slight increase in selectivity to heavier weight hydrocarbons. The impregnated catalysts showed very little difference in activity and selectivity but the co-precipitated catalyst showed a decrease in activity after the catalyst was microwave heated. A slight increase in selectivity to lower weight olefins and heavier molecular weight hydrocarbons was noted after microwave heating. The TPSR (Temperature programmed surface reaction) results revealed that this may be due to the stronger adsorption of CO on the surface of the catalyst after microwave heating. A similar trend was observed for catalysts promoted with 0.1 wt% potassium i.e. a slight increase in selectivity to heavier weight hydrocarbons after microwave heating.Item Organometallic chemistry of some manganese and zirconium complexes: A green chemistry approach(2006-10-27T07:30:31Z) Stanley, ManziniThe solventless reaction between Mn(CO)4(PPh3)Br and PPh3 as neat reagents using FTIRS was conducted and the activation enthalpy change of formation was found to be 143 ± 19 kJmol-1 while the activation entropy change of formation was 104 ± 7 Jmol-1K-1. The same reaction was also carried out in chloroform and the activation enthalpy change of formation was found to be 146 ± 8 kJmol-1 while the activation entropy change of formation was 114 ± 6 Jmol-1K-1. When the reaction was conducted in TCE solution, the activation enthalpy and entropy changes of formation were 137 ± 6 kJmol-1 and 97 ± 5 Jmol-1K-1 respectively. The solventless reaction of Mn(CO)4(PPh3)Br with PPh3 in KBr matrix using DRIFTS was also conducted and the activation enthalpy change of formation was found to be 169 ± 28 kJ.mol-1 while the activation entropy change of formation was 204 ± 57 J.mol-1.K-1. The sample preparation method, the type of support and the particle size of the support material influenced the reaction rate. The soventless reaction Mn(CO)4LBr + L → Mn(CO)3L2Br + CO [L= P(p-C6H4-R)3, R = Ph, MeO, Cl, F] in KBr using DRIFTS was also studied. It was found that the electronic effects of the ligand already attached on the metal complex influenced the rate of the reaction. An optical microscopy study of the reaction Mn(CO)4LBr + L' → Mn(CO)3LL'Br + CO [L= P(p-C6H4-R)3, R = H, Ph, MeO] was undertaken in an attempt to reconcile the wellbehaved reaction kinetics of the solventless reactions with solventless reactions by observing the microscopic behaviour of the reagents. The reactions were observed to go through a melt phase at temperatures much lower than the lowest melting point of the reagents, provided the reagents were in contact with each other. Isolated reagents neither reacted nor melted. The molten reagent thus served as a medium that allowed the diffusion of the reagents and products to ensure well-behaved kinetics. Investigation using 31P NMR demonstrated that the dissociation of the attached phosphine ligands also iii iv took place. The evidence obtained using the various techniques enabled the elucidation of the reaction mechanism. The solventless reaction, (η5-C5H5)2ZrCl2 + Na+RCOO-, R = C6H5, p-C6H4-NO2, p-C6H4- NH2 → (η5-C5H5)2ZrCl(RCOO) + NaCl did not occur but the reaction was found to take place in the NMR solvent. Single crystal XRD study of (η5-C5H5)2ZrCl(RCOO) R = C6H5, p-C6H4-NO2 revealed that the carboxylato ligand was coordinated in a bidentate fashion. The reaction of chlorobis(η5-cyclopentadienyl)hexylzirconium(IV) with internal hexene isomers failed to yield terminal olefins even under harsh experimental conditions. Isomerisation reactions using substituted zirconium metallocenes also failed to produce the terminal olefin. The reaction of Cp2ZrCl2 / n-BuLi with internal hexenes yielded a stoichiometric amount of 1-hexene. The reaction was found to be catalytic in Cp2ZrCl2 but limited by the amount of n-BuLi.