published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / Industrial and Manufacturing Systems Engineering / Master / Master of Philosophy

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. October 2013. Johannesburg, South Africa. / We report the role of a double step heat treatment process in the synthesis of novel GaN nanostructures (NSs) using a two stage furnace following a catalyst free vapour-solid growth mechanism. Morphological analysis revealed that GaN NSs were composed of rod-like structures with average diameter of 250 nm and accumulated particulates of GaN with diameter of ~ 12 – 16 nm providing enhanced surface area. The wurtzite phase of GaN nanorods of agglomerated nanoclusters was synthesized at temperatures as low as 750 °C. An X-ray photoelectron spectroscopic study confirmed formation of GaN. The surface areas of the GaN NSs were high at ~20 m2/g with respect to that expected for solid nanorod structures. The GaN NSs were of high crystallinity and purity as revealed by structural studies. Raman spectral analysis showed stronger intensity of the A1(LO) mode with respect to that for E2(high) mode indicating the high electronic quality of the sample. A photoluminescence study revealed the dominant presence of a defect band around 1.7-2.1 eV corresponding to nitrogen di-vacancies. Subsequent annealing in NH3 has demonstrated a compensation of the defect state and evolution of a band edge peak with possible hydrogen compensation of surface states. We also report the role of activated carbon on Ga2O3 to make GaN/C nanostructure composites using a single stage furnace. TEM analysis showed that GaN/C nanostructures gave different morphologies with different ratios of GaN/C. The surface areas of these materials showed an increase as the ratio of activated carbon was increased. PXRD showed that a ratio of Ga2O3: C of 1:0.5 (w/w) was sufficient to form GaN. TGA revealed that the ratios of Ga2O3: C of 1:0.5 – 1:2 gave materials that were thermally stable. Raman spectra showed that the material had excellent electronic properties. The material with a Ga2O3/C 1:2 ratios showed a poor gas response due to the change in reference value of resistance with the variation of hydrogen concentration. iv This study also provides the first investigation of GaN as a catalyst support in hydrogenation reactions. The GaN NSs were synthesized via chemical vapour deposition (CVD) in a double stage furnace (750 ºC) while nitrogen doped carbon spheres (NCSs) were made by CVD in a single stage furnace (950 ºC). TEM analysis revealed that the GaN NSs were rod-like with average diameters of 200 nm, while the NCSs were solid with smoother surfaces, and with diameters of 450 nm. Pd nanoparticles (1 and 3% loadings) were uniformly dispersed on acid functionalized GaN NSs and NCSs. The Pd nanoparticles had average diameters that were influenced by the type of support material used. The GaN NSs and NCSs were tested for the selective hydrogenation of cinnamaldehyde in isopropanol at 40 and 60 °C under atmospheric pressure. A comparative study of the activity of the nanostructured materials revealed that the order of catalyst activity was 3% Pd/GaN >3% Pd/NCSs > 1% Pd/NCSs > 1% Pd/GaN. However, 100% selectivity to hydrocinnamaldehyde (HCALD) was obtained with 1% Pd/GaN at reasonable conversion rates.

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Thesis (M. Phil.)--University of Hong Kong, 2010. / Includes bibliographical references. Also available in print.

Thesis (Ph. D.)--Ohio State University, 2001. / Title from first page of PDF file. Document formatted into pages; contains xiii, 165 p.; also contains graphics (some col.). Includes abstract and vita. Advisor: Jonathan P. Pelz, Dept. of Physics. Includes bibliographical references (p. 160-165).

Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2003. / Includes bibliographical references (leaves 62-66). Also available in electronic version. Access restricted to campus users.

Thesis (M. Phil.)--University of Hong Kong, 2002. / Includes bibliographical references (leaves 58-61).

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Thesis (M.S.)--Ohio University, August, 2001. / Title from PDF t.p.