Synthesis and functionalization of gallium nitride nanostructures for gas sensing and catalyst support
Date
2014-01-10
Authors
Kente, Thobeka
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Abstract
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.
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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.
Description
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.