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Title: Tungsten oxide and tungsten oxide based heterogeneous nanostructure thin films : synthesis via AACVD, characterisation, growth mechanism, and application in photocatalysis
Author: Ling, M.
ISNI:       0000 0004 7231 0980
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2016
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Tungsten oxide thin films with one-dimensional (1D) nanostructure (e.g. nanorod (NR)) show enhanced performances for gas sensing, catalysis and photocatalysis due to a large surface-to-volume ratio, high crystallinity, reduction of light reflection and relative high collection efficiency of charge carriers. This thesis details the use of aerosol assisted chemical vapour deposition (AACVD) to deposit tungsten oxide NR array thin films via optimising the deposition conditions (e.g temperature, solvent, precursor and substrate). The tungsten oxide NR array thin films were able to be grown directly on glass, quartz, silica and alumina substrates. Based on observation of the change of tungsten oxide morphologies from planar to NR on traversing from the inlet to outlet of an AACVD reactor, where the actual substrate temperature changed from 339 to 358 °C, a ‘kinetic competition’ mechanism was proposed to describe the relation between deposition parameters and the morphology formed during AACVD. In this mechanism the formation of planar and wave-like morphologies is due to the competition between rperp (perpendicular growth rate) and ri (parallel growth rate contributed by nucleation rate). When rperp is around 7 times faster than ri, the formation of tungsten oxide NR is favoured. The difference between rperp and ri is attributed to planar defects which appear in the direction perpendicular to the NR growth suppressing ri. These planar defects induced by oxygen vacancy may also be responsible for the observed dislocation loops with size (~0.4 to 1.5 nm) providing with the associated strain field generated leading to a quantum-spatial-confinement effect which modifies the band structure of WO3 NR array thin films. During photodegradation of stearic acid (SA) WO3 NR with length around 1200 nm (deposited for 5 mins) gave the highest photocatalytic activity, and the WO3 NR were more than 2 μm in length reduced tungsten states (W4+) were observed via NIR and in XPS spectra which lead to lower photocatalytic activity. In order to improve photocatalytic activity of plain WO3 NR arrays, AACVD was used to grow noble metal (Au, Pt, Pd and Ru) and metal oxide (PdO, RuO2, Co2O3, CuOx and TiO2) 4 NPs supported on 1D WO3 nanorod arrays with size of the NPs (1.9 to 7.3 nm) is directly controlled by the deposition time (0.5 to 36 minutes). Hybrid nanostructures of Au/WO3 (1 min, with particle mean size 3.0 nm), Pt/WO3 (10 min, 3.0 nm) and PdO/WO3 (5 min, 5.6 nm) increased photocatalytic activity by 40 to 50% compared to undecorated plain WO3 NR array thin films.
Supervisor: Blackman, C. ; Palgrave, R. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available