Cross, AJ; (2014) The synthesis, characterisation and functional testing of metal oxide thin films. Doctoral thesis , UCL (University College London).

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Abstract

This thesis is concerned with the deposition of titanium dioxide thin films using chemical vapour deposition (CVD). The work emphasises the influence of deposition parameters on the properties of the resultant film. The materials have been designed with a wide range of potential applications in mind, from air and water purification to energy production and medical implant devices. Where possible, tests have been conducted to assess the efficacy of the material for these applications. The first chapter highlights a number of technologically important applications of titanium dioxide in order to demonstrate the motivation for research in this area. Some theoretical concepts are presented which are fundamental to understanding the behaviour of titanium dioxide. The principles behind chemical vapour deposition and the photocatalytic tests used in this work are discussed. The second chapter focusses on the use of three different metal substrates in TiO2 CVD. Firstly, the phase of TiO2 deposited on steel substrates was investigated. It was shown that the grade of steel had little influence with predominantly anatase films being formed in the vast majority of areas analysed. Since each grade of steel has different mechanical properties and is used in different applications, this work demonstrates that the photocatalytic properties of anatase films can be endowed to a large range of products. Secondly, TiO2 was shown to be adherent to a flexible substrate, namely aluminium foil. It was demonstrated from XPS analysis that aluminium ions did not diffuse into the TiO2, which remained photocatalytically active. A photocatalyst on a lightweight, flexible substrate offers several advantages over glass which has been the most frequently employed substrate to date. Thirdly, titanium dioxide was coated onto an alloy of cobalt, chromium and molybdenum, CoCrMo. The alloy is of interest for biomedical implants but suffers from poor biocompatability. By coating its surface with TiO2, it was shown to enhance osteogenic differentiation. Chapter three investigates nitrogen doped titanium dioxide for its potential as a visible light photocatalyst. A novel synthetic strategy was employed in which the amount of oxygen precursor was varied to determine if this would affect the position of the nitrogen in the titanium dioxide crystal structure. Differences were observed in the nitrogen XPS signal, the absorption profile, surface morphology and photocatalytic activity under both UV and visible irradiation. Visible light activity was observed for a sample made with lower amounts of oxygen precursor under UV and visible light. Finally, brookite, a rare, metastable form of titanium dioxide, was formed by atmospheric pressure chemical vapour deposition, APCVD. A brief literature review discusses the potential applications and some of the previously employed synthetic routes to brookite formation. Three sets of conditions are reported for the first known synthesis of brookite by atmospheric pressure chemical vapour deposition. A possible mechanism for brookite formation is hypothesised. This work concludes with a summary of the key findings from the experimental work and possible avenues for further research.

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