Shaw, B-JA; (2017) Surface Science Studies of Model Metal Oxide Systems. Doctoral thesis , UCL (University College London).

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Abstract

The work presented in this thesis details the investigation of two important metal oxides: ultrathin films of CeO2(111) and single crystal rutile TiO2(110). Near ambient pressure photoelectron spectroscopy (NAPPES) has been used to investigate the effects of temperature, and water and oxygen adsorption on the band gap state of rutile TiO2(110). This state has long been attributed to bridging oxygen vacancies, hydroxyls and, more recently, also to subsurface titanium interstitials and oxygen vacancies. In this experiment the non-stoichiometric rutile TiO2(110) surface was exposed to water and oxygen at an array of temperatures whilst monitoring the band gap state and other core-level spectra. Our results show that temperature, oxygen and high pressures of water have a marked effect on the band gap state of rutile TiO2(110). Scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) have been employed to study the structure and growth of ultrathin CeO2(111) films supported on Pt(111) and Rh(111). Atomically resolved STM images of these ultrathin films have been obtained, which allowed for examination of the defect structure of these thin films, in addition to the nucleation sites of certain adsorbates. To this effect, the behaviour of individual Au atoms at room temperature on this surface has been investigated, whereby it was observed that the single Au adatoms exhibited no preferential adsorption site. The growth and structure of iron nanowires supported on rutile TiO2(110) has also been investigated with low energy electron microscopy (LEEM), X-ray photoemission electron microscopy (XPEEM), scanning tunnelling microscopy (STM) and atomic force microscopy ABSTRACT 4 (AFM). The nanowires were grown by the deposition of iron onto the substrate, which was held at an elevated temperature. The magnetic domain structure was studied using X-ray magnetic circular dichroism (XMCD) and spin-polarised low energy electron microscopy (SPLEEM). The chemical analysis of the nanowires was performed via micro X-ray photoelectron spectroscopy (μ-XPS) and X-ray absorption spectroscopy (XAS). Our experimental results indicate that the growth orientation of the nanowires is dictated by the anisotropy of the substrate, i.e. the nanowires grow along the [001] direction of the substrate. The iron nanowires possess an iron core appear and the larger structures appear to be encapsulated by mixed Fe-Ti oxides, possible a combination of FeTiO3 and α-Fe2O3, whilst the smaller ones display a (3×3) oxygen overlayer. A magnetic contrast was only observed in XMCD, which we attribute to the difference in sensitivity between this technique and SPLEEM.

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