Surface science of ultrathin metal oxide films

The properties of metal oxide surfaces are key to their diverse technological applications. However, the semiconducting nature of metal oxides presents a problem - many surface science techniques are electron based and thus require samples to be conducting. As such, bulk crystal studies of metal oxides by techniques such as photoemission spectroscopy (PES) and scanning tunneling microscopy (STM) are limited to reduced surfaces. Alternatively, thin films of a metal oxide can be synthesised on a suitable conducting substrate that mimic the bulk crystal surface whilst having sufficient conducting character to use these techniques. CeO2 is an important material found in three-way catalysts that remove pollutants from the exhaust gas of modern automobiles. Key to this application is the ability of reduced ceria to store and release oxygen depending on the composition of the exhaust. The addition of noble metals such as Pd to the ceria surface greatly improves the efficiency of pollutant conversion evidenced by X-ray photoelectron spectroscopy (XPS) by reducing the ceria. Resonance photoemission spectroscopy (RESPES) has been used to investigate the eect of Pd on ceria CeO2-x(110) thin films grown on a Pt(111) substrate. RESPES is more surface specific than XPS and thus reveals more information on the surface layers of ceria films. TiO2(110) is the most studied metal oxide surface, and has a multitude of applications. Its chemistry with two of the most abundant chemical species - water (H2O) and oxygen (O2) - is thus very important. H2O has been shown to dissociate on TiO2 surfaces. TiO2 thin films grown on W(100) were used as model system to study the chemistry of the reaction between TiO2 and H2O, and subsequently the reaction of hydrated surfaces with O2 using XPS. STM was used to examine the morphology of TiO2(110) films grown on W(100)-(2 x 1)-O, changes with film thickness and methods of improving surface smoothness. The first detailed STM images showing row structure of TiO2(110) films grown on W are shown.