Understanding Metal Oxide Surfaces at the Atomic Scale: STM Investigations of Bulk-defect Dependent Surface Processes
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Understanding Metal Oxide Surfaces at the Atomic Scale: STM Investigations of Bulk-defect Dependent Surface Processes Ulrike Diebold Department of Physics, Tulane University New Orleans, LA 70118, U.S.A. ABSTRACT Surface defects are important in oxide surface chemistry, because they change not only the surface geometric structure, but also affect the local electronic structure. Scanning Tunneling Microscopy (STM) images with atomic-scale resolution, in combination with area-averaging surface spectroscopies, is an ideal tool to study local surface defects and their relationship to surface reactivity. We report STM results on TiO2(110) surfaces which show the surprising influence of bulk defects on surface properties. The reduced crystals used in this and other surface science studies contain Ti interstitials and oxygen vacancies. Re-oxidation at elevated temperatures results in the growth of additional TiO2 layers with Ti coming from the bulk of the crystal and O from the gas phase. This often result in partially incomplete surface structures with many undercoordinated atoms. The desorption behavior of elemental S, dosed at room temperature, depends on the reduction state of the sample. This is explained by a mechanism where desorption from a weaklybound precursor state competes with the availability of new adsorption sites in the form of oxygen vacancies which migrate from the bulk to the surface. INTRODUCTION Metal oxides are fascinating, yet complicated, materials which exhibit an extremely wide range of physical and chemical properties. They are used in a variety of technical applications. An incomplete list includes uses in catalysis, photocatalysis, solar panels, gas sensors, as protective or optical coatings, in biocompatible materials, in Li-based batteries, electrochromic devices, and as potential gate insulators for the new generation of MOSFETS. The surface and interface properties of metal oxides play a role in all these applications, and sometimes even dominate device performance. Consequently, a deeper understanding of metal oxide surfaces would impact a wide range of fields. While some progress has been made in recent years, the fundamental knowledge of metal oxide surfaces is still rather incomplete and far from the level that has been achieved for elemental semiconductors or metals. Local imperfections such as point or line defects, step edges, and impurities are important in surface chemistry because they provide adsorption sites with special steric configurations. Bonding in metal oxides is covalent with a high degree of ionic character, hence, defects change the local surface electronic structure as well. Consequently, surface defects are particularly important in oxide surface chemistry. Scanning probe techniques are capable of revealing surface geometric and geometric structure at the atomic scale and AA5.1.1
lend themselves naturally to investigate the relationship between these nanoscale features and surface adsorption/reaction processes. One single-crystalline model system, the rutile TiO2(
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