First-principles Studies of Surface Defects of Model Metal-Oxide Semiconductors

In this thesis, three different model metal-oxide semiconductor systems will be discussed. First, the impact of hydroxyl vacancies, OHvac, on the geometry, electronic structure, and mechanical properties of single-walled aluminosilicate, (Al2SiO7H4)N, and aluminogermanate, (Al2GeO7H4)36, nanotubes is investigated. It is found that, with the exception of one OHvac localised on the outer wall of the (Al2GeO7H4)36 tube, these defects induce occupied and empty states in the band gap. Those states are found to be highly localised both in energy and in real space. Different magnetisation states are also found, depending on both the chemical composition and the specific side with respect to the tube cavity. The focus of the thesis then shifts to one of the most important and well-studied metaloxide surfaces, the rutile TiO2(110) surface. The reactivity of the surface is revisited, in view of the discrepancy between theory and experiment on the interaction between molecular oxygen and surface hydroxyls. This discrepancy is resolved by proposing that excess charge, associated with the oxygen vacancy and originating from Ti interstitials, is present on the surface. This surface charge opens new reaction channels not theoretically possible otherwise. The study utilises hybrid Density Functional Theory (DFT) calculations and Scanning Tunneling Microscopy (STM) simulations to provide evidence for the proposed surface charging. The last part of the thesis focuses on another surface of TiO2, the (011) surface. TiO2(011) has recently attracted attention due owing to its reported high photocatalytic activity. Several proposed structures of the surface are inconsistent with each other. Recent developments, based on Surface X-Ray Diffraction (SXRD) data and DFT simulations, now agree on a new structure. In this part a review of the various structures is provided and further evidence is given on the validity of the new proposal by providing further insight on the appearance of the surface on the STM.