Ab initio simulation of molecular processes on oxide surfaces

Ab initio calculations based on both density functional theory (DFT) and Hartree-Fock (HF) methods are used to investigate the energetics and equilibrium structure of the stoichiometric and reduced TiO2 (110) surface, the adsorption of potassium on the (100) surface and of water on the (110) surface. It is shown that DFT and HF predictions of the relaxed ionic positions at the stoichiometric surface agree well with each other and fairly well with recent X-ray diffraction measurements. The inclusion of spin polarisation is shown to have a major effect on the energetics of the reduced surface formed by removal of bridging oxygens. The gap states observed to be induced by reduction are not reproduced unless spin polarisation is included. Static and dynamic DFT calculations on adsorbed water on TiO2 (110) confirm that dissociation of H2O leads to stabilisation at low coverages, but suggest a more complex picture at monolayer coverage, in which there is a rather delicate balance between molecular and dissociated geometries.