Origin of defect states on the surface ofTiO2

We have investigated the electronic structure of oxygen vacancies on ${\mathrm{TiO}}_2$(110), including atomic relaxation. The study of different defect sites shows that the experimentally observed gap state at 0.7 eV below the conduction-band edge is indeed due to an oxygen vacancy. This state, however, is not due to the removal of an O atom from a surface bridging site, as frequently proposed, but results from a subsurface oxygen vacancy. In this configuration, there is a maximum reduction of the screening between surface cations.