Surface electronic structure of TiO2: Atomic geometry, ligand coordination, and the effect of adsorbed hydrogen

The intrinsic electronic surface-state structure in the region of the bulk band gap for the (110), (100), and (001) surfaces of Ti${\mathrm{O}}_2$ (rutile) has been determined by fracturing single-crystal samples in ultrahigh vacuum and measuring their ultraviolet photoemission spectra. None of the faces exhibits an appreciable density of surface states in the bulk band gap, in disagreement with recent calculations for the Ti${\mathrm{O}}_2$(001) surface by Kasowski and Tait. The atomic geometry of both perfect and defect surfaces is examined, and the incomplete screening of pairs of surface cations at defect sites is suggested to give rise to occupied band-gap surface states rather than the coordinative unsaturation of surface cations. Hydrogen-exposure experiments indicate that Ti${\mathrm{O}}_2$ surfaces may not interact as strongly with hydrogen as has been suggested.