O deficiency in the rutile TiO2 (110) surface:: Ab initio quantum chemical investigation of the electronic properties

The (110) surface of rutile TiO2 (110) has been modeled using a density functional theory (DFT) plane-wave pseudo-potential method (CASTEP). In this study, 6 and 9 atomic-layer slabs have been examined. The stoichiometric surface converges to a low-spin solution in both cases with a density of states (DOS) similar to that for the bulk. O deficiencies are introduced by the removal of neutral O atoms thus leaving a neutral model with a surfeit of 2 e(-) per vacancy. This results in the partial filling of the Ti t(2g) conduction band orbitals and a compensatory shift in the Fermi level. The reduced surface converges to a high-spin solution in all cases, with the excess spin located within the previously unoccupied Ti t(2g) orbitals. Removal of the bridging surface O atoms results in an excess spin of 2 electrons per unit cell with approximately one-half that for removal of in-plane surface O atoms and subsurface O atoms. The removal of O atoms from the surface leads to an increase of the band gap, with the largest increase due to the removal of in-plane 3-fold coordinated surface O atoms, and the smallest one due to the removal of subsurface O atoms. (C) 2006 Wiley Periodicals, Inc.