Atomic geometry and STM simulations of a TiO2(1 1 0) surface upon formation of an oxygen vacancy and a hydroxyl group

Abstract

The atomic geometry of a TiO2(1 1 0) surface upon creation of an oxygen defect site and formation of a hydroxyl group was investigated using 3 × 1, 2 × 2 supercells by spin polarized density functional theory calculations. It was found that both the removal of a bridging O atom and the formation of an OH group lead to distortion in the atomic positions of the neighboring atoms depending on the choice of the unit cell used in the calculations. The scanning tunneling microscopy (STM) simulations performed using the 2 × 2 unit cell suggest that both an oxygen vacancy and a hydroxyl group should be observed experimentally as a bright protrusion but of different shapes. The O vacancy exhibits a spherical shape whereas the OH group was elongated perpendicular to the [0 0 1] direction. In contrast, in the 3 × 1 supercell, the OH group appears as a bright spot while the oxygen defect looks darker. These findings clearly suggest that a proper geometry is necessary to reproduce experimental STM images of an oxygen vacancy and a hydroxyl group on a TiO2(1 1 0) surface.