Identifying Trapped Electronic Holes at the Aqueous TiO2 Interface

Trapping of photogenerated holes near or at the surface is believed to be a crucial step in the photo-oxidation of water and organic pollutants by TiO2. The detailed nature of these localized states is however still a matter of debate. Here, we investigate this question for the ruffle TiO2(110) water interface using ab initio molecular dynamics simulation methods based on a state-of-the-art hybrid density functional. We identify the reactive surface trapped holes as the OH center dot on five-coordinated terminal Ti and its deprotonated O center dot-. Our calculations show that the large reorganization energies hold the key to reconciling some of the conflicting interpretations of spectroscopic and thermodynamic measurements reported in the literature. We also observe an asymmetry in reorganization energies owing to the pinning of the valence band of TiO2. This has important implications for the understanding of the heterogeneous electron transfer kinetics driving photo-oxidation.