Defects, Adsorbates, and Photoactivity of Rutile TiO2 (110): Insight by First-Principles Calculations.

We investigate the effect of adsorbates on the structure and photoabsorption of reduced TiO2 by first-principles calculations of rutile TiO2(110) in the presence of both titanium interstitials (Tiint's) and adsorbed water or methanol. Our results show that while Tiint's prefer to reside in deep inner layers when the surface is clean, they tend to diffuse toward the surface in the presence of water or methanol. This migration is due to the mutual stabilization of the adsorbates and Tiint defects in the near-surface region. We also find that adsorbed water/methanol changes the orbital character and localization sites of the excess electrons associated with the Tiint. These results can explain why the adsorption of water and methanol enhances the photoabsorption of the reduced TiO2(110) surface.