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 (Ti_int's) and adsorbed water or methanol. Our results show that while Ti_int'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 Ti_int 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 Ti_int. These results can explain why the adsorption of water and methanol enhances the photoabsorption of the reduced TiO₂(110) surface.