Bulk and surface polarons in photoexcited anatase TiO₂

Using hybrid functional electronic structure calculations, we have investigated the structure and energetics of photogenerated electrons and holes in the bulk and at the (101) surface of anatase TiO₂. Excitons formed upon UV irradiation are found to become self-trapped, consistent with the observation of temperature-dependent Urbach tails in the absorption spectrum and a large Stokes shift in the photoluminescence band of anatase. Electron and hole polarons are localized at Ti³⁺ and O^- lattice sites, respectively. At the surface, the trapping sites generally correspond to undercoordinated Ti³⁺_5c and O^-_2c surface atoms or to isolated OH species in the case of a hydroxylated surface. The polaron trapping energy is considerably larger at the surface than in the bulk, indicating that it is energetically favorable for the polarons to travel from the bulk to the surface. Computed one-electron energy levels in the gap and hyperfine coupling constants compare favorably with oxidation potential and EPR measurements.