Stability of excess electrons introduced by Ti interstitial in rutile TiO₂(110) surface

The surface chemistry of rutile TiO₂ (110) has long been ascribed to surface bridging oxygen vacancies (VO) and accompanying excess electrons. However, recently there has been debate whether titanium interstitials (Tiint), a subsurface defect, participates in the surface reactions of TiO₂ (110). We used a combination of ab initio molecular dynamics and static density functional theory calculations to systematically investigate the spatial distribution of excess electrons introduced by Ti^int. We found that these excess electrons form polarons and that the most stable structure had one polaron located at the Ti_int site, and the other three at second-layer Ti sites below five-coordinate Ti sites. This behavior of Tiint providing excess electrons to the near-surface region is similar to that of VO, which suggests that Tiint may contribute to the surface chemical reactions of TiO₂ (110).