Subsurface Polaron Concentration As a Factor in the Chemistry of Reduced TiO₂ (110) Surfaces

Surface reactivity of rutile TiO₂ (110) surfaces has long been ascribed to bridging oxygen vacancies (V_O), but recently, excess electrons introduced by donor defects are being considered as the main players. However, the spatial distribution of them is not yet clear due to difficulties in interpreting filled state images of scanning tunneling microscopy (STM). In this study, several different images available in the literature are consistently interpreted using density functional theory (DFT). The key factors are polarons in the second layer below Ti_5c row (Ti_5c-2nd polarons) and a temperature dependence of their concentration. Bright blobs in the experimental images are interpreted as Ti_5c-2nd polarons. At 78 K, their concentration reaches 33.3% ML, where 1 ML is defined as the density of (1 × 1) unit cells, regardless of V_O coverage. In contrast, at 5 K, it is twice the V_O coverage. This discrepancy is understood by the ionization of donor defects other than V_O, most probably subsurface Ti interstitials, and subsequent diffusion of polarons to Ti_5c-2nd sites at high temperature. This mechanism explains seemingly contradicting reports on oxygen chemisorption on this surface, which suggests that the so-called oxygen-vacancy model needs to be modified at temperature above at least 78 K.