Charge Behavior of Terminal Hydroxyl on Rutile TiO₂(110)

Titanium dioxide (TiO₂) is of considerable interest as a photocatalyst and a catalyst support. Surface hydroxyl groups (OH) are the most common adsorbates on the TiO₂ surface and are believed to play crucial roles in their applications. Although the characteristics of bridging hydroxyl (OH_br) have been well understood, the adsorption structure and charged states of terminal hydroxyl (OH_t) have not yet been experimentally elucidated at an atomic scale. In this study, we have investigated an isolated OH_t on the rutile TiO₂(110) surface by atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). We found that OH_t is in a negatively charged state. The unique characteristic of OH_t is different from that of OH_br and involves the amphoterism and diversity of catalytic reactions of TiO₂.