Studies of Surface Wettability Conversion on TiO₂ Single-Crystal Surfaces

Reversible surface wettability conversion on titanium dioxide (TiO 2 ) single crystals has been achieved, and its mechanism has been examined by means of contact angle measurement and X-ray photoelectron spectroscopy (XPS). A UV light illuminated TiO 2 single-crystal surface exhibited a 0° contact angle for both water and oil, indicative of a highly amphiphilic surface against its native hydrophobic surface. This was ascribed to photoreduction of surface Ti 4+ to Ti 3+ at definite sites, leading to preferential adsorption of dissociative water on top. A long-term storage of the highly amphiphilic surface resulted in reconversion of the surface wettability. It was found that the amphiphilic-to-hydrophobic reconversion is due to the replacement of the adsorbed hydroxyl groups by oxygen, which returns the surface geometric and electronic structures similar to the native TiO 2 surface. The result of angle-resolved XPS measurement revealed that the surface reactions occurred at the uppermost layers of the single crystals. By comparing the reactivities of (110), (100), and (001) single-crystal surfaces, it was concluded that oxygen bridging sites played an important role in the surface wettability conversions.