Studies of Surface Wettability Conversion on TiO₂ Single-Crystal Surfaces

TLDR

The wettability changes are driven by photoreduction of Ti⁴⁺ to Ti³⁺ that promotes water adsorption, and by hydroxyl replacement with oxygen that restores the native TiO₂ surface structure. UV illumination renders TiO₂ single crystals amphiphilic with 0° water and oil contact angles, a state that reverts to hydrophobic after storage, with XPS indicating surface reactions confined to the top layers and oxygen bridging sites being key to the conversion.

Abstract

Reversible surface wettability conversion on titanium dioxide (TiO2) 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 TiO2 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 Ti4+ to Ti3+ 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 TiO2 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.