Effects of thermal and evacuating treatments on photo-induced hydrophilic conversion at TiO2 surfaces

Abstract

It is well known that the amount of physisorbed water molecules decreases by thermal and evacuating treatments at anatase TiO2 surfaces. However, the initial water contact angle of the anatase TiO2 surface coated on the glass was little changed (from 24°±1° to 23°±1°) and the hydrophilicity did not change by the heating treatment at 150 °C for 1 hour. On the other hand, the value of its initial contact angle increased remarkably (from 24°±1° to 67°±1°), and the TiO2 surface was hydrophobicized by the evacuation at approximately 5×10-6 Pa. These observations may show that not only the amount of physisorbed water molecules but also the structure, i.e., OH groups of TiO2 surfaces are related to the hydrophilicity. The photo-decreasing rate of the water contact angle under the ultra violet (UV) illumination at the anatase TiO2 surface (the photo-induced hydrophilic conversion rate) increased by the heating treatment at 150 °C for 1 hour. However, photo-induced hydrophilic conversion rates were little changed by the evacuating treatment at approximately 5×10-6 Pa. These results suggest that the photo-induced hydrophilic conversion phenomena are not necessarily affected by the initial amount of physisorbed water molecules.