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

It is well known that the amount of physisorbed water molecules decreases by thermal and evacuating treatments at anatase TiO2 surfaces. (1- 3)) However, the initial water contact angle of the anatase TiO2 surface coated on the glass was little changed (from 24 degrees +-/ 1 degrees to 23 degrees +/- 1 degrees) and the hydrophilicity did not change by the heating treatment at 150 degreesC for 1 hour. On the other hand, the value of its initial contact angle increased remarkably (from 24 degrees +/- 1 degrees to 67 degrees +/- 1 degrees), and the TiO2 surface was hydrophobicized by the evacuation at similar to 5x10(-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 degreesC for 1 hour. However, photo-induced hydrophilic conversion rates were little changed by the evacuating treatment at similar to 5x10(-6) Pa. These results suggest that the photo-induced hydrophilic conversion phenomena are not necessarily affected by the initial amount of physisorbed water molecules.