Thermoinduced and Photoinduced Sustainable Hydrophilic Surface of Sputtered-TiO2 Thin Film

To achieve self-cleaning at a low maintenance cost, we investigated the possibility of obtaining a sustainable hydrophilic surface of TiO2 thin film. As the hydrophilicity of TiO2 films fabricated by FTS has not yet been studied, we deposited TiOx using FTS, and then TiO2 was formed through additional treatment. Hydrophilic surfaces were obtained by thermoinduced and photoinduced methods. UV irradiation led to the conversion of Ti4+ to Ti3+ in the lattice structure and an increase in the number of OH groups on the surface, and annealing induced the formation of Ti3+ defect sites, as well as organic degradation and changes in the crystal structure. Through the annealing process, the water contact angle of as-deposited film was decreased from 78.7° to 35.7°, and crystallinity changed from amorphous to anatase. These changes contributed to the formation of a hydrophilic surface and reduced the water contact angle by up to 10.8°. After the formation of a hydrophilic surface through annealing and UV irradiation, the sample returned to its original state. We confirmed that the water contact angle of the returned sample was decreased through exposure to sunlight; it reduced the water contact angle of the returned sample by 15.2°. Thus, the results revealed that the crystallinity influences the hydrophilicity and its sustainability for TiO2 films under sunlight.