Acceleration of wettability switching on TiO2 thin films under ultraviolet irradiation and direct current bias voltage

Abstract

This study reports the results of combining ultraviolet (UV) light irradiation and direct current (DC) bias voltage on the acceleration of the photoresponsive wettability switching of TiO2 thin films. According to the mechanism of wettability switching reported previously, hydrophilic surface termination is caused by the reaction between the photogenerated holes and bridging oxygen sites on the TiO2 surface. In this study, we investigate the effect of applying DC bias voltage to TiO2 thin films to intentionally polarize holes toward the TiO2 surface for increasing and accelerating the generation of absorption sites. The wettability switching width of the contact angles from the hydrophobic state to hydrophilic state was virtually doubled and the switching time of the contact angles was reduced to approximately 1/10th by combining a DC bias voltage of 0.9 V and UV irradiation, compared to TiO2 thin films processed only with UV irradiation. This photoresponsive wettability switching based on TiO2 photocatalysts is a promising technology for wireless liquid manipulation as amplification of the switching width and acceleration of the switching response of the surfaces are important for practical applications.