Mechanism of Photoinduced Superhydrophilicity on the TiO2 Photocatalyst Surface

The physicochemical properties of the H2O molecules adsorbed on TiO2 surfaces during UV light irradiation were fully investigated by near-infrared (NIR) absorption spectroscopy. It was found that the H2O molecules adsorbed on the TiO2 surfaces desorb during UV light irradiation by the heating effect of the light source. Since the amount of the H2O adsorbed on the TiO2 surfaces decreased, the distribution of the hydrogen bonds within the H2O molecules decreased, resulting in a decrease in the surface tension of the H2O clusters. The decrease in the surface tension of H2O under UV light irradiation was found to be one of the most important driving forces in which the H2O clusters on the TiO2 surface spread out thermodynamically, forming H2O thin layers. The partial elimination of the hydrocarbons from the TiO2 surface by the photocatalytic complete oxidation was seen to be the other important factor, providing free spaces on the surface where the H2O clusters could spill over and spread out to form the thin H2O layers. Moreover, the temperature changes of the TiO2 powder samples during UV light irradiation were found to show a good correspondence with the changes in the contact angle of the H2O droplets on the TiO2 thin film surfaces. Especially the time scale for the hydrophilic conversion on the TiO2 surfaces under UV light irradiation was in good agreement with the decrease in the amount of H2O molecules adsorbed on the TiO2 surfaces but not the amount of the hydrocarbons eliminated by the photocatalytic oxidation reactions, showing that the adsorption and desorption of H2O molecules are generally quite sensitive to the temperature changes of solid surfaces.