Reversible wettability control of TiO2 surface by light irradiation

Titanium dioxide (TiO2) thin films with various surface morphologies were prepared on the substrates of (1 0 0) plane of strontium titanate by a metalorganic chemical vapor deposition process. These TiO2 thin films became highly hydrophilic by ultraviolet (UV) irradiation, and returned to the initial relatively hydrophobic state by visible-light (VIS) irradiation. The hydrophobic–hydrophilic conversion induced by UV irradiation, is due to the increase in dissociated water adsorption on the film surface. In contrast, the conversion from hydrophilic to hydrophobic by VIS irradiation is caused by the elimination of water adsorbed on the surface due to the heat generated. Changes of water contact angle between hydrophilic states and hydrophobic ones strongly depended on the roughness of the film surface. The wettability of the TiO2 surface was enabled to convert between hydrophilic and hydrophobic reversibly by alternative UV and VIS irradiation.