Effect of relative humidity on water contact angle on TiO₂ single crystals: behavior of photoinduced hydrophilicity after turning off UV irradiation

Abstract

Hydrophobic to hydrophilic conversion of TiO₂ has been reported to occur based on measurements of the contact angle between water and a hydrophobic substrate surface as a function of UV irradiation time. The formation of -OH groups is considered to be responsible for the hydrophilicity; therefore, it is possible that the relative humidity in the atmosphere has an effect on the hydrophilicity response. In this study, TiO₂ single crystals were first exposed to UV light at different relative humidity levels and then the UV light was switched off and the water contact angle was measured as a function of time. There was a linear relationship between the contact angle and time. Contact angles 30 s after the water drop first fell on the surface of the substrate after UV irradiation were smaller than those before UV irradiation. However, the contact angle was not dependent on the relative humidity. The slope of the contact angle vs. time curve before/after UV irradiation was dependent on the relative humidity.