Photoinduced surface wettability conversion of ZnO and TiO2 thin films

The photoinduced surface wettability conversion reactions of ZnO and TiO2 t hin films were investigated by means of water contact angle measurement and X-ray photoelectron spectroscopy. Before ultraviolet (UV) illumination, Zn O and TiO2 films exhibited water contact angles of similar to 109 and simil ar to 54 degrees, respectively. UV illumination turned both surfaces to hig hly hydrophilic with water contact angles smaller than 10 degrees, Storage in the dark reconverted the highly hydrophilic films to their original stat es. Reversible surface wettability conversion reactions were achieved by al ternate UV illumination and storage in the dark on both the films. The simi lar behaviors of wettability conversion observed on ZnO and TiO2 surfaces s uggest that they follow a similar conversion mechanism. Preferential adsorp tion of water molecules on the photogenerated surface defective sites is as cribed to the formation of highly hydrophilic ZnO and TiO2 surfaces. Achiev ement of highly hydrophilic ZnO and TiO2 surfaces by high-temperature annea ling and Ar+ sputtering provided supporting evidence for the explanation th at surface defective sites play crucial roles in causing the surface wettab ility conversion reactions.