The improved photo-induced hydrophilicity of ZnO by adjusting the solution concentration

The microstructure, surface topography and wetting properties of zinc oxide (ZnO) thin films, which were grown at different concentrations of zinc nitrate hexahydrate, were measured by x-ray diffractometry, scanning electron microscopy and water contact angle apparatus. It is revealed that at concentrations larger than 0.01 mol l(-1), one peak corresponds to the (111) direction of the cubic phase of ZnO, except for six peaks of the hexagonal phase of ZnO, was observed. As the concentration increases, the average diameter and density of the ZnO nanorods increase but the area proportion of the nonpolar planes in the outermost surface increase first, then decrease. All the ZnO thin films exhibit hydrophobic behavior. After 120 min of ultraviolet irradiation, the wettability of the ZnO surfaces change from hydrophobicity to hydrophilicity. The photo-induced change of the water contact angle increases first then decreases with the increase of concentration. The differences of photo-induced hydrophilicity may be determined by the transition of the wetting model, the area proportion of the nonpolar planes and the surface roughness.