Vacuum ultraviolet responded rapid reversible conversion of super-hydrophobicity and super-hydrophilicity of ZnO films

ZnO films, with rapid reversible transition properties of super-hydrophobicity and super-hydrophilicity under vacuum ultraviolet (VUV) irradiation, were prepared by a simple and low-temperature solution method by controlling the pH value with hydrofluoric acid. The surface of the film exhibits hierarchical structure with nanostructure on the micro-spheres mimicking to the taro leaf surface. The fresh film shows the water contact angle (WCA) of 151°,turning into a super-hydrophilic (WCA〈5°) one after 30 min VUV irradiation. The super-hydrophilic film can be recovered to super-hydrophobic through being placed in the dark for 6 d. The wettability of the film can be reversibly switched circularly by the alternation of VUV irradiation and dark storage. The transition from super-hydrophobic to super-hydrophilic is more quickly than that of the past reports due to the use of VUV and the special hierarchical structure of the ZnO film surface. This study is expected to promote the applications of ZnO materials in micro-fluidic devices.