Zn-ZnO superhydrophobic thin films were prepared by thermal oxidation of sputtered Zn. The superhydrophobicity observed in these coatings was attributed to multiscale roughness in the nanometric range only. The higher scale roughness was due to the combination of nanoclusters (solid regions) and air gaps while the lower scale roughness was due to the textured surface created by the fusion of individual ZnO nanocrystals to form the nanoclusters. The superhydrophobicity in these coatings was observed only for an optimum combination of solid regions (i.e., nanoclusters) and air pockets. Experimental evidences were provided to demonstrate that an additional micron-scale roughness on the substrate did not affect the wettability of the coating. It was observed that the wettability of the Zn-ZnO coatings changed from hydrophobic to superhydrophobic during the initial 24h after deposition. This occurred due to the outward self-diffusion of Zn to the surface and its subsequent oxidation under ambient conditions. Field-emission scanning electron microscopy and atomic force microscopy were used to demonstrate the morphological changes while micro-Raman spectroscopy was used to record the chemical changes on the coating surface as a result of the outward diffusion and subsequent oxidation of Zn. Studies have also been carried out to determine the effect of ultra-violet irradiation on the Zn-ZnO coatings. The ultra-violet irradiation transformed the Zn-ZnO surface from superhydrophobic to hydrophilic. Studies based upon field-emission scanning electron microscopy, atomic force microscopy, micro-Raman spectroscopy, and roughness profilometry showed that this transformation was due mainly to the morphological changes that occurred in addition to the chemical changes taking place on the ZnO surface under the influence of ultra-violet irradiation. The ultra-violet irradiation disturbed the optimum density of air pockets, leading to the loss of superhydrophobicity.

Nanometric Multiscale Rough Zn-ZnO Superhydrophobic Thin Films: Self-Diffusion of Zinc and Effect of UV Irradiation. Barshilia, H.C., Tej, K.R.S., Devi, L.M., Rajam, K.S.: Journal of Applied Physics, 2010, 108[7], 074315