Nanometric multiscale rough Zn–ZnO superhydrophobic thin films: Self-diffusion of zinc and effect of UV irradiation

Zn–ZnO superhydrophobic thin films have been prepared by thermal oxidation of sputtered Zn. The superhydrophobicity observed in these coatings is 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 has been observed only for an optimum combination of solid regions (i.e., nanoclusters) and air pockets. Experimental evidences have been provided to demonstrate that an additional micron-scale roughness on the substrate does not affect the wettability of the coating. It has been observed that the wettability of the Zn–ZnO coatings changes from hydrophobic to superhydrophobic during the initial 24 h after deposition. This occurs due to the outward self-diffusion of Zn to the surface and its subsequent oxidation under ambient conditions. Field-emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) 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 UV irradiation on the Zn–ZnO coatings. The UV irradiation transformed the Zn–ZnO surface from superhydrophobic to hydrophilic. Our studies based on FESEM, AFM, micro-Raman spectroscopy, and roughness profilometry show that this transformation was mainly due to the morphological changes that occur in addition to the chemical changes taking place on the ZnO surface under the influence of UV irradiation. The UV irradiation disturbs the optimum density of air pockets, leading to the loss of superhydrophobicity.