On the origin of the surface superhydrophobicity of rough-textured inorganic materials with intrinsic hydrophilicity

Abstract

In mechanism, the surface superhydrophobicity is often thought to be the result of trapping-air within the grooves leading to a composite solid-liquid-air interface. However, the mechanism cannot reasonably reflect why the rough surfaces of intrinsically hydrophilic materials are capable of behaving hydrophilicity or superhydrophobicity. In this work, several typical rough-textured inorganic materials (i.e. metal oxide, sulfide, selenide and halide) endowed with intrinsic hydrophilicity are taken as example to reveal the superhydrophobic origin of intrinsically hydrophilic materials. Their wettability for the same rough-textured surface is usually hydrophilic when dried in N₂, while is hydrophobicity or superhydrophobicity when in O₂. The distinct difference in the wettability is closely related to the anion vacancies such as oxygen-, sulfide- and halogen-ion ones. From the generation of H₂O₂ in water-droplet on their rough-textured surfaces, it is found that the H₂O₂ yield increases with an increase of their hydrophobicity, and decreases with enhancing their hydrophilicity, indicating an evidence dependence of their surperhydrophobicity on the absorption of O₂^2- on their surfaces. DFT calculation shows that the introducing Va can give a higher adsorption-energy for oxygen molecular species (especially O₂^2-) than N₂ and H₂O. Hence, we propose that the presence of abundant Va on the intrinsically hydrophilic inorganic materials can result in the formation of preferential O₂^2- adsorption layer in the grooves, which endows these inorganic materials with superhydrophobicity. Our results can provide a further insight into the origin of superhydrophobicity for intrinsically hydrophilic materials, and guide the design of water-proof functional surfaces