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

Abstract

In mechanism, surface superhydrophobicity is often thought to be the result of the trapped air within the grooves of a superhydrophobic surface, 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 showing 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 examples to reveal the superhydrophobic origin of intrinsically hydrophilic materials. The wettability of these rough-textured surfaces is usually hydrophilic when dried in N₂, while it is hydrophobic or superhydrophobic when dried in O₂. This distinct difference in wettability is closely related to anion vacancies of anions such as oxide, sulfide and halide ions. From the generation of H₂O₂ in water droplets on the rough-textured surfaces, it is found that the H₂O₂ yield increases with an increase in their hydrophobicity and decreases with an increase in their hydrophilicity, indicating an evident dependence of their surperhydrophobicity on the absorption of O₂²⁻ on their surfaces. DFT calculation shows that introducing V_a can give a higher adsorption-energy for molecular oxygen species (especially O₂²⁻) than N₂ and H₂O. Hence, we propose that the presence of abundant V_a on intrinsically hydrophilic inorganic materials can result in the formation of preferential O₂²⁻ adsorption layer in their grooves, which endows these inorganic materials with superhydrophobicity. Our results can provide further insight into the origin of superhydrophobicity in intrinsically hydrophilic materials and guide the design of water-proof functional surfaces.