Enhanced reversible wettability conversion of micro-nano hierarchical TiO2/SiO2 composite films under UV irradiation

A new type of micro-nano hierarchical TiO2/SiO2 composite film, which is formed by nanoparticles of TiO2 deposited on microspheres of SiO2, has been prepared through a convenient and size-controllable method. A water-resistant agent (Aquapel PPG Co., USA) is grafted onto the composite films to achieve surface hydrophobicity. The morphologies, formations, and wettability of the novel films were investigated by field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and water contact angle (WCA), respectively. The results indicate that these composite films can achieve reversible wetting and are reproducible over several cycles of ultraviolet (UV) exposure and dark storage. The specific micro-nano structures enabled significant variation in wetting, due to the effect on both the hydrophilic and hydrophobic states of these tunable surfaces. Our study demonstrated that the wettability transition increased with decreased TiO2/SiO2 size ratios, which resulted in a maximum wetting contrast in WCA of 103.8°, which is nearly twice the contrast of TiO2 nano-structured film. A simple theoretical model was proposed to elucidate the correlation between size ratio and WCA, which indicated good agreement with the experimental data. These findings offer a new method for the design of intelligent surfaces with tailored and reversible wettability.