Effect of surface morphology on wettability conversion

A hierarchical structural surface of TiO₂ film with reversibly light-switchable wettability between superhydrophobicity and superhydrophilicity on metal substrate was fabricated through simply dip-coating method from TiO₂ precursor solution containing TiO₂ nanoparticles with the average diameter 25 nm (P25), followed by heat-treatment and modification with fluoroalkylsilane (FAS) molecules. The morphology, phase and crystallographic structure, and chemical composite of the as-prepared TiO₂ film were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The wettability of TiO₂ film was characterized by a drop shape analyzer. The water contact angle of superhydrophobic TiO₂ film was up to 165.6°. Under UV irradiation, the water contact angle decreased and the superhydrophobic TiO₂ film became superhydrophilic because of hydroxyl groups absorption on the TiO₂ surface. Meanwhile, the surface morphology of TiO₂ film, which resulted from the TiO₂ nanoparticles added in TiO₂ precursor solution, had a significant effect on the wettability conversion of TiO₂ film and enhanced the switch from hydrophobicity to hydrophilicity. The wettability could be reversibly switched between superhydrophobicity and superhydrophilicity via alternation of UV exposure and dark storage.