Superhydrophobic TiO₂ surfaces: Preparation, photocatalytic wettability conversion, and superhydrophobic-superhydrophilic patterning

We report herein the preparation and UV-stimulated wettability conversion of superhydrophobic TiO₂ surfaces, as well as the preparation of superhydrophilic-superhydrophobic patterns by use of UV irradiation through a photomask. A CF₄ plasma was used to roughen smooth TiO₂ sol-gel films to produce a nanocolumnar morphology, and subsequent hydrophobic modification with octadecylphosphonic acid (ODP) rendered the roughened surfaces superhydrophobic. The superhydrophobic properties of these surfaces were evaluated by both static and dynamic water contact angle (CA) measurements. It was found that the surface morphology of the TiO₂ film, which was dependent on the etching time, has a great influence on the observed superhydrophobic properties. The nanocolumnar surface morphology exhibited large water CA and small contact angle hysteresis (CAH); this is discussed in terms of the Wenzel equation and the Cassie-Baxter equation. Under low-intensity UV illumination (1 mW cm^-2), the superhydrophobic TiO₂ surface underwent a gradual decrease of water CA and finally became superhydrophilic, due to photocatalytic decomposition of the ODP monolayer. Readsorption of ODP molecules led to the recovery of the superhydrophobic state. This UV-stimulated wettability conversion was employed to prepare superhydrophilic stripes (50 and 500 μm wide) on a superhydrophobic TiO ₂ surface. The pattern was able to guide water condensation, as well as the evaporation of a polystyrene microsphere suspension, due to the extremely large wettability contrast between superhydrophobic and superhydrophilic areas.