Natural hydrophobicity and reversible wettability conversion of flat anatase TiO₂ thin film

Flat anatase TiO2 thin film deposited at room temperature shows the natural hydrophobicity, which is destroyed by 400 °C vacuum annealing. On the basis of the analysis of surface composition and structure, the origin of hydrophobicity of the flat TiO2 film can be identified as (1) approximately fully stoichiometric TiO2 and (2) hydrocarbon adsorbates on the film surface. We further validate that interfacial water molecules near the surface of the as-prepared TiO2 film are oriented in the hydrophobic hydration structure via Fourier transform infrared/attenuated total reflection. Moreover, the as-prepared TiO2 film also shows a smart surface reversibly switched between hydrophobicity and super-hydrophilicity. During the recovery process of hydrophobicity, the irradiated films show the wettability with water contact angle of 107 ± 1.7, 72 ± 2.5, 80 ± 1.1, and 17 ± 1.3° corresponding to after a week of exposure to ambient air, O2, CF4, and Ar, respectively. It can be strongly reinforced that the stoichiometry and the adsorbates both play an important role in forming the hydrophobic TiO2 films.