The surface wettability of TiO₂ nanotube arrays: which is more important—morphology or chemical composition?

The wettability of TiO₂ nanotube arrays (TiO₂-NTAs) synthesized by electrochemical anodization was intensively investigated. It was found that annealing temperature of TiO₂-NTAs has significant effect on the hydrophilicity of TiO₂-NTAs. With the increase of annealing temperature, the fluorine element content on TiO₂-NTAs surface decreases, which results in decrease in water contact angle and increase in hydrophilicity for TiO₂-NTAs. The reason is that F⁻ ions escape from the lattice and oxygen vacancies are created at the two coordinated oxygen bridging sites at TiO₂-NTAs surface after annealing in argon atmosphere. And these defects can in turn increase the affinity for hydroxyl ions formed by dissociation of chemisorbed water molecules and thereby form hydrophilic domains. In addition, TiO₂ crystal becomes well organized gradually with the increase of annealing temperature, F⁻ ions are not favored to exist in the lattice and thus escape from the lattice. Less F element content results in better hydrophilicity of TiO₂-NTAs.