Surface Wettability of ZnO-Loaded TiO₂ Nanotube Array Layers

Herein we report on the synthesis and the effects of gradual loading of TiO₂ nanotube array layers with ZnO upon surface wettability. Two-step preparation was chosen, where TiO₂ nanotube layers, grown in a first instance by anodization of a Ti foil, were gradually loaded with controlled amounts of ZnO using the reactive RF magnetron sputtering. After crystallization annealing, the formerly amorphous TiO₂ nanotubes were converted to predominantly anatase crystalline phase, as detected by XRD measurements. The as-prepared nanotubes exhibited a well-aligned columnar structure, 1.6 μm long and 88 nm in diameter, and a small concentration of oxygen vacancies. Ti²⁺ and Ti³⁺ occur along with the Ti⁴⁺ state upon sputter-cleaning the layer surfaces from contaminants. The Ti²⁺ and Ti³⁺ signals diminish with gradual ZnO loading. As demonstrated by the VB-XPS data, the ZnO loading is accompanied by a slight narrowing of the band gap of the materials. A combined effect of material modification and surface roughness was taken into consideration to explain the evolution of surface super-hydrophilicity of the materials under UV irradiation. The loading process resulted in increasing surface wettability with approx. 33%, and in a drastic extension of activation decay, which clearly points out to the effect of ZnO-TiO₂ heterojunctions.