Surface Wettability of ZnO-Loaded TiO(2) Nanotube Array Layers

Herein we report on the synthesis and the effects of gradual loading of TiO(2) nanotube array layers with ZnO upon surface wettability. Two-step preparation was chosen, where TiO(2) 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 TiO(2) 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. Ti(2+) and Ti(3+) occur along with the Ti(4+) state upon sputter-cleaning the layer surfaces from contaminants. The Ti(2+) and Ti(3+) 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-TiO(2) heterojunctions.