Preparation and Photocatalytic Properties of ZnO Deposited TiO₂ Nanotube Arrays by Anodization.

ZnO doped TiO₂ nanotube arrays (ZnO-TNTs) with an average diameter of 6080 nm and an average length of 24 m were prepared on the Ti substrate by a one-step anodizing method using NH₄F/ethylene glycol as the electrolyte. The phase structure, morphology, chemical composition, photocatalytic property and mechanism of TNTs were studied by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, transmission electron microscopy, UV-vis diffuse reflectance spectra, photoluminescence and photocatalytic degradation of methylene blue under visible light. The results showed that highly ordered ZnO doped TNTs were successfully prepared by anodization. ZnO nanoparticles were dispersively distributed inside the walls of TNTs. ZnO-TNTs having a different amount of ZnO was prepared by adjusting the concentration of Zn(NO₃)₂·6H₂O in the electrolyte. It was found that by changing doping amount of ZnO, the width of the TiO₂ band gap and the recombination rate of photo-generated electron-hole pairs also changed. The TNTs which doped with 1 mM ZnO showed the best degradation rate of methylene blue (MB). At the concentration of 1 cm²·mL of ZnO-TNTs, the degradation rate reached at the level of 81.9% with 8 mg·L methylene blue aqueous solution. Compared with undoped TNTs, the band gap of ZnO-TNTs reduced from 3.25 eV to 2.75 eV, and the recombination rate of photo-generated electron-hole pairs decreased significantly. The film of ZnO-TNTs prepared by the one-step anodizing method depicted excellent photocatalytic properties under visible light.