Effect of Crystallization Methods on Morphology and Photocatalytic Activity of Anodized TiO2 Nanotube Array Films

Highly ordered TiO2 nanotube array (TNs) thin films were prepared by electrochemical anodization of titanium foil in a mixed electrolyte solution containing Na2SO4, H3PO4, NaF, and sodium citrate and then treated by calcination, vapor-thermal, and hydrothermal methods, respectively. The as-prepared samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The photocatalytic activity of the film samples was evaluated by photocatalytic degradation of methyl orange (MO) aqueous solution under UV light irradiation. The production of hydroxyl radicals ((OH)-O-center dot) on the surface of UV-irradiated samples was detected by a photoluminescence technique using terephthalic acid as a probe molecule. The transient photocurrent response was measured by several on off cycles of intermittent irradiation. It was found that post-treatment exhibited a great influence on the morphology, crystallization, and photocatalytic activity of TNs thin films. Calcination and vapor-thermal treatment had no great influence on surface morphology and architecture of the TNs films. On the contrary, for the hydrothermal treated samples, the nanotube array structures were completely destroyed, and only aggregated anatase panicles were observed. The vapor-thermal-treated films showed higher photocatalytic activity than the calcined and hydrothermal treated films. This was attributed to the fact that the vapor-thermal-treated films had better crystallization than the calcined films and remained tubular structures compared with the hydrothermal-treated samples. Good crystallization and remaining tubular structures resulted in the highest photocatalytic activity of the vapor-thermal-treated films.