The effects of annealing temperature on photocatalytic activity of TiO2 nanotubes grown by anodic oxidation

Highly ordered and self-organized TiO2 nanotube arrays were grown on titanium foil by an anodic oxidation method in an electrolyte containing ethylene glycol, ammonium fluoride (NH4F) (0.4% wt), and deionized water (5% wt). As-prepared amorphous titanium dioxide (TiO2) nanotube arrays were annealed at various ambient temperatures for 1 hour. The crystal phase characterization of TiO2 nanotube arrays was characterized using X-Ray diffraction (XRD). In addition, phase transition was controlled via high temperature XRD (HT-XRD). The morphology of nanotube arrays was analyzed using a field emission scanning electron microscope (FESEM). The effect of annealing temperature on photoelectrochemical performance of prepared samples was evaluated by electrochemical impedance spectroscopy (EIS) and photo-electrocatalytic degradation of Rhodamine B (RhB) dye. Results indicated that nanotube morphology and photo-electrocatalytic performance was dramatically altered at an annealing temperature of 700 °C. Nanotubular morphology was totally deformed and no photo-electrocatalytic activity was observed due to anatase to rutile phase transition at 900 °C. The pure anatase nanotube arrays have the highest photoelectrochemical activity at 500 °C annealing temperature due to better electron transport, lower recombination rate, good crystallinity, and nanotubular morphology.