Effect of acid solution, fluoride ions, anodic potential and time on the microstructure and electronic properties of self-ordered TiO2 nanotube arrays

Self-ordered TiO2 nanotube arrays were fabricated by the anodization of Ti foil in acid solution containing fluoride ions in the present work. The influences of the acid solution, fluoride ions, anodic potential and time on the microstructure and electronic properties of TiO2 nanotube arrays (TNTs) were also systematically investigated by SEM, Raman spectroscopy, EIS and Mott-Schottky analysis. The SEM results show that TNTs are formed in a double-layer structure, i.e., as the upper TNTs and a lower barrier film. The lengths of TNTs and the thickness of the barrier film increased with increasing potential and time, reached maximum value in phosphoric acid solution containing NH4F, and reached minimum value in oxalic acid solution containing NaF. The crystallinity of TNTs was enhanced with increasing anodic potential and time, and TNTs fabricated in NH4F solution had the best crystallinity. The influences of the acid solution, fluoride salt, anodic potential and time on the electronic properties of TNTs were similar to those on the crystallinity.