Fast-rate formation of TiO2 nanotube arrays in an organic bath and their applications in photocatalysis

Abstract

In this work, 18.5 µm titanium oxide (TiO2) nanotube arrays were formed by the anodization of titanium (Ti) foil in ethylene glycol containing1 wt% water and 5 wt% fluoride for 60 min at 60 V. The fast growth rate of the nanotube arrays at 308 nm min − 1 was achieved due to the excess fluoride content and the limited amount of water inethylene glycol used for anodization. Limited water content and excess fluoride inethylene glycol inhibited the formation of a thick barrier layer by increasing thedissolution rate at the bottom of the nanotubes. This eased the transport oftitanium, fluorine and oxygen ions, and allowed the nanotubes to grow deep intothe titanium foil. At the same time, the neutral condition offered a protectiveenvironment along the tube wall and pore mouth, which minimized lateral andtop dissolution. Results from x-ray photoelectron spectra revealed that theTiO2 nanotubes prepared in ethylene glycol contained Ti, oxygen (O) and carbon (C)after annealing. The photocatalytic activity of the nanotube arrays produced wasevaluated by monitoring the degradation of methyl orange. Results indicate that ananotube with an average diameter of 140 nm and an optimal tube length of 18.5 µm with a thin tube wall (20 nm) is the optimum structure required to achieve highphotocatalytic reaction. In addition, the existence of carbon, high degree of anatasecrystallinity, smooth wall and absence of fluorine enhanced the photocatalytic activity ofthe sample.