A general approach to the fabrication of Sn-doped TiO2 nanotube arrays with titanium vacancies for supercapacitors

In general, it is more difficult to realize the metal doping of TiO2 in comparison with the nonmetal doping. Currently, metal-doped TiO2 is mainly prepared by hydrothermal treatment, sol-gel method and high-energy ion implantation. However, these methods have some drawbacks such as cumbersome steps and the need for the use of environmentally harmful reagents or expensive equipment. Here, we report a general approach to realize the metal doping in TiO2 nanotube arrays (NTAs) by using low-melting-point metals as dopant sources. As an example, after thermal annealing of the anodized TiO2 NTAs together with tin metal with a lower melting point in argon atmosphere, Sn-doped TiO2 NTAs are successfully achieved. Unlike common TiO2, the as-prepared Sn-doped TiO2 NTAs have structural defects of Ti vacancies, showing p-type conductivity. Moreover, the Sn-doped samples exhibit significantly enhanced supercapacitive performances compared to the undoped counterpart. Especially, the presented doping methodology is conceptually different from the previous works and is much simpler and more effective, which offers a general technique to realize metal doping of TiO2 NTAs.