Anodic TiO2 nanotube supercapacitors enhanced by a facile in situ doping method

In this work, the capacitive properties of anodic TiO2 nanotubes were enhanced by a facile in situ doping method. The tuning of TiO2 anodization was realized by adding Fe(NO3)3 into the aqueous HF solution as the dopant. The morphology, structure, chemical composition and capacitive properties of the as-prepared TiO2 nanotubes were characterized by various methods. It is found that N, F, and Fe elements can be doped into TiO2 nanotubes during the anodization process. The effect of doping concentration on the capacitive properties of TiO2 nanotubes was also investigated. With the optimum doping concentration of ~ 0.02 M, the doped TiO2 nanotubes exhibited a capacitance of 1.11 mF cm−2 at the scan rate of 100 mV s−1, much higher than that of the undoped TiO2 nanotubes. The X-ray photoelectron spectroscopy (XPS) results indicated the presence of N, F, and Fe in the TiO2 lattice and absorbed F on the TiO2 surface, both of which are believed to be the cause for the capacitance enhancement of the doped TiO2 nanotubes.