N-Doped TiO2 Nanotube Arrays: Synthesis by Anodization in an Ionic Liquid ([BMIM]BF4) and Assessment of Photocatalytic Property

N-doped TiO2 nanotube (NT) arrays were prepared by anodization in electrolytes containing HF, ionic liquid 1-n-butyl-3-methyl-imidazolium tetrafluoroborate ([BMIM]BF4) and water. The ionic liquid, [BMIM]BF4, was used as the nitrogen source. The surface morphology and the composition of samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR) and UV-Vis diffuse reflectance spectra (DRS). The photocatalytic activities of samples were investigated by the degradation of methylene blue (MB) under spherical xenon lamp. The results show that the degradation rate of MB over N-doped TiO2 NT arrays is obviously higher than that over undoped TiO2 NT arrays. The above results are caused by the increased oxygen vacancies and the decreased band gap.