The electrolyte contained 0.5 wt% NH 4F, 5 mL H 2O, and 195 mL ethylene glycol. A DC power supply (WYK-6010, 0–60 V, and 0–10 A) was used to control the experimental current and voltage for 1.2 h. The anodization was carried out in a two-electrode electrochemical cell with a graphite sheet as the cathode at a constant potential 60 V. The titanium sheets were cleaned by sonicating in 1 : 1 acetone and ethanol solution, followed by being rinsed with deionized water and dried in airstream. The titanium foils (0.6 mm thick, 99.5% purity, and cut in 1 cm × 2 cm) were used as the substrates for the growth of the TiO 2 nanowire arrays. In this work, the N-doped nanoball films were prepared and their photocatalytic activities were evaluated by the degradation of methyl blue under visible light irradiation. TiO 2 nanostructure materials displayed high performance for their potential in improving photocatalytic activity because of their high surface area. Macak et al., and Shankar et al., and Allam and El-Sayed pointed out that the morphology, crystallinity, composition, and illumination geometry of nanotube arrays were critical factors in their performance as photoelectrodes. Recently, it was recognized that compared with metal doping (Ca 2+, Sr 2+, and Ba 2+), transition metal ions (Fe 3+, Cr 6+, Co 3+, and Mo 5+ ), rare earth cations (La 3+, Ce 3+, Er 3+, Pr 3+, Gd 3+, Nd 3+, and Sm 3+), and some nonmetal doping (C, S, and F ), nitrogen-doped TiO 2 exhibited a valid process for narrowing the band gap and demonstrated a more appropriate solution for extending the photocatalytic activity of TiO 2 into the visible region. It is of great significance to enlarge the TiO 2 absorption band border to visible light range and to improve the photocatalytic efficiency of the TiO 2 that can be used in visible light irradiation. However, the wideband gap of TiO 2 (3.2 eV for the anatase phase and 3.0 eV for the rutile phase) needs ultraviolet (UV) light for electron-hole separation, which is only 5% of the natural solar light.

Introductionĭue to the strong photocatalytic activity, antiphotocorrosion ability, biologic compatibility, and chemical stability of TiO 2, TiO 2 has become the most promising photocatalyst. It is indicated that there may be two main reasons for the enhanced photocatalytic activity: the increase of O vacancy and photocatalytic reactivity surface area in nitrogen-doped samples.

The concentrations of methyl blue reduce to nearly 44% after 4-hour photodecomposition test by nitrogen-doped sample. The nitrogen-doped samples exhibited significant enhanced absorption in visible light range, narrowing band gap from 3.2 eV to 2.8 eV and the smaller nanoball diameter size. TiO 2 nanoball films of nitrogen doping and no doping were prepared by anodic oxidation method.

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