Rapid TiO2 Nanotubes Formation in Aged Electrolyte and Their Application as Photocatalysts for Cr(VI) Reduction Under Visible Light

Heterogeneous photocatalysis based on semiconductor oxide is a promising technology for treating polluted industrial waste water. Cr(VI) is an example of toxic and mobile water pollutant originated mainly from industrial processes like steel industry and electroplating. The removal of Cr(VI) can be done by photoreduction on a surface of photocatalysts like TiO2 nanotubes (TNTs). Here we describe a fast-rate formation of highly ordered TNTs via two steps anodization processes in an aged electrolyte containing fluoride-ethylene glycol. First step anodization was done for template formation as well as for saturating the electrolyte with TiF6 ² - and second anodization was done to grow TNTs. The template was formed by removing initial anodic layer through sonication. Anodization voltage was kept constant at 60 V. After an hour of anodization, 22.70 μm long TNTs were formed with the rate of ~350 nm/min. The as-anodized TNTs were then annealed in N2 gas at different temperatures. The annealed TNTs were then used as photocatalyst for the photoreduction of Cr(VI) under sunlight in a parabolic trough reflector. The TNTs annealed at 450 °C in nitrogen show the best photoreduction activity with ~80% Cr(VI) removal after 5 h exposure to sunlight. This is due to the presence of anatase TiO2 as well as a possibility of nitrogen insertion, which may have narrowed the band gap of the oxide.