Formation of grassy TiO nanotube thin film by anodisation in peroxide electrolyte for Cr(VI) removal under ultraviolet radiation.

Arrays of TiO nanotubes (TiO NTs) with grassy surfaces were observed on titanium foil anodised at 60 V in fluorinated ethylene glycol (EG) with added hydrogen peroxide (HO). The grassy surface was generated by the chemical etching and dissolution of the surface of the TiO NTs walls, which was accelerated by the temperature increase on the addition of HO . Upon annealing at 600 °C, the grassy part of the TiO NTs was found to consist of mostly anatase TiO whereas the bottom part of the anodic oxide comprised a mixture of anatase and rutile TiO. The TiO NTs were then used to reduce hexavalent chromium (Cr(VI)) under ultraviolet radiation. They exhibited a rather efficient photocatalytic effect, with 100% removal of Cr(VI) after 30 min of irradiation. The fast removal of Cr(VI) was due to the anatase dominance at the grassy part of the TiO NTs as well as the higher surface area the structure may have. This work provides a novel insight into the photocatalytic reduction of Cr(VI) on grassy anatase TiO NTs.