Formation of grassy TiO₂ nanotube thin film by anodisation in peroxide electrolyte for Cr(VI) removal under ultraviolet radiation

Arrays of TiO₂ nanotubes (TiO₂ NTs) with grassy surfaces were observed on titanium foil anodised at 60 V in fluorinated ethylene glycol (EG) with added hydrogen peroxide (H₂O₂). The grassy surface was generated by the chemical etching and dissolution of the surface of the TiO₂ NTs walls, which was accelerated by the temperature increase on the addition of H₂O₂ . Upon annealing at 600 °C, the grassy part of the TiO₂ NTs was found to consist of mostly anatase TiO₂ whereas the bottom part of the anodic oxide comprised a mixture of anatase and rutile TiO₂. The TiO₂ 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 TiO₂ 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 TiO₂ NTs.