Photocatalytic performance of freestanding tetragonal zirconia nanotubes formed in H₂O₂/NH₄F/ethylene glycol electrolyte by anodisation of zirconium

ZrO₂ nanotubes (ZrNTs) were produced by anodisation of zirconium foil in H₂O₂/NH₄F/ethylene glycol electrolyte. The as-anodised foils were then soaked in the anodising electrolyte for 12 h. Soaking weakens the adherence of the anodic layer from the substrate resulting in freestanding ZrNTs (FS-ZrNTs). Moreover, the presence of H₂O₂ in the electrolyte also aids in weakening the adhesion of the film from the foil, as foil anodised in electrolyte without H₂O₂ has good film adherence. The as-anodised FS-ZrNTs film was amorphous and crystallised to predominantly tetragonal phase upon annealing at >300 °C. Annealing must, however, be done at <500 °C to avoid monoclinic ZrO₂ formation and nanotubes disintegration. FS-ZrNTs annealed at 450 °C exhibited the highest photocatalytic ability to degrade methyl orange (MO), whereby 82% MO degradation was observed after 5 h, whereas FS-ZrNTs with a mixture of monoclinic and tetragonal degraded 70% of MO after 5 h. (paper)