Photoreduction of Cr(VI) in wastewater by anodic nanoporous NbO formed at high anodizing voltage and electrolyte temperature.

In this study, nanoporous anodic film was produced by anodization of niobium, Nb in a fluoride ethylene glycol electrolyte. The effect of anodization voltage and electrolyte temperature was studied to find an optimum condition for circular, ordered, and uniform pore formation. The diameter of the pores was found to be larger when the applied voltage was increased from 20 to 80 V. The as-anodized porous film was also observed to comprise of nanocrystallites which formed due to high field-induced crystallization. The nanocrystallites grew into orthorhombic NbO after post-annealing treatment. The Cr(VI) photoreduction property of both the as-anodized and annealed NbO samples obtained using an optimized condition (anodization voltage: 60 V, electrolyte temperature: 70 °C) was compared. Interestingly, the as-anodized NbO film was found to display better photoreduction of Cr(VI) than annealed NbO However, in terms of stability, the annealed NbO presented high photocatalytic efficiency for each cycle whereas the as-anodized NbO showed degradation in photocatalytic performance when used continually.