Anodic growth of nanotubular TiO2/Nb2O5 on Ti-Nb alloys for photocatalytic Cr(VI) removal

Hexavalent chromium [Cr(VI)] is a highly toxic and persistent environmental pollutant, posing serious risks to human health and ecosystems. Its photocatalytic reduction to the more benign trivalent chromium [Cr(III)] remains a critical challenge in environmental remediation. This study investigates the formation of nanotubular oxide layers on titanium-niobium (Ti-Nb) alloy pellets via anodization to form a semiconductor photocatalyst for enhanced Cr(VI) reduction. The alloy pellets were prepared using powder metallurgy with Nb contents of 20, 40, and 60 wt% added to Ti powders. After sintering, X-ray diffraction confirmed that Ti-20Nb, Ti-40Nb, and Ti-60Nb primarily contained α-Ti with minor β-Ti phases, with β-Ti increasing as Nb content increased. Anodization of these alloy pellets in a fluoride electrolyte produced well-ordered nanotubular oxide layers with longest nanotubes observed on Ti-40Nb. The anodic oxide was initially amorphous but transformed into a mixed TiO₂/Nb₂O₅ structure upon annealing. While Ti-20Nb and Ti-40Nb were dominated by anatase TiO₂, Ti-60Nb exhibited rather amorphous oxide with HRTEM can see nanocrystallites of TiO₂ and Nb₂O₅. Photocatalysis experiments demonstrated that Ti-60Nb showed the best Cr(VI) reduction performance, achieving complete reduction under UV light within 120 min and 90 % reduction under solar irradiation, outperforming pure Ti. The enhanced performance is attributed to the formation of TiO₂/Nb₂O₅ junctions, which improve charge carrier dynamics suppressing electron-hole recombination.