Formation of Titanium Dioxide Nanotubes on Ti-Ni Alloys by Anodization for Cr(VI) Removal from Wastewater

This study evaluated the synthesis of nanotubes (NTs) on a Ti-Ni alloy substrate and application as a photocatalyst for the reduction of toxic Cr-(VI). Ti-Ni alloys (with Ni contents of 1, 5, 10, 20, and 40 wt %) prepared via the powder metallurgy method were anodized under parameters that promote NT formation. Field-emission scanning electron microscopy revealed that NT structures were formed only on Ti-Ni alloys containing <10 wt % Ni, while higher Ni content (>20 wt %) led to less defined, nanoporous morphologies. Post-anodization annealing in air promoted the formation of anatase TiO₂, while subsequent hydrogenation in an Ar-H₂ atmosphere introduced Ti^{3 +} defects and oxygen vacancies without compromising the NT integrity. XPS analysis confirmed the presence of Ni^{2 +} and Ti^{3 +} states, which effectively narrowed the bandgap from 2.31 eV (pure TiO₂) to 1.65 eV (Ti-10%Ni), enabling enhanced absorption in the visible-light region. The Ti-10%Ni sample exhibited outstanding photocatalytic performance, achieving complete Cr-(VI) removal (100%) under sunlight in 90 min and under UV in just 60 minsignificantly outperforming undoped TiO₂ (70% removal). This enhanced activity is attributed to synergistic effects of Ni doping, Ti^{3 +}-induced midgap states, and suppressed charge recombination, offering a promising route for efficient and sustainable photocatalytic water treatment applications.