Fabrication of visible-light-responsive titanium dioxide layer on titanium using anodic oxidization in nitric acid

A visible-light-responsive titanium dioxide (TiO2) layer was fabricated by anodizing a Ti plate in aqueous nitric acid (HNO3) solutions, followed by annealing at 723 K for 5 h in air. The predominant structure of the oxide layer was TiO2 with an anatase structure that contained ca. 1 at% of incorporated nitrogen. The crystallite size of the anatase TiO2 was enlarged as anodizing voltage increased, but the atomic ratio of the incorporated nitrogen was almost constant. The TiO2 layers could degrade methylene blue (MB) solution under an UV light at 370 nm, exhibiting photocatalytic activity. The activity increased monotonically as the crystallite size of the oxide increased. The TiO2 layers with their crystallite size exceeding a specific value showed excellent response under visible light at 420, 450, and 505 nm. Overall, the results of this study showed that anodic oxidization of Ti in HNO3 solution is a promising technique for fabrication of a visible-light-responsive TiO2 layer.