Enhanced photocatalytic activity of rutile TiO₂ prepared by anodic oxidation in a high concentration sulfuric acid electrolyte

The photocatalytic characteristics of TiO₂ prepared by anodic oxidation in an electrolyte with a high concentration of sulfuric acid were investigated, focusing on the crystallinity and microstructure of the oxide. The predominant phase in the anodic oxide changes from anatase to rutile with the sulfuric acid concentration; the change appears when this concentration is 0.4 M. Nanosized pores appear in the oxide when the concentration of sulfuric acid is greater than 0.1 M, and the surface area increases with the sulfuric acid concentration. Structural analysis revealed that the inhomogeneous lattice strain in both anatase and rutile is reduced to almost zero, implying that the oxide contains a small number of recombination sites; as a result, the probability of the extinction of the photogenerated charges is low. The methylene blue (MB) bleaching test shows that the photocatalytic activity improves as the concentration of sulfuric acid increases and that the characteristics of rutile are better than those of anatase. It is concluded that rutile-structured TiO₂ exhibits the best photocatalytic activity among the investigated oxides, due to its high crystallinity and porous microstructure.