Catalyst-Doped Anodic TiO2 Nanotubes: Binder-Free Electrodes for (Photo)Electrochemical Reactions

Nanotubes of the transition metal oxide, TiO2, prepared by electrochemical anodization have been investigated and utilized in many fields because of their specific physical and chemical properties. However, the usage of bare anodic TiO2 nanotubes in (photo)electrochemical reactions is limited by their higher charge transfer resistance and higher bandgaps than those of semiconductor or metal catalysts. In this review, we describe several techniques for doping TiO2 nanotubes with suitable catalysts or active materials to overcome the insulating properties of TiO2 and enhance its charge transfer reaction, and we suggest anodization parameters for the formation of TiO2 nanotubes. We then focus on the (photo)electrochemistry and photocatalysis-related applications of catalyst-doped anodic TiO2 nanotubes grown on Ti foil, including water electrolysis, photocatalysis, and solar cells. We also discuss key examples of the effects of doping and the resulting improvements in the efficiency of doped TiO2 electrodes for the desired (photo)electrochemical reactions.