Self-organized TiO2 nanotubes enabled by anodizing Ti foils are intensively studied for their various potential applications in energy and environment fields.Lately, much attention has been paid to the experimental discovery, made by either programmed rapid annealing or week-long soaking of the as-anodized nanotubes, that reveals that the tube wall of an anodic TiO2 nanotube comprises of two distinct layers.