The photocatalytic activity of the photoanodes mainly depends on the generation and separation of photogenerated electron–hole pairs and the efficiency of electron-transfer on their surfaces.A responsive photocurrent usually reflects the photocatalytic activity of the electrode. The transient photocurrent responses of TiO2 NTs and Fe-modified TiO2 NTs electrodes under visible-light irradiation ( l > 420 nm) at a bias potential of 0.4 V are shown in Fig. 5. A considerable rise in the photocurrent responses can be observed for all the electrodes under visiblelight irradiation, and the on-off cycles of photocurrent are reproducible. This is due to the generation and separation of photo-generated electron–hole pairs at the Fe-modified TiO2NTs/electrolyte interface.Holes move to the surface of TiO2 NTs, where they are trapped or captured by reduced species inthe electrolyte, while the electrons are transported to the back contact substrate via the walls of TiO2 NTs.