High sub-band gap response of TiO2 nanorod arrays for visible photoelectrochemical water oxidation

Developing new methods to use visible light is important for TiO2 photocatalysis. This research reported that the TiO2 rutile nanorod (NR) array shows high sub-band gap visible light response. By means of electrochemical cyclic voltammetries, the intrinsic occupied gap states of TiO2 NR array were identified. It was seen that the gap states can be switched from filled states to empty states by visible light illumination, which play the role of relay in promoting electrons from the valence band to the conduction band of TiO2 NRs after absorbing two visible photons. In a consequence, the TiO2 NR array shows the obvious anodic photocurrents for water photoelectrochemical oxidation and the photocatalytic degradation of methyl orange under visible light illumination. This study revealed the significance of the deep occupied gap states of TiO2 NR arrays, which are usually considered as recombination centers. Different from conventional photocatalysis arising from inter-band electron transition, the present research shows a different pathway to use visible light in TiO2 photocatalysis effectively, which should be highly meaningful for designing visibly-active TiO2 photocatalysts.