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Graphical abstract Highlights • Deep gap states of TiO 2 nanorod array were identified by cyclic voltammetries. • Spatial location, energy position, and chemical entity of gap states were revealed. • High sub-band gap response was observed in photoelectrochemical water oxidation. Abstract Developing new methods to use visible light is important for TiO 2 photocatalysis. This research reported that the TiO 2 rutile nanorod (NR) array shows high sub-band gap visible light response. By means of electrochemical cyclic voltammetries, the intrinsic occupied gap states of TiO 2 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 TiO 2 NRs after absorbing two visible photons. In a consequence, the TiO 2 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 TiO 2 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 TiO 2 photocatalysis effectively, which should be highly meaningful for designing visibly-active TiO 2 photocatalysts. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Charge interfacial transfer from Au/TiO 2 nanorod array to electrolyte was studied. • Au/TiO 2 perimeter sites were shown to be important for charge interfacial transfer. • Au-induced defects at Au/TiO 2 perimeter can promote electron interfacial transfer. Abstract In this research, by means of electrochemical cyclic voltammetries (CVs), the pathways of charge carrier interfacial transfer (IT) from TiO 2 and Au/TiO 2 rutile nanorod (NR) arrays to electrolyte were studied. For pure TiO 2 , the electron IT to O 2 takes place via both the conduction band (CB) states and the surface states (SS). The UV light illumination increases the electron IT via CB states, but inhibits that via SS pathway. The CV results also evidenced that the Au nanoparticles (NPs) on TiO 2 surface play dual roles of electron storage and electron IT pathway. Our results showed that the Au-induced defects, locating at Au/TiO 2 perimeter, form an important additional pathway for electron IT to O 2. UV light illumination increases the electron IT via Au NPs and Au-induced SS. In addition, we also revealed a special IT pathway of hot holes via the Au/TiO 2 perimeter contributes to the full water oxidation under visible light illumination. Therefore, the Au/TiO 2 perimeter region plays dual roles in the IT of both electrons and holes. Basing on these results, the IT pathways of charge carriers in Au/TiO 2 were illustrated from the viewpoint of energy band diagram. The photocatalytic degradation of methyl orange, photoelectrochemical oxidation of water, and photogeneration of free hydroxyl radicals (OH) were studied under UV and visible light illuminations. The association between the charge carrier IT pathways and the photocatalytic properties was also discussed. [ABSTRACT FROM AUTHOR]