We report facile and effective surface-functionalization of TiO2 nanotubes array (NTs) via a TiCl3-mediated solution treatment and its effects on the charge transport and transfer properties for photoelectrochemical (PEC) water-oxidation. TiO2 NTs with similar to 5 mu m length were prepared by hydrothermal-etching a TiO2 nanorods array. Subsequently, TiO2 NTs were treated with an aqueous TiCl3 solution at 80 degrees C to generate surface oxygen vacancies and to deposit a TiO2 nano-branch layer on the side-walls of TiO2 NTs, and these modifications were confirmed by X-ray photoelectron spectroscopy and transmission electron microscopy. Through electrochemical impedance spectroscopy analysis, we found that the TiCl3-mediated surface-functionalization of TiO2 NTs significantly improves the charge carrier transport and transfer properties, owing to the increase in the charge carrier density (due to the generation of surface oxygen vacancies) and surface roughness (due to the formation of nano-branches), respectively. The TiCl3 treatment considerably improves the incident photon-to-current conversion efficiency (IPCE) and photocurrent density of TiO2 NTs (especially at low-bias potentials) during the PEC water-oxidation, and the treated material demonstrates a maximum IPCE of similar to 93% and a photocurrent density of similar to 2.25 mA/cm(2) at 1.23 V versus the reversible hydrogen electrode. (C) 2018 Elsevier Inc. All rights reserved.