Facile and controllable surface-functionalization of TiO₂ nanotubes array for highly-efficient photoelectrochemical water-oxidation

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 ∼5 μm length were prepared by hydrothermal-etching a TiO₂ nanorods array. Subsequently, TiO₂ NTs were treated with an aqueous TiCl₃ solution at 80 °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 ∼93% and a photocurrent density of ∼2.25 mA/cm² at 1.23 V versus the reversible hydrogen electrode.