Role of Cl Ion Desorption in Photocurrent Enhancement of the Annealed Rutile Single-Crystalline TiO₂ Nanorod Arrays

TiO₂ nanorods arrays (NRAs) have been considered as very promising photoanode materials in photoelectrochemical (PEC) cells. However, the performance of TiO₂ NRAs still requires substantial improvement in order to reach the goal of practical applications. Annealing treatment of TiO₂ NRAs can help to improve the PEC performance, but the mechanism is still not yet fully understood. In this work, we systematically investigated the optical and electronic properties, as well as the PEC performance of the thermally treated rutile single-crystalline TiO₂ NRAs. Surprisingly, we recorded a maximum photocurrent density of 1.38 mA/cm² at 1.3 V versus reversible hydrogen electrode for TiO₂ NRAs annealed in O₂, which is about 28 times higher than that of the pristine TiO₂ NRAs. We further revealed that the surface adsorbed Cl ions largely suppress the photoresponse of the TiO₂ NRAs as they serve as recombination centers and block the adsorption of water molecules to the surface of TiO₂ NRAs. The enhancement in photocurrent after annealing in O₂ is due to the desorption of the Cl ions, filling of the surface V_o, expansion of the depletion layer, and increase of the grain size. Our results shed light on the effect of annealing on the PEC performance of TiO₂ NRAs and offer guidance for annealing of other semiconductor materials.