Simple but Effective Way To Enhance Photoelectrochemical Solar-Water-Splitting Performance of ZnO Nanorod Arrays: Charge-Trapping Zn(OH)₂ Annihilation and Oxygen Vacancy Generation by Vacuum Annealing

This study presents an effective and the simplest method to substantially improve the photoelectrochemical water-splitting ability of hydrothermally grown ZnO nanorod arrays (NRAs). In the hydrothermal growth of ZnO NRAs, unwanted Zn(OH) 2 species are formed, which act as trapping sites of photoexcited charges. We found that those inherent charge-trapping sites could be annihilated by the desorption of the hydroxyl groups upon vacuum annealing above 200 °C, which resulted in an enhancement of the charge-separation efficiency and photocurrent density. Another drastic increase in the photocurrent density occurred when ZnO NRAs were treated with annealing at higher temperature (700 °C), which can be attributed to the introduced oxygen vacancies acting as shallow donors in the ZnO crystal lattice. The removal of the charge-trapping Zn(OH) 2 and the generation of oxygen vacancies were confirmed by photoluminescence (PL) and XPS analyses. The ZnO NRAs treated by this simple method yield a photocurrent density of 600 μA/cm 2 at 1.23 V RHE under 1 sun illumination, which is 20 times higher than that obtained from as-grown ZnO NRAs. This study presents a highly efficient way of increasing the bulk electric conductivity and photoelectrochemical activity of metal oxide nanorods without requiring the introduction of any extrinsic dopants.