Oxygen-deficient WO 3 via high-temperature two-step annealing for enhanced and highly stable water splitting.

When grown according to the conventional one-step annealing process at 550 °C, WO3 exhibits low photocurrent density and inferior photocurrent retention. In contrast, after a two-step annealing process at first 550 °C and second 700 °C, WO3 contains a higher concentration of oxygen deficiencies acting as shallow donors, thus leading to improved charge separation. Importantly, this results in a substantial photocurrent increase and higher material stability, two criteria that are of utmost importance for efficient photoelectrochemical water splitting.