Continuous energy-level distribution of ZnXCd1−XS induced by gradient oxygen doping for efficient photoelectrochemical water splitting

Abstract

Fermi-level pinning caused by surface defect states severely limits charge separation and interfacial charge transfer in photoelectrochemical systems. Herein, a gradient oxygen-doped Zn_xCd_1−XS photoanode (O-ZCS) with a broadened electronic state distribution was constructed via a defect-induced Fick diffusion strategy, leading to a spatially graded electronic energy landscape with depth-dependent band-edge modulation. The optimized O-ZCS photoanode delivers a photocurrent density of 10.72 mA cm⁻² at 1.23 V (vs. RHE) and achieves a maximum applied-bias photon-to-current efficiency (ABPE) of 7.44% at 0.25 V (vs. RHE). Depth-profile XPS measurements and density functional theory calculations indicate that oxygen incorporation occurs preferentially from the surface to the subsurface region of Zn_XCd_1−XS. The incorporated oxygen passivates surface sulfur-vacancy sites through the formation of Cd–O bonds, thereby inducing a graded band–edge distribution and alleviating Fermi-level pinning. This gradient electronic structure promotes photogenerated carrier separation, suppresses surface recombination, and improves interfacial oxidative charge-transfer kinetics. This work demonstrates that gradient non-metal doping is an effective strategy for energy-level engineering in metal sulfide photoanodes and provides insights into the rational design of efficient photoelectrochemical energy-conversion systems.