Field-channel synergistic engineering in gradient-doped 2D overlayers for high-efficiency solar water splitting

Achieving high solar-to-hydrogen (STH) efficiency in photoelectrochemical (PEC) water splitting is fundamentally limited by the challenge of simultaneously optimizing light absorption and charge separation in photoanodes. Herein, we overcome this bottleneck by fabricating ~20 nm thick B:C3N5-x nanosheets featuring a macroscopic gradient of B dopants and N vacancies on Mo:BVO via a substrate-mediated stabilization strategy. This unique architecture not only broadens the visible-light absorption range of BiVO4 (BVO), but also establishes a robust macroscopic dipole field. Originating from the breaking of inversion symmetry and synergistically reinforced by localized interlayer B-N bonding, this dipole field provides a powerful internal driving force for ultrafast carrier separation. Consequently, achieving a record 2.89% ABPE, the Mo:BVO/B:C3N5-x/NiFeBi photoanode delivers an outstanding 8.52% STH efficiency in tandem with a perovskite solar cell. This work demonstrates that combining gradient engineering with interlayer coupling effectively reconciles the intrinsic trade-off between optical thickness and charge extraction. Furthermore, this strategy presents a versatile blueprint for rationally designing of complex 2D/3D heterostructures for next-generation solar energy conversion and related applications.