Gradient Valence Engineering Synchronizes Charge-Carrier and Catalytic Dynamics for Efficient Solar Water Oxidation

The efficiency of photoelectrochemical water splitting is constrained by the kinetic mismatch between ultrafast charge separation and slow catalytic turnover. Inspired by the spatiotemporal precision of photosystem II, we designed a redox-engineered BiVO₄/Fe-HOTP (BVO/R-Fe-HOTP) photoanode, with 'R-' denoting the sample subjected to sequential NaBH₄ reduction and O₂ oxidation treatment (where HOTP refers to the 2,3,6,7,10,11-hexaoxidotriphenylene multidentate ligand). This architecture establishes a programmable valence gradient that bridges charge separation and catalytic water oxidation. Through controlled redox engineering, we grew an amorphous Fe-HOTP layer on BVO, establishing a continuous transition from electron-rich Fe^δ+ (δ < 2), at the interface, to highly oxidized Fe³⁺, at the outer surface. Under light illumination, surface Fe³⁺ is further oxidized to Fe⁴⁺, generating active redox sites that enable a turnover frequency (TOF) of 82 s^-1. This architecture reduces interfacial band offsets for ultrafast hole injection and establishes a built-in potential gradient that extends carrier lifetime to 0.03 s. Thus, the BVO/R-Fe-HOTP photoanode delivers a photocurrent density of 6.1 mA cm^-2 at 1.23 V_RHE and, when coupled with a Si solar cell, achieves unbiased solar water splitting with a solar-to-hydrogen efficiency of 4.58%. These results establish gradient valence engineering as an effective strategy for synchronizing charge-carrier and catalytic dynamics.