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

ABSTRACT

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 Feδ+ (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, 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 VRHE 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.