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 3+, 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 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.
Xin et al. (Mon,) studied this question.