Multifunctional Bi 2 O 2 S Nanosheet Interface Enabling Co-Pi-Coupled ZnFe 2 O 4 Nanorods for Enhanced Solar-Driven Water Oxidation

This work reports a multifunctional interfacial engineering strategy using Bi 2 O 2 S nanosheets and a Co-Pi catalyst to enhance the photoelectrochemical (PEC) water oxidation performance of ZnFe 2 O 4 photoanodes in a neutral electrolyte. Although ZnFe 2 O 4 possesses suitable band-edge positions, chemical stability, and visible-light absorption, its PEC activity is severely limited by surface recombination and sluggish charge transport. By integrating low-band gap Bi 2 O 2 S nanosheets as a light-harvesting, surface-passivating, and hole-transport layer, and coupling them with a Co-Pi cocatalyst, the optimized ZnFe 2 O 4 /Bi 2 O 2 S/Co-Pi photoanode delivers a photocurrent density of 0.34 mA cm –2 at 1.23 V RHE, representing an approximately 10-fold enhancement over pristine ZnFe 2 O 4 . A cathodic shift of ∼180 mV in the onset potential and an applied bias photon-to-current efficiency exceeding 0.5% are also achieved under neutral pH conditions. Mechanistic investigations reveal that Bi 2 O 2 S effectively passivates oxygen-vacancy-related surface states and forms a favorable type-II heterojunction with ZnFe 2 O 4, leading to Fermi-level depinning, enhanced interfacial band bending, and improved photogenerated charge separation. The Co-Pi cocatalyst further accelerates interfacial hole injection and oxygen evolution reaction kinetics, resulting in a charge-transfer efficiency of ∼63% and reduced interfacial resistance. Overall, this study demonstrates that multifunctional Bi 2 O 2 S nanosheets, combined with an earth-abundant cocatalyst, provide an effective route to improving the efficiency and stability of spinel ferrite photoanodes for solar fuel generation in neutral aqueous environments.