Revealing the new role of surface states in interfacial charge transfer at zinc ferrite photoanodes for efficient photoelectrochemical water splitting

Abstract

Surface states (SS) as the charge trapping sites play an extremely important role in designing high-performance photoelectrodes, which are generally thought to cause adverse surface charge recombination for zinc ferrite (ZnFe₂O₄, ZFO) photoanodes. However, the in-depth understanding of the relationship between SS and photoelectrochemical performance is still ambiguous. Herein, the modification of SS energy level and density at ZFO photoanodes was achieved via surface NaBH₄ chemical reduction and NiFeOOH co-catalysts loading. Combined with advanced spectroelectrochemical techniques, a new type of SS was discovered. Specifically, after optimizing surface structures of ZFO photoanodes via the surface modification, the SS would transform from the traditional intrinsic SS as the unfavorable recombination center to the non-intrinsic SS as the beneficial “charge storage station”, allowing for an impressively steep increase in the surface charge transfer efficiency and achieving a record high photocurrent of 1.70 mA/cm² at 1.50 V_RHE. This work presents comprehensive analysis about the controversial role of ZFO photoanodes’ SS and establishes the detailed response laws between SS and PEC water splitting performance. It is expected to guide future photoelectrode design via surface modification strategies.