Expediting hole transfer surface states in hematite-based composite photoanodes.

Regarding the indirect hole transfer route in hematite-based photoelectrodes, the widely accepted viewpoint is that the FeO states act as a hole transfer medium, while other types of surface states act as recombination centers. Alternatively, it has rarely been reported that the recombining surface states may contribute to the charge transport in modified photoelectrodes. In this study, we employed CoCr layered double hydroxide (LDH)/FeO and CoCr LDH/Zr:FeO as research models to investigate the distinct charge transfer pathways in composite photoanodes. Different from the adverse role of surface states at ∼0.7 V the reversible hydrogen electrode (r-SS) in the bare hematite photoelectrodes (FeO or Zr:FeO), the r-SS in the composite photoanodes (CoCr LDH/FeO or CoCr LDH/Zr:FeO) served as a hole transfer station to induce high-valent Co cations, and the position of r-SS determined the onset potential of the composite photoelectrodes. Moreover, the FeO states still acted as active intermediates to transport numerous holes to the cocatalyst, which enhanced the charge utilization efficiency at 1.23 V the reversible hydrogen electrode (RHE) to a large extent. Besides, a noteworthy fact is that Zr doping increased the number of active FeO states, which significantly contributed to the enhancement in current density. However, it led to a delayed onset potential because of the positively shifted surface states (r-SS and FeO). Evidently, the different surface state distributions between FeO and Zr:FeO gave rise to anisotropic charge transfer and recombination behavior in the composite photoanodes. This study gives extensive insight into the hole transfer route in composite photoanodes and reveals the surface state-tuning effects of dopants and cocatalysts, which are significant for a deep understanding of the surface states and optimal design of composite photoanodes surface state modulation.