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

Abstract

Regarding the indirect hole transfer route in hematite-based photoelectrodes, the widely accepted viewpoint is that the FeIV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)/Fe2O3 and CoCr LDH/Zr:Fe2O3 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 versus the reversible hydrogen electrode (r-SS) in the bare hematite photoelectrodes (Fe2O3 or Zr:Fe2O3), the r-SS in the composite photoanodes (CoCr LDH/Fe2O3 or CoCr LDH/Zr:Fe2O3) 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 FeIVO states still acted as active intermediates to transport numerous holes to the cocatalyst, which enhanced the charge utilization efficiency at 1.23 V versus the reversible hydrogen electrode (RHE) to a large extent. Besides, a noteworthy fact is that Zr doping increased the number of active FeIV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 FeIVO). Evidently, the different surface state distributions between Fe2O3 and Zr:Fe2O3 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 via surface state modulation.