Expediting hole transfer kinetics through surface state modulation of a Ru-FeOOH and FeNi(OH)_x dual-layer cocatalyst coated Zr-Fe₂O₃ photoanode for boosting photoelectrochemical water splitting

The potential of a hematite photoanode for photoelectrochemical (PEC) water oxidation is primarily constrained by its sluggish water oxidation kinetics due to severe charge recombination at recombination surface states (r-SS). Abundant r-SS are responsible for a Fermi level pinning effect, which causes a potential drop in the Helmholtz layer. Tuning surface states is crucial for improving charge transfer at the semiconductor-electrolyte interface (SEI). In this context, cocatalyst loading is a potential strategy for promoting charge transfer by modulating intermediate surface states (i-SS). Accordingly, Ru-incorporated FeOOH and microwave-assisted FeNi(OH)_x dual-layer cocatalysts (DLCs) were coated on a Zr-doped Fe₂O₃ photoanode to investigate surface charge transfer kinetics at the SEI. The optimized Zr-HT/Ru-FeOOH/FNH photoelectrode achieved a photocurrent density of 2.27 mA cm⁻² with a surface charge separation efficiency of 96.6% at 1.23 V vs. RHE. The DLC modulated the i-SS and facilitated hole transfer, leading to Fermi level de-pinning through passivation of r-SS. Moreover, cocatalyst loading increased the surface-active area, which enhanced the number of active sites available for charge transfer reactions. During PEC water splitting, the optimized Zr-HT/Ru-FeOOH/FNH photoanode generated 38.2 and 18.5 μmol h⁻¹ of H₂ and O₂ gases, respectively. The approach of using dual-layer cocatalysts provides a new technique for stimulating and advancing the effectiveness of metal oxide photoanodes by tailoring surface states for PEC water oxidation.