Tuning the surface states by in-situ Zr/Hf co-doping and MoO₃ hole transport layer modification for boosting photoelectrochemical performance of hematite photoanode

Hematite (α-Fe₂O₃) is a potential photoanode material for photoelectrochemical (PEC) water splitting; nevertheless, its PEC performance is constrained by substantial bulk and surface charge recombination rates. Herein, the effect of in-situ Zr/Hf dual-ion doping and the MoO₃ hole transport layer (HTL) on regulating the surface states of α-Fe₂O₃ photoanodes for effective water oxidation is investigated. The co-doping improves bulk properties by enhancing the electrical conductivity, thereby facilitating hole transfer via intermediate surface states (i-SS). Furthermore, the MoO₃ HTL passivates the recombination surface states (r-SS), thus alleviating the Fermi level pinning, resulting in an improved open-circuit photovoltage. As a result, a novel Zr/Hf-HT:MoO₃ photoanode attains a photocurrent density of 2.34 mA cm⁻² at 1.23 V vs. RHE (1.23 V_RHE), which is 123% higher than that of Bare-HT. The Zr/Hf-HT:MoO₃ photoanode achieves 86 and 61.3% of surface charge separation and charge transfer efficiencies at 1.23 V_RHE, respectively, representing 26 and 100% enhancements over those of Bare-HT. Lastly, the FeNi(OH)_x cocatalyst coated Zr/Hf-HT:MoO₃ photoanode reaches a photocurrent density of 2.62 mA cm⁻² at 1.23 V_RHE with 98% stability and generates 47.8 and 22 μmol h⁻¹ of H₂ and O₂ gases, respectively.