Phosphonic Acid Porphyrin Assemblies Boost Surface Kinetics and Water Oxidation of α‐Fe 2 O 3 Photoanodes

Abstract In photoelectrochemical (PEC) water oxidation, holes in the valence band (VB) of α‐Fe 2 O 3 have restricted conversion efficiency due to their short diffusion distance and poor oxidation kinetics. To address this limitation, molecular interfacial engineering is employed with a porphyrin self‐assembler to modulate hole dynamics at the α‐Fe 2 O 3 surface. Three porphyrins with different peripheral groups—TPPP (─PO 3 H 2 ), TSPP (─SO 3 H), and TCPP (─CO 2 H) are incorporated as hole transport layer (HTLs) at α‐Fe 2 O 3 /co‐catalyst (FeNiOOH) interface. Their anchoring groups ensure robust molecular assembly formation, facilitating interfacial hole transfer. PEC characterizations reveal that phosphonic acid‐terminated interfacial charge transfer resistance. The optimized FeNiOOH/TPPP/α‐Fe 2 O 3 photoanode achieves remarkable enhancements, with photocurrent density and applied bias photon‐to‐current efficiency boosted by 6.7‐fold and 14‐fold, respectively, compared to pristine α‐Fe 2 O 3 . In situ scanning photoelectrochemical microscopy and intensity‐modulated photocurrent spectroscopy are employed to quantitatively elucidate the interfacial charge transfer kinetics. It is demonstrated that TPPP significantly suppressed the recombination of photogenerated charge carriers and enhanced hole transport within α‐Fe 2 O 3 , leading to accelerated surface reaction kinetics. This work establishes a promising molecular engineering for dynamic interfacial charge management, offering a viable pathway toward designing high‐performance PEC water‐splitting systems.