Balancing Passivation and Conductivity: The Critical Role of Carbon Layer Thickness in Fe ₂ O ₃ /Fe ₃ O ₄ Heterojunction Photoanodes

Hematite (α-Fe₂O₃) is a promising photoelectrocatalyst for water oxidation, yet it suffers from intrinsic limitations including poor electrical conductivity, low charge-separation efficiency, an ultrashort hole-diffusion length, and a high onset potential, which hinder its practical performance. While constructing the Fe₂O₃/Fe₃O₄ heterojunction has proven effective in enhancing the photoelectrochemical properties of hematite, severe charge recombination at the heterojunction/electrolyte interface remains a critical challenge. In this work, a controllable synthesis strategy was employed to fabricate a series of core-shell Fe₂O₃/Fe₃O₄ heterojunction (T-ASSt@Cx) photoanodes with precisely tuned carbon-layer thicknesses using anodized stainless steel (T-ASSt) as the substrate. Through systematic investigations, a dual-regulatory function of the carbon overlayer on the photoelectrochemical performance was revealed. Specifically, an optimal moderate carbon layer (2-3 nm) could simultaneously passivate surface defects and serve as an efficient electron-extraction and transport pathway, significantly enhancing the charge-separation efficiency of the heterojunction from 47.7% to 83.7% at 1.23 V vs RHE and inducing a 360 mV cathodic shift in onset potential. In contrast, insufficient carbon layer thickness fails to suppress surface charge recombination, while excessive thickness severely hinders electrolyte access to the heterojunction and impedes hole transfer kinetics. DFT calculations demonstrate that the carbon nanolayer optimizes the adsorption of OER intermediates, significantly reduces the reaction energy barrier, and promotes charge transfer kinetics through interfacial electron redistribution. This study not only elucidates the critical balance between charge separation and mass transport governed by carbon overlayer thickness, but also provides interfacial engineering strategies for the rational design of high-performance, stable carbon-encapsulated heterojunction photoelectrocatalytic systems.