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

) 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.