Engineering Direct S-Scheme Heterojunctions with Ultrafast Interfacial Charge Transfer: A Case Study on 2-Dimensional α-Fe2O3/Cu2O Interfaces

Longer wavelengths of light contain less energy but comprise more of the solar spectrum, making them important to incorporate into any process aiming for high efficiency. Here, we developed a novel redox-mediated synthetic mechanism to construct a heterojunction with strongly coupled interfaces. Specifically, an α-Fe₂O₃/Cu₂O/CuO nanosheet composite was synthesized, forming an S-scheme α-Fe₂O₃/Cu₂O electronic interface, a burgeoning class of materials designed to upconvert longer wavelengths of light and utilize solar energy more effectively. Through a series of experiments including X-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV-Vis) diffuse reflectance spectroscopy (UV-Vis-DRS), electrochemical impedance spectroscopy (EIS), and photocatalytic measurements, we were able to fully confirm the electronic structure of the α-Fe₂O₃/Cu₂O interfacial heterojunction. These characterizations demonstrate the S-scheme flow of electrons, which is further supported by COMSOL numerical simulations. The successful formation of the S-scheme heterojunction is made possible through the direct Fe-O-Cu covalent bonding at the interface. These bonds provide ultrafast interfacial charge transfer pathways on picosecond time scales followed by long-lived charge-separated states, as quantified by our transient optical experiments. The proposed redox-mediated synthetic strategy provides a valuable guideline for constructing effective solid heterojunctions with strongly coupled interfaces, which are desirable for various applications in catalysis, energy storage, electronics, photovoltaics, and beyond.