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.