Observation of charge transfer cascades in α-Fe2O3/IrOx photoanodes by operando X-ray absorption spectroscopy

Abstract

Electrochemical devices for energy conversion and storage are central for a sustainable economy. The performance of electrodes is driven by charge transfer across different layer materials and an understanding of the mechanistics is pivotal to gain improved efficiency. Here, we directly observe the transfer of photogenerated charge carriers in a photoanode made of hematite (α-Fe₂O₃) and a hydrous iridium oxide (IrO_x) overlayer, which plays a key role in photoelectrochemical water oxidation. Through the use of operando X-ray absorption spectroscopy (XAS), we probe the change in occupancy of the Ir 5d levels during optical band gap excitation of α-Fe₂O₃. At potentials where no photocurrent is observed, electrons flow from the α-Fe₂O₃ photoanode to the IrO_x overlayer. In contrast, when the composite electrode produces a sustained photocurrent (i.e., 1.4 V vs. RHE), a significant transfer of holes from the illuminated α-Fe₂O₃ to the IrO_x layer is clearly demonstrated. The analysis of the operando XAS spectra further suggests that oxygen evolution actually occurs both at the α-Fe₂O₃/electrolyte and α-Fe₂O₃/IrO_x interfaces. These findings represent an important outcome for a better understanding of composite photoelectrodes and their use in photoelectrochemical systems, such as hydrogen generation or CO₂ reduction from sunlight.