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

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 (a-Fe2O3) and a
hydrous iridium oxide (IrOx) 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 a-Fe2O3. At
potentials where no photocurrent is observed, electrons flow from
the a-Fe2O3 photoanode to the IrOx 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
a-Fe2O3 to the IrOx layer is clearly demonstrated. The analysis of
the operando XAS spectra further suggests that oxygen evolution
actually occurs both at the a-Fe2O3/electrolyte and a-Fe2O3/IrOx
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.