The role that the α-Fe2O3/NiFeOOH interface plays in dictating the oxygen evolution reaction (OER) mechanism on hematite has been a source of intense debate for decades, but the chemical characteristics of this interface and its function are still ambiguous and subject to speculation. In this study, we employed operando X-ray absorption spectroscopy to investigate the interfacial dynamics at the α-Fe2O3/NiFeOOH interface. We uncovered the spontaneous formation of a FeOOH interfacial layer under (photo)­electrochemical conditions. This FeOOH interfacial layer plays a role in the surface passivation of hematite and in accumulating the (photo)­generated holes upon external potential application. This hole-accumulation process leads to the extraction of more (photo)­generated holes from hematite before releasing them to NiFeOOH to carry out the water-splitting reaction, and it also explains the reason for the delay in the nickel oxidation process. Based on these observations, we propose a model where NiFeOOH acts mainly as an OER catalyst and a facilitator of holes extraction from hematite, while the interfacial FeOOH layer acts as a surface passivation and hole-accumulation overlayer.

The authors present a table-top soft x-ray absorption spectrometer, accomplishing investigations of the near-edge x-ray absorption fine structure (NEXAFS) in a laboratory environment. The system is based on a low debris plasma ignited by a picosecond laser in a pulsed krypton gas jet, emitting soft x-ray radiation in the range from 1 to 5 nm. For absorption spectroscopy in and around the “water window” (2.3–4.4 nm), a compact helium purged sample compartment for experiments at atmospheric pressure has been constructed and tested. NEXAFS measurements on CaCl2 and KMnO4 samples were conducted at the calcium and manganese L-edges, as well as at the oxygen K-edge in air, atmospheric helium, and under vacuum, respectively. The results indicate the importance of atmospheric conditions for an investigation of sample hydration processes.

A compact spectrometer for laboratory NEXAFS measurements, allowing quantitative chemical analysis in the soft X-ray spectral range (250–1000 eV).