(Invited) Towards a Mechanistic Understanding of the Oxygen Evolution Reaction on Hematite: Operando Characterization of Band Bending and Interfacial Chemical Speciation Using Sum Frequency Generation Spectroscopy

Abstract

Hematite is an attractive photoanode for the oxygen evolution reaction (OER) due to its cheap price, relatively small band gap, stability in alkaline solution and a valence band energy suitable for OER catalysis. However, insight into the OER mechanism, and thus the capability to rationally modify this material to optimize performance, is limited by the challenge in relating band bending, surface state population and chemical reactivity operando. Conventionally these issues are addressed by some combination of electrochemical impedance spectroscopy (EIS), photocurrent/voltage kinetics, and linear optical spectroscopies in the visible. While important, all of these approaches suffer from significant limitations: e.g. EIS and related techniques integrate over all charge transfer processes in the device while only those at the hematite/water interface are of interest (and thus require a challenging to validate model to quantify) and UV/Vis absorption measurements (whether steady-state or time resolved) are bulk-sensitive and probe spectrally broad features that are challenging to relate to surface structure.Sum Frequency Generation (SFG) spectroscopy is interface-specific by its symmetry selection rules, can straightforwardly probe interfacial chemical speciation if one of the two incident fields is in the infrared, and is sensitive to the amplitude of interfacial electric fields. These attributes make the technique particularly suitable for characterization of interfacial chemical speciation, double layer and space charge layer structure at the working electrode/electrolyte interface within thin-film spectroelectrochemical cells. Here I describe our recent efforts to characterize chemical speciation and interfacial field at a hematite/electrolyte interface, in the presence and absence of external illumination, as one moves from the Hematite flat band potential through the onset of the OER. Using vibrationally resonant SFG spectroscopy, at infrared wavelengths > 10 micron, we quantify the population of surface holes, i.e. Fe(IV)=0 groups, as the OER turns on. Concurrently we quantify the development of the space charge potential using non-resonant SFG. Recent work has argued, largely from current or optical transients in the visible under chopped illumination, that the OER on Hematite proceeds by a multi-hole mechanism. Because our approach allows for the simultaneous quantification of interfacial field and chemical speciation we directly address this mechanism and the circumstances under which it is expected to apply. More generally, the role of interfacial field in oxidation catalysis has been an object of much current interest. The approach described here – in which interfacial speciation and field can be quantified operando – offers an ability to explore this relationship not possible via any other combination of experimental approaches.