Competitive Photo-Oxidation of Water and Hole Scavengers on Hematite Photoanodes: Photoelectrochemical and Operando Raman Spectroelectrochemistry Study

Hematite (α-Fe₂O₃) is one of the most studied photoanode materials due to its stability in alkaline electrolytes and visible light absorption. However, its reported performance lags significantly behind its theoretical limit. Toward determining routes for efficient photo-oxidation on hematite, we investigated the role of hole-trapping surface states as reaction intermediates using sacrificial reductant reagents as hole scavengers, H₂O₂ and FeCN. Photoelectrochemical characterization at low scavenger concentrations and intermediate potentials, as opposed to previous studies at high scavenger concentrations, has shown the reaction mechanism to include competitive photo-oxidation between water molecules and the hole scavengers, similar for both H₂O₂ and FeCN. Using operando Raman spectroelectrochemistry, we show similar transient features for both scavengers, interpreted as scavenger adsorption to a two-site reaction intermediate participating in the photogenerated hole transfer of water photo-oxidation, hence the competition. These findings strengthen the significance of hole-trapping surface states for water photo-oxidation on hematite and the previously suggested two-site reaction pathway for efficient hole transfer in this reaction. A better understanding of the mechanisms of photoelectrochemical water splitting can assist in improving the efficiency of solar hydrogen production.