A First-Principles Study on the Role of an Al₂O₃ Overlayer on Fe₂O₃ for Water Splitting

Understanding the role of an overlayer material on a catalyst is crucial for improving catalytic activity. Iron­(III) oxide (α-Fe₂O₃) is a widely studied catalyst commonly used for solar water splitting. Recently, the water splitting efficiency with α-Fe₂O₃ was enhanced by deposition of an α-Al₂O₃ overlayer. In order to understand the origin of this improvement, we perform first-principles calculations with density functional theory + U on the α-Fe₂O₃(0001) surface with an α-Al₂O₃ surface overlayer. We find catalysis is unfavorable directly over α-Al₂O₃ and rather takes place over α-Fe₂O₃ exposed areas. In agreement with experiment, we find that α-Al₂O₃ coverage decreases the overpotential required for water oxidation on α-Fe₂O₃. We explain this improvement through the decrease in the work function of α-Fe₂O₃ upon α-Al₂O₃ coverage that aids in extracting electrons during the water oxidation reaction. We suggest that selecting an overlayer with a smaller work function than that of the catalyst as a strategy for future development of better catalysts.