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

Understanding the role of an overlayer material on a catalyst is crucial for improving catalytic activity. Iron(III) oxide (alpha-Fe2O3) is a widely studied catalyst commonly used for solar water splitting. Recently, the water splitting efficiency with alpha-Fe2O3 was enhanced by deposition of an alpha-Al2O3 overlayer. In order to understand the origin of this improvement, we perform first-principles calculations with density functional theory + U on the alpha-Fe2O3(0001) surface with an alpha-Al2O3 surface overlayer. We find catalysis is unfavorable directly over alpha-Al2O3 and rather takes place over a-Fe203 exposed areas. In agreement with experiment, we find that alpha-Al2O3 coverage decreases the overpotential required for water oxidation on alpha-Fe2O3. We explain this improvement through the decrease in the work function of alpha-Fe2O3 upon aAl(2)O(3) 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.