Covalent Fixation of Surface Oxygen Atoms on Hematite Photoanode for Enhanced Water Oxidation

Abstract

Suppression of surface states is one of the general issues for metal oxide photoanodes in water oxidation. For hematite (α-Fe2O3), the surface states are mainly attributed to Fe3+/Fe2+ redox couples in oxygen deficient regions (surface oxygen vacancies). To date, most of the passivation overlayers against surface states are metal oxides. However, oxygen vacancies are prevalent for most metal oxides. This is because their formation in metal oxides is often thermodynamically favorable. In contrast, the formation of oxygen vacancies is more energy-consuming when oxygen atoms are covalently bonded. On the basis of this understanding, we propose a new strategy to transform the surface of Fe2O3 into amorphous iron phosphate (denoted "Fe-Pi"), where the oxygen atoms are "covalently fixed" in phosphate (PO43–). As a result, the oxygen vacancies are decreased and the surface states are effectively suppressed. The onset potential of corresponding photoanode shifts negatively by 0.15 V and the photocurrent density increases by 4.2 (simulated sunlight) and 4.1 (visible light) times. The suppression of surface states by amorphous Fe-Pi overlayer is then confirmed by series of electrochemical analysis. This work is expected to create new opportunities for optimizing the performance of Fe2O3 and other metal oxide photoanodes.