Roles of Surface States in Cocatalyst-Loaded Hematite Photoanodes for Water Oxidation

Photoelectrochemical water oxidation over bare hematite (α-Fe 2 O 3 ) involved physical–chemical processes through surface states. It is found that the surface state with a higher oxidative energy (S1) served as a reaction intermediate for water oxidation, while the other surface state with a lower oxidative energy (S2) was basically catalytically inactive and induced charge recombination. Since a cocatalyst is commonly loaded on α-Fe 2 O 3 to enhance water oxidation, the physical–chemical processes between the surface states and the cocatalyst are supposedly important but remain unclear. In the present study, we elucidated such processes by employing photoelectrochemical impedance spectroscopy. We found that, in a CoPi-loaded α-Fe 2 O 3, S1 no longer served as an intermediate for water oxidation and was mostly passivated. At the same time, S2 became the key intermediate for water oxidation by serving as a hole reservoir, prolonging the hole lifetime, and finally transferring holes to CoPi. This study reveals the surface chemistry for optimizing cocatalyst-loaded α-Fe 2 O 3, which is different from that for bare α-Fe 2 O 3 .