Determination of photoelectrochemical water oxidation intermediates on haematite electrode surfaces using operando infrared spectroscopy.

Semiconductor electrodes capable of using solar photons to drive water-splitting reactions, such as haematite (α-Fe2O3), have been the subject of tremendous interest over recent decades. The surface has been found to play a significant role in determining the efficiency of water oxidation with haematite; however, previous works have only allowed hypotheses to be formulated regarding the identity of relevant surface species. Here we investigate the water-oxidation reaction on haematite using infrared spectroscopy under photoelectrochemical (PEC) water-oxidation conditions. A potential- and light-dependent absorption peak at 898 cm(-1) is assigned to a Fe(IV)=O group, which is an intermediate in the PEC water-oxidation reaction. These results provide direct evidence of high-valent iron-oxo intermediates as the product of the first hole-transfer reaction on the haematite surface and represent an important step in establishing the mechanism of PEC water oxidation on semiconductor electrodes.