Observation and Alteration of Surface States of Hematite Photoelectrodes

Hematite prepared by atomic layer deposition (ALD) was found to exhibit photocurrents when illuminated by near-infrared light (lambda = 830 nm), whose energy is smaller than the band gap of hematite. The phenomenon was inferred to be a result of valence band to surface state transition. The influence of surface states on the thermodynamics of the hematite/water interface was studied under open-circuit conditions. It was discovered that the equilibrium potential of the hematite surface was more negative than water oxidation potential by at least 0.4 V. With a NiFeOx coating by photochemical decomposition of organometallic precursors, the equilibrium potential of hematite was restored to water oxidation potential, and the photoresponse under 830 nm illumination was annihilated. Therefore, the states were rationalized by the chemical status at the electrode surfaces, and this hypothesis was supported by similar observations on other metal oxide electrodes such as TiO2.