Observation and Alterations of Surface States on Hematite Photoelectrodes

As a prototypical material for solar water splitting, hematite represents an earth abundant compound whose band gap promises a potentially high solar-to-hydrogen conversion efficiency (up to 16%).  Hematite materials also present many challenges, e.g., poor light absorption near the band edge, short hole diffusion distance, and slow charge transfer kinetics.  These properties found in various other metal oxides are all present in hematite, making it a good research tool to test material design and treatment strategies that is of generic importance.  One particular challenge of hematite pertaining to surface states is the low V_ph generation, typically <0.4 V. The consequence is manifested by the late turn-on characteristics in the current-density-voltage (J-V) plots.  Recent studies by us confirmed that low V_ph, not high overpotential requirement, is the reason for the late turn-on. Here we present our recent study on the origin of the low V_ph. Hematite prepared by atomic layer deposition (ALD) was found to exhibit photocurrents when illuminated by near infrared light (l = 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 states 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 hematite surface was more negative than water oxidation potential by at least 0.4 V.  With a NiFeO_x 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.