Electrochemistry and photoelectrochemistry of iron(III) oxide

Abstract

Iron(III) oxide has been extensively studied as a possible n-type semiconductor for use in solar photoelectrolysis cells. However, its properties have remained curiously elusive; even such fundamental properties as bandgap and flat-band potential are still controversial, and this uncertainty has hindered any rational evaluation of the use of the material in solar cells. In this paper, an extensive study of the surface and bulk properties of both single-crystal and polycrystalline Fe₂O₃ is reported. Surface pretreatment is found to have a major effect on the photoelectrochemical properties. Even in properly treated samples, however, the photocurrent onset is found to be delayed due to the small value of the faradaic rate constant for the oxidation of water, and a semi-quantitative treatment of this case is provided. Improper surface treatment is shown to lead to a substantial conversion of the surface of Fe₂O₃ to Fe₃O₄; this introduces a large fraction of recombination sites at the surface that not only delay d.c. photocurrent onset to very anodic potentials but also reduce the observed efficiency to remarkably low values.