Open‐Circuit Photopotentials at Doped α ‐ Fe2 O 3 Electrodes in Aqueous Solution

Abstract

The influence of redox couples in aqueous solution on the interface energetics of chemically and thermally doped polycrystalline α‐Fe2O3 photoanodes has been investigated. Experiments were carried out at several pH values. Results are compared to those obtained from a single crystal electrode. It is shown that the potential of the semiconductor under illumination shifts toward positive values when the energy level of the redox couple in solution is increased. The role that grain boundaries in the sintered electrodes may play, the effect of pH and the redox couple concentration are discussed.

The Transient Photocurrent and Photovoltage Behavior of a Hematite Photoanode under Working Conditions and the Influence of Surface Treatments

“…The deviation could possibly be a manifestation of Fermi level pinning induced by the presence of surface states under the conduction band that play the role of recombination centers. Fermi level pinning has already been observed for hematite photoanodes in several investigations. ,, …”

“…Surface states causing Fermi level pinning have been observed through several analytical techniques, at energetic levels between the flat band potential and a potential 0.5 V more anodic, corresponding to applied voltages between 0.5 and 1.0 V vs RHE. ,, These energetic traps are localized too high in energy for holes to react with water, and it is therefore unlikely that charge transfer occurs from these surface states. Nevertheless, the presence of these surface states is detrimental for the onset of the photocurrent, even if charge transfer for water photolysis with hematite is occurring with holes localized energetically in the valence band or in other intraband gap states (closer to the valence band).…”

“…Fermi level pinning has already been observed for hematite photoanodes in several investigations. 32,44,45 Figure 4b shows the TPV amplitude (ΔV SS and ΔV SS,H2O2 ) induced by the blue light pulse as a function of the measured steady state photopotential before the light pulse. In the electrolyte containing H 2 O 2 (broken lines, empty markers), the linear increase in ΔV SS,H2O2 , observed up to 1.3 V, is consistent with a rise in the Fermi level of electrons due to the increase in photogenerated electrons by light absorption.…”

Metal Oxide Photoelectrodes for Solar Fuel Production, Surface Traps, and Catalysis

“…A slope of 1 can be observed for Fe 2 O 3 in carefully prepared single-crystalline electrodes, 27 but for electrodes prepared via solution-based approaches or other nanostructuring techniques, slopes less than 1 are observed. 44 For example, in hematite films prepared by spray pyrolysis, a slope of only 0.54 was observed when varying the V redox from −0.1 to 0.22 V versus Ag/AgCl. This suggests that the Fermi level of the electrode is partially pinned; a change of 0.1 V in the redox potential induces a change of 0.05 V in the photopotential.…”

“…A plot of V OC versus V redox should thus exhibit a slope of 1 in the ideal case. A slope of 1 can be observed for Fe 2 O 3 in carefully prepared single-crystalline electrodes, but for electrodes prepared via solution-based approaches or other nanostructuring techniques, slopes less than 1 are observed . For example, in hematite films prepared by spray pyrolysis, a slope of only 0.54 was observed when varying the V redox from −0.1 to 0.22 V versus Ag/AgCl.…”