Constraints to the flat band potential of hematite photo-electrodes

Abstract

We revisit the fundamental constraints that apply to flat band potential values at semiconductor photo-electrodes. On the physical scale, the Fermi level energy of a non-degenerate semiconductor at the flat band condition, EF(FB), is constrained to a position between the conduction band, EC, and the valence band, EV,: |EC| < |EF(FB)| < |EV| throughout the depth of the semiconductor. The same constraint applies on the electrode potential scale, where the values are referenced against a common reference electrode: UC(FB) < UF(FB) < UV(FB). Some experimentally determined flat band potentials appear to lie outside these fundamental boundaries. In order to assess the validity of any determined flat band potential, the boundaries set by the conduction band and the valence band must be computed on both scales a priori, where possible. This is accomplished with the aid of an analytical reconstruction of the semiconductor|electrolyte interface in question. To illustrate this approach, we provide a case study based on synthetic hematite, α-Fe2O3. The analysis of this particular semiconductor is motivated by the large variance in the flat band potential values reported in the literature.

Flat band potential determination: avoiding the pitfalls

“…Its determination can also help to estimate the positions of band edges in new materials. We have reported previously 17 that for a single material there can be a wide dispersion in at band potential values in the literature, which cannot be reconciled by differences in experimental conditions employed in their determination. Furthermore, a large proportion of these values were found to be outside the range predicted theoretically.…”

Interfacial band-edge energetics for solar fuels production

“…12 As discussed later in this article, the surface dipole can lead to variations of the band edge position that amount to several hundred meV. 13 In addition, external surface dipoles can be added that modify the surface energy levels, since in general the electrostatics of the solution are invariant.…”

Carbon‐Coated Hematite Electrodes with Enhanced Photoelectrochemical Performance Obtained through an Electrodeposition Method with a Citric Acid Additive

“…If there is no www.chemelectrochem.org charge accumulation at the hematite/electrolyte interface, the flat-bandp otentials hould equal the work functiond ifference between hematite and the electrolyte. [32] Empirical observations suggest that the flat-band potentialisoften approximately equal to the onset potential, particularly in the case with the addition of ah ole scavenger. [33] In this study, however,t he determined flat-band potentials are cathodically shifted by around0 .2 Vc ompared to the onset potentialo ft he photocurrent (insertedp anel of Figure4c).…”

“…The discrepancy might be because the model used (the MS model) is too simplified. [32] As the recombination of bulk hematite electrons with long-lived holes accumulated at the hematite/electrolyte interface is faster than water oxidation, an additional bias must be applied to sustain the energetic barrier and prevents ubsequent recombination.T his overpotential consideration could partially account for the discrepancy between the onset potentialo f photocurrent and the flat-band potential. [34] Accordingly,a n appropriate interpretation would be that the surfacec arbon layer could anodically shift the flat-bandp otentialt oac ertain degree.…”