Abstract
Iron oxides such as hematite (α-Fe2O3) play an important role in diverse fields ranging from biogeochemistry to photocatalysis. Here we perform calculations of both the electron and electron hole polaron structures and associated reorganisation energies for a series of bulk iron oxides: hematite (α-Fe2O3), lepidocrocite (γ-FeOOH), goethite (α-FeOOH) and white rust (Fe(OH)2). Through the use of gap-optimized hybrid functionals and large supercells under periodic boundary conditions, we remove some of the complications and uncertainties present in earlier cluster model calculations. It is found that while the electron hole polaron in these materials generally localises onto a single iron site, the electron polaron localises across two iron sites of the same spin layer as a consequence of the lower reorganisation energy for electrons compared to holes. An exception to these trends is the hole of goethite, which according to our calculations does not form a localised polaron.
Highlights
With increasing environmental concerns, it is necessary to pursue more sustainable and more efficient materials
The density of states is provided in the Electronic supplementary information (ESI).† This results in a vertical state where 51% of the excess spin density is localised over parallel spin iron atoms, and following nuclear relaxation the majority of the excess hole localises over a single iron atom with an average contraction of the local Fe–O bond lengths of 0.069 Å
The difference is most likely related to the contrast between the steric strain of the crystal environment compared to the higher degrees of conformational freedom among solvating water molecules as well as the significantly higher optical dielectric constant of the iron oxides compared to water
Summary
It is necessary to pursue more sustainable and more efficient materials. An approximation of the polarisation of the full bulk crystal in response to the presence of the electron polaron, referred to as the outer-sphere reorganisation energy, was calculated from Marcus theory as 0.17 eV Their results are consistent with the small electron polaron model, with the excess electron localising over a single iron atom. Calculations of the both the electron and electron hole polaron structures and associated reorganisation energies are performed for a series of iron oxides: hematite (a-Fe2O3), lepidocrocite (g-FeOOH), goethite (a-FeOOH) and white rust (Fe(OH)[2]). This series enables examination of a variety of structural effects including hydration state, hydrogen bonding and the valence of bulk iron.
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