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Otte, Katrin; Schmahl, Wolfgang W.; Pentcheva, Rossitza (2012): Density functional theory study of water adsorption on FeOOH surfaces. In: Surface Science, Vol. 606, No. 21-22: pp. 1623-1632
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Using density functional theory (DFT) calculations with an on-siteCoulomb repulsion term, we study the composition, stability, andelectronic properties of the most common FeOOH surfaces goethite(101),akaganeite(100), and lepidocrocite(010), and their interaction withwater. Despite the differences in surface structure, the trends insurface stability of these FeOOH polymorphs exhibit remarkablesimilarities. We find that the reactivity and the binding configurationof adsorbates depend strongly on the coordination of surface iron: atthe fourfold coordinated Fe2 site water is chemisorbed, whereas at thefivefold coordinated Fe1 water is only loosely bound with hydrogenpointing towards the surface. Our results show that the oxidation stateof surface iron can be controlled by the surface termination whereferryl (Fe4+) species emerge for oxygen terminated surfaces and ferrousiron (Fe2+) at iron and water terminations leading to a reduced bandgap. In contrast, the fully hydroxylated surfaces, identified as stablesurface configurations at standard conditions from the surface phasediagram, show electronic properties and band gaps closest to bulk FeOOHwith ferric surface iron (Fe3+). Only in the case of goethite(101), atermination with mixed surface hydroxyl and aquo groups is stabilized.