Structure, charge distribution, and electron hopping dynamics in magnetite (Fe 3O 4) (1 0 0) surfaces from first principles

• Published in: Geochimica et cosmochimica acta 2010-08, Vol.74 (15), p.4234-4248
• Main Authors: Skomurski, Frances N., Kerisit, Sebastien, Rosso, Kevin M.
• Format: Article
• Language: English
• Subjects: AVAILABILITY; CHARGE DISTRIBUTION; ELECTRONS; Environmental Molecular Sciences Laboratory; FREE ENERGY; MAGNETITE; magnetite, or surface, electron transfer rates, vacuum, hydrated, quantum mechanics, structure, charge distribution, bulk; MONOCRYSTALS; PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; POLARONS; REDOX REACTIONS; SURFACES; VALENCE; WATER
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2010.04.063

For the purpose of improving fundamental understanding of the redox reactivity of magnetite, quantum-mechanical calculations were applied to predict Fe 2+ availability and electron hopping rates at magnetite (1 0 0) surfaces, with and without the presence of adsorbed water. Using a low free energy surface reconstruction (½-Fe tet layer relaxed into the Fe oct (1 0 0) plane), the relaxed outermost layer of both the hydrated and vacuum-terminated surfaces were found to be predominantly enriched in Fe 2+ within the octahedral sublattice, irrespective of the presence of adsorbed water. At room temperature, mobile electrons move through the octahedral sublattice by Fe 2+–Fe 3+ valence interchange small polaron hopping, calculated at 10 10–10 12 hops/s for bulk and bulk-like (i.e., near-surface) environments. This process is envisioned to control sustainable overall rates of interfacial redox reactions. These rates decrease by up to three orders of magnitude (10 9 hops/s) at the (1 0 0) surface, and no significant difference is observed for vacuum-terminated versus hydrated cases. Slower hopping rates at the surface appear to arise primarily from larger reorganization energies associated with octahedral Fe 2+–Fe 3+ valence interchange in relaxed surface configurations, and secondarily on local charge distribution patterns surrounding Fe 2+–Fe 3+ valence interchange pairs. These results suggest that, with respect to the possibility that the rate and extent of surface redox reactions depend on Fe 2+ availability and its replenishment rate, bulk electron hopping mobility is an upper-limit for magnetite and slower surface rates may need to be considered as potentially rate-limiting. They also suggest that slower hopping mobilities calculated for surface environments may be amenable to Fe 2+–Fe 3+ site discrimination by conventional spectroscopic probes.