PNNL

Abstract

For the purpose of improving fundamental understanding of the redox reactivity of magnetite, quantum mechanical calculations were applied to predict Fe2+ availability and electron hopping rates at magnetite (100) surfaces, with and without the presence of adsorbed water. Using a low free energy surface reconstruction (½ Fetet layer relaxed into the Feoct (100) plane), the relaxed outermost layer of both the hydrated and vacuum-terminated surfaces were found to be predominantly enriched in Fe2+ within the octahedral sublattice, irrespective of the presence of adsorbed water. At room temperature, mobile electrons move through the octahedral sublattice by Fe2+-Fe3+ valence interchange small polaron hopping, calculated at 1010-1012 hops/second 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 (109 hops/second) at the (100) 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 Fe2+-Fe3+ valence interchange in relaxed surface configurations, and secondarily on local charge distribution patterns surrounding Fe2+-Fe3+ valence interchange pairs. These results suggest that, with respect to the possibility that the rate and extent of surface redox reactions depend on Fe2+ 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 hopping mobilities in magnetite nanoparticles may be slower than for bulk single crystals, towards time-scales amenable to Fe2+-Fe3+ site discrimination by conventional spectroscopic probes.