Electron Small Polarons and Their Mobility in Iron (Oxyhydr)oxide Nanoparticles

Associate professor Cathrine Frandsen et al. has just published a study in Science on the kinetics of electron mobility in rust nanoparticles. Comparisons between different phases revealed that short-range structural topology, not long-range order, dominates the electron atom-to-atom-hopping rate.

Jordan E. Katz, Xiaoyi Zhang, Klaus Attenkofer, Karena W. Chapman, Cathrine Frandsen, Piotr Zarzycki, Kevin M. Rosso, Roger W. Falcone, Glenn A. Waychunas, and Benjamin Gilbert

“Electron Small Polarons and Their Mobility in Iron (Oxyhydr)oxide Nanoparticles”, Science 337 (2012) 1200

distorted_lattice_site_generated_by_electron_injected_into_an_iron_III__oxide_

Rust is a poor conductor. Nevertheless, thermally activated electron hopping in the interior of iron (oxyhydr)oxides can be important for a number of reactions like interfacial redox-processes where for instance Fe(III) with acceptance of an electron may dissolve as Fe2+(aq). The study published in Science by Katz et al. has used 3-7 nm particles of iron(oxyhydr)oxides with dye molecules attached and a laser to generate electron transfer onto the nanoparticles.  With time-resolved X-ray spectroscopy it has been possible to follow the electrons’ atom-to-atom hopping rates. The measurements together with ab initio calculations show that the electron transport is associated with formation of electron small polarons, that being lattice distorted sites of Fe(II) that moves through the crystal by electron  hopping at a rate of 1-5 “hops” per ns, depending on the crystal structure.  By measuring the electron hopping rates for different iron(oxyhydr)oxides, the studies demonstrate that Fe(II) detachment from the crystal is rate-limiting in the overall dissolution reaction. Moreover, the studies show that electron hopping in the iron oxides is not a bottleneck for the growth of microbes that use these mineral as electron acceptors. The protein-to-mineral electron transfer rate is slo

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Cathrine Frandsen
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DTU Physics
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