title: Proton Dynamics on Goethite Nanoparticles and Coupling to Electron Transport.
abstract: The surface chemistry of metal oxide particles is governed by the charge that develops at the interface with aqueous solution. Mineral transformation, biogeochemical reactions, remediation, and sorption dynamics are profoundly affected in response. Here we report implementation of replica-exchange constant-pH molecular dynamics simulations that use classical molecular dynamics for exploring configurational space and Metropolis Monte Carlo walking through protonation space with a simulated annealing escape route from metastable configurations. By examining the archetypal metal oxide, goethite (α-FeOOH), we find that electrostatic potential gradients spontaneously arise between intersecting low-index crystal faces and across explicitly treated oxide nanoparticles at a magnitude exceeding the Johnson-Nyquist voltage fluctuation. Fluctuations in adsorbed proton density continuously repolarize the surface potential bias between edge-sharing crystal faces, at a rate slower than the reported electron-polaron hopping rate in goethite interiors. This suggests that these spontaneous surface potential fluctuations will control the net movement of charge carriers in the lattice.
authors.affiliation: Institute of Physical Chemistry, Polish Academy of Sciences , 01-224 Warsaw, Poland.
authors.forename: Piotr
authors.initials: P
authors.lastname: Zarzycki
authors.affiliation: Pacific Northwest National Laboratory , Richland, Washington 99354, United States.
authors.forename: Dayle M
authors.initials: DM
authors.lastname: Smith
authors.affiliation: Pacific Northwest National Laboratory , Richland, Washington 99354, United States.
authors.forename: Kevin M
authors.initials: KM
authors.lastname: Rosso