REACTIVE FE(II) AND ELECTRON HOPPING MOBILITIES IN SPINEL FERRITE NANOPARTICLES

Iron oxide nanoparticles are high-surface area minerals that are widespread in the environment. Because of the ferric/ferrous iron redox couple, these nanophases are important participants in natural redox cycles in sediments. In particular, changes in geochemistry and mineralogy across a redox boundary are tightly coupled to the fate of electrons transferred to and from these nanophase minerals. For example, spinel ferrite nanoparticles are one important source and sink for reactive Fe(II) equivalents due to their topotactic solid-solution property and stable multi-valent nature. Our team has been performing multiple lines of research focused on understanding the distribution and availability of Fe(II) in this class of materials. Compositionally controlled Fe_3-xTi_xO₄ titanomagnetite nanoparticles, in which the structural Fe(II)/Fe(III) ratio is intentionally tuned by the Ti(IV) content, were synthesized for reduction kinetics studies using pertechnetate as the electron accepting probe molecule. The particles were found to include structural Ti(IV) into the octahedral sublattice with concomitant increase in lattice Fe(II) up to x = 0.35. Characterization involving in situ XRD, Mössbauer spectroscopy, XANES/EXAFS, and ex situ techniques such as TEM and XMCD enabled analysis of the nanoparticle properties and Fe(II)/Fe(III) ratio before and after reaction. Combined with structural and magnetic information from first principles calculations, and the collective information has allowed discrimination of different reactive pools of ferrous iron, including Fe(II) in the octahedral sublattice and sorbed Fe(II). Calculations of Fe(II) electron hopping mobilities for x=0 compositions suggest a strong dependence on the proximity to the (100) surface. Finally, we have also examined the fate of Fe(II) electrons photoinjected into the initially ferric endmember maghemite (γ‑Fe₂O₃), in which iron redox dynamics followed at the nanosecond timescale using x-ray spectroscopy and first principles calculations suggest picosecond electron hopping rates. These lines of research are converging on a picture of rapid electron exchange dynamics between iron oxide nanoparticles and their environment, with profound implications for the biogeochemistry of the subsurface.