HR: 09:15h
AN: U51C-05    [Abstracts]
TI: Band-edge electronic structure modification in iron oxide and oxyhydroxide nanoparticles
AU: * Gilbert, B
EM: BGilbert@lbl.gov
AF: Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, United States
AB: Transition metal oxide and oxyhydroxide nanoparticles are the focus of considerable current interest in geochemistry because of their roles in biogeochemical redox cycles. Much progress has been made in understanding the structure and phase relationships in mineral nanoparticles, but the effects of small size and modified surface structure on reactivity remains an outstanding problem. Common environmental nanoparticles have been shown to exhibit enhanced chemical reactivity relative to bulk mineral surfaces, but the origin of this behavior is not well established. We studied the electronic structure component of mineral reactivity by comparing oxygen K-edge soft x-ray absorption and emission spectra of three phases of iron oxide and oxyhydroxide nanoparticles with spectra obtained from bulk minerals of the equivalent phases. These spectroscopies probe the electronic states in the vicinity of the semiconductor band gap that are important for interfacial (photo)electrochemical processes. Both the conduction and valence band states of goethite (α-FeOOH) nanoparticles exhibit considerable broadening that is likely associated with surface and interior disorder and which leads to an effective reduction in the band gap. By contrast, the band gaps of hematite (α- Fe₂O₃) and maghemite (γ-Fe₂O₃) nanoparticles are increased as a consequence of shifts in the threshold positions of the valence and conduction band, respectively. The origins of these electronic structure modifications are presently uncertain. Nevertheless, the data directly predict that the reducing potential of conduction band electrons in maghemite nanoparticles and the oxidizing potential of hematite valence band electrons are enhanced relative to the bulk minerals, and that simple photochemical studies can be used to validate these predictions.
DE: 5109 Magnetic and electrical properties (0925)
SC: Union [U]
MN: 2007 Joint Assembly