Oxygen K-Edge Emission and Absorption Spectroscopy of Iron Oxyhydroxide Nanoparticles

摘要

Transition metal oxide and oxyhydroxide nanoparticles are the focus of considerable current interest in geochemistry. 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 soft x-ray absorption and emission spectra of bulk goethite (-FeOOH) with spectra obtained from ~ 6 nm FeOOH nanoparticles and larger FeOOH nanoparticles obtained by hydrothermal coarsening. The semiconductor band gap is reduced in the FeOOH nanoparticles, mainly due to the presence of additional states in the upper valence band. We performed ab initio simulation of the electronic structure of oxygen sites at the 010 surface of goethite, and observe that oxygen sites with reduced metal coordination contribute to the O 2p DOS at higher binding energy. Hence we conclude that FeOOH nanoparticle surfaces are more disordered than the surfaces of goethite, and that this structural component is likely the dominant cause of enhanced rates of reductive dissolution.