T68 Structure, Properties, and Geochemistry of Nanoparticles, Nanoclusters, and Nanocomposites in Biogeochemical Systems II: In Honor of Benjamin Gilbert, Recipient of the 2010 MSA Award

Sunday, 31 October 2010
1:30 PM - 5:30 PM
Mile High Ballroom 2AB/3AB

ULTRAFAST X-RAY AND OPTICAL TRANSIENT ABSORPTION SPECTROSCOPY OF THE REDUCTIVE DISSOLUTION OF IRON OXIDE NANOPARTICLES

Paper number: T68-5
Program: Technical Sessions
Time: 2:35 PM – 2:50 PM

Authors

Abstract

The reduction of Fe(III) is one of the most important chemical changes that takes place in the development of anaerobic soils and sediments, and the reductive dissolution of iron-bearing minerals by microbes plays a critical role in this process. Despite its importance in biogeochemistry, many questions remain about the mechanism of this electron transfer reaction, in part because the speed of the fundamental chemical steps renders them inaccessible to conventional study. Time-resolved X-ray spectroscopy is a technique that can overcome this limitation and measure changes in oxidation state and structure occurring on the ultrafast timescale. We use this approach, in conjunction with femtosecond optical spectroscopy, to measure electron transfer rates and the speciation of Fe atoms in iron oxide nanoparticles following laser excitation of a surface-bound dye molecule.

Both maghemite and ferrihydrite nanoparticles were synthesized by co-precipitation of Fe3+ in basic solution with a varying amount of Fe2+, giving particles 1.5-3 nm in diameter. UV/vis absorption and fluorescence spectroscopy showed that the dye, 2,7-dichlorofluorescein, binds strongly to the particle surface and that its emission is quenched fully when bound. Under steady-state photoexcitation, dye-sensitized particles evolve Fe2+(aq) over time, indicating electron transfer from sensitizer to particle followed by reductive dissolution. Pump-probe X-ray absorption spectroscopy measurements at the Fe K edge show the transient formation of reduced iron species within ~100 ps following laser excitation of the dye molecule, which is not seen. Ultrafast optical spectroscopy of the sensitizer molecule shows electron injection occurs within the X-ray pulse length. No transient species were observed in control experiments using unsensitized particles.nanoparticles. These data represent the first direct real-time observation of the dynamics of ferrous ion formation and its subsequent transformations in iron oxide.