Iron is the most abundant transition metal in the Earth's crust, and naturally occurring iron oxide minerals play an important role in environmental redox reactions. Although iron redox reactions are well known and widely studied, their precise mechanisms are not well understood. Recent work showed that iron oxide redox reactions involve electron transfer pathways within the solid, suggesting that overall reaction rates could be dependent on electron mobility. We used pump-probe spectroscopy to investigate electron injection, mobility and recombination in ferrihydrite nanoparticles sensitized by fluorescein derivatives. We acquired optical transient absorption spectra as a function of particle size and under a variety of solution conditions, and used cryogenic transmission electron microscopy to determine the aggregation state of the nanoparticles. We observed electron recombination over timescales that spanned picoseconds to microseconds and kinetics that were fit with a stretched exponential decay function. The recombination rates were weakly affected by nanoparticle size and aggregation state, suspension pH, or the injection of multiple electrons per nanoparticle. We conclude that electron mobility in ferrihydrite is limited by electron trapping at defect sites. These deep trap states reduce the influence of factors such as surface charge on electron mobility.

Electron mobility and trapping in ferrihydrite nanoparticles

251st National Meeting (2016)

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