Iron is the most abundant transition metal in the Earth's crust, and naturally occurring iron oxide minerals play a commanding role in environmental redox reactions. Although iron oxide redox reactions are well-studied, their precise mechanisms are not fully understood. Recent work has shown that these involve electron transfer pathways within the solid, suggesting that overall reaction rates could be dependent upon electron mobility. Initial ultrafast spectroscopy studies of iron oxide nanoparticles sensitized by fluorescein derivatives supported a model for electron mobility based on polaronic hopping of electron charge carriers between iron sites, but the constitutive relationships between hopping mobilities and interfacial charge transfer processes has remained obscured. We developed a coarse-grained lattice Monte Carlo model to simulate the collective mobilities and lifetimes of these photoinjected electrons with respect to recombination with adsorbed dye molecules for essential nanophase ferrihydrite and tested predictions made by the simulations using pump-probe spectroscopy. We acquired optical transient absorption spectra as a function of the 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 biphasic electron recombination kinetics over time scales that spanned from picoseconds to microseconds, the slower regime of which was fit with a stretched exponential decay function. The recombination rates were weakly affected by the nanoparticle size and aggregation state, suspension pH, and injection of multiple electrons per nanoparticle. We conclude that electron mobility indeed limits the rate of interfacial electron transfer in these systems, with the slowest processes relating to escape from deep traps, the presence of which outweighs the influence of environmental factors, such as pH-dependent surface charge.
Bibliographical noteFunding Information:
Benjamin Gilbert was supported by the Office of Science, Office of Basic Energy Sciences (BES), United States Department of Energy (DOE), under Contract DE-AC02-05CH11231. Kevin M. Rosso was supported by the Chemical Sciences, Geosciences, and Biosciences Division, BES, DOE, through the Geosciences Program at Pacific Northwest National Laboratory (PNNL). Jennifer A. Soltis was supported by the University of Minnesota through the Nanostructural Materials and Processes Program, the Department of Chemistry Newman and Lillian Bortnick Fellowship, and the Council of Graduate Students Thesis Research Travel Grant. Jennifer A. Soltis and R. Lee Penn were also supported by the National Science Foundation (NSF, 0957696). Piotr Zarzycki was supported by the National Science Center (DEC-2016/22/E/ST4/00446). The TA studies were performed at the Molecular Foundry, Lawrence Berkeley National Laboratory, which is supported by BES, DOE, under Contract DE-AC02-05CH11231. The cryo-TEM studies were carried out in the Characterization Facility, University of Minnesota, a member of the NSF-funded Materials Research Facilities Network (www.mrfn.org) via the Material Research Science and Engineering Center (MRSEC) program. A portion of this research was performed using Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the Office of Biological and Environmental Research, DOE, and located at PNNL.
© 2017 American Chemical Society.
- charge separation
- electron transfer