Electron transfer dynamics at complex aqueous iron oxide interfaces controls Fe(II)/Fe(III) cycling and consequently metal sequestration and lability in the photic zone of freshwater sources. Comprehension of underlying mechanisms and kinetics will also contribute to model development to inform the design, and predict the performance, of nano-iron based water purification devices and low-cost catalysts. Hematite nanoparticles are a common form of Fe(III)-oxide in natural waters and an excellent test system. The autocatalytic mechanism of reductive dissolution of iron oxides proceeds as follows: (1) adsorption/absorption of the electron donor, (2) interfacial electron transfer and release of the oxidized donor, (3) production of the reduced iron (II) cation, and (4) back reaction of Fe(II) with hematite. Electrons donated into iron (III) oxides are mobile over nanoscale distances despite moderate to low bulk electrical conductivity derived from low-charge carrier densities, low hole mobilities, and rapid recombination. Time-resolved pump-probe techniques, especially transient absorption spectroscopy (TAS), allow us to study iron (III) and iron (II) electron exchange dynamics in the presence of a photoexcited transient population of excess electrons on picosecond to nanosecond time scale as a function of solution pH, concentration, and nanoparticle size. We employ time-resolved TAS to determine the reaction dynamics and electron transfer properties between aqueous rhodamine B dye molecules and ~ 100 nm-sized hematite nanoparticles and how they depend on solution pH, solvent type, and concentration. Preliminary work has shown that rhodamine B binds to hematite and provides strong evidence that the bound molecule transfers an electron to hematite from the excited state. Our TAS results indicate a substantially longer lifetime of the photo-excited state for the dye in the presence of hematite, consistent with electron transfer to the oxide.

Electron transfer dynamics between hematite nanoparticles and natural organic matter at aqueous interfaces

ACS Fall 2021

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