Rust—iron oxide—is a poor conductor of electricity,
which is why an electronic device with a rusted battery usually won’t work.
Despite this poor conductivity, an electron transferred to a particle of rust
will use thermal energy to continually move or “hop” from one atom of iron to
the next. Electron mobility in iron oxide can hold huge significance for a
broad range of environment- and energy-related reactions, including reactions
pertaining to uranium in groundwater and reactions pertaining to low-cost solar
energy devices. Predicting the impact of electron-hopping on iron oxide
reactions has been problematic in the past, but now, for the first time, a
multi-institutional team of researchers, led by scientists at the U.S. Department
of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have
directly observed what happens to electrons after they have been transferred to
an iron oxide particle.
“We believe this work is the starting point for a
new area of time-resolved geochemistry that seeks to understand chemical
reaction mechanisms by making various kinds of movies that depict in real time
how atoms and electrons move during reactions,” says Benjamin Gilbert, a
geochemist with Berkeley Lab’s Earth Sciences Division and a co-founder of the
Berkeley Nanogeoscience Center who led this research. “Using ultrafast
pump-probe X-ray spectroscopy, we were able to measure the rates at which
electrons are transported through spontaneous iron-to-iron hops in redox-active
iron oxides. Our results showed that the rates depend on the structure of the
iron oxide and confirmed that certain aspects of the current model of electron
hopping in iron oxides are correct.”
Gilbert is the corresponding author of a paper in Science
that describes this work. The paper is titled “Electron small polarons and
their mobility in iron (oxyhydr)oxide nanoparticles.” Co-authoring the paper
were Jordan Katz, Xiaoyi Zhang, Klaus Attenkofer, Karena Chapman, Cathrine
Frandsen, Piotr Zarzycki, Kevin Rosso, Roger Falcone, and Glenn Waychunas.
At the macroscale, rocks and mineral don’t appear
to be very reactive—consider the millions of years it takes for mountains to
react with water. At the nanoscale, however, many common minerals are able to
undergo redox reactions—exchange one or more electrons—with other molecules in
their environment, impacting soil and water, seawater as well as fresh. Among
the most critical of these redox reactions is the formation or transformation
of iron oxide and oxyhydroxide minerals by charge-transfer processes that cycle
iron between its two common oxidation states iron(III) and iron(II).
“Because iron(II) is substantially more soluble
than iron(III), reductive transformations of iron(III) oxide and oxyhydroxide
minerals can dramatically affect the chemistry and mineralogy of soils and
surface,” Gilbert says. “In the case of iron(III) oxide, the reduction to
iron(II) can cause mineral dissolution on a very fast timescale that changes
the mineralogy and water flow pathways. There can also be a mobilization
of iron into solution that can provide an important source of bioavailable iron
for living organisms.”
Gilbert also noted that many organic and inorganic
environmental contaminants can exchange electrons with iron oxide phases. Whether
it is iron(III) or iron(II) oxide is an important factor for degrading or
sequestering a given contaminant. Furthermore, certain bacteria can transfer
electrons to iron oxides as part of their metabolism, linking the iron redox
reaction to the carbon cycle. The mechanisms that direct these critical
biogeochemical outcomes have remained unclear because mineral redox reactions
are complex and involve multiple steps that take place within a few billionths
of a second. Until recently these reactions could not be observed, but things
changed with the advent of synchrotron radiation facilities and ultrafast X-ray
spectroscopy.
“Much like a sports photographer must use a camera
with a very fast shutter speed to capture an athlete in motion without
blurring, to be able to watch electrons moving, we needed to use a exceedingly
short and very bright (powerful) pulse of X-rays,” says Jordan Katz, the lead
author on the Science paper who is now with Denison University. “For
this study, the X-rays were produced at Argonne National Laboratory’s Advanced
Photon Source.”
In addition to short bright pulses of X-rays, Katz
said he and his co-authors also had to design an experimental system in which
they could turn on desired reactions with an ultrafast switch.
“The only way to do that on the necessary timescale
is with light, in this case an ultrafast laser,” Katz says. “What we
needed was a system in which the electron we wanted to study could be
immediately injected into the iron oxide in response to absorption of light.
This allowed us to effectively synchronize the transfer of many electrons into
the iron oxide particles so that we could monitor their aggregate behavior.”
With their time-resolved pump-probe spectroscopy
system in combination with ab initio calculations performed by
co-author Kevin Rosso of the Pacific Northwest National Laboratory, Gilbert,
Katz and their colleagues determined that the rates at which electrons hop from
one iron atom to the next in an iron oxide varies from a single hop per
nanosecond to five hops per nanosecond, depending on the structure of the iron
oxide. Their observations were consistent with the established model for
describing electron behavior in materials such as iron oxides. In this model,
electrons introduced into an iron oxide couple with phonons (vibrations of the
atoms in a crystal lattice) to distort the lattice structure and create small
energy wells or divots known as polarons.
“These electron small polarons effectively form a
localized lower-valence metal site, and conduction occurs through
thermally-activated electron hopping from one metal site to the next,” Gilbert
says. “By measuring the electron hopping rates we were able to experimentally
demonstrate that iron(II) detachment from the crystal is rate-limiting in the
overall dissolution reaction. We were also able to show that electron hopping
in the iron oxides is not a bottleneck for the growth of microbes that use
these mineral as electron acceptors. The protein-to-mineral electron transfer
rate is slower.”
Katz is excited about the application of these
results to finding ways to use iron oxide for solar energy collection and
conversion.
“Iron oxide is a semiconductor that is abundant,
stable and environmentally friendly, and its properties are optimal for
absorption of sunlight,” he says. “To use iron oxide for solar energy
collection and conversion, however, it is critical to understand how electrons
are transferred within the material, which when used in a conventional design
is not highly conductive. Experiments such as this will help us to design new
systems with novel nanostructured architectures that promote desired redox
reactions, and suppress deleterious reactions in order to increase the
efficiency of our device.”
Adds Gilbert, “Also important is the demonstration
that very fast, geochemical reaction steps such as electron hopping can be
measured using ultrafast pump-probe methods.”