Direct Observations of Electron Hopping in Iron Oxide


Iron oxide (rust) is a poor electrical conductor, but electrons in iron oxide can use thermal energy to hop from one iron atom to another. A Berkeley Lab experiment has now revealed exactly what happens to electrons after being transferred to an iron oxide particle. (Image courtesy of Benjamin Gilbert, Berkeley Lab)


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 oneatomof 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.”

吉尔伯特是在杂志上描述这项工作的论文的相应作者。本文标题为“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.

在宏观尺度、岩石和矿物不出现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).




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.



“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.”

这项研究受到了美国国务院of Energy’s Office of Science, which also supports the Advanced Photon Source.

Image: Benjamin Gilbert, Berkeley Lab

1 Comment论“氧化铁电子跳跃的直接观察”

  1. Madanagopal, V.C.|2012年9月10日上午7:52|Reply

    Gilbert`s observation of hopping of electron from ferric oxide (Iron II) to triferric tetroxide (Iron III) is an oxidation phenomena, by rusting process. He wonders that eventhough rusting iron is bad conductor, transfer of electron takes place by hopping. What about chlorophyl pigments which is also bad conductor? Sun`s ultraviolet makes the electron hop from pigments and oxidises water molecule to break into Hydrogen and Oxygen and aids photosynthesis. Here photo-electric effect draws out the electron. Thermo-electric effect is pulling out electron in the said case. Photon hitting an electron like a base-ball was already established in Raman effect and Compton effect where photon fails to remove an electron but undergoes modulation itself as Stokes line and Anti-Stokes line. I commend the X-Ray spectroscopic studies which proves the quantum nature of photons. Thank You.

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