On the hop: iron oxide nanoparticles

Pulsed perspective

 

High-intensity X-rays have allowed an international team to investigate how electrons hop around within nanoparticles of iron oxide for the first time. The work could offer new insights not only into the motion of electrons in rust, but also how one of the most abundant minerals affects the condition of soil and water around it. The same insights might also inform developments in electron-transfer processes in biology and solar energy conversion.

Redox-active ions and proteins are well-known as involving iron oxide in electron exchange reactions. But, scientists were not so clear as to exactly how electrons hop from atom to atom within a particle or how fast that happens. The same process controls charge collection in solar energy devices involving metal oxides, and thus this work may have relevance to new energy technologies.

Researchers from Argonne National Laboratory, the Lawrence Berkeley National Laboratory, the Technical University of Denmark, Pacific Northwest National Laboratory, and the Polish Academy of Sciences collaborated on the work that looks at how electron transfer to iron(III) oxides creates iron(II) sites in the mineral. They point out that the iron(II) sites are not fixed but because iron(II) is more soluble than iron(III) when electrons hop on to iron atoms at the mineral surface, any iron(II) generated can dissolve. This ultimately affects the chemistry and mineralogy of soils and surface waters.

Such iron ion mobility is important in the movement of environmental contaminants because so many bind readily to iron oxide surfaces, most notably uranium. Dissolution of iron species will inevitably lead to the more rapid dissemination of such contaminants. It is likely that the same event that causes iron oxide to dissolve (reduction) would cause dissolved uranium ions to precipitate as solid uranium(IV) oxide. "Our study has important consequences in this area, but the details of how the contaminants would be affected in varying situations is quite complex," Katz adds.

"Another very important aspect of iron geochemistry is that it is an essential nutrient for life, and the only form that is 'bioavailable' is when it is dissolved in water, and so the iron(II) form. This way the reduction of iron(III) to iron(II) makes the iron locked up in minerals available to living organisms. This is of tremendous consequence!" team member Jordan Katz told SpectroscopyNOW.

"We believe that this work is the starting point for a new area of time-resolved geochemistry," explains LBNL's Benjamin Gilbert. "Time-resolved science seeks to understand chemical reaction mechanisms by making various kinds of 'movies' that depict in real time how atoms and electrons move during reactions. We have imported some of these ideas and approaches into geochemistry, and are very excited about the future possibilities."

The electron transfer processes taking place in iron oxide nanoparticles are on the pico-to-nano second timescale, depending on the precise internal structure of the particles and the temperature. The team adapted a pump-probe technique in which they could use a laser light to excite a dye molecule on the surface of the iron and so inject an electron into the particle. Femtosecond optical transient absorption spectroscopy showed how very fast the electron injection process is and they then snapped shots of the electron-transfer process inside the iron oxide particle using ultra-short pulse X-rays to get a sub-nanosecond view.

"This opens up studies with many other semiconductor materials," explains team member Xiaoyi Zhang. "The same technique of using a light-induced electron transfer to initiate chemistry can be applied to studies of solar cells, hydrogen generation, catalysis and electrochemical (battery) energy storage. It can provide new insights into how electrons or energy flow inside materials."

III to II transition on D and B

The team used two beamlines, 11-ID-D and 11-ID-B at the Advanced Photon Source in their studies. The former provided a close-up view of individual electron hops as fast as 80 picoseconds. APS upgrades might make these snapshots even shorter. Beamline 11-ID-B, meanwhile, is a dedicated pair-distribution-function beamline, which offers information about the particles' crystal structure that is unavailable to other techniques. In combination data from both beamlines provide what the researchers believe is the most complete picture of electron hopping in these minerals so far and of the transformation of iron(III) to iron(II) in solids.

Katz points out that each researcher in the collaboration foresees different benefits to their field of this same research. "As far as our future direction with this work, I think each of the collaborators would have a slightly different answer!" he told SpectroscopyNOW. "Some of us are interested in applying this now proven technique to other systems of interest to study other redox reactions in real time with X-ray spectroscopy," he says. For instance, it has geochemical significance, implications for improving solar energy conversion devices and allowing theoreticians to validate their work on electron transfer in different types of solids.