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 team of researchers, have directly observed what happens to electrons after they have been transferred to an iron oxide particle.

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).

The team 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.

With their time-resolved pump-probe spectroscopy system in combination with ab initio calculations the team 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.

Researchers are excited about the application of these results to finding ways to use iron oxide for solar energy collection and conversion.

This story is reprinted from material from the
Berkeley Lab, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.