An international team of researchers has used pioneering electron microscopy techniques to discover an important mechanism behind the reaction of metallic nanoparticles with the environment.
Crucially, the research led by the Univ. of York and reported in Nature Materials, shows that oxidation of metals proceeds much more rapidly in nanoparticles than at the macroscopic scale. This is due to the large amount of strain introduced in the nanoparticles due to their size which is over a thousand times smaller than the width of a human hair.
Improving the understanding of metallic nanoparticles—particularly those of iron and silver—is of key importance to scientists because of their many potential applications. For example, iron and iron-oxide nanoparticles are considered important in fields ranging from clean fuel technologies, high-density data storage and catalysis, to water treatment, soil remediation, targeted drug delivery and cancer therapy.
The research team, which also included scientists from the Univ. of Leicester, the National Institute for Materials Science and the Univ. of Illinois at Urbana-Champaign used the unprecedented resolution attainable with aberration-corrected scanning transmission electron microscopy to study the oxidization of cuboid iron nanoparticles and performed strain analysis at the atomic level.
Lead investigator Dr. Roland Kröger, from the Univ. of York’s Dept. of Physics, said: “Using an approach developed at York and Leicester for producing and analyzing very well-defined nanoparticles, we were able to study the reaction of metallic nanoparticles with the environment at the atomic level and to obtain information on strain associated with the oxide shell on an iron core.
“We found that the oxide film grows much faster on a nanoparticle than on a bulk single crystal of iron—in fact many orders of magnitude quicker. Analysis showed there was an astonishing amount of strain and bending in nanoparticles which would lead to defects in bulk material.”
The scientists used a method known as Z-contrast imaging to examine the oxide layer that forms around a nanoparticle after exposure to the atmosphere, and found that within two years the particles were completely oxidized.
Corresponding author Dr. Andrew Pratt, from York’s Dept. of Physics and Japan’s National Institute for Materials Science, said: “Oxidation can drastically alter a nanomaterial’s properties – for better or worse—and so understanding this process at the nanoscale is of critical importance. This work will therefore help those seeking to use metallic nanoparticles in environmental and technological applications as it provides a deeper insight into the changes that may occur over their desired functional lifetime.”
The experimental work was carried out at the York JEOL Nanocentre and the Dept. of Physics at the Univ. of York, the Dept. of Physics and Astronomy at the Univ. of Leicester and the Frederick-Seitz Institute for Materials Research at the Univ. of Illinois at Urbana-Champaign.
The scientists obtained images over a period of two years. After this time, the iron nanoparticles, which were originally cube-shaped, had become almost spherical and were completely oxidized.