Electron microscopy gives new insight into metal nanoparticle oxidation

An international team of researchers has used electron microscopy techniques to discover an important mechanism behind the reaction of metallic nanoparticles with the environment.

The research led by York University 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.

According to York University, improving the understanding of metallic nanoparticles – particularly those of iron and silver – is of key importance to scientists because of their many potential applications.

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 included scientists from Leicester University, Japan’s National Institute for Materials Science, and the University of Illinois at Urbana-Champaign, used the resolution attainable with aberration-corrected scanning transmission electron microscopy to study the oxidisation of cuboid iron nanoparticles and performed strain analysis at the atomic level.

In a statement, lead investigator Dr Roland Kröger, from York University’s Department of Physics, said: ‘Using an approach developed at York and Leicester for producing and analysing 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 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 oxidised.

Corresponding author Dr Andrew Pratt, from York’s Department 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 Department of Physics at York University, the Department of Physics and Astronomy at Leicester University and the Frederick-Seitz Institute for Materials Research at the University 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 oxidised.