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 shows that oxidation of metals - the process that describes, for example, how iron reacts with oxygen, in the presence of water, to form rust - 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 used the unprecedented 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.

Lead investigator Dr Roland Kröger, from the University of York’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 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 oxidised.

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.

Professor Chris Binns, from the University of Leicester, said: “For many years at Leicester we have been developing synthesis techniques to produce very well-defined nanoparticles and it is great to combine this technology with the excellent facilities and expertise at York to do such penetrating science. This work is just the beginning and we intend to capitalise on our complementary abilities to initiate a wider collaborative programme.”

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