Nanoparticles and the environment

Metallic nanoparticles have many possible uses – including clean fuel technology and high density storage – but their potential effects on their environment remain unknown.

An international team of researchers have used pioneering electron microscopy techniques to reveal an important mechanism behind the reaction of these metallic nanoparticles in the environment.

In their paper published in Nature Materials, researchers showed that oxidation of metals proceeds much faster in nanoparticles than at the macroscopic scale. The team – which includes scientists from the Universities of Leicester and York – believe this is due to the large amount of strain introduced into the nanoparticle due to their size.

In particular, the team used aberration-corrected scanning transmission electron microscopy to study the oxidation of cuboid iron nanoparticles and performed strain analysis at the atomic level.

“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,” said lead investigator Dr Roland Kröger, from York’s Department of Physics.

“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 materials”

A method known as Z-contrast imaging was used to examine the oxide layer that forms around a nanoparticle after exposure to the atmosphere; researchers found that within two years, the particles were completely oxidised. The iron nanoparticles – which were originally cube-shaped – had become almost spherical.

“Oxidation can drastically alter a nanomaterial’s properties – for better or worse – and so understanding this process at the nanoscale is of critical importance,” said corresponding author Dr Andrew Pratt, also from York’s Department of Physics. “This work will therefore help those seeking to use metallic nanoparticles in environmental and technological applications as it provided a deeper insight into the changes that may occur over their desired functional lifetime.”

Enhanced oxidation of nanoparticles through strain-mediated ionic transport http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat3785.html