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Abstract:
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.

York researchers discover important mechanism behind nanoparticle reactivity

Heslington, UK | Posted on November 3rd, 2013

Crucially, the research led by the University of York and reported in *Nature
Materials*, 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, which also included scientists from the University of
Leicester, the National Institute for Materials Science, Japan and the
University of Illinois at Urbana-Champaign, USA, 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.

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 the University of York, the Department of Physics
and Astronomy at the University of Leicester 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.

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."

The research was supported by a Max-Kade Foundation Visiting Professorship
stipend to Dr Kröger and financial support from the World Universities
Network (WUN). The Engineering and Physical Sciences Research Council
(EPSRC) funded the initial stages of the project (EP/D034604/1).

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About University of York
The University of York was founded in 1963 with 200 students. Since then, it has expanded to 10,000 students and has over 30 academic departments and research centres.

Academic excellence

From its inception, the University has concentrated on strong viable departments and teaching and research of the highest quality. The quality of York's teaching has received many accolades. York and Cambridge top the teaching league with the highest scores in official teaching assessments.

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