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Make mine a decaf: Breakthrough in knowledge of how nanoparticles grow

July 23, 2015

A team of researchers from the University of Leicester and France's G2ELab-CNRS in Grenoble have for the first time observed the growth of free nanoparticles in helium gas in a process similar to the decaffeination of coffee, providing new insights into the structure of nanoparticles.

Nanoparticles have a very large surface area compared with their volume and are often able to react very quickly. This makes them useful as catalysts in chemical reactions and they are often used in sports equipment, clothing and sunscreens.

In a paper published by the Journal of Physical Chemistry Letters and funded by the Royal Society, The Leverhulme Trust, the British Council and CONACYT, the teams from the University of Leicester's Department of Physics and Astronomy and the CNRS in Grenoble measured how helium ions cluster with neutral helium atoms and grow into nanoparticles.

During the study they examined how helium ions drift through a cell filled with helium atoms. When the pressure of helium was increased the researchers observed a decrease in the mobility of the ions.

Dr Klaus von Haeften from the University of Leicester's Department of Physics and Astronomy, who has received a Visiting Professorship from the University Joseph Fourier, said: "We concluded that the increased pressure forced more and more helium atoms to bind to the ions gradually, until the clusters grew to nanometre-sized particles. This process continued until the nanoparticles reached the maximum size possible which also depended on the temperature.

"Further increase of the pressure was found to reduce the size, which we interpreted as compression. These size changes could then be followed in great detail. For low and moderate pressures the size changed rather rapidly whereas in the high pressure region the changes were slow."

By analysing how quickly the particle volume changed with pressure the researchers were able to investigate the structure of the nanoparticles.

Nelly Bonifaci from the G2ELab-CNRS said: "At low and moderate pressure the nanoparticles were much softer than solid helium and we concluded that they must be liquid. At high pressures they became progressively harder and eventually solid."

Dr von Haeften added: "By choosing helium we were able to study a system of greatest possible purity and our results are therefore very precise. Similar processes occur in the decaffeination of coffee in high pressure carbon dioxide, in dry cleaning and in chemical manufacturing. In all these processes nanoparticles grow. By knowing their size we can much better understand these processes and improve them."

This is the first time that researchers have been able to observe the growth of free nanoparticles in a large range of pressure in gaseous helium.

Frédéric Aitken from the G2ELab-CNRS added: "Our work is an important benchmark for the research on the formation and size of nanoparticles."
-end-
The original article 'Formation of Positively Charged Liquid Helium Clusters in Supercritical Helium and their Solidification upon Compression' has appeared in the Journal of Physical Chemistry Letters and is available at http://dx.doi.org/10.1021/acs.jpclett.5b01159

Notes to editors:

For more information please contact Dr Klaus von Haeften on kvh6@leicester.ac.uk

About the Leverhulme Trust:

The Leverhulme Trust was established by the Will of William Hesketh Lever, the founder of Lever Brothers. Since 1925 the Trust has provided grants and scholarships for research and education; today it is one of the largest all-subject providers of research funding in the UK, distributing approximately £80 million a year. For more information: http://www.leverhulme.ac.uk@LeverhulmeTrust

About the British Council

The British Council is the UK's international organisation for cultural relations and educational opportunities. We create international opportunities for the people of the UK and other countries and build trust between them worldwide.

We work in more than 100 countries and our 8,000 staff - including 2,000 teachers - work with thousands of professionals and policy makers and millions of young people every year by teaching English, sharing the arts and delivering education and society programmes.

We are a UK charity governed by Royal Charter. A core publicly-funded grant provides 20 per cent of our turnover which last year was £864 million. The rest of our revenues are earned from services which customers around the world pay for, such as English classes and taking UK examinations, and also through education and development contracts and from partnerships with public and private organisations. All our work is in pursuit of our charitable purpose and supports prosperity and security for the UK and globally.

For more information, please visit: http://www.britishcouncil.org. You can also keep in touch with the British Council through http://twitter.com/britishcouncil and http://blog.britishcouncil.org/.

University of Leicester

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