How gold nanoparticles melt
Image: Tracking shape changes in Au nanoclusters.
Using aberration corrected scanning transmission electron microscopy, Wales-based researchers have pin-pointed exactly how nanoparticles melt.
As Professor Richard Palmer from the College of Engineering at the University of Cardiff says: "Although melting behaviour was known to change on the nanoscale, the way in which nanoparticles melt was an open question.”
“Given that the theoretical models are now rather old, there was a clear case for us to carry out our new imaging experiments to see if we could test and improve these theoretical models," he adds.
To investigate surface heating of gold nanoparticles, 2 to 5 nm diameter clusters were deposited onto carbon membranes.
These clusters were then imaged using an in-situ heating stage in a 200 keV JEOL 2100F aberration corrected scanning transmission electron microscope, with observations later supported by large-scale quantum mechanical simulations.
As reported in Nature Communications, the researchers discovered that at elevated temperatures the nanoparticles formed a solid core-liquid shell structure.
Cluster surface melting temperatures were found to show evidence of size-dependent melting point suppression, with the cluster core melting temperatures being significantly greater than predicted by existing models of free clusters.
Caption: Shape changes in Au nanoclusters, indicating cluster surface melting at high temperatures. Images of two individual clusters containing 561 and 2530 atoms are shown. [Swansea University]
"We were able to prove the dependence of the melting point of the nanoparticles on their size and for the first time see directly the formation of a liquid shell around a solid core in the nanoparticles over a wide region of elevated temperatures, in fact for hundreds of degrees,” highlights Palmer.
"This helps us to describe accurately how nanoparticles melt and to predict their behaviour at elevated temperatures,” he adds. “This is a science breakthrough in a field we can all relate to - melting - and will also help those producing nanotech devices for a range of practical and everyday uses, including medicine, catalysis and electronics."
Research is published in Nature Communications.