Although melting behaviour is known to change at the nanoscale, the way nanoparticles melt has remained an open question.

Now, in a paper published in Nature Communications, researchers at Swansea University describe an experiment in which they imaged gold nanoparticles, with diameters ranging from two-to-five nanometres, as they were heated.

The researchers imaged the nanoparticles using an aberration corrected scanning transmission electron microscope, according to Prof Richard Palmer, who led the research.

They then carried out large-scale quantum mechanical calculations, to simulate their results.

“We established that the nanoparticle melts from the outside in, so you have a liquid skin, which is floating around a solid core,” said Palmer. “The melting point of the nanoparticles depends very strongly upon their size.”

Understanding how nanoparticles melt, and predicting their behaviour at elevated temperatures, could help in a range of applications, Palmer said.

In catalysis, for example, chemical reactions typically take place at elevated temperatures on small particles, which have a much lower melting point than their bulk metals.

“If you have a catalyst consisting of small particles, it may be that the molecules that are coming in to react are actually encountering a liquid surface, rather than the solid surface that you would have supposed,” he said. “So you may be able to tune the chemical activity by controlling the nature of the surface of these particles, by controlling the temperature at which the reaction takes place.”

The research could also be used to improve an additive manufacturing process known as sintering, in which small particles are merged together to create a desired material. The sintering process begins at a temperature at which the surface of the particles start to melt, said Palmer.

“So by choosing nanoparticles of different sizes, you could lower the temperature where sintering takes place, or indeed raise it,” he said.