Researchers from McGill University have discovered that ultrasonication — normally used to tear materials apart — can instead stick them together to form porous, functional microparticles with potential applications in medicine, alternative energy and more.

Two years ago, PhD student David Basset was using high-frequency sound waves to blast apart a porous ceramic material coated with phosphates when he noticed something unusual. “The particles appeared to be breaking apart, but when we’d come back later, they’d be re-aggregated,” says Jake Barralet, Basset’s supervisor and a materials scientist in the Faculties of Dentistry and Medicine at McGill.

Ultrasonic waves are known to induce cavitation bubbles, which in turn lead to short-lived, tiny and extremely localized areas of high temperature, up to many thousands of degrees Celsius. According to Barralet and his team, those high temperatures appear to be capable of fusing the phosphate ions with the surface of the nanoparticles, causing them to aggregate.

The process is much gentler than traditional aggregation methods like sintering, which requires partial melting of the nanoparticles. As such, it’s allowed the team to create a stunning array of functional nanoparticles, which they recently described in Advanced Materials. For example, nanoparticles of cobalt phosphate are used for photocatalytic splitting of water. Using the ultrasonic phosphate technique, Barralet and his team were able to fuse nanoparticles into larger, easier-to-handle aggregates, without losing their functionality. “There’s a very slight reduction in surface area, in the five per cent range,” says Barralet. “But the catalytic properties are unchanged.” Even better, the bond created by the technique seems electrically conductive, which means that electrodes that are currently two dimensional can be made three dimensional. More efficient water-splitting catalysts could advance the field of solar fuels, which uses the splitting of water into hydrogen and oxygen as a way of storing solar energy.

In the paper, the team reports how they went on to replicate their success with a wide variety of other catalytically important materials, including titania (TiO2), nickel oxide, silver and carbon nanotubes. Their future work will focus on the possibility of using the porous materials for drug delivery. “You can encapsulate bioactive compounds, functional enzymes and all sorts of things,” says Barralet. “It was a chance discovery, but we were lucky in being able to work out backwards what was going on and reproduce it in different systems.”