The researchers, led by Stephen Whitelam and Jeffrey Urban, obtained their result thanks to a unique optical probe that they developed in their laboratory. The probe is capable of directly imaging metal nanocrystals as they undergo various phase transformations during reactions with hydrogen gas. This is the first time that phase transformations in nanocrystals have ever been observed in this way, says the team.

Phase transformations in nanomaterials are profoundly different from those occurring in bulk materials since the surface area of nanocrystals is much greater and affects the thermodynamics and kinetics of particle nucleation. Urban and colleagues’ optical probe consists of a stainless steel airtight cell with optical windows and heating elements, connected to a high vacuum pump. The experiment involves collecting luminescence spectra (with a confocal Raman microscope) of palladium nanocrystals measuring between around 10 and 120 nm across as they interact with hydrogen gas. "The apparatus is able to monitor tiny variations in luminescence as hydrogen is absorbed," explains Urban, "which allows us to determine how the size of the nanocrystals affects the intrinsic thermodynamic and kinetics of hydriding and dehydriding phase transformations."

The researchers also employed a statistical mechanical model to quantify their experimental observations for the differently sized palladium nanocrystals. The results show a clear and direct correlation between nanocrystal size and luminescence.

"Metal nanocrystals emit light, but metal hydrides do not, so we can directly monitor the progress of the phase transition by measuring the emitted light while adding hydrogen gas, and changing the temperature and pressure," Urban told "This allows us to ‘watch’ the phase transition for different sizes of nanocrystals – for example, small nanocrystals change their luminescence very quickly whereas larger nanocrystals change more slowly."

The findings could be useful for optimizing hydrogen storage kinetics in such nanosystems, he adds.

The California team now plans to look at the effects of dopants on phase transformations in metal nanocrystals and study how different gases, not just hydrogen, interact with these structures.

The present research is detailed in Nature Materials doi:10.1038/nmat3716.

Further reading

Nanocrystals: size does matter (Nov 2009)
Nanocubes give better shaped crystals (Jul 2007)
Tomography technique images nanocrystals (Mar 2013)