Nanoparticle maps shock researchers
Image: Individual nanoparticles of the same substance, palladium, with different properties [Svetlana Alekseeva]
Sweden-based researchers have developed a novel method to map grain boundaries in nanoparticles, unexpectedly discovering that nanoparticles of the same material have different properties.
The latest results pave the way to a better understanding of the nanomaterials that are already used in catalytic converters, fuel cells, electronic devices and more.
While the influence of grain boundaries on the properties of bulk materials is widely documented, researchers have yet to fully investigate effects in nanoparticles.
And while electron back-scatter diffraction has been the method of choice to characterise grain boundary microstructures, it lacks the spatial resolution for studying nanocrystalline materials.
Given this, Professor Christoph Langhammer from Physics at the Chalmers University of Technology, and colleagues, turned to plasmonic nanospectroscopy with TEM and transmission Kikuchi diffraction (TKD) to map grain boundaries in individual palladium nanoparticles during hydrogenation phase transformation.
As Langhammer points out: "Multichannel single particle plasmonic nanospectroscopy enables measurements of the the individual response from up to 10 nanoparticles simultaneously, during both hydrogen absorption and desorption."
"We combined it with TEM and TKD... [which is] an advance compared with state-of-the-art methods, where only sequential measurements of individual nanoparticles are possible, meaning artifacts due to measurement-to-measurement variation cannot be avoided," he adds.
To optimise analysis, the researchers used TEM 'windows', consisting of a 40 nm electron-transparent silicon nitride membrane coated with a 10 nm reflective chromium layer.
Palladium nanoparticles were fabricated on the silicon nitride membranes, with the chromium layer then added for plasmonic nanospectroscopy, enhancing the intensity of light back-scattered from the nanoparticles.
As the researchers highlight, the mirror layer enabled multichannel single-particle plasmonic nanospectroscopy based on enhanced visible light back-scattering under darkfield illumination.
The chromium layer was then etched away from the TEM window, ready for TEM analysis.
The researchers went onto obtain brightfield TEM images of the nanoparticles using a FEI Titan 80-300, operated at an accelerating voltage of 300 kV.
For TKD analysis, the researchers used a state-of-the-art Bruker OPTIMUS detector with the electron beam entering the specimen from the backside and the detector collecting the Kikuchi diffraction pattern in transmission mode.
Using this combination of methods, the researchers revealed the details of the nanoparticle-grain boundary structure, type and orientation.
They also found correlation between the length and type of grain boundary in individual nanoparticles and hydride-formation pressure.
"Our experiments clearly showed how the reaction with hydrogen depends on the the way in which the nanoparticles are constructed," highlights Langhammers' colleague, Svetlana Alekseeva. "It was surprising to see how strong the correlation was between properties and response, and how well it could be predicted theoretically.”
The properties and response patterns differ for individual nanoparticles of the same substance, palladium, and these in turn determine the properties and responses of the nanoparticles when they come into contact with other substances. [Svetlana Alekseeva]
Crucially, the researchers also produced maps of individual palladium nanoparticles, with different grains depicted different coloured fields; some particles consist of a large number of grains, others have fewer grains.
“Our work indicates that not everything is what it seems; it’s the details that are crucial," says Langhammer. "To understand if and why nanoparticles are hazardous to humans, animals or nature, we need to look at them individually. Our new method now allows us to do this.”
Research is published in Nature Communications.