Stanford researchers record reactions of self-healing nanoparticles

A group of researchers at Stanford University has recorded reactions of self-healing nanoparticles with one of the most advanced microscopes in the world.

The group, led by Jen Dionne, associate professor of materials science and engineering and senior author of the paper detailing their work published Monday in Nature Communications, recorded atoms moving in and out of nanoparticles less than 100 nanometers in size, with a resolution approaching 1 nanometer.

"The ability to directly visualize reactions in real time with such high resolution will allow us to explore many unanswered questions in the chemical and physical sciences," Dionne was quoted as saying in a news release. "While the experiments are not easy, they would not be possible without the remarkable advances in electron microscopy from the past decade."

Their experiments focused on hydrogen moving into palladium, a class of reactions known as an intercalation-driven phase transition. The reaction is physically analogous to how ions flow through a battery or fuel cell during charging and discharging.

For the experiments, the Dionne lab created palladium nanocubes, a form of nanoparticle, that ranged in size from about 15 to 80 nanometers, and then placed them in a hydrogen gas environment within an electron microscope. The researchers knew that hydrogen would change both the dimensions of the lattice and the electronic properties of the nanoparticle. They thought that, with the appropriate microscope lens and aperture configuration, techniques called scanning transmission electron microscopy and electron energy loss spectroscopy might show hydrogen uptake in real time.

After months of trial and error in the lab 18 feet, or about 5.5 meters, below the Engineering Quad of Stanford University, the results were extremely detailed, real-time videos of the changes in the particle as hydrogen was introduced.

Following these videos, they examined the nanocubes during intermediate stages of hydrogenation using a second technique in the microscope, called dark-field imaging, which relies on scattered electrons. In order to pause the hydrogenation process, the researchers plunged the nanocubes into an ice bath of liquid nitrogen mid-reaction, dropping their temperature to 100 degrees Kelvin, namely minus 173 degrees Celsius or minus 280 Fahrenheit.

These dark-field images served as a way to check that the application of the electron beam hadn't influenced the previous observations and allowed the researchers to see detailed structural changes during the reaction.

While most electron microscopes operate with the specimen held in a vacuum, the microscope used for this research has the advanced ability to allow the researchers to introduce liquids or gases to their specimen. "We benefit tremendously from having access to one of the best microscope facilities in the world," said Tarun Narayan, co-lead author of the study and recent doctoral graduate from the Dionne lab.

The researchers saw the atoms move in through the corners of the nanocube and observed the formation of various imperfections within the particle as hydrogen moved within it. However, "the nanoparticle has the ability to self-heal," said Dionne. "When you first introduce hydrogen, the particle deforms and loses its perfect crystallinity. But once the particle has absorbed as much hydrogen as it can, it transforms itself back to a perfect crystal again."

This ability of the nanocube to self-heal, described as imperfections being "pushed out" of the nanoparticle by the researchers, makes it more durable, a key property needed for energy storage materials that can sustain many charge and discharge cycles.

As the efficiency of renewable energy generation increases, the need for higher quality energy storage is more pressing than ever.

For its part, the Dionne lab has many directions it can go from here. The team could look at a variety of material compositions, or compare how the sizes and shapes of nanoparticles affect the way they work, and, soon, take advantage of new upgrades to their microscope to study light-driven reactions. Endit