By Renée Haran
Mixing atoms in infinite combinations is the stuff of many a materials scientist’s dreams. In the world of nanoceramics or tiny inorganic materials, finding a winning combination unlocks a treasure trove of possibilities, including creating bespoke materials, overcoming the limitations of traditional materials, and enhancing active sites in catalysts, among others.
However, there are rules that govern whether scientists and engineers can mix two or more solids. Elements must have similar sizes, structures, electron attraction, and combining capacities to form a solid solution.
Despite these challenges, a group of researchers led by Jeff Urban and Chaochao Dun from the Foundry’s Inorganic nanostructures facility, in collaboration with Mark T. Swihart’s lab from the University at Buffalo (SUNY) department of chemical and biological engineering, are bending the rules of atomic mixology.
The team developed a rapid flame-based aerosol method that forces two typically unmixable elements to blend into a uniform solid.
Using a method where materials are synthesized under conditions far from thermodynamic equilibrium, they have successfully created a variety of solid ceramic nanoshells, including those containing nickel and other combinations of paired elements. The researchers published their findings in Nature Communications.
“It’s like trying to mix oil and water,” said Dun. “No matter how hard you try, they separate. Similarly, in ceramics, many metals or metal oxides don’t combine due to fundamental chemical differences.”
By leveraging rapid heating followed immediately by rapid cooling, the researchers can overcome traditional thermodynamic limits. Overcoming thermodynamic limits, even if only briefly, enables the creation of compositions that were previously unattainable.
The newly formed materials also remain semi-stable unless they are re-exposed to extreme conditions. Dun likens the aerosol-based flame process to flash-freezing a topping-rich ice cream before it melts.
“If you mix ice cream with a bunch of incompatible toppings, like hot caramel and frozen nuts, normally they’d separate. But if you flash-freeze the mix with liquid nitrogen the moment you stir, you can lock everything in. You get a dessert with a brand-new texture and flavor,” said Dun.
These findings open doors to the creation of novel nanoceramics, including high-entropy ones incorporating five or more metallic elements, as well as catalysts with record-breaking stability for converting carbon dioxide into syngas – a mixture of gases used for energy, among other energy applications.
In addition to mixing the unmixable, the novel method employed by the researchers enables precise control over the composition, particle size, shape, and structure of the resulting ceramic nanoshells. The researchers can “fine-tune” the part of the shell that is effective in speeding up reactions for a variety of applications.
“It’s like the difference between chocolate chip cookies with big chips or mini chips. You can even have one of those cookies where you just stick one big chocolate kiss in the middle,” said Swihart.
The researchers discovered an “encapsulated exsolution” phenomenon during the reduction of nickel aluminum oxide, forming ultra-stable nickel particles within porous aluminum oxide nanoshells.
Extending on Swihart’s cookie analogy, Dun likens the encapsulated exsolution to a chocolate chip cookie cake, where instead of rising to the surface, the nickel nanoparticles are embedded within the aluminum shell, making it more stable.
The embedded structures demonstrated superior performance compared to conventional catalysts.
“The catalyst produced by encapsulated exsolution is much more stable against clumping (sintering),” Swihart said. “If the nickel atoms can wander around and find each other, then the small particles grow into bigger particles, so now your chocolate chip cookie has morphed into a cookie with one big chocolate kiss in the middle. In that case, the catalyst would become much less active.”
Finally, using advanced electron microscopy and synchrotron X-ray techniques at the Lawrence Berkeley National Laboratory’s Advanced Light Source, the researchers were able to correlate atomic-level structure with macroscopic catalytic performance, bridging materials science, chemical engineering, and catalysis with their unified platform.
Dun said this binary metal oxide research serves as an “appetizer” for what he has coined the “high-entropy trilogy.”
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The research team’s ‘atomixologists’ have been busy combining all manner of materials, including high-entropy oxides that exceed 25 elements integrated into a single crystalline structure, high-entropy alloys, and multi-element metal-organic framework systems (MOFs) – a class of porous, crystalline materials built from multiple building blocks.
Dun believes the high-entropy trilogy research between University at Buffalo and Berkely Lab will pave the way for future advancements in the field of materials science. As the researchers continue to explore the literal millions of atomic “cocktails” they can create with their flame-based aerosol method, they anticipate uncovering additional insights and applications that could transform the landscape of materials science.
“You know, it’s like how we have stars (with planets) everywhere, but not all of them have life. But if we keep exploring them, I believe that someday we’ll find (another) one that has water,” said Dun.
“That’s what we’re doing here. We will keep making all these materials, and there’s no guarantee that all the materials are good, but there is a lot of possibility that we can make one that’s very good.”
