A nanocrystal is an extraordinarily tiny piece of material - composed of anywhere from a few to a few thousand atoms - in which atoms are arranged in a precise, ordered structure. Think of it like taking a piece of gold and shrinking it down to the size of a few hundred atoms. It's still gold, still crystalline, just almost incomprehensibly small.
Nanocrystals are in the transistors inside computers and smartphones, in smartphone displays and TV screens, in the gold-nanoparticle sensors that power COVID and pregnancy tests, and in the pipes of your car exhaust system, among countless other innovations. Their small size gives them a dramatically higher ratio of surface area to volume, making them especially useful as catalysts - materials that speed up chemical reactions without being consumed in the process.
Matteo Cargnello, associate professor of chemical engineering in the School of Engineering, and his lab have worked extensively on developing nanocrystals to drive catalytic reactions more efficiently. Their most recent project, a collaboration with researchers at the Korea Advanced Institute of Science and Technology (KAIST) in South Korea, aimed to move beyond simple, single-metal nanocrystals and build something more complex and challenging: a particle containing five different metals working together.
"We work with mostly metallic nanocrystals for catalysis, and we're interested in mixing multiple metals into one specific nanocrystal," said Cargnello. "When you start mixing two, three, four different metals, the properties of the final material may be very different than the properties of the individual single-metal nanocrystals."
The research team uncovered a surprising phenomenon: When five different metals are combined to synthesize a nanocrystal, the resulting particles are actually more uniform, not less. The discovery opens a new chapter in the preparation of nanomaterials, and could have direct implications for the future of hydrogen fuel. The details of this collaboration - helmed by Cargnello, Professor Hee-Tae Jung and former visiting graduate student Jeesoo Yoon from KAIST, and Jinwon Oh, PhD '26, with collaborators from the chemical company BASF - were published May 7 in Science.
The surprise
The team chose to work with ruthenium, a precious but highly active metal, as their starting point, then attempted to add four cheaper, more abundant materials: iron, cobalt, nickel, and copper. The motivation was both scientific and economic. Ruthenium is expensive, and if adding base metals could enhance or maintain its performance, manufacturers could use less of it.
Out of 31 possible product combinations, one emerged: a single, uniform five-metal nanocrystal in which all five elements were present in consistent proportions across every particle.
But creating a nanocrystal with five metals in controlled, uniform proportions is far from straightforward. Each metal behaves differently in solution. Each metal precursor reduces - meaning it transforms into its pure metallic form - at different speeds and temperatures. The expectation, shared widely in the field, was that combining more metals would produce more chaos: a jumble of different particle types with wildly varying compositions.
Instead, the team found the opposite. When two metals were combined with ruthenium, the results were messy: a mixture of different particle types with inconsistent sizes and compositions. Adding a third metal didn't help much either. But when they pushed to four total metals, and then to five, something remarkable happened. The chaos resolved. Out of a theoretical field of 31 possible product combinations, essentially one product emerged: a single, uniform five-metal nanocrystal in which all five elements were present in consistent proportions across every particle.
"The surprising discovery is that when you start adding more elements, all the way up to five, it's the opposite of what we expected," Cargnello said. "All five elements together in a single nanocrystal ended up looking like a single product."
Copper sets the stage
Through careful time-lapse experiments - collecting samples at different temperatures and analyzing them step by step - the team pieced together exactly what happened. Copper, it turns out, is the key to the whole process.
Among the four base metals, copper is the most "noble," meaning it converts to its metallic state most readily under the reaction conditions. It deposits first onto the ruthenium seed particles, but critically, it doesn't mix into the ruthenium. Instead, it sits alongside it, forming what researchers call a heterodimer: a nanocrystal with two distinct, side-by-side domains. In other words, rather than the two metals dissolving into one another the way copper and zinc dissolve to form brass, they remain as separate but physically joined regions within a single particle.
That copper-ruthenium structure then acts as a kind of chemical invitation for the other metals. Cobalt and nickel, which have affinities for ruthenium and copper, respectively, deposit next. Iron, the most difficult to reduce, arrives last and envelopes the growing particle from the outside. The result is an onion-like structure with ruthenium at the core, copper nestled beside it, cobalt and nickel forming intermediate shells, and an outer iron-rich layer.
The surprising discovery is that when you start adding more elements … it's the opposite of what we expected. All five elements together in a single nanocrystal ended up looking like a single product.Matteo CargnelloAssociate Professor of Chemical Engineering
The process, the team discovered, is self-organizing. The immiscibility of ruthenium and copper, or their refusal to fully blend, like oil and water, paradoxically creates the scaffold that brings everything else together in order.
A catalyst four times more powerful
Beyond the synthesis breakthrough, the team demonstrated that these five-metal nanocrystals perform significantly better than their single-metal counterparts in a reaction of growing industrial importance: ammonia decomposition.
Ammonia is already the most produced chemical in the world, and it is increasingly seen as a promising carrier for hydrogen energy. Because hydrogen gas is difficult to store and transport, it can instead be chemically combined with nitrogen to form ammonia, which is far easier to liquefy and ship. At the destination, ammonia is decomposed back into hydrogen and nitrogen, and the hydrogen used as a clean fuel source. The catch is that this decomposition requires high temperatures, often above 600 C, which places enormous demands on whatever catalyst drives the reaction.
When tested in this application, the five-metal nanocrystal catalyst outperformed a standard ruthenium catalyst by a factor of four in reaction rate. More striking still, it maintained that performance even after being held at 900 C for 12 hours - conditions that caused the single-metal ruthenium catalyst to degrade significantly. The multimetallic particles resisted sintering, a common failure mode in which high temperatures cause tiny particles to agglomerate into larger, less effective ones.
If we see the same promising results that we've been observing in the lab, there is the potential for these materials to be translated directly into industrial application.Matteo Cargnello
BASF, the German chemical company that co-funded and collaborated on the research, is now testing the catalysts under conditions closer to real industrial settings. Cargnello, who has worked with BASF for a decade and seen previous lab discoveries translated into commercial use, is cautiously optimistic.
"If we see the same promising results that we've been observing in the lab, there is the potential for these materials to be translated directly into industrial application," he said.
For now, the most immediate impact may be the design rules the team has established: a roadmap for how researchers might combine metals while accounting for which ones mix and which ones don't to reliably produce complex, uniform nanocrystals.
In a field where controlling five ingredients at the atomic scale was once considered nearly intractable, that clarity is itself a discovery worth building on.
