Bimetallic nanoparticles are minuscule grains a few dozen to hundreds of atoms in size. But they hold out remarkable promise as vehicles for a number of applications, and researchers had no accurate, flexible general means for creating them until now.
Traditional methods, says Elam, a chemist at Argonne National Laboratory, lack the precision to make a batch of purely bimetallic nanoparticles. Rather, they yield a mixture of both bimetallic and monometallic nanoparticles, and these different nanoparticles have different chemical properties.
There are two major forms of bimetallic nanoparticles that scientists try to produce. One type, called core-shell, has one metal completely surrounding the other, like the candy coating over the chocolate center of a Tootsie Pop. In the other configuration, called an alloy, the metals are homogeneously mixed at the atomic scale so that atoms of both metals are present on the surface of the nanoparticle.
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Predictions made by theoretical calculations say that both types of bimetallic nanoparticles can be outstanding catalysts in applications such as biofuels and fuel cells. However, scientists have not had a general strategy to synthesize either type of nanoparticle on any surface and for a broad range of different metals.
In an effort to overcome such limitations, Elam and his team turned to atomic layer deposition. Atomic layer deposition (ALD) is a technique used in semiconductor manufacturing, in which extremely thin sheets of material are laid on top of each other one at a time.
Each time an ALD cycle is executed, a new sheet of material just a few atoms thick is deposited. ALD had been used in the past to create a variety of materials with customizable chemical and electrical properties, but until now researchers had not been able to selectively grow bimetallic nanoparticles with enough control to create successful catalysts.
Core-shell and Alloy Nanoparticles
Atomic layer deposition has been used earlier to grow single-metal nanoparticles on surfaces. This new breakthrough lets scientists grow the second metal only on the first metal, and not on the surrounding surfaces.
The key is to carefully control the growth temperature and selection of the chemicals used. Using this strategy, the Argonne researchers could make both core-shell and alloy nanoparticles while controlling the particle composition and particle size on a variety of different surfaces.
“It’s like being able to customize a car with the exact features you want it to have,” Elam said. “Once we’ve created these custom nanoparticle catalysts, we can pass them on to our scientific colleagues for a test drive.”
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