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May 28, 2004
Hollow Nanocrystals and How to Mass Produce Them

Recently Yadong Yin and his colleagues in Paul Alivisatos's laboratory were experimenting with ways to modify the surfaces of nanocrystals — particles only a few billionths of a meter in size, comprised of only a few thousand atoms. After exposing cobalt nanocrystals to sulfur, they examined the results under a transmission electron microscope.

What they saw came as a surprise: the myriad solid crystals had all turned into hollow spheres.

Transmission electron microscope images track the formation of hollow nanocrystals.

Alivisatos is director of Berkeley Lab's Materials Sciences Division and a professor of chemistry at the University of California at Berkeley. His laboratory, in which Yin is a postdoctoral fellow, has been the source of numerous discoveries and inventions on the nanoscale. But while the mass production of nanoscale hollow spheres would be an exciting addition to the repertoire, it presented a puzzle.

"After we talked about what we'd observed, we decided that a process called the Kirkendall effect was responsible for the hollowing out of the spheres," says Yin.

The Kirkendall effect. At the boundary between two solids diffusing into each other at different rates, for example zinc and copper, their alloy (brass) grows in the direction of the faster-moving species (zinc). Unfilled voids are left behind and coalesce into large pores. (After Preston Huey, Science)

Discovered in 1942, the Kirkendall effect describes what happens when two solids diffuse into each other at different rates. The boundary between two metals, zinc and copper for example, is formed by a growing layer of alloy — brass, in this case — which expands in the direction of the faster-moving species, zinc.

This was the clue to Ernest Kirkendall's discovery that the atoms of the two solids don't change places directly; rather diffusion occurs where voids open, making room for atoms to move in. In the wake of the faster-moving material, large pores or cavities form as unfilled voids coalesce.

"Most of the time, people don't like this," says Yin. "It can be a big problem in welding, for example. But we saw the possibilities."

Yin and his colleagues realized that in a sulfide-coated cobalt nanocrystal, cobalt atoms rapidly move outward, leaving voids behind, while sulfur atoms move sluggishly inward. A rind of cobalt sulfide forms as they mix. Meanwhile the inner voids coalesce. Once all the cobalt has diffused into the sulfide, what's left is an empty sphere.

"We have lots of practice with cobalt nanoparticles, so we used cobalt to demonstrate the effect," says Yin. "We made hollow nanospheres of cobalt oxide, cobalt sulfide, and cobalt selenide." Then, to show that the process worked for metals generally, they also made hollow nanospheres of iron oxide and cadmium sulfide. "It gave us the confidence to say we could make hollow nanocrystals with many different materials."

On the nanoscale, the Kirkendall effect explains why a fast-diffusing cobalt nanocrystal leaves a hollow center behind as it moves into a surrounding sulfide-compound shell. (After Preston Huey, Science)

The nanospheres were remarkably uniform: depending on the proportions of the starting materials, their hollow centers were 40 to 70 percent as big as the initial crystal, but hole size varied no more than 13 percent in any given batch. This uniformity and versatility suggested a wide range of applications including drug delivery systems, optics, electronics, and selective chemical reactors, all on the nanoscale.

To demonstrate the possibilities, Yin and his colleagues in Alivisatos's laboratory consulted their neighbors in Gabor Somorjai's laboratory. Somorjai, also a member of the Lab's Materials Sciences Division and the UC Berkeley Chemistry Department, is an international authority on catalysis who has experimented with arrays of nanocrystals on surfaces.

Somorjai encouraged the researchers to isolate catalyst particles, such as platinum nanocrystals, inside their hollow shells. Compared to catalysts on open surfaces or in the channels of porous structures, catalysts confined in this way could reduce secondary reactions and deliver just the desired reaction products in the desired amounts.

To enclose catalytic platinum nanocrystals inside a solid shell, the researchers begin with platinum seeds (left), then coat them with cobalt which, when oxidized, evolves into hollow nanocrystals.

"At first I thought it would be impossible," says Yin, "but then we saw how we could do it." The synthesis began with platinum nanocrystals, or "seeds." Cobalt was added to form structures with platinum cores and cobalt shells.

Now the outer cobalt shells were oxidized, launching the familiar Kirkendall-like process: the cobalt diffused rapidly outward, the oxygen slowly inward, forming hollow spheres of cobalt oxide with platinum seeds rattling around in their centers.

In a test, the confined catalysts efficiently promoted the reaction of ethylene (C2H4) and hydrogen to form ethane (C2H6). One puzzle remained: how did the ethylene and hydrogen get inside the cobalt oxide shells to reach the platinum, and how did the ethane get out?

Yadong Yin and his colleagues use a three-necked flask to create hollow nanocrystalline structures in a "one-pot" process. (Photo Roy Kaltschmidt)

"We assume these small molecules are traveling along grain boundaries through the shell," Yin says, "but this is not completely understood. It's something we're still studying."

Meanwhile, the manufacturing process is remarkable in itself. "Unlike a lot of organic chemistry where you have many steps and lose a lot of material, this is a one-pot synthesis," says Yin. "You just add things, one at a time. You don't have to take anything out to purify the products in each step. You keep 100 percent of what you've made."

It's the beginning of mass production of complex nanoscale structures whose practical applications, already demonstrated, have barely begun.

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