The Sciences

Cages of Carbon

By Tim FolgerSep 1, 1993 12:00 AM


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There is a lot of hollow space inside the carbon latticework known as a buckyball--space that can be used to trap an atom or two.

To some chemists, buckyballs are the best thing since the Bunsen burner. The soccer ball- shaped molecules, which are built of 60 or more carbon atoms, have been the focus of intense research ever since their discovery eight years ago. Now chemists have found ways to use buckyballs and other fullerenes (named for their striking resemblance to Buckminster Fuller’s geodesic domes) as molecular cages to trap atoms.

Among the recently incarcerated atoms is helium, a most unlikely chemical captive. Helium is one of the noble gases--noble because it does not form attachments to other atoms in the usual way, by sharing electrons with them. No compounds of helium had been known before, says Martin Saunders, one of the chemists at Yale who used buckyballs to cage helium.

The standard method of making buckyballs--passing an electric current through graphite electrodes in a helium-filled chamber--takes advantage of helium’s nobility. (If the chamber were filled with ordinary oxygen-rich air, say, the carbon atoms vaporizing off the graphite rods would burn like charcoal on a grill instead of assembling themselves into buckyballs.) But Saunders suspected that some of the balls might, by pure chance, grow around some free-floating helium atoms. To confirm these suspicions, he and his colleagues heated buckyballs to about 1300 degrees, reasoning that the high temperature would break some of the carbon bonds and allow the trapped atoms to escape. Sure enough, they found traces of helium.

But only traces: not many helium atoms get trapped by accident. To increase their yield, the chemists took a batch of preformed buckyballs and baked them in helium at high pressure. The heat apparently again broke some carbon bonds, but this time the high pressure caused helium from outside the cage to flow in before the bonds reformed. When the chemists later measured the helium in this batch, Saunders says, they found nearly 5 million times more than would be found in a typical sample of buckyballs.

Helium-impregnated buckyballs, Saunders thinks, would make superb chemical tracers. They could be used, for instance, to track the spread of pollutants from their point of discharge into a river. Helium can be detected in extremely small quantities, says Saunders, because there are no other helium compounds around in the atmosphere. You could dissolve a microgram of it in an Olympic-size swimming pool, and if you stirred it thoroughly, you would be able to detect the helium and the buckyball in one drop of water. Buckyballs and helium, says Saunders, are more benign than the radioactive tracers that are sometimes used.

Helium isn’t the only element to be jailed in a carbon cage recently. Researchers at SRI International in Menlo Park, California, have trapped crystals of lanthanum dicarbide inside fullerenes--but ones far larger than the buckyballs Saunders used. Each of these fullerenes consists of hundreds of thousands of carbon atoms assembled into cages that are nested one inside another, like tiny Russian dolls. In making the fullerenes, Rodney Ruoff and his colleagues at SRI drilled a hole in one of the graphite electrodes and filled it with lanthanum oxide. When current flowed through the electrode, it vaporized both the graphite and its lanthanum oxide filling. The lanthanum atoms, now freed, latched onto two carbon atoms, forming crystals of lanthanum dicarbide.

Somehow those crystals made it into the innermost of the nested fullerenes. The SRI researchers sent their sample to Shekhar Subramoney, an electron microscopist at Du Pont Experimental Station in Wilmington, Delaware, for analysis. Within three days Subramoney was on the phone from his darkroom calling me, says Ruoff. He said, ‘I’ve never seen anything like this in my life.’ The cavities were filled with the lanthanum dicarbide.

Lanthanum dicarbide, says Ruoff, reacts very strongly with water vapor and oxygen and rapidly degrades in air. But the fullerene container has kept air out, protecting a lanthanum sample for more than six months. Whereas the work of Saunders and his colleagues shows that fullerenes can be used to bind elements that are so inert they ordinarily won’t react with anything, the SRI group’s results suggest just the opposite application: fullerenes may offer a way to package and protect substances so reactive and delicate that chemists have trouble holding on to them at all.

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