High hopes. Could a gas-phase process yield nanotubes at nanoprices?

Nanotubes by the Kilo

Researchers say they have developed a new chemical process to produce large amounts of carbon nanotubes with relative ease. The find, announced on 3 October at a meeting of the American Vacuum Society in Boston, could bring down the production costs of nanotubes and help researchers apply them in a range of new materials and devices.

One hundred times as strong as steel and able to conduct like either metals or semiconductors, carbon nanotubes have long been touted for uses as down to earth as lightweight fuel tanks and car bumpers and as fanciful as cables for elevators into space. The hitch, so far, has been that the most promising tubes--single-walled nanotubes (SWNTs), consisting of one layer of carbon atoms arrayed like rolled-up chicken wire--can be made only by the thimbleful and can cost up to $2000 a gram.

Five years ago, a team led by Richard Smalley at Rice University in Houston came up with a new production method that blasts a graphite target with lasers in the presence of catalytic metal particles. The intense heat generated by the lasers turns the graphite into a vapor of carbon atoms, which the metal particles then help to coalesce into nanotubes. But the laser apparatus is expensive and has yielded only about 300 grams of SWNTs in the past 2 years.

In search of better results, Smalley and his colleagues looked for ways to make both the starting materials and the catalyst gaseous. The key turned out to be a molecule called iron pentacarbonele, which has an iron atom surrounded by five carbon monoxide (CO) groups. They spray this compound along with additional CO into a chamber heated to about 1000°C. The heat rips the CO arms off the iron atoms, leaving the lone atoms energetically unhappy and eager to bond with one another to form more stable clusters. And--as in the laser scheme--those metal clusters excel at producing SWNTs. Meanwhile, the high temperature also causes CO molecules to react with one another to form the more stable CO2, leaving behind lone carbon atoms, which quickly find the iron nanoparticles and begin to grow a SWNT.

Because the gas-phase synthesis is akin to the way bulk plastics are made today, the new scheme has potential to be scaled up to make industrial quantities. "Within the next year we should easily be able to produce 10 kilograms of this stuff [in the lab]," says Smalley.

"It's a very important development that nanotubes can be made in big quantities," says Walt de Heer, a nanotube expert at the Georgia Institute of Technology in Atlanta. "It implies that [their] price will come down, and this could allow their use as large-scale construction materials," he says. Still, de Heer cautions that inexpensive ingredients don't guarantee low costs: Related carbon molecules called buckyballs can be made from cheap starting materials, he points out, yet they remain more expensive than gold.

Related sites
Richard Smalley's home page