Researchers report that by simply adding a little water vapor to a standard recipe, they've hit upon a new, highly efficient way to grow carbon nanotubes. The advancement could make the molecules much cheaper to produce and expand their applicability.
Since their discovery 13 years ago, carbon nanotubes have been nanotechnology's poster child. The tiny straw-shaped molecules are stronger than steel, flexible, and conductive and have been touted as the right stuff for everything from chemical sensors to wires for nanoscale computer circuitry. But their exorbitant cost (more than 30 times the price of gold) has limited their payoff so far.
Today, most nanotubes are grown with the help of vanishingly small catalyst particles that help carbon atoms in a vapor develop into tiny tubes. Yet, most of these catalysts stop working after just a minute of operation as carbon atoms cover the catalyst particles with an amorphous coating that prevents nanotubes from taking shape. Nanotube researchers can remove the amorphous carbon simply by adding pure oxygen. But it works a little too well and quickly oxidizes--or burns--the growing nanotubes.
"We figured we needed a weak oxidizer that will not damage the carbon nanotubes," says Kenji Hata, a physicist at the National Institute of Advanced Industrial Science and Technology in Tsukuba, who led the current study, reported in the 19 November issue of Science. The group decided to look at water, Hata says, because water readily reacts with carbon to create carbon monoxide and molecular hydrogen. When Hata tuned his apparatus to add about 100 parts per million of water to hydrocarbons--the source of the carbon for the growing tubes--and other inert carrier gases, the water reacted with the amorphous carbon from the catalyst particles but didn't damage the growing nanotubes. As a result, virtually all of the catalyst particles remained active and quickly produced a forest of nanotubes growing up from a surface. And because of the high efficiency of the growth process, the resulting crop of nanotubes ends up nearly free of catalyst contaminants. If the approach can be scaled up, it could significantly drop the price of nanotubes, opening the door to new commercial applications.
"The results are quite remarkable and will lead to much follow-up," says Hongjie Dai, a chemist and nanotube expert at Stanford University.