Pure Nanotubes: Just Add Water

NANOMATERIALS

To grow large amounts of pure single-walled carbon nanotubes (SWNTs), a group of Japanese researchers found they need only add one simple ingredient: water. The discovery could lead to an inexpensive, large-scale method for making nanotubes that can be used without further purification.

Chemical vapor deposition is a standard way to make SWNTs, but the process is plagued by amorphous carbon impurities that coat catalyst particles, slowing or stopping the synthesis. Kenji Hata, Don N. Futaba, and colleagues at the Research Center for Advanced Carbon Materials in Tsukuba, Japan, report they were able to overcome this problem and make SWNTs that are 99.98% pure by adding a controlled amount of water vapor to the growth atmosphere [Science, 306, 1362 (2004)]. They speculate that, in combination with metal particles from the catalyst, the water oxidizes any amorphous carbon that could build up on the catalyst.

By using this water-assisted growth process, the researchers create dense, vertically aligned SWNT "forests" that quickly reach millimeter heights. In one run, the group made a 2.5-mm-tall SWNT thicket on a 7- by 7-mm silicon wafer in just 10 minutes.

The process works with a variety of catalysts commonly used to grow SWNTs. The team can regenerate the catalytic substrate by slicing the nanotubes off with a razor blade.

The Tsukuba researchers also found they could build highly organized SWNT structures having almost any desired shape by lithographically patterning the catalysts prior to nanotube growth. One pattern of catalyst islands produced an array of cylindrical pillars, while flexible structures that look like sheets of paper grew from a pattern of thin stripes.