For Pure Nanotubes, Just Add Water

Washing away impurities with water turns out to be as good for growing carbon nanotubes as it is for keeping a clean house.

Carbon nanotubes show great promise as building blocks for molecular machines, high-speed electronics and super-strong materials, but it has proven difficult to reliably grow large amounts of pure carbon nanotubes and to keep the growth process orderly.

The long, rolled-up sheets of carbon atoms can be narrower than a single nanometer, have useful electrical, optical, and magnetic properties, and are stronger by weight than steel. A nanometer is one millionth of a millimeter, or the span of 10 hydrogen atoms.

Researchers from the Japanese National Institute of Advanced Industrial Science and Technology (AIST) have added water to the standard method of manufacturing carbon nanotubes to produce tall, dense, vertically-aligned stands of pure nanotubes. The researchers have made stands of single-wall carbon nanotubes as tall as 2.5 millimeters and 99.98 percent pure. Individual nanotubes range from one to three nanometers in diameter.

The purity of the nanotubes makes the usual post-growth purification process unnecessary. This makes the method quicker, less expensive and less likely to damage the nanotubes than existing processes, according to the researchers. Nanotubes produced using the method are appropriate for many fields, including biology, medical implants, chemistry, electronics and magnetics research.

This carbon block consists of densely-packed single-wall carbon nanotubes. The block was made by adding water to the traditional chemical vapor method of forming nanotubes. Source: AIST

Ordinarily nanotube growth stops after about a minute as disorganized carbon soot accumulates. Water removes this amorphous carbon layer, making more catalyst particles active so that more nanotubes grow, and keeping them active longer to produce taller nanotubes. The high density of the nanotubes causes them to grow vertically rather than in many directions.

The method could be used to mass produce carbon nanotubes within five years, and for practical applications within ten years. The work appeared in the November 19, 2004 issue of Science.

-- TECHNOLOGY RESEARCH NEWS

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