The technique resulted in vertically-aligned nanotube forests that were easy to separate from the catalysts. The nanotubes had a carbon purity of more than 99.98%.

"Every current carbon nanotube application can benefit from these results," Kenji Hata of AIST's Research Center for Advanced Carbon Materials told nanotechweb.org. "The elimination of costly, time-consuming and potentially damaging post-growth purification processes translates into as-grown samples pure enough to use in various fields such as biology, chemistry and magnetic research. Furthermore, the use of carbon nanotubes in field emission - where impurities in the carbon nanotube emitters can seriously affect performance - can also benefit from our results."

Hata and colleagues grew the nanotubes by ethylene CVD, using argon or helium with hydrogen that contained a controlled amount of water vapour. They reckon the presence of water improved the activity and lifetime of the catalyst by removing amorphous carbon from its surface during the deposition process.

The team used the technique for a range of catalysts, including iron nanoparticles and sputtered metal thin-films on silicon wafers, quartz and metal foils. The process produced nanotubes up to 2.5 mm long in around 10 minutes. In standard ethylene CVD growth, in contrast, the catalysts only remain active for about a minute.

The researchers were able to remove the nanotube forest structure from the substrate with a razor blade. After this, the substrate was still catalytically active and was able to grow further nanotube forests.

"The easy removal of the vertically-aligned forests makes for easy handling," added Hata. "And the ability to 're-grow' nanotubes on the used catalytic surfaces applies directly to mass-production purposes."

The team believes it could apply water-assisted growth to systems such as methane and acetylene CVD or to the growth of multiwalled carbon nanotubes.

"We are constantly trying to improve our system and to better understand the role of water in carbon nanotube synthesis in order to further prolong growth time to the centimetre and metre length scales," said Hata.

The researchers reported their work in Science.