"[Double-walled carbon nanotubes] have higher thermal and chemical stability than single-walled carbon nanotubes, and can be applied to gas sensors, dielectric devices, nanoelectronic devices, nanocomposites and emitters etc," Takuya Hayashi of Shinshu University told nanotechweb.org.

The double-walled nanotubes have a coaxial structure, containing two concentric graphene cylinders. To date, techniques for making the materials have generally produced mixtures of double- and single-walled nanotubes contaminated with metal particles, amorphous carbon and multilayer carbon nanotubes.

"By improving the catalytic chemical vapour deposition process, we have successfully produced a relatively high yield of double-walled carbon nanotubes," said Hayashi. "Followed by the simple purification process, we have obtained - for the first time - a nanopaper made of high-purity double-walled carbon nanotubes."

Hayashi and colleagues used a chemical vapour deposition technique with a conditioning catalyst of molybdenum at one end of the furnace and an iron nanotube catalyst in the middle of the furnace. The conditioning catalyst caused the growth of more double-walled than single-walled nanotubes; the researchers believe this may be due to an increase in the amount of active carbon species.

Following deposition, the team used a two-step purification process. A hydrochloric acid treatment removed the iron catalyst and supporting material, while oxidation in air removed amorphous carbon and chemically active single-walled carbon nanotubes.

A final filtration step produced a flexible sheet of pure double-walled nanotube paper. The process yielded more than 95% double-walled nanotubes. The tubes were of two types: they had a inner-to-outer diameter ratio of 0.77:1.43 or 0.90:1.60.

"This high-purity double-walled nanotube material will help exploration and exploitation of the interesting properties expected in double-walled nanotubes," said Hayashi.

To demonstrate the flexible nature of the paper, the team fashioned some of the sheets into an origami samurai helmet (see figure).

Now the researchers, who reported their work in Nature, plan to improve the process further in order to "stimulate basic research and broaden the application field".