Credit: © ACS

The production and purification of carbon nanotubes on large scales is a major challenge in nanotechnology. Now, Robert Haddon and colleagues1 at the University of California, Riverside and Carbon Solutions have shown that high-speed centrifugation can be used to separate pre-purified single-walled carbon nanotubes (SWNT) from carbon nanoparticles in an industrially viable process on a bulk scale of 10 grams.

The purification method involves first, the pre-purification steps: (1) nitric acid treatment, which removes the metal catalysts, and (2) low-speed centrifugation and decantation, which removes the amorphous carbonaceous impurities. The pre-purified mixture is then subjected to high-speed centrifugation and similar decantation steps to remove the remaining carbon nanoparticle contaminants. The separation process is based on the relative stability of the nanotubes and nanoparticles under the influence of gravity, which, in turn, depends on their net surface charge. Nitric acid treatment from the pre-purification step leaves carbon atoms at the nanotube ends and defect sites functionalized with carboxylic acid groups, resulting in negative charges on the SWNT surface. The carbon nanoparticles that are more stable and resistant to chemical attack have fewer such carboxylate groups and, therefore, bear less net charge.

Haddon and colleagues reported purification recovery factors of 90% for the high-speed centrifugation step, which produced forces 20,000 times that of gravity. However, the approach only works for nanotubes that form stable dispersions in water.