Purification of Single-Wall Carbon Nanotubes by Selective Microwave Heating of Catalyst Particles

A new scalable method for purification of single-wall carbon nanotubes (SWNTs) by using microwave heating in air is developed. The microwaves couple to the residual metal catalyst, raising significantly the local temperature leading to both the oxidation and rupturing of the carbon passivation layer over the metal catalyst particles. With this protective carbon coating weakened or removed, a mild acid treatment is then sufficient to remove most of the metal in the sample, leaving the nanotubes in tact. Using microwave processing and 4 M HCl acid reflux for 6 h we were able to remove residual metal (Ni, Y) in arc-discharge SWNTs to a level lower than 0.2 wt % (∼0.04 at. %). Results from transmission and scanning electron microscopy, Raman spectroscopy, and thermogravimetric studies were discussed.