Efficient gas-phase purification using chloroform for metal-free multi-walled carbon nanotubes

Abstract

Gas-phase treatments using chloroform (CHCl3) and water vapor were used to remove metal impurities (MIs) from as-prepared multi-walled carbon nanotubes (AP-MWCNTs), which is a non-destructive way to produce highly pure CNTs with extremely low MI content and with retention of CNT properties. After purification using CHCl3, the MIs in the AP-MWCNTs had significantly decreased to 12 ppm, representing a purification efficiency of 99.8%. This purification method, applicable to various AP-MWCNTs with high contents and different compositions of MIs, is more effective than liquid-phase purification using acids and does not damage the CNT structure and morphology. This moderate purification is due to a combination of factors. Chlorine radicals and hydrochloric acid from the thermal decomposition of CHCl3 react with MIs encased by graphitic carbon layers to form metal chlorides that sublime at high temperature. Hydrogen, oxygen, and water vapor generated during the formation of the metal chlorides etch graphitic carbon layers and lead to the formation of new pores and cracks, which provide fast reaction and diffusion paths. During metal purification, the MWCNT surface becomes lightly coated with chlorine or chlorinated carbon, which are eliminated from the surface post-treatment using water vapor.