High yield purification of multiwalled carbon nanotubes by selective oxidation during thermal annealing

Abstract

Multiwalled carbon nanotubes were synthesized by electric arc discharge method in helium ambient with the pressure of 400 Torr and then purified by thermal annealing. During the annealing in air, the quartz tube in which the raw samples were placed was rotated in order to expose evenly the nanotubes and the carbonaceous particles to the air. The carbonaceous particles were presumably etched away by the selective oxidation with faster etching rate than nanotubes. This gives rise to very high yield of about 40%. It was found from Raman scattering measurements that the ratio of the intensity of G-line peak (1583 cm −1) to that of D-line peak (1285 cm −1) increased drastically by this purification process. Our density-functional tight-binding calculations clearly show that the desorption energy barrier of a C–O pair from the nanotube edge is 2.48 eV, higher than 0.3∼2.1 eV from an amorphous carbon, confirming the current approach of purification by the selective oxidation.