Large-scale synthesis and HRTEM analysis of single-walled B- and N-doped carbon nanotube bundles

Bundles of B- and N-doped single-walled carbon nanotubes (SWNTs) containing up to ∼10 at% B and up to ∼2 at% N were synthesized at high yields under thermo–chemical treatment of pure C SWNT bundles and B2O3 in a flowing nitrogen atmosphere. The bundles were characterized by means of high-resolution transmission electron microscopy and electron energy loss spectroscopy. The effects of synthesis temperature (1503–1773 K) and time (30–240 min) on the B and N contents and yield of the SWNT bundles were determined. The maximum yield of the B- and N-doped SWNT bundles was obtained under synthesis at 1553 K over 30 min. Atomic structure and morphology of individual SWNTs in the bundles, in particular, packing of doped SWNTs, helicity distribution, encapsulation of fullerene-like clusters, diameter and shell number variations were studied. The synthesized SWNTs in the bundles were stacked in a honeycomb array with the uniform inter-tube spacing of ∼0.3 nm. No preferable orientation for the graphene-like tubular shells was found, i.e. both zigzag and armchair edges were observed with approximately equal proportions. Frequently, diameter increase took place for the outer tubes in a bundle and for isolated SWNTs. C-based or BN-based fullerene-like encapsulates were observed in individual SWNTs. Carbon oxidation by the B2O3 vapor and B and N substitution for C is thought to underlie the doping of C SWNTs. The substitution reaction temperature–time limits with respect to the morphological stability of B- and N-doped SWNT bundles are finally elucidated.