13C NMR study of 13C-enriched single-wall carbon nanotubes synthesized by catalytic decomposition of methane

Abstract

13C NMR spectra and spin-lattice relaxation times were measured for single-wall carbon nanotubes with 99.9 and 50.0% 13C enrichments and natural abundance (1.1% 13C) prepared by catalytic decomposition of CH 4. The 13C isotropic shift is about 116 ppm from tetramethylsilane, being estimated from magic-angle-spinning (MAS) spectra. The value does not depend on the degree of the 13C enrichment. The 13C MAS NMR spectra show two additional small peaks at 171 and 152 ppm, which are ascribed to carbon species at defects or edges. The line widths of the main isotropic peak in MAS spectra are about 30 ppm, the origin of which is mostly chemical shift dispersion, reflecting a distribution of diameter and helicity. The line width in the 13C static spectra originates from chemical shift dispersion, chemical shift anisotropy and dipole–dipole interactions between 13C spins as well as between 13C and 1H spins at defects or edges. 1H NMR spectra confirm the presence of H-containing species. The 13C spin-lattice relaxation is dominated presumably by interaction with magnetic impurities.