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

C-13 NMR spectra and spin-lattice relaxation times were measured for single-wall carbon nanotubes with 99.9 and 50.0% C-13 enrichments and natural abundance ( 1.1% C-13) prepared by catalytic decomposition of CH4 The C-13 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 C-13 enrichment. The C-13 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 C-13 static spectra originates from chemical shift dispersion, chemical shift anisotropy and dipole-dipole interactions between C-13 spins as well as between C-13 and H-1 spins at defects or edges. H-1 NMR spectra confirm the presence of H-containing species. The C-13 spin-lattice relaxation is dominated presumably by interaction with magnetic impurities. (C) 2003 Elsevier Ltd. All rights reserved.