13C NMR investigation of carbon nanotubes and derivatives☆
Introduction
Since a decade, nanotube-based materials have been extensively studied. Recently the functionalization of carbon nanotubes was realized, [1], [2], [3], [4], but the structural and chemical modifications are still unknown. In particular the nature and the number of chemical functions and the possibility to recover the as-grown nanotubes are still controversial. Then it appears very important to study such materials in detail and to identify their different chemical characteristics. We report on high resolution 13C NMR of functionalized SWNT. The NMR signature is compared to the one of purified SWNT and discussed in terms of possible chemical modifications like oxidative damage of the molecular structures.
Section snippets
Materials
Electric arc discharge is used to vaporize carbon in the presence of catalyst mixture (Ni:Y or Pt:Rh) in order to produce carbon nanotubes. The materials as collected from the chamber contain a high density of nanotubes and other forms of carbon like fullerenes, graphitized carbon structures, amorphous carbon as well as encapsulated catalyst in graphitic shells. Our purification method is based on an oxidation with nitric acid, a filtration step and a high temperature annealing. Our chemical
Results and discussion
Fig. 1 presents the static and MAS spectra of highly purified SWNT. MAS experiment averages Chemical Shift Anisotropies and reveals a unique isotropic line at 126 ppm which is the well-known 13C NMR signature of the nanotube [6]. The linewidth of about 50 ppm is due to a distribution of isotropic shifts related to the different chiralities and diameters of SWNT present in the sample [7]. The static spectrum shows 300 ppm of CSA that is typical for an sp2 carbon hybridization. After oxidation
Theoretical predictions
The physical origin of the shift in the Larmor frequency is the local modification of the magnetic field due to the neighborhood of the resonant nuclear spin. If the studied material is a pure carbon compound, the principal cause of the shift are the electrons while the other 13C acts through dipolar broadening. This electronic contribution is separated in two parts, respectively, called the chemical shift tensor which is the effect of the orbital motion and the Knight shift which we
Acknowledgements
This work was supported by FUNCARS European network No. HPRN-CT-1999-00011 and KISTEP under the contract No. 98-I-04-A-026, Ministry of Science and Technology (MOST), Korea.
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Original version presented at the International Workshop on Quantum Transport in Synthetic Metals II, Seoul, Korea, 28–31 August 2000.