${}^{13}\mathrm{C}$ NMR spectroscopy of carbon nanohorns

We report the results of a carbon-13 nuclear magnetic resonance spectroscopic investigation of the structure of carbon nanohorn aggregates (CNHs). The results show that CNHs consist of two components, characterized by different chemical shifts and spin lattice relaxation $\left(T_1\right)$ behavior. The first component has a chemical shift of $124\mathrm{ppm}$ and displays rapid spin-lattice relaxation behavior and is assigned to the nanotubelike horns on the particles’ surfaces. The second component has a chemical shift of 116 ppm and much slower spin-lattice relaxation behavior and is assigned to the graphitelike part of the CNH aggregrate. The results of integrated peak area measurements indicate a 1:2 ratio of nanohorns to the graphitelike substrate. The absence of a clear Korringa behavior for the temperature dependence of $T_1$ and the lack of a Knight shift ruled out any metallic behavior and indicated instead behavior characteristic of semiconductor materials with paramagnetic centers due to structural defects providing an effective relaxation mechanism in the nanohorn domains. We also observed an anomalous change in $T_1$ near $17\mathrm{K}$ in the nanohorn domains suggesting the development of an antiferromagnetic correlation between localized electron spins.

Figure 1

(Color online) TEM images of carbon nanohorns. The inset shows a schematic diagram of a typical hornlike outer part of CNHs.

Figure 2

(a) Static ${}^{13}\mathrm{C}$ NMR spectrum for 97% ${}^{13}\mathrm{C}$ enriched CNHs taken at RT in a magnetic field of $8.4\mathrm{T}$. The inset shows a static NMR spectrum for CNHs with natural abundance obtained at $80\mathrm{K}$ in the same magnetic field. These spectra were obtained by spin echo mapping method. (b) Magic angle spinning NMR spectrum for 97% ${}^{13}\mathrm{C}$ enriched CNHs spun at $15.000\mathrm{kHz}$ taken at $300\mathrm{K}$ in a magnetic field of $14.1\mathrm{T}$. The asterisks stand for spinning sidebands. The system frequency was $150.870\mathrm{MHz}$.

Figure 3

(a) Inversion recovery spin-lattice relaxation time measurement. The spectra recovered asymmetrically. (b) Two-Lorenzian deconvolution of the central peak in the MAS spectrum. The peak positions for the fast and slow relaxation components are $124\mathrm{ppm}$ and $116\mathrm{ppm}$, respectively.

Figure 4

(a) Magnetization recovery curve (closed squares) in a progressive saturation $T_1$ measurement taken at the peak maximum $\left(122\mathrm{ppm}\right)$ at $280\mathrm{K}$. The solid and dashed lines represent the double exponential and single exponential fitting curves, respectively. (b) Temperature dependence of the magnetization recovery profile (symbols) and the two exponential fits (solid lines).

Figure 5

(a) Temperature variation of static ${}^{13}\mathrm{C}$ NMR spectra. Below $40\mathrm{K}$, the center of gravity of the spectrum shifted downfield slightly and the spectra were somewhat broadened. (b) Temperature variation of the spin-lattice relaxation rate $\left(T_1^{-1}\right)$ for the fast (closed circles) and slow (open squares) components of relaxation. The peak near $17\mathrm{K}$ is an indication of strong antiferromagnetic correlations that develop between the localized spin states belonging to the nanohorn part. Inset shows the temperature variation of the pre-exponential factor.