EXAFS investigations of iodine-doped carbon nanotubes

We report an x-ray absorption fine structure study at the iodine-$K$ edge of the local structure in iodine-doped carbon nanotubes. The iodine-carbon host interaction is shown to be weaker in multiwalled carbon nanotubes (MWNTs) than in single-walled carbon nanotubes (SWNTs). Iodine species are only localized at the surface of the external tube for MWNTs, whereas iodine species enter inside SWNTs. For doped SWNTs, both the experimental and the theoretical EXAFS spectra allow us to establish the structure of the iodine chain as disordered pentaiodide at the saturation level.

Figure 1

EXAFS spectra at the iodine $K$ edge of the unwashed (a) I-MWNTs and (b) I-SWNTs.

Figure 2

Experimental EXAFS signal of the I-MWNTs sample (a) and theoretical ab initio spectrum of ${\mathrm{I}}_2$ crystal clusters (b).

Figure 3

Back Fourier transform of the experimental spectrum and the fit taking into account one shell of iodine and one shell of carbon; inset: Fourier transform $\mathrm{FT}\left[{\mathrm{k}}^3\chi \right(k\left)\right]$ of the experimental EXAFS spectrum; note that the difference between the peak position $\left(2.5\mathrm{\AA }\right)$ and the real $\mathrm{I}\text{−}\mathrm{I}$ distance $\left(2.73\mathrm{\AA }\right)$ is due to a phase term.

Figure 4

Raman spectra of iodine doped SWNTs: Top, carbon solution samples; bottom and middle, HIPCO samples. Micro-Raman spectra were measured using the $514.5\mathrm{nm}$ excitation line.

Figure 5

Experimental and simulated EXAFS spectra of several probable environments around the iodine absorbing atom. Inset: back scattering amplitudes of the iodine-iodine and iodine-carbon pair of atoms; drawings of the different $({\mathrm{I}}_n{)}^{-}$ chains.