Adsorption of Argon on Carbon Nanotube Bundles and its Influence on the Bundle Lattice Parameter

We report experimental studies of the adsorption characteristics and structure of both ${}^{36}\mathrm{A}\mathrm{r}$ and ${}^{40}\mathrm{A}\mathrm{r}$ on single-wall carbon nanotube bundles. The structural studies make use of the large difference in coherent neutron scattering cross section for the two Ar isotopes to explore the influence of the adsorbate on the nanotube lattice parameter. We observe no dilation of the nanotube lattice with ${}^{40}\mathrm{A}\mathrm{r}$, and explain the apparent expansion of this lattice upon ${}^{36}\mathrm{A}\mathrm{r}$ adsorption by the location of the adsorbed Ar atoms on the outer bundle surface.

Figure 1

Schematic section of a nanotube bundle containing 37 nanotubes with a diameter of $(17\pm 1)\AA$. The dispersion of the nanotube diameter gives rise to an imperfect lateral ordering and heterogeneous interstitial channels (IC) providing possible adsorption sites for atoms and small molecules (black dots drawn to the size of a particle with $4\AA$ diameter). Other adsorption sites are the grooves (G) and the rounded outer surface of the bundle (S).

Figure 2

Isoteric heat of adsorption for Ar adsorbed on single-wall carbon nanotube bundles. The high binding energy sites ($3.6\mathrm{k}\mathrm{c}\mathrm{a}\mathrm{l}/\mathrm{m}\mathrm{o}\mathrm{l}$ or $156\mathrm{m}\mathrm{e}\mathrm{V}/\mathrm{\text{atom}}$) correspond to the adsorption in the grooves (G) and in the largest interstitial channels (IC), whereas the low energy binding sites ($2.4\mathrm{k}\mathrm{c}\mathrm{a}\mathrm{l}/\mathrm{m}\mathrm{o}\mathrm{l}$ or $104\mathrm{m}\mathrm{e}\mathrm{V}/\mathrm{\text{atom}}$) correspond to the adsorption, at higher coverages, on the rounded part of the outer graphene surface of the bundles (S).

Figure 3

(a) Neutron diffraction spectra of the bare SWNTB sample (broken line) and upon adsorption of $2.7\mathrm{m}\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{g}$ (solid line) of ${}^{36}\mathrm{A}\mathrm{r}$. Vertical dotted lines: Positions of the Bragg peaks expected for a hexagonal packing of nanotubes with a lattice spacing of $17\AA$. (b) Solid trace: Difference between the diffraction spectra in (a), revealing the changes induced upon adsorption. Dotted trace: Same but for adsorption of $2.7\mathrm{m}\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{g}$ of ${}^{40}\mathrm{A}\mathrm{r}$. Vertical dashed and dotted lines indicate expected Ar peak positions (see text).

Figure 4

(a) Neutron diffraction spectrum of the bare SWNTB sample in the vicinity of the (10) bundle lattice peak fitted by the sum of a polynomial of order three (background) and a Gaussian peak (solid lines). (b) Diffraction spectra (after background removal) of the bare SWNTB sample (open circles) and after adsorption of 0.68 and $2.7\mathrm{m}\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{g}$ (squares and upper triangles, respectively) of ${}^{36}\mathrm{A}\mathrm{r}$ (filled symbols) and ${}^{40}\mathrm{A}\mathrm{r}$ (open symbols). The solid lines are Gaussian fits to the data. (c) (10) diffraction peak obtained from a simulation of a bare seven (10,10)-SWNT bundle (open circles) and after ${}^{36}\mathrm{A}\mathrm{r}$ adsorption. The two ${}^{36}\mathrm{A}\mathrm{r}$ coverages correspond to the filling of the grooves (solid squares) and the complete coverage of the outer surface layer (solid triangles) and can thus be compared to the spectra (same symbols) in (b). Solid lines are guides to the eye.