EXAFS study of rubidium-doped single-wall carbon nanotube bundles

The local structure around the rubidium ions inserted in single-wall carbon nanotube bundles (Rb-doped SWCNT) is studied by Rb $K$-edge extended x-ray-absorption fine structure (EXAFS). The dependence of the local order around the rubidium ions is investigated as a function of the time of doping (i.e., as a function of the stoichiometry of the sample). The first coordination shell of the rubidium ions, related to the distance between rubidium and the first nearest-neighboring carbon atoms, has a clear time doping dependence. Comparison between ab initio simulations of the EXAFS spectra and experimental data questions the interstitial site (between three tubes) as the preferential insertion site in SWCNT bundles. The results indicate that the rubidium ions are mainly located inside the tubes and around the bundles. The results are in good agreement with combined x-ray and neutron diffraction experiments performed on the same samples.

Figure 1

The EXAFS oscillations $\chi \left(k\right)$ of Rb-doped SWCNT samples (top). The EXAFS oscillations $\chi \left(k\right)$ of $\mathrm{Rb}{\mathrm{C}}_{24}$ and $\mathrm{Rb}{\mathrm{C}}_8$ intercalation graphite compounds (bottom). Comparison between experimental results and FEFF8 calculations is done. The first simulation is performed with a cluster of 4 shells up to $5\mathrm{\AA }$ including the $\mathrm{Rb}–\mathrm{Rb}$ contributions. The second simulation is performed by considering the three carbon shells up to $4.7\mathrm{\AA }$ from the adsorbing atom. Inset: Rb $K$-edge absorption spectra of $\mathrm{Rb}{\mathrm{C}}_8$ and $\mathrm{Rb}{\mathrm{C}}_9$. Spectra are vertically shifted for clarity.

Figure 2

The pseudoradial functions, $\mathrm{FT}\left({\mathrm{k}}^3\chi \left(k\right)\right)$ of the different Rb-doped SWCNT (top) and Rb-doped graphite compounds: $\mathrm{Rb}{\mathrm{C}}_{24}$ and $\mathrm{Rb}{\mathrm{C}}_8$ (bottom). The fits are performed using backscattering amplitude and total phase shift extracted from FEFF8 calculations (square). Note that the difference between the peak position $\left(2.7\mathrm{\AA }\right)$ and the real $\mathrm{Rb}–\mathrm{C}$ distance $\left(3.15\mathrm{\AA }\right)$ is due to a phase term.

Figure 3

The different local structures used in the FEFF calculations for clusters of $4.7\mathrm{\AA }$ around Rb. The curvature corresponds to the one of a (10,10) nanotube. For site 1 (Rb on graphene sheet), the first coordination shell is related to a $\mathrm{Rb}–\mathrm{C}$ distances of $3.16\mathrm{\AA }$, and the number of carbons on this shell is six. The second coordination shell is related to a $\mathrm{Rb}–\mathrm{C}$ distance at $4\mathrm{\AA }$, with six carbon atoms on this shell. The third $\mathrm{Rb}–\mathrm{C}$ distance coordination shell is related to a $\mathrm{Rb}–\mathrm{C}$ distance of $4.70\mathrm{\AA }$, with twelve carbon atoms on this shell. For site 2 (Rb on inner SWCNT surface), six $\mathrm{Rb}–\mathrm{C}$ distances are found in the $3.016–3.16\mathrm{\AA }$ range, six $\mathrm{Rb}–\mathrm{C}$ distances between $3.5–4\mathrm{\AA }$, and twelve $\mathrm{Rb}–\mathrm{C}$ distances between 4 and $4.7\mathrm{\AA }$. For site 3 (Rb on outer SWCNT surface) six $\mathrm{Rb}–\mathrm{C}$ distances are found in the $3.12–3.23\mathrm{\AA }$ range, eight $\mathrm{Rb}–\mathrm{C}$ distances between 4 and $4.7\mathrm{\AA }$. For site 4 (Rb on groove) four $\mathrm{Rb}–\mathrm{C}$ distances are found between $3.11–3.28\mathrm{\AA }$, six $\mathrm{Rb}–\mathrm{C}$ distances from $3.5\text{to}4.7\mathrm{\AA }$, and eighteen from $3.9\text{to}4.7\mathrm{\AA }$. For site 5 (Rb in interstitial site), ten $\mathrm{Rb}–\mathrm{C}$ distances are found between $2.77–3.31\mathrm{\AA }$, sixteen $\mathrm{Rb}–\mathrm{C}$ distances from $3.6\text{to}3.98\mathrm{\AA }$, and fifteen from $4.15\text{to}4.7\mathrm{\AA }$.

Figure 4

Comparison between the experimental Rb $K$-edge EXAFS oscillations, $\chi \left(k\right)$, measured in the saturation-phase of Rb-doped SWCNT $\left(\mathrm{Rb}{\mathrm{C}}_9\right)$, and FEFF8 calculations for the different Rb sites.