The diffusion of dilute metallic lithium across the surface and into the bulk of atomically clean highly oriented pyrolytic graphite in ultrahigh vacuum is reported. Auger electron spectroscopy and a surface-dependent chemical reaction were utilized to monitor the coverage, oxidation state, and diffusion of Li. A very small diffusion activation energy (0.16 ± 0.02 eV; 15.4 ± 1.8 kJ/mol) is found for Li surface diffusion across the graphite basal plane. A model involving diffusion-rate-limited Li atom transport across the basal plane of graphite through step edge sites into the interior is proposed. These measurements indicate that the diffusion coefficient, D, for Li is very large (D(300 K) ≈ 5 × 10 −6 cm 2 s −1 ).

Single-walled carbon nanotubes (SWNTs) exhibit high surface areas and precisely defined pores, making them potentially useful materials for gas adsorption and purification. A thorough understanding of the interactions between adsorbates and SWNTs is therefore critical to predicting adsorption isotherms and selectivities. Metallic (M-) and semiconducting (S-) SWNTs have extremely different polarizabilities that might be expected to significantly affect the adsorption energies of molecules. We experimentally and theoretically show that this expectation is contradicted, for both a long chain molecule (n-heptane) and atoms (Ar, Kr, and Xe). Temperature-programmed desorption experiments are combined with van der Waals corrected density functional theory, examining adsorption on interior and exterior sites of the SWNTs. Our calculations show a clear dependence of the adsorption energy on nanotube diameter but not on whether the tubes are conducting or insulating. We find no significant experimental or theoretical difference in adsorption energies for molecules adsorbed on M- and S-SWNTs having the same diameter. Hence, we conclude that the differences in polarizabilities between M- and S-SWNTs have a negligible influence on gas adsorption for spherical molecules as well as for highly anisotropic molecules such as n-heptane. We expect this conclusion to apply to all types of adsorbed molecules where van der Waals interactions govern the molecular interaction with the SWNT.

The doping of single-walled carbon nanotubes (SWNTs) under ultrahigh vacuum by Li atoms has been explored experimentally and theoretically. The chemical effect of Li in breaking the C−Cl bond in chloromethane has been observed. Temperature programmed desorption (TPD) experiments show that at low coverage CH3Cl is physisorbed to the undoped SWNT sample, exhibiting a desorption process near 178 K. The CH3Cl desorption peak shifts to about 240 K for lithiated SWNTs, indicating an increase in binding energy of about 0.16 eV. More importantly, the integrated intensity of the CH3Cl desorption peak is dramatically reduced in the lithiated SWNT case, and CH3, C2H6, or related species are not observed in significant quantities in the TPD experiments up to a temperature of 500 K. This strongly indicates that CH3Cl reacts on lithiated SWNTs to produce an irreversibly bound species. Products LiCl and Li2Cl2 are observed to desorb near 700 K. Density functional theory calculations present possible reaction mechanisms and clues to the fate of the reacted CH3Cl. Our calculations show that at least two Li atoms are required to dissociate CH3Cl through a low-energy pathway. The products of the reaction are LiCl and CH3. The CH3 is chemically bound to defect sites on the nanotubes, and CH3 + CH3 radical recombination is suppressed. The second Li atom acts catalytically to lower the reaction energy required to break the C−Cl bond in CH3Cl. Furthermore, the CH3 bound to SWNT defect sites is observed to dehydrogenate at high temperatures in molecular dynamics simulations. This study indicates the potential for Li doping of SWNTs and other high surface area carbons to produce highly dispersed reaction centers for the destruction of toxic materials containing carbon−chlorine bonds.