title: Experimental and theoretical comparison of gas desorption energies on metallic and semiconducting single-walled carbon nanotubes.
abstract: 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.
authors.affiliation: Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, USA.
authors.forename: Lynn
authors.initials: L
authors.lastname: Mandeltort
authors.forename: De-Li
authors.initials: DL
authors.lastname: Chen
authors.forename: Wissam A
authors.initials: WA
authors.lastname: Saidi
authors.forename: J Karl
authors.initials: JK
authors.lastname: Johnson
authors.forename: Milton W
authors.initials: MW
authors.lastname: Cole
authors.forename: John T
authors.initials: JT
authors.lastname: Yates
mesh_terms: D000327:Adsorption; D005740:Gases / Q000737:chemistry*; D006536:Heptanes / Q000737:chemistry; D008670:Metals / Q000737:chemistry*; D037742:Nanotubes, Carbon / Q000737:chemistry*; D011789:Quantum Theory; D012666:Semiconductors; D013499:Surface Properties; D013816:Thermodynamics
chemical_list: D005740:Gases; D006536:Heptanes; D008670:Metals; D037742:Nanotubes, Carbon