(21e) Noble Gas Adsorption On Carbon Nanotubes and Metal Surfaces (Pd, Pt, Cu): A Van Der Waals Density Functional Study

We have used the recently developed van der Waals density functional approach [1] to study dispersion-dominated systems consisting of noble gases (Ar, Kr, Xe) adsorbed on carbon nanotubes and metal surfaces [Pd(111), Pt(111), Cu(111)]. We have found that the conventional PBE functional fails to capture the van der Waals interaction between noble gases and carbon nanotubes or metal surfaces. In contrast, the vdW-DF approach gives considerable improvement in the description of the adsorption energies. For the carbon nanotube systems, the vdW-DF prediction of the Xe adsorption energies on the outside of the (10,0), (15,0), and (20,0) nanotubes are -0.11, -0.14, and -0.15 eV, respectively, while the gases are essentially unbound with PBE, having energies of 0.01 to -0.02 eV. Surprisingly, no difference in the potential energy was found for semiconducting and metallic carbon nanotubes with either the PBE or the vdW-DF functionals, indicating that the adsorption energies for rare gases on carbon nanotube are not strongly influenced by differences in the electronic structure of the nanotubes. The adsorption energies predicted from classical potentials are smaller than those obtained with the vdW-DF functionals by about 10-35%. We have also seen similar differences between PBE and the vdW-DF functionals for the adsorption energies of the noble gases on various metal surfaces. As an example, the potential energy for Xe on Pd(111) is plotted in Figure 1. The calculated adsorption energy is about -0.33 eV with vdW-DF, while the PBE result is much smaller (-0.05 eV). The experimental value is reported to be -0.32 eV [2]. In agreement with experiments, both PBE and vdW-DF functionals prefer the on-top adsorption site. Our results suggest that the vdW-DF functional can be used to describe accurately weakly bound systems within a density functional theory formalism. [1] M. Dion, H. Rydberg, E. Schröder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Lett., 92, 246401 (2004). [2] J.F. Zhu, H. Ellmer, H. Malissa, T. Brandstetter, D. Semrad, and P. Zeppenfeld, Phys. Rev. B, 68, 045406 (2003). Figure 1. The adsorption energy of Xe on metal surface Pd(111) calculated using the PBE and vdW-DF functionals.