Adsorption of M Species and M2 Dimers (M = Cu, Ag, and Au) on the Pristine and Defective Single-Walled Carbon Nanotubes: A Density Functional Theory Study

We studied the adsorption of neutral M, charged M, and M-2 dimers (M = Cu, Ag, Au) on the pristine singlewalled carbon nanotubes (SWNTs) as well as on the Stone-Wales and vacancy sites by means of the B3LYP/6-310 (d,p) hybrid density functional method. Our results for neutral metal atoms on the pristine and defective SWNTs agree very well with previous periodic calculations. The binding affinity trend of metal species toward the pristine and defective SWNTs is in the order of M+ > M- > M. This implies that the transfer of electron density between metal species and the nanotubes, the electrostatic attraction, and the Pauli repulsion play an important role in the M-SWNT system. From the Mulliken population analysis, the transfer of electron density induces a positive charge of the metal species and a negative charge of the carbon atoms of nanotubes. As far as the adsorption energy is concerned, the metal species are likely to deposit on the defect site, particularly on the vacancy site, rather than on the pristine tube. To explore the reactivity of the M-SWNT complexes which can serve as a gas sensor as well as a catalyst, the interaction between a CO molecule and a metal atom deposited on the atomic vacancy was also examined. We also found that the adsorption energy per atom decreases from the metal atom to metal dimers in line with the fact that metal-metal cohesion dominates over metal-SWNT interaction. Finally, based on calculated interaction energies, dimerization of adsorbed atoms on the defect sites is not particularly favored compared to dimerization on the pristine tube except for that of Ag and Au atoms on the vacancy site.