First-principles study of Ru atoms and clusters adsorbed outside and inside carbon nanotubes.

First-principles calculations were performed to investigate the binding energies, geometric structures, and electronic properties of 4d transition metal (TM) (particularly, Ru), atoms, and clusters adsorbed outside/inside the single-walled or double-walled carbon nanotubes. The equilibrium adsorption structures of the TM atoms depend on the valence electron configuration of the metal atoms. Due to curvature effect, all TM atoms adsorbed inside and outside (6, 6) carbon nanotubes donate different amounts of electrons to the nanotube, with a nearly constant difference of about 0.5 electrons/TM atom. The analysis of electronic density of states revealed hybridization between the p electrons from C and the d electrons from Ru, which results in charge transfer from metal to carbon. The amount of charge transfer shows systematical trend with the electronegativity of 4d TMs. When Ru atom or cluster adsorbs on double-walled nanotubes, the effect of charge transfer is slightly enhanced with regard to the single-walled nanotubes.