Ab initio study of hydrogen adsorption to single-walled carbon nanotubes

We perform the density-functional and quantum chemical calculations of adsorption of a hydrogen molecule to various kinds of single-walled carbon nanotubes (SWNT’s). The potential energy barrier height (PBH) for the dissociative adsorption of a hydrogen molecule onto the outer wall of a nanotube decreases as the tube diameter decreases. In contrast, the PBH for ${\mathrm{H}}_2$ penetration into an open-ended nanotube increases as the tube diameter decreases, independent of the atomic geometry, i.e., zigzag or armchair structures. ${\mathrm{H}}_2,$ however, cannot adsorb to the inner wall of any type of nanotube. These results on the structure-dependent ${\mathrm{H}}_2$ adsorption to SWNT’s are reasonably explained by the ${\mathrm{sp}}^2-{\mathrm{sp}}^3$ rehybridization caused by the interaction between the adsorbing hydrogen and carbon atoms of SWNT’s.