Density functional study of molecular hydrogen coverage on carbon nanotubes

Density functional calculations of the adsorption of molecular hydrogen on the external surface of (5, 5), (6, 4), (8, 1) and (16, 2) carbon nanotubes have been carried out. Binding energies of single molecules have been studied as a function of orientation of the molecules and type of nanotube. We have found weak adsorption, with binding energies near 100 meV/molecule in the most stable configurations. The binding energies on metallic and semiconducting nanotubes are similar. When the nanotube surface is fully covered with one molecule per graphitic hexagon, the binding energy per molecule decreases for some nanotubes due to repulsive interactions between neighbor molecules. For the same reason, direct adsorption of a single hydrogen layer with a coverage of more than one molecule per graphitic hexagon is not possible, even at low temperatures. However, adsorption of two layers (14.3 wt% hydrogen adsorbed when all the surface is covered) leads to binding energies between 40 and 80 meV/molecule, although the molecules of the outer layer are more weakly bound compared to those of the inner one. All the small calculated binding energies indicate that substantial adsorption is only possible at very low temperatures.