Hydrogen chemisorption on sp 2 -bonded carbon: Influence of the local curvature and local electronic effects

We report on the interaction of hydrogen with sp2-bonded carbon which has been investigated on graphite (0001), single-walled carbon nanotubes and C60 multilayer films. These substrates have been chosen to represent a large range of curvature in the carbon network. The photoelectron spectroscopy study of the samples treated with atomic hydrogen and low-energy hydrogen ions reveals that hydrogen is chemisorbed on the basal plane of the sp2-bonded carbon networks, as evidenced by the lowered emission from π-derived states and a lowering of the electron work function of up to 1.3 eV. The hydrogen adsorption energy barrier is found to strongly depend on the local curvature of the carbon network whereby the barrier is lowered with increasing curvatures. Whereas in the case of C60 and single-walled carbon nanotubes, hydrogen chemisorption can be achieved by exposure to atomic hydrogen, the chemisorption on graphite (0001) requires hydrogen ions of low kinetic energy (∼1 eV). Furthermore, the adsorption energy barrier is found to increase with hydrogen coverage. The scanning tunnelling microscopy study of individual adsorption sites on the graphite (0001) surface reveals long-ranged (∼5 nm) electronic effects observed as a (sqrt(3)×sqrt(3))R30° superstructure in the local density of states. It is shown that this superstructure is due to the scattering of delocalized electron wavefunctions at the point defects. The resulting standing waves induce a redistribution of the local density of states which is directly related to the point-like Fermi surface of graphite.