Hydrogen adsorption on ${\mathrm{sp}}^2$-bonded carbon: Influence of the local curvature

The interaction of atomic hydrogen and low-energy hydrogen ions with ${\mathrm{sp}}^2$-bonded carbon is investigated on the surfaces of ${\mathrm{C}}_{60}$ multilayer films, single-walled carbon nanotubes, and graphite (0001). These three materials have been chosen to represent ${\mathrm{sp}}^2$-bonded carbon networks with different local curvatures and closed surfaces (i.e. no dangling bonds). Chemisorption of hydrogen on these surfaces reduces emission from photoemission features associated with the π electrons and leads to a lowering of the work function up to 1.3 eV. It is found that the energy barrier for hydrogen adsorption decreases with increasing local curvature of the carbon surface. Whereas in the case of ${\mathrm{C}}_{60}$ and single-walled carbon nanotubes, hydrogen adsorption can be achieved by exposure to atomic hydrogen, the hydrogen adsorption on graphite (0001) requires $H^{+}$ ions of low kinetic energy (∼1 eV). On all three materials, the adsorption energy barrier is found to increase with coverage. Accordingly, hydrogen chemisorption saturates at coverages that depend on the local curvature of the sample and the form of hydrogen (i.e., atomic or ionic) used for the treatment.