Electronic Structure of Carbon Nanowalls andHydrogen Adsorption

Abstract

The structure of carbon nanowalls (CNWs) was investigated by transmission electron microscopy (TEM), which indicated clearly the existence of the bent graphene structure in boundary regions between the crystallites of CNWs. The electronic structure of CNWs was measured by ultraviolet photoelectron spectroscopy (UPS) in comparison with highly oriented pyrolitic graphite (HOPG). The interlayer band of CNWs was observed at the same energy as that in HOPG, while the σ − π hybridized bands were observed at the same binding energy as in carbon nanotubes. Combination of UPS and temperature programmed desorption (TPD) measurements investigated deuterium adsorption which preferably occurred in the boundary regions and enhanced the density of states associated with the electronic states of the boundary regions. The first-principles calculation was employed for the bent coronene molecule as a model of the boundary region between the crystallites of CNWs to elucidate the mechanism of the hydrogen adsorption to CNWs. The hydrogen adsorption energies onto both on-top and hollow sites become greater as the bending angle increases, since the electronic structure of the carbon atom at the adsorption site changes from sp² to sp³ hybridization character, clarified by natural bond orbital analysis.