Adsorption of H₂O molecules at the open ends of singlewalled carbon nanotubes

The adsorption of H₂O molecules on the open ends of carbon nanotubes (CNTs) have been investigated using the self-consistent-charge density functional tight binding (SCC-DFTB) method. An H₂O molecule dissociates into H and OH fragments at the armchair nanotube edge with an adsorption energy of -3.52 eV and relatively small activation barrier height of less than 0.35 eV. The Fermi level shifts upward by 0.14 eV, and the density of states near the Fermi level does not change appreciably. In the case of the adsorption on the zigzag nanotube edge, an H₂O molecule dissociates into H and OH fragments with an activation barrier height of 0.12 eV. They adsorb onto two nearby carbon atoms at the open end. The adsorption energy is -4.84 eV, larger in magnitude than that on the armchair edge and the distortion of the local geometry under H and OH fragments is very small. The Fermi energy shifts upward by 0.17 eV, and the density of states at the Fermi level is enhanced. After the adsorption of H₂O, the Fermi energy shifts upward in the most stable configurations over both CNTs. This will decrease the work function of the nanotubes and the turn-on voltage of the field emission current. The enhancement of the field emission current is expected to be larger for the zigzag nanotube, since the shift of the Fermi energy is larger and the density of states is enhanced near the Fermi energy.