Theoretical study of acetylene adsorption on armchair nanotubes

We have performed first principles total energy calculations to investigate the adsorption of an acetylene molecule inside and outside carbon nanotubes (CNT). We have calculated the binding energies and the electronic properties of a single acetylene molecule interacting with (n,n) CNTs (n = 4–7). The exchange-correlation potential energies were treated in the generalized gradient approximation (GGA) and the local density approximation (LDA). In all cases there is an energy barrier to axially introduce the C2H2 molecule into the CNTs. For the (6,6) and (7,7) tubes the small energy barriers suggest that the acetylene molecule could be introduced spontaneously at room temperature. The local density of states of the nanotube plus C2H2, together with charge density plots show that the influence of C2H2 is weak and the original properties of the nanotubes are only slightly changed. For (4,4) the energy barriers are very large, indicating a high energetic cost to introduce the molecule. For (5,5), LDA and GGA give conflicting results: LDA finds that it is favorable for the acetylene molecule to be inside the nanotube, while GGA predicts the opposite. However, in the first case, the molecule has little mobility in the radial direction, and it is restricted to positions very close to the axis of the tube. For the (6,6) and (7,7) nanotubes, we find that the local density of states of the CNT plus C2H2 are very similar to the superposition of the isolated NTs and acetylene densities of states. These results, together with charge density plots show that the influence of C2H2 is weak and the original properties of the nanotubes are only slightly changed.