Adsorption capacity of H_2O, NH_3, CO, and NO_2 on the pristine graphene

摘要

First-principles together with statistical mechanics calculations have been performed to study the adsorption behavior of H_2O, NH_3, CO, and NO_2 on the pristine graphene. In the first-principles calculations, we find that the most recent van der Waals (vdW) density functional vdW-DF2 gives even larger binding energies (E_b) that those obtained with the local density approximation, indicating vdW-DF2 may be inappropriate for describing the interaction between these molecules and graphene. With the potential energy curves of the molecules on graphene calculated by the density functional theory, the adsorption capacity (n) of the molecules on the pristine graphene is calculated with the statistical mechanics method. NO_2 has the largest n of the order of 10~8 cm~(-2) among the four molecules on graphene at room temperature and concentration of 1.0 ppm, but still smaller by almost two order than that on graphene devices estimated from the experimental results. This is probably due to the strong binding of NO_2 to the graphene edges with terminating oxygen atoms with E_b, as large as 1.0 eV. The calculations of the adsorption capacity of small polar molecules on the pristine graphene and comparison with the experimental values may contribute to the understanding of the mechanism and designing of graphene based gas sensors.