Flexibility in the Graphene Sheet: The Influence on Gas Adsorption from Molecular Dynamics Studies

Despite being considered completely rigid in most studies, graphene is really flexible leading to out-of-plane movements. In this work, the influence of such flexibility on the adsorption of methane and nitrogen on graphene is studied using molecular dynamics. Indeed, we have used intramolecular force fields for graphene with in plane and out-of-plane components that allow for describing the movements and deformations of the graphene sheets and providing a more realistic description of the adsorbent. In addition, intermolecular force fields validated at the CCSD(T) level are used. We show that considering the movement of graphene in the adsorption study significantly improves the performance, especially in combination with molecular models that average interactions, such as pseudo-atoms. In these cases, when the interactions are directional, as in methane, the inclusion of flexibility makes the obtained results closer to those of the much more precise atomistic results, making its use highly recommended. Uptakes, adsorption isotherms, and z-density profiles prove that graphene is a very promising candidate for adsorption of methane, while for nitrogen, high pressures are required to obtain high yields.