Tunable Energy Barrier for Intercalation of a Carbon Nanotube into Graphene Nanosheets: A Molecular Dynamics Study of a Hybrid Self-Assembly

Molecular dynamics (MD) simulations were utilized to explore the energetics of formation of a graphene–carbon nanotube (CNT) hybrid in an aqueous environment, resulting from the intercalation of a single-walled CNT into a gallery defined by two parallel graphene sheets. It was found that the formation of a graphene–CNT hybrid can be divided into three processes involving (a) exfoliation of graphene sheets by repulsive interactions, (b) intercalation of a CNT into the graphene gallery associated with an activation energy barrier, and (c) spontaneous self-assembly/association of constituent CNT and graphene sheets driven by hydrophobic interactions or electrostatic attraction, leading to the formation of a three-dimensional hybrid. In contrast with pristine graphene sheets, ionic functionalization makes graphene sheets more hydrophilic and enhances their exfoliation in water, resulting in a significant lowering of the CNT intercalation barrier by nearly 150 kcal/mol. The simulations predict that the lowest intercalation barriers would arise for cases where both the energetic cost for graphene exfoliation and steric repulsions between the incoming CNT and graphene sheets are the lowest. Once the CNT moves past the barrier, its further incorporation into the graphene gallery is spontaneous and is assisted by strong hydrophobic interactions between the CNT and graphene surfaces. The most stable hybrid complex was observed when the CNT and graphenes are functionalized with oppositely charged ionic groups.