(420m) Understanding the Stabilization of Liquid-Phase Exfoliated Graphene In Polar Solvents: Molecular Dynamics Simulations and Kinetic Theory of Colloid Aggregation

Authors: 
Shih, C., Massachusetts Institute of Technology
Lin, S., Massachusetts Institute of Technology
Blankschtein, D., Massachusetts Institute of Technology
Strano, M., Massachusetts Institute of Technology


Understanding the solution phase dispersion of pristine, unfunctionalized graphene is important for the production of conducting inks and top-down approaches to electronics. It can also be used as higher-quality alternatives to chemical vapor deposition. In this talk, we discuss a theoretical framework which utilizes molecular dynamics simulation and the kinetic theory of colloid aggregation to inform the mechanism of stabilization of liquid-phase exfoliated graphene sheets in N-methylpyrrolidone (NMP), dimethylformamide (DMF), N,N’-dimethylsulphoxide (DMSO), γ-butyrolactone (GBL), and water (H2O).

By calculating the potential of mean force between two solvated graphene sheets using molecular dynamics (MD) simulations, we found that the dominant barrier hindering the aggregation of graphene is the last layer of confined solvent molecules between the graphene sheets, resulting from the strong affinity of the solvent molecules for graphene. The origin of the energy barrier responsible for repelling the sheets is the steric repulsions between solvent molecules and graphene before the desorption of the confined single-layer solvent layer.

We have formulated a kinetic theory of colloid aggregation to model the aggregation of graphene sheets in the liquid phase in order to predict stability using the simulated potential of mean force. With only one adjustable parameter, the average collision area, which can be estimated from experimental data, our theory can describe the experimentally observed degradation of the single-layer graphene fraction in NMP. We use these results to rank potential solvents according to their ability to disperse pristine, unfunctionalized graphene and find that NMP ≈ DMSO > DMF > GBL > H2O, consistent with the wide-spread use of the first three solvents for this purpose.