Stabilization of 2D graphene, functionalized graphene, and Ti2CO2 (MXene) in super-critical CO2: a molecular dynamics study

Abstract

The stabilization of nanoparticles is a main concern to produce an efficient nanofluid. Here, we carry out Molecular Dynamics (MD) modeling to evaluate the stabilization of 2D graphene, oxygen- and hydrogen-functionalized graphene, and Ti₂CO₂ MXene nano-sheets in carbon dioxide (CO₂) liquid under supercritical conditions. The rheological properties (i.e., viscosity and self-diffusion) of the nanofluids with the corresponding nano-sheets are also calculated. The results show that pristine graphene aggregates in the liquid and the adsorbed CO₂ molecules on the graphene surfaces could not provide enough repulsive forces to avoid their aggregation. The stabilization of graphene is improved by surface functionalization of graphene with oxygen and hydrogen functional groups. This is because, first, the presence of the corresponding surface groups can alter the electrostatic forces of the graphene nano-sheets and, second, a larger number of CO₂ molecules attracted by the functionalized nano-sheets can provide additional repulsive forces between the nano-sheets. The results demonstrate that Ti₂CO₂ MXene nano-sheets aggregate severely although they are covered by oxygen surface terminations and attract more CO₂ molecules than the pristine graphene. This is attributed to the strong interlayer coupling between the MXene nano-sheets so that the electrostatic repulsion of the MXene surfaces could not overcome the interlayer coupling. Based on our results, the stabilization of the corresponding 2D nano-sheets in CO₂ liquid is in the order: oxygen-functionalized graphene > hydrogen-functionalized graphene > pristine graphene > Ti₂CO₂ MXene. The calculation of the rheological properties of the nanofluids with different nano-sheets demonstrates that immersing the nano-sheets in the CO₂ liquid affords a superior viscosity enhancement as much as possible to be dispersed in the liquid. This study expands the potential applications of these 2D materials in producing SC-CO₂ based nanofluids.