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Exfoliation and dispersion of 2D materials in polar solvents: A molecular simulation approach

Authors : Titas Kumar Mukhopadhyay and Ayan Datta*


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Abstract

Among the various novel synthetic routes for 2D materials, liquid phase exfoliation (LPE) has emerged to be most effective. Nevertheless, the key material properties guiding the hierarchical steps for LPE are still unknown. Herein, through all atom molecular dynamics simulations and free energy calculations we compare the exfoliation and re-aggregation tendencies of five common 2D materials namely graphene, blue and black phosphorene, hexagonal boron nitride and molybdenum disulfide, in polar solvents water and DMSO. It is revealed that, larger molecules such as DMSO efficiently intercalates between material layers during sonication and interact predominantly through van der Waals interactions, thereby stabilizing the dispersed state and increasing the exfoliation efficacy. Small molecules e.g. water cannot stabilize the individual 2D layers through

solvation and therefore, the intercalated water molecules are easily ejected from in between the nanosheets, eventually leading to re-combination. Solvation free energy calculations show that solvation of a single nanosheet is thermodynamically favorable in both solvents although the nature of balance between the enthalpic and entropic parts is fundamentally different for water and DMSO. From the calculation of exfoliation free energies it is shown that the ease of exfoliation follows the trend: MoS2 > Blue Phosphorene > Black Phosphorene > Hexagonal Boron Nitride > Graphene.

Keyword

2D Materials, liquid phase exfoliation, molecular dynamics, potential of mean force, solvation free energy.