Solvent Exfoliation of Transition Metal Dichalcogenides: Dispersibility of Exfoliated Nanosheets Varies Only Weakly between Compounds

We have studied the dispersion and exfoliation of four inorganic layered compounds, WS2, MoS2, MoSe2, and MoTe2, in a range of organic solvents. The aim was to explore the relationship between the chemical structure of the exfoliated nanosheets and their dispersibility. Sonication of the layered compounds in solvents generally gave few-layer nanosheets with lateral dimensions of a few hundred nanometers. However, the dispersed concentration varied greatly from solvent to solvent. For all four materials, the concentration peaked for solvents with surface energy close to 70 mJ/m(2), implying that all four have surface energy dose to this value. Inverse gas chromatography measurements showed MoS2 and MoSe2 to have surface energies of similar to 75 mJ/m(2), in good agreement with dispersibility measurements. However, this method suggested MoTe2 to have a considerably larger surface energy (similar to 120 mJ/m(2)). While surface-energy-based solubility parameters are perhaps more intuitive for two-dimensional materials, Hansen solubility parameters are probably more useful. Our analysis shows the dispersed concentration of all four layered materials to show well-defined peaks when plotted as a function of Hansen's dispersive, polar, and H-bonding solubility parameters. This suggests that we can associate Hansen solubility parameters of delta(D) similar to 18 MPa1/2, delta(P) similar to 8.5 MPa1/2, and delta(H) similar to 7 MPa1/2 with all four types of layered material. Knowledge of these properties allows the estimation of the Flory-Huggins parameter, chi, for each combination of nanosheet and solvent. We found that the dispersed concentration of each material falls exponentially with chi as predicted by solution thermodynamics. This work shows that solution thermodynamics and specifically solubility parameter analysis can be used as a framework to understand the dispersion of two-dimensional materials. Finally, we note that in good solvents, such as cyclohexylpyrrolidone, the dispersions are temporally stable with >90% of material remaining dispersed after 100 h.