Liquid phase exfoliation (LPE) offers a scalable preparation towards colloids of two-dimensional (2D) Van der Waals (VdW) solids. Over the years, LPE has been rationalized within the context of solution thermodynamics. We outline this approach by introducing a method for the LPE of rhenium disulfide (ReS2), a promising novel semiconducting 2D material. By screening LPE in various solvents, we retrieve optimal conditions for high yield ReS2 exfoliation in solvents characterized by similar Hildebrand and Hansen solubility parameters – a result in agreement with the solution thermodynamics approach. Besides, our thermodynamic analysis points out that the current method to describe LPE fails to account for the considerable tolerance on the solubility parameter mismatch. In other words, the window for solvent exfoliation is much broader than solution thermodynamics allows for based on the size of the colloid. We combine our experimental dataset on ReS2 with an extensive survey of literature studies to reconcile the too-broad exfoliation window. Starting from standard Flory-Huggins theory, we address inconsistencies in the current line of thought and propose a more coherent way of working based on the critical exchange parameter χc for stable polymer solutions. In such a framework, suitable exfoliation solvents result in exchange parameters below a critical value χc. Notably, the predicted solubility parameter window for suitable exfoliation solvents based on such a criterium agrees well with the experimental data available for a wide range of 2D VdW solids. Therefore, we conclude that solution thermodynamics describes LPE, provided that the solubility parameter window is linked to the critical exchange parameter for solvent/solute immiscibility.

Thermodynamic perspective on liquid phase exfoliation of two dimensional van der Waals solids

ACS Spring Meeting 2021

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