Aggregation Behavior of 2D Nanomaterials beyond Graphen

We report, for the first time, the aggregation behavior of the three most popular emerging 2D nanomaterials (NMs) beyond graphene viz., MoS2, WS2, and h-BN in aquatic media. These materials have been exfoliated in aqueous suspensions using ethanol/water mixture and studied under varying concentrations of both monovalent (NaCl) and divalent (CaCl2) electrolytes that are representative of natural aquatic environment. In addition, Suwanee River Natural Organic Matter (SRNOM) has also been used to analyze the effect of natural macromolecules on 2D NM aggregation. An increase in electrolyte concentration resulted in compression of the electric double-layer of the negatively charged 2D NMs, thus displaying classical Derjaguin-Landau-Verwey-Overbeek (DLVO) type interaction. The critical coagulation concentration (CCC) has been estimated as 37 mM, 60 mM, and 19 mM NaCl and 3 mM, 7.2 mM, and 1.3 mM CaCl2 for MoS2, WS2, and h-BN respectively. Theoretical predictions of CCC by DLVO theory have been found comparable to the experimental values when the theory is modified to take the dimensionality of the materials into account. The electrostatic repulsion force has been attributed as the major DLVO force governing the colloidal stability of MoS 2and WS2 while the van der Waals attraction has been found to govern colloidal stability of h-BN. The presence of SRNOM stabilizes the colloids significantly possibly by electrosteric repulsion. The results of this study can be used to understand the fate and transport of 2D NMs in the environment as well as their suitability in various applications.