Production of aqueous dispersions of inorganic graphene analogues by exfoliation and stabilization with non-ionic surfactants

The production of stable aqueous suspensions of several inorganic graphene analogues [MoS2, WS2 and hexagonal BN (h-BN)] by exfoliation of the corresponding bulk layered materials via sonication has been investigated, with a particular focus on the use and efficacy of non-ionic surfactants as dispersing agents. For the two metal dichalcogenides, some non-ionic surfactants afforded highly concentrated dispersions (up to several milligrams per milliliter), outperforming dispersions produced with an ionic surfactant or in water-alcohol mixtures in the absence of surfactant, which were taken as reference systems. Furthermore, suspensions with metal dichalcogenide to surfactant concentration ratios as high as 2.4-3.5 could be attained through appropriate processing of the as-prepared suspensions, which should be advantageous for the preparation of materials and devices with minimal interference from the surfactant. In the case of h-BN, all surfactants failed to yield suspensions with concentration significantly above that achieved in water alone, which was attributed to the chemical peculiarities of h-BN platelets exfoliated in water via sonication. The suspensions produced with the most successful non-ionic surfactants exhibited long-term stability (months) and were made up of platelets with lateral dimensions from 50 up to a few hundred nanometers and thicknesses of a few to several nanometers. Raman spectroscopy analysis suggested that edge effects dominate the detailed spectral features for the MoS2 and WS2 platelets, in particular the position of their Raman bands. Such results indicate that extreme caution must be exercised when using this technique to gauge the thickness of small-sized MoS2/WS2 platelets, such as those typically produced by liquid-phase exfoliation approaches. Overall, the present results should facilitate the manipulation and use of these two-dimensional materials in several prospective application areas, such as biomedicine or photocatalysis.