Date Published: April 27, 2016
Publisher: Public Library of Science
Author(s): Viviane Forsberg, Renyun Zhang, Joakim Bäckström, Christina Dahlström, Britta Andres, Magnus Norgren, Mattias Andersson, Magnus Hummelgård, Håkan Olin, Vipul Bansal.
Many solution processing methods of exfoliation of layered materials have been studied during the last few years; most of them are based on organic solvents or rely on surfactants and other funtionalization agents. Pure water should be an ideal solvent, however, it is generally believed, based on solubility theories that stable dispersions of water could not be achieved and systematic studies are lacking. Here we describe the use of water as a solvent and the stabilization process involved therein. We introduce an exfoliation method of molybdenum disulfide (MoS2) in pure water at high concentration (i.e., 0.14 ± 0.01 g L−1). This was achieved by thinning the bulk MoS2 by mechanical exfoliation between sand papers and dispersing it by liquid exfoliation through probe sonication in water. We observed thin MoS2 nanosheets in water characterized by TEM, AFM and SEM images. The dimensions of the nanosheets were around 200 nm, the same range obtained in organic solvents. Electrophoretic mobility measurements indicated that electrical charges may be responsible for the stabilization of the dispersions. A probability decay equation was proposed to compare the stability of these dispersions with the ones reported in the literature. Water can be used as a solvent to disperse nanosheets and although the stability of the dispersions may not be as high as in organic solvents, the present method could be employed for a number of applications where the dispersions can be produced on site and organic solvents are not desirable.
Transition metal dichalcogenides (TMDs) in their bulk form have been known and studied for decades [1–5] but research around these materials has faced a revival during the past few years partly due to the advances in graphene research [6, 7]. The absence of a band gap in pristine graphene, which is also a layered material, further motivated studies of semiconductor materials such as molybdenum disulfide (MoS2). Metal atoms are sandwiched between two layers of chalcogen atoms in layered materials such as the TMD MoS2 where, for instance, strong covalent forces hold the individual atoms within each layer together and the layers are kept together by weaker van der Waals forces [7, 9]. During exfoliation these weaker forces are overcome, resulting in very thin films and possibly single nanosheets of the layered material. MoS2 is one of the most studied TMD and has mainly been employed in catalysis [4, 10] and as a lubricant . Several novel properties appear when thinning down to a single or few nanosheets of the material . The band gap shift from the near infrared to the visible range, for example, makes these materials especially interesting for optoelectronics  such as photovoltaic applications . Bulk MoS2 has an indirect band gap of 1.2 eV  and by exfoliating the material the number of layers is reduced and the band gap can increase up to 1.9 eV  for single nanosheets, where for instance, a transition to a direct band gap semiconductor was observed .
The MoS2 (Molybdenum IV sulfide) powder (99% < 2 μm) was purchased from Aldrich and used as supplied. The exfoliation process consisted of two steps. The first one, mechanical exfoliation, we did using an orbital sander to exfoliate the bulk MoS2 powder for 2 min between sand papers. The second step was the liquid exfoliation, in which we used the processed powder from the first step to produce the dispersions, which were then liquid exfoliated by sonication. In this study we introduced a method to disperse MoS2 in water involving mechanical and liquid exfoliation. The nanosheets were successfully exfoliated according to the AFM and TEM imaging. The dimensions that we obtained were in the same range of the liquid processed MoS2 dispersions reported by other authors [22, 31], and the concentration achieved by this method was half of the reported concentration achieved by liquid exfoliation in organic solvents at the same liquid exfoliation conditions . Although the half time t1/2 of the dispersions in water were much lower than the ones prepared according to other methods  (see Table 1), we accounted for the advantages of using an environmental friendly solvent and no additional chemicals for the dispersion stability. The present method can be employed for large scale production of nanosheets at lower costs and in more environmental friendly conditions as if organic solvents are to be employed in conditions that may overcome the drawback of the lower stability of the dispersions in water only without additives. Nevertheless, we observed that even 4 months after the sample preparation, there were still particles dispersed on a sample prepared by the presented method. Source: http://doi.org/10.1371/journal.pone.0154522