Exfoliated MoS2 in Water without Additives.

Viviane Forsberg,Britta Andres,Joakim Bäckström,Håkan Olin,Magnus Hummelgård,Magnus Norgren,Mattias Andersson,Renyun Zhang,Christina Dahlström,Vipul Bansal

doi:10.1371/journal.pone.0154522

Abstract

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.

Highlights

Transition metal dichalcogenides (TMDs) in their bulk form have been known and studied for decades [1,2,3,4,5] 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 second step was the liquid exfoliation, in which we used the processed powder from the first step to produce the dispersions, which were liquid exfoliated by sonication
The nanosheets were successfully exfoliated according to the accessed force microscopy (AFM) and transmission electron microscopy (TEM) imaging

Summary

Introduction

Transition metal dichalcogenides (TMDs) in their bulk form have been known and studied for decades [1,2,3,4,5] but research around these materials has faced a revival during the past few years partly due to the advances in graphene research [6, 7]. 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. Several novel properties appear when thinning down to a single or few nanosheets of the PLOS ONE | DOI:10.1371/journal.pone.0154522 April 27, 2016

Methods

Results

Conclusion