Preparation of High Concentration Dispersions of Exfoliated MoS₂ with Increased Flake Size

Abstract

Solvent exfoliation of inorganic layered compounds is likely to be important for a range of applications. However, this method generally gives dispersions of small nanosheets at low concentrations. Here we describe methods, based on sonication of powdered MoS 2 in the solvent N-methyl-pyrrolidone, to prepare dispersions with significantly increased lateral nanosheet size and dispersed concentration. We find the concentration to scale linearly with starting MoS 2 mass allowing the definition of a yield. This yield can be increased to ~40% by controlling the sonication time, resulting in concentrations as high as 40 mg/ml. We find the nanosheet size to increase initially with sonication time reaching ~700 nm (for a concentration of ~7.5 mg/ml). At longer sonication times the nanosheets size falls off due to sonication induced scission. The nanosheets produced by such methods are relatively thin and have no observable defects. We can separate the dispersed nanosheets by size using controlled centrifugation. This allows us to produce dispersions with mean flake size of up to ~2m. However, such large flakes are noticeably thicker than the standard nanosheets. We demonstrate that such nanosheets can be mixed with polymers to form composites. While standard nanosheets result in no improvement in composite mechanical properties, addition of size-selected nanosheets results in significant improvements in composite modulus and strength.

Large-Scale Production of Size-Controlled MoS2 Nanosheets by Shear Exfoliation

“…These images clearly show planar objects with length of ~100 nm and width roughly half that, and are consistent with what is usually observed for exfoliated layered compounds. 6,22,39 To further analyse these objects, a few drops of supernatant were deposited onto Si/SiO2 and analysed by atomic force microscopy (AFM). Shown in figure 1D are two representative AFM images of the deposited objects.…”

“…This results in the production of nanosheets which are stabilised against aggregation via the interaction with the liquid. 12,15,17,18 This method has been used to successfully produce dispersions of nanosheets of a range of materials, including graphene, 8,19 BN,6,20 and various transition metal dichalcogenides 6,10,21,22 such as MoS2 and WSe2. Using sonication-assisted exfoliation, dispersions of volumes typically in the range of hundreds of millilitres can be produced.…”

Tandem intercalation strategy for single-layer nanosheets as an effective alternative to conventional exfoliation processes

“…Single-layer or few-layer nanosheets can be obtained from bulk TMCs by several exfoliation techniques such as dry mechanical cleavage with Scotch tape 10,11 or solution-based exfoliation processes [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] . Intercalation of alkali metals by using elemental metals 12,13 (for example, potassium) or organo-alkali compounds (for example, butyllithium [14][15][16] and more recently sodium naphthalenide 17 ) has been a widely used method for the exfoliation of layered materials.…”

“…While the electrochemical exfoliation method has been developed as a faster and controllable exfoliation protocol, complicated electrochemical set-ups present challenges for the mass production of single-layer nanosheets [18][19][20][21] . Alternatively, a sonication method in the presence of solvents 22,23 such as N-methyl-pyrrolidone has been demonstrated for the successful exfoliation of wide range of TMCs, but the products are often mixed with single-and multi-layer TMCs with potential degradation of samples due to the harsh sonication conditions required to overcome the interlayer interactions [24][25][26] . As stated here, current methodologies have their own advantages and disadvantages, but none of them are yet perfect 17,26 , although it is true that these are effective for large-size layered nanostructures.…”

“…Alternatively, a sonication method in the presence of solvents 22,23 such as N-methyl-pyrrolidone has been demonstrated for the successful exfoliation of wide range of TMCs, but the products are often mixed with single-and multi-layer TMCs with potential degradation of samples due to the harsh sonication conditions required to overcome the interlayer interactions [24][25][26] . As stated here, current methodologies have their own advantages and disadvantages, but none of them are yet perfect 17,26 , although it is true that these are effective for large-size layered nanostructures. The development of mild exfoliation strategies that can circumvent the key harsh process conditions (for example, sonication and H 2 gas formation) has been challenging and, therefore, new and better routes to obtain single-layer TMCs in a simple, efficient and reproducible manner are currently being pursued.…”