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

Abstract

In order to fulfil their potential for applications, it will be necessary to develop large-scale production methods for two-dimensional (2D) inorganic nanosheets. Here we demonstrate the large-scale shear-exfoliation of molybdenum disulphide nanosheets in aqueous surfactant solution using a kitchen blender. Using standard procedures, we measure how the MoS2 concentration and production rate scale with processing parameters. However, we also use recently developed methods based on optical spectroscopy to simultaneously measure both nanosheet lateral size and thickness, allowing us to also study the dependence of nanosheet dimensions on processing parameters. We found the nanosheet concentration and production rates to depend sensitively on the mixing parameters (the MoS2 concentration, Ci; the mixing time, t; the liquid volume, V; and the rotor speed, N). By optimising mixing parameters, we achieved concentrations and production rates as high as 0.4 mg/ml and 1.3 mg/min respectively. Conversely, the nanosheet size and thickness were largely invariant with these parameters. The nanosheet concentration is also extremely sensitive to the surfactant concentration. However, more interestingly the nanosheet lateral size and thickness also varied strongly with the surfactant concentration. This allows the mean nanosheet dimensions to be controlled during shear exfoliation at least in the range ~40-220 nm for length and ~2-12 layers for thickness. We demonstrate the importance of this by showing that the MoS2 nanosheets prepared using different surfactant concentrations, and so displaying different nanosheets sizes, perform differently when used as hydrogen evolution catalysts. We find the nanosheets produced using high surfactant concentrations, which gives smaller flake sizes, perform significantly better, consistent with catalysis occurring at nanosheet edges. Finally, we also demonstrate that shear exfoliation using a kitchen blender is not limited to MoS2 but can also be achieved for boron nitride and tungsten disulphide. ToC fig3

Photoluminescence from Liquid‐Exfoliated WS2Monomers in Poly(Vinyl Alcohol) Polymer Composites

“…In liquid phase exfoliation, the main parameters are initial concentration of WS2 (Ci), processing/sonication time, t, power, P, and volume, V, as well as stabilizer concentration, CPVA, and centrifugation conditions. 27,59 As dispersed concentrations tend to scale monotonically with initial concentration and inputted energy density ( / P t V  ), 27,60 these parameters are important, but not very interesting. More interesting are the stabilizer concentration and centrifugation conditions as both these parameters tend to control not only the dispersed concentration but also the nanosheet size and so presumably the monolayer content.…”

“…[19][20][21][22][23] For composite applications, because large quantities of 2D nanosheets are usually required, probably the most suitable nanosheet production method is liquid phase exfoliation (LPE). In this method, layered crystals, usually in powdered form, are exfoliated by ultrasonication 24,25 or shear mixing, 26,27 usually in appropriate solvents or surfactant solutions.…”

“…A significant advantage of this approach is its versatility; it has been used to exfoliate graphene, 24,[27][28][29][30][31] BN, 15 a range of TMDs 25,26,32,33 and TMOs 34,35 as well as black phosphorus [36][37][38][39][40] and a host of other 2D materials. 41,42 Another advantage is the inherent processability of nanosheet dispersions.…”

Exciton formation assisted by longitudinal optical phonons in monolayer transition metal dichalcogenides

“…The confinement of correlated charge carriers or excitons to a narrow region of space in low dimensional transition metal dichalcogenides (TMDCs) leads to unique photoluminescence properties [13][14][15][16][17][18][19][20][21][22] that are otherwise absent in the bulk configurations [23]. The availability of state-of-the art exfoliation techniques [24][25][26][27] enable fabrication of low dimensional transition metal dichalcogenides that is useful for applications [13,15,[28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43]. The excitonic processes that determine the performance of TMDC-based electronic devices include defect assisted scattering and trapping by surface states [44], decay via exciton-exciton annihilation [45][46][47], phonon assisted relaxation [48], and capture by mid-gap defects through Auger processes [49].…”

Graphene reflux: improving the yield of liquid-exfoliated nanosheets through repeated separation techniques

“…[13][14][15][16] A typical lab-scale exfoliation procedure is as follows: the parent material is added to a solvent (often containing a dispersant) and exfoliation is carried out by an external energy source such as sonication, shear mixing, or high pressure homogenization. [17][18][19][20] Liquid-phase exfoliation typically produces a mixture of nanosheets with varying thickness, as well as unexfoliated parent material. The number of layers in nanosheets (directly associated with thickness) dictate the quality of the dispersion, so a secondary step is required to separate nanosheets based on their thickness.…”