Letter pubs.acs.org/JPCL
Liquid-Phase Exfoliation of MoS2 Nanosheets: The Critical Role of Trace Water Amit Gupta, Vaishali Arunachalam, and Sukumaran Vasudevan* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
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S Supporting Information *
ABSTRACT: Sonication-assisted liquid-phase exfoliation of layered materials in suitable organic solvents offers a simple scalable route for the production of 2D nanomaterials. N-methyl-2-pyrrolidone (NMP) is one of the most efficient solvents for liquid-phase exfoliation of a variety of layered solids, including MoS2. We show here that trace water present in NMP is crucial for the stability of MoS2 nanosheets in NMP dispersions. In the absence of water, the sheets are fragmented and chemically unstable. Using solution NMR techniques, 2D nuclear Overhauser effect and spin−lattice relaxation measurements, supported by classical molecular dynamics simulations, we are able to establish the role of water molecules in stabilizing the dispersion. We show that water molecules are localized at the Mo-terminated edges of the MoS2 sheets, thereby inhibiting chemical erosion of the sheets, and they also exhibit enhanced interactions with the solvent NMP molecules, leading to the stability of the dispersion.
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models is to lower the difference in surface energies between the solvent and the layered material.11−13 It has been suggested that the Hansen solubility parameter, which is the square root of dispersive, polar, and H-bonding components of the cohesive energy of a material, is an appropriate parameter for identifying solvents for efficient liquid-phase exfoliation of layered materials, the best solvents being those where the parameters of the solvent and nanomaterial match. A more recent refinement has suggested that both the polar and dispersive components of the surface energies of the solvent and solid have to be individually matched for efficient delamination and to obtain stable dispersions.14 On the basis of these parameters, a large number of solvents and solvent combinations have been screened, and solvents that are efficient for liquid-phase exfoliation of different layered materials have been identified.11,14 It has been widely recognized that N-methyl-2-pyrrolidone (NMP) is one of the most efficient solvents for the liquid-phase exfoliation of a variety of layered solids, including MoS2 and other transition metal dichalcogenides.5,11,15 This has been attributed to the fact that the surface tension of NMP at ∼40 mJm−2 matches closely with the surface energy of many layered materials.11 Sonication-assisted exfoliation of MoS2 in NMP, for example, is known to provide stable dispersions with concentrations up to 40 mg/mL of the dispersed nanosheets.16 We discovered, quite by accident, that when dry NMP, free of water, was used for the exfoliation of MoS2, the concentrations of the dispersions were small and sonication resulted in MoS2 nanoparticles rather than nanosheets. However, when trace
xfoliation of layered materials such as graphite and transition metal dichalcogenides into mono- or few-layers is of significant interest for both fundamental studies and potential applications.1−4 Two-dimensional layered materials are characterized by their strong in-plane bonding and weak van der Waals coupling between the layers. Exfoliation can be achieved mechanically to yield 2D nanosheets of high quality but is generally limited by its lack of scalability.5 Sonicationassisted liquid-phase exfoliation of layered compounds in appropriate solvents or aqueous surfactant solutions is one of the most promising and simplest routes for the production of 2D materials on a large scale. Sonication results in the exfoliation of the layered crystal into single- and multilayer nanosheets that are then stabilized by interaction with the solvent or the surfactant present in the dispersion.4−6 The method has the advantage of extreme simplicity and scalability and is the method of choice for the production of nanosheets for fabrication as films or composites.7,8 Liquid-phase exfoliation has the advantage over lithium intercalation− exfoliation methods in that it is not an air-sensitive process nor does it involve chemical reactions and consequently provides 2D nanosheets with high crystallinity. Sonication-assisted exfoliation of layered solids is now widely accepted as the technique that can provide dispersions of single- or few-layered nanosheets in large concentrations. Sonication results in shear forces, arising from the collapse of cavitation bubbles, that can peel off the layers.9,10 The choice of solvent is critical; it must facilitate the delamination process and be able to sustain stable dispersions with high concentrations of the exfoliated 2D inorganic nanosheets. Various phenomenological models have been advanced to rationalize, and predict, the right choice of solvent, (or the combination of solvents) that can achieve these objectives.5 The central theme of most © 2016 American Chemical Society
Received: October 17, 2016 Accepted: November 14, 2016 Published: November 14, 2016 4884
DOI: 10.1021/acs.jpclett.6b02405 J. Phys. Chem. Lett. 2016, 7, 4884−4890
Letter
The Journal of Physical Chemistry Letters
Figure 1. (a) Digital image of MoS2 dispersions in dry NMP. (b) Transmission electron micrograph (TEM) of the MoS2−NMP dispersion. The two types of particles are indicated. (c) HRTEM of the darker particles (the electron diffraction pattern is shown in the inset) and (d) the corresponding EDS spectra. (e) TEM of the lighter particles (the electron diffraction pattern is shown in the inset) and (f) corresponding EDS spectra.
Figure 2. (a) Digital image of MoS2 dispersions in NMP containing 0.1 mole fraction of water. (b) Scanning electron micrograph of the sheets obtained from the MoS2−NMP (H2O 0.1 mole fraction) dispersion (the electron diffraction pattern is shown in the inset). (c) HRTEM image of a single sheet and (d) corresponding EDS spectra.
used in the preparation of the dispersions, were recorded (Supporting Information, Figure S2). No additional resonances, other than those that can be assigned to NMP, were observed in the NMR spectra, but the GC-MS data showed trace amounts of N-methylsuccinimide; this impurity, however, was present in the solvent prior to sonication. The dispersions were centrifuged at 4000 rpm (RCF 1475G), and only the clear fractions were used for subsequent studies. An important consideration in the present experiments was that care was taken at all stages, sonication as well as centrifugation, to ensure that there was no exposure of the dispersions to the atmosphere (details of the preparative methods and procedures for estimating MoS2 are described in the Supporting Information, section S1). The effect of trace water on the dispersions is visually quite striking. In dry NMP, the dispersions are pale, but when trace water is present, the dispersions are quite dark, indicating a much higher concentration of exfoliated MoS2 nanosheets in the dispersion (Figures 1a and 2a; see also Supporting Information Figure S1). Electron microscopy images show that there are more fundamental differences in the nature of the dispersions. Transmission electron microscope (TEM) images were recorded for dispersions drop-coated on carbon-Formvarcoated grids and subsequently dried at 50 °C under vacuum. The images for dispersions obtained in dry NMP showed polydisperse near-circular particles, with diameters typically ranging from 5 to 50 nm (Figure 1b). Closer examination revealed that there are two types of particles. The darker
amounts of water were present in the NMP, there was an increase in the concentration of the dispersions with increased lateral dimensions of the MoS2 nanosheets. (While this work was in progress, a similar observation on the importance of the presence of trace water in NMP for obtaining dispersions of MoS2 nanosheets at large concentration was reported.17) Here we use electron microscopy and two-dimensional NMR techniques from the solution chemist’s toolbox18−20 along with classical molecular dynamics (MD) simulations of the dispersions to investigate the critical role of water molecules in providing stable dispersions of MoS2 nanosheets in NMP. The three techniqueselectron microscopy, NMR spectroscopy, and MD simulationsexplore different facets of the same phenomena, and the molecular picture that emerges is more complex than what phenomenological models suggest. We show here that water molecules bind to the Mo-exposed edges of the MoS2 nanosheets, preventing chemical erosion of the edges that would otherwise have resulted in clusters rather than sheets. The localized water molecules in turn interact with the solvent NMP molecules present at the periphery of the nanosheets. MoS2 dispersions were prepared by sonicating crystalline MoS2 powders (Sigma-Aldrich,
