Abstract
Layered materials present attractive and important properties due to their two-dimensional (2D) structure, allowing potential applications including electronics, optoelectronics and catalysis. However, fully exploiting the outstanding properties will require a method for their efficient exfoliation. Here we present that a series of layered materials can be successfully exfoliated into single- and few-layer nanosheets using the driving forces coming from the phase inversion, i.e., from micelles to reverse micelles in the emulsion microenvironment built by supercritical carbon dioxide (SC CO2). The effect of variable experimental parameters including CO2 pressure, ethanol/water ratio and initial concentration of bulk materials on the exfoliation yield have been investigated. Moreover, we demonstrate that the exfoliated 2D nanosheets have their worthwhile applications, for example, graphene can be used to prepare conductive paper, MoS2 can be used as fluorescent label to perform cellular labelling and BN can effectively reinforce polymers leading to the promising mechanical properties.
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Introduction
Following the advent of graphene in 2004, it was soon recognized that in addition to the composition and arrangement of atoms in materials, dimensionality plays a critical role in determining their fundamental properties1,2. Over the past decade, some significant developments have been spearheaded by research into two-dimensional (2D) layered nanomaterials considering that they have potential applications in a wide range of fields such as electronics, bio-sensors, catalysis and energy storage, etc3,4,5,6,7,8,9,10,11,12,13. Obviously, successful scale-up fabrication of high-quality layered materials is the prerequisite for achieving their full potential14,15. Micromechanical cleavage is a simple method, but it is on a small scale and has disadvantages in commercial high-end applications16. Intercalation technique is a useful alternative top-down method to exfoliate layered materials by using various kinds of intercalates, such as alkali metals and transition-metal halides17,18,19,20. However, ion intercalation-based methods have drawbacks associated with their sensitivity to ambient conditions. Coleman and other researchers have made a major breakthrough via the sonication-assisted exfoliation in nonvolatile organic solvents or mixed-solvents, like dimethylsulphoxide (DMSO), N-methyl-pyrrolidinone (NMP), N-vinyl-Pyrrolidinone (NVP), to produce mono- and few-layer nanosheets21,22,23,24. It is a simple liquid exfoliation method following the mechanism that the surface energy of the solvent must match that of the solute, but the employment of nonvolatile organic solvents will impede further applications of layered nanomaterials and bring the negative impact to the environment.
Besides the methods referred above, surfactant-assisted exfoliation is of particular interest25,26,27. First, the used solvent is water and so it is benign to environment. Second, the application of surfactants caters for the exfoliation demand that it is necessary to enhance the ratio of surface to mass and to form larger interface. It is well-known that in a micelle, the hydrophobic tail of the surfactant points towards the core while the polar head group forms an outer shell. Similarly, surfactant may also aggregate in non-polar organic solvent, wherein the structure was referred as reverse micelles28. As an excellent and green alternative to conventional organic solvents29,30,31, supercritical carbon dioxide (SC CO2) possesses an important property that it can assist surfactant-water solutions to build the reverse-micelle emulsions microenvironment and the phase behaviour of emulsions microenvironment can be manipulated by tuning the physical property of solution32,33,34. Specifically, just as a “switch” for the molecular aggregation of surfactants, the tuning of the aggregation behaviours of surfactants by CO2 is reversible, which can be realized by simply pressurization and depressurization35.
In this study, for the first time we present a highly facile, efficient and versatile method to exfoliate a series of layered materials by virtue of SC CO2 to switch the phase inversion from micelles to reverse micelles in the emulsion microenvironment. We have systematically explored the typical layered materials such as graphene, MoS2, WS2 and BN in the emulsions microenvironment of CO2/surfactant/H2O system and obtained the detail information about optimal solution condition of their efficient exfoliation. Experimental results demonstrate that the driving force coming from the phase inversion from micelles to reverse micelles is efficient for the exfoliation and as well as the curvature transition of surfactants and the phase behaviours of micelles in the emulsions can be manipulated by changing formulation variables, such as CO2 pressure and ethanol/water ratio36,37,38. Further the exfoliated graphene can be used to prepare high-conductive paper, MoS2 nanosheets can be used as broad-spectrum fluorescent label to perform cellular labelling and BN nanosheets can be used to efficiently reinforce the mechanical properties of polymer. So this strategy utilizing reverse-micelle-induced method for exfoliation of layered materials has great potential application in electronic, biotechnology, mechanics, energy and information storage, etc.
Results
Production of layered materials
Figure 1 shows the schematic diagram of the exfoliation process of layered materials in the emulsions microenvironment of the CO2/surfactant/H2O system. Since PVP can adsorb on the surface of these layered materials through the strong hydrophobic interactions between PVP chains and surface of layered materials25, How to cite this article: Wang, N. et al. High-efficiency exfoliation of layered materials into 2D nanosheets in switchable CO2/Surfactant/H2O system. Sci. Rep. 5, 16764; doi: 10.1038/srep16764 (2015).Additional Information
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Acknowledgements
We are grateful for the National Natural Science Foundation of China (Nos. 51173170, 50955010 and 20974102), the financial support from the Innovation Talents Award of Henan Province (114200510019) and the Program for New Century Excellent Talents in University (NCET-08-0667).
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Q.X. proposed the concept and conducted the research. N.W. conceived and designed the experiments, N.W., S.S.X., M.C., Y.H.Q. and H.X.L. performed the experiments and the standard characterizations. N.W., S.S.X., M.C. and Y.H.Q. analysed the data. Q.X., N.W. and B.X.H. co-wrote the paper. All authors discussed the results and reviewed the manuscript.
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Wang, N., Xu, Q., Xu, S. et al. High-efficiency exfoliation of layered materials into 2D nanosheets in switchable CO2/Surfactant/H2O system. Sci Rep 5, 16764 (2015). https://doi.org/10.1038/srep16764
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DOI: https://doi.org/10.1038/srep16764
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