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Figure 2. Production of 2D nanomaterials via liquid-phase exfoliation of bulk layered crystals. Reprinted from reference [23]. Copyright 2016, Wiley-VCH. the exfoliation procedure is shown in Figure 3a.[31] A single and few layers of BN, MoS2, and WS2 nanosheets were also obtained via directly exfoliating their powder materials using SC CO2 assisted with ultra- sound by Wang’s group.[32] In addition to this, Li et al.[33] reported a procedure of shear-assisted SC CO2 exfoliation for producing high-qual- ity and large-quantity graphene nanosheets (Figure 3b). 2.2.2. Reverse-Micelle-Induced Exfoliation of Layered Materials Because of the nonpolar property as well as weak van der Waals forces of SC CO2, a dispersion of a hydrophilic or lipophilic phase in a CO2 continuous phase can be formed.[34–36] Thus, the reversed micelles can be built with the aid of surfactant–water solutions in SC CO2. The phase behavior of the reverse-micelle emulsions microenvironment can be manipulated by tuning the physical property of solution, such as pres- sure and temperature. A series of 2D layered materials, such as gra- phene, MoS2, WS2, and BN, have been successfully exfoliated into single- or few-layer nanosheets in the emulsion microenvironment built by SC CO2 based on the driving forces arising from the phase inversion, i.e., from micelles (CO2 in water) to reverse micelles (water in CO2) (Figure 4).[37–39] The effect of CO2 pressure and ethanol/wa- ter ratio on the exfoliation yield of layered materials was investigated. Among a series of surfactants, nonionic surfactant polyvinylpyrrolidone (PVP), with the excellent wetting property and 2D plane structure, is confirmed to have superlative efficiency for exfoliation of the layered materials. The integrity and uniformity of the lattice structure of these nanosheets displayed in high-resolution TEM micrographs (HRTEM) suggest that no defects and deformation are introduced during the exfoliation process (Figure 5). AFM measurements further confirm that the exfoliated MoS2 nanosheets consist of two to four monolayers. Sim- ilarly, the thicknesses of graphene (3–5 nm), WS2 (1–3 nm), and BN (2–6 nm) nanosheets indicate that they also exist as few-layer nanosheets in dispersions. Moreover, the calculated concentration of MoS2 and grapheme can be approximately 0.139 and 0.137 mg mL1 individually, which are higher than those concentrations reported. The exfoliation efficiency of these layered materials is attributed to the syn- ergistic effect of surfactant and SC CO2. 2.2.3. Pickering Emulsion-Induced Exfoliation of Layered Materials Besides the construction of the emulsion microenvironment of CO2/ surfactants/H2O system, the building of pickering emulsion in water– ethanol–CO2 is another attractive strategy to exfoliate 2D layered mate- rials. Take MoS2 as a typical example, MoS2–water pickering emulsion droplets can be formed by dispersing MoS2 nanosheets at the interface between water and CO2–ethanol driven by the synergetic effect of SC CO2, ethanol and mechanical stirring (Figure 6).[40] In this study, the effects of the different volume fractions of ethanol on the exfoliation of MoS2 nanosheets in the SC CO2/ethanol/water system were studied, and the experimental results indicated that the concentration of single- or few-layered MoS2 nanosheets dispersion can reach the highest as the ethanol volume fraction reaching 80%. The as-prepared MoS2 nanosheets have excellent crystallinity, and no defects and deformation can be observed. Furthermore, the thickness of the exfoliated MoS2 nanosheets is 1.1–1.6 nm, indicating single- or few-layer nanosheets are obtained successfully. Energy Environ. Mater. 2018, 1, 46–60 48 © 2018 Zhengzhou UniversityPDF Image | CO2 and 2-Dimensional Nanomaterials with Green Chemistry
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