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Figure 6. a) Possible mechanism for the exfoliation process of MoS2 in the pickering emulsion microenvironment. b) Photographs of the CO2/ethanol/water system with CO2 pressures of 16 MPa at 313.2 K with different ethanol volume fractions. Reprinted from reference [40]. Copyright 2015, The Royal Society of Chemistry. Figure 7. Different polymorphs or phases of single-layer and stacked single- layer transition metal dichalcogenides: a) 1T phase, b) 2H phase, and c) 3R phase. Reprinted from reference [11]. Copyright 2015, The Royal Society of Chemistry. The successful exfoliation of these 2D layered nanomaterials brings us more possibilities in various applications. For example, grapheme was used to prepare conductive paper, MoS2 can be used as fluorescent label to perform cellular labeling, and BN can effectively reinforce poly- mers with promising mechanical properties. 2.3. SC CO2-Induced Phase Engineering of 2D Materials Phase transitions can be employed to regulate the properties of a mate- rial with significant technological value. Unlike graphene, TMDs have the specially bandgap dependent electronic properties. And another important unique feature of TMDs is that they can form different crystal polytypes depending on the different coordination modes between transition metals and chalcogen atoms and/or stacking orders between layers.[11] There are three main polymorphs of TMDs, that is, 1T, 2H, and 3R, corresponding to the octahedral, trigonal prismatic, and rhom- bohedral phases, respectively (Figure 7). The best well-known poly- morph structures are metallic (1T) and semiconducting (2H).[41,42] The existence of metallic phase can improve the electrical conductivity effectively, which is appropriate for electrocatalysis. Several approaches have been explored to perform phase modification, such as chemical reactions, electron irradiation, lithium intercalation, and strain/pres- sure-induced route.[29,43–48] Phase modification via chemical reactions has been reported for some TMDs such as MoS2, WS2, and MoSe2. Lin et al.[49] proposed that the phase transformation of chemically exfoli- ated MoS2 can easily convert one to another due to intralayer atomic plane gliding and the according atomic displacements. Wang et al.[50] put forward a shear mechanism of the 2H-1T phase transition, which was confirmed by testing the lithium intercalation process in MoS2 nanosheets via an in situ high-resolution transmission electron micro- scopy (TEM) approach. However, uncontrollable phase composition of the resultant nanosheets, tedious post-treatment on the residual lithium ions, and rigorous experimental conditions such as high temperature and high pressure limit their scalable production. As an ideal external stimulus, SC CO2 can induce the phase transformation of layered mate- rials effectively. In recent study, Qi et al.[51] proposed a strategy to realize the forma- tion of 1T-MoS2 nanosheets using SC CO2 system (Figure 8). Theoreti- cal calculations and experimental results demonstrated the strain in single-layer MoS2 nanosheets generated from the different adsorption strength of CO2 on 2H and 1T phases facilitated the transformation of partial 2H-MoS2 to 1T-MoS2. To further fabricate a high ratio of 2D metallic 1T phase MoS2 nanosheets, Tong et al.[52] put forward a reverse-micelle-induced strain engineering with the assistance of SC CO2 (Figure 9). The key of the successful transfer of the 1T phase from 2H phase depends on the strain of single-layer nanosheets originating from the curvature transition, which can be formed during the forma- tion process of reverse micelles. The relative content of the 1T phase was estimated to be more than 90%. The existence of the 1T phase can greatly enhance the photoelectrocatalytic performance. Besides, the topotactic phase transformation of MoO3 from orthorhombic phase (a- MoO3) to metastable hexagonal phase (h-MoO3) can be also realized with the assistance of SC CO2.[53] 2.4. SC CO2-Assisted Construction of 2D Heterostructures Atomically thin 2D nanosheets prepared from their bulk counterparts can be used as promising building blocks to construct functional hybrid materials, owning to extraordinary properties, such as ultrathin thick- ness, high specific surface area, and excellent electronic properties. Heterostructures prepared from these ultrathin 2D nanomaterials have brought about unprecedented opportunities for fundamental scientific studies and device engineering.[54] The heterostructures consisting of dissimilar 2D materials include vertical (stacked layer by layer through Energy Environ. Mater. 2018, 1, 46–60 51 © 2018 Zhengzhou UniversityPDF Image | CO2 and 2-Dimensional Nanomaterials with Green Chemistry
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