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Yumei Ren, PhD student received her BE degree at Henan University of Urban Construction (2012). Currently, she is a PhD student at the College of Materials Science & Engineering of Zhengzhou University under the supervi- sion of Prof. Qun Xu. Her research interests focus on the synthesis, assembly, and appli- cations of two-dimensional nanosheets and their composites. Prof. Dr. Qun Xu obtained her PhD in Physical Chemistry from the Institute of Chem- istry, Chinese Academy of Sci- ence in 1999. She now directs a research group in the fabri- cation of low-dimensional materials and exploring the relevant applications in energy storage and conversion. Her work focuses on developing new materials systems and reliable strategies for efficient light harnessing and utilization. tension owning to it is just in one phase. These fascinating properties make it is superior in advanced materials processing. Firstly, the solubil- ity of a solute in SCF is a function of the fluid density, which can be tuned by adjusting the temperature and pressure. Secondly, the gas-like viscosities accelerate the mass and heat transfer. Thirdly, the zero surface tension can provide a good wetting of the surface as well as facilitate a better penetration of the reactants or solvents into the materials struc- ture. Besides, SCF is easy to separate by releasing pressure. Among all the SCFs, SC CO2 is the most popular one due to CO2 is inert, nonflammable, nontoxic, inexpensive, and readily available from the byproducts of many industrial processes, and it has moderate critical temperature and pressure (31.1°, 73.8 bar) (Figure 1).[22] Therefore, SC CO2 has been recognized as a nontoxic and environmental benign green solvent which could replace many conventional organic solvents used in traditional materials processing. 2.2. Layered 2D Nanomaterials Exfoliated with the Assistance of SC CO2 Layered materials are an interesting and potentially useful material class, making them an exciting starting material for further applications. Sin- gle- or few-layer 2D nanosheets exfoliated from bulk crystals can dra- matically enhance the range of properties depend on their atomic-layer thickness and 2D morphology. To date, different approaches have been reported to prepare the ultrathin 2D nanosheets. Among the available methods, liquid-phase exfoliation has been considered to be one of the most suitable routes for producing the atomically thin nanosheets, which was promoted and developed by Coleman and his col- leagues.[23–26] The methods of production of 2D nanomaterials via liq- uid-phase exfoliation of bulk layered crystals are illustrated in Figure 2. Coleman et al.[27] have performed a series of organic solvents used in the liquid exfoliation of the layered compounds, such as graphite, MoS2, WS2, and BN. Shen et al. also provided a rational strategy to exfoliate 2D materials using efficient solvents. In this study, the surface tension of between components solvents and 2D materials should match, for the purpose of obtaining the high exfoliation efficiency and the stabilization of the suspensions.[28] Besides, the presence of inter- calants such as Li ions and organic molecules can also facilitate the exfo- liation of bulk layered crystals into ultrathin 2D nanomaterials. For example, Voiry et al.[29] prepared exfoliated WS2 nanosheets using n- butyl lithium (BuLi), the formed LixWS2 by lithium intercalated to bulk WS2 powder can be easily exfoliated. However, a number of significant disadvantages limit these techniques. Most important, the as-prepared nanosheets lack of control over thickness and lateral size distributions. Moreover, the use of high-boiling organic solvents can bring the nega- tive impact to the environment. Therefore, it is required to explore eco- friendly methods and SC CO2-assisted liquid exfoliation has shown its great potential. 2.2.1. Ultrasound/Shear-Assisted Exfoliation of Layered Materials Supercritical carbon dioxide, owning to its high diffusivity, zero inter- face force, low viscosity, and small molecule size, can easily intercalate and exfoliate the layered materials. Several works have been reported the exfoliation of graphite and other 2D materials into single or fewer layers with the assistance of SC CO2. Xu et al.[30] confirmed that CO2 molecules can be used as a “wedge” in the exfoliation of graphene. Gao et al. reported an ultrasonic-assisted production of graphene in SC CO2 using a pressure reactor coupled with an ultrasonic generator, in which the production yield reached more than 50% and 93% of the obtained graphene sheets were less than three layers. The schematic drawing for Figure 1. Phase diagram of CO2. Reprinted from reference [22]. Copyright 2000, Nature Publishing Group. Energy Environ. Mater. 2018, 1, 46–60 47 © 2018 Zhengzhou UniversityPDF Image | CO2 and 2-Dimensional Nanomaterials with Green Chemistry
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