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REVIEW Supercritical Carbon Dioxide Building Close Ties Between CO2 and Functional Two-Dimensional Nanomaterials with Green Chemistry Strategy Yumei Ren and Qun Xu* Two-dimensional nanomaterials represented an emerging class of nanomaterials that have attracted dramatically increasing attention in both academia and industries. With the development of green chemistry, supercritical carbon dioxide with unique properties can be used as excellent reaction media for two-dimensional nanomaterials preparation and processing. In this review, we summarize the recent state-of-art progress on the ultrathin two-dimensional nanomaterials fabricated with the assistance of supercritical carbon dioxide, including carbon dioxide-induced liquid exfoliation, phase engineering, and the formation of amorphous materials, as well as the construction of heterostructures and novel functional nanomaterials. Based on the potential and versatility of supercritical carbon dioxide in materials processing and synthesis, we will also give some personal perspectives on the existing challenges and future research directions. 1. Introduction Two-dimensional (2D) nanomaterials, in the wake of the growing research of graphene, have gained intensive research attention and shown great promise for a wide range of applications.[1,2] Inspired by the recent advancements in graphene, numerous graphene-like ultra- thin 2D nanomaterials, such as hexagonal boron nitride (h-BN), transi- tion metal dichalcogenides (TMDs), and transition metal oxides (TMOs), have been fueled vigorous investigations to date, bringing about new breakthroughs in condensed matter physics, material science, chemistry, and nanotechnology.[3,4] Ultrathin 2D nanomaterials with single- or few-atoms thick endow the unique appealing features including highly exposed active edge sites, large specific surface area, high flexibility, and good mechanical strength, making them promising for applications in electronics/opto- electronics, catalysis, biotechnology, energy storage and conversion, and so on.[5,6] A wide range of 2D materials with interesting properties have been reported. Specially, as a typical class, ultrathin 2D TMDs nanosheets, such as MoS2, WS2, and WSe2, offer new opportunities for practical applications.[7] One of TMDs’ unique features is that they have layer-dependent electronic and optical properties.[8,9] An indirect-to- direct bandgap transition of TMDs nanosheets can be realized with Y. Ren, Prof. Q. Xu College of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450052, China E-mail: qunxu@zzu.edu.cn DOI: 10.1002/eem2.12005 Energy Environ. Mater. 2018, 1, 46–60 46 tuning the number of layers. Another is their capability to form various crystal phases.[10,11] Based on the different coordination models between transition metal and chalcogen atoms and/or stacking orders between layers, different polymorphisms with diverse electronic proper- ties such as metallic (1T) and semiconducting (2H) phase can be achieved. To explore the full potential of the 2D layered materials, well- defined heterostructures created by dissimilar atomic crystals have opened up unprecedented opportunities to study the quantum electronics at the nanoscale.[12,13] The dramatically properties and various potential applications of the layered 2D nano- materials greatly stimulate the development of various reliable synthetic methods for the pro- duction of the atomically thin 2D nanomaterials with desired composition and physical/chemical and optical/electronic properties.[10,14] Top-down and bottom-up are two well-established strategies to process them. However, the disadvantages of either exces- sive use of solvents or the required harsh growth conditions such as high temperature and high vacuum limit their widespread usage.[8,15] The growing need for environmentally benign products and processes renders more attention focused on the concepts of green chemistry and sustainability both in industry and academia.[16,17] The supercritical fluid (SCF) technique worked as a green and effective means in materi- als synthesis and processing, as well as possessing many potential bene- fits in the context of sustainability, has been an important field in green chemistry.[18–20] Here, in this review, we describe the basics and unique advantages of supercritical carbon dioxide (SC CO2) in materials pro- cessing and then focus on the recent advances in the fabrication of 2D nanomaterials promoted by the application of SC CO2. Finally, the chal- lenges and opportunities of this field are discussed in this review. 2. SC CO2-Assisted Preparation and Functionalization of 2D Nanomaterials 2.1. Supercritical Carbon Dioxide A SCF is defined as a fluid whose temperature and pressure are above its critical state. Compared to the liquid and gas phases, SCF as a single phase exhibits many superior physical properties.[18,21,22] The solubil- ity, viscosity, diffusivity, and the density of SCF can be easily tuned by changing the pressure and temperature. Moreover, it has zero surface © 2018 Zhengzhou UniversityPDF Image | CO2 and 2-Dimensional Nanomaterials with Green Chemistry
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