Ultrasound exfoliation of inorganic analogues of graphene
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Ultrasound exfoliation of inorganic analogues of graphene Václav Štengl1*, Jiří Henych1,2, Michaela Slušná1,2 and Petra Ecorchard1 Abstract High-intensity ultrasound exfoliation of a bulk-layered material is an attractive route for large-scale preparation of monolayers. The monolayer slices could potentially be prepared with a high yield (up to 100%) in a few minutes. Exfoliation of natural minerals (such as tungstenite and molybdenite) or bulk synthetic materials (including hexagonal boron nitride (h-BN), hexagonal boron carbon nitride (h-BCN), and graphitic carbon nitride (g-C3N4)) in liquids leads to the breakdown of the 3D graphitic structure into a 2D structure; the efficiency of this process is highly dependent upon the physical effects of the ultrasound. Atomic force microscopy (AFM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) were employed to verify the quality of the exfoliation. Herein, this new method of exfoliation with ultrasound assistance for application to mono- and bilayered materials in hydrophobic and hydrophilic environments is presented. Keywords: Ultrasound; Exfoliation; Graphene inorganic analogues
Background Mechanical exfoliation, called the ‘scotch tape method’ [1], was the first method used for the preparation of single-layer graphene from natural graphite. Subsequently, through the utilization of this principle, other layered materials that are so-called inorganic analogues of graphene (IAG), such as MoS2 [2,3] and WS2 [4], hexagonal boron nitride (h-BN) [5], hexagonal boron carbon nitride (h-BCN), and graphitic carbon nitride (g-C3N4) (see Figure 1), were exfoliated. The current state of knowledge about the synthesis of IAGs is gathered below. Some recent attempts to obtain ultrathin MoS2 include the preparation of monolayered MoS2 flakes that were mechanically exfoliated from a piece of commercially available crystalline MoS2 sample by the scotch tape method [6]. Joensen et al. [7] exfoliated MoS2 into monolayers by intercalation with lithium followed by a reaction with water. Chemically exfoliated MoS2 was also prepared via lithium intercalation using a solution of butyllithium in hexane. However, this method resulted in loss of semiconducting properties of the pristine MoS2, * Correspondence: [email protected] 1 Materials Chemistry Department, Institute of Inorganic Chemistry AS CR, v.v.i., Řež 250 68, Czech Republic Full list of author information is available at the end of the article
due to the structural changes that occurred during Li intercalation [8,9]. Yao et al. [10] reported on a method for the fabrication of monolayers and multilayers of BN, MoS2, and graphene utilizing a combination of low-energy ball milling and sonication. Ball milling generates shear and compression, which can cleave the layered materials into the 2D nanosheets. Exfoliated WS2 was also prepared using ultrasonic treatments with n-butyllithium in hexane; this process was more difficult than the exfoliation of MoS2 [8,9] due to the resist
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