Synthesis and Properties of MXenes
Various procedures are described in the preparation of MXenes by using the etching methods with HF or other fluoride solutions. The effects of the reaction conditions on properties of the MXenes are discussed. In addition, various properties of the synthe
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Synthesis and Properties of MXenes
2.1 Introduction MXenes, as a new group of two-dimensional (2D) structured materials, have attracted extensive attentions from all around the world. The 2D structures are obtained, as the element A is removed from the parent phase of Mn+1 AXn . In the unit cell of the layered hexagonal phases of, Mn+1 AXn , there are two formula units, in which the M layers are constructed through the close packing of the X atoms. The spaces in between the octahedral sites are filled with the X atoms, whereas the layers of a atoms serve to interleave the Mn+1 Xn layers [1]. The laminated structures exhibit strong anisotropic characteristics, because the M-X bond could have combined ionic, covalent and metallic natures, while the M-A bond is of just metallic behavior. The adjacent layers in the structures are strongly bonded, so that the exfoliation of Mn+1 AXn is different from those of graphite and transition metal dichalcogenides (TMDs), in which the bonding is through the weak van der Waals interactions [2]. The strengths of the M-X and M-A are different, whereas the A layers have higher reactivity. Therefore, the A layers can be removed relatively easily, if suitable etching reagents are used. As a consequence, layer structured MXene, or Mn+1 Xn Tx , is thus obtained. In addition, MXene usually contains surface-terminating functional groups, denoted as Tx , which include oxygen (=O), hydroxyl (–OH) and fluorine (–F). These functional groups are evidently formed during the etching process and linked to the M atoms in the MXenes. The thickness of Mn+1 Xn Tx is directly dependent on the value of n. For instance, for the values of n = 1, 2 and 3, they are single, double and triple octahedral blocks, respectively. More recently, MXene with n = 4 has also been synthesized, which has a formula of Mo4 VC4 , containing five atomic layers of transition metals [3]. The element A in the parent phases of Mn+1 AXn , which is selectively removed, is most likely Al in the materials reported in the open literature. The M site usually hosts transition metals, such as Ti, V, Cr, Nb, Ta, Zr, Mo, etc. The element A can also be taken away by using thermal etching at high temperatures, but the layered structures could be negatively affected or even totally destroyed [4]. In addition, both © Springer Nature Switzerland AG 2021 Z. Xiao et al., MXenes and MXenes-based Composites, Engineering Materials, https://doi.org/10.1007/978-3-030-59373-5_2
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2 Synthesis and Properties of MXenes
elements M and A can be etched through high temperature chlorination reaction, which results in porous products, known as carbide derived carbons (CDC), instead of layered structures [5]. In this regard, chemical etching is much preferred to develop 2D structured MXenes. As the first 2D MXene, Ti3 C2 , was obtained by using selective etching of Al from the layered hexagonal ternary carbide, Ti3 AlC2 , with hydrofluoric acid (HF) solution to be the etching agent at room temperature (RT) [6]. In fact, the Ti3 AlC2 belongs to the fami
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