Introduction

Abstract

A brief introduction is presented, which is followed by the description on the arrangement of the book.

Two-dimensional (2D) materials have formed a new family of low-dimensional materials, which have drawn numerous attentions of the research community. Although various 2D materials have been proposed and synthesized, it is necessary to mention the titanium carbide, Ti₃C₂, which was first obtained by removing the Al atoms from the hexagonal ternary carbide, Ti₃AlC₂, through selective etching with aqueous hydrofluoric acid (HF) solution [1]. There are nearly one hundred similar ternary carbides and nitrides which are similar to Ti₃AlC₂, which have a general chemical formular of M_n+1AX_n, with M, A and X to stand for early transition metals, elements from the groups of IIIA or IVA and carbon/nitrogen, respectively, while n could be integer of 1‒3 [2]. Moreover, the M_n+1AX_n phases can be present as solid solutions, with different combinations of elements at the sites of M, A and X. As a result, the number of M_n+1X_n should unlimited [3,4,5,6,7,8,9,10,11,12,13,14,15,16].

There are two formula units in each unit cell of the layer-structured hexagonal phases of M_n+1AX_n, with the M layers to be strongly adhered by the X atoms that are filled in between the octahedral sites, while the M_n+1X_n layers are sandwiched by the A atom layers [17, 18]. As a consequence, the structures are of laminar architecture, thus having anisotropic characteristics. The M-X bond is a mixture of ionic, covalent and metallic behaviors, whereas the M-A is a pure metallic bond. The M_n+1AX_n nanolayers are strongly bonded due to the bonding characteristics. In comparison, relatively weak van der Waals force is usually dominant the layer-structured materials, like graphite and transition metal dichalcogenide compounds (TMDs) [19]. As a result, they can be readily exfoliated through mechanical action to form 2D materials.

Owing to the difference in bonding properties, the strengths of the M-X and M-A interactions are different, so that the A layers can be taken away, thus forming M_n+1X_nT_x layers, where T_x stands for surface functional groups, including =O, –OH and –F, which are linked to the M atoms on the surfaces generated during the etching reaction process. The layer thickness of the M_n+1X_nT_x items is determined by the value of n, i.e., the number of the building blocks. They are single, two and three building blocks for n = 1, 2 and 3, respectively. This newly emerged group of materials are named as MXenes, in order to demonstrate the elimination of the component A from the initial compounds of M_n+1AX_n and the 2D characteristic structure of graphene.

Besides the extensive studies on properties and applications of MXenes, the 2D materials have also been employed to form hybrids or composites, for a wide range of potential applications [20,21,22]. In this book, the advancement of MXenes and their nanohybrids and nanocomposites, in terms of synthesis, characterization and utilization. The synthesis and processing of representative MXenes will be covered in Chap. 2. In Chap. 3, the fabrication and characterization of MXenes-based hybrids and composites will be presented and discussed. The applications of MXenes in energy storage and conversion will be described in Chap. 4, such as anode materials of batteries, electrodes of supercapacitors, storage of hydrogen and so on. Other applications, including biosensing, environmental remediation, piezoelectric effects and electromagnetic interference (EMI) and shielding/absorption, etc., will be summarized in the last chapter.