Properties of MXene

MXenes are a family of two-dimensional nanomaterials of transition metal carbides and nitrides, among the newest and most famous two-dimensional materials. In 2011, the first sample of MXenes was produced by selective extraction of highly bonded layered crystal structures and identified with the general formula Mn+1XnTx. In this formula, M represents transition metals (such as Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and Mn) and n = 1–3. X is carbon, nitrogen, or both, and Tx is surface-terminating functional groups (such as O, OH, F, and Cl). MXenes are generally synthesized by selective etching layer A elements from ternary mixtures of MAX phases with hexagonal structure (P6/mmc) as starting material. The name “MXenes” is derived from the original MAX phases with the selective deletion of “A” and the addition of “ene” to emphasize the similarity to two-dimensional materials like graphene. The term MAX, a large group of ternary carbides and nitrides with a hexagonal layer structure, reflects the chemical composition Mn+1AXn, which “A” is one of the elements of the III and IV groups of the A group. The weaker metallic bond in M-A compared to the M-X covalent bond/metallic/mixed ionic facilitates the selective etching of A atomic layers from MAX phases. MXenes appear with unique properties compared to their parent phase, the MAX phase, and the most important characteristic of MXenes is that the properties of MXenes can be controlled by the method of synthesis from the selected MAX phase. The electronic, magnetic, optical, mechanical, topological, and other properties can significantly be affected by the dimensions and morphology of a material. Due to their unique 2D layered structure, hydrophilic surface characteristics and high metallic conductivity, high electrical and thermal, mechanical properties, and good chemical stability are exhibited by MXenes. The intrinsic structure and chemical composition of MXenes are derived from transition metals with d orbitals, which show attractive physical and chemical properties. MAX phases have the nature of ceramics with high structural hardness and the characteristics of metals, such as good electrical and thermal conductivity. Similar to the MAX phase, two-dimensional MXenes also have excellent electrical conductivity due to the metallic properties of the transition metal and carbon or nitrogen that make it up. However, some MXenes remarkably show semiconducting behavior, which is desirable for chemical gas sensors. The surface of MXenes is covered by hydroxyl, oxygen, or fluorine, and the ratio of each of these compounds or elements is mainly determined by the synthesis method. The electrical properties of MXenes are significantly affected by the surface terminals resulting from wet chemical synthesis. Surface performance is one of the most attractive features of MXenes, which can adjust their properties. The richness of chemistry and surface functions of MXenes leads to highly variable and adjustable physical and chemical properties, large elastic modulus, outstanding optical properties, high specific ionic capacity, and compatibility with water and organic solvents. Considering the unique features of MXenes, this chapter examines the properties of MXenes and their relationship with their structure and synthesis.