MXenes: trends, growth, and future directions

Abstract

more than a decade ago created a new family of 2D transition metal carbides, nitrides, and carbonitrides [1]. Because top-down selective etching was used for Ti 3 C 2 synthesis from a ternary carbide (Ti 3 AlC 2), which belongs to the large family of MAX phases [2], it was clear since the discovery of the first MXene that many more 2D compositions are possible. Soon after, additional MXenes with different transition metals and solid solutions were reported [3], establishing MXenes as a family of 2D materials with a chemical formula of M n+1 X n T x. To date, M covers groups 3 to 6 transition metals, X is carbon or nitrogen, and T represents the surface terminations, which include groups 16 and 17 or the periodic table and hydroxyl and imido groups (Fig. 1). With the recent discovery of oxygen substitution in carbide MXenes [4] and formation of oxycarbides, X can include oxygen as well (at least in solid-solution MXenes). MXenes can have different numbers of M-X-M layers, which is shown by n, and ranges from 1 to 4, and x in T x is ≤ 2 [5]. Since our ACS Nano editorial in 2019 [6], the landscape of MXenes has changed from the composition and application perspectives. The range of MXene compositions has expanded in all four components of the MXene formula, M, X, T and n in M n+1 X n T x. As for M, full-range solid solutions of M, such as (Ti,V) 2 CT x , (Ti,Nb) 2 CT x , (V,Nb) 2 CT x , allow precise control over the desired properties of MXenes [7]. In 2019, a MXene with five layers of M (n = 4), Mo 4 VC 4 T x , was discovered, which added a new level of structural control to the MXenes and 2D materials, in general [8]. Having nine (11 or more, if surface terminations are considered) atoms in cross section, this is the "thickest" of 2D materials reported today with high bending stiffness which may lead to unique mechanical and electromechanical applications. In 2021, high-entropy MXenes with multiple M elements were discovered [9], with five compositions reported within about a year (Fig. 2) by different research groups around the world [10-12]. In parallel, additional solid solutions on the X site, that is carbonitrides, were reported [12, 13]. While the control of surface terminations (T) and achieving uniform surface groups was only explored in computational studies in the 2010s, MXenes with uniform surface terminations, such as oxygen, imido group, sulfur, chlorine, selenium, bromine, tellurium, and even no surface terminations (bare), were synthesized in the 2020. In this study, a general method to add and remove surface terminations by the use of molten salts has been developed [14] and produced the first MXenes with superconductivity at < 8 K, such as Nb 2 C(NH) and Nb 2 CS 2. Additionally, in 2022, transition metal carbo-chalcogenides were discovered that are M 2 C layers with surfaces terminated by chalcogens (such as S). These phases are produced by exfoliation of layered transition metal carbo-chalcogenides, and compositions such as Nb 2 CS 2 and Ta 2 CS 2 have been reported [15], which opens new ways to bridge the gap between MXenes and other 2D materials. All these discoveries have increased the number of MXene compositions to 46 (only M, X, and n variations), without consideration of multiple compositions for each solid solution and surface terminations, which would drastically expand the list (Fig. 2). In parallel to the increase in the number of MXene compositions , the percentage of different applications in the total number of MXene publications was also changed from the end of 2018 till now (compare Fig. 3a and b). These charts are based on our Web of Science search with different topics. First, we looked at the topic of MXene (search MXene