Two-dimensional (2D) Ti3C2 discovery at Drexel University 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 Ti3C2 synthesis from a ternary carbide (Ti3AlC2), 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 Mn+1XnTx. 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 Tx is ≤ 2 [5].

Fig. 1
figure 1

Periodic table showing elements used to build MXenes. The green background shows metals (M elements), gray background—nonmetals (X elements) and dark yellow background—terminating elements (T). The striped background shows rare-earth elements that can be components of in-plane ordered MAX structures. The elements with diagonal stripes are only reported in MXene precursors (MAX phases). Oxygen is shown with gray diagonal stripes because of the recently discovered oxycarbide MXene

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 Mn+1XnTx. As for M, full-range solid solutions of M, such as (Ti,V)2CTx, (Ti,Nb)2CTx, (V,Nb)2CTx, allow precise control over the desired properties of MXenes [7]. In 2019, a MXene with five layers of M (n = 4), Mo4VC4Tx, 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 [10,11,12]. In parallel, additional solid solutions on the X site, that is carbonitrides, were reported [12, 13].

Fig. 2
figure 2

MXene structures and compositions reported to date. The top row shows structures of mono-M MXenes. The second row shows solid solutions (their compositions are marked in green below). The third row shows in-plane and out-of-plane ordered double-M MXenes (their compositions are marked in red). The fourth row shows an ordered divacancy structure, which has only been reported for the M2C MXenes, making an M4/3C composition due to 1/3 of all atom positions being vacant in each M layer (their compositions are marked in orange color). The fifth row shows high-entropy MXenes (their compositions are marked in violet). This table includes both experimentally (marked in blue) and theoretically (marked in gray) explored compositions of MXenes. Surface terminations are not included. This table includes phases that are synthesized via bottom-up or phase transformation of other phases, such as W2N, V2N, and Mo2N

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 Nb2C(NH) and Nb2CS2. Additionally, in 2022, transition metal carbo-chalcogenides were discovered that are M2C 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 Nb2CS2 and Ta2CS2 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 as a research topic on the Web of Science), which revealed more than 12,000 publications till October 2022 (Fig. 3c). Interestingly, more than 10,000 of those publications have the word “MXene” in their abstracts. In our previous editorial on the rise of MXenes [6], where we analyzed MXenes’ publications till the end of 2018, there were only ~ 1300 publications. Comparing the numbers after less than four years indicates an order of magnitude increase in the total number of publications (Fig. 3c). Another interesting aspect of this rapid growth is the expansion of MXenes to new areas of research, such as biomedical, mechanical, and electronics and electromagnetics. In 2019, almost half (47%) of all MXene publications were focused on their energy and catalytic applications (Fig. 3b), while these topics are currently ~ 30% of total publications on MXenes (Fig. 3a). This does not mean that the research on energy storage/conversion or catalytic applications has slowed down, as the number of publications in energy-related areas has increased more than eightfold. The increase in the percentage of new research areas in the total number of MXene publications is mostly due to the rapid growth of those new fields.

Fig. 3
figure 3

Explored applications and properties of MXenes to date. a, b The pie chart in a shows the ratio of publications in each explored application of MXenes with respect to the total number of publications on MXenes, and is compared to a similar chart with the same colors from 2018 (b). c The bar chart shows the cumulative number of MXene publications per year. d Comparison of the total number of publications on various MXenes shows domination of Ti3C2

MXenes in biomedical applications is one of those emerging areas of research. There are currently more than 1,100 publications on biomedical applications of MXenes, while this number was only about 80 by the end of 2018, as the field only emerged in 2017. Many new biomedical applications of MXenes emerged or greatly expanded since 2018, such as photothermal cancer therapy, diagnostics, theranostics, dialysis and epidermal/implantable electronics. Another area that experienced rapid penetration of MXenes is microwave absorption, electromagnetic interference shielding, and communication (antennas and RFID tags), which has seen a 20-fold increase since 2018. High electrical conductivity of MXene films and coatings, tunable surface chemistry and ease of solution processing are some of the main contributors to the rapid growth in the electromagnetic applications [16]. All synthesized MXenes are electrically conductive, which is unique in the 2D materials world. More recently, it was shown that MXenes can be used as nanometer-thin interconnects with a breakdown current density of 108 A/cm2 for a Ti3C2Tx and Nb4C3Tx single flakes (two orders of magnitude higher compared to copper), which is important for further shrinkage of the size of microelectronics in accordance with Moore’s law. Another emerging area of research is tribology, due to MXenes' weak interflake bonding combined with their strong intraflake bonds [17], like in other 2D materials. However, MXene’s functionalized and functionalizable surface differentiate them from other 2D and layered solid lubricants such as graphene, BN and MoS2.

While MXenes are growing fast, the effect of MXene precursors, mostly MAX phases, on properties of the resulting MXenes composition was mostly neglected for a long time. In 2022, it was shown that the Ti3C2Tx precursor, Ti3AlC2, can contain substitutional oxygen in its structure. For example, up to 30 at.% of oxygen in the carbon layer was found in Ti3AlC2 made via the conventional method of synthesis form elemental precursors using a stoichiometric ratio mix of 2TiC:1Al:1Ti, while no oxygen was detected in Ti3AlC2 using the modified non-stoichiometric mix of 2TiC:2Al:1.25Ti powders [4]. The resulting MXene has shown greatly improved oxidation resistance and conductivity compared to the oxygen-containing Ti3C2Tx [18]. Formation of oxycarbides (and probably oxynitrides) is important for controlling not only the properties but also the structures of MXenes. In our synthesis of the first n = 4 MAX phase, Mo4VAlC4, we used 0.05 mol of V2O3 in the elemental powder mixture to stabilize this phase [8]. Apparently, oxygen substitution may enable new MAX and MXene structures which are not stable in the pure M-X system. Beyond the X sites, a better understanding of M sites in ordered double-metal and high-entropy MAX and MXenes is needed to characterize ordering, intermixing, and in-plane segregation of elements in M layers [4, 19]. Defect engineering is another frontier in controlling properties of MXenes.

Majority of MXene publications are still focused on Ti3C2Tx, with ~ 70% of all published articles (increase from 2018), followed by Ti2CTx, V2CTx, Nb2CTx, and Mo2CTx (Fig. 3d). The growing interest on Ti3C2Tx is due to several reasons, including the best developed synthesis and processing protocols, leading to high quality, large size and the highest electrical conductivity to date, and availability of the precursor. Titanium, carbon and aluminum are abundant and inexpensive elements, promising inexpensive products with no limitations in terms of raw materials. However, more attention to the topic of MXene synthesis and the effects of precursors is needed. As the demand for MXenes increases, more MAX and MXene compositions become commercially available. However, as there are still no standards for MAX or MXene, some of the commercially available products might not have the required quality for research or manufacturing. More collaboration between the industry and academia, which may be facilitated by MXene Association, for which Graphene and 2D Materials serves as an official publication, is needed to identify high-quality MAX phases and MXenes that can be used for research and commercial purposes.

The MXene research community is making progress, as many of the challenges listed in Ref. [20] are being tackled and many have been solved in the past two years. As mentioned earlier, MXenes with uniform surface terminations have been realized. High-entropy MXenes are now a part of the MXene family. Oxycarbide MXenes have been demonstrated. Effects of compositions on properties of several solid-solution MXenes have been reported. MXene shelf-life and stabilization against oxidation have become a major topic for research and several methods are investigated to prevent MXenes’ oxidation [21, 22] or fabricate MXenes with higher oxidation stability [18]. This resulted in increasing environmental stability of single-layer MXenes in solution and in air, with lifetime of V2C extended from less than a day to several months and Ti3C2 from weeks to over a year. The conductivity of Ti3C2 films (MXene “paper”) reached values over 20,000 S/cm and values of conductivity over 10,000 S/cm were maintained after 2 years of storage. Wet chemical etching in acidic solutions has been scaled up to kilogram quantities, while non-aqueous etchants, fluorine-free and electrochemical MAX etching methods have been published. The molten salt method has been used to synthesize MXenes from non-Al MAX phases. However, many more challenges listed in Refs. 4 and 20 (which we will not repeat here) remain and addressing them will require joint efforts of the MXene community and collaborators. In return, one can expect exciting new fundamental science related to quantum confinement in 2D MXene sheets with an unprecedented level of control over their structure, composition and properties, novel chemistry in confinement between MXene sheets, as well as solutions to practical problems in energy, electronics, environmental and healthcare fields.