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The Rise of MXenes MXenes (pronounced “maxenes”), a fast-growing family of 2D materials. In a 2D flake of MXene, n + 1 (n = 1−3) layers of early transition metals (M, elements in blue in Figure 1) are interleaved with n layers of carbon or nitrogen (X, elements in gray in Figure 1), with a general formula of Mn+1XnTx. The Tx in the formula represents the surface terminations, such as O, OH, F, and/or Cl (elements in orange in Figure 1), which are bonded to the outer M layers.3 Atomic schematics of three types of MXenes are shown at the bottom of Figure 1a. The variety of compositions and structures of MXenes has led to the formation
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or the past 15 years, starting with the discovery of the unique physical properties of single-layer graphene, twodimensional (2D) materials have been widely researched. This interest led to both a new wave of research on known 2D materials, such as metal dichalcogenides and boron nitride, and the discovery of many new 2D materials.1,2 Although many of these materials remain subjects of purely academic interest, others have jumped into the limelight due to their attractive properties, which have led to practical applications. Among the latter are carbides and nitrides of transition metals known as
Figure 1. Periodic tables showing compositions of MXenes and MAX phases. (a) Elements used to build MXenes. The bright blue elements represent MXenes that have not been yet experimentally confirmed. The schematics of three typical structures of MXenes are presented at the bottom. (b) Elements used to build MAX phases, MXenes, and their intercalated ions. The elements with blue striped background are only reported in MXene precursors (MAX phases), and their MXenes have not yet been synthesized. The elements on the red background are the A elements in MAX phases that can potentially be selectively etched to make MXenes. The green background shows the cations that have been intercalated into MXenes to date. As per the legend at the bottom, 1M and 1A indicate the formation possibility of a single (pure) transition metal and A element MAX phase (and MXene). SS indicates the existence of solid solutions in transition metal atomic planes (blue) or A element planes (red); 2M shows the formation possibility of an ordered double-transition metal MAX phase or MXene (either in-plane or outof-plane).5 Published: August 27, 2019 © 2019 American Chemical Society
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DOI: 10.1021/acsnano.9b06394 ACS Nano 2019, 13, 8491−8494
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Cite This: ACS Nano 2019, 13, 8491−8494
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Figure 2. MXene compositions reported to date. The top row shows structures of mono-M MXenes; the second row shows double-M solid solutions (SS). The double-M SS compositions are marked in green. The third row shows ordered double-M MXenes, and their compositions are marked in red. The fourth row shows an ordered divacancy structure, which has only been reported for the M2C MXenes, making a M1.33C composition due to ∼33 atom % of vacancies in the M layers, and their compositions are marked in pink. This table includes both experimentally (marked in blue) and theoretically (marked in gray) explored compositions of MXenes. Surface terminations are not shown. Adapted with permission from ref 3. Copyright 2017 Springer Nature.
Conference on MXenes covered numerous aspects of the basic science and applications of MXenes, including synthesis, structure, and properties, as well as applications in energy storage and conversion, environment and catalysis, separation membranes, medicine, optics, and electronics. This variety of topics represents a major expansion in MXene applications compared to the first conference, which focused on MXenes for energy. MXene symposia in Europe, the United States, and China are scheduled for the end of this year and spring and summer 2020, respectively. Several factors are contributing to the rapid expansion of the field. The first evidence for significant growth of MXenes is the number of research institutions that are studying MXenes and have already published their work in peer-reviewed journals (more than 750 institutions from 50 countries).5 This intense research has led to fast growth in the number of synthesized compositions. The first MXene (Ti3C2Tx) was discovered at Drexel University in 2011,6 with no prior prediction for the stability of such 2D compounds. Since then, more than 30 MXene compositions have been published (marked in blue in Figure 2), and dozens more have been explored by computational methods (marked in gray in Figure 2). A unique feature of MXenes comes into play when two transition metals are mixed in a MXene structure. In addition to the formation of expected solid solutions, such as (Ti,Nb)CTx (marked in green in Figure 2), transition metals can form ordered structures in a single 2D MXene flake, either by forming atomic sandwiches of transition metals planes (for n ≥ 2) such as Mo2TiC2Tx, or in-plane (n = 1) ordered structures such as (Mo2/3Y1/3)2CTx. Although ordered
At least 100 stoichiometric MXene compositions and a limitless number of solid solutions offer not only unique combinations of properties but also a way to tune them by varying ratios of M or X elements. of a large and rapidly expanding family of 2D materials. MXenes (Figure 1a), their precursor MAX phases, and intercalated metal ions in MXenes (Figure 1b) serve as embodiments of the fundamental principles of chemistry, showing how the elements can be used as building blocks to form a variety of nanomaterials. To mark the 150th anniversary of Dmitri Mendeleev’s nowiconic periodic table of the elements, the United Nations General Assembly and UN Educational, Scientific, and Cultural Organization have proclaimed 2019 the International Year of the Periodic Table of Chemical Elements.4 The MAX and MXene compositions nicely illustrate the power of the periodic table. In the past 2 years, the MXenes field has seen significant increases in the numbers of research areas and publications.5 The 2nd International Conference on MXenes, held in May 2019 at the Beijing University of Chemical Technology in Beijing, China, attracted 450 participants, more than double the number of attendees of the first conference, organized at Jilin University in Changchun, China, a year earlier, an indication of the quickly growing interest in this family of materials. The 2nd International 8492
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Figure 3. Explored applications and properties of MXenes to date. The center pie chart shows the ratio of publications in each explored application/property of MXenes with respect to the total number of publications on MXenes. The middle pie chart ring, with the same colors, shows the starting year for exploration of each application/property of MXenes. NB: although there may be one or two papers published prior to the mentioned year, we considered a year with several significant publications as the starting (breakthrough) year for each slice. The outer ring shows the ratio of publications on Ti3C2Tx MXene versus all other MXene compositions (M2XTx, M3X2Tx, and M4X3Tx).5
synthesize new MAX phases as well as fluorine-free MXenes.16 This method can significantly widen experimental research on MXenes, as those who are interested in MXenes but do not want to work with any hydrofluoric acid (HF)-containing or HFforming chemicals in their laboratories can now synthesize MXenes. MXenes have a unique combination of properties, including the high electrical conductivity and mechanical properties of transition metal carbides/nitrides; functionalized surfaces that make MXenes hydrophilic and ready to bond to various species; high negative zeta-potential, enabling stable colloidal solutions in water; and efficient absorption of electromagnetic waves, which have led to a large number of applications. MXene applications are presented as the center pie chart in Figure 3.5 The second ring in Figure 3 shows the year in which the first papers reported on each application. The first explored application of MXenes was in energy storage, which remains a large proportion of MXene activities. The use of MXenes in the biomedical field, although only 2 years old, has become one of the hottest research topics with studies on photothermal therapy of cancer, theranostics, biosensors, dialysis, and neural electrodes.5,17 Another area in which MXene research is taking over from other nanomaterials is in electromagnetic applications, including electromagnetic interference shielding and printable antennas.18 In other fields, including electronic and structural applications, most of the published studies are theoretical with relatively few experimental papers, and many predicted properties, such as ferromagnetism or topological insulators, have yet to be validated experimentally.
MXenes were synthesized in 2014 and reported in 2015,7 many new compositions have been synthesized in this subfamily of MXenes (marked in red in Figure 2). The rush in synthesizing new ordered 2D carbide phases brought excitement to the MAX phase research community. Researchers who have been studying MAX phases for the past two decades started to develop new MXene precursors, mainly MAX phases but also other layered carbides and nitrides. Since 2017, researchers have synthesized about 30 new ordered double-transition metal MAX phases and explored their properties, including their magnetic characteristics.8−10 Transition metals that are only reported in MAX phases are marked with blue stripes in Figure 1b. In the past 2 years, computational studies on MXenes and their precursors have predicted hundreds of possible compositions.10−14 The formation of solid solutions on M and/or X sites offers possibilities for the synthesis of an infinite number of nonstoichiometric MXenes and an attractive opportunity to finely tune properties by mixing different transition metals or creating carbonitrides. There are ongoing attempts to produce 2D borides and thereby to add another X element to the system. Another important advance witnessed in the past year is the development of processes for fluoride-free synthesis of MXenes. Most of the initially published MXene synthesis routes involved fluoride-containing compounds, either aqueous or molten salts.3 An electrochemical fluoride-free synthesis route, for example, in dilute hydrochloric acid, was recently reported for Ti2CTx MXene synthesis.15 However, scaling up selective electrochemical etching methods to large production volumes may be a challenging task. In 2019, Huang et al. used molten ZnCl2 salt to 8493
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(3) Anasori, B.; Lukatskaya, M. R.; Gogotsi, Y. 2D Metal Carbides and Nitrides (MXenes) for Energy Storage. Nat. Rev. Mater. 2017, 2, 16098. (4) Seijo, B. C. 2019: The Year the Periodic Table Gets Its Due. C&EN 2019, 97, 24−25. (5) Anasori, B.; Gogotsi, Y. 2D Metal Carbides and Nitrides (MXenes), Structure, Properties and Applications; Springer: Berlin, 2019. (6) Naguib, M.; Kurtoglu, M.; Presser, V.; Lu, J.; Niu, J.; Heon, M.; Hultman, L.; Gogotsi, Y.; Barsoum, M. W. Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2. Adv. Mater. 2011, 23, 4248−4253. (7) Anasori, B.; Xie, Y.; Beidaghi, M.; Lu, J.; Hosler, B. C.; Hultman, L.; Kent, P. R. C.; Gogotsi, Y.; Barsoum, M. W. Two-Dimensional, Ordered, Double Transition Metals Carbides (MXenes). ACS Nano 2015, 9, 9507−9516. (8) Tao, Q.; Lu, J.; Dahlqvist, M.; Mockute, A.; Calder, S.; Petruhins, A.; Meshkian, R.; Rivin, O.; Potashnikov, D.; Caspi, E. a. N.; Shaked, H.; Hoser, A.; Opagiste, C.; Galera, R.-M.; Salikhov, R.; Wiedwald, U.; Ritter, C.; Wildes, A. R.; Johansson, B.; Hultman, L.; Farle, M.; Barsoum, M. W.; Rosen, J. Atomically Layered and Ordered Rare-Earth i-MAX Phases: A New Class of Magnetic Quaternary Compounds. Chem. Mater. 2019, 31, 2476−2485. (9) Petruhins, A.; Lu, J.; Hultman, L.; Rosen, J. Synthesis of Atomically Layered and Chemically Ordered Rare-Earth (RE) i-MAX Phases; (Mo2/3RE1/3)2GaC with RE= Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Mater. Res. Lett. 2019, 7, 446−452. (10) Dahlqvist, M.; Lu, J.; Meshkian, R.; Tao, Q.; Hultman, L.; Rosen, J. Prediction and Synthesis of a Family of Atomic Laminate Phases with Kagomé-Like and In-Plane Chemical Ordering. Sci. Adv. 2017, 3, No. e1700642. (11) Ashton, M.; Hennig, R. G.; Broderick, S. R.; Rajan, K.; Sinnott, S. B. Computational Discovery of Stable M2AX Phases. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 94, 054116. (12) Dahlqvist, M.; Petruhins, A.; Lu, J.; Hultman, L.; Rosen, J. The Origin of Chemically Ordered Atomic Laminates (i-MAX); Expanding the Elemental Space by a Theoretical/Experimental Approach. ACS Nano 2018, 12, 7761−7770. (13) Rajan, A. C.; Mishra, A.; Satsangi, S.; Vaish, R.; Mizuseki, H.; Lee, K.-R.; Singh, A. K. Machine-Learning Assisted Accurate Band Gap Predictions of Functionalized MXene. Chem. Mater. 2018, 30, 4031− 4038. (14) Frey, N. C.; Wang, J.; Vega Bellido, G. I. n.; Anasori, B.; Gogotsi, Y.; Shenoy, V. B. Prediction of Synthesis of 2D Metal Carbides and Nitrides (MXenes) and their Precursors with Positive and Unlabeled Machine Learning. ACS Nano 2019, 13, 3031−3041. (15) Sun, W.; Shah, S.; Chen, Y.; Tan, Z.; Gao, H.; Habib, T.; Radovic, M.; Green, M. Electrochemical Etching of Ti2AlC to Ti2CTx (MXene) in Low-Concentration Hydrochloric Acid Solution. J. Mater. Chem. A 2017, 5, 21663−21668. (16) Li, M.; Lu, J.; Luo, K.; Li, Y.; Chang, K.; Chen, K.; Zhou, J.; Rosen, J.; Hultman, L.; Eklund, P.; Persson, P. O. Å.; Du, S.; Chai, Z.; Huang, Z.; Huang, Q. An Element Replacement Approach by Reaction with Lewis Acidic Molten Salts To Synthesize Nanolaminated MAX Phases and MXenes. J. Am. Chem. Soc. 2019, 141, 4730−4737. (17) Cheng, L.; Wang, X.; Gong, F.; Liu, T.; Liu, Z. 2D Nanomaterials for Cancer Theranostic Applications. Adv. Mater. 2019, 1902333. (18) Sarycheva, A.; Polemi, A.; Liu, Y.; Dandekar, K.; Anasori, B.; Gogotsi, Y. 2D Titanium Carbide (MXene) for Wireless Communication. Sci. Adv. 2018, 4, No. eaau0920.
To date, more than 70% of all MXene research has focused on the first discovered MXene, Ti3C2Tx. The exploration of this MXene is so extensive that, for many researchers, the name MXene has become synonymous with Ti3C2Tx, and they use it without specifying the composition, which can be confusing as there are numerous structures and compositions of MXenes. At least 100 stoichiometric MXene compositions and a limitless number of solid solutions offer not only unique combinations of properties but also a way to tune them by varying ratios of M or X elements. The large, underexplored family of MXenes and their unique combination of properties opens the door to a variety of different applications, and the possibilities of new compositions make us believe that we are still in the early days of MXene research and many exciting discoveries are to come.
Yury Gogotsi,* Associate Editor
Babak Anasori*
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Indiana University−Purdue University Indianapolis
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Yury Gogotsi: 0000-0001-9423-4032 Babak Anasori: 0000-0002-1955-253X Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS.
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ACKNOWLEDGMENTS Y.G. would like to thank the U.S. Department of Energy for continuous funding of his research on MXenes from the initial discovery (Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies under Contract No. DE-AC02-05CH11231, subcontract 6951370 under the Batteries for Advanced Transportation Technologies (BATT) Program) to exploration of energy-related applications via the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the Office of Science, Office of Basic Energy Sciences. Research of Y.G. and B.A. on fundamental properties of MXenes is currently supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Grant No. DESC0018618.
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REFERENCES
(1) Zhang, H. Ultrathin Two-Dimensional Nanomaterials. ACS Nano 2015, 9, 9451−9469. (2) Wee, A. T. S.; Hersam, M. C.; Chhowalla, M.; Gogotsi, Y. An Update from Flatland. ACS Nano 2016, 10, 8121−8123. 8494
DOI: 10.1021/acsnano.9b06394 ACS Nano 2019, 13, 8491−8494
