MXenes: Two-Dimensional Building Blocks for Future Materials and Devices Cite This: ACS Nano 2021, 15, 5775-5780 Read Online ACCESS Metrics & More Article Recommendations S ince the synthesis of Ti 3 C 2 was reported in 2011, 1 we have seen tremendous growth in research on synthesis, characterization, and applications of two-dimensional (2D) carbides and nitrides named MXenes. It was, in fact, an article in ACS Nano in 2012 that reported the syntheses of M 2 X, M 3 X 2 , and M 4 X 3 and announced the birth of an entirely new large family of 2D materials. 2 M 5 C 4 was reported in ACS Nano in 2020, further increasing the structural diversity of 2D carbides and carbonitrides. The general formula of MXenes is M n+1 X n T x , where M represents a transition metal, X represents carbon or nitrogen, n can be from 1 to 4, and T x indicates terminations on the surface of the outmost transition metal layers (Figure 1). 3 The possibility of in-plane and out-of-plane ordering of the metal atoms brings the number of possible structures to well over 100. 4 Surface terminations increase it by another order of magnitude, while the possibility of forming solid solutions on M and X sites, as well as mixed terminations, leads to a potentially unlimited number of 2D materials with distinct properties. ACS Nano is one of the leaders in publishing cutting-edge research on MXenes and guiding the research community through Editorials and Perspectives, 5-7 which outline future research directions. The number of papers on MXenes that we publish increases every year, proportional to the number of submissions received. For example, 18 papers reporting research on MXenes appeared in ACS Nano in the first 3 months of 2021. This virtual issue highlights many of the key findings in the field that have been published in ACS Nano. Of course, due to length limitations, we could only include a small portion of all the excellent MXene papers from our journal in this virtual issue. We emphasize the most cited and most frequently accessed recent publications, representing various methods of MXene synthesis, processing, property character- ization, and applications. In this Editorial accompanying the virtual issue, we introduce the variety of MXene compositions; describe their unique place in the materials world, their fundamental properties, and the parameters governing them; briefly summarize syntheses and control of surface termi- nations of MXenes; and discuss their promise in selected applications, as well as challenges in the field. ■ PLACE OF MXENES IN THE MATERIALS WORLD The discovery of fascinating properties in single- and few-layer graphene attracted attention to many other 2D materials. 8,9 After initial studies focusing on exfoliation of other van der Waals bonded solids, such as BN or transition metal dichalcogenides, 10 2D silicon and germanium structures that do not have van der Waals bonded layered precursors were demonstrated. 11-13 MXenes2D carbides and nitrides of transition metals that are produced by selective etching of strongly bonded layered solids, such as MAX phasesfollowed in 2011. 1 Why do MXenes stand out among so many already available 2D materials? MXenes provide 2D sheets with metallic electrical conductivity, a property that was largely missing from the palette of 2D materials, most of which are semiconductors, semimetals, or dielectrics. Moreover, MXenes offer reported conductivity values up to 20,000 S/cm, 14 higher strength and stiffness compared to other solution-processed 2D materials, scalable solution syntheses (kg batches), sufficient environmental stability for a large variety of applications, biocompatibility, and aqueous solution processing without surfactants. One can consider them to be hydrophilic, water-dispersible 2D metals or electrically conductive clay (MXenes’ rheological properties are similar to that of clay). 15 There are dozens of MXenes already available, and a potentially infinite number of compositions is possible. Moreover, further control of properties can be achieved by reversible chemical/electrochemical intercalation, applied potential, or illumination. MXenes show metallic conductivity, but their Fermi levels can be tuned by external stimuli, almost like in semiconducting materials. Those combinations of properties of MXenes are valuable for many applications. A combination of conductivity and redox ability enables energy storage, conductivity and catalytic ability allow electrocatalysis, conductivity plus transparency are needed for transparent conductors and heaters, conductivity combined with color enables photonic and optoelectronic devices, tunable plasmon resonance can be used in photo- thermal therapy, photocatalysis, and surface-enhanced Raman spectroscopy. 16 Published: April 27, 2021 Editorial www.acsnano.org Published 2021 by American Chemical Society 5775 https://doi.org/10.1021/acsnano.1c03161 ACS Nano 2021, 15, 5775-5780 Downloaded via 50.31.9.168 on September 10, 2021 at 11:26:21 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.