The Rise of MXenes F or the past 15 years, starting with the discovery of the unique physical properties of single-layer graphene, two- dimensional (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 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 M n+1 X n T x . The T x 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 Published: August 27, 2019 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 out- of-plane). 5 Editorial www.acsnano.org Cite This: ACS Nano 2019, 13, 8491-8494 © 2019 American Chemical Society 8491 DOI: 10.1021/acsnano.9b06394 ACS Nano 2019, 13, 8491-8494 Downloaded via 107.173.52.20 on August 28, 2019 at 10:36:23 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.