The Future of MXenes

Abstract

W e are entering the era of new materials: materials that can be assembled from nanoscale building blocks, unlike all previous material generations from the Stone Age to the Silicon Age.Two-dimensional (2D) materials provide nanometer and subnanometer-thin "bricks" for such assembly.If needed, organic molecules and polymers can serve as mortar, but van der Waals (vdW) or electrostatic forces can also provide a strong bonding between the 2D layers.To make this vision real and start assembling materials, structures, and devices from nanoparticles, we need many building blocks with a large variety of physical and chemical properties.The separation of single layers of graphene and demonstration of unique physics in this atomically thin sp 2 carbon layer attracted attention to other 2D materials.Initially, known vdW-bonded materials such as MoS 2 and boron nitride (BN) were explored, but later 2D layers were created from elements like Si, Ge, or B and compounds that do not have weakly bonded layered precursors.MXenes belong to the latter group. 1 They are carbides, nitrides, oxycarbides, and carbonitrides of early transition metals, such as Ti, Nb, Mo, etc. (Figure 1).However, they occupy a special place in the large class of 2D materials for several reasons.First, they come in a large variety of structures with 2-5 layers of early transitional metal atoms (M elements) connected by 1-4 layers of nonmetal atoms (X = C, N, O).Moreover, multiple M metals can be combined in one structure, forming in-plane and out-of-plane ordered MXenes or random solid solutions, including high-entropy MXenes with 3-5 metals (Figure 2).The hexagonal structure of MXene is a (111) slice of the cubic NaCl structure of bulk cubic carbide/nitride.The surface of MXenes can be terminated with oxygen, OH, amines, halogens, and chalcogens; antimony and phosphorus were added to this group recently. 2Considering a dozen transition metals, carbon and nitrogen, four archetypical MXene structures, and the known monatomic surface terminations, at least a thousand stoichiometric compositions may be possible.With solid solutions on M and X sites and mixed surface terminations, an infinite number of 2D materials can be created in this system. 3hus, MXenes are taking us into the era of atomistic design and chemical assembly of new 2D materials.This was only possible in the world of organic molecules in the past, including covalent organic frameworks (COF) and metalorganic frameworks (MOFs).More than 50 MXenes have been reported to date.Considering solid solutions with various ratios of elements and distinct surface terminations defining MXene properties, the number of experimentally created MXenes probably approaches a hundred.Advances in computational materials science, machine learning, and

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