Double transition-metal MXenes: Atomistic design of two-dimensional carbides and nitrides
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Introduction There has been significant interest in the design of two-dimensional (2D) materials since the characterization of single layer graphene in 20041 to meet rapidly evolving demands for advanced materials in technological applications, including energy storage,2–4 electronics,5 membranes,6,7 catalysts,8 and sensors.9,10 MXenes are a large family of 2D materials, discovered in 2011,11,12 which offer a unique combination of electronic,13–15 optical,16–18 mechanical,19–21 and colloidal properties.22,23 MXenes are few-atoms-thick 2D sheets with a general formula of Mn+1XnTx. In each MXene flake, n + 1 (n = 1–4) layers of a transition metal (M) are interleaved with n layers of carbon or nitrogen (X).24,25 The Tx in the formula represents surface terminations, including =O, –OH, –F, and –Cl, which are bonded to the outer M layers.24,26,27 MXenes have high metallic conductivity (up to 15,100 S⋅cm–1 in Ti3C2Tx
film form),28 are optically transparent (absorbing 3% of visible light/nm thickness),18 exhibit a high modulus of elasticity (330–400 GPa),19–21 and can act as electromagnetic interference shields29 and electrochemically active materials.24 MXenes synthesis is a top-down approach, by selectively etching the A-layers from three-dimensional (3D) crystalline layered carbides and nitrides,30,31 mostly MAX phases. In a MAX phase, Mn+1Xn layers are bonded with an atomic layer of an A-group element, which is usually a group 13 to 16 element (Al, Ga, Si, Ge, P, and As).32 Three types of MAX phase structures, M2AX, M3AX2, and M4AX3, are shown in Figure 1a. MAX phases are usually synthesized by reactive sintering of their elemental powder in stoichiometric ratios, such as 3M:1A:2C for M3AC2, at temperatures above 1300°C under controlled atmosphere.32 The M–A bonds are primary bonds, which makes mechanical exfoliation challenging.
Weichen Hong, Department of Mechanical and Energy Engineering and Integrated Nanosystems Development Institute, Purdue School of Engineering and Technology, Indiana University–Purdue University Indianapolis, USA; [email protected] Brian Wyatt, Department of Mechanical and Energy Engineering and Integrated Nanosystems Development Institute, Purdue School of Engineering and Technology, Indiana University–Purdue University Indianapolis, USA; [email protected] Srinivasa Kartik Nemani, Department of Mechanical and Energy Engineering and Integrated Nanosystems Development Institute, Purdue School of Engineering and Technology, Indiana University–Purdue University Indianapolis, USA; [email protected] Babak Anasori, Department of Mechanical and Energy Engineering and Integrated Nanosystems Development Institute, Purdue School of Engineering and Technology, Indiana University–Purdue University Indianapolis, USA; [email protected] doi:10.1557/mrs.2020.251
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