Two-Dimensional, Ordered, Double Transition Metals Carbides (MXenes)

TLDR

Two-dimensional materials with greater chemical diversity and structural complexity are more likely to exhibit unique properties; in the studied MXenes, outer Mo and Cr layers sandwich inner carbide layers, controlling the flakes’ chemical and electrochemical behavior. The study aims to predict two new families of ordered 2D carbide MXenes, M'₂M″C₂ and M'₂M″₂C₃, using density functional theory. The authors employed density functional theory to predict the new MXenes and then synthesized Mo₂TiC₂Tₓ, Mo₂Ti₂C₃Tₓ, and Cr₂TiC₂Tₓ to experimentally confirm the predictions. Experimental validation revealed distinct electrochemical behavior between Mo₂TiC₂Tₓ and Ti₃C₂Tₓ, demonstrating that the new MXenes expand the 2D material family with diverse structures, chemistries, and potential properties.

Abstract

The higher the chemical diversity and structural complexity of two-dimensional (2D) materials, the higher the likelihood they possess unique and useful properties. Herein, density functional theory (DFT) is used to predict the existence of two new families of 2D ordered, carbides (MXenes), M'2M″C2 and M'2M″2C3, where M' and M″ are two different early transition metals. In these solids, M' layers sandwich M″ carbide layers. By synthesizing Mo2TiC2Tx, Mo2Ti2C3Tx, and Cr2TiC2Tx (where T is a surface termination), we validated the DFT predictions. Since the Mo and Cr atoms are on the outside, they control the 2D flakes' chemical and electrochemical properties. The latter was proven by showing quite different electrochemical behavior of Mo2TiC2Tx and Ti3C2Tx. This work further expands the family of 2D materials, offering additional choices of structures, chemistries, and ultimately useful properties.