Two-dimensional, ordered, double transition metals carbides (MXenes)

We use[1] density functional theory to predict the existence of two new families of 2D ordered carbides (MXenes), M'$_2$M''C$_2$ and M'$_2$M''$_2$C$_3$, where each M is a different early transition metal. Synthesizing Mo$_2$TiC$_2$T$_x$, Mo$_2$Ti$_2$C$_3$T$_x$, and Cr$_2$TiC$_2$T$_x$ (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 Mo$_2$TiC$_2$T$_x$ and Ti$_3$C$_2$T$_x$. This work further expands the family of 2D materials, offering additional choices of structures, chemistries, and ultimately useful properties. [1] B. Anasori et al. ACS Nano {\bf 9} 9507 (2015). DOI: 10.1021/acsnano.5b03591