Unveiling Termination Preferences and Screening of Structural Space in Multi-Metal MXenes.

Abstract

MXenes are promising two-dimensional materials for energy storage, catalysis, and electronics; however, our atomistic understanding of the stability mechanisms that rule the stability and physicochemical properties of multiple-metal MXenes is far from satisfactory. In this study, we investigated the configurational space, structural parameters, energetic stability, and the electronic properties of the MXenes (<i>M'M</i> <i>″</i>) _<i>n</i>+1(<i>X'X</i> <i>″</i>) _<i>n</i> O₂ family, where M = Mo, Cr, Mn, Nb, V, Ti, Y, and X = C, N, B. We used density functional theory calculations within the Perdew-Burke-Ernzerhof functional including Hubbard corrections for the Cr, Mn, and V <i>d</i>-states. We identified a correlation between the magnitude of the occupation of the <i>M d</i> _{<i>z</i> ²} -states and the preferential occupation of the O-sites on the MXene surface. Specifically, a reduced occupation of the <i>d</i> _{<i>z</i> ²} -states leads to an energetic preference for the face-centered cubic sites, which is observed in most systems, except for Mo₂CO₂, CrMoNO₂, MoVCO₂ (Mo side), and MoNbNO₂, where a higher occupancy of the <i>d</i> _{<i>z</i> ²} -states promotes a preference for hexagonal close-packed sites. The in-plane configuration, that is, with metals or <i>X</i> mixed in the same layer, is more stable for MnNbCO₂, MoNbNO₂, NbYBO₂, Ti₃CNO₂, and Nb₃CNO₂ MXenes, while the out-of-plane configuration, that is, with <i>M</i> or <i>X</i> separated in different layers, minimized the total energy for MoVCO₂, CrMoNO₂, and Ti₂NbCNO₂. Furthermore, we found a clear correlation between the work function and surface area and the chemical composition. As expected, the majority of our compositions are metallic, which is advantageous for applications of these materials as electrodes, e.g., in electrochemistry applications.