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 (M′M″)n+1(X′X″)nO2 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 d-states. We identified a correlation between the magnitude of the occupation of the M dz2-states and the preferential occupation of the O-sites on the MXene surface. Specifically, a reduced occupation of the dz2-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 dz2-states promotes a preference for hexagonal close-packed sites. The in-plane configuration, that is, with metals or X 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 M or X 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.