Atomistic Origin of Phase Stability in Oxygen-Functionalized MXene: A Comparative Study

Oxygen-functionalized MXene, M2CO2 (M = group III–V metals), are emergent formidable two-dimensional (2D) materials with a tantalizing possibility for device applications. Using first-principles calculations, we perform an intensive study on the stability of fully O-functionalized (M2CO2) MXenes. Depending on the position of O atoms, the M2CO2 can exist in two different structural phases. On one side of MXene, the O atom occupies a site which is exactly on the top of the metal atom from the opposite side. On the other side, the O atom can occupy either the site on the top of the metal atom of the opposite side (BB′ phase) or on the top of the C atom (CB phase). We find that for M = Sc and Y the CB phase is stable, whereas for M = Ti, Zr, Hf, V, Nb, and Ta the stable phase is BB′. The electron localization function, the atom-projected density of states, the charge transfer, and the Bader charge analyses provide a rational explanation for the relative stability of these two phases and justify the ground state structure by giving information about the preferential site of adsorption for the O atoms. We also calculate the phonon dispersion relations for both phases of M2CO2. The BB′-Sc2CO2 and the CB-Ti2CO2 are found to be dynamically unstable. Finally, we find that the instability of BB′-M2CO2 (M = Sc and Y) originates from the weakening of M–C interactions, which manifest as a phonon mode with imaginary frequency corresponding to the motion of C atom in the a–b plane. The insight into the stability of these competing structural phases of M2CO2 presented in this study is an important step in the direction of identifying the stable phases of these 2D materials.