The role of oxygen in the synthesis of the Mo₂CT_<i>x</i> MXene from the Mo₂Ga₂C MAX phase

MAX phases are layered materials that have been known for over 60 years and MXenes are their metaphorical children, a family of two-dimensional (2D) materials that appeared in 2011. MXenes are often synthesised by etching out the layers of the A element of their parent MAX phases. The remaining 2D MXene layers are very reactive and spontaneously become covered by a layer called the surface termination. The Mo₂C MXene was first synthesised in 2015, by using hydrofluoric acid to remove the Ga layers from the Mo₂Ga₂C MAX phase, and its surface termination was determined to be mostly composed by O groups. While the layers of Mo and C in Mo₂Ga₂C are arranged in a face-centred cubic manner, those of Mo₂C prefer a hexagonal close-packed stacking. This implies that, at some point during the etching process, a phase transition should occur because of the replacement of Ga by O. Herein, we employ density-functional theory calculations to analyse the thermodynamics of this process by assessing the stability of the two stacking phases of the Mo₂Ga₂C MAX phase, as the Ga atoms are gradually replaced by O ones. Among other results, we identify the preferred locations of O and F atoms in the Mo₂Ga₂C lattice, propose a possible explanation for the greater presence of O atoms over F ones on the surface of as-synthesised Mo₂C MXenes, evaluate the interaction between O atoms in Mo₂Ga₂C crystals, estimate the energy of Ga-Ga bonds, and lastly, show that the relative stability of the two stacking phases of Mo₂Ga₂C is reversed when approximately half of the Ga content is replaced by O.