The role of oxygen in the synthesis of the Mo2CTx MXene from the Mo2Ga2C MAX phase

Abstract

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.