Structural and electronic properties of the titanium carbide MXene with variable sublattice oxygen composition

MXenes are two-dimensional materials that are traditionally composed of transition metal layers intercalated by carbon (carbides) or nitrogen (nitrides) atomic layers. Recently, it was experimentally demonstrated that most, if not all, carbide MXenes synthesized to date are in fact oxycarbides. This includes the titanium carbide (Ti3C2) MXene that is deeply analyzed in this work using a first-principles approach based on density functional theory (DFT). In particular, we studied the stability of the substitutional oxygen defect and clusters thereof in this MXene. In agreement with the experimental suggestions, we found that the oxycarbide forms (Ti3C2-xOx, x ∈ [0, 2]) are structurally and energetically very stable, at least up to 75 % replacement of the carbon atoms with oxygen. If all the carbon is replaced, the surface transitions to a different geometry, in which its atomic layers are aligned in an ABA fashion. Substitutional O clusters in Ti3C2 impose almost no lattice strain, even at high O concentrations, explaining the delay in the identification of oxycarbide MXenes. We found that the Ti3C2-xOx MXenes display metallic behaviour regardless of the oxygen concentration on the carbon layer.