2D MXenes Monolayers are More Thermally Stable than Multilayers

2D MXenes have demonstrated potential applications in optoelectronic devices. The thermal stability of these ultrathin films must be carefully considered due to ambient humidity, unavoidable heating caused by light absorption, current flow, <i>etc.</i> Here, we use in-situ spectroscopic ellipsometry to study Ti₃C₂T_x MXenes oxidation and evaluate the resultant structures using AFM. We demonstrate that degradation can be dramatically suppressed upon heating in ambient conditions, after various pre-heat treatments in a vacuum. Contrary to conventional wisdom, our work shows that Ti₃C₂T_x MXene monolayers are more thermally stable than their multilayer counterparts. We discuss how to improve thin film stability through thermal drying in a vacuum and establish the appropriate conditions for each type of film. Density functional theory calculations elucidate possible reasons for the improved stability of monolayers. Our findings provide important design rules for Ti₃C₂T_x-based devices, which is very important for the further market application of MXenes.