Surface Chemistry–Driven Oxidation Mechanisms in Ti₃C₂T_<i>x</i> MXenes

Ti₃C₂T _<i>x</i> is a leading compound within the MXenes family and can find host in widespread applications. It is synthesized by selectively etching layers from the Ti₃AlC₂ precursor, and this process typically introduces surface terminations, T _<i>x</i> , such as -OH, =O, or -F. However, the aggressive chemical conditions required for its preparation, as well as exposure to air, humidity, and heat, can lead to impurity phases that potentially compromise its desirable properties. Herein, a two-step oxidation process is revealed during heat treatment, where initial oxidation occurs between layers without altering the integrity of the Ti₃C₂-layered structure, followed by the formation of anatase TiO₂ at elevated temperatures. The process is carefully monitored using <i>in situ</i> Raman spectroscopy and <i>in situ</i> microwave conductivity measurements, applied to Ti₃C₂T _<i>x</i> prepared using various etching techniques involving concentrated hydrofluoric acid, LiF + HCl, and HF + HCl mixtures. The oxidation process is heavily influenced by the synthesis route and surface chemistry of Ti₃C₂T _<i>x</i> , with fluoride and oxyfluoride groups playing a pivotal role in stabilizing the anatase phase. The absence of these groups, in contrast, can lead to the formation of rutile TiO₂.