Surface Chemistry-Driven Oxidation Mechanisms in Ti3C2T x MXenes.

Ti3C2T x 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 x , 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 in situ Raman spectroscopy and in situ microwave conductivity measurements, applied to Ti3C2T x 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 x , 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.