Unraveling MXene Oxidation Mechanisms Under Realistic Gas Environments by In Situ Gas‐Cell TEM

ABSTRACT The environmental instability of Ti 3 C 2 T x MXenes remains a critical bottleneck impeding their practical deployment in optoelectronics and energy storage. While oxidative degradation is known to compromise their performance, the atomic‐scale dynamics governing this process under realistic environments remain elusive. Here, we present a comprehensive in situ gas‐cell TEM study to visualize the oxidation pathways of Ti 3 C 2 T x under controlled O 2 and H 2 O vapor atmospheres. We identify two distinct, environment‐dependent oxidation mechanisms. In an O 2 environment, strong chemisorption drives a rapid, thermodynamically driven phase transformation characterized by severe lattice strain and the formation of a heterogeneous mixture of rutile and anatase TiO 2 . Conversely, water vapor induces a hydrolysis‐dominated process that initiates at a lower temperature (∼220°C) due to facile surface reactions, yet proceeds with notably sluggish, self‐limiting kinetics. This kinetically moderated pathway preferentially stabilizes a single‐phase anatase structure with a relaxed lattice, attributed to hydroxyl‐mediated passivation. Corroborated by DFT calculations and EELS, we establish an adsorption hierarchy (O 2 > H 2 O) that dictates the reaction barriers and phase evolution. These findings provide a fundamental understanding of MXene degradation, suggesting that excluding moisture is paramount for low‐temperature stability, while preventing oxygen exposure is critical during high‐temperature processing to preserve structural integrity.