The mechanism of room-temperature oxidation of a HF-etched Ti₃C₂T_x MXene determined via environmental transmission electron microscopy and molecular dynamics

The oxidation chemistry of two-dimensional transition metal carbide MXenes has brought new research significance to their protection and application. However, the oxidation behavior and degradation mechanism of MXenes, in particular with time under oxygen conditions at room temperature, remain largely unexplored. Here, several experimental and theoretical techniques are used to determine a very early stage of the oxidation mechanism of HF-etched Ti3C2Tx (a major member of MXenes and T-x = surface functional groups) in an oxygen environment at room temperature. Aberration-corrected environmental transmission electron microscopy coupled with reactive molecular dynamics simulations show that the crystal plane-dependent oxidation rate of Ti3C2Tx and oxide expansion are attributed to differences in the coordination and charge of superficial Ti atoms, and the existence of the channels between neighboring MXene layers on the different crystal planes. The complementary x-ray photoelectron spectroscopy and Raman spectroscopy analyses indicate that the anatase and a tiny fraction of brookite TiO2 successively precipitate from the amorphous region of oxidized Ti3C2Tx, grow irregularly and transform to rutile TiO2. Our study reveals the early-stage structural evolution of MXenes in the presence of oxygen and facilitates further tailoring of the MXene performance employing oxidation strategy.