How Much Oxygen Can a MXene Surface Take Before It Breaks?

Abstract Tuning and tailoring of surface terminating functional species hold the key to unlock unprecedented properties for a wide range of applications of the largest 2D family known as MXenes. However, a few routes for surface tailoring are explored and little is known about the extent to which the terminating species can saturate the MXene surfaces. Among available terminations, atomic oxygen is of interest for electrochemical energy storage, hydrogen evolution reaction, photocatalysis, etc. However, controlled oxidation of the surfaces is not trivial due to the favored formation of oxides. In the present contribution, single sheets of Ti 3 C 2 T x MXene, inherently terminated by F and O, are defluorinated by heating in vacuum and subsequentially exposed to O 2 gas at temperatures up to 450 °C in situ, in an environmental transmission electron microscope. Results include exclusive termination by O on the MXene surfaces and eventual supersaturation ( x > 2) with a retained MXene sheet structure. Upon extended O exposure, the MXene structure transforms into TiO 2 and desorbs surface bound H 2 O and CO 2 reaction products. These results are fundamental for understanding the oxidation, the presence of water on MXene surfaces, and the degradation of MXenes, and pave way for further tailoring of MXene surfaces.