Reactivity\nof Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> MXene\nwith Atomic Hydrogen: Tuning of Surface Terminations\nby Halogen Removal and Reversible O to OH Conversion

Two-dimensional transition metal carbides or nitrides,\nso-called\nMXenes, hold the prospect of a proactive emergence as innovative catalysts\nand device components owing to the specific qualities gained from\nthe chemical species that functionalize the layers. Tuning the nature\nand the number of the surface terminations becomes the key factor\nfor the effective use of MXenes in technology. This study explores\nthe capability of H atoms to modify the surface composition of Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> flakes. While\nexposing the sample at room temperature to H atoms, the change of\nits surface chemical state is followed by synchrotron radiation X-ray\nphotoelectron spectroscopy. It turns out that halogen terminations\nare progressively and substantially removed. In parallel, the O terminations\nare partially converted into OH groups, the O/OH ratio being possibly\ncontrolled by the OH–OH repulsion. The dramatic surface composition\nchange leaves the valence state of the Ti atoms almost unchanged.\nDensity functional theory simulations of the valence band spectra\nfor different Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> model systems identify all spectral features and model the change\nof the electronic properties around the Fermi level. Heating the hydrogenated\nsample to 400 K removes the OH groups, leaving the MXene surface deprived\nof most of the pristine terminations, thus giving way to new, application-oriented\nfunctionalization.