Reactivity of Ti₃C₂T_<i>x</i> MXene with Atomic Hydrogen: Tuning of Surface Terminations by Halogen Removal and Reversible O to OH Conversion

Two-dimensional transition metal carbides or nitrides, so-called MXenes, hold the prospect of a proactive emergence as innovative catalysts and device components owing to the specific qualities gained from the chemical species that functionalize the layers. Tuning the nature and the number of the surface terminations becomes the key factor for the effective use of MXenes in technology. This study explores the capability of H atoms to modify the surface composition of Ti3C2Tx flakes. While exposing the sample at room temperature to H atoms, the change of its surface chemical state is followed by synchrotron radiation X-ray photoelectron spectroscopy. It turns out that halogen terminations are progressively and substantially removed. In parallel, the O terminations are partially converted into OH groups, the O/OH ratio being possibly controlled by the OH–OH repulsion. The dramatic surface composition change leaves the valence state of the Ti atoms almost unchanged. Density functional theory simulations of the valence band spectra for different Ti3C2Tx model systems identify all spectral features and model the change of the electronic properties around the Fermi level. Heating the hydrogenated sample to 400 K removes the OH groups, leaving the MXene surface deprived of most of the pristine terminations, thus giving way to new, application-oriented functionalization.