Chemical Origin of Termination-Functionalized MXenes: Ti3C2T2 as a Case Study

MXenes represent a burgeoning family of two-dimensional (2D) functional materials with a variety of applications that highly rely on termination mediated surface functionalization, but the understanding of termination is limited. Here, we take Ti3C2T2 (T = O, F, OH, and H) as an example of MXenes, to demonstrate how termination stabilizes the Ti3C2 monolayer matrix by saturating the nonbonding valence electrons of the surface Ti atom by the low-energy orbitals of the termination. This is achieved by orbitally resolved density of states analysis by simply yet efficiently manipulating the internal coordination of the octahedral crystal field to match exactly the Cartesian coordination. Highly degenerate 3d orbitals of surface Ti split in such a way that it exhibits pseudogaps whose widths predict a stability order Ti3C2O2 > Ti3C2F2 > Ti3C2(OH)(2) > Ti3C2H2 > Ti3C2, consistent with Bader charge analysis, thermodynamic calculations, and experimental results. This new criterion could have implications in the general context of ubiquitous termination phenomena of MXenes and other relevant termination-functionalized 2D materials.