Disclosing Trends on C(1s) Core-Level Binding Energies of MXene Flakes: Metal, Size and Edge Effects

The C-(1s) core-level binding energies (CLBE) of realistic MXene finite systems were investigated using density functional theory (DFT). The ΔSCF approach was employed to explicitly account for final state effects. MXene flakes with M₂CT _x stoichiometry (M = Sc, Y, Ti, Zr, Hf, V, Nb, and Ta, where T _x denotes surface termination), consisting of 18 stoichiometric units, were constructed in both pristine (M₂C)₁₈ and O-functionalized (M₂CO₂)₁₈ forms. CLBEs were computed for all C atoms in each flake using the PBE functional, which was found to correlate linearly with results from the more accurate hybrid PBE0 functional. An increase in CLBEs is observed across periods, driven by decreasing atomic radii and increasing metal electronegativity, whereas trends along groups follow a behavior similar to that of first ionization potentials. O-functionalized flakes exhibit higher CLBEs due to the electronegative effect and the structural distortion caused by the O termination. This enables a clear distinction between pristine and O-terminated materials in XPS spectra. In addition, the flake size effect on central carbons in (Ti₂C) _n and (Ti₂CO₂) _n flakes, with n up to 216, is negligible, supporting the extrapolation of the present results for larger systems. Overall, the C-(1s) XPS spectra of MXene flakes are expected to be centered around the CLBE of the central C atom, with peak widths determined by the difference between the maximum and minimum CLBE values.