A first-principles study of multilayer Ti ₃ C ₂ T _x MXene model

We proposed a more realistic albeit slightly complicated multilayer Ti₃C₂T_x model and performed a comprehensive theoretical study of its structural and electronic properties. In this work, we constructed various multilayer Ti₃C₂T_x structures considering different concentrations of hydrofluoric acid (HF; 5, 10, and 48 wt%) as the etchant. The validity of our ternary mixed O/OH/F-terminated Ti₃C₂T_x multilayer models is confirmed by the consistency of the calculated d-spacing (9.60 ± 0.07 Å), simulated X-ray diffraction (XRD) spectra and the predicted adhesion energy (0.77 ± 0.15 J m^-2) with the reported experimental measurements. The uniform terminated and mixed terminated multilayer Ti₃C₂T_x exhibit metallic characteristics, similar to those of monolayer Ti₃C₂T_x. We found a stronger interaction between the interlayers with OH-rich ternary mixed terminated Ti₃C₂T_x surfaces, due to the formation of hydrogen bonds between the hydroxyl groups and adjacent layers of F/O terminal groups as supported by the crystal orbital Hamilton population (COHP) calculation. From this finding, we propose that multilayer Ti₃C₂T_x etched with a strong HF acid could be easily exfoliated into monolayer sheets due to smaller adhesion energy. Based on this work, we believe that the current findings will offer a fundamental understanding and a useful baseline multilayer model for the future investigation of the hydrogen and ion storage and diffusion properties in the MXene multilayer application.