Investigation of vacancy-ordered Mo1.33C MXene from first principles and x-ray photoelectron spectroscopy

MXenes are a comparatively young class of 2D materials, composed of transition-metal carbides/nitrides of the general formula ${\mathrm{M}}_{n+1}{\mathrm{X}}_{n}{\mathrm{T}}_{x}$, where T represents surface terminations, typically O, OH, and/or F. Recently, a new type of MXene with vacancy ordering was discovered, $\mathrm{M}{\mathrm{o}}_{1.33}\mathrm{C}{\mathrm{T}}_{x}$, with conduction and capacitance superior to the MXene counterpart without vacancies, $\mathrm{M}{\mathrm{o}}_{2}\mathrm{C}{\mathrm{T}}_{x}$. We here present a theoretical evaluation of $\mathrm{M}{\mathrm{o}}_{1.33}\mathrm{C}{\mathrm{T}}_{x}$ based on first-principles calculations, where $x=2$ and T is O, F, OH, or a mixture thereof. In addition to structural evaluation upon vacancy formation, we identify preferred terminations as well as termination sites, and resulting dynamical stability and electronic properties. For mixed terminations, the mixing energy is evaluated. We show that while $\mathrm{M}{\mathrm{o}}_{2}\mathrm{C}$ is typically O terminated, mixed terminations with a high F content are suggested for $\mathrm{M}{\mathrm{o}}_{1.33}\mathrm{C}{\mathrm{T}}_{x}$, which in turn gives the highest metallicity out of all the configurations investigated. In addition, the results indicate a strong tuning potential of the band gap through choice of terminations, with an electronic structure changing between insulating and metallic depending on termination(s) and their configuration. We also performed x-ray photoelectron spectroscopy to identify and quantify the terminating species on the MXene, as well as their respective binding energies. The experimental results are consistent with the theoretical analysis, and combined they suggest an explanation to the MXene chemistry as well as the reported high conductivity of $\mathrm{M}{\mathrm{o}}_{1.33}\mathrm{C}{\mathrm{T}}_{x}$.