Identifying the Atomic Layer Stacking of Mo₂C MXene by Probe Molecule Adsorption

Abstract

A density functional theory study is presented here aimed at investigating whether the atomic stacking on the new family of two-dimensional MXene materials has an influence on their adsorption properties and whether these properties can provide information about this structural feature. To this end, the Mo 2 C MXene, exhibiting two nearly degenerate crystal structures with either ABC or ABA atomic stacking, is chosen as a case study. The study of the adsorption of CO, CO 2 , and H 2 O on both polymorphs of Mo 2 C reveals substantial differences that could be used in experiments to provide information about the atomic stacking of a given sample. Particularly, we show that the asymmetric and symmetric stretching modes of the adsorbed CO 2 and the CO stretching mode are clear features that allow one to identify the stacking of atomic layers of the Mo 2 C MXene. The present finding is likely to apply to other MXenes as well.

Bandgap engineering of MXene compounds for water splitting

“…41 The core electrons and their interaction with the valence electrons were represented using projector augmented wave (PAW) pseudopotentials, 42 while the valence electrons were described using a plane wave basis set with an optimal kinetic energy cutoff of 415 eV, as previously tested and validated. 16,17,26,27 Spin-polarized calculations were carried out using the Perdew-Burke-Ernzerhof (PBE) 43 exchange-correlation functional within the generalized gradient approximation (GGA), 44 with additional calculations applying the PBE0 hybrid functional for a more reliable bandgap estimation. 45 The convergence criteria for the optimizations were set at 10 −6 eV for the self-consistent eld steps in all cases, and optimizations were considered converged when forces acting on atoms were less than 0.01 eV Å −1 .…”

“…On top of that, MXenes do not necessarily have to follow the regular ABC stacking inherited from the precursor MAX phases, and some pristine and O-terminated MXenes, with T x = O 2 , have been shown to prefer to adopt an ABA stacking, 26 which influences their electronic properties. 27 Thus, in the interest of determining which MXenes are good candidates for possible photocatalytic applications, e.g. water splitting, the bandgap and band structure of these compounds must be inspected as a function of composition, stacking, and termination.…”

Effect of oxygen termination on the interaction of first row transition metals with M2C MXenes and the feasibility of single-atom catalysts

“…Finally, it must also be noted that a different ABA stacking is possible for the MXene central layers, which sometimes is more favorable than that inherited from the MAX phase and may have an impact on the properties of the adsorbed molecules. 60,61 In the present work, only the ABC stacking of the M 2 C MXenes was considered.…”

MXenes à la Carte: Tailoring the Epitaxial Growth Alternating Nitrogen and Transition Metal Layers

“…A set of MXene seeds with M 2 X formula has been studied, including M elements from groups IV (Ti, Zr, Hf), V (V, Nb, Ta), and VI (Cr, Mo, W), whereas X = C or N, totaling 18 M 2 X MXenes. Their basal (0001) surfaces have been described through p (3×3) supercell slab systems, as done in former studies. ,, In all calculations, the MXene atomic positions as well as those of adsorbate atoms or molecules were fully optimized by total energy minimization. The employed supercells have a vacuum length along the (0001) basal cell vector of at least 10 Å, enough to practically nullify the interactions among replicated slabs or the adsorbates.…”