MXenes atomic layer stacking phase transitions and their chemical activity consequences

Two-dimensional (2D) transition-metal nitrides and carbides (MXenes), containing a few atomic layers only, are novel materials which have become a hub of research in many applied technological fields, ranging from catalysis, to environmental scrubber materials, up to batteries. MXenes are obtained by removing the A element from precursor MAX phases, and it is for this reason that it is often assumed that the resulting 2D material displays the MAX atomic layer stacking---an ABC sequence with trigonal $({D}_{3d})$ symmetry. By means of density functional theory calculations, including dispersion, this work thoroughly explores the stability of alternative ABA stacking, with ${D}_{3h}$ hexagonal symmetry, for a total of 54 MXene materials with ${\mathrm{M}}_{2}\mathrm{X}, {\mathrm{M}}_{3}{\mathrm{X}}_{2}$, and ${\mathrm{M}}_{4}{\mathrm{X}}_{3}$ stoichiometries ($\mathrm{M}=\mathrm{Ti}$, Zr, Hf, V, Nb, Ta, Cr, Mo, or W; and $\mathrm{X}=\mathrm{C}$ or N), revealing that for clean MXenes, the ABA stacking is fostered (i) by the number of $d$ electrons in M, (ii) when $\mathrm{X}=\mathrm{N}$ rather than $\mathrm{X}=\mathrm{C}$, and (iii) when the surface is terminated by oxygen adatoms. The results suggest that stacking phase transitions are likely to take place under working operando conditions, surmounting affordable layer sliding energy barriers, in accordance with the experimentally observed layer distortions in $\mathrm{M}{\mathrm{o}}_{2}\mathrm{N}$. Finally, we tackled the adsorptive and catalytic capabilities implications of such layer phase transition by considering ${\mathrm{N}}_{2}$ adsorption, dissociation, and hydrogenation on selected ABC and ABA stacked MXenes. Results highlight changes in adsorption energies of up to \ensuremath{\sim}1 eV, and in ${\mathrm{N}}_{2}$ dissociation energy barriers of up to \ensuremath{\sim}0.3 eV, which can critically change the reaction step rate constant by three to four orders of magnitude for working temperatures in the 400--700 K range. Consequently, it is mandatory to carefully determine the atomic structure of MXenes and to use models with the most stable stacking when inspecting their chemical or physical properties.