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 $\left(D_{3d}\right)$ 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 ∼1 eV, and in ${\mathrm{N}}_2$ dissociation energy barriers of up to ∼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.

Figure 1

Top (left panels) and side (right panels) views of $p$(3 × 3) MXene supercells. The top images represent ABC stacking, while the bottom ones feature ABA stacking. Green (gray) spheres denote M (X) atoms. Tags are shown for high-symmetry relevant surface sites, including bridge (B), top (T), hollow (H), hollow metal (${\mathrm{H}}_{\mathrm{M}}$), and hollow carbon/nitrogen (${\mathrm{H}}_{\mathrm{X}}$, in practice ${\mathrm{H}}_{\mathrm{C}}$ or ${\mathrm{H}}_{\mathrm{N}}$) positions. The dashed orange rhombus represents the boundaries of the periodic supercell.

Figure 2

Plots of $\mathrm{\Delta }{E}_{\mathrm{stack}}$ as a function of the M element for several MXene widths, $n$, X compositions —C in black, N in blue— with and without O termination. The dashed red line denotes the equal energetic stability of ABC and ABA stacking, with negative values indicating preference for the latter.

Figure 3

Side views of the sliding steps taken by the ${\mathrm{W}}_2\mathrm{C}$ (top), ${\mathrm{W}}_3{\mathrm{C}}_2$ (middle), and ${\mathrm{W}}_4{\mathrm{C}}_3$ (bottom) MXene surfaces in their transition from ABC to ABA stacking. Below each transition step, the values of the reaction step energy change, Δ$E$, and its energetic barrier, $E_{\mathrm{b}}$, are given in eV and per formula unit.

Figure 4

Side view of the sliding steps taken by $\mathrm{C}{\mathrm{r}}_2\mathrm{C}{\mathrm{O}}_2$ MXene surface during its ABC→ABA stacking transition. Below each transition step, the values of the reaction step energy change, Δ$E$, and its energetic barrier, $E_{\mathrm{b}}$, are given in eV.

Figure 5

Side views of the optimized ${\mathrm{W}}_2\mathrm{N}{\mathrm{O}}_x$ MXene (0001) surface, with ABC (top) or ABA (bottom) stacking, and with $N_{\mathrm{O}}=0$ (left), $N_{\mathrm{O}}=1$ (middle), both with $C_{3v}$ symmetry, and $N_{\mathrm{O}}=2$ (right), with $C_{1h}$ symmetry. Note the severe surface distortion that occurs when ${\mathrm{W}}_2\mathrm{N}$ with ABC stacking and one surface fully O-covered, adsorbs two O atoms on the other side.

Figure 6

Side views of a ${\mathrm{W}}_2\mathrm{N}{\mathrm{O}}_2$ MXene surface with ABC (left) and ABA (right) stacking. Dashed lines are guides to the eye; notice the asymmetry of the hollow sites occupied by the O atoms in the case of ABA.

Figure 7

Gibbs free adsorption energies, $G_{\mathrm{ads}}$, as a function of temperature, $T$, for each fraction of O coverage (from the clean surface in black to the full monolayer of O adatoms in red), at a constant pressure of 1 bar (left). The $T/p_{{\mathrm{O}}_2}$ phase diagram (right) showing the oxidation transition zone (pink) for ${\mathrm{W}}_2\mathrm{N}$, separating the pristine (white area) and the fully covered (red) surface regions.