posted on 2017-03-15, 00:00authored byTeck Leong Tan, Hong Mei Jin, Michael B. Sullivan, Babak Anasori, Yury Gogotsi
2D transition metal carbides and
nitrides known as MXenes are gaining
increasing attention. About 20 of them have been synthesized (more
predicted) and their applications in fields ranging from energy storage
and electromagnetic shielding to medicine are being explored. To facilitate
the search for double-transition-metal MXenes, we explore the structure–stability
relationship for 8 MXene alloy systems, namely, (V1–xMox)3C2, (Nb1–xMox)3C2, (Ta1–xMox)3C2, (Ti1–xMox)3C2, (Ti1–xNbx)3C2, (Ti1–xTax)3C2, (Ti1–xVx)3C2, and (Nb1–xVx)3C2, with 0 ≤ x ≤ 1, using high-throughput computations. Starting
from density-functional theory calculated formation energies, we used
the cluster expansion method to build quick-to-compute interactions,
enabling us to scan through the formation energies of millions of
alloying configurations. For the Mo-rich MXenes, (M11–xMox)3C2 (where M1: Ti, V, Nb, Ta) Mo atoms prefer to occupy the surface
layers, and ordering persists to high temperatures, based on our Monte
Carlo simulations. When Ti is alloyed with Nb or Ta, in the Ti-rich
MXenes, Ti atoms prefer the surface layers (e.g., Ti–C–Nb–C–Ti
sequence), and in the Nb- or Ta-rich MXenes, Ti occupies only one
surface layer and the other two layers are Nb or Ta (e.g., Ti–C–Nb–C–Nb),
exhibiting asymmetric ordering. However, alloying Ti with V results
in solid solutions across all compositions. (Nb1–xVx)3C2 phase separates at lower temperatures but forms solid solutions
at synthesis temperatures. Postsynthesis annealing at moderate temperatures
(800 to 1000 K) increases the ordering for all the compositions. Lastly,
by investigating the stability of their precursor MAX phases and surface-terminated
MXenes, we discuss the synthesis possibilities of highly ordered MXenes.