Synthesis of Mo₄VAlC₄ MAX Phase and Two-Dimensional Mo₄VC₄ MXene with Five Atomic Layers of Transition Metals

MXenes are a family of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides with a general formula of M_<i>n</i>+1X_<i>n</i>T_<i>x</i>, in which two, three, or four atomic layers of a transition metal (M: Ti, Nb, V, Cr, Mo, Ta, etc.) are interleaved with layers of C and/or N (shown as X), and T_<i>x</i> represents surface termination groups such as -OH, ═O, and -F. Here, we report the scalable synthesis and characterization of a MXene with five atomic layers of transition metals (Mo₄VC₄T_<i>x</i>), by synthesizing its Mo₄VAlC₄ MAX phase precursor that contains no other MAX phase impurities. These phases display twinning at their central M layers which is not present in any other known MAX phases or MXenes. Transmission electron microscopy and X-ray diffraction were used to examine the structure of both phases. Energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and high-resolution scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy were used to study the composition of these materials. Density functional theory calculations indicate that other five transition metal-layer MAX phases (M'₄M″AlC₄) may be possible, where M' and M″ are two different transition metals. The predicted existence of additional Al-containing MAX phases suggests that more M₅C₄T_<i>x</i> MXenes can be synthesized. Additionally, we characterized the optical, electronic, and thermal properties of Mo₄VC₄T_<i>x</i>. This study demonstrates the existence of an additional subfamily of M₅X₄T_<i>x</i> MXenes as well as a twinned structure, allowing for a wider range of 2D structures and compositions for more control over properties, which could lead to many different applications.