Using Multiple M-Elements to Understand the Chemistry and Structure of MXenes

MXenes are potentially the largest class of 2D materials discovered so far. With a general formula of M_n+1X_nT<i>_x</i>, M is an early transition metal (Ti, V, Nb, Ta, etc.), X is C and/or N, T<i>_x</i> represents the surface groups (-O, -OH, -F, -Cl), and <i>n</i> = 1–4, over 30 stoichiometric phases have already been discovered, with many more predicted computationally. This class of materials has been widely studied owing to their exceptional properties, including hydrophilicity, scalability, mechanical strength, thermal stability, redox capability, and ease of processing. Because MXenes inherit their structure from MAX phase precursors, understanding MAX phase synthesis leads to control over flake size, defect density, and chemical composition of the resultant MXene. One understudied, yet important class of MXenes are solid-solution MXenes, where multiple elements are randomly distributed within the M layers. Herein, a set of multi-M chemistries (Mo, V, Ti, Nb) are used to study the effect of structure and chemistry on MXenes. While solid-solution MXenes have unique and tunable chemical, optical, and electronic properties, they also enable the formation of novel MXenes that cannot exist otherwise. By choosing specific chemistries, we can then begin to understand fundamental aspects of MXene chemistry and structure.