Direct Synthesis and Chemical Vapor Deposition of Two-Dimensional Transition Metal Carbides and Nitrides (MXenes)

MXenes are a large family of two-dimensional (2D) transition metal carbides and nitrides that follow a generalized formula of M<i>_n</i>₊₁X<i>_n</i>T<i>_x</i> (<i>n</i> = 1-4), where M stands for early transition metal (such as Ti, V, or Nb), X is C or N, and T is surface terminations (for example, -O, -F, and -Cl). Interest in 2D MXenes continues to grow because of their promising performance in energy storage, electromagnetic interference (EMI) shielding, and catalysis.<br/>Most MXene synthesis methods are fundamentally top-down approaches, which involve the conversion of a MAX phase (A is typically Al or Si) precursor through selective etching of the A layers in hydrofluoric acid-containing solutions or other etchants such as Lewis acidic molten salts and HCl. Recent developments have begun to focus on bottom-up routes due to superior atom economy and a greater degree of synthetic control in such methods.<br/>We demonstrated two direct ways, bypassing the MAX phase intermediates, to prepare one of the most widely used MXenes, Ti₂CCl₂. One is based on the high-temperature reaction of a stoichiometric mixture of precursors. The other uses chemical vapor deposition (CVD). We further pushed MXene bottom-up synthesis to a new level by expanding the scope of MXene available through direct synthesis. We show C₂Cl₄ and other analogous similar precursors can directly synthesize high-purity Ti₂CCl₂ along with Zr₂CCl₂, and Nb₂CCl₂ MXenes. This method implies simple halocarbon molecules of varying stoichiometries can enable a general and scalable route to various MXenes. The high reactivity of molecular precursors increases the nucleation rate, so that the lateral sizes of MXene flakes remain small, on the order of tens of nanometers. The large surface-to-volume ratio helps with the solution procession of MXenes, and is ideal for surface functionalization of MXenes.<br/>The direct synthesis routes save time and minimize hazardous waste production. Besides, these new routes offer synthetic modalities not compatible with traditional methods, especially in the cases where corresponding MAX phases are not available. Considering the extensive possibility of element combinations, the direct synthesis method could substantially expand the variety of the MXene family. These benefits are expected to improve the efficiency of scaling production and expedite converting lab-scale discoveries into industrial-level applications.<br/><br/>Reference:<br/>1. Wang, D.; Zhou, C. K.; Filatov, A. S.; Cho, W. J.; Lagunas, F.; Wang, M. Z.; Vaikuntanathan, S.; Liu, C.; Klie, R. F.; Talapin, D. V., Direct synthesis and chemical vapor deposition of 2D carbide and nitride MXenes. <i>Science </i><b>2023,</b> <i>379</i> (6638), 1242-1247.