Since their discovery in 2011, the family of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, MXenes, have grown to encompass more than 100 stoichiometric compositions, providing materials for a variety of applications such as energy storage, wireless communication, and photothermal therapy because of their redox-active surfaces, high electrical conductivities of >15,000 S cm^-1, and plasmon resonance behavior. The next step to expand upon such promising results is to integrate MXenes into devices for commercial and consumer use, which will rely on scaling up the synthesis of MXenes to industrial levels. This transition will require understanding the MAX etching mechanism and kinetics to determine the limiting parameters, understanding the relationship between the extent of reaction and chemistry and properties of the resulting MXenes, and speeding up the synthesis process to reduce costs. Moreover, while the synthesis protocols for the first and most widely studied MXene, Ti₃C₂T_x, were developed through years of systematic experiments, reaching the same level of optimization for every member of the MXene family would be time-consuming. Thus, there is a need to develop new strategies for correlating MAX composition with etch-ability. To address the above problems, herein, we develop a simple H₂ gas collection system for the in-situ monitoring of the MAX etching reaction. We use this system to correlate the effect of various parameters such as temperature, hydrofluoric acid concentration, and MAX particle size on the properties of the resulting MXenes and by fitting with Avrami reaction kinetic models, to better understand the reaction mechanism. Overall, our results reveal that for Ti₃AlC₂, etching time can be decreased by >60% without significant oxidation or degradation of the resulting Ti₃C₂T_x MXene. Fundamentally, our results reveal a new basis for optimizing the MXene synthesis process which can be used to investigate MAX phases that have not been etched successfully, like Cr₂AlC, which is predicted to have interesting magnetic properties.

Mark Anayee

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