Kinetics of Ti₃AlC₂ Etching for Ti₃C₂T_<i>x</i> MXene Synthesis

The family of two-dimensional (2D) carbides and nitrides called MXenes has grown to encompass numerous structures and compositions. MXenes have been explored in a variety of applications such as energy storage, wireless communication, optoelectronics, and medicine because of their high electrical conductivity, redox-active surfaces, plasmonic behavior, and other attractive properties. Knowledge of the process kinetics is of fundamental importance for synthesis and property control of MXenes. Prediction of the optimal processing time as a function of various parameters will also facilitate scaling wet chemical synthesis of MXenes for industrial use. Herein, we performed a systematic study of the kinetics of the MAX phase precursor etching reaction for topochemical MXene synthesis by collecting and tracking the evolution of the byproduct H2 gas. For the Ti3AlC2 MAX to Ti3C2Tx MXene conversion, we investigated the influence of critical parameters, such as etchant composition, concentration, temperature, and MAX particle size, on the etching kinetics and developed an empirical predictive model to determine the optimal synthesis conditions given any input parameters. We tested a set of 12 kinetics models as well as a model-free fitting method and found the best agreement with the experimental results from three models and the model-free method (R2 > 0.990). The measured apparent activation energies ranged from 54.2 to 55.7 kJ/mol. Overall, our results suggest NH4HF2 as the most efficient etchant, such that the etching time required to produce Ti3C2Tx can be reduced to a few hours. We also demonstrated the importance of separating MAX powders based on particle size into narrow fractions. Finally, we discuss how this method can be improved and applied to study yet-to-be synthesized MXenes and how the MAX/MXene transformation can serve as a platform to model reactions under confinement.