Layer-by-layer mechanism of the MAX phase to MXene transformation

Abstract

MXenes are the fastest growing family of two-dimensional (2D) materials with potential for applications from energy storage to biomedicine, sensing, and electromagnetic shielding. Despite significant progress in MXene synthesis through selective etching of layered MAX phase precursors, limited understanding of the fundamental etching mechanism and kinetics hinders rational optimization of the process. Here, we monitored the etching process using in situ and ex situ techniques at the single-particle and ensemble levels. Our work shows that etching nucleation is instantaneous and etching occurs layer by layer. Through analytical modeling, we found that etching of V₂AlC is diffusion-limited. In contrast, etching of Ti₂AlC and Ti₃AlC₂ is reaction-interface-limited, with an additional surface reaction limitation for Ti₃AlC₂ accounting for more than one-quarter of the total etching time. Overall, our work provides significant insights into the MAX phase etching mechanism and kinetics and an overview of the tools and techniques available to investigate etchable layered materials.