Mechanism and Kinetics of Topochemical Etching for Synthesis of 2D MXenes from Layered MAX Phases

MXenes represent a rapidly growing family of 2D transition metal carbides and/or nitrides, which exhibit unique combinations of properties, including metallic conductivity, hydrophilic surface chemistry, redox-active surface, and plasmonic behavior that make them attractive as electrodes for pseudocapacitive energy storage to coatings for electromagnetic interference shielding, transparent conducting displays for optoelectronics, conductive yarns for functional textiles, implantable electrodes for medicine, and many other applications.<br/><br/>MXenes are typically derived via selective etching of atomically thick ‘A’ layers from precursor layered MAX phases using HF-based aqueous etchants. However, the chemical etching process is only somewhat optimized for one composition, and is nonetheless often slow, with low-yield, and results in defects in the materials. Thus, understanding the fundamental mechanism and kinetics of the etching reaction is needed to rationally explore etching of yet-to-be synthesized MXene compositions and optimize etching of existing MXene compositions. Despite their importance, such studies have been challenging because of the atomic thickness of the A-element layers being etched and the aggressive etchants that hinder in situ studies.<br/><br/>Here, we spatially track the etching front in 3D (using analytical setups for tracking byproduct gas evolution), 2D (using cross-sectional imaging of partially etched particles), and 1D (using in situ optical microscopy and profilometry of single particles). Thus, we show the role of surface oxide and binary carbide impurities on the nucleation of etching, and we discovered a layer-by-layer growth process for the etching front. From kinetic modeling, we show that etching varies from reaction-limited to diffusion-limited for various MAX compositions. Overall, our work enables rational high-throughput optimization of synthesis for all MXene compositions, scaling up synthesis towards industrial levels, and tailored synthesis for specific applications.