Synthesis and Chemistry of Two-Dimensional Transition Metal Carbides (MXenes)

The family of two-dimensional (2D) transition metal carbides and nitrides (MXenes) has grown to encompass numerous structures and compositions. MXenes have shown properties that make them attractive for applications ranging from energy storage to electronics and medicine, including high electrical conductivity, redox-active surfaces, and plasmonic behavior. MXenes are typically produced through topochemical etching of atomically thick layers from precursor layered MAX phases using corrosive aqueous etchants. However, the harsh synthesis conditions lead to various defects in the MXene lattice that make them susceptible to degradation and limit their practical storage, distribution, and application. Herein, we investigate the chemistry of the precursor MAX phases and demonstrate the existence of oxycarbides; and explore the influence of MAX stoichiometry on the chemical and thermal stability of the resulting MXenes. We also probe the mechanism and kinetics of the MAX etching reaction for MXene synthesis using in situ analytical and microscopic techniques and demonstrate the influence of critical etching parameters on the structure, chemistry, and properties of the resulting MXenes. We further develop kinetic models that allow prediction of the optimal synthesis conditions, and which make the synthesis more scalable, environmentally friendly, and efficient.