Effect of Alkali Metal Cations on the High Temperature Stability of Ti₃C₂T_x and Mo₂TiC₂T_x MXenes

Two-dimensional transition metal carbides, known as MXenes, have found wide use in energy storage and catalysis applications. Although MXenes have been used in these applications, few studies have investigated the use of MXenes’ interior transition metal carbide/nitride core and abundant surface groups as nanosized building blocks toward extreme environment nanoceramics. In this presentation, we illustrate the high-temperature behavior of Ti₃C₂T<i>_x</i> and Mo₂TiC₂T<i>_x</i> MXenes up to 2,000 °C. In addition, we exhibit the role of surface-adsorbed alkali metal cations (Na⁺, K⁺, Ca²⁺) on improving the phase stability and controlling the phase transformation of these MXenes using <i>in situ </i>two-dimensional x-ray diffraction (XRD²) up to 1,100 °C using and <i>ex situ </i>XRD²up to 2,000 °C. We also present the changes in phase transformation of non-decorated and alkali metal cation decorated Mo₂TiC₂T<i>_x</i> MXenes using <i>in situ</i> scanning transmission electron microscopy (STEM) up to 800 °C. The combination of <i>in situ</i> XRD² and STEM methods with <i>ex situ </i>XRD², x-ray photoelectron spectroscopy, and thermogravimetric analysis demonstrate the effect of alkali metal cations on improving the phase stability and controlling the phase transformation of MXenes. This presentation assists in furthering development of MXenes as a diverse and tunable family of nanoceramics for ultra-high temperature applications.