High-temperature stability and phase transformations of titanium carbide (Ti₃C₂T_x) MXene

Two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, known as MXenes, are under increasing pressure to meet technological demands in high-temperature applications, as MXenes can be considered to be one of the few ultra-high temperature 2D materials. Although there are studies on the stability of their surface functionalities, there is currently a gap in the fundamental understanding of their phase stability and transformation of MXenes' metal carbide core at high temperatures (>700 °C) in an inert environment. In this study, we conduct systematic annealing of Ti₃C₂T_xMXene films in which we present the 2D MXene flake phase transformation to ordered vacancy superstructure of a bulk three-dimensional (3D) Ti₂C and TiC_ycrystals at 700 °C ⩽T⩽ 1000 °C with subsequent transformation to disordered carbon vacancy cubic TiC_yat higher temperatures (T> 1000 °C). We annealed Ti₃C₂T_xMXene films made from the delaminated MXene single-flakes as well as the multi-layer MXene clay in a controlled environment through the use ofin situhot stage x-ray diffraction (XRD) paired with a 2D detector (XRD²) up to 1000 °C andex situannealing in a tube furnace and spark plasma sintering up to 1500 °C. Our XRD²analysis paired with cross-sectional scanning electron microscope imaging indicated the resulting nano-sized lamellar and micron-sized cubic grain morphology of the 3D crystals depend on the starting Ti₃C₂T_xform. While annealing the multi-layer clay Ti₃C₂T_xMXene creates TiC_ygrains with cubic and irregular morphology, the grains of 3D Ti₂C and TiC_yformed by annealing Ti₃C₂T_xMXene single-flake films keep MXenes' lamellar morphology. The ultrathin lamellar nature of the 3D grains formed at temperatures >1000 °C can pave way for applications of MXenes as a stable carbide material 2D additive for high-temperature applications.