High-temperature stability and phase transformations of titanium carbide (Ti3C2T x ) MXene

Abstract

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 as one of the few high temperature 2D materials. Although there are studies on the stability of their surface functionalities, there is a gap in fundamental understanding of the phase stability and transformation of the metal carbide core at high temperatures (> 700 °C) in an inert environment. In this study, we conduct systematic annealing of Ti₃C₂T_x MXene films in which we present the 2D MXene flake phase transformation to ordered vacancy superstructure of a bulk three-dimensional (3D) Ti₂C phase and 3D bulk TiC_y crystals at 700 ᵒC ≤ T ≤ 1000 ºC with subsequent transformation to disordered carbon vacancy cubic TiC_y during annealing at higher temperatures (T > 1000 ºC). We annealed Ti₃C₂T_x MXene films made from the delaminated MXene single-flakes as well as the multi-layer MXene clay powder in a controlled environment through the use of in-situ hot stage x-ray diffraction (XRD) paired with a 2D detector (XRD²) up to 1000 °C and ex-situ annealing in a tube furnace and spark plasma sintering up to 1500 °C. Our XRD² analysis 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_x form. While annealing the multi-layer MXene create TiC_y grains with cubic and irregular morphology, the grains of 3D Ti₂C and TiC_y formed by annealing the Ti₃C₂T_x MXene single-flake films keep the 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.