High-temperature stability and phase transformations of titanium carbide (Ti ₃ C ₂ T _<i>x</i> ) 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 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_<i>x</i>MXene films in which we present the 2D MXene flake phase transformation to ordered vacancy superstructure of a bulk three-dimensional (3D) Ti₂C and TiC_<i>y</i>crystals at 700 °C ⩽<i>T</i>⩽ 1000 °C with subsequent transformation to disordered carbon vacancy cubic TiC_<i>y</i>at higher temperatures (<i>T</i>> 1000 °C). We annealed Ti₃C₂T_<i>x</i>MXene films made from the delaminated MXene single-flakes as well as the multi-layer MXene clay in a controlled environment through the use of<i>in situ</i>hot stage x-ray diffraction (XRD) paired with a 2D detector (XRD²) up to 1000 °C and<i>ex situ</i>annealing 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_<i>x</i>form. While annealing the multi-layer clay Ti₃C₂T<i>_x</i>MXene creates TiC_<i>y</i>grains with cubic and irregular morphology, the grains of 3D Ti₂C and TiC_<i>y</i>formed by annealing Ti₃C₂T_<i>x</i>MXene 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.