Multi-year study of environmental stability of Ti3C2Tx MXene films

Abstract

MXenes are a family of two-dimensional (2D) carbides and nitrides with extraordinary electrical, optical, chemical, and electrochemical properties. There is a perception that MXenes are unstable and degrade quickly, limiting potential applications and requiring specific storage conditions to last for a long time. It primarily comes from studies on delaminated MXenes flakes in dilute dispersions and samples from defective or non-stoichiometric precursors when MXene research was in its infancy. In the years since then, significant developments in synthesis, processing, and understanding of their chemistry have led to dramatic increases in the environmental stability of many MXenes, especially the widely studied Ti3C2Tx. However, previous studies focused primarily on MXene dispersion, while in the majority of applications, MXenes are processed into films soon after synthesis. Herein, we analyze Ti3C2Tx free-standing films aged from 4 to 10 years through structural and morphological characterization along with electrical conductivity measurements to reveal the effect, or lack thereof, of prolonged storage under ambient conditions. Further, we show that the decrease in electronic conductivity over time is caused mainly by water uptake by the hydrophilic surfaces of MXenes, which can be removed by vacuum annealing. As a result, the conductivity of the films can be partially or almost completely recovered.