A general method for precise modification of –O termination on MXenes by low-pressure flash annealing

Abstract

The physicochemical properties and application performance of MXenes are fundamentally linked to their surface chemistry. Specifically, two-dimensional (2D) MXenes terminated with the –O group exhibit remarkable potential in a wide range of applications, such as energy storage, catalysis, and electronics. However, conventional synthetic techniques, such as prolonged high-temperature annealing in an electric furnace, usually cause irreversible structural damage or undesirable phase transformations of 2D MXene slabs into bulk 3D structures, resulting in a substantial reduction in the surface area and a consequent decline in application performance. Herein, we propose a general method for the precise modification of –O termination on MXenes by employing low-pressure flash annealing (LP-FA) of raw MXenes with complex and diverse terminations, guided by Le Chatelier's principle. The low-pressure environment facilitates gas-generation solid–solid reactions at relatively lower temperatures, thereby promoting the removal of superficial –F terminations and the formation of –O terminations on MXenes. Additionally, the rapid heating rate (10³ K s⁻¹) and short duration (∼5 s) of flash annealing, coupled with a lower peak annealing temperature, effectively prevent structural damage or layer-by-layer stacking of MXene slabs and enhance the potential applications of –O terminated MXenes. As an illustration of practical applications, we have demonstrated that the –O terminated Nb₂CT_x MXene exhibits an exceptional capacity of 420 mA h g⁻¹ at a current density of 50 mA g⁻¹, along with remarkable stability over more than 3000 cycles of Li-ion charge/discharge testing. This performance positions it among the highest-performing MXene anode materials. Consequently, our LP-FA method introduces an additional parameter, beyond the conventional temperature and time factors, to modulate general modifications and advance the industrial application of –O terminated MXene-based advanced functional materials.