2D single- and few-layered MXenes: synthesis, applications and perspectives

Transition metal carbides and nitrides (MXenes), as a promising class of two-dimensional inorganic compounds, are materials composed of a few atomic layers of transition metal carbides, nitrides, or carbonitrides. The properties of an MXene can be ingeniously regulated according to the external strain and the number of layers in the crystals and thin films. Single- and few-layered MXenes (SFL-MXenes) show preferable properties, such as a low ion diffusion barrier, low open-circuit voltage, and high specific surface area. In this review, we comprehensively summarize current advances in SFL-MXene research. The fabrication of SFL-MXenes via both top-down and bottom-up strategies is summarized in detail. Top-down strategies involve HF etching, in situ HF etching, molten salt etching, and electrochemical etching methods. Bottom-up strategies involve chemical vapor deposition, self-assembly, and template-assisted growth approaches. Additionally, applications of SFL-MXenes, including energy storage, energy conversion, sensing, optoelectronic, optical device, electromagnetic, and environmental applications, are illustrated. Finally, future challenges faced by SFL-MXenes and their application potential are summarized. With increasing progress in fabrication approaches using controlled synthesis, the unusual properties of SFL-MXenes can be unveiled and exploited for various prospective applications.