Solvent-free one-step green synthesis of MXenes by “gas-phase selective etching”

Two-dimensional transition metal carbides and nitrides (MXenes) have attracted extensive attention due to their unique electronic properties and various applications for energy harvesting. However, the current synthesis procedures involve using fluoride-based aqueous solutions via reacting with Al-containing MAX phases. Besides, fluorine-free, cost-effective, and straightforward synthetic methods are rarely reported and need to be explored. Here, a solvent-free one-step gas-phase dry etching method, named "Gas-Phase Selective Etching", was first proposed and used to etch different elements A (Al, Si, Sn, etc.) of MAX phases using the strong oxidation ability of halogens and hydrogen halides gases (Cl2, Br2, I2, HCl, HBr, and HI). The gas-phase etching can achieve a pure product of MXenes coupled with specific gas-induced functional groups. Meanwhile, MXenes powder can be directly obtained without the removal processes of etchants and by-products. Furthermore, the multilayer-MXene with -Cl functional groups (Ti3C2Cl2) has strong oxidation resistance and superior rate performance of capacitance. It provides us with an intelligent, cost-effective synthesis strategy that has a high yield and is safe, allowing for the promising mass production of MXenes.