**2. Synthetic Methods**

The MXene flakes are commonly synthesized by using hydrofluoric acid or a mixed solution of lithium fluoride and hydrochloric acid to selectively remove the "A" layer from the ternary MAX phase, where the M represents early transition metals (e.g., Ti, Mo, V), the A represents IIIA or IVA group elements (e.g., Al, Ga, Si, Ge), and the X represents C and/or N. The MXenes usually possess accordion-like hexagonal lattices which result from the original metallic backbone, weak M-A bond, and strong M-X bond [39]. Meanwhile, some surface terminal groups (e.g., -OH, -O, -F) are formed during the bond breaking and binding processes, which provides more active sites as well as suitable interlayer spacing (Figure 1). Till now, the routinely used method for synthesizing the MXene and its derivatives is hydrofluoric acid (HF) etching. Early works revealed that HF is the most effective etchant to selectively react with the "A" layer atoms and continual out-diffusion to exfoliate the MAX phase (such as Ti3AlC2). Those works also proved that the MAX phases are inert in the conventional acids (e.g., H2SO4, HCl, and HNO3), alkaline liquors (e.g., NaOH), and salt solutions (e.g., NaCl and Na2SO4). Nevertheless, HF reagent is a hazardous poison with a highly corrosive property to the human body. The direct use of HF raises the potential for causing considerable safety and environmental problems [40]. Thus, many mild etchants have been developed (e.g., NH4HF2, or a mixture of LiF/HCl). It was noted that the diverse routes will bring different surface functional groups such as fluoride (-F), hydroxyl (-OH), and/or oxygen (-O) on MXenes and give rise to different levels of delamination, and hence, the preparation process will directly determine the properties of the final MXenes, e.g., metallic, semi-metallic, and semiconducting types. To

understand the formation fundamentals from the MAX phase to the MXene sheets, three basic approaches will be discussed in the sections (e.g., wet-chemical etching in hydrofluoric acid, in situ HF-forming method, and freeze-and-thaw-assisted (FAT) method).

**Figure 1.** (**a**) The elements of the periodic table for MAX phases. (**b**) Structures of M2AX, M3AX2, and M4AX3 phases [41].
