*2.1. Wet-Chemical Etching*

The first MXene of Ti3C2 was obtained from the MAX phase of Ti3AlC2 in the HF solution after successful and selective removal of Al atoms. Subsequently, various kinds of MXene members have been developed by using the wet-chemical etching in hydrofluoric acid. Interestingly, after the etching, the deficiencies of the Al atoms change the structure from undamaged cube to accordion-like cube due to the stable bond between M-X and the weak bonding to the A-M layers. In addition, the as-formed Al deficiencies able to expose more Ti atoms to the atmosphere and further facilitate the exposed terminations to combine with other functional groups. Hence, the extraction processes help to vary the surface chemistries and change their band gap, which results in a higher surface chemical activity of MXene in contrast to MAX phase. The reaction formula for the selective etching process can be ascribed as following [20]:

$$\rm Ti\_3AlC\_2\ (s) + HF(aq) = AlF\_3(aq) + 3/2H\_2(g) + Ti\_3C\_2\ (s) \tag{1}$$

$$\rm Ti\_3C\_2(s) + 2H\_2O(aq) = Ti\_3C\_2(OH)\_2(s) + H\_2(g) \tag{2}$$

$$\rm Ti\_3C\_2(s) + 2HF(aq) = Ti\_3C\_2F\_2(s) + H\_2(g) \tag{3}$$

where Equation (1) represents the selective reaction of A layers with HF etchant while Equations (2) and (3) always react together and represent the growth mechanism of the function groups on the exposed Ti atoms [42].
