**3.2. Microwave-assisted method**

In recent years, microwave-assisted chemical synthesis strategy has become a well-established technique to promote and enhance chemical reactions. The main advantages of this method are represented by much shorter reaction time (generally in only a few minutes) and higher energy efficiency when comparing to other conventional strategies. Due to these advantages, some 2D nanomaterials can be prepared by this way conveniently, such as SnO2, α-Ni(OH)2, K0.17MnO2 and CuSe [33–37].

To understand it more clearly, let us take α-Ni(OH)2 as an example (**Figure 7**) [36]: firstly, precursors were prepared by starting materials of Ni(NO3)2·6H2O, urea, deionized water and ethylene glycol at given proportions. Then, the resulting solution is transferred into a home‐ made round-bottomed flask and treated under microwave irradiation in a microwave reactor at 700 W for several minutes. Finally, the green powder is obtained by centrifugation and washed several times with distilled water and absolute ethanol. After that, the powder was dried in vacuum at 80°C for 12 h. Detailed structure information for the synthesized Ni(OH)<sup>2</sup> nanosheets is unraveled by FESEM and TEM images in **Figure 7b**–**d**. In comparison with traditional wet-chemical syntheses, the microwave-assisted liquid-phase growth can shorten the reaction time to less than 20 min. Particularly, the ultrathin α-Ni(OH)2 nanosheets exhibit a maximum specific capacitance of 4172.5 Fg−1 at a current density of 1 Ag−1.

**Figure 7.** (a) Schematic illustration for the synthesis of nanosheets. (b) High magnification FESEM images of α-Ni(OH)<sup>2</sup> nanosheets, (c) TEM image (the inset showing SAED pattern) and (d) a planar HRTEM image [36].
