*2.3.1. Ultrathin nanosheets of MoS2 through electrochemical lithium-intercalated exfoliation process*

To control the step of the formation of LixMoS2 compound, Zhang's group developed a simple method—electrochemical lithiation process—to control Li-intercalated and subsequent exfoliation in water or ethanol [15]. Lithium intercalation of MoS2 is performed in a test cell. The bulk MoS2 is prepared as cathode. The electrochemical intercalation is performed using galvanostatic discharge at a current density of 0.05 mA. After the discharge process, the product is washed with acetone to remove any residual electrolyte, followed by exfoliation and ultrasonication in distilled water or ethanol. Eventually, the suspension is centrifuged and washed many times with water.

As shown in **Figure 3b**–**d**, high-quality and ultrathin MoS2 nanosheets can be obtained by this simple method. The lateral size of nanosheets is about several hundred nanometers with a near transparency. Electron diffraction pattern of a flat area of the nanosheet (**Figure 3c**) and the corresponding high-resolution transmission electron microscope (HRTEM) image (**Fig‐ ure 3d**) show that the hexagonal lattice structure is formed by this method, showing clearly a lattice spacing of 2.7 Å assigned to the (100) planes. This strategy can also been applied to prepare other single-layer semiconducting nanosheets, such as WS2, TiS2, TaS2 or graphene.
