*2.4.2. (Co2+-Co3+)-LDHs*

change exfoliation is a normal method to get this kind of 2D materials [19–23]. The scheme of ion-change exfoliation is shown in **Figure 4a** [19]. In the next section, we will take LiCoO2 and

**Figure 4.** (a) Schematic illustration of the osmotic swelling to exfoliation process [19]. (b) AFM image and height profile of the exfoliated cobalt oxide adsorbed onto PEI-coated mica substrate. Photograph of the colloidal suspension of co‐ balt oxide. (c) The visible light is illuminated from the side of the beaker to demonstrate the Tyndall scattering effect

LiCoO2 is a kind of cation-exchange layered metal oxides. Seong-Ju's group delaminated LiCoO2 into monolayer recently [23]. The procedure of exfoliating could be described as follows: A proton-exchange reaction for the generation of layered LixH1-xCoO2 is carried out

[23]. Co2+–Co3+ LDH nanosheets: (d) AFM image; (e) TEM image [24].

*2.4.1. LiCoO2*

(Co2+-Co3+)-LDHs as examples to illuminate this method [23, 24].

8 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

According to Sasaki's report [24], 0.5 g of the sample is dispersed into an aqueous solution (500 mL) that contains 2.5 M NaClO4 and 2.5 mM HCl. This mixed solution is carefully degassed by purging with nitrogen gas for conversion into ClO4 − -LDH (addition of HCl is crucial for preventing carbonate, CO3 2− contamination from air). After purging with nitrogen gas, the reaction vessel is tightly capped and shaken for 1 day at room temperature. The sample is filtered, washed with degassed water and air-dried. The obtained ClO4 <sup>−</sup> LDH (0.1 g) is mixed with formamide (100 mL) in a conical beaker. After an ultrasonic treatment for 30 min, a translucent colloidal suspension is obtained. The suspension is further centrifuged to remove possible nonexfoliated particles. AFM and TEM images shown in **Figure 4d** and **e** provided direct evidence for successfully exfoliation of the samples.
