*2.3.2. Ultrathin nanosheets of MoS2 through lithium-intercalated exfoliation*

Recently, Chhowalla's group successfully prepared monolayer MoS2 through lithiumintercalated and exfoliation [18]. In detail, they immersed 3 g bulk MoS2 crystals in certain concentration of butyllithium solution in hexane for 2 days in a flask filled with argon gas to obtain lithium intercalation compound. Exfoliation is achieved immediately after this process (within 30 min to avoid deintercalation) by ultra-sonicating LixMoS2 in water for 1 h. The mixture is centrifuged several times to remove excess lithium in the form of LiOH and unexfoliated material.

Commercial MoS2 powder (**Figure 3e**) was used to prepare highly monodisperse monolayer MoS2 nanosheets through Li intercalation and exfoliation. As shown in **Figure 3f** and **g**, the lateral size of product is 300∼800 nm and the average thickness is about 1∼1.2 nm, which is larger than the dimension of 0.65∼0.7 nm reported for mechanically exfoliated MoS2 mono‐ layers. This discrepancy may be explained by surface corrugation due to the distortions and the presence of adsorbed or trapped molecules. The absence of any sheets below the thickness values and no evidence of step edges on the nanosheets surface suggest that they consist of monolayers. The selected area electron diffraction (SAED) patterns indicate hexagonal symmetry of the atomic arrangement and that individual sheets consist of a single-crystal domain (**Figure 3h**). All of the results undoubtedly confirm that MoS2 with monolayer thickness has been successfully synthesized.

#### **2.4. Ion-change exfoliation**

layers. The black phosphorus nanosheets show excellent photodegradation of organic

Ultrasonic exfoliation is incapable of peeling off a single layer of 2D nanostructure; herein, lithium (Li) intercalation process is introduced for synthesizing single sheets. The scheme of lithium-intercalated exfoliation strategy is shown in **Figure 3a** [15]. The formation of LixXS2 compound is a key step in lithium intercalation process, and this reaction can be tuned to control the yield of single layers [16, 17]. The yield of this strategy for obtaining single-layer transition metal dichalcogenide is nearly 100%, while some challenges still remain. The first one is that the experiment is carried out at high temperature for long durations. Also, the lithium intercalation must be controlled carefully to obtain single-layer nanosheets, while preventing the formation of metal nanoparticles and precipitation of Li2S. To make readers understand this method more in-depth, in the following, we would take MoS2 as an example

**Figure 3.** (a) Electrochemical lithiation process for the fabrication of 2D nanosheets from the layered bulk material. (b– d) Morphology characterization of MoS2 exfoliated by electrochemical lithium-intercalated and exfoliation process [15].

(e–h) Morphology characterization of MoS2 exfoliated by lithium-intercalated exfoliation process. [18].

components such as DPBF and MO.

to introduce this strategy.

**2.3. Lithium-intercalated and exfoliation**

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

Although the exfoliation strategy mentioned previously is convenient to delaminate van der Waals solids into ultrathin nanostructure, it is hard to exfoliate layered ionic solids such as LiCoO2 or LDHs. This is because these ionic solids have strong ionic bonds in the layers. Ionchange 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 (Co2+-Co3+)-LDHs as examples to illuminate this method [23, 24].

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

*2.4.1. LiCoO2*

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 in an aqueous solution of HCl (1 M, 1 mg mL−1 of LiCoO2) at room temperature for 3 days. During the proton-exchange reaction, HCl solution is replaced with fresh solution every day. Exfoliation of the layered cobalt oxide is achieved by the intercalation of tetrabutylammonium (TBA) cation into the layers of LixH1-xCoO2. After the reaction, incompletely exfoliated particles were separated from the colloidal suspension through centrifugation (6000 rpm for 10 min), resulting in a pure colloidal suspension. The powdered samples of completely exfoliated nanosheets are collected from the pure colloidal suspension through high-speed centrifugation at 17,000 rpm for 30 min.

AFM of the as-obtained sample provides direct evidence for the exfoliation of layered cobalt oxide nanosheets (**Figure 4b**). The thickness of LiCoO2 nanosheets is about 1.2 nm, which is slightly thicker than the crystallographic thickness of individual cobalt oxide layers. The observation of Tyndall phenomenon from the pure suspension, characteristic of colloidal suspensions, provides strong evidence for the exfoliation of layered cobalt oxide nanosheets too (**Figure 4c**).
