**2.1. Micromechanical exfoliation**

world. As a consequence, more and more 2D materials have been synthesized successfully,

Ideal 2D materials belong to those with only one atom or several atoms thickness and infinite lateral size. The reliable synthesis of single- and few-layer 2D materials is an essential first step for characterizing the layer-dependent changes in their properties, as well as for providing pathways of their integration into a multitude of applications [1]. For instances, when the thickness of metal Co was reduced to only one or several atoms, the catalytic activity of carbon dioxide reduction would be improved greatly when comparing to bulk metal Co [3]. Singlelayer MoS2 has been synthesized by mechanical exfoliation strategy and exhibits excellent performance in the field of gas sensors and phototransistors. It is worth noting that when bulk MoS2 is exfoliated into monolayer, the type of band gap would change from indirect type to direct one [1, 4]. Another important example is that single-layered Co(OH)2 can realize an

The emergence of these novel properties is the driving force for the rapid development of research in ultrathin 2D nanosheets, which has been on the forefront of scientific disciplines including chemistry, physics, materials, science, medicine and biology. However, the quest for methods of producing 2D materials with controlled thickness and lateral size has been always a challenging subject. This may be caused by the anisotropic crystal growth and strong chemical bonds in crystal structure. The common classification of crystalline structures according to the type of chemical bonds could be divided into van der Waals solids, layered ionic solids and nonlayered materials [1]. Every synthetic strategy has its own merits and demerits in preparing different kind of materials. Therefore, in this chapter, we concentrate on the different synthetic methods for synthesizing two-dimensional crystals. According to the principle of generating two-dimensional materials, we can divide the synthetic strategies into top-down and bottom-up strategies. The distinction between these two general classifi‐ cations is based on the processes involved in the creation of the nanometer-sized structures [6]. In the bottom-up approach, nanoscale materials are constructed from atomic or molecular precursors that are allowed to react and grow in size or self-assemble into more complex structures. By contrast, the top-down approach carves nanoscale structures by controlled

In top-down nanofabrication, the crafting of ultrathin nanosheets is achieved through physi‐ cal-based or through chemical-based process. The physical top-down approach employs the use of mechanical force or ultrasonic wave to exfoliate layered van der Waals solids into singleand few-layer 2D materials, whereas the chemical top-down strategy relies essentially on chemical reaction that brought about by ion exchange or by the application of heat and so on. In this section, a brief overview is presented of the top-down synthetic process for 2D nano‐

ultrahigh energy density in all-solid-state asymmetric supercapacitor [5].

showing great promise for in many applications.

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

removal of materials from larger or bulk solids.

structures, including their advantages and limitations.

**2. Top-down strategy**

Micromechanical exfoliation has been used to exfoliate graphite into single-layer graphene by Novoselov and Geim for the first time [2]. Mechanical exfoliation is a straightforward method to obtain only one- or few-layer nanosheets, which well maintains the crystal structure and properties. Beside this, versatility and low cost of this method make it highly popular for synthesizing 2D materials and extremely convenient for fundamental research. Even so, this method is limited to the laboratory research and seems impossible to scale up for industrial production. Although large size and high quality of ultrathin nanosheets can be obtained by this way, this method is applicable only for layered van der Waals solids. The nanosheets of layered ionic solids and nonlayered materials cannot be obtained by this strategy. Addition‐ ally, several other factors (stoichiometry and stacking orders) play the key roles in successful fabrication of monolayer MX2 nanostructures by mechanical exfoliation. Herein, we take graphene as an example to introduce this method.

Graphene is a monolayer sheet of carbon, showing only one atom thickness but extending indefinitely in two dimensions, which is the typical representative of 2D materials. Many astonishing properties have been discovered for graphene, which include better electrical and thermal conductivity, mechanical strength and optical purity. The procedure of microme‐ chanical exfoliation is very simple. **Figure 1** illustrates the process of mechanical exfoliation [7].

**Figure 1.** An illustrative procedure of the Scotch-tape–based micromechanical cleavage for graphene [7].

The exfoliation mechanics of this method are utilization of mechanical force to exfoliate graphite from Scotch tape. If one takes great pain to repeat this normal force over and over, the graphitic layer would become thinner and thinner, and eventually one can obtain singlelayer graphene.

#### **2.2. Ultrasonic exfoliation**

Ultrasonic exfoliation is an effective strategy to delaminate van der Waals solids into singleor few-layer nanosheet. Compared to the mechanical exfoliation, this method is more effective and higher productive. The details for ultrasonic exfoliation process are shown in **Figure 2** [8]. As indicated in **Figure 2a**, sonication time and suitable solvents play the key roles in exfoliation. Suitable solvents are those with appropriate surface energies. In good solvents, the exfoliated nanosheets are stabilized against reaggregation. Otherwise, for "bad" solvents, reaggregation and sedimentation will occur. In recent reports, different types of organic solvents have been used as a dispersing medium for delaminating van der Waals solids (listed in **Table 1**) [8–14]. Although this method has many advantages, it is hard to get high-purity single-layer 2D material, which is a primary need for electronic applications. To make readers understand this method more in-depth, in the following, we will take black phosphorus (BP) as an example to introduce this strategy.

**Figure 2.** (a) An illustrative procedure of sonication-assisted exfoliation. The layered crystal is sonicated in a solvent, resulting in exfoliation and nanosheet formation [8]. (b) TEM image of ultrathin BP nanosheets. (c–d) AFM image and corresponding height image of ultrathin BP nanosheets [13].



1 A is absorbance; l is the path length of the beam of light through the material sample. A/l at fixed wavelength was used to estimate the mass remaining in the supernatant.

**Table 1.** The best 20 solvents for each material [9].

**2.2. Ultrasonic exfoliation**

introduce this strategy.

corresponding height image of ultrathin BP nanosheets [13].

**Solvent A/l (AU)1**

Ultrasonic exfoliation is an effective strategy to delaminate van der Waals solids into singleor few-layer nanosheet. Compared to the mechanical exfoliation, this method is more effective and higher productive. The details for ultrasonic exfoliation process are shown in **Figure 2** [8]. As indicated in **Figure 2a**, sonication time and suitable solvents play the key roles in exfoliation. Suitable solvents are those with appropriate surface energies. In good solvents, the exfoliated nanosheets are stabilized against reaggregation. Otherwise, for "bad" solvents, reaggregation and sedimentation will occur. In recent reports, different types of organic solvents have been used as a dispersing medium for delaminating van der Waals solids (listed in **Table 1**) [8–14]. Although this method has many advantages, it is hard to get high-purity single-layer 2D material, which is a primary need for electronic applications. To make readers understand this method more in-depth, in the following, we will take black phosphorus (BP) as an example to

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

**Figure 2.** (a) An illustrative procedure of sonication-assisted exfoliation. The layered crystal is sonicated in a solvent, resulting in exfoliation and nanosheet formation [8]. (b) TEM image of ultrathin BP nanosheets. (c–d) AFM image and

**Solvent A/l (AU)**

**670 nm**

**Solvent A/l (AU)**

**630 nm**

**BN MoS2 WS2**

**300 nm**

 Cyclohexylpyrrolidone (CHP) 100 NVP 100 DMSO 100 *N*-dodecyl-pyrrolidone (N12P) 61 N8P 98 NVP 92 Benzyl benzoate 44 N12P 97 NMF 90 Isopropanol 44 CHP 88 N12P 84 *N*-Octyl-pyrrolidone (N8P) 44 NMP 80 DMEU 75 *N*-vinylpyrrolidone (NVP) 41 DMEU 73 DMF 73

Single-layer BP is a very promising two-dimensional material that can be the substitution of graphene due to its exceptional electronic properties. The direct band gap of BP can be tuned from 0.3 eV in the bulk to 1.5 eV in the monolayer. Recently, Xie's group successfully prepared pristine 2D black phosphorus through direct ultrasonic exfoliation in organic solvent [13]. Briefly, they dispersed 50 mg of bulk black phosphorus in 100 mL of distilled water, which was bubbled with argon to eliminate the dissolved oxygen molecules for avoiding the oxidation. Then, sonicating the mixture solution in ice water for 8 h (Note: keeping the system at a relatively low temperature is important). After ultrasonic treatment, the resultant disper‐ sions were centrifuged at 1500 rpm for 10 min to remove the unexfoliated component and the supernatant was collected for further use.

Ultrathin nanosheet thus obtained show lateral size of about several hundred nanometers (see TEM image in **Figure 2b**), and the ultrathin thickness is indicated by the near transparency of the sheets. AFM image and the corresponding height distribution (**Figure 2c** and **2d**) show that the measured height is about 2.0 nm, consistent with the four individual black phosphorus layers. The black phosphorus nanosheets show excellent photodegradation of organic components such as DPBF and MO.

#### **2.3. Lithium-intercalated and exfoliation**

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 to introduce this strategy.

**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].
