**3.1. Exfoliation**

promoting their catalytic activities. This ratio increases with the reduction in the number of

(2) Abundant uncoordinated surface atoms. During a catalytic process, the reaction substrates are absorbed onto the surface atoms of the catalyst and then dissociated into highly reactive intermediates, which makes these surface atoms catalytically active sites. Certainly not all the surface atoms can efficiently participate in catalytic reactions. As a matter of fact, the adsorp‐ tion and dissociation often take place on the coordinately unsaturated sites that are thermo‐ dynamically instable [16]. In the 2D materials, there are much more uncoordinately unsaturated atoms on the surface, which enhances their activities in catalysis. It is worth noting that the basal surfaces of catalysts might be catalytically inert in some certain reactions [14, 16]. Nevertheless, this feature does not diminish the advantage of 2D materials since catalytic activity can still arise from the active sites located along the edges of the nanosheets. For example, MoS2 is a typical and efficient electrocatalyst for HER with higher activity at the edge compared to the basal part. The research indicates that atoms at the edges have lower coordi‐ nation number than those on the surface, thereby providing reaction sites with higher catalytic

(3) Planar structure with atomic thickness. In general, the electrocatalytic performances of a catalyst are not only governed by the intrinsic nature and the number of active sites but also determined by the electron transfer between the active site and the supporting electrode [36]. From this point of view, the unique planar structure with atomic thickness of 2D materials is a decisive advantage for catalysis. The density of states should be significantly increased due to surface distortion of 2D materials, which favours the electron transport along the 2D conducting channels with high mobility as well as between its interface with other components or media, facilitating the electron diffusion between the catalytically active sites and the supporting electrode [37]. Furthermore, the unique planar structure of 2D materials also makes them ideal loading substrates for the assembly or growth of various novel hybrid catalysts. Other building blocks can be readily loaded on the flat surface or stacked layer by layer for

(4) Excellent solution dispersity. 2D materials often show excellent dispersity in certain solution whose type depends on their synthetic method. Thus far, exfoliation in aqueous solution is the most widespread method to produce 2D materials [39]. Most of the resulted materials are thus dispersible in water to facilitate further processing and applications towards green chemistry. In general, their superior dispersity to other counterparts is enabled by high surface area as well as large portion of uncoordinated surface atoms. The high specific surface area keeps nanosheets from precipitation, while electrostatic repulsion between the nanosheets

(5) Highly tunable properties. One major difference between 2D TMDs and other 2D materials, such as graphene, is their high anisotropy and unique crystal structure. For this reason, the material properties of 2D TMDs can be effectively tuned in a wide range through different methodologies including reducing dimensions, intercalation, heterostructure, alloying, gating, pressure, lighting, and so forth [27, 40–44]. For example, through the intercalation of guest ions, the carrier densities of 2D TMDs can be tuned by multiple orders of magnitude,

atomic layers and hypothetically reaches its maximum value in monolayer.

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

activity [34, 35].

various catalytic applications [38].

further prevents their agglomeration.

Exfoliation is a top‐down assembly method, in which physical and chemical driving forces are used to achieve separation of bulk materials. In the exfoliation of 2D materials, the precursors are usually their bulk counterparts. During the exfoliation, external or internal driving force is needed to weaken and eventually overcome the van der Waals force between adjacent layers. It can use mechanical force, such as friction or shear forces, or chemical force, in which the free energy or externally added electrochemical energy provides the driving force. Exfoliated sheets must typically be stabilized to prevent aggregation and re‐stacking using surfactants, polymers, solvents, or liquid‐liquid interfaces that trap and stabilize the exfoliated sheets [39, 46, 47].

It is the successful isolation of graphene from graphite using scotch tape that sparks the tremendous interest in exfoliating 2D TMD materials [13]. This mechanical exfoliation possesses the advantages of being highly reproductive and is quite suitable to fabricate single devices for research purposes and build devices based on all‐layered materials [12, 48]. Nevertheless, mechanical exfoliation is not suitable for large‐scale production due to the absence of layer number and lateral size control capability and it also suffers from low yield and contaminates monolayer surfaces with the adhesive polymer [32]. The limitations on throughput can be overcome by exfoliation in the liquid phases [46, 47, 49]. In general, direct sonication of a layered host is carried out in a solvent chosen to stabilize the exfoliated sheets and sometimes selected based on matching surface tension to solid surface energies. Although this method can partially exfoliate TMDs into few‐layer materials, only a very low yield of monolayer TMDs can be produced.

In addition to the direct dry or liquid‐phase exfoliation, a two‐step process, ion intercalation followed by exfoliation, is able to produce TMDs with a higher yield. Lithium, sodium, or potassium ions are intercalated into the interlayer space and form ion‐intercalated compounds, which can be further sonicated in water or organic solvents to form TMD dispersions [50]. Exfoliation of the bulk TMD crystals can also be achieved using organolithium compounds. For example, the *n*‐BuLi reacts chemically with TMDs, forming Li‐intercalated compounds [51]. The compounds are further exfoliated by the reaction of Li with water. A variety of TMD sheets, including MoS2, TiS2, TaS2, and WS2, can be produced by this method with the lateral size up to few microns (**Figure 2a**).

**Figure 2.** Preparation methods for TMDs. (a) Chemical exfoliation process of TMDs. Reproduced with permission from Zheng et al. [50]; copyright 2014 Nature Publishing Group, (b) CVD growth of TMDs. Reproduced with permission from Shi et al. [59]; copyright 2014 Royal Society of Chemistry, (c) hydrothermal growth of MoS2 on reduced graphene oxide. Reproduced with permission from Li et al. [83]; copyright 2011 American Chemical Society, and (d) colloidal synthesis of TMDs. Reproduced with permission from Yoo et al. [85]; copyright 2014 American Chemical Society.

Electrochemical exfoliation has been used for several decades for exfoliation and restacking of layered materials to generate novel compounds [14]. It proceeds through the electrochemical insertion of an ion (such as Li+ ) into the host crystal. This destabilizes the crystal while inducing a phase change at the same time (Eq. (1)).

$$\rm{MoOS\_2 + xLi^+ + xe^- \to Li\_xMoS\_2} \tag{1}$$

$$\rm Li\_xMoS\_2 + xH\_2O \to MoS\_2 + \frac{x}{2}H\_2 + xLiOH \tag{2}$$

Placing the intercalated material in polar solvents forces hydrolysis of the lithiated species and formation of single‐sheet colloidal suspensions (Eq. (2)) [52, 53]. The yield of this method is nearly 100% but requires long reaction times and careful exfoliation to prevent destruction. This method may be one of the most promising for large‐scale fabrication of true monolayer materials [14, 52, 54, 55].

#### **3.2. Chemical vapour deposition**

Chemical vapour deposition (CVD) is an important and widely used technique for growing inorganic materials, which yields large, high‐quality single crystals of oxide and chalcogenide materials with morphologies ranging from nanoribbons, plates, to monolayers [56–58]. In a typical CVD process, source powder(s) or molecular precursor in solution is heated. A carrier gas (e.g. argon, nitrogen, or forming gas) transports the vapour‐phase precursors downstream to substrates that are placed in a region of appropriate temperature for nucleation of TMDs (**Figure 2b**). Optimization of substrate choice, molecular precursors, and reaction geometry can facilitate growth of monolayers [59]. Compared with chemical exfoliation, the CVD method is more efficient in growing TMD monolayer films on substrates (SiO2/Si [60] or Au [61]), with high quality and controllable thickness [46, 62].

Several CVD synthesis methods for TMDs have been studied, such as sulphurization of a transition metal or metal oxide thin film, thermal decomposition of thio‐salts, and vapour‐ phase transport method [63–70]. Furthermore, Li and co‐workers developed a growth method of TMDs via vapour‐phase chemical reaction of transition metal oxide and chalcogen, which can control the thickness and crystallinity of TMDs [71, 72]. Specifically, metal oxide MoO3 is used as transition metal source and it undergoes a two‐step reaction. The suboxide MoO3-*<sup>x</sup>* is firstly produced during the reaction, which further serves as an intermediate to react with chalcogen vapour (sulphur) and forms the monolayer TMDs with a triangular shape.

Further studies show that the formation of TMDs is controlled by the surface energy of substrate. The aromatic molecules can significantly enhance the wetting between precursors and the substrate surfaces and thus promote the nucleation and lateral growth of TMDs [73]. TMDs' single‐crystal domain with lateral size up to several hundred micrometres can be produced by optimising the vapour‐phase reaction conditions [74]. The direct vapour‐phase reaction of transition metal oxide and sulphur/selenium has been widely adopted to produce TMDs including MoS2, WS2, MoSe2, and WSe2 [62, 75–77].
