**3.3. Wet chemical approaches**

**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

*MoS xLi xe Li MoS* 2 2 *<sup>x</sup>*

22 2 2 2 *<sup>x</sup>*

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

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

) into the host crystal. This destabilizes the crystal while inducing

*<sup>x</sup> Li MoS xH O MoS H xLiOH* + ® ++ (2)

+ - + +® (1)

insertion of an ion (such as Li+

materials [14, 52, 54, 55].

**3.2. Chemical vapour deposition**

a phase change at the same time (Eq. (1)).

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

Wet chemical approaches are bottom‐up methods which offer a potentially powerful alterna‐ tive to exfoliation and CVD. It can be used to synthesize TMDs with thicknesses ranging from the monolayer to hundreds of layers [78, 79]. Compared with CVD method, the reaction temperatures are much lower, and the produced materials are exceptional uniform and with low defect density. Thanks to diverse wet‐chemical methods, the materials can be doped by adding other reagents during growth and one can also use ligand chemistry to cap the material's surface in order to modify or protect the surface [80–82]. Moreover, wet‐chemical methods are often easily translated into larger‐scale manufacturing processes, which may facilitate the commercialization of TMD materials. By selection of environmentally precursors and solvents, solution‐based methods can be adapted to adhere to principles of green chem‐ istry and manufacturing [80].

A traditional wet‐chemical approach to chalcogenides involves hydrothermal or solvothermal growth (**Figure 2c**) [83]. Taking the synthesis of MoS2 nanoflakes as an example, in a typical procedure, (NH4)6Mo7O24·4H2O and thiourea have been utilized as the precursors for the Mo and S elements in hydrothermal reactions [84]. After reaction in a Teflon‐lined stainless steel autoclave, the low‐quality MoS2 flakes with abundant active sites are obtained.

Colloidal synthesis is a well‐established technique for synthesizing TMDs [80]. In a typical process, similar to other colloidal synthetic routes, a cold solution of precursor chemicals is injected into a hot solvent or a one‐pot route can also be adopted where precursors are mixed together and then heated (up to 320 °C). Recently, it was reported that monolayer TMDs such as TiS2, HfS2, and ZrS2 could be synthesized by a novel colloidal referred to as "diluted chalcogen continuous influx" [85]. In this method, the delivery rate of a chalcogen source (such as H2S or CS2) to a transition metal halide precursor in solution was controlled to be slow enough to favour the lateral (2D) growth over 3D growth (**Figure 2d**).

Although the wet‐chemical approaches may unavoidably alter the lattice structure of thin TMDs and introduce extrinsic defects during exfoliation process, these defects may be helpful in electrocatalytic reactions [14, 16, 22].
