# Solution-Based Synthesis and Processing of Metal Chalcogenides for Thermoelectric Applications
## 2 Solution-Based Approaches for Synthesis of Metal Chalcogenides
### Liquid-Based Exfoliation Method  
Ever since graphene was discovered, two-dimensional (2D) nanomaterials have gained tremendous interest in the research world due to their unique and extraordinary optical, electronic, thermal, and mechanical characteristics compared to their bulk counterparts [30; 31]. In the nanostructure engineering of thermoelectric materials, the commonly used strategy to improve thermoelectric performance is to simultaneously reduce the thermal conductivity and enhance the electrical conductivity [10]. It has been demonstrated that lattice scattering of the phonons in nanoscaled layered thermoelectric materials would be able to effectively reduce the material's thermal conductivity as the formation of nanostructures brings about lots of boundaries which act as effective phonon scattering centers [32]. Meanwhile, the charge carrier mobility on the 2D thin layers with higher crystallinity is greatly enhanced compared to the bulk material [33]. Thus, the performance of bulk thermoelectric materials has been improved with 2D nanostructures [34]. Currently, several methods for production of 2D metal chalcogenides have been developed, including the bottom-up synthesis by vapor-phase deposition [27; 35; 36], solvothermal method [37], and top-down approach by micromechanical cleavage [38]. However, those methods always suffer from harsh conditions such as high vacuum, high temperature, and/or slow throughput. The liquid exfoliation method is regarded as a highly scalable and achievable solution-based method for producing highly crystalline 2D nanomaterials with good yields in mild conditions [39; 40; 41]. The method utilizes high shearing or ultrasonication on crystal layers formed in their respective stabilizing liquids (such as surfactants, polymer solutions, ionic liquids, etc.) to produce 2D nanosheets. Net exfoliation energy is decreased by interactions of the molecules at the nanosheet-liquid interface, preventing the nanosheets from aggregating. The obtained dispersions are stable, and the nanosheets can be synthesized at a concentration of more than 1 g L\({}^{-1}\). The liquid exfoliation method has been used in the synthesis of various 2D materials, including transition metal dichalcogenides, graphene, boron nitride, transition metal oxides, phosphorene, MXenes, and so on [39]. For Bi\({}_{2}\)Te\({}_{3}\), each crystal layer is made up of approximately 1-nm thick quintuple sheets where the five atoms is covalently bonded in the order of Te1-Bi-Te2-Bi-Te1. The bulk material is then made up of weakly interacting stacked quintuple nanosheets [42]. Given the weak van der Waals forces of interaction between the layers, the 2D nanostructures can thus be formed by intercalation chemistry with exfoliation. In 2009, [PERSON] and coworkers reported the lithium intercalation and exfoliation of layered Bi\({}_{2}\)Te\({}_{3}\) and Bi\({}_{2}\)Se\({}_{3}\) (Figure 2a) [43]. The intercalation of the lithium ions between the nanosheets is due to the reducing power of electrons solvated in liquid ammonia. The intercalated materials can be exfoliated in water with the assistance of ultrasonication to create colloidal suspensions. Dialysis can then be used to remove the lithium ions after exfoliation. The colloidal suspensions can be coated onto different substrates to form 2D flimsy oriented thin films. Very recently, [PERSON] et al. prepared S-doped SnSe nanosheets by hydrothermal lithium intercalation and subsequent exfoliation from bulk ingots [44]. The substitution of S atoms is beneficial for the phonon scattering from many atomic disorders and nanosized boundaries, leading to effective reduction in thermal conductivity. Furthermore, through solution-phase chemical transformation of the obtained SnSe\({}_{1-x}\)S\({}_{x}\) nanosheets using tartaric acid, which can partially dissolve Sn\({}^{2+}\) ions, a highly porous structure can be generated with further decreased thermal conductivity due to the scattering effect caused by the nanoscale porous boundaries.