# Solution-Based Synthesis and Processing of Metal Chalcogenides for Thermoelectric Applications
## 2 Solution-Based Approaches for Synthesis of Metal Chalcogenides
### Hydrothermal or Solvothermal Method  
Hydrothermal synthesis involves the growth of crystals from submicron to nanometer scale by chemical reaction in an aqueous solution at elevated temperature and high pressure. This method has been widely used since the 1960s for the synthesis of functional nanomaterials with different specific shapes and sizes [12; 49]. Precise control over the internal reaction conditions, including pressure, time, pH value, concentration, and organic additives or templates, and external reaction environment, such as microwave or conventional heating, is necessary for the successful synthesis of the inorganic semiconducting nanostructures. By varying these two internal and external conditions, different hydrothermal synthesis methods have been developed [50]. To have greater control over the size, shape, reactivity, and phase of the product, the solvothermal method can be employed by using more different organic solvents rather than water. The solvent properties such as viscosity and polarity can influence the transport behavior and solubility of the reagents in the liquid medium. Compared to hydrothermal synthesis, solvothermal synthesis showed a higher tolerance as it could be carried out in a solvent with a higher reaction temperature [12]. Figure 3: (**a**) Schematic of the electrochemical exfoliation process for generating 2D nanosheets. (**b**) Experiment set-up of a two-electrode system using Bi\({}_{2}\)Te\({}_{3}\) crystal attached to a copper tape as the working electrode and a Pt foil as the counter electrode, which is separated from the solution through a dialysis membrane in order to avoid direct contact of exfoliated material with the electrode surface. (**c**) Exfoliation occurred by applying cathodic (-10 V) voltage to the working electrode for two minutes. (**d**) Exfoliation occurred by applying repeated alternating anodic (+10 V) and cathodic (-10 V) voltages to the working electrode for two minutes. Readapted with permission from American Chemical Society [48]. Since the properties of inorganic semiconductors largely depend on their size, shape, and crystallinity, the highly controllable hydrothermal/solvothermal method has exhibited extraordinary advantages in the synthesis of inorganic semiconducting nanostructures due to simple manipulation, high yield, low energy consumption, good crystallinity, homogeneity, and narrow size distribution. By using this method, a number of nanostructured metal chalcogenides (based on Bi\({}_{2}\)Te\({}_{3}\), Cu\({}_{2}\)Se, and SnSe) with different morphologies (Figure 4), such as nanostrings [51], nanorods [52; 53], nanowires [54], nanobelts [55], nanoplates [56; 57; 58; 59; 60; 61; 62; 63], nanoflowers [56; 64], and nanopowder [65], have been synthesized and evaluated for thermoelectric applications. [PERSON] et al. reported highly crystalline \(\beta\)-phase Cu\({}_{2}\)Se nanosheets prepared through heating the mixture of CuO, SeO\({}_{2}\), NaOH, and polyvinylpyrrolidone (PVP) in ethylene glycol at 230 \({}^{\circ}\)C for 24 h. After sparking plasma sintering, the obtained Cu\({}_{2}\)Se pellets exhibit an ultralow thermal conductivity (0.2 W m\({}^{-1}\)K\({}^{-1}\)) due to strong phonon scattering by high-density small-angle grain boundaries in the nanostructured material, which leads to a considerably high ZT of 1.82 at 850 K. [PERSON] et al. reported the preparation of nanostructured Bi\({}_{2}\)Te\({}_{3}\) with different morphologies (such as nanotubes, needle-shaped nanowires, nanoflakes, and flower-like clusters) by a facile hydrothermal treatment using sodium dodecyl benzene sulfonate (SDBS) and PVP as surfactants [43]. Results show that the formation of nanostructures can be readily controlled and tuned by the use of different surfactants and varying the reaction temperatures. Another significant advantage of the hydrothermal/solvothermal method is its great convenience for incorporation of different levels of element doping via addition of the corresponding precursors in the reaction mixture. [PERSON] et al.