# Solution-Based Synthesis and Processing of Metal Chalcogenides for Thermoelectric Applications
## 2 Solution-Based Approaches for Synthesis of Metal Chalcogenides
### Electrodeposition Method  
The electrodeposition method has been considered as an effective process to fabricate metallic coatings on a substrate surface through a cathodic reduction reaction in either an aqueous or an organic solution containing the metal ions to be electrodeposited [32; 79; 80; 81]. The substrate material (such as indium-tin oxide (ITO), silicon, copper, porous anodic alumina (PAA) membranes, etc.) is used as the cathode to attract positively charged metallic ions and reduce them to their metallic form as a coating on the surface. The experimental parameters, such as electrolytes, reagent concentrations, nature of electrodes, temperature, and applied electrical conditions, could have effects on the properties of the electrodeposited films. This electrochemical synthesis process is scalable, simple, rapid, and cost-effective and it gives great control of the thickness, crystallinity, composition, and morphology. Electrodeposition has been extensively applied to fabricate various metal chalcogenide thermoelectric materials (Figure 7), such as Bi\({}_{2}\)Te\({}_{3}\) films [82; 83; 84; 85; 86; 87; 88; 89], Bi\({}_{2}\)Te\({}_{3}\) nanowire arrays [90; 91; 92; 93], Bi\({}_{2}\)Te\({}_{3}\) nanotube arrays [94], Cu\({}_{2}\)Te films [95; 96], Cu\({}_{2}\)Te nanowires [97], SnSe films [98; 99; 100; 101], and so on. Figure 6: (**a**) Schematic illustration of synthesis route; the inset picture shows the weight of 40 mmol (theoretical yield) synthesized Cu\({}_{2}\)Se powder. (**b**–**e**) TEM images of Cu-, Pb-, and Bi-based chalcogenide nanostructures. Readapted with permission from Elsevier Ltd. [70]. Figure 7: SEM images of (**a**) planar and (**b**) cross-sectional views of the electrodeposited Bi\({}_{2}\)Te\({}_{3}\) films; (**c**) highly ordered nanopores in an anodic alumina membrane (AAM) template; (**d**) side view of the electrodeposited Bi\({}_{2}\)Te\({}_{3}\) nanowires array partially embedded in AAM. Surface SEM images of electrodeposited Cu\({}_{2x}\)Se films at (**e**) 1000 and (**f**) 10,000 magnification. SEM images of (**g**) surface and (**h**) cross-sectional views of the electrodeposited SnSe films. Readapted with permission from the American Chemical Society [72], The Minerals, Metals and Materials Society [95], and Elsevier Ltd. [100]. In an early study, [PERSON] et al. prepared both n-type and p-type Bi\({}_{2}\)Te\({}_{3}\) films via electrodeposition from a nitric acid bath containing Bi\({}^{3+}\) and HTeO\({}^{2+}\) via tuning the applied potential [86]. Most of the successive work on electrodeposition of Bi\({}_{2}\)Te\({}_{3}\) films was carried out in a similar acidic aqueous solution of 1 M HNO\({}_{3}\), which can only dissolve about 20 mM Te species. The low concentration of metal ions and mass transport in the solution resulted in inhomogeneous composition, a limited growth rate, and low film thickness (<10 \(\upmu\)m). [PERSON] and coworkers investigated organic electrolyte ethylene glycol, which provided advantages such as high metal ion solubility and wide electrochemical window, for electrodeposition of thick Bi\({}_{2}\)Te\({}_{3}\) films (up to 67 \(\upmu\)m) with uniform composition [85]. [PERSON] et al. reported that using sodium lignocellulosic surfactant in the reaction can lead to reduction in Bi\({}_{2}\)Te\({}_{3}\) crystallite size to 20.8 nm and improvement of film alignment onto the preferred (110) orientation [87]. Recently, [PERSON] et al. prepared electrodeposited films of a Cu-Se binary compound from solutions containing CuSO\({}_{4}\), H\({}_{2}\)SeO\({}_{3}\), and H\({}_{2}\)SO\({}_{4}\). The copper contents and morphology of the films can be tuned via shifting the depositing potential [95]. The highest power factor for electrodeposited copper selenide film was 0.13 \(\upmu\)W m\({}^{-1}\) K\({}^{-2}\) with a Seebeck coefficient of 56.0 \(\upmu\)V K\({}^{-1}\).