Morphology-dependent thermoelectric properties of mixed phases of copper sulfide (Cu2−xS) nanostructures synthesized by hydrothermal method

In this research work we have synthesized copper sulfide (Cu2−xS) nanostructures by the hydrothermal method. The post-sulfurization was performed by using tube furnace to reduce sulfur deficiency and to minimize impurities present in the crystal-lattice. The samples were synthesized by various pH values of precursor solution. The samples were characterized by XRD, RAMAN spectroscopy, SEM, Hall measurements and Seebeck effect. XRD analysis revealed the formation of hexagonal covellite (CuS) and chalcocite (Cu2S) phases of copper sulfide. Despite these phases XRD data also showed the dominating hexagonal digenite (Cu1.8S) phase of copper sulfide. We observed a decreasing trend in average crystalline size with the increasing pH values of the samples from the XRD data. We have found maximum average crystalline size of 39.1679 nm at pH value of 4. The formation of LO mode of hexagonal covellite phase of copper sulfide was observed from RAMAN analysis. The SEM images revealed the formation of flower-like nanostructures at the lowest pH value. It strengthened our argument that the crystallinity of nanostructures is inversely proportional to the pH value. We have achieved maximum values of Seebeck coefficient and conductivity at room temperature as 73.920 µV/K and 0.909 S/cm, respectively. The Seebeck coefficient and conductivity graphs showed an increasing trend due to the energy filtering effect in mesoporous structure. The maximum value of power factor was 4.91 × 10–7 Wm−1 K−2.