Synthesis and investigation of thermoelectric properties of Cu-doped bismuth sulfide (Bi₂S₃) nanostructures: an experimental approach

In this study, we have prepared Bi₂S₃ nanostructures as it has good thermoelectric characteristics, and to enhance the thermoelectric effects, we doped the pure Bi₂S₃ nanostructures with two different concentrations of copper (Cu). These nanostructures were synthesized at 450 °C. X-ray diffraction (XRD) confirmed the successful synthesis of all samples with an orthorhombic crystal structure. The average crystallite size of Bi₂S₃ was 26 nm, and after doping of Cu of two different concentrations, reduced crystallite sizes, i.e., 21 nm and 16 nm, were recorded. UV–visible spectroscopy showed that the band gap of Bi₂S₃ nanostructures increased after doping it with Cu. The electrical conductivity measured by the LCR meter showed an increasing trend with increasing doping concentration. DC conductivity was increased, and resistivity is observed to be decreased after doping of Cu in Bi₂S₃ nanostructures. This might be due to the contribution of charge carriers from Cu. Enhanced Seebeck coefficients measured in the temperature range of 303–366 K were recorded after every doping concentration. In the end, the power factor was calculated for all three materials, and a surprising increase in power factor was recorded for Bi_1.85Cu_0.15S₃ as compared to Bi₂S₃ and Bi_1.95Cu_0.05S₃ nanostructures. Power factors were recorded as \(\sim\) 0.15 \(\mathrm{\mu W}/{\mathrm{mK}}^{2}\), \(\sim\) 0.58 \(\mathrm{\mu W}/{\mathrm{mK}}^{2}\), and \(\sim\) 1.50 \(\mathrm{\mu W}/{\mathrm{mK}}^{2}\) for Bi₂S₃, Bi_1.95Cu_0.05S₃, and Bi_1.85Cu_0.15S₃ nanostructures, respectively. These all-recorded data prove the suitability of Cu-doped Bi₂S₃ nanostructures for the conversion of heat into electricity, i.e., for thermoelectric devices.