Effect of Bismuth substitution on the enhancement of thermoelectric power factor of nanostructured BixCo3-xO4

Bismuth Cobalt Oxide (BixCO3-xO4) nanoparticles with different compositions (x = 0, 0.025, 0.05, 0.1, 0.2) were prepared by chemical precipitation method. The structural, morphological and thermal properties of the prepared samples were studied by XRD, SEM, FTIR and TG&DTA analysis. X-ray diffraction analysis shows that pure phase of Cobalt oxide was formed till x 0.05 and while increasing the Bi concentration (0.05 x 5 0.2) mixed phases of CO3O4, Co2O3, CoO and separate phase of Bi2O3 were formed. The diffraction peaks were reasonably shifted due to substitution of Bi2+ ions. XPS analysis conforms the presence of mixed valance states of Co and presence of Bi with their binding states in the samples. The electrical resistivity and Seebeck coefficient were measured for BixCo3-xO4 (0 <= x <= 0.2) at different temperatures. It was observed that the electrical resistivity decrease till x 5 0.05 due to the substitution of Bi ions in Cobalt lattice and increases at higher x values (0.05 5 X <= 0.2) due to the formation of Bi2O3 phase. The Bi substitution has considerably reduced the electrical resistivity by one order when completely dissolved in the cobalt oxide lattice at lower x values. The Seebeck coefficient value gradually increased for all samples of BixCo3-xO4 (0 <= x <= 0.2). The power factor was calculated from electrical resistivity and Seebeck coefficient and the maximum power factor of 0.025 mu Wm K-1 (-2) was obtained for Bi0.2Co2.8O4 sample at 530 K. The experimental results revealed that the Bi substitution have promising effect on the thermoelectric properties of nanostructured BixCo3-xO4 (0 <= x <= 0.2).