Nanostructured thermoelectric cobalt oxide by exfoliation/restacking route

Abstract

We report on enhanced thermoelectric (TE) properties of two-dimensional (2D)-nanostructured cobalt oxides prepared by the exfoliation/restacking method. Polycrystalline [(NaxCoO2)/(CaO)n] (x = 0.48-0.58 and n = 0.14-0.23) were synthesized by restacking the exfoliated Na0.7CoO2 with CaCl2. The composite oxide of [(NaxCayCoO2)/(LizCoO2)m] (x = 1.8, y = 1.2, and z = 0.24; m = 5) was produced with a 2D-superlattice structure, in which the exfoliated NaxCoO2 and LizCoO2 nanolayers were alternately stacked. The substitution of Ca2+ for Na+ of the restacked compounds decreases the carrier density, which results in increases of Seebeck coefficient and resistivity. The doping of the restacked compounds was found to be comparable to that of NaxCoO2 (x > 0.85). Despite the decreased vacancy of interlayer cations, the present restacked compounds show reduced thermal conductivity compared to the pristine sample. Due to the decrease of in-plane thermal conductivity, figure of merit (ZT) values of the restacked compounds increase in comparison to that of the pristine samples, which shows the beneficial effect of the nanostructure on the thermoelectric property. According to the increasing amount of calcium during the restacking process, the carrier density decreased by lower hole doping, resulting in the increased in-plane Seebeck coefficients, however, the resulting ZT value remains between 0.3∼0.4 because of the increased thermal conductivity due to higher sintering temperature. It has been found that the cobalt oxide exfoliated by tetramethylammonium (TMA) exhibits more significantly reduced thermal conductivity than that by octylamine (OA), resulting in a larger ZT value. This result implies that the nanolayer of NaxCoO2 exfoliated by TMA is thinner than by OA, which results in more significant phonon scattering in two-dimensional CoO2 layer.