An alternative, faster and simpler method for the formation of hierarchically porous ZnO particles and their thermoelectric performance†
We devised an alternative, faster and simple method to prepare hierarchically nanoporous ZnO powders (NZnO) via a quick interfacial reaction (double emulsion) from the mixtures of zinc nitrate hexahydrate (solution, Zn(NO3)2·6H2O), n-hexane (C6H14), ammonium bicarbonate (NH4HCO3), Tween 80 (C64H124O26) and Span 80 (C24H44O6), which was calcined at 450 °C. Due to the micro/nanopore structures, the obtained NZnO powders had a larger surface area (∼69.7 m2 g−1) than the commercial ZnO powders. For the thermoelectric evaluation, the textured NZnO pellets (T-NZnOP) were prepared by low pressure spark plasma sintering (SPS) using the newly synthesized NZnO powders (NZnO). In this study, the synthesized NZnO powders that were made into T-NZnOP (sample pellets) showed a significantly lower thermal conductivity with distinctive electrical properties when compared to the bulk commercial ZnO powders. The thermoelectric enhancement can be attributed to the nanopore distribution found in the porous T-NZnOP material which demonstrates the potential usefulness of this method for other porous oxide thermoelectric (TE) materials.