Engineering thermoelectric and mechanical properties by nanoporosity in calcium cobaltate films from reactions of Ca(OH)2/Co3O4 multilayers.

Controlling nanoporosity to favorably alter multiple properties in layered crystalline inorganic thin films is a challenge. Here, we demonstrate that the thermoelectric and mechanical properties of Ca3Co4O9 films can be engineered through nanoporosity control by annealing multiple Ca(OH)2/Co3O4 reactant bilayers with characteristic bilayer thicknesses (b t ). Our results show that doubling b t , e.g., from 12 to 26 nm, more than triples the average pore size from ∼120 nm to ∼400 nm and increases the pore fraction from 3% to 17.1%. The higher porosity film exhibits not only a 50% higher electrical conductivity of σ ∼ 90 S cm-1 and a high Seebeck coefficient of α ∼ 135 µV K-1, but also a thermal conductivity as low as κ ∼ 0.87 W m-1 K-1. The nanoporous Ca3Co4O9 films exhibit greater mechanical compliance and resilience to bending than the bulk. These results indicate that annealing reactant multilayers with controlled thicknesses is an attractive way to engineer nanoporosity and realize mechanically flexible oxide-based thermoelectric materials.