Mesoporous Ag₂Se as a model n-type thermoelectric: soft-templated synthesis and thermal transport engineering

Mesoporous structuring is a promising route to suppress thermal conductivity in thermoelectrics by enhancing phonon scattering, yet mesoporous thermoelectric materials remain scarcely explored. Here, we report the synthesis and thermoelectric evaluation of mesoporous Ag₂Se as a model n-type system. Mesoporous Ag₂Se thin films are obtained by block-copolymer-directed soft-templated electrodeposition using PS-b-PEO micelles, which interact with Ag⁺ and Se(iv) species and direct the formation of orthorhombic β-Ag₂Se frameworks with a high density of mesopores. By tuning the molecular weight of copolymers and applying an optimized deposition potential of -0.48 V vs. Hg/Hg₂SO₄ (saturated K₂SO₄), we realize narrowly distributed pore diameters of 8, 12, and 16 nm and a concomitant deposition in crystallite size. The films are delaminated from ITO substrates and pressed into pellets while preserving both the mesoporous architecture and the β-Ag₂Se lattice. For the Ag₂Se-12 nm sample, the thermal conductivity at room temperature is reduced from 0.93 W m^-1 K^-1 for nonporous Ag₂Se to 0.40 W m^-1 K^-1, with moderate degradation of the power factor, resulting in a room-temperature zT of 0.98 (56% enhancement). These results establish mesostructural engineering in Ag₂Se as an effective strategy to reduce the thermal conductivity, providing a design principle for next-generation thermoelectrics.