Realizing zT>2 in Environment-Friendly Monoclinic Cu₂ S-Tetragonal Cu_1.96 S Nano-Phase Junctions for Thermoelectrics

Phase-junction nanocomposites, made of nanograins with the same composition but different phases, offer a platform to optimize the physiochemical performance of materials. Herein, we demonstrate a straightforward strategy to synthesize Cu_2-x S phase-junction nanocomposites by retaining surface 1-dodecanethiol (DDT) ligands, in contrast to the traditional method that strips the ligands. As a result, phase junctions between a conventional monoclinic (m) phase and an unconventional metastable tetragonal (t) phase are obtained. The significantly improved power factor is obtained due to the doping of the t-phase. The phase-junction interfaces reduce thermal conductivity. Finally, surface regulation of phase junctions pushes the peak zT to 2.1 at 932 K, being the highest reported for environment-friendly metal sulfides. This work provides a paradigm to optimize thermoelectric performance by controlling phase junctions through surface-ligand tuning.