Thermoelectric Performance of Surface-Engineered Cu_1.5-xTe-Cu₂Se Nanocomposites

Cu_2-xS and Cu_2-xSe have recently been reported as promising thermoelectric (TE) materials for medium-temperature applications. In contrast, Cu_2-xTe, another member of the copper chalcogenide family, typically exhibits low Seebeck coefficients that limit its potential to achieve a superior thermoelectric figure of merit, zT, particularly in the low-temperature range where this material could be effective. To address this, we investigated the TE performance of Cu_1.5-xTe-Cu₂Se nanocomposites by consolidating surface-engineered Cu_1.5Te nanocrystals. This surface engineering strategy allows for precise adjustment of Cu/Te ratios and results in a reversible phase transition at around 600 K in Cu_1.5-xTe-Cu₂Se nanocomposites, as systematically confirmed by in situ high-temperature X-ray diffraction combined with differential scanning calorimetry analysis. The phase transition leads to a conversion from metallic-like to semiconducting-like TE properties. Additionally, a layer of Cu₂Se generated around Cu_1.5-xTe nanoparticles effectively inhibits Cu_1.5-xTe grain growth, minimizing thermal conductivity and decreasing hole concentration. These properties indicate that copper telluride based compounds have a promising thermoelectric potential, translated into a high dimensionless zT of 1.3 at 560 K.