Effects of Grain Size on the Thermoelectric Properties of Cu2SnS3: An Experimental and First-Principles Study

Cu-Sn-based sulfides are earth-abundant and nontoxic compounds of special interest for low-cost energy harvesting applications. In the present work, we have investigated the effect of grain size on the thermoelectric properties of Cu2SnS3 (CTS). Three dense CTS samples with nanometric grains were produced by mechanical alloying combined with spark plasma sintering, preserving the small size of crystalline domains to 12, 25, and 37 nm, respectively. The experimental results show that the Seebeck coefficient (S) and electrical resistivity (ρ) decrease with decreasing domain sizes, while the thermal conductivity (κ) increases. A smaller domain size correlates with a lower resistivity and a degenerate semiconductor-like behavior due to higher carrier concentration. At the same time, our synthesis method leads to materials with very low lattice thermal conductivity, thanks to the nanometric size of grains and structural disorder. As a result, the sample with the smallest grain size exhibits the highest zT of ∼0.4 at 650 K. First-principles density functional theory (DFT) simulations on various CTS crystallite surfaces revealed localized states near the Fermi level and the absence of band gap, indicating the metallic nature of the surfaces. Various CTS systems were tested by DFT, showing the following order of increasing formation energy: stoichiometric CTS, Cu vacancy, Cu-rich, Sn vacancy, and Sn-rich.