Ternary sulfides BaLa2S4 and CaLa2S4 as promising photocatalytic water splitting and thermoelectric materials: First-principles DFT calculations.

Both ternary sulfides, BaLa 2 S 4 and CaLa 2 S 4 , are found to possess a suitable band gap for absorbing visual light; moreover, the E C B and E V B are at right positions allowing the photo-generated electron/hole pairs to split water. The valence band of BaLa 2 S 4 (CaLa 2 S 4) are characterized by high peaks at −1 eV and −2 eV, and strong hybridization of S/La- p and Ba- s (Ca- s / p) states. These features help to enhance the separation of electron/hole pairs. Meanwhile, the conduction band has high peak at 3 eV, which is mostly constructed by hybridization of La- f , La- d and Ba- s (Ca- s / p) states. The conduction band's smooth dispersion is favorable for inter-band transition, thus it enhances the absorption rate of two considered compounds. The Ba/Ca/La–S bonds are predominantly covalent with electronegativity difference of about 31–38%. The large difference in effective mass between electron and hole enhances the separation of electron/hole pairs. CaLa 2 S 4 and BaLa 2 S 4 are p-type semiconductors with positive thermopower S of about 229.8 μ V / K and 234.8 μ V / K , respectively, at Fermi level (μ = 0) and temperature 300 K. Both materials show good thermoelectric performance for hole concentration being in the range 1 × 1019–1x1021 cm-3. The electronic figure of merit Z T e marks the highest value of about 0.95 and 0.97 at 2 × 1019 cm-3 for CaLa 2 S 4 and BaLa 2 S 4 , respectively. Image 1 • Band structure of Ba/CaLa 2 S 4 is suitable for light absorption and water splitting. • Electronic structure is favorable for electron/hole separation and charge mobility. • BaLa 2 S 4 and CaLa 2 S 4 are p-type semiconductors. • Figure of merit is 0.95 (CaLa 2 S 4) and 0.97 (BaLa 2 S 4) at 2x1019 cm-3. [ABSTRACT FROM AUTHOR]