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

Both ternary sulfides, BaLa2S4 and CaLa2S4, are found to possess a suitable band gap for absorbing visual light; moreover, the ECB and EVB are at right positions allowing the photo-generated electron/hole pairs to split water. The valence band of BaLa2S4 (CaLa2S4) are characterized by high peaks a t-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 be-tween electron and hole enhances the separation of electron/hole pairs. CaLa2S4 and BaLa2S4 are p-type semiconductors with positive thermopower S of about 229.8 mu V/K and 234.8 mu V/K, respectively, at Fermi level (mu = 0) and temperature 300 K. Both materials show good thermoelectric performance for hole concentration being in the range 1 x 10(19)-1 x 10(21) cm-(3.) The electronic figure of merit ZT(e) marks the highest value of about 0.95 and 0.97 at 2 x 10(19) cm(-3) for CaLa2S4 and BaLa2S4, respectively. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.