Photocatalytic reduction of Cr(VI) by AgIn5S8/ZnIn2S4 heterojunction under visible light: Experimental and density functional theory study

Ternary metal sulfides have excellent light absorption properties, but their high recombination rate of photo generated carriers limits their practical application. To address this problem, this work constructed a novel nanoflower−like AgIn5S8/ZnIn2S4 heterojunction and explored its removal performance for Cr(VI) in water. AgIn5S8 was prepared by the solvothermal method, whereas ZnIn2S4 and AgIn5S8/ZnIn2S4 heterojunction was synthesized via the hydrothermal method. The morphological characteristics of AgIn5S8/ZnIn2S4 heterojunction showed that the AgIn5S8 particles uniformly distributed on the surface of ZnIn2S4 with a flower like structure. The obtained 10% AgIn5S8/ZnIn2S4 exhibited a higher specific surface area (75.07 m2/g) than AgIn5S8 (20.91 m2/g) and ZnIn2S4 (58.30 m2/g), with the maximum Cr(VI) removal efficiency of 88.5% at an initial concentration of 20 mg/L. The photocatalytic reduction of Cr(VI) by AgIn5S8/ZnIn2S4 conformed to the pseudo-first-order kinetic model. The superoxide radicals (·O2−) and photogenerated electrons (e−) were identified as the major active substances for Cr(VI) photoreduction according to the electron paramagnetic resonance (EPR) and free radical trapping experiments. The Mott−Schottky analysis demonstrated that the band structures of AgIn5S8 and ZnIn2S4 were compatible. Density functional theory (DFT) calculations showed that AgIn5S8/ZnIn2S4 heterojunction enhanced the separation efficiency of photogenerated carriers through an internal electric field (IEF). These results demonstrate that type–II AgIn5S8/ZnIn2S4 heterojunctions have the potential for effective removal of Cr(VI) in wastewater under visible light irradiation.