Mechanism study on the photocatalytic efficiency enhancement of MoS2 modified Zn–AgIn5S8 quantum dots

Abstract

In this work, colloidal quantum dots (QDs) of environmentally friendly multinary semiconductor Zn–AgIn₅S₈ are used for heterostructure photocatalysts with hydrothermally deposited MoS₂ focusing on the mechanism study of charge transfer processes in the photocatalytic system. The influence of MoS₂ is systematically investigated on the structure, optical properties and photocatalytic activity of the Zn–AgIn₅S₈/MoS₂ nanocomposites, where the compromise of the catalytic activity and light shielding effect and increased stability is observed with increasing MoS₂. The optimized Zn–AgIn₅S₈/MoS₂-5% composite exhibits high photocatalytic activity for nearly complete degradation of Rhodamine B within 4 min under visible light irradiation. The composite photocatalysts also show good activity on degradation of other organic pollutants including a colorless antibiotic, tetracycline. Photocatalytic mechanism study proves that the main oxidizing species is the superoxide radical formed by the reaction of photogenerated electrons and oxygen molecules, for which MoS₂ mainly acts as a cocatalyst that promotes the electron transfer from the Zn–AgIn₅S₈ QDs. Time-resolved photoluminescence spectroscopy further reveals the increased charge separation by MoS₂ from the photoluminescence quenching and lifetime decrease of the QDs, especially that of the long-lived trap states. Moreover, more efficient charge transfer from the dyes is also observed with Zn–AgIn₅S₈/MoS₂, which is distinctively different from that with the Zn–AgIn₅S₈ QDs, indicating stronger interaction between the dye and photocatalyst with the introduction of MoS₂. Based on these results, a continuous charge flow mechanism is proposed, where the electron transfer can be promoted by MoS₂ not only from the QDs but also from the dyes that are all converted to superoxide radicals and contributed to the photocatalytic activity increase.