Construction of a direct Z-scheme ZnS quantum dot (QD)-Fe2O3 QD heterojunction/reduced graphene oxide nanocomposite with enhanced photocatalytic activity

A novel direct Z-scheme ZnS quantum dot (QD)-Fe2O3 QD heterojunction/reduced graphene oxide (rGO) (GZxFy) nanocomposite was successfully synthesized by a simple hydrothermal method. The surface of ZnS and Fe2O3 QDs was modified by -COOH and -OH groups, respectively. ZnS QDs can be covalently bonded with Fe2O3 QDs by the dehydration reaction to form the ZnS QDs-Fe2O3 QDs heterojunctions. The GZxFy nanocomposites displayed the highest photodegradation efficiency of 96.45% (40 min, UV light) and 90.17% (480 min, visible light) for methylene blue when the mass ratio of ZnS QDs to Fe2O3 QDs was 1:3. The excellent photocatalytic activity could be attributed to the enhanced light-harvesting ability, high large specific surface area, efficient interfacial charge-carrier separation and transfer as well as the low charge transfer resistance. Ultraviolet photoelectron spectroscopy and radical trapping experiments were used to confirm the Z-scheme mechanism formed between ZnS QDs and Fe2O3 QDs, and verify the electron transfer direction for UV or visible light-driven photocatalytic reactions. GZ1F3 nanocomposites showed the excellent superparamagnetic behavior, which enabled its rapid magnetic recycle from the solution within 95 s in a magnetic field.