Construction of a p-n heterojunction based on magnetic Mn 0.6 Zn 0.4 Fe 2 O 4 and ZnIn 2 S 4 to improve photocatalytic performance

To improve the photocatalytic performance of Mn 0.6 Zn 0.4 Fe 2 O 4 (MZFO) and ZnIn 2 S 4 (ZIS) for organic pollutants, the p-n MZFO@ZIS heterojunctions with different weight percentage (10 ~ 40%) of MZFO are constructed from spent batteries and added indium ion via a green bioleaching and hydrothermal method. Structural, optical, and photocatalytic properties for the heterojunctions are investigated systematically by XRD, FT-IR, SEM–EDX, TEM, BET, VB-XPS, UV–vis DRS, PL, etc. The results confirm that p–n junction significantly improves the visible light adsorption and the separation efficiency of photogenerated carriers. Specifically, MZFO-25%@ZIS shows the highest photodegradation performance toward Congo red (CR), and its reactive kinetic constant is about 9.6, 7.8, and 7.0 times higher than that of P25 TiO 2 , MZFO, and ZIS, respectively, and MZFO-25%@ZIS still possesses a high reusability and simple magnetic separation after 5 cycles of reuse. The radical trapping experiments and electronic paramagnetic resonance (EPR) tests show that · O 2 − , · OH, and h + are the most important active substance for degrading CR. The pathways for the CR degradation are further proposed based on the intermediate analysis. DFT + U calculations confirm that the high charge density of Zn–O, Fe–O, and Zn–S bonds in the MZFO and ZIS molecules provides the electrons for the sufficient production of free radicals. This work also provides a novel high value-added strategy for the green utilization of spent batteries.