ZnO/NiMoO4n-n heterojunction with enhanced carrier separation for persulfate-assisted sonocatalytic removal of methylene blue.

A series of ZnO/NiMoO4 composites was prepared and assessed for sonocatalytic removal of methylene blue (MB), used as a representative dye pollutant. Among the synthesized materials, the composite containing 30 wt% ZnO (ZN-30) displayed the highest activity. Optimal conditions comprised a catalyst dosage of 1.0 g/L, an initial solution pH of 7.20, ultrasonic power of 500 W, and an initial MB concentration of 3 mg/L. At these settings, removal of MB by ZN-30 reached 96.43 ± 1.71% within 120 min. The corresponding efficiencies for ZnO and NiMoO4 alone were 64.89 ± 2.76% and 64.25 ± 0.81%, respectively. The enhanced sonocatalytic activity was attributed to the ZnO/NiMoO4 heterojunction, which limited recombination of electron-hole (e--h+) pairs generated during ultrasonic activation. The major species driving MB removal were hydroxyl radical, h+ and superoxide anion radical. Introducing K2S2O8 (15 mmol/L) enabled 91.03 ± 0.76% MB removal within 30 min, substantially reducing the reaction time. ZN-30 maintained good stability and reusability during MB removal. These findings demonstrate the efficacy of heterojunction construction in enhancing sonocatalytic performance and support ZnO/NiMoO4 composites for organic-pollutant removal through sonocatalysis.