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The water pollution due to industrial effluents causes a great health problem. Hence, it is important to treat wastewater before discharging to the environment. In this work, water hyacinth ( Eichhornia Crassipes ) plant extract mediated ZnO, CO _3 O _4 , and p-CO _3 O _4 /n-ZnO composite catalysts were synthesized by green co-precipitation routes. The resulting samples were characterized by x-ray diffractometer (XRD), scanning electron microscope (SEM), Fortier transform infrared (FT-IR), and with other instruments. The catalytic activities of ZnO, Co _3 O _4 , and Co _3 O _4 /ZnO were tested for MB dye degradation under visible light irradiation. The catalytic degradation of MB with p-CO _3 O _4 /n-ZnO composite catalyst was 95.5%; while 72% and 79% of MB dye was degraded by ZnO and Co _3 O _4 catalysts, respectively. The kinetic rate constants (k) in the degradation of MB dye with ZnO, Co _3 O _4 , and p-CO _3 O _4 /n-ZnO composite catalysts were also 0.014 min ^−1 , 0.018 min ^−1 , and 0.028 min ^−1 , respectively. The results showed that the presence of plant extract during the synthesis of the catalysts makes the catalyst more active and enhances the catalytic performances. Moreover, the formation of p-n junction in the p-CO _3 O _4 /n-ZnO catalyst also facilitates the photogenerated electron–hole separation and further enhances the catalytic efficiency. Hence, the formation of p-n junction is the key factor for enhancing the photodegradation of MB dye under visible light irradiation and the plant extract mediated catalyst synthesis also further improves its performance.