0-D/3-D heterojunction composite constructed by decorating transition metal oxide nanoparticle on peony-like ZnO hierarchical microstructure for improved photodegradation of palm oil mill effluent

A series of transition metal oxide nanoparticles decorated on peony-like hierarchical porous ZnO (MO/ZnO, where MO = CuO, WO3 and Nb2O5) was synthesized using a precipitation-ultrasonic method. The X-ray diffraction, energy dispersive X-ray and fourier-transform infrared spectroscopy findings indicated the existence of both MO and ZnO in the composites. The field-emission scanning electron microscope and transmission electron microscope observations revealed that the MO nanoparticles were well dispersed onto peony-like ZnO hierarchical microstructure. Furthermore, the enhanced visible light absorption abilities of MO/ZnO were confirmed by UV–vis diffuse reflectance spectroscopy. The photocatalytic effectiveness of MO/ZnO was investigated by palm oil mill effluent (POME) degradation under UV irradiation. Among them, Nb2O5/ZnO composite exhibited the best photodegradation efficiency (91.7%) and the highest rate constant (0.0102 min−1) after 240 min irradiation. Moreover, a complete decolorization of POME was observed after 30 min irradiation using Nb2O5/ZnO composite. The phytotoxic effects of POME against the Vigna radiata seeds also reduced greatly after photocatalytically treated using Nb2O5/ZnO composite. Meantime, the product revealed great photocatalytic stability with the degradation efficiency retained more than 80% after 4 cycles. The radical trapping experiment demonstrated that hydroxyl radical and photogenerated hole played major role in POME degradation. The outstanding photoactivity of Nb2O5/ZnO composite can be explicated by efficient charge separation between Nb2O5 and ZnO where the evidence can be obtained from photoluminescence and terephthalic acid-photoluminescence probing analyses. The present study to construct these heterojunction composites can provide an innovative view to design highly effective photocatalytic system for the treatment of industrial wastewater. •Precipitation-ultrasonic preparation of transition metal oxide decorated ZnO.•Material properties enhanced upon loading of transition metal oxides.•Nb2O5/ZnO showed the best photoactivity in palm oil mill effluent degradation.•Hydroxyl radical and photogenerated hole played major role in photodegradation.•Improved charge separation by heterojunction composite.