One-step synthesis of phosphorus-doped g-C3N4/Co3O4 quantum dots from vitamin B12 with enhanced visible-light photocatalytic activity for metronidazole degradation

In this study, Co 3 O 4 quantum dots decorated phosphorus-doped g-C 3 N 4 nanosheets was synthesized though one-step thermal polymerization method using vitamin B12 (VB12) as precursor of cobalt and phosphorus mixed with melamine. The prepared photocatalysts were characterized by X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), BET, transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (DRS), photoluminescence (PL) and electrochemical measurements. Appropriate amount of introduced VB12 can improve photocatalytic activities while excess caused inhibition. The sample with the weight ratio of VB12 in precursors at 1% exhibited the optimum photodegradation efficiency for metronidazole (MTZ), about 2 times as higher as the removal rate of pure g-C 3 N 4 . Several effects on the photocatalytic activity was also investigated and the results showed that enough dissolved oxygen and presence of Fe 3+ was found to promote MTZ photodegradation, while NO 3 − , Ca 2+ and HCO 3 − would inhibit it. Superoxide radical O 2 − was identified as the dominate active specie in photodegradation process and possible enhancement mechanism of introduced VB12 for photocatalytic activities was proposed. It was inferred that the synergistic effect of generated Co 3 O 4 quantum dots and P-doped g-C 3 N 4 , including increasing the separation efficiency of photo-inducted electrons and holes, inhibiting the recombination process of them and decreasing the band gap energy, could be the reason for the better photocatalytic activity. At last, MTZ degradation pathway in the photosystem was determined.