Preparation and tetracycline degradation performance of MoO₃-Cu₂O/CN ternary photocatalyst

With the frequent use of antibiotics, a series of water pollution problems have been caused, which directly pose a potential threat to human ecosystems. Due to the structurally stable properties of antibiotics, traditional wastewater treatment methods cannot completely remove them. Therefore, developing an efficient and sustainable method to remove antibiotics plays a crucial role in the treatment of water pollution.

Advanced oxidation processes (AOPs) are widely used in water resource management because of their ability to produce strong oxidizing free radicals and effectively decompose pollutants. As a method that can produce highly oxidizing free radicals, photocatalysis technology mainly uses reactive oxygen species (ROS) generated by photocatalyst in the light to oxidize and decompose pollutants, which has the advantages of high efficiency and environmental protection. Graphitic carbon nitride (g-CN) has become the focus of photocatalysis research due to its advantages of low cost, high stability and suitable band gap. However, the rapid electron and hole recombination rate and low surface area of the original g-CN severely limited its application in the field of photocatalysis. So far, the construction of Z-type heterojunction can not only promote the effective separation of carrier, but also maintain the strong REDOX ability of photogenerated electrons and holes, which is one of the effective ways to significantly improve the photocatalytic activity of original g-CN. Therefore, in this work, we first grew MoO in situ on g-CN (CN) by hydrothermal method, and then electrodeposited CuO on its surface to construct MoO-CuO/CN ternary composite photocatalyst. The catalyst was characterized by XRD, SEM, TEM, XPS and FTIR, and the successful preparation of the composite photocatalyst was proved.

Using tetracycline as the target pollutant, the degradation effect of prepared photocatalyst on tetracycline was studied. Studies have shown that after combining MoO with CN, the tetracycline degradation activity is significantly improved, and the ability to degrade tetracycline is further improved by continuing to load CuO. Within 150 min, the tetracycline degradation rate of the optimal sample 1.5 MoO-CuO-100/CN was 97.75%. The mechanism was investigated by free radical capture experiment and electron paramagnetic resonance spectroscopy (EPR). It was confirmed that hydroxyl radical (·OH) and superoxide radical (·O) were the main active substances in the photocatalytic process. The valence band and conduction band positions of CN, MoO and CuO are calculated based on various tests. It is shown that the double Z-type heterojunction is formed by the ternary composite photocatalyst. In addition, the photoelectric chemical test shows that compared with the original CN, the terpolymer has the smallest fluorescence, the largest photocurrent and the smallest impedance value, which indicates that the construction of the mass structure not only reduces the recombination rate of photogenerated electrons and holes, but also facilitates the interfacial charge transfer and improves the degradation effect of tetracycline. Finally, the intermediate products in the photocatalytic degradation of tetracycline were analyzed by color mass coupling (LC-MS), which further verified that hydroxyl radical (·OH) and superoxide radical (·O) were the main active substances in the photocatalytic process.Conclusion: Under visible light, 1.5 MoO-CuO-100/CN composite has the best degradation effect on tetracyclines, and the degradation rate at 150 min is 97.75%, which is 2.2 and 1.5 times that of CN (45.28%) and 1.5 MoO/CN (63.24%), respectively. The mechanism was investigated by free radical capture experiment and electron paramagnetic resonance spectroscopy (EPR). It was confirmed that hydroxyl radical (·OH) and superoxide radical (·O) were the main active substances in the photocatalytic process. The valence band and conduction band positions of CN, MoO and CuO are calculated based on various tests. It is shown that the double Z-type heterojunction is formed by the ternary composite photocatalyst. At the same time, the improvement of photocatalytic activity is mainly due to the construction of double Z mechanism, which widens the visible light absorption range, retains the hole electrons with high REDOX ability, and reduces the recombination rate of photogenerated electrons and holes. The results of stability experiment show that the degradation rate of tetracyclic z is still more than 90% after four cycles of the prepared catalyst, which has excellent stability and can be recycled. This work can provide theoretical guidance for the construction of double Z-type heterojunction.