IntroductionWith the increasing environmental protection and sustainable development, conventional water pollution treatment methods have some challenges in practical applications. Photocatalytic technology has potential advantages such as energy conservation and environmental protection, providing an effective and sustainable method for wastewater treatment. The design and synthesis of photocatalysts are crucial in photocatalytic reactions. Silver molybdate material has some advantages of controllable morphology, unique physicochemical properties, having potential application prospects. However, the narrow photoresponse range and the high recombination efficiency of photo generated charge carriers restrict its application in the field of photocatalysis. In this study, a ZnO/Ag₂MoO₄ composite photocatalyst was prepared by a hydrothermal method with ZnO. The purpose was to improve the photocatalytic activity of silver molybdate via constructing heterojunctions. The photocatalytic performance of different proportions of composite materials for the degradation of organic pollutants was investigated. In addition, the optical stability of the catalyst and the active free radicals involved in the reaction were also analyzed.MethodsZnO/Ag₂MoO₄ composite catalysts with different mass fractions of 10%, 50%, and 90% ZnO were synthesized by a hydrothermal method with ZnO, AgNO₃, and Na₂MoO₄·2H₂O, respectively.The photocatalytic degradation of organic pollutants was carried out in a photocatalytic reactor. 50 mg of photocatalyst was dispersed in 50 mL of Rhodamine B (RhB) or meloxicam (MLX) solution in each experiment. Prior to photocatalytic reaction, the suspension was stirred in the darkness for 30 min to obtain the absorption-desorption equilibrium. Afterwards, the suspension was placed under a xenon lamp (300 W) for photocatalytic degradation. At each interval, RhB or MLX solution with catalyst was sampled and centrifuged to remove the solid catalyst. The concentration of RhB or MLX was determined by a UV-Vis spectrophotometer. The similar experimental steps were used for the cyclic experiments. After each cyclic experiment, the solid catalyst was separated and washed with water and ethanol to continue the next cyclic experiment.To determine the main active species involved in the degradation of RhB, free radical capture experiments were conducted. The free radical capture experiment followed the same steps as the normal degradation experiment. Benzoquinone, ethanol, silver nitrate, and tert-butanol were added as a scavenger for superoxide radicals, holes, electrons, and hydroxyl radicals during the reaction.Results and discussionZnO/Ag₂MoO₄ composite catalysts with different ratios were prepared by a hydrothermal method. The results indicate the effective preparation of ZnO/Ag₂MoO₄ without other impurity elements. Based on the XRD patterns, the sharp diffraction peaks of ZnO/Ag₂MoO₄ composites indicate a good crystallinity. The SEM images show that ZAM-7 is a block shaped structure with uneven size, and ZnO is deposited in a columnar form on the surface of ZnO/Ag₂MoO₄. The minimum size of ZAM-7 is approximately 100-200 nm. The specific surface areas of ZnO, ZnO/Ag₂MoO₄, and ZAM-7 are 4.5795 m²/g, 0.0921 m²/g, and 2.6339 m²/g, respectively. The specific surface area of ZAM-7 is 28 times greater than that of ZnO/Ag₂MoO₄, indicating that the composition of Ag₂MoO₄ and ZnO is beneficial for increasing the specific surface area of the catalyst, thus providing more active sites for photocatalytic reactions.The photocatalytic performance of ZnO/Ag₂MoO₄ composite materials is investigated using RhB and MLX as simulated organic pollutants. After 90-min illumination, the removal rates of RhB by ZnO, Ag₂MoO₄, and ZAM-7 are 40.1%, 13.8%, and 89.6%, respectively. The rate constants of ZnO, Ag₂MoO₄, and ZAM-7 are 1.19×10^-2 min^-1, 0.22×10^-2 min^-1, and 3.60×10^-2 min^-1, respectively. The degradation of meloxicam is also carried out with ZAM-7. When the catalyst dosage is 50 mg, the MLX concentration is 100 mg/L, the degradation rate of MLX by the catalyst can reach 80.6% after 270 min visible light irradiation.ConclusionsZnO/Ag₂MoO₄ composite photocatalysts were prepared by a hydrothermal method. The minimum size of ZnO/Ag₂MoO₄ composite catalysts was 100-200 nm. ZnO was deposited in a prismatic shape on the surface of Ag₂MoO₄. ZnO/Ag₂MoO₄ composite catalyst exhibited a superior photocatalytic degradation activity for both organic dyes and drugs. Under the optimal reaction conditions, the degradation rate of RhB by the composite catalyst could reach 95.0%, and the degradation rate of the drug MLX could reach 80.6%. The maximum reaction rate constants for the degradation of RhB by composite catalysts were 3 times and 16 times greater than those of ZnO and Ag₂MoO₄, respectively. The improvement of photocatalytic activity after the composition of Ag₂MoO₄ and ZnO was attributed to the improvement of photogenerated carrier separation efficiency, the increase of visible light absorption intensity, the reduction of band gap width, and the increase of active sites. The cyclic experiment of the catalyst showed that ZnO/Ag₂MoO₄ composite catalyst had a good optical stability. The results could provide a useful reference for the development and design of photocatalysts for water pollution treatment.