Investigation On Three-dimensional Porous ZnWO₄ And Its Heterojunction Photocatalysts Prepared By Vacuum Freeze-drying

In recent years,as the problem of water pollution has intensified,people's demand for water resources has increased,and higher requirements have been placed on the quality of the living environment.Therefore,how to effectively control water pollution has become an urgent problem to be solved.In the study of water pollution control,semiconductor photocatalysis technology is low in cost,high in pollution control efficiency and no secondary pollution to the environment.Because of these unique advantages,semiconductor photocatalysis technology has become the focus of attention.Zinc tungstate?ZnWO₄?with the black tungsten ore structure has high application value in the fields of photoluminescence,optical fiber,scintillation materials,magnetism and catalysis.However,recently,ZnWO₄with micro-nano structure still has the following three problems:First,the powder ZnWO₄ is easy to agglomerate,resulting in greatly reduced active sites,and the photocatalytic performance is affected.Second,the techniques commonly used to prepare ZnWO₄,such as templates.Most of the preparation processes are complicated and difficult to apply in mass production of ZnWO₄.Third,because ZnWO₄ has a slightly wider band gap,it can only generate photogenerated electron-hole pairs under ultraviolet excitation,and the utilization rate of sunlight is extremely low.To this end,in order to solve the above problems,the three-dimensional porous ZnWO₄ nanomaterials were prepared by vacuum freeze-drying method,which reduced agglomeration and could be applied in mass production,and the ZnWO₄ nanomaterials were doped with WO₃ and ZnO to form heterojunctions thereby improving the separation efficiency of photogenerated electron-hole pair,which greatly increased the photocatalytic activity of the WO₃/ZnWO₄ and ZnO/ZnWO₄ composite heterojunctions.The specific research contents are as follows:?1?PVA/H₃PW₁₂O₄₀/C₄H₆O₄Zn composite materials were prepared by vacuum freeze-drying technology,using high molecular weight polyvinyl alcohol?PVA?/phosphorus tungsten heteropoly acid(H₃PW₁₂O₄₀)/zinc acetate dihydrate?C₄H₆O₄Zn·2H₂O?as the precursor.Then,through the high-temperature calcination process,porous ZnWO₄,WO₃ and WO₃/ZnWO₄ heterojunction materials with three-dimensional structure,macroporous pores and mesoporous pores,uniform pores and controlled morphology were prepared.During the experiment,the ratio of phosphotungstic acid to zinc acetate was adjusted by adjusting the mass of zinc acetate dihydrate in the precursor to prepare pure phase zinc tungstate,tungsten oxide and different WO₃ loadings and observe the different topographical features of the three.The photocatalytic degradation ability to the dye Rhodamine B?RhB?were studied under full-spectrum illumination conditions.The results show that the specific surface area of the WO₃/ZnWO₄ heterojunction material prepared by us is larger than that of pure phase zinc tungstate,which is attributed to its mesoporous structure,and the photocatalytic properties of the composite heterojunction are improved compared to that of pure phase zinc tungstate and pure phase tungsten oxide,and as the WO₃ loading increases,the photocatalytic properties of the heterojunction become better and better,thanks to the larger specific surface area brought about by the appearance of mesopores,which increases the active sites of the catalytic reaction,and the constructed heterojunction reduces the recombination probability of photogenerated electron-hole pairs,enabling carriers to diffuse to the surface of the material and participate in the reaction.?2?Using PVA/H₃PW₁₂O₄₀/H₃PW₁₂O₄₀ aqueous solution as precursor,similarly,three-dimensional porous ZnWO₄,ZnO and ZnO/ZnWO₄ heterojunction materials were prepared by vacuum freeze-drying combined with high-temperature calcination,and the effect of ZnO on the photocatalytic properties of ZnO/ZnWO₄ heterojunction materials was investigated by changing the amount of phosphotungstic acid in the precursor to change the loading of ZnO in the composite heterojunction.The results show that the photocatalytic properties of the composite heterojunction are better than the pure phase zinc tungstate and pure phase zinc oxide,and the photocatalytic properties of the sample become better with the increase of ZnO loading.Compared with the first part,the photocatalytic performance of ZnO/ZnWO₄ heterojunction is greatly improved compared to that of WO₃/ZnWO₄heterojunction,which means that doping ZnO is more beneficial to improve the photocatalytic properties of zinc tungstate than doping WO₃.The capture experiments show that superoxide radicals play the most important role in the photocatalytic reactions of these two heterojunctions,and the photocatalytic degradation efficiency of ZnO/ZnWO₄ heterojunctions is higher because the conduction band of ZnO is higher than that of WO₃,which is more conducive to the generation of superoxide radicals.