Rationally Controlled Fabrication And Catalytic Performance Of Zinc Oxide Based Composites

With the rapid development of the global modernization process,energy crisis and enviromental contamination have become two major problems restricting the development of human society.Therefore,seeking efficient solutions to these problems has become a research hotspot recently.Semiconductor photocatalysis technology can convert solar energy into chemical energy and can realize redox reactions with lots of advantages,such as low energy consumption,high performance and cost efficiently.It has received extensive attention from research scholars in recent years.Due to its suitable bandgap,stable physicochemical properties and abundant active sites,zinc oxide（ZnO）has been widely studied in the field of photocatalysis.However,owning to the narrow photoresponse range and high recombination rate of photogenerated electrons and holes,the photocatalytic activity and its application of pure ZnO are greatly restricted.This thesis focuses on the structural control and catalytic performance optimization of ZnO-based photocatalytic materials.Based on a comprehensive review of its research status,a series of new ZnO-based composites were successfully prepared through various synthetic strategies,such as ion doping,semiconductor integrating,heterojunction constructing and built-in electric field building.The main research results are summarized as follows:1.In order to improve the photocatalytic performance of the ZnO-based photocatalysts,a Z-scheme SnO_<sub>2/ZnO photocatalyst was designed by rationally adjustion of the Sn:Zn molar ratio to control the morphology of the final product.The crystal phase composition,microstructure,light absorption performance and mechanism of formation for the prepared photocatalyst were also discussed.The photocatalytic performance of SnO_<sub>2/ZnO composites with different morphologies was evaluated by the photocatalytic degradation experiment of methylene blue（MB）.The results showed that SnO_<sub>2/ZnO composites with different morphologies have similar crystal structures.In comparison with that of pure ZnO,the spherical SnO_<sub>2/ZnO photocatalyst has the largest specific surface area and the best photogenerated electron-hole separation efficiency.Under the light irradiation,the degradation rate of MB reached 95.49%within 60 min,which is 2.63 times than that of the pure ZnO.It also exhibited better degradation efficiencies in the degradation of three quinolone antibiotics.Mechanism studies showed that the conduction band and valence band positions of SnO_<sub>2and ZnO are well matched and form a typical Z-scheme structure.This structure effectively promotes the separation of electrons and holes and enhances the photocatalytic activity.In addition,the main active species in the reaction system is·OH.2.Based on the research of SnO_<sub>2/ZnO composites,the In-doped oxygen vacancy ZnO photocatalyst（In-OV-ZnO）was further designed and synthesized by introducing defect structure on the surface of ZnO.X-ray photoelectron spectroscopy（XPS）and photoluminescence spectroscopy（PL）characterization proved that the In-OV-ZnO photocatalyst contains a large number of oxygen vacancies.In addition,ultraviolet diffuse reflectance spectroscopy（UV-Vis DRS）and photocurrent tests showed that the presence of oxygen vacancies and In ions is beneficial to improve the light absorption capacity of the catalyst and the separation efficiency of carriers.The photocatalytic hydrogen production experiment showed that the photocatalytic hydrogen production rate of In-OV-ZnO sample reached 2346.2μmol·g^-1·h^-1under the light irradiation,which was much higher than that of ZnO under the same conditions(409.58μmol·g^-1·h^-1).The photocatalytic degradation experiments showed that the degradation rates of MB and MO by In-OV-ZnO reached 96.84%and 90.05%,respectively.Mechanism studies showed that·O₂^-plays a major role in the photocatalytic degradation reaction.3.Three-dimensional ZnIn₂S₄/ZnO photocatalysts were prepared using ZnO nanorods as templates,which further improved the visible light response range of ZnO-based composites.Various characterization tests showed that ZnIn₂S₄nanosheets were successfully grown on the surface of rod-shaped ZnO,forming a three-dimensional multi-level layered structure.The diameter of ZnO nanorods is about 400 nm,the length is about 5μm,and the thickness of ZnIn₂S₄nanosheets grown on the sidewalls is about 5 nm.The ZnIn₂S₄/ZnO photocatalysts have good light absorption capacity and more surface active sites,which lead to the better performance in both photocatalytic hydrogen production and photodegradation of pollutants.The photocatalytic performance experiment showed that the photocatalytic degradation rate of MB over ZnIn₂S₄/ZnO sample reached 95.83%under 90 min visible light irradiation.After 6 h of photoreaction,the hydrogen production rate reached 3348.3μmol·g^-1·h^-1,which was 1.4times than that of In-OV-ZnO photocatalyst.The·OH is the main active species in the ZnIn₂S₄/ZnO reaction system.4.To make full use of ZnO’s dual characteristics of semiconductor and piezoelectricity,the interface band design and heterojunction were used to promote charge separation and transfer.Using ZnO nanorods as the substrate,two piezoelectric/photocatalytic reaction systems were constructed by p-n heterojunction Ag₃PO₄/ZnO and core-shell structure Cu₂O/ZnO nanorod array.The results showed that the p-n heterojunction array prepared by growing Ag₃PO₄particles on the surface of the ZnO nanoarray by the co-precipitation method and the piezoelectric/photocatalytic effect was significantly improved,and the degradation efficiency of the MB reached 98.16%within 30 min.The Cu₂O/ZnO core-shell structure nanoarray prepared by the ion-layer adsorption method has a degradation efficiency of 99.08%for MB within 30min,which is 1.67 times than that of ZnO under the same conditions.Radical capture experiments evidenced that in the Ag₃PO₄/ZnO catalytic system,·OH and·O₂^-are the main active species,while the main active species in the Cu₂O/ZnO catalytic reaction system is h⁺.