Preparation And Photocatalytic Performances Of Tungstate ?ZnWO₄,Bi₂WO₆?-based And G-C₃N₄-based Visible Light-driven Photocatalytic Materials

The development of novel high-efficiency visible-light-driven photocatalysts is of great importance for photocatalysis research.In this thesis,fabrication of the heterojunction and doping ions were adopted,on a purpose of enhancing the separation and the transportation of the photogenerated electron-hole pairs.Therefore,the photocatalytic performance of semiconductor would be boosted.ZnWO₄,Bi₂WO₆,and g-C₃N₄ are popular semiconductor photocatalysts.Two series of ZnWO₄-based,one series of Bi₂WO₆-based,two series of g-C₃N₄-based composited materials,and one series of Bi₂WO₆-based doped materials were prepared.The as-prepared photocatalysts were characterized by various techniques.We have achieved some positive results,and our work provides an effective approach to design and develop high-activity,high-stability,and visible-light-driven photocatalysts.The specific work includes the following aspects:1.Preparation of ZnWO₄-based semiconductor photocatalysts and its photocatalytic mechanism research.?1?Novel heterojunction photocatalysts In₂S₃/ZnWO₄ were prepared by hydrothermal and surface-functionalized method,and the optimized In₂S₃/ZnWO₄ ratio was tuned to explore their visible-light photocatalytic activity for RhB degradation.The heterojunction structure was formed by In₂S₃ nanoparticles grew on the primary ZnWO₄ nanorods.Compared with pure ZnWO₄,the heterojunction composites possess larger specific surface area and stronger visible-light absorption.Remarkably,In₂S₃/ZnWO₄ composites exhibited much higher photocatalytic activity than that of the individual In₂S₃ and ZnWO₄.The results of photocurrent and PL revealed that the formation of the heterojunction between In₂S₃ and ZnWO₄ induced the excellent separation and transportation of the photogenerated charges.Then,a possible reaction mechanism for the excellent photocatalytic activity of In₂S₃/ZnWO₄ composites was proposed.?2?One-dimensional FeWO₄@ZnWO₄/ZnO heterojunction photocatalysts were prepared by a two-step hydrothermal method.The heterojunction structure was formed through the dispersing of FeWO₄ nanoparticles on the surface of ZnWO₄/ZnO nanorods.The heterojunction photocatalyst possessed strong visible light absorption capacity.The photocatalytic activities of the prepared photocatalysts were evaluated by degradation of RhB,MB,and phenol under visible light irradiation.Remarkably,the FeWO₄@ZnWO₄/ZnO composite showed superior photocatalytic activity than that of individual FeWO₄ and ZnW04/ZnO.The enhanced photocatalytic activity could be attributed to the interfacial transportation of photogenerated holes between FeWO₄ and ZnWO₄/ZnO substrate,leading to more effective separation and faster transfer of carriers in the heterostructures,which was confirmed by photocurrent responses and PL analysis.Radical trapping experiments indicated that ·OH were the major reactive species.Moreover,a possible mechanism for the excellent photocatalytic activity of FeWO₄@ZnWO₄/ZnO composites was proposed.This finding may pave a way for the further better construction of a ZnWO₄-based heterojunction to improve the visible light photocatalytic performance.2.Preparation of Bi₂WO₆-based semiconductor photocatalysts and its photocatalytic mechanism research.?1?Samarium and nitrogen co-doped Bi₂WO₆ nanosheets were successfully synthesized by using a hydrothermal method.The crystal structures,morphology,elemental compositions,and optical properties of the prepared samples were investigated.The incorporation of samarium and nitrogen ions into Bi₂WO₆ was proved by XRD,EDS,and XPS.DRS spectroscopy indicated that the samarium and nitrogen co-doped Bi₂WO₆ possessed strong visible-light absorption.Remarkably,the samarium and nitrogen co-doped Bi₂WO₆ exhibited higher photocatalytic activity than single-doped and pure Bi₂WO₆ under visible-light irradiation.Radical trapping experiments indicated that h+ and ·O2-were the main active species.The results of PL and photocurrent measurements demonstrated that the recombination rate of the photogeneratcd electrons and holes pairs was greatly depressed.The enhanced activity was attributed to the synergistic effect of the in-built Sm3+/Sm2+ redox pair centers and the N-doped level.The mechanism of the excellent photocatalytic activity of Sm-N-Bi₂WO₆ is also discussed.?2?Direct Z-scheme MoS₂/Bi₂WO₆ heterojunctions were prepared by a facile method.The heterostructure was formed by loading MoS₂ nanoparticles on the surface of Bi₂WO₆ nanosheets.The as-prepared MoS₂/Bi₂WO₆ composites showed superior photocatalytic activity than that of individual MoS₂ and Bi₂WO₆.The optimal composite with 4 at%MoS₂ content exhibited the highest photocatalytic activity.Based on the active species trapping experiments,PL analysis,and photocurrent responses,the possible enhanced photocatalytic mechanism could be ascribed to a direct Z-scheme heteroj unction system which can not only increase the separation efficiency of photogenerated electronhole pairs but also possess a splendid oxidation and reduction ability for high photocatalytic performances.3.Preparation of g-C₃N₄-based semiconductor photocatalysts and its photocatalytic mechanism research.?1?A novel Eu₂O₃/g-C₃N₄ redox heterojunction was designed and fabricated by a facile in-situ growth strategy.Compared with pure g-C₃N₄,the as-prepared Eu₂O₃/g-C₃N₄ redox heterojunction showed extremely enhanced photocatalytic performance for decontamination of MB,RhB,and tetracycline under visible-light irradiation.The presence of self-redox couple of Eu3+/Eu2+ was identified by XPS analysis before and after photoreaction.The boosted photocatalytic mechanism was explored systematically via DRS,PL,and photocurrent measurements.Moreover,the radical trapping and EPR experiments revealed that ·O2-and h+ were the main active species,and small amount of ·OH radicals were also generated.The enhanced photocatalytic activity was derived from the effective separation of photogenerated electron-hole pairs by forming Eu₂O₃/g-C₃N₄ heterostructure with self-redox center.In particular,the self-redox couple of Eu3+/Eu2+ can lead to more effective separation of photogenerated carrier,as well as sustainable production of ·O2-radical species.It is anticipated that our work could provide a new insight into development of g-C₃N₄-based redox heterojunction with highly-visible-light photocatalytic activity.?2?Novel g-C₃N₄/La₂Ti₂O₇ heterojunction photocatalysts were designed and constructed.The results revealed that La₂Ti₂O₇ nanosheets were well attached to the surface of g-C₃N₄ and the composites had good visible-light absorption properties.Significantly,compared to pure La₂Ti₂O₇ and g-C₃N₄,the composites exhibited outstanding photocatalytic performance under visible-light irradiation.The ·O2-and h+ were the major active species.The enhanced photocatalytic activity could be derived from the effective transportation of photo generated electrons from g-C₃N₄ to La₂Ti₂O₇.The electrons with high reduction ability can generate more active species to participate in the photodegradation reaction.