Preparation,Characterization,and Properties Of Bismuth Tungstate And Bismuth Molybdate Based Photocatalysts

Semiconductor photocatalysis is regarded as a potential technology to solve energy crisis and environment problems.Some traditional photocatalysts with wide band gaps,like TiO₂ and ZnO,could absorb and utilize ultraviolet light,but do not respond to visible light?40%of solar-light?.Thus,it is urgent to develop novel visible-light photocatalysts.Bismuth tungstate?Bi₂WO₆?and bismuth molybdate?Bi₂MoO₆?,the layered Aurivillius-type compounds,are excellent visible-light photocatalysts with narrow band gaps,which show great potential for industrial application.However,the high recombination efficiency of photogenerated electrons and holes is still a problem that needs to be resolved to further enhance the photoactivity of Bi₂WO₆ and Bi₂MoO₆.Various ways have been developed to deal with this problem,such as morphology control,element doping,and preparation of complexes.First of all,the structure-performance relationship of Bi₂WO₆ nanosheets is researched in this thesis.Second,the mechanism for the photoactivity enhancement of Fe-doped Bi₂WO₆ is discussed.Finally,the functions of indium hydroxide?In?OH?₃?in the In?OH?₃/Bi₂WO₆ and In?OH?₃/Bi₂MoO₆ heterojunctions are analyzed.In summary,this thesis explains the structure-performance relationship of Bi₂WO₆ nanosheets and provides an understanding of the photoactivity improvement,which provide guidance for industrial application.Based on the above research,the main contents and conclusions of this thesis are listed as follows:1.Synthesis and photocatalytic activity of bismuth tungstate nanosheets with tunable thicknessThe Bi₂WO₆ synthesized by hydrothermal or solvothermal methods tends to form 2D nanosheets.The reduction of the nanosheet size is considered as an important reason for the photoactivity enhancement of the Bi₂WO₆ nanostructure.However,owing to the fact that the nanosheet size refers to both the nanosheet thickness and the lateral size?L?,it is essential to understand whose change plays the main role for the photoactivity improvement.Bi₂WO₆ nanosheets with dominant exposed?010?facets and various sizes were hydrothermally synthesized via pH adjustment of precursor suspensions.As pH increases from<1 to 8,resultant nanosheets exhibit decreased specific surface area,improved crystallinity and photoabsorption,increased thicknesses,and decreased photoactivity in degradation of rhodamine B?RhB?,methylene blue?MB?,and Eosin Y?EY?under visible light irradiation.The photoactivity of the Bi₂WO₆ sample obtained at pH<1 is about 6,100,and 25 times of that at pH = 8 for RhB,MB,and EY degradations,respectively.The photoactivity enhancement is ascribed to the reduction of the H.The photocatalytic efficiencies are inversely proportional to the reduction of H₂ when the nanosheets can be penetrated by incident light.This work reveals the structure-performance relationship of Bi₂WO₆ nanosheets and provides significant guidance for preparation of high efficient two-dimensional photocatalysts.2.Iron-doped bismuth tungstate with excellent photocatalytic performanceMetal doping of bismuth-based oxide photocatalysts have been a research hotspot in recent years to enhance the photocatalytic performance,but the research on mechanism of doping-induced photoactivity improvement is still incomplete.To illustrate completely and clearly the mechanism,Fe-doped Bi₂WO₆,as an example,was synthesized via a facile hydrothermal method.It exhibits much higher photoactivity than pure Bi₂WO₆ in degradation of Rhodamine B?RhB?and salicylic acid?SA?.By replacing W⁶⁺,the Fe³⁺ doping obviously narrows the energy bandgap and generates many oxygen vacancies which can increase the adsorption of both RhB and SA,and the introduced defect and Fe doping energy levels in the bandgap of Bi₂WO₆ significantly increase the separation efficiency of photoinduced charge carriers.These lead to the considerable photoactivity enhancement of Fe-doped Bi₂WO₆.The photoactivity of Fe-doped Bi₂WO₆ is superior than most reported modified Bi₂WO₆ photocatalysts.This work illustrates in detail the mechanism for photoactivity improvement of Fe-doped Bi₂WO₆,benefiting for research of other metal doped photocatalysts.3.Facile synthesis of indium hydroxide nanosheet/bismuth tungstate hierarchical microsphere heterojunction with enhanced photocatalytic performanceVarious Bi₂WO₆-based heterojunctions have been constructed to enhance the photocatalytic performance,but hydroxide/Bi₂WO₆ heterojunctions were rarely reported.To illustrate function of hydroxides in the heterojunctions for charge separation and photoactivity enhancement,In?OH?₃/Bi₂WO₆ heterojunctions were simply prepared for the first time via in situ growth of In?OH?₃ nanosheets on surfaces of Bi₂WO₆ hierarchical microspheres in a chemical precipitation process.Construction of the heterojunction benefits from formation of In-O-Bi/W bonds at the interface between Bi₂WO₆ and In?OH?₃.Photoluminescence spectroscopy and time-resolved fluorescence spectroscopy,demonstrate that the In?OH?₃/Bi₂WO₆ heterojunction exhibits accelerated separation of photoinduced charge carriers which results in increased generation rates of reactive oxygen species and enhanced photocatalytic degradation efficiencies for Rhodamine B and resorcinol,in comparison with pure Bi₂WO₆.The heterojunction shows high chemical stability and satisfactory recyclability.This work provides a new Bi₂WO₆-based heterojunction and,more importantly,deep insight into function of hydroxides in the heterojunction,which can direct preparation of other hydroxide-containing heterojunctions.4.Facile synthesis of indium hydroxide nanosheet/bismuth molybdate hierarchical microsphere heterojunction with enhanced photocatalytic performanceHeterojunction construction is one of the most important ways to enhance the photocatalytic performance of Bi₂MoO₆,but hydroxide/Bi₂MoO₆ heterojunctions were rarely reported.Based on the study of In?OH?₃/Bi₂WO₆,this strategy was extended to Bi₂MoO₆ to investigate its universality.In?OH?₃/Bi₂MoO₆ heterojunctions were simply prepared for the first time via in situ growth of In?OH?₃ nanosheets on surfaces of Bi₂MoO₆ hierarchical microspheres in a chemical precipitation process.Construction of the heterojunction benefits from formation of In-O-Bi/Mo bonds at the interface between Bi₂MoO₆ and In?OH?₃.Photoluminescence spectroscopy,time-resolved fluorescence spectroscopy,and photoelectrochemical tests demonstrate that the In?OH?₃/Bi₂MoO₆ heterojunction exhibits considerably accelerated separation of photoinduced charge carriers which results in increased generation rates of reactive oxygen species and enhanced photocatalytic degradation efficiencies for Rhodamine B,salicylic acid,and resorcinol,in comparison with pure Bi₂MoO₆.The heterojunction shows high chemical stability and satisfactory recyclability.This work provides a new Bi₂MoO₆-based heterojunction and,more importantly,deep insight into function of hydroxides in the heterojunction,which can direct preparation of other hydroxide-containing heterojunctions.