Synthesis And Photocatalytic Hydrogen Generation From Water Splitting Of Cd_0.5Zn_0.5S Nanocomposite

With an increasing concern about the global energy crisis and environmental pollution caused by excessive consuming of fossil fuels,the utilization of solar energy for the production of hydrogen from water is of great practical significance and broad application prospects.Hydrogen is a clean and environment friendly fuel with high energy density.Since the first report on photocatalytic splitting of water on Ti02 electrodes was published in 1972,H2 production from water splitting using semiconductors as photocatalysts from water under solar-light irradiation has been widely recognized as one of the most promising way to get hydrogen.However,TiO₂ has no visible-light response due to its large band gap of 3.2 eV.In order to make better use of sunlight,it has become an important research topic in the field of material chemistry to fabricate photocatalysts with high activity and good stability driven by visible light.The Cd_xZn_1-xS ternary solid solution with adjust able energy gaps is a group of excellent photocatalysts for H2 production under visible light irradiation.It owns the advantages of both CdS and ZnS,and has good resistance of photocorrosion and good performance under visible light irradiation.However,Cd_xZn_1-xS also has the shortcoming of low visible-light utilization and rapid recombination of photogenerated electron-hole pairs.Solving this problem means further improve the catalytic activity of solid solution as well as the stability of catalysts.In order to further optimize the photocatalytic performance of Cd_xZn_1-xS,it is necessary to combine Cd_xZn_1-xS with other semiconductor materials to form heterojunction and to construct an effective interface.Based on this idea,we first select WO₃ as the cocatalyst.WO₃ has some promising properties,such as high carrier mobility,good chemical and thermostability,as well as good resilience to photocorrosion in aqueous solution.Therefore,a nanocomposite of WO₃ and Cd_xZn_1-xS was prepared and the catalytic activity for hydrogen evolution from water was promoted significantly.WS₂ with graphene-like transition metal dichalcogenides?TMDCs?,endows some special properties,such as unique morphology,good conductivity,and large surface area.It is also ideal building blocks for hybrid materials.The hybride photocatalyst of Cd_xZn_1-xS/WS₂ show much better catalytic activity than the preminary Cd_xZn_1-xS.The mechanisms of the visible light catalytic hydrogen production were calarified by different experimental measurements.The concrete contents of this thesis include:1:An overview on the background,research progress,and problems of Cd_xZn_1-xS solid solution as photocatalytic hydrogen production catalyst.2:Research on WO₃ doped Cd_0.5Zn_0.5S solid solution.Cdo.5Zno.5S/WO₃ hereojuntion nanocomposites with different ratio of Cdo.5Zno.5S and WO₃ were successfully synthesized by a one-step hydrothermal method.H2 evolution rate of Cd_0.5Zn_0.5S/WO₃ achieved a maximum when the loading amount of Cd_0.5Zn_0.5S on W03 reached about 60 wt%with an QE of 16.7%at 420 nm,which is about 2 times higher than that of Cd_0.5Zn_0.5S catalyst alone.Pt doped Cd_0.5Zn_0.5S/WO₃ catalyst were also prepred.Z-scheme heterojunction system was formed and the photocatalytic H2-production performance was improved further.3:Cd_0.5Zn_0.5S/WS₂ nanocomposites with different amounts of Cd_0.5Zn_0.5S deposited on WS₂ were prepared by a one-step hydrothermal method with thioacetamide as sulfur source.Intimate heterojunctions between Cd_0.5Zn_0.5S and WS₂ were identified by TEM and HRTEM technologies in this material.The chemical composition and valence of the elements were characterized by XPS experiments.The band energies were characterized by UV-vis diffusive reflectance spectroscopy?DRS?.H2 evolution rate of Cd_0.5Zn_0.5S/WS₂ achieved a maximum when the loading amount of Cd_0.5Zn_0.5S on WS₂ reached about 41 wt%with an apparent quantum efficiency of 23.18%at 420 nm.The excellent photocatalytic H2 generation activity of Cd_0.5Zn_0.5S/WS₂ can be attributed to the presence of WS₂,which can act as electron pool and provide a large number of active sites for photocatalytic reactions due to the large specific surface area and mesopores structure.4:GaP was used for the first time as a single photocatalyst for H2-production with different sacrifical agents.It was showed that Pt loading on surface of GaP can promote the catalytic activity dramatically.In summary,introduction of another kind of semiconductor co-catalyst to Cd_0.5Zn_0.5S?CZS?can form heterogeneous catalyst containing heterojunctions.The presence of heterojunction can efficiently hindered the recombination of electron and hole paires and redcued the large over-potential of H+ reduction,and then improve the hydrogen production performance.