Study On Synthesis,characterization And Photocatalytic Performance Of Knb O3-based Composite Materials

Carbon dioxide emission and the non-renewable feature of fossil fuels have caused serious environmental pollution.Therefore,it is crucial to develop new photocatalysis and new energy technologies and to use more environmentally friendly ways to produce renewable energy.Among the various energy sources,solar energy is considered to be one of the cleanest and most environmentally friendly renewable energy sources and can be developed and used as a substitute for fossil fuels.The use of solar light energy to photolyze water for hydrogen production and environmental pollution remediation is becoming more common as a result of different processes like photocatalysis,photoelectrocatalysis,and electrocatalysis.Therefore,it is always a goal to develop new single materials or composite materials with highly efficient photocatalytic activity.To date,different photocatalysts have been developed.However,most developed photocatalysts have a narrow range of solar absorption spectrum.Therefore,researchers have been making great efforts to develop new photocatalyst materials with a wide response to solar absorption spectrum.Potassium niobate（KNbO₃）has been widely attracted much attention due to its advantages of chemical stability,low toxicity,low cost,simple preparation.More importantly,KNbO₃has a perovskite structure,which is very beneficial to the migration of carriers in photocatalytic reactions.Therefore,KNbO₃ in this thesis was used as the base material and a series of KNbO₃-based composite photocatalysts were prepared.By means of composite photocatalysts,the improvement in the reduction of Cr（VI）pollutant and hydrogen production via photocatalyticly splitting water is realized.The main research content is as follows:（1）KNbO₃ was synthesized by hydrothermal method and KNbO₃/Cd Se composite photocatalysts were then prepared by precipitation method.The properties of the composite photocatalysts were further studied by a series of characterizations like XRD,XPS,and SEM,and the ability of photocatalytic reduction of Cr（VI）was tested.Experimental results show that the Cd Se particles are attached to the KNbO₃ surface,and the two are closely bonded.Compared with pure KNbO₃ and Cd Se,when the ratio of KNbO₃ to Cd Se is 1:10,KNbO₃/Cd Se composite shows the best photocatalytic activity.After 30 min of illumination,the efficiency of photocatalytic reduction of Cr（VI）can reach 94%.The cycle test also shows that KNbO₃/Cd Se composites are very stable.Meanwhile,the experiments of trapping agents show that·O₂^-and·OH radicals are the main active species for reducing Cr（VI）.In the composites,the formation of heterojunctions between KNbO₃ and Cd Se promotes the effective separation of photogenerated electron-hole pairs,thus improving the photocatalytic efficiency of reduction of Cr（VI）.（2）KNbO₃/Sn S₂ composite photocatalyst was synthesized by hydrothermal method.The crystal structure,micromorphology,element distribution,optical properties and photocatalytic reduction of Cr（VI）were further measured and analyzed by several characterization methods.The results showed that the KNbO₃/Sn S₂ composite photocatalyst was tightly bonded and had clear boundaries,and the KNbO₃/Sn S₂ heterojunction was successfully synthesized.Optical and electrochemical tests showed maximum visible light adsorption and charge transfer.After40 min of visible light irradiation,the KNbO₃/Sn S₂（1:3）system can reduce 95.4%of Cr（VI）,showing excellent photocatalytic efficiency and good cycle stability.The trapping experiments showed that the main active species in the photocatalytic reaction is·O₂^-.Therefore,KNbO₃/Sn S₂ composite photocatalyst can be used to effectively control chromium pollution.（3）(Sr_0.6Bi_0.305)₂Bi₂O₇（SBO）single phase and KNbO₃/SBO composite photocatalytic materials were successfully synthesized by hydrothermal method,and the different properties of the composite photocatalysts were studied by XRD,XPS and SEM characterization methods.The performance of photocatalytic hydrogen production was tested to verify the photocatalytic activity of the prepared composite catalyst.Experiments on hydrogen production showed that when the molar ratio of KNbO₃ to SBO is 800:1,the photocatalytic performance of composite is the best.After 4 h of simulated sunlight,the photocatalytic hydrogen production of KNbO₃/SBO（800:1）reaches 86μmol·g^-1·h^-1,which is about 39.5 times as high as that of the pure phase KNbO₃.The photogenerated electrons of KNbO₃ move to the conduction band of SBO under the action of an internal electric field and the introduction of methanol can also effectively inhibit the rapid recombination of photogenerated electrons and holes,which promotes the hydrogen production rate via splitting water and significantly improves the photocatalytic efficiency.