Design Of Hollow CeO₂-based Photocatalyst And Its Activity Enhancement Mechanism

With the development of human society,the development and utilization of environment-friendly energy haves attracted more and more attention.As a new energy technology that fully utilizes solar energy to degrade organic matters,to hydrolyze hydrogen?oxygen?,and to convert carbon dioxide,photocatalysis has become a research hotspot in this field.Rare earth compounds are considered to be photocatalytic semiconductors with great potential due to their unique electronic structure,and cerium oxide has attracted attention due to good ultraviolet light responsiveness and stability.However,the relatively poor light absorption capacity of cerium oxide and the high probability of electron-hole pair compounding limit its application in the field of photocatalysis.Designing a cerium oxide-based catalyst with good ultraviolet-visible absorption capability and high-efficiency electron-hole pair separation and transfer ability plays an important role in improving its photocatalytic efficiency.Therefore,in this thesis,starting from theoretical simulation calculation,the relationship between structure and performance was discussed,hollow cerium oxide-based composite photosemiconductor materials were designed and synthesized,and theris photocatalytic performance via chemical?physical?adsorption,doping,and morphology regulation was optimized.The structure,morphology and photocatalytic properties of the system were systematically characterized.The electron structure,electron-hole pair separation and transfer mechanism of the composite materials were comprehensively analyzed by means of photoelectric chemical performance test and transient/steady state fluorescence spectrum,and the mechanism of photocatalytic enhancement was discussed.The specific research contents and conclusions are as follows:?1?The electronic structure and surface charge density of carbon quantum dot composite hollow cerium oxide microspheres?CQDs/h-CeO₂?were theoretically calculated by first-principles principle.The results showed that after the introduction of CQDs,the band gap of composite materials could be significantly reduced,and the visible light response ability was greatly improved.The charge transfer of the interface was forecasted by differential charge density theoretical calculation.Theoretically,the introduction of CQDs prevented the recombination of photogenerated electrons and holes,which was favorable for photocatalytic reactions.The NCQDs/h-CeO₂composite hollow microspheres were prepared by hydrothermal method and impregnation method.The structure and photocatalytic properties of the NCQDs/h-CeO₂ were analyzed.It was found that the light absorption edge of NCQDs/h-CeO₂ was around 600 nm,which was significantly redshifted compared with that of h-CeO₂?450 nm?,indicating that NCQDs composite broadened the light absorption range of materials.The flat band potential of NCQDs/h-CeO₂ was-0.73 eV,which was significantly more negative than h-CeO₂?-0.68 eV?.The electrochemical impedance spectroscopy?EIS?test showed that,the radius of curvature of NCQDs/h-CeO₂ was smaller than that of h-CeO₂,which indicated that the transmission rate of the surface photogenerated carrier of NCQDs/h-CeO₂ was higher than that of h-CeO₂,contributing to the improvement of photocatalytic activity.Under visible light irradiation,the degradation rate of NCQDs/h-CeO₂ could reach 2.35 times that of h-CeO₂.?2?Fe/h-CeO₂ photocatalytic materials with different Fe doping amount were prepared by a hydrothermal method with ferrous sulfate and lanthanum nitrate as main materials.The structure and photocatalytic properties of the materials were systematically studied.The results showed that Fe doping could broaden the light absorption range of the materials to visible light and effectively increase the specific surface area of the materials.Besides,0.25%Fe/h-CeO₂ exhibited the highest activity of photocatalytic degradation of RhB,and the degradation efficiency could be twice that of h-CeO₂.The electronic structure and surface charge density of Fe/h-CeO₂ were theoretically calculated by first-principles.The results showed that the band gap of h-CeO₂ and Fe/h-CeO₂ were 1.62 eV and 1.29 eV,respectively.After doped with Fe,the band gap of Fe/h-CeO₂ decreased significantly compared with pure CeO₂.FT-IR,XPS,EIS and photoelectric chemistry test results showed that a small amount of Fe doping changed the position of conduction band and valence band energy level of h-CeO₂ and increased the number of oxygen vacancy.The high transfer rate of photocarriers on the surface of Fe/h-CeO₂ material contributed to the improvement of photocatalytic activity.?3?A series of Fe-MOFs/CeO₂ composite photocatalysts were synthesized by a hydrothermal method using cerium oxide hollow microspheres as a carrier,naphthalene dicarboxylic acid as an organic ligand and iron as metal centers.The results showed that Fe-MOFs/CeO₂ contained hollow CeO₂ microspheres with a diameter of about 500 nm as the carrier,and the surface was a burr-like shell coated with Fe-MOFs.The thickness of the burr shell was about 100 nm.After composite with MOF,the band gap of Fe-MOFs/CeO₂ could be reduced to 1.62 eV,which was lower than pure h-CeO₂ and MOFs,indicating that light response range and band structure of the samples could be effectively improved after composite with MOF.The results of photoelectrochemical test showed that the MOFs complex could introduce a naphthalene ring with a?-?conjugated structure,which made the photogenerated electron hole transport path shorter and reduced the recombination probability of photogenerated electron-hole in the bulk phase.Meanwhile,the coating of MOFs could not only improve the adsorption performance of samples,but also improved the charge transfer characteristics between the surface of samples and electrolyte,which effectively reduced the charge transfer resistance between samples and electrolyte,and further improved the photocatalytic performance of samples.After 24 h of dark adsorption and 2 h of visible light irradiation,the Fe-MOFs/CeO₂ and h-CeO2degraded 72%and 45%RhB,respectively.?4?Multilayer cerium oxide/yttrium oxide composite hollow microspheres were prepared by means of hydrothermal and heat treatment with yttrium carbon spheres as a template.The structure and photocatalytic properties of composite microspheres were studied systematically.The results showed that the core layer of multilayer microspheres was mainly Y₂O₃,and the shell layer was mainly CeO₂.There was a large gap between layers,which was combined in the form of"point contact".Compared with pure h-CeO₂ hollow spheres,the introduction of Y₂O₃ core layer could change the electronic structure of the materials and reduced the band gap.Because it was difficult to form compact heterogeneous interface between layers of YCe composite microspheres,and there were very few channels for electron and hole transmission in the system,the charge transfer resistance of the composite microspheres were increased dramatically compared with h-CeO₂.It hindered the photoproduction electronic-hole transferred to the photocatalytic activity center of the surface of the materials,and reduced the photocatalytic activity.