Synthetic Design And Photocatalytic Performance Of Ceria-based Multi-component Core/Shell Particles

Cerium oxide（Ce O₂）has the characteristics of strong oxygen storage capacity,stable Ce³⁺/Ce⁴⁺redox and abundant oxygen vacancies,thus it has been fully applied in the field of photocatalysis.However,traditional Ce O₂photocatalysts have limitations such as easy aggregation,wide band gap,narrow photoresponse range,and easy recombination of photo-generated electron-hole pairs,which restrict the photocatalytic activity of Ce O₂.In this paper,multi-component core/shell composite particles were designed and synthesized with conductive polymer polyaniline（PANI）as the intermediate layer of polymer/Ce O₂,carbon spheres（CS）as the core,and coated with Ce O₂.The photocatalytic activity of the composite particles was further improved by doping modification with rare earth metals and transition metals.The composite particles were characterized by field emission scanning electron microscope（FESEM）,transmission electron microscope（TEM）,fourier transform infrared（FT-IR）spectra,X-ray diffraction（XRD）,thermogravimetric analyzer（TGA）,photoluminescence（PL）,Raman spectroscopy（Raman）,ultraviolet-visible（UV-vis）spectrophotometer and X-ray photoelectron spectroscopy（XPS）.Finally,the photocatalytic degradation performance of the composite particles towards methylene blue（MB）was tested by a photochemical reactor.The main results are as follows:（1）The PANI was fully swollen to the PS surface by the interfacial polymerization method,and then the Ce O₂was coated by the in-situ chemical deposition method to prepare the core/shell composite particles PS/PANI/Ce O₂.The characterization results show that PANI with a particle size of 10-15 nm adheres to the PS surface in the form of particles,and the surface of the particles coated with Ce O₂has a distinct roughness with a particle size of 260-280 nm.Using 10 mg L^-1of MB as the target pollutant,the photocatalytic degradation performance of the composite particles was investigated under ultraviolet-light irradiation.The degradation results show that the degradation rate of the composite PS/PANI/Ce O₂modified by PANI increases to 97.0%,and the reaction rate constant increases to 10.64×10^-3min^-1,which are higher than those of PS/Ce O₂(the degradation rate is 72.7%,and the reaction rate is 3.94×10^-3min^-1).This may be attributed to the synergistic effect of PANI and Ce O₂in the polymer/Ce O₂multi-component system.PANI can effectively inhibit the recombination of Ce O₂photo-generated electron-hole pairs,so that more hydroxyl radicals（·OH）and superoxide radicals(·O^2-)are generated during the photocatalytic process to degrade MB molecules.（2）CS with a wide particle size distribution was prepared by a simple hydrothermal method,and the composite particles CS/Ce O₂were prepared by in-situ chemical deposition method.The characterization by FESEM and TEM confirm that the particle surface has obvious roughness,showing a clear core/shell structure,and the thickness of the shell layer is10-15 nm.The PL test shows that CS can effectively promote the separation of electron-holes of Ce O₂.And XPS result shows that a lot of Ce³⁺and oxygen vacancies exist on the surface of CS/Ce O₂.Ce³⁺and oxygen vacancies can combine with O₂to form·O^2-,and oxygen vacancies can capture electrons and promote the separation of photo-generated electron-hole pairs.Using 10 mg L^-1of MB as the target pollutant,the photocatalytic degradation performance of the composites was researched under visible-light irradiation.The results show that the photo-degradation rate of CS/Ce O₂increases to 87.36%,and the reaction rate constant is 3.45×10^-2min^-1,indicating that the photocatalytic activity of the novel core/shell composites CS/Ce O₂is significantly improved.The enhanced photocatalytic activity of the composites originates from the fact that the CS framework can provide an electron transport channel for Ce O₂,which can inhibit the recombination of photo-generated electron-hole pairs,thereby generating more·OH and·O^2-.（3）CS/Ce O₂is modified by in-situ chemical deposition method with rare earth metals and transition metals as doping elements.The surface of the doped composites has obvious roughness,showing a core/shell structure,and the thickness of the shell layer is 15-18 nm.It is proved that doping modification has little effect on the morphology of composite particles.The PL results show that the doping modification can further promote the electron-hole separation.XPS results show that the Ce³⁺and oxygen vacancy concentrations of the doped composites are increased.This may be because Ce⁴⁺is converted into Ce³⁺and forms oxygen vacancies under the promotion of doping ions.The results of photocatalytic degradation of MB show that the photocatalytic degradation efficiencies of the composites modified by rare earth metals La,Sm,Gd,and Dy are 99.95%,99.95%,99.29%,and 99.34%,respectively.The degradation rates are increased by 12.59%,12.59%,11.93%,and 11.98%,respectively.The reaction rate constants are12.52×10^-2min^-1,12.52×10^-2min^-1,8.25×10^-2min^-1and 8.38×10^-2min^-1,respectively,which are as much as that of CS/Ce O₂by 3.63,3.63,2.39and 2.43 times.Moreover,the degradation efficiency of transition metals Y,Cu,Mn and Zr are improved to 95.77%,98.79%,99.95%and 98.75%,respectively.And the reaction rate constants are 5.27×10^-2min^-1,7.36×10^-2min^-1,12.54×10^-2min^-1and 7.30×10^-2min^-1.The photocatalytic degradation of transition metals-doped composites are also greatly improved.It is proved that the doping modification can effectively improve the photocatalytic activity of CS/Ce O₂.Without H₂O₂participating in the photocatalytic degradation experiments,the photodegradation efficiencies of doped transition elements Cu and Mn decrease by 15.8%and 9.78%to 82.99%and 90.17%.Under the same conditions,the photodegradation efficiency of the composites doped with La and Sm only decrease by 6.44%and 3.69%.This may be due to the photocatalytic and photo-Fenton dual pathway reaction between transition metals and H₂O₂,which promotes the enhanced photocatalytic degradation rate.