Structural Design And Photocatalytic Activity Of Silica-Ceria Multi-Phase Composite Particles

CeO₂ has the advantages of high surface lattice defect density,large light absorption threshold,and easier desorption of adsorbed oxygen.However,CeO₂ has the shortcomings such as easy agglomeration,wide band gap,narrow light response range,and easy recombination of photogenerated electrons and holes.And these result in its limited applications in environment and energy fields.In this paper,mesoporous silicas with large specific surface areas were synthesized as the support of CeO₂-based composite materials.The silica-ceria binary composite photocatalysts were prepared.The silica cores with large specific surface areas have strong adsorption abilities for methylene blue（MB）.The CeO₂shells have photocatalytic degradation activities.The photocatalytic performance of the composite particles is enhanced by the synergy between silica cores and CeO₂ shells.In order to further improve its photocatalytic performance,the synthesized composite particles were modified by ion doping,changing the calcination atmosphere,and forming heterojunctions.The structure and photocatalytic properties of the composites were characterized by field emission scanning electron microscopy（FESEM）,transmission electron microscopy（TEM）,X-ray diffraction（XRD）,nitrogen adsorption desorption,photoluminescence spectroscopy（PL）,Raman spectroscopy（Raman）,X-ray photoelectron spectroscopy（XPS）,and ultraviolet-visible spectroscopy（UV-Vis）.Besides,the photocatalytic degradation performance towards MB of the obtained photocatalysts was investigated under ultraviolet light irradiation.The main results are as follows:（1）The worm-like mesoporous silicas（W-mSiO₂）microspheres obtained by the improved St（?）ber method were used as the cores.The CeO₂nanoparticles were coated by in-situ chemical precipitation method to synthesize W-mSiO₂/CeO₂ composite particles.In addition,W-mSiO₂/Ce_0.83Er_0.17O₂ and W-mSiO₂/Ce_0.83Er_0.17O₂-N₂ composite particles were synthesized through modification such as doping Er³⁺and changing the calcination atmosphere.Compared to the commercial CeO₂,the improved photocatalytic performance of the composite particles W-mSiO₂/CeO₂（86.9%vs 17.7%）may be due to the synergy between the W-mSiO₂ cores and the CeO₂ shells.The W-mSiO₂ cores with high specific surface areas have good adsorption performance for MB.So,MB is concentrated near the shell CeO₂.In addition,the W-mSiO₂ cores increase the specific surface area of the CeO₂ shells,resulting in an increase in the number of active sites.Moreover,Er³⁺doping leads to an increased oxygen vacancy concentration and Ce³⁺ratio,and a narrower band gap.Thence,it increases the separation efficiency of photogenerated electron-hole pairs and the concentration of strong oxidizing groups such as·OH and·O₂^-,resulting in the further improvement in photocatalysis performance of composite particles（93.1%vs 86.9%）.The non-oxidizing gas N₂ causes a part of Ce³⁺to remain in the composite particles without being completely oxidized to Ce⁴⁺.Then,the oxygen defect concentration is further increased,and the photocatalytic activity is further enhanced.Therefore,modifications such as Er³⁺doping and non-oxidizing atmosphere（nitrogen）calcination atmosphere are helpful to enhance the photocatalytic degradation reaction activity of composite particles for MB.（2）The three-dimensional dendritic channel mesoporous silicas（D-mSiO₂）microspheres synthesized by the oil-water two-phase layered method were used as the cores and coated with CeO₂ nanoparticles to get D-mSiO₂/CeO₂ composite particles.The CeO₂ shell layer was doped with different contents of Sm³⁺to prepare D-mSiO₂/Ce_1-xSm_xO_2-δcomposite particles.The photocatalytic performance of composite particles shows a trend of first increasing and then decreasing with the increase of doping amount.This can be attributed to the lattice distortion caused by Sm³⁺doping when x=0-0.3,which leading to the CeO₂ oxygen vacancy concentrations and Ce³⁺ratios increased synchronously.These results to an increase in the separation efficiency of photogenerated electron-hole pairs,and an increase in the concentration of active groups such as·OH and·O₂^-with strong oxidizing properties.When x>0.3,Sm³⁺becomes the recombination center,which reduces the separation efficiency of photogenerated electron-hole pairs.This results in the concentration of active groups such as·OH and·O₂^-with strong oxidizing properties decreasing.Therefore,Sm³⁺doping may increase the recombination rate of light-generated holes and electrons,but it may also become a recombination center and reduce the separation efficiency.（3）D-mSiO₂ microspheres were used as the cores and coated with two layers.TiO₂,Cd S,and Zn O were used as the inner layer,and CeO₂ was used as the outer layer.D-mSiO₂/TiO₂/CeO₂,D-mSiO₂/Cd S/CeO₂,and D-mSiO₂/Zn O/CeO₂ with heterojunction were prepared.In this paper,compared to D-mSiO₂/CeO₂,the photocatalytic performance of all composite particles forming heterojunctions has been improved.The photocatalytic performance of D-mSiO₂/TiO₂/CeO₂ is prominent（97.9%）.This can be attributed to the heterojunction formed by TiO₂ and CeO₂ can improve the separation efficiency of photogenerated electrons and holes.It increases the active groups such as·OH and·O₂^-with strong oxidizing properties,and finally leads to the improvement in degradation efficiency of MB.Therefore,in this system,semiconductor coupling to construct a heterojunction is an effective way to improve the photocatalytic activity of composite particles.