Structural Optimization And Photocatalytic Activity Of Core-shell Structure Composite Particles Of RE³⁺:NaYF₄@CdS Quantum Dots Sensitized Nanocrystalline TiO₂

TiO₂ is one ideal photocatalytic material,because of its low cost,nontoxicity,acid and alkali resistance,corrosion resistance,high chemical stability,excellent chemical properties and stability.However,the band gap of TiO₂ is 3.2 eV,and it can only absorb ultraviolet light with a wavelength of less than 387.5 nm which is only5%of the energy in the solar.Therefore,the sunlight utilization of TiO₂ is very low,limiting its widely use in practice.In order to improve the solar light absorption of TiO₂,TiO₂ need to be modified.In this paper,the absorption of TiO₂ to solar light is enhanced by the synergistic effect of upconversion luminescence and sensitization of CdS quantum dots?CdS QDs?.Upconversion luminescence can improve the utilization of TiO₂ in near-infrared light.The sensitization of CdS QDs can extend the photoresponse range of TiO₂ to the visible region,which improves the utilization of visible light by TiO₂.There are several main aspects:?1?RE³⁺:NaYF₄ up-conversion luminescent material was prepared by solvothermal method.The TiO₂ shell was formed on the surface of RE³⁺:NaYF₄ by using tetrabutyl titanate hydrolysis to prepare the core-shell structure RE³⁺:NaYF₄@TiO₂ composite material.The upconversion luminescent nanomaterial RE³⁺:NaYF₄ was as the core,and the CdS QDs sensitized TiO₂ as the shell.The photocatalysts were characterized by X-ray diffraction,transmission electron microscopy and fluorescence spectra.?2?Under infrared light?wavelength 975 nm?,compared with TiO₂,the photocatalytic performance of NaYF₄:Yb,Tm@TiO₂ composite particles increased by 56%.That is,the introduction of NaYF₄:Yb,Tm upconverting nanomaterials can improve the utilization of TiO₂ to near-infrared light.The reason is that NaYF₄:Yb,Tm can emit 348 nm and 366 nm ultraviolet light absorbed by TiO₂ when excited by infrared light.However,the near infrared photocatalytic performance of NaYF₄:Yb,Er@TiO₂ is not as good as that of TiO₂.The reason is that NaYF₄:Yb,Er emits only visible light when excited by infrared light.After introducing CdS QDs,the photoresponse range of TiO₂ can be extended to the visible region.Therefore,RE³⁺:NaYF₄@TiO₂ was composited with CdS QDs.Compared with that of titanium dioxide,the visible light catalytic performance of NaYF₄:Yb,Tm@TiO₂/CdS QDs and NaYF₄:Yb,Er@TiO₂/CdS QDs was respectively increased by 31%and 21%under visible light irradiation?wavelength>420 nm?.At the same time,the photocatalytic performance of RE³⁺:NaYF₄@TiO₂/CdS QDs was also investigated under white light?wavelength 300 nm²500 nm?.Compared with that of titanium dioxide,the white light catalytic performance of NaYF₄:Yb,Tm@TiO₂/CdS QDs and NaYF₄:Yb,Er@TiO₂/CdS QDs was respectively increased by 73%and 17%under white light irradiation.?3?The photocatalytic properties of NaYF₄:Yb,Tm@TiO₂/CdS QDs and NaYF₄:Yb,Er@TiO₂/CdS QDs composites were compared.The results show that the photocatalytic activity of NaYF₄:Yb,Tm@TiO₂/TiO₂CdS QDs composite particles is56%higher than that of NaYF₄:Yb,Er@TiO₂/CdS QDs composite particles.The reason for this is that the ultraviolet light at 348 nm and 366 nm and visible light at476 nm can be absorbed by TiO₂/CdS QDs in the up-conversion fluorescence emitted by NaYF₄:Yb,Tm.But the upconversion fluorescence emitted by NaYF₄:Yb,Er is mainly at 526 nm,544 nm and 658 nm,which is outside the absorbable range of TiO₂/CdS QDs.In other words,the up-conversion fluorescence emitted by NaYF₄:Yb,Er is not absorbed by TiO₂/CdS QDs.