Double Rare Earth Doped Materials NaXF₄(X=Bi?Y):Energy Transfer And Application In Luminescence And Photocatalysis

Rare earth luminescent materials have been widely used in various fields,including national defense,aerospace,lighting and energy,due to their high color purity and excellent stability.According to different luminescence principles,rare earth luminescent materials can be divided into down-conversion and upconversion materials.Among them,the former is mainly used in the display and lighting industry,and the latter can be used in photocatalysis,biological imaging,therapy and other fields.Energy transfer is a universal physical phenomenon in rare earth luminescent materials,which has a decisive influence on the luminescent properties of materials.For example,in the down-conversion material,energy transfer between rare earth ions can change the light absorption range of raw material and adjust the luminescent color.In the upconversion material,the infrared light can be utilized effectively by energy transfer,and the purpose of efficient utilization of solar energy is achieved.In this paper,the energy transfer behavior and effect on the properties of double rare earth ions in Na XF₄(X=Bi,Y)materials were studied.The main research contents of this paper are listed as follows:1.Synthesis of Ce³⁺/Tb³⁺co-doped NaBiF₄ and NaBiF₄:Yb³⁺,Tm³⁺/TiO₂composites:A series of Ce³⁺/Tb³⁺co-doped NaBiF₄ phosphors were synthesized by chemical precipitation method at room temperature.Nuclear magnetic resonance measurement and X-ray diffraction demonstrated that Ce³⁺/Tb³⁺was successfully doped into NaBiF₄ matrix.In the NaBiF₄:Ce³⁺,Tb³⁺phosphors,the emission of Tb³⁺was significantly enhanced by the energy transfer process of Ce³⁺?Tb³⁺.Luminescence spectra and fluorescence lifetime confirmed the existence of energy transfer between Ce³⁺and Tb³⁺in NaBiF₄ matrix.When the concentration of Tb³⁺is10mol%,the energy transfer efficiency can reach about 70.1%.NaBiF₄:Yb³⁺,Tm³⁺/TiO₂ composites were synthesized by the same preparation method.The fluorescence spectra and UV-Vis absorption spectra proved that TiO₂ could effectively utilize the ultraviolet light emitted by NaBiF₄:Yb³⁺,Tm³⁺by the upconversion process,which is expected to make up for blanks in the utilization of infrared light by TiO₂.The experimental results show that NaBiF₄ is unstable in water system and changes into Bi F₃ after catalytic reaction.NaBiF₄:Yb³⁺,Tm³⁺/TiO₂ is not suitable to be directly used as catalyst for dye degradation in water system.2.Synthesis of NaYF₄:Yb³⁺,Tm³⁺@TiO₂ composites:NaYF₄:Yb³⁺,Tm³⁺@TiO₂ composites with different molar ratios were synthesized by hydrothermal and mixing-calcination methods,and their luminescence and photocatalytic properties were studied.SEM showed that TiO₂ existed in two forms in different proportions of the composite.When the content of TiO₂ was higher than 50%,TiO₂ could effectively coat NaYF₄,and the surplus scattered TiO₂ would exist in the form of small particles.In the other case,most of the TiO₂ aggregated into small spheres,and a small amount adhered to the hexagonal prism of NaYF₄.The energy transfer between the two components of the composite was confirmed by the upconversion emission spectra and absorption spectra.The photocatalytic activity of NaYF₄@TiO₂ composite evaluated by degradation of methyl orange(MO)was further studied in near-infrared light and full spectrum.Under the irradiation of near-infrared light,the degradation amount of MO by NaYF₄@TiO₂ compound was increased by about 20%compared with that of individual components.Under full spectrum irradiation,the samples with 80%and 50%TiO₂ content showed better photocatalytic performance,and the first-order reaction rate constant k value was 0.0361min^-1 and 0.0298min^-1.This is related to two factors,one is the utilization of infrared light reflected in the fluorescence spectrum,and the other is the coating degree caused by different composite proportions.Radical scavenger tests showed that the hole and superoxide radical were the main active species for this photocatalytic reaction.