Preparation Of Rare Earth Doped Cerium Oxide Down-conversion Luminescent Materials,Properties And Its Enhancement Mechanism

In recent years,CeO₂ has also shown good application prospects in fluorescent labeling,bio-pharmaceutical tracking,treatment and solar cells due to its cubic phase fluorite structure,good thermal stability,chemical stability and excellent oxidation resistance.However,studies have shown that the unbalanced charge between Ce⁴⁺and Ln³⁺ions easily introduces oxygen vacancies into the matrix material and acts as a center of fluorescence quenching.Therefore,a rare earth doped ceria conversion luminescent material was prepared by the combination of hydro-thermal and co-precipitation,and its fluorescence intensity was improved by double or triple doping,and its structure,properties and fluorescence enhancement mechanism were investigated.The main research work is as follows:（1）Li⁺-doped CeO₂:Eu³⁺was synthesized by different doping methods.The effects of different doping modes of Li⁺ions on the structure and spectral properties of CeO₂:Eu³⁺and the fluorescence enhancement mechanism of Li⁺doped CeO₂:Eu³⁺were investigated.The results show that the Li⁺ion in the CeO₂:Eu³⁺down-conversion material is a substitution substitution for Ce⁴⁺,and the oxygen vacancy content in the matrix is increased due to the charge imbalance between Ce⁴⁺and Li⁺.Combined with fluorescence spectroscopy,it was found that an increase in oxygen vacancies reduced the symmetry of Eu³⁺and exhibited a fluorescence quenching center.In the Li⁺indirect doped CeO₂:Eu³⁺down-conversion material,Li⁺does not produce a Ce⁴⁺substitution,but exhibits a fluxing effect.The analysis shows that Li⁺does not undergo hydro-thermal process when Li⁺is indirectly doped with CeO₂:Eu³⁺,so that Li⁺does not have enough energy to overcome the ionic radius and the large difference of charge between Li⁺and Ce⁴⁺.Therefore,Li⁺indirectly doped CeO₂:Eu³⁺exhibits the role of flux,and promotes the diffusion of ions and accelerates the crystallization of the matrix material,resulting in higher crystallinity and less oxygen vacancies in the matrix material,achieving CeO₂:Eu³⁺fluorescence intensity enhancement.（2）The Bi³⁺-doped CeO₂:Eu³⁺down-conversion luminescent materials were synthesized by hydro-thermal co-precipitation.The effects of Bi³⁺ion doping on the local environment and spectral properties of Eu³⁺and the fluorescence enhancement mechanism of Bi⁺doped CeO₂:Eu³⁺were investigated.The analysis results show that the Bi⁺doped CeO₂:Eu³⁺is consistent with the diffraction peak position in the CeO₂ standard card JCPDS 34-0394.At the same time,the Bi³⁺ion in the Bi³⁺doped CeO₂:Eu³⁺down-conversion material is a substitution substitution for Ce⁴⁺,and the oxygen vacancy content in the matrix is increased due to the charge imbalance between Ce⁴⁺and Bi³⁺.However,XPS results show that Bi³⁺doping reduces the Ce³⁺concentration in the CeO₂:Eu³⁺lattice.Combined with the results of fluorescence spectroscopy,the excitation spectra of 365 nm and the characteristic transition strength of CeO₂:Eu³⁺excited at 365 nm at 593 and613 were significantly enhanced with Bi³⁺doping.At the same time,the ratio of the electric dipole transition（⁵D₀-⁷F₂）and the magnetic dipole transition（⁵D₀-⁷F₁）of Eu³⁺is reduced.This result shows that Bi³⁺has strong interaction with near oxygen vacancies and promotes the increase of the number of Ce⁴⁺-O-Eu³⁺clusters in CeO₂:Eu³⁺,resulting in energy transfer between O^2-to Ce⁴⁺charge transfer transition and Eu³⁺.It is enhanced to effectively increase the fluorescence intensity of CeO₂:Eu³⁺at 365 nm.（3）Li⁺and Sm³⁺co-doped CeO₂:Eu³⁺down-conversion luminescent materials were synthesized by hydro-thermal co-precipitation.The local structure and spectral properties of Li⁺ion doping on CeO₂:Sm³⁺,Eu³⁺were investigated.The analysis results show that the Li⁺and Sm³⁺co-doped CeO₂:Eu³⁺down-conversion materials are consistent with the diffraction peak positions in the CeO₂ standard card JCPDS 34-0394,and the sample still exhibits cubic fluorite-type crystal CeO₂.However,the intensity of the CeO₂:Eu³⁺diffraction peak is reduced and the lattice parameter of the matrix is reduced.Analysis of fluorescence spectra showed that when Sm³⁺was doped with CeO₂:Eu³⁺,the down-conversion luminescence of Sm³⁺was significantly reduced,and the characteristic transition of Eu³⁺was significantly enhanced.It is revealed that there is energy transfer in Sm³⁺and Eu³⁺in the ceria matrix.The energy transfer is through the resonance energy transfer between the ⁴G_5/2/2 energy level of the excited state of Sm³⁺and the⁵D₀ energy level of Eu³⁺.When further doped with Li⁺,the fluorescence intensity of CeO₂:Sm³⁺,Eu³⁺is further enhanced.Structural analysis shows that Li⁺doping increases the oxygen vacancy content.However,it has been found that an increase in oxygen vacancies is beneficial to enhance the energy transfer between Sm³⁺and Eu³⁺,thereby further enhancing the fluorescence intensity.