Hydrothermal Synthesis Of Rare Earth-doped β-NaY_1-xGd_xF₄（0≤x≤1.00） And Upconversion And Downconversion Luminescent Properties

Fluoride-based luminescent materials are considered as excellent host lattices for up-conversion（UC） and down-conversion（DC） luminescence in terms of their low phonon energy（<400 cm-1）, high emission efficiency, high refractive index and quantum efficiency. Therefore, they are widely applied in the field of lighting device, multifunctional bioimaging and photodynamic therapy. Moreover, chemical composition and crystal structure together with shape and dimensionality of the crystals are now considered as particularly important factors that influence the chemical and/or physical properties of materials. The optical properties and the luminescence efficiency of the fluoride luminescent materials can therefore be improved and enhanced by control the crystal phase, morphology, grain size and composition of the final crystals.In this article, we present a facile and effective hydrothermal route to synthesize a series of rare earth-doped β-NaY_1-xGd_xF₄（0≤x≤1.00） luminescent materials. The variations of the structure, morphology and luminescent properties of the obtained crystals were analysed. The main results are list as follows:（1） The β-NaGdF₄: Eu³⁺ and β-NaYF₄: Yb³⁺/Tm³⁺ microcrystals were synthesized by a chelator-capped hydrothermal method and the influences of the Ln³⁺/Na F molar ratio, Ln³⁺/Na₃ Cit molar ratio and p H value of the initial solution on microstructures and UC and DC luminescent properties of the final crystals were discussed. The results indicate that:（1） Under the optimal processing conditions, i.e. the Ln³⁺/Na F molar ratio of 1:6, Ln³⁺/Na3 Cit molar ratio of 1:3 and the p H value among the range of 8-11, the obtained β-NaGdF₄: 2 mol% Eu³⁺ microcrystals possess homogenous, monodisperse and uniform microstructure. β-NaYF₄: Yb³⁺/Tm³⁺ microcrystals with pure phase, homogeneous distribution and smooth surface can be obtained under the optimal Ln³⁺/Na F molar ratio of 1:7, Ln³⁺/Na3 Cit molar ratio of 1:2 and p H value of 9.（2） The emission intensity of the as-synthesized β-NaGdF₄: 2 mol% Eu³⁺ crystals depends on their grain size and the adsorbed quantity of Cit^3- ions, and the microcrystals with larger grain size and smaller amount of capped Cit^3- ions can exhibit greater luminescence emission intensity.（3） Upon excitation of the host at 271 nm, the energy absorbed by Gd³⁺ ions can be transferred to Eu³⁺ ions in the β-NaGdF₄ luminescent materials and the characteristic luminescence at 590 nm and 614 nm of Eu³⁺ are observed. Under excitation at 980 nm, the energy absorbed by Yb³⁺ ions can be transferred to Tm³⁺ ions in the β-NaYF₄ microcrystals and the characteristic luminescence of f-f transitions of Tm³⁺ is observed.（2） The β-NaGdF₄: Eu³⁺/Tb³⁺ submicrocrystals were synthesized by Na₃Cit-capped hydrothermal method and the influence of the Eu³⁺/Tb³⁺ molar ratio on microstructures and photoluminescent properties of the final crystals were discussed. The results indicate that:（1） The structure, morphology and grain size of the β-NaGdF₄ host lattice were independent on the Eu³⁺/Tb³⁺ molar ratio. All the obtained products are hexagonal quasi-spherical crystals with the grain size about 220-240 nm.（2） The absorbed excitation energy migrates from Gd³⁺-based lattice to Tb³⁺ and Eu³⁺, and the characteristic green, red emission can be obtained under a single excitation wavelength in β-NaGdF₄: Tb³⁺ and β-NaGdF₄: Eu³⁺ samples, respectively.（3） When appropriately modulating the Tb³⁺/Eu³⁺ doping concentrations, various emission colors can be obtained and tuned from red, orange-red, pink, white and blue-green to green through single excitation energy, which might be attributed to the ET from Tb³⁺ to Eu³⁺ in the β-NaGdF₄: Tb³⁺/Eu³⁺ host. The efficiency of the energy transfer increases initially and then decreases with less Eu³⁺ and more Tb³⁺ in the β-NaGdF₄ host lattice.（3） The β-NaY_1-xGd_xF₄: Yb³⁺/Tm³⁺（0≤x≤1.00） solid solution microcrystals were synthesized by Na3Cit-capped hydrothermal method and the effect of Gd³⁺ ions substitution on microstructures and UC luminescent properties of the final crystals were investigated in detail. The results indicate that:（1） The β-NaY_1-xGd_xF₄: Yb³⁺/Tm³⁺（0≤x≤1.00） solid solution microcrystals can be formed with the substitution of Y³⁺ by Gd³⁺. When x increases from 0 to 0.20, more β-NaGdF₄ nuclei can be obtained and higher electronic charge density on the surface of the obtained crystals, resulting in higher nucleation rate and lower growth rate of the solid solution microcrystals. Finally, the grain size of the synthesized microcrystals decreases gradually. When x=0.35, the obtained products change from larger microprism β-NaYF₄ to smaller spherical β-NaGdF₄ crystals.（2） When x=0.04, the synergy balance of surrounding symmetry of the Ln³⁺ and grain size can be kept, resulting in solid solution microcrystals with higher UC luminescent intensity.