Accepted: November 18, 2019 | Published Online: November 20, 2019

Citation: Li J, Chen Q, Yu H, Cao B, Liu Z (2019) Synthesis and Luminescent Properties of Monodisperse Spherical RE₂O₃:Eu³⁺ (RE = Gd, Y, Lu, La) Phosphors. Int J Earth Sci Geophys 5:029

Copyright: © 2019 Li J, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Abstract

We studied in detail the effects of morphology and size on the properties of RE₂O₃: Eu³⁺ (RE = Gd, Y, Lu, La) phosphors, the relationship between the coordination environment of Eu³⁺ and fluorescence properties. Through the comparative test of RE₂O₃ (RE = Gd, Y, Lu, La) fluorescent matrix doping with the same activation (Eu³⁺), the fluorescence properties difference of the four kinds of phosphors was analyzed. Spherical RE₂O₃: Eu³⁺ (RE = Gd, Y, Lu, La) phosphors were successfully prepared by the urea-based homogeneous precipitation method. The particles of these phosphors have perfect spherical shape, narrow size distribution, and non-agglomeration except La₂O₃: Eu³⁺ particles. The (Gd_0.95Eu_0.05)₂O₃, (Y_0.95Eu_0.05)₂O₃, and (Lu_0.95Eu_0.05)₂O₃ samples exhibit strong red emission at ~611 nm (⁵D₀-⁷F₂ transition of Eu³⁺) under excitation wavelength of ~258 nm (CTB of Eu³⁺, 8S7/2-6IJ transition of Gd³⁺), but the (La_0.95Eu_0.05)₂O₃ sample exhibit weak red emission at ~630 nm (⁵D₀-⁷F₂ transition of Eu³⁺) under excitation wavelength of 290 nm (CTB (charge transfer band) of Eu³⁺). Among them, (Gd_0.95Eu_0.05)₂O₃ phosphors have the best fluorescent properties due to the energy transfer of Gd³⁺→Eu³⁺ which further enhance the red emission at ~611 nm (⁵D₀-⁷F₂ transition of Eu³⁺). All the (RE_0.95Eu_0.05)₂O₃ phosphors in this work have similar CIE chromaticity coordinates (~0.65 ± 0.01, ~0.34 ± 0.01), high color purity, and similar fluorescence lifetimes (~1.70 ± 0.1). Therefore, the excellent type of (Gd_0.95Eu_0.05)₂O₃ phosphors in this work will be expected to be widely used in lighting and display areas.

Keywords

RE₂O₃: Eu³⁺ phosphors, Urea-based homogeneous precipitation, Luminescence property

Introduction

Recently, RE₂O₃ (RE = Gd, Y, Lu, La) have been interestingly studied owing to their excellent physical and chemical properties, monodisperse spherical particle morphology and potential applications as matrix in the fields of luminescence devices [1-9]. Eu³⁺ activated RE₂O₃ is well-known red-emitting phosphors widely used in the white LEDs [10-13], and displays such as plasma display panels (PDP) [14], field emission displays (FED) [3,15-21], and cathode-ray tubes (CRT) [22].

Based on the above situation, we want to study the fluorescence properties of RE₂O₃: Eu³⁺ (RE = Gd, Y, Lu, La) phosphors by analyzing the morphology and size of powders, and the coordination environment of Eu³⁺. However, the researchers are more inclined to fully study a single type of phosphor, including the synthesis and fluorescence properties of the phosphor. There is no research report on the comparative analysis of properties of RE₂O₃: Eu³⁺ (RE = Gd, Y, Lu, La) phosphors. In addition, we want to compare and analyze the fluorescent powders synthesized by the same activator Eu³⁺ with RE₂O₃ (RE = Gd, Y, Lu, La) fluorescent matrix, and it is expected to have a more direct comparison of its fluorescence performance, and the application prospect of each phosphor is analyzed. Among which, the phosphors possess high color purity (> 90%) is needed which is an important performance of phosphor used in display field [23].

Therefore, the situation and fluorescence properties of RE₂O₃: Eu³⁺ (RE = Gd, Y, Lu, La) was systematically studied and analyzed in this work. This is mainly for the three following reasons: (1) RE₂O₃: Eu³⁺ phosphor particles have strong luminescence intensity owing to good dispersion and sphericity [24,25]; (2) Eu³⁺ as one of the most widely red phosphor activator, was usually used in a lot of studies on the fluorescence properties in single matrix, but there are few studies on the difference of fluorescence properties of Eu³⁺ in different matrices. Furthermore, the Gd³⁺ may sensitize red emission of Eu³⁺ (⁵D₀-⁷F_1,2 transition of Eu³⁺) via efficient Gd³⁺→Eu³⁺ energy transfer, which can enhance the red emission of Eu³⁺ [26-32]; (3) Although the synthetic methods of rare-earth oxide phosphors are diversified, the solid phase method is much more studied than the chemical liquid phase method, so the chemical liquid phase method is used in this work.

Experimental Procedure

In this paper, the precursor was prepared by the urea-based homogeneous precipitation method, and then calcined at 1273 K to obtain the RE₂O₃: Eu³⁺ (RE = Gd, Y, Lu, La) phosphors. Crystalline phases of the RE₂O₃: Eu³⁺ (RE = Gd, Y, Lu, La) phosphors had been examined and confirmed by XRD. Particle size and morphology parameters of the phosphors were verified by FE-SEM. The luminescent properties were investigated by the PLE/PL spectroscopy and decay kinetics analysis. Therefore, we gave a detailed description of the synthesis, crystalline phases, morphology and luminescent properties of the RE₂O₃: Eu³⁺ (RE = Gd, Y, Lu, La) phosphors.

The raw materials used in this work are rare-earth oxides (Gd₂O₃, Y₂O₃, Lu₂O₃, La₂O₃ and Eu₂O₃, 99.99% pure, Huizhou Ruier Rare Chemical Hi-Tech Co. Ltd., Huizhou, China), CO(NH₂)₂•12H₂O (AR, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China), and HNO₃ (AR, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China). All the reactants were directly involved in the reaction and need not be further purified.

Rare-earth oxides were dissolved in hot nitric (353 K) acid to obtain rare-earth nitrates. The mother salt solution was achieved through mixing the RE(NO₃)₃ solutions according to the formula of (RE_1-xEu_x)₂O₃ (RE = Gd, Y, Lu, La, and x = 0.05). The CH₄N₂O (with 40 times the molar quantities of cations) was added in the mother solution, and the total volume was kept at 500 mL. The mixed solutions were firstly homogenized under stirring for 60 min at 298 K and then heated to 363 ± 1 K within 60 min. The precipitation was assembled through centrifuge after aging treatment for 2 h at 363 ± 1 K, and washed repeatedly with deionized water and alcohol to remove impurities. The wet precipitation was dried in air at 353 K for 24 h. The precursor was finally calcined at 1273 K for 4 h to obtain the resultant particles. Compared to the traditional synthesized methods, such as solid-state reaction [22], co-precipitation [8,26,32], hydrothermal method [2,3,17,27], and so on, the (RE_0.95Eu_0.05)₂O₃ (RE = Gd, Y, Lu) powders synthesized by urea-based homogeneous precipitation method possesses good dispersion and perfect spherical shape [21,23,25].

Crystalline phases were characterized by powder X-ray diffraction (D8-ADVANCE, BRUKER Co. Ltd., Germany) using nickel-filtered CuKα radiation under 40 kV/40 mA and a scanning speed of XRD diffraction angle (2θ) is 4°/min; Particle morphologies were observed via FE-SEM (QUNATAFEG-250, FEI Co. Ltd., USA); The photoluminescence spectra of the (RE_1-xEu_x)₂O₃ phosphors were measured under excitation light of a 150 W Xe lamp on the fluorescence spectrophotometer (FP-6500, JASCO Co. Ltd., Japan).

Result and Discussion

Figure 1 shows the XRD patterns of (RE_0.95Eu_0.05)₂O₃ (RE = Gd, Y, Lu, La) precursors calcined at 1273 K. It can be seen that all of the samples calcined at 1273 K have sharp characteristic peaks. The pure phase (Gd_0.95Eu_0.05)₂O₃, (Y_0.95Eu_0.05)₂O₃, and (Lu_0.95Eu_0.05)₂O₃ were formed indicating that the addition of Eu³⁺ did not alter the crystal structure, but the (La_0.95Eu_0.05)₂O₃ diffraction peaks shift towards large angle owing to the smaller radius of Eu³⁺ than La³⁺. The radius of Gd³⁺, Y³⁺, Lu³⁺, La³⁺ and Eu³⁺ are 1.053 Å, 1.019 Å, 0.977 Å, 1.16 Å and 1.066 Å, respectively. Because the ion radius of La³⁺ is larger than that of Eu³⁺, the lattice of this phosphor shrank when Eu³⁺ replaces the La³⁺ in the cell, resulting in the drifts of XRD diffraction peak to a large angle.

The scanning electron microscopy morphologies of the (RE_0.95Eu_0.05)₂O₃ (RE = Gd, Y, Lu, La) precursors and precursors calcining at 1273 K were presented in Figure 2. It can be seen that all four precursors have excellent spherical morphology and dispersity indicating that the urea-based homogeneous precipitation method can stably obtain the samples of this morphology. The average particle size of (Gd_0.95Eu_0.05)₂O₃ (a), (Y_0.95Eu_0.05)₂O₃ (b), (Lu_0.95Eu_0.05)₂O₃ (c), and (La_0.95Eu_0.05)₂O₃ (d) precursors were ~500 nm, ~170 nm, ~230 nm, and ~280 nm, respectively. The difference in particle size among the four precursors is mainly due to different nucleation density. By comparing the FE-SEM photos (e, f, g), the former three precursors calcining at 1273 K can maintain excellent monodisperse spherical morphology, but the phosphor particle of (La_0.95Eu_0.05)₂O₃ (h) was sintered. The doping of Eu³⁺ makes the lattice structure of (La_0.95Eu_0.05)₂O₃ severe shrinkage, leading to many defects in precursor of (La_0.95Eu_0.05)₂O₃, which makes the spherical precursor unable to maintain the original morphology at 1273 K.

Figure 3 shows the excitation spectra of (RE_0.95Eu_0.05)₂O₃ (RE = Gd, Y, Lu, La) monitoring at 611 nm, 610 nm, 610 nm and 626 nm emission (the ⁵D₀-⁷F₂ transition of Eu³⁺), respectively. (Gd_0.95Eu_0.05)₂O₃, (Y_0.95Eu_0.05)₂O₃ and (Lu_0.95Eu_0.05)₂O₃ phosphors have a broad PLE band at ~258 nm and (La_0.95Eu_0.05) ₂O₃ phosphors is at ~290 nm. Both of the two excitation peaks are due to a CTB (charge transfer band), namely, the electronic transition from the 2p orbital of O^2- to the 4f orbital of Eu³⁺ activators [33]. The electronegativity of Gd, Y, Lu and La are 1.2, 1.22, 1.27 and 1.1, respectively. Due to the electronegativity of La is lower than that of Gd, Y and Lu, La has weaker ability to attract electrons caused the excitation peak of (La_0.95Eu_0.05)₂O₃ phosphors to shift towards large angle. However, the irregular morphology of (La_0.95Eu_0.05)₂O₃ phosphor particles leads to the weakest intensity of excitation peak. It is noteworthy that the strongest excitation peak at ~258 nm stems from the characteristic transition of (Gd_0.95Eu_0.05)₂O₃ and 8S7/2-6IJ of Gd³⁺ overlapped, implying the Gd³⁺→Eu³⁺ energy transfer. As marked in the figure, the other bands were mainly f-f transition of Eu³⁺ located at ~362 nm (7F0-5D4), ~387 nm (7F0-5G2), ~394 nm (7F0-5L6) and ~465 nm (7F0-5D2), respectively.

According to the PLE spectra, the optimum excitation wavelength was determined. Based on this, the emission spectra of the (RE_0.95Eu_0.05)₂O₃ (RE = Gd, Y, Lu, La) phosphor were achieved under the optimum wavelength shown in Figure 4. The four typical emission bands originating from electron transition of Eu³⁺ ion can be noticed: ⁵D₀-7F1 (~587 nm), ⁵D₀-⁷F₂ (~611 nm, the strongest), ⁵D₀-7F3 (~654 nm), and ⁵D₀-7F4 (~689 nm) [7,10-13,19,22-26,32,33]. Furthermore, the weak emission peak at ~630 nm is also derived from the ⁵D₀-⁷F₂ transition of Eu³⁺ ion. The 611 nm emission intensity of (Gd_0.95Eu_0.05)₂O₃ is higher than other samples due to three reasons: (1) The efficient Gd³⁺→Eu³⁺ energy transfer enhanced the red emission of (Gd_0.95Eu_0.05)₂O₃ as revealed by PLE (Figure 3). (2) Due to the electronegativity of Gd is lower than that of Y and Lu, Gd has weaker ability to attract electrons and lower transferred energy. However, the luminescence intensity of (La_0.95Eu_0.05)₂O₃ phosphor at 626 nm is weakest due to it has irregular morphology. (3) The surface defects of (Gd_0.95Eu_0.05)₂O₃ phosphor particle are less than other samples owing to maximum particle size and minimal surface area. Besides, the strongest emission peak (626 nm) of (La_0.95Eu_0.05)₂O₃ shift towards large angle compared with other three kinds of phosphors implying the electronegativity of La is lower than that of Gd, Y and Lu. On the other side, it is well-known that the Eu³⁺ occupy S6 and C2 sites in Ln2O3 oxides. The different spectra between Gd₂O₃, Lu₂O₃, Y₂O₃ and La₂O₃ indicate the different Eu³⁺ sites in these oxides.

The CIE chromaticity coordinates for the emission of the (RE_0.95Eu_0.05)₂O₃ (RE = Gd, Y, Lu, La) phosphors under the respective wavelength excitation are shown in Figure 5. The CIE chromaticity coordinate of (Gd_0.95Eu_0.05)₂O₃ is determined to be (0.64, 0.35) with vivid red color via 254 nm UV excitation from a hand-held UV lamp, as shown in inset. The chromatic coordinate of (Y_0.95Eu_0.05)₂O₃ was calculated to be (0.66, 0.33), and the samples of (Lu_0.95Eu_0.05)₂O₃ and (La_0.95Eu_0.05)₂O₃ were calculated to have the uniform color coordinates (x, y) of (0.66, 0.34). The corresponding color temperature was calculated according to the following formulas:

$$T\text{ = -437}{n}^3+3601n^2-6861n+5514.31\text{ (1)}$$

and

$$n\text{ = }\left(x-0.332\right)/\left(y-0.1858\right)\text{ (2)}$$

where T is color temperature of phosphors, (x, y) is the color coordinate of the light source, n is variable.

The color temperatures of the samples were ~2768 K (Gd₂O₃:Eu³⁺), ~3806 K (Y₂O₃:Eu³⁺), and ~3390 K (La₂O₃:Eu³⁺, Lu₂O₃:Eu³⁺), respectively. Further observation is that all the (RE_0.95Eu_0.05)₂O₃ (RE = Gd, Y, Lu, La) phosphors have similar CIE chromaticity coordinate of (0.65 ± 0.01, 0.34 ± 0.01), and (Gd_0.95Eu_0.05)₂O₃ has a lower color temperature.

The color purity is an important property of the phosphor chromaticity property, and we can use the following formulas to calculate the color purity:

$$Colorpurity\text{ = }\frac{\sqrt{{\left(x-x{}_i\right)}^2+{\left(y-y_i\right)}^2}}{{\left(x_d-x{}_i\right)}^2+{\left(y_d-y_i\right)}^2}\times 100\%\text{ (3)}$$

where (x, y) is the color coordinate of the light source, (x_i, y_i) is the CIE of an equal-energy illuminant with a value of (0.3333, 0.3333), and (x_d, y_d) is the chromaticity coordinate corresponding to the dominant wavelength of the light source. Through consulting literature, we can obtain the (x_d, y_d) chromaticity coordinate of (Gd_0.95Eu_0.05)₂O₃ and (La_0.95Eu_0.05)₂O₃ were (0.6658, 0.3340) and (0.7006, 0.2993), respectively, and the (x_d, y_d) color coordinates of (Y_0.95Eu_0.05)₂O₃ and (Lu_0.95Eu_0.05)₂O₃ are the same as those of (Gd_0.95Eu_0.05)₂O₃. By substituting the coordinates of (x, y), (x_i, y_i), and (x_d, y_d) in Eq. (3), the color purities of (Gd_0.95Eu_0.05)₂O₃, (Y_0.95Eu_0.05)₂O₃, (Lu_0.95Eu_0.05)₂O₃ and (La_0.95Eu_0.05)₂O₃ are determined to be 92.38%, 98.26%, 98.28% and 88.59% respectively. The results show that (RE_0.95Eu_0.05)₂ O₃ series phosphors have excellent red emission and high color purity.

The PL thermal properties of phosphors for LEDs are necessary technological parameters. Thus, the effect of temperature on luminescence and activation energy of thermal quenching were studied via the PL spectra in 298-573 K temperature range, the results were shown in Figure 6a. The shape and positions of PL bands do not change with the temperature variation, but the emission intensity of Eu³⁺ in (Gd_0.95Eu_0.05)₂ O₃ decreases with the temperature increasing. Since the energy transfer effect is also affected by temperature, the rate of decline is different. The decrease of emission intensity is due to the thermal quenching caused by the thermal activation of the crossing point between the ground and the excited state. The emission intensities of Eu³⁺ at 373 K could retain about 99.30% of their corresponding initial values at 298 K. In order to investigate the temperature-dependent thermal quenching phenomenon, the activation energy can be achieved using the Arrhenius equation:

$$\text{In}\left(\frac{I_0}{I}-1\right)\text{ = 1n }A-\frac{E_{\alpha }}{kT}\text{ (4)}$$

where E_α and T represent the objective activation energy and temperature (K), respectively. A is a constant and k is the Boltzmann constant (8.626 × 10^-5 eV). I₀ is the integrated emission intensity at room temperature and I is the integrated emission intensity at differently operated temperatures. The ln(I₀/I-1) versus 1/kT activation energy graph for thermal quenching of the (Gd_0.95Eu_0.05)₂ O₃ phosphor is plotted in Figure 6b, which shows that the slope of the best fitting line is -0.4631, thus the activation energy E_α is approximately 0.4631 eV. The relatively high activation energy achieved in this work indicates that it possesses good thermal stability and is an excellent candidate for application in LEDs.

Figure 7 shows the decay kinetics for 610-626 nm emission of the (RE_0.95Eu_0.05)₂O₃ phosphors calcined at 1273 K. The decay curve can be fitted via the formula:

$$I\text{ = }A\text{exp}\left(-t/{\tau }_R\right)+B\text{ (5)}$$

where I is the relative fluorescence intensity, t is the fluorescence lifetime, τ_R is the decay time. A and B are constants. Figure 7a, Figure 7b, Figure 7c and Figure 7d displays the lifetime value of Gd, Y, Lu and La, which are 1.76 ms, 1.80 ms, 1.70 ms and 1.61 ms, respectively. The results show that the fluorescence lifetimes of (RE_0.95Eu_0.05)₂O₃ phosphors are similar.

Conclusions

The (RE_0.95Eu_0.05)₂O₃ phosphors have been synthesized by the urea-based homogeneous precipitation method at 1273 K. The (RE_0.95Eu_0.05)₂O₃ (RE = Gd, Y, Lu) powders with uniform particle morphology exhibit strong red emission at ~611 nm (⁵D₀-⁷F₂ transition of Eu³⁺) under excitation wavelength of ~258 nm (CTB of Eu³⁺, 8S7/2-6IJ transition of Gd³⁺), but the (La_0.95Eu_0.05)₂O₃ powders were sintered and exhibit weak red emission at ~630 nm (⁵D₀-⁷F₂ transition of Eu³⁺) under excitation wavelength of 290 nm (CTB of Eu³⁺). Furthermore, the presence of Gd³⁺ excitation bands on the PLE spectra monitoring the Eu³⁺ emission directly proves Gd³⁺→Eu³⁺ energy transfer, which can further enhance the red emission of the (Gd_0.95Eu_0.05)₂O₃ phosphors. All the (RE_0.95Eu_0.05)₂O₃ (RE = Gd, Y, Lu, La) phosphors in this work have similar CIE chromaticity coordinates at (~0.65 ± 0.01, ~0.34 ± 0.01). And the color temperatures and color purities of the samples were ~2768 K, 92.38% (Gd₂O₃:Eu³⁺), ~3806 K, 98.26% (Y₂O₃:Eu³⁺), ~3390 K, 98.28% (Lu₂O₃:Eu³⁺), ~3390 K and 88.59% (La₂O₃:Eu³⁺), respectively. Fluorescence decay analysis found that the (RE_0.95Eu_0.05)₂O₃ (RE = Gd, Y, Lu, La) phosphors in this work have similar lifetime (~1.70 ± 0.1). Therefore, the (Gd_0.95Eu_0.05)₂O₃ phosphors will be widely used in lighting and display fields. In general, we studied more deeply and comprehensively the relationship between the morphology and fluorescence properties of (RE_0.95Eu_0.05)₂O₃ (RE = Gd, Y, Lu, La) phosphors than the previous reported results.

Acknowledgements

This work was supported in part by the National Natural Science Foundation of China (No. 51402125 and 51602042), China Postdoctoral Science Foundation (No. 2017M612175), the Special Fund for the Postdoctoral Innovation Project in Shandong Province (No. 201603061), the Research Fund for the Doctoral Program of University of Jinan (No. XBS1447), the Natural Science Foundation of University of Jinan (No. XKY1515), the Science Foundation for Post Doctorate Research from the University of Jinan (No. XBH1607), the Natural Science Foundation of Shandong Province (No. ZR2016QL004).