Synthesis and Properties of a Red Na5Zn2Gd1-x(MoO4)6: xEu3+ Phosphor

1. Introduction

White light-emitting diode (WLED) has attracted wide attention due to its advantages of high luminous efficiency, long lifetime and low power consumption and environmental friendliness [1,2]. The red phosphors can be excited by blue and near-ultraviolet in particular. The commercial sulfide phosphors CaS: Eu²⁺, Y₂O₂S: Eu³⁺ show limitations, such as being chemically unstable and having a short lifetime under ultraviolet radiation, thus hindering the development of a highly efficient white LED [3,4,5]. Therefore, more interest is being focused on the search for a stable red phosphor with intense absorption in the near-UV to the blue spectral region [6,7]. With strong physical and chemical stability, molybdates have been investigated as potential red emitting phosphors. It is well known that Eu³⁺ is an rare earth ion generating red light and can be effectively stimulated by blue light and UV light to achieve red emission [8,9,10,11,12]. Molybdates have been widely used as host materials to phosphors based on their low phonon energy, outstanding chemical and physical properties, good thermal stability, and strong charge transfer zone in the ultraviolet region [13,14]. Eu³⁺-doped molybdates are red-emitting phosphors due to the ⁵D₀→⁷F_J spin and parity-forbidden transition. Currently, some ternary molybdate compounds have emerged as promising laser materials [15,16,17,18]. Based on relevant studies, the triple molybdates have a layered structure which is tetrahedral, and four O^2- ions are positioned in four corners of the tetrahedron [19]. The MoO₄^2- isolated island-like tetrahedron was distributed in the crystal, in which the compounds of divalent ions (such as Zn, Ba, Mg, and Sr) were coordinated by six oxygen atoms, and the O^2- were from the MoO₄^2- tetrahedral group [20]. In this work, a new kind of triple molybdate compound Na₅Zn₂Gd_1-x(MoO₄)₆: xEu³⁺ (0.03≤x≤0.35) red phosphor with scheelite structure was prepared, and its luminescent properties were studied in detail for the first time. The obtained phosphor compared with commercial red phosphor Y₂O₂S: Eu³⁺ is closer to standard color coordinate values by the color coordinate calculated, indicating that the red phosphor has high color purity.

2. Experimental

2.1. Phosphors Preparation

The red phosphors Na₅Zn₂Gd_1-x(MoO₄)₆: xEu³⁺ (0.03≤x≤0.35) were prepared using citrate method with raw materials of Zn(NO₃)₂·6H₂O (AR), (NH₄)₆Mo₇O₂₄·4H₂O (AR), C₆H₈O₇·2H₂O (AR), NaOH (AR), Eu₂O₃ (99.99%) and Gd₂O₃ (99.99%). The detailed experimental process is similar to that in Refs. [8,9]. Firstly, rare-earth oxides (Eu₂O₃, Gd₂O₃) were dissolved in 1:1 HNO₃ under vigorous stirring, and the excess HNO₃ was removed under heating. Then a stoichiometric amount of (NH₄)₆Mo₇O₂₄·4H₂O, NaOH, and Zn(NO₃)₂ solution was obtained by dissolving in deionized water. Subsequently, 0.1 mol L^-1 Gd(NO₃)₃ and 0.1 mol L^-1 Eu(NO₃)₃ solution added to the Zn(NO₃)₂ solution mix well and then citric acid dissolved in a round-bottomed flask. NaOH was added to the other in the (NH₄)₆Mo₇O₂₄·4H₂O solution stirring to dissolve and poured into a round bottom flask and stirred uniformly, and 60 ℃ for 4 h after the pH of the solution was adjusted to 7-8 (with white precipitate). The resulting sample was placed in a drying oven at 120 ℃ for 10h, after the water was gradually reduced to give a brown powder precursor, then calcined in the furnace at 800 ℃ for 2 h, and cooled. As a result of the last process, white powders of the Na₅Zn₂Gd(MoO₄)₆: Eu³⁺ were synthesized.

2.2. Characterizations

X-ray powder diffractions (XRD) of the samples were examined using the Bruker D8-Advance diffractometer. The crystallization processes of the complex precursors were examined by thermogravimetry differential thermal analysis (TG-DTA, DTG-6OH, Harriet Island ferry ), using a heating rate of 10 ◦C/min in an air atmosphere. The infrared spectrum was measured on a Nicolet-6700 Fourier transform infrared spectrometer, KBr pellet. The morphology of the samples was characterized by Japan's Hitachi S-3400N scanning electron microscopy. Luminescent properties of the samples were carried out using a Japan's Hitachi F-4500 fluorescence spectrometer with a 150W-Xenon lamp under working voltage 400V, scan rate 1200 nm/min, and the excitation and emission slits were set at 2.5 nm. All the measurements were performed at room temperature.

3. Results and Discussion

3.1. TG-DTA Analysis of Na₅Zn₂Gd_1-x(MoO₄)₆: x Eu³⁺

To choose the best thermal decomposition temperature for the samples, the TG-DTA curves of the Na₅Zn₂Gd(MoO₄)₆: Eu³⁺ precursors were studied, as shown in Figure 1. The TG curve shows two distinct weight loss steps upto 500 ℃ and no further weight loss was registered between 500 and 1000 ℃. The weight loss is related to the decomposition of the organic matrix. The DTA curve consist of three exothermic peaks at 220, 331, 420 ℃. The peak at 220 and 331 ℃ indicate that the thermal events can be associated with the exhaustion of organic species of surface absorbed water and citric acid, and the third exothermic peak at 420 ℃ is due to the crystallization of Na₅Zn₂Gd (MoO₄)₆ powder from the amorphous component. However, the particle size distribution of the sample was more uniformity at 800 ℃ than at other thermal decomposition temperature.

3.2. Structure and Morphology of Na₅Zn₂Gd_1-x(MoO₄)₆: x Eu³⁺

Figure 2a shows the XRD patterns of Na₅Zn₂Gd_1-x(MoO₄)₆: xEu³⁺ (x= 0.0, 0.25) annealed at 800℃. All of the diffraction peaks coincide well with the standard data of Na_2.2Zn_0.9(MoO₄)₂ (JCPDS NO. 49-0898) and no extra peaks from any impurities are detected, which indicates that all the phosphors adopt a monoclinic powllite structure of Na_2.2Zn_0.9(MoO₄)₂ and Eu³⁺ doping do not impact the crystal structure of the host matrix. The diffraction peaks at 2θ= 12.94°is almost invisible caused by the preferred orientation of the sample, which is no crystal growth along the (020) plan orientation. The diffraction peak intensity at 30.28 is significantly weaker than that in the standard card, thus indicating that the existence of an incomplete crystal structure on the Na₅Zn₂Gd_1-x(MoO₄)₆: xEu³⁺ surface can interfere with the diffraction of the X-rays, resulting to a reduced intensity of the diffraction peak.

The morphology of the Na₅Zn₂Gd_0.75(MoO₄)₆: 0.25Eu³⁺ samples annealed at 800 ℃ was determined using SEM analysis. Figure 2b,c shows the different magnification scanning electron images. It can be seen that the Na₅Zn₂Gd_0.75(MoO₄)₆: 0.25 Eu³⁺ samples with the layered monoclinic structure are substantially rectangular, the length from 1 to 4 μ m, and the width between 0.5~1 μm larger individual particles. Furthermore, VESTA software was used to simulate its unit cells, as shown in Figure 2d. The crystal structure contains MoO₄^2- tetrahedral and (Zn, Gd)O₆ octahedrons, a three-dimensional structure, and Na atoms in a cavity formed in the polyhedron. The tetracoordination of molybdate connects with the sides of the layer by the oxygen vertex, and (Zn, Gd)O₆ octahedra are connected between the common edge. Each layer consists of passing corner-connected GdO₆ octahedrons and MoO₄^2- tetrahedra. Adjacent layers connected through ZnO₆ octahedral form Na₅Zn₂Gd(MoO₄)₆ scheelite layered structure, which is consistent with the information conveyed by XRD.

3.3. FTIR spectra of Na₅Zn₂Gd_0.75(MoO₄)₆: 0.25 Eu³⁺

Figure 3 shows the IR spectra of Na₅Zn₂Gd(MoO₄)₆: Eu³⁺, the peaks of the precursor at 3431 cm^-1 is the stretching vibration of -OH group from water and citric acid and the absorption peak at 1396 cm^-1 is attributed to stretching vibration of C-O bond. The peak at 1630 cm^-1 belong to -C=O bond, and the carbonyl group red-shifts due to the formation of complex [21]. The absorption peak of H₂O and CO₂ decreased significantly after the precursor was sintered at 800 ℃ for 2 h. The absorption peak of citric acid was almost vanishing. As for the calcined samples, a series of strong absorption bands, corresponding to the appearance of MoO4^2-, can be observed in the region of 500-1000 cm^-1, indicating the formation of Na₅Zn₂Gd_0.75(MoO₄)₆: 0.25 Eu³⁺. The absorption peaks at 702, 899, 1000 cm^-1 were should be attributed to the bending vibration and asymmetric stretching modes of Mo-O bond asymmetric stretching vibrations of MoO₄^2-. And there is the stretching vibration of the Eu-O bond at 549 cm^-1.

3.4. Luminescent Properties of Na₅Zn₂Gd(MoO₄)₆: Eu³⁺

Figure 4 shows the excitation spectrum of Na₅Zn₂Gd(MoO₄)₆: Eu³⁺ phosphor calcined at 800 ℃by monitoring the emission wavelength at 614 nm. It can be seen that the excitation spectrum consists of broad band from 250-350 nm which corresponds to the charge transfer transition O^2-→Eu³⁺ and narrow excitation peaks from 360 to 500 nm, which is attributed to the intra-configurational f-f transitions of Eu³⁺ ions [22]. The absorption peak at 395 nm and 465 nm is attributed to the f-f transition of Eu³⁺ ions, corresponding to ⁷F₀→⁵L₆ and ⁷F₀→⁵D₂ electronic transition, respectively. The new triple molybdate Na₅Zn₂Gd(MoO₄)₆: Eu³⁺ can be excited by blue light 465 nm prepared by the method, indicating that the phosphor matching with a wide output wavelength of the blue LED chip.

The emission spectrum of Na₅Zn₂Gd(MoO₄)₆: Eu³⁺ phosphor is obtained under the excitation wavelength at 465 nm as shown in Figure 5. The emission band is observed at 614 nm, which is ⁵D₀→⁷F₂ transition of Eu³⁺, much higher than at 595 nm (⁵D₀→⁷F₁), the ratio of intensity I_5D0-7F2/I_5D0-7F1 is about 10. It indicates that Eu³⁺ occupies a site or sites without an inversion center, and low symmetry. In addition, another emission peak at 580 nm are attributed to ⁵D₀-⁷F₀ transition of Eu³⁺ ions. Therefore, Na₅Zn₂Gd(MoO₄)₆: Eu³⁺ phosphors is a phosphor that is excited by blue light and emits strong red light. The ⁵D₀→⁷F₁ transition is mainly magnetic allowed (magnetic dipole transition) in the spectra of Eu³⁺. At the same time, the ⁵D₀→⁷F₂ is the only center in the absence of inversion symmetry conditions have low sensitivity to mandatory electric dipole transition. Thus Eu³⁺ was asymmetrically occupied in the Na₅Zn₂Gd(MoO₄)₆ lattice sites [23,24,25].

3.5. Effect of Doped-Eu³⁺ Concentration

The different intensity of emission spectra under 465 nm excitation corresponding to ⁵D₀→⁷F₂ transition of Na₅Zn₂Gd_1-x(MoO₄)₆: Eu_x³⁺ (x= 0.03, 0.04, 0.05, ..., ..., 0.35) phosphor after annealing at 800 ℃ for 2 h is shown in Figure 6. It can be seen that the shapes and positions of all the Na₅Zn₂Gd_1-x(MoO₄)₆: Eu_x³⁺ samples are the same in addition to their relative luminescence intensity. Thus, with the increased concentration of Eu³⁺ in the host, the luminescent intensity has enhanced and reached a maximum at a concentration of x=0.25. Rare earth ion itself has a lot of energy levels. Thus, with the increased concentration of Eu³⁺ in the host, the luminescent intensity has enhanced and reached a maximum at a concentration of x=0.25. As the Eu³⁺ doping is further increased, the distance between Eu³⁺ ions decreases. The concentration quenching phenomenon occurs during excitation, thus the luminous intensity decreases. The energy loss at the emission level initiated by the cross-relaxation between the excited ions might be the main reason causing the concentration quenching [26]. The fluorescent emission of Eu³⁺ was mainly due to the transitions from metastable level ⁵D₀ to level ⁷F₂ . Considerable excited ions on the ⁵D₀ level were transferred from the ⁵D₀ level to the ⁷F₂ level due to the cross-relaxation process; therefore, several useful excited ions were consumed through the radiation and relaxation processes and the fluorescence emission was thereby reduced lead to the luminous intensity decrease. The optimum molar concentration of 25 mol% in Eu³⁺ ions doped Na₅Zn₂Gd_1-x(MoO₄)₆: Eu_x³⁺ phosphor.

The emission spectra of Na₅Zn₂Gd_0.75(MoO₄)₆: 0.25Eu³⁺ samples with different ratios of citric acid with metal ions (C: M = 0.5:1, 1:1, 1.5:1, 2:1, 2.5:1 and 3:1) at 800 ℃ annealing temperature were measured, as shown in Figure 7. The luminescent intensity of samples increased with the trend of first increases and then decreased with the rise in the ratio of citric acid and metal ions. Thus, it can be seen that the luminous intensity reaches a maximum when the ratio of citric acid and metal ion at 2:1. The pH values with a constant molar ratio of citric acid to metal ions effect the emission intensities of Na₅Zn₂Gd_1-x(MoO₄)₆: xEu³⁺ due to the luminescent performance at different pH value should be ascribed to the different morphology of the products.

3.6. CIE Color Coordinates of Na₅Zn₂Gd_0.75(MoO₄)₆: 0.25 Eu³⁺

According to the emission spectrum data of Na₅Zn₂Gd_0.75(MoO₄) ₆: 0.25 Eu³⁺ (the ratio of citric acid and the metal cation is 2:1) samples under 465 nm excitation, the CIE chromaticity diagram shown in Figure 8. The color coordinates of the emission spectra of the sample (0.658, 0.341) was located in the red region. It is closer to the NTSC standard values (0.670, 0.330) after comparing with the U.S. National Television Standards Committee (NTSC) and the color coordinates (0.48, 0.50) of the commercial Y₂O₂S: Eu³⁺ red phosphor, which means the Na₅Zn₂Gd_0.75(MoO₄) ₆: 0.25 Eu³⁺ is a stable red phosphor for white LEDs.

4. Conclusions

The novel Na₅Zn₂Gd_1-x(MoO₄)₆: xEu³⁺ ( x = 0.03, 0.04, 0.05, ..., 0.25, ..., 0.35) series red phosphors successfully prepared with sol-gel method, and the optimal synthesis condition was investigated based on luminescent relative intensity. The results of XRD and SEM show samples with scheelite layered structure, the length from 1 to 4 μ m, and wide with between 0.5~1 μm. The strongest emission peaks was located at 614 nm under the 465 nm excitation,which is corresponding to ⁵D₀→⁷F₂ transition of Eu³⁺ indicated that the Na₅Zn₂Gd(MoO₄)₆: Eu³⁺ crystal has a strong and pure red emission. The red phosphor with the color coordinate values (0.658, 0.341) was almost close to the international standard color coordinate values (0.670, 0.330) and thus it is a promising red light material candidate for creating near UV light and blue light in phosphor-converted white LEDs.