Synthesis and Properties of a Red Na₅Zn₂Gd_1−x(MoO₄)₆: xEu³⁺ Phosphor

Abstract

Novel Eu³⁺-doped Na₅Zn₂Gd(MoO₄)₆ triple molybdate phosphors were fabricated by the sol-gel method. The structure, morphology, and luminescent properties have been characterized by X-ray diffraction (XRD), thermogravimetric differential thermal analysis (TG-DTA), scanning electron microscopy (SEM), FTIR spectroscopy, and luminescence spectroscopy. The results indicated that the synthesized Na₅Zn₂Gd_1−x(MoO₄)₆: xEu³⁺ phosphor consisted of a pure phase with monoclinic structure. Under excitation at 465 nm, the Na₅Zn₂Gd_1−x(MoO₄)₆: xEu³⁺ phosphor exhibits an intensive red emission band around 610 nm corresponding to the transition of ⁵D₀→⁷F₂ which is much higher than that ⁵D₀→⁷F₁ at 594 nm, which was appropriate for a blue LED. According to the influence of the synthesis conditions, the phosphors showed the highest emission intensity when the doping concentration of Eu³⁺ was 25 mol.% and the molar ratio of citric acid to metal ions was 2:1. Na₅Zn₂Gd_0.75(MoO₄)₆: 0.25 Eu³⁺ with the color coordinates (x = 0.658, y = 0.341) is a more stable red phosphor for blue-based white LEDs than the commercial Y₂O₂S: Eu³⁺ red phosphor (0.48, 0.50) due to its being closer to the NTSC standard values (0.670, 0.330).

1. Introduction

The white light-emitting diode (WLED) has garnered extensive attention due to its merits 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 widely known that Eu³⁺, a rare earth ion that generates red light, can be effectively stimulated by blue light and UV light to achieve red emission [8,9,10,11,12]. Molybdates have been widely utilized as host substances for phosphors because of their low phonon energy, remarkable chemical and physical characteristics, excellent thermal stability, and strong charge transfer region in the ultraviolet range [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 O²⁻ ions are positioned in four corners of the tetrahedron [19]. The MoO₄²⁻ 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 O²⁻ ions were from the MoO₄²⁻ tetrahedral group [20]. The enhanced luminescent properties were achieved in some phosphate-molybdates (such as Na₂Gd(PO₄)(MoO₄)) [21,22]. Therefore, it would be very interesting to investigate the luminescent properties of Eu³⁺ ion-activated phosphate-molybdates. In this work, a new kind of triple molybdate compound Na₅Zn₂Gd_1−x(MoO₄)₆: xEu³⁺ red phosphor with a monoclinic powellite structure was prepared, and its luminescent properties were studied in detail for the first time. We have reported the structure and luminescent properties, including the diffuse reflectance spectra, excitation and emission spectra, life decay curves, and the color purity of Na₅Zn₂Gd_1−x(MoO₄)₆: xEu³⁺. The values of the band gap energy of the obtained phosphors (Eg = 2.68–2.70 eV) are smaller compared to the same type of doped phosphors such as CaMoO₄: xRE³⁺ (Eg = 3.84–3.93 eV) [23], with prolonged lifetime and 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⁺ (x = 0.15, 0.18, 0.20, 0.22, 0.25, 0.28, 0.30, and 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, the corresponding amounts of Gd(NO₃)₃ and Eu(NO₃)₃ solutions (depending on the ratio x) were added to the Zn(NO₃)₂ solution and mixed thoroughly. Then, citric acid was dissolved in a round-bottomed flask. In another vessel, NaOH was added to the (NH₄)₆Mo₇O₂₄·4H₂O solution, stirred to dissolve it, and then poured into a round-bottom flask and stirred uniformly. After adjusting the pH of the solution to between 7 and 8 (resulting in white precipitate), it was maintained at 60 °C for 4 h. The resulting sample was placed in a drying oven at 120 °C for 10 h. As the water gradually evaporated, a brown powder precursor was obtained. Finally, it was calcined in a furnace at 800 °C for 2 h, and then cooled; white powders of the Na₅Zn₂Gd(MoO₄)₆: Eu³⁺ were synthesized.

2.2. Characterizations

X-ray powder diffraction (XRD) of the samples was implemented by means of a Bruker D8-Advance diffractometer (BrukeR, Hong Kong, China). The crystallization procedures of the complex precursors were explored through thermogravimetric differential thermal analysis (TG-DTA, DTG-6OH, Harriet Island ferry). A heating rate of 10 °C/min was utilized in an air atmosphere. The infrared spectrum was measured on a Nicolet-6700 Fourier transform infrared spectrometer (Thermo Fisher Scientific, Beijing, China）with KBr pellets. The morphology characteristics of the samples were determined by Japan’s Hitachi S-3400N scanning electron microscopy (Hitachi, Tokyo, Japan). The UV–vis diffuse reflectance spectra were recorded by a Shimadzu (UV-2550, Hitachi, Tokyo, Japan) near-infrared UV–visible spectrophotometer. The luminescent properties of the samples were assessed using a Japanese Hitachi F-4500 fluorescence spectrometer (Hitachi, Tokyo, Japan) equipped with a 150 W-Xenon lamp. The working voltage was set at 400 V, and the scan rate was 1200 nm/min. The excitation and emission slits were set at 2.5 nm. The luminescence decay curves were measured by an Edinburgh FLS1000 spectrometer (Edinburgh Instruments Ltd., Livingston, UK) equipped with a pulsed Xenon lamp as the excitation source. All the measurements were conduced at room temperature.

3. Results and Discussion

3.1. TG-DTA Analysis of Na₅Zn₂Gd_1−x(MoO₄)₆: xEu³⁺

To choose the best thermal decomposition temperature for the samples, the TG-DTA curves of the Na₅Zn₂Gd(MoO₄)₆: Eu³⁺ precursor were studied, as shown in Figure 1. The TG curve shows two distinct weight loss steps up to 500 °C. No further weight loss is observed between 500 and 1000 °C. The weight loss is associated with the decomposition of the organic matrix. The DTA curve consist of three exothermic peaks at 220, 331, 420 °C. The peaks at 220 and 331 °C imply that the thermal occurrences could be related to the depletion of organic species, surface absorbed water, and citric acid. The third exothermic peak at 420 °C is attributed to the crystallization of Na₅Zn₂Gd (MoO₄)₆ powder from the amorphous component. However, the particle size distribution of the sample was more uniform at 800 °C than at other thermal decomposition temperatures.

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 °C. All of the diffraction peaks are highly consistent with the standard data of Na_2.2Zn_0.9(MoO₄)₂ (JCPDS NO. 49-0898) and no extra peaks originating from any impurities are observed, suggesting that all the phosphors exhibit a monoclinic powellite structure (C2/c) of Na_2.2Zn_0.9(MoO₄)₂. The doping of Eu³⁺ does not have an impact on the crystal structure of the host matrix. Additionally, with the change in the x value the lattice parameters are increased slightly due to the ionic radii of Eu³⁺ (0.95 Å) being very close to that of Gd³⁺ (0.94 Å). The diffraction peak at 2θ = 12.94° is almost invisible caused by the preferred orientation of the sample, which has no crystal growth along the (020) plan orientation. The diffraction peak intensity at 30.28 is significantly weaker than that in the standard card, which may be attributed to the relatively low crystallinity of the Na₅Zn₂Gd_1−x(MoO₄)₆: xEu³⁺ and the poor orderliness of the crystal.

The morphology of the Na₅Zn₂Gd_0.75(MoO₄)₆: 0.25Eu³⁺ samples annealed at 800 °C 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 a layered monoclinic structure are substantially rectangular, the length being from 1 to 4 μm, and the width between 0.5~1 μm for larger individual particles. Furthermore, VESTA software was used to simulate its unit cells, as shown in Figure 2d. The crystal structure contains MoO₄²⁻ tetrahedra and (Zn, Gd)O₆ octahedra, 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 edges. Each layer consists of passing corner-connected GdO₆ octahedrons and MoO₄²⁻ tetrahedra. Adjacent layers connected through ZnO₆ octahedra form a 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 FTIR spectra of Na₅Zn₂Gd(MoO₄)₆: Eu³⁺. For the precursor, the peak at 3431 cm⁻¹ corresponds to the stretching vibration of the -OH group from water and citric acid. The absorption peak at 1396 cm⁻¹ is attributed to the stretching vibration of the C-O bond. The peak at 1630 cm⁻¹ is associated with the -C=O bond. The carbonyl group shows a red shift because of the formation of complex [21]. The absorption peaks of H₂O and CO₂ decreased significantly after the precursor was sintered at 800 °C for 2 h. The absorption peak of citric acid was almost vanishing. In the case of the calcined samples, a set of strong absorption bands that are in line with the appearance of MoO4²⁻ can be detected in the region scanning from 500 to 1000 cm⁻¹. This suggests the formation of Na₅Zn₂Gd_0.75(MoO₄)₆: 0.25 Eu³⁺. The absorption peaks at 702, 899, 1000 cm⁻¹ should be ascribed to the bending vibration and asymmetric stretching modes of Mo-O bond asymmetric stretching vibrations of MoO₄²⁻, and there is the stretching vibration of the Eu-O bond at 549 cm⁻¹.

3.4. UV–Vis Diffuse Reflectance Spectra of Na₅Zn₂Gd_1−x(MoO₄)₆: xEu³⁺

Figure 4a shows the UV diffuse reflectivity spectra of Na₅Zn₂Gd_1−x(MoO₄)₆: xEu³⁺. The absorption edges of Na₅Zn₂Gd_1−x(MoO₄)₆: xEu³⁺ (x = 0.00, 0.05, 0.25, 0.35) at different concentrations of Eu³⁺ are 450, 462, 465, and 465 nm, respectively. With the increasing concentration, a red-shift occurs for the absorption edges, which is attributed to the f-f electronic transitions of Eu³⁺ ions. Meanwhile, Figure 4b shows the Tauc plots obtained from absorption spectra data, and the band gap value was calculated by the Kubelka–Munk equation [22]. The band gaps of Na₅Zn₂Gd_1−x(MoO₄)₆: xEu³⁺ (x=0.00, 0.05, 0.25, 0.35) are 2.72 eV, 2.70, 2.69, and 2.68 eV, respectively. The band gap values are close to the same type of doped phosphors such as BMO: Eu³⁺-500 (Eg = 2.67 eV) [23]. A smaller band gap value is more conducive to absorbing energy to complete the electronic transition process [24].

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

Figure 5 shows the excitation spectrum of the Na₅Zn₂Gd(MoO₄)₆: Eu³⁺ phosphor calcined at 800 °C with the emission wavelength monitored at 614 nm. It is evident that the excitation spectrum comprises a broad band from 250–350 nm, which corresponds to the charge transfer transition from O²⁻ to Eu³⁺. Additionally, there are narrow excitation peaks from 360 to 500 nm, which are attributed to the intra-configurational f-f transitions of Eu³⁺ ions [25]. The absorption peaks at 395 nm and 465 nm are attributed to the f-f transitions of Eu³⁺ ions, corresponding to ⁷F₀→⁵L₆ and ⁷F₀→⁵D₂ electronic transitions, respectively. The new triple molybdate Na₅Zn₂Gd(MoO₄)₆: Eu³⁺ can be excited by blue light at 465 nm prepared by the method, indicating that the phosphor matches with the 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 6. The emission band is observed at 614 nm, which is the 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 is attributed to the ⁵D₀-⁷F₀ transition of Eu³⁺ ions. Therefore, Na₅Zn₂Gd(MoO₄)₆: Eu³⁺ phosphor 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 to have low sensitivity to mandatory electric dipole transitions. Thus, Eu³⁺ was asymmetrically occupied in the Na₅Zn₂Gd(MoO₄)₆ lattice sites [26,27,28].

3.6. Effect of Eu³⁺ Doping Concentration

The different intensity of emission spectra under 465 nm excitation corresponding to ⁵D₀→⁷F₂ transition of Na₅Zn₂Gd_1−x(MoO₄)₆: xEu³⁺ (x = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, and 0.35) phosphors after annealing at 800 °C for 2 h are shown in Figure 7. It can be seen that the shapes and positions of all the Na₅Zn₂Gd_1−x(MoO₄)₆: xEu³⁺ 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. The rare earth ion itself has a lot of energy levels. As the doping of Eu³⁺ is further increased, the distance between Eu³⁺ ions shrinks. The phenomenon of concentration quenching occurs during excitation, resulting in a decrease in luminous intensity. The energy loss at the emission level initiated by the cross-relaxation between excited ions may be the main cause of concentration quenching [29]. The fluorescent emission of Eu³⁺ is mainly due to the transitions from the metastable level ⁵D₀ to the level ⁷F₂. A significant number of excited ions on the ⁵D₀ level are transferred from the ⁵D₀ to the ⁷F₂ level due to the cross-relaxation process. Consequently, several useful excited ions are consumed through radiation and relaxation processes, and the fluorescence emission is thereby reduced, leading to a decrease in luminous intensity. The optimum molar concentration is 25 mol% in Eu³⁺ ion-doped Na₅Zn₂Gd_1−x(MoO₄)₆: xEu³⁺ phosphor.

The emission spectra of the Na₅Zn₂Gd_0.75(MoO₄)₆: 0.25Eu³⁺ 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) were measured, as shown in Figure 8. The luminescent intensity of samples increased initially and then decreased as the ratio of citric acid to metal ions increased. Thus, it can be observed that the luminous intensity reaches a maximum when the ratio of citric acid to metal ions is 2:1. The pH values with a constant molar ratio of citric acid to metal ions affect the emission intensities of Na₅Zn₂Gd_1−x(MoO₄)₆: xEu³⁺. This is because the luminescent performance at different pH values should be attributed to the different morphologies of the products.

3.7. Luminescence Decay Lifetimes of Na₅Zn₂Gd_1−x(MoO₄)₆: xEu³⁺

The luminescence decay curves of Na₅Zn₂Gd_1−x(MoO₄)₆: xEu³⁺ were measured by monitoring the 615 nm emission of ⁵D₀→⁷F₂ under excitation of 465 nm. Figure 9 shows the luminescence decay curves of Eu³⁺-activated Na₅Zn₂Gd_1−x(MoO₄)₆ phosphors with various doping concentration. As a result, the decay curves of all measured samples exhibit a single exponential behavior and can be well fitted by the following equation:

I(t) = I₀ + Aexp (−t/τ)

where I(t) is the emission intensity at time t, I₀ is the initial emission intensity, and τ is the lifetime. A is the coefficient and τ is the lifetime.

By linear fitting, the lifetimes for the Na₅Zn₂Gd(MoO₄)₆ phosphors were obtained as 656, 668, 714, and 702 µs when the Eu³⁺ doping concentration was 0.05, 0.15, 0.25, and 0.35, respectively (see inset of Figure 9). As we can see, there is little difference among the lifetimes of the Na₅Zn₂Gd_1−x(MoO₄)₆: xEu³⁺ phosphors with different Eu³⁺ doping concentrations, indicating a weak concentration quenching behavior of Eu³⁺ in the present host.

3.8. 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 to the metal cation is 2:1) under 465 nm excitation, the CIE chromaticity diagram is shown in Figure 10. The color coordinates of the emission spectra of the sample (0.658, 0.341) were located in the red region. When compared with the color coordinates of the commercial Y₂O₂S: Eu³⁺ (0.48, 0.50) red phosphor, it is closer to the U.S. National Television Standards Committee (NTSC) standard values (0.670, 0.330), which means the Na₅Zn₂Gd_0.75(MoO₄) ₆: 0.25 Eu³⁺ is a stable red phosphor for white LEDs.

4. Conclusions

A series of the Na₅Zn₂Gd_1−x(MoO₄)₆: xEu³⁺ (x = 0.03, 0.04, 0.05, …, 0.25, …, 0.35) red phosphors was 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 a monoclinic powellite structure. The value of the band gap energy of Na₅Zn₂Gd_0.75(MoO₄)₆: 0.25Eu³⁺ was 2.69 eV, which is more conducive to absorbing energy to complete the electronic transition process. The strongest emission peak was located at 614 nm under the 465 nm excitation, which corresponds to the ⁵D₀→⁷F₂ transition of Eu³⁺ indicating that the Na₅Zn₂Gd(MoO₄)₆: Eu³⁺ crystal has a strong and pure red emission. The red phosphor has color coordinate values of (0.658, 0.341), which is close to the NTSC system standard for red chromaticity. Consequently, it is a promising candidate red phosphor for near-UV-based WLEDs.