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Tungstate flux-grown Y10W2O21:Eu3+ phosphors with enhanced particle size and luminescence properties
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Tungstate flux-grown Y10W2O21:Eu3+ phosphors with enhanced particle size and luminescence properties ⁎
Tzu-Chin Chiena, Chii-Shyang Hwanga, , Yung-Tang Nienb, Masahiro Yoshimuraa a b
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan Department of Materials Science and Engineering, National Formosa University, Yunlin 63201, Taiwan
A R T I C L E I N F O
A BS T RAC T
Keywords: Yttrium-tungsten oxide Photoluminescence Thermal quenching Tungstate flux Particle size
Eu0.5Y9.5W2O21 strong red emission phosphors were synthesized via a conventional solid-state reaction method with environmentally friendly tungstate flux (M2WO4, M=Li, Na, K). The growth process of the Eu0.5Y9.5W2O21 phosphors was studied using X-ray diffraction, scanning electron microscopy, and thermogravimetry/ differential thermal analysis. The photoluminescence properties, including emission and excitation spectra and thermal stability, were also investigated. The experimental results indicate that the particle size and morphology of the Eu0.5Y9.5W2O21 phosphors depend on the type and content of tungstate flux. The luminescence intensities of the phosphors were effectively enhanced by the tungstate flux. The results also show that the Eu0.5Y9.5W2O21 phosphors are very suitable for use in near-ultraviolet light-emitting diode (LED) chips (392 nm) and blue LED chips (465 nm).
1. Introduction
morphology and size distribution [4–7]. The choice of flux should be based on its ability to melt at the recrystallization temperature and avoid the formation of any secondary phases or luminescence-killing centers. The most common fluxes used for manufacturing phosphors are H3BO3, NH4Cl, NaCl, and alkali/alkaline-earth fluorides. Fluoride fluxes are commonly used in the high-temperature synthesis of siliconnitride- or oxynitride-based phosphors, and clearly promote the luminescence properties and particle size [8–10]. However, concerns remain regarding environmental pollution related to fluoride fluxes, in terms of toxicity, corrosion of kilns, and staff health [11]. Therefore, it is imperative to find a superior flux that can be applied at high temperatures. According to the literature, single-crystal calcium tungstate has been grown from M2WO4 (M=Li, Na, and K) fluxes [12,13]. Recently, single-crystal calcium tungstate phosphors were synthesized via the flux growth method [14]; the phosphor emission intensity was improved due to the high crystal quality. Other tungstate compounds, such as Na2RE(PO4)(WO4) (RE=Y, Tb-Lu) and Bi2WO6, also have higher quality single crystals when Na2WO4 flux is applied [15,16]. Some laser materials of KY(WO4)2 and KGd(WO4)2 crystals were grown via a top-seed solution growth method with K2WO4 flux [17,18]. Moreover, the tungstate flux M2WO4 (M=Li, Na and K) can dissolve easily in water. Based on these earlier reports, it is evident that little attention has been devoted to examining the influence of tungstate flux type on the particle size distribution and luminescence properties of phosphors. To address this, the present work focuses on
Tungstate compounds have been widely investigated as potential hosts for Eu3+-activated phosphors due to their high efficiency, narrow line-width emission peak centered at 615 nm, and broad excitation band in the near-UV region [1]. Previously, we studied an yttriumtungsten oxide, namely Y10W2O21, as a phosphor host candidate due to its high melting temperature [2]. The (EuxY1-x)10W2O21 phosphor was synthesized via a solid-state reaction and calcined at 1400 °C for 6 h in air. It exhibited two strong excitation peaks, one being near-UV (392 nm) and the other blue light (465 nm). The two dominant emission peaks at 612 and 627 nm were caused by the substitutions of multi-coordinate sites of Y3+(coordination numbers=7, 8) by Eu3+. Additionally, this phosphor had good thermal stability and quantum efficiency. However, the (Y1-xEux)10W2O21 particles formed agglomerations, roughly dozens of micrometers in size, which affected the photoluminescence (PL) properties of the phosphors. A fluorescent material for commercial phosphor-converted light-emitting diodes (pcLEDs) requires a high luminescence yield and a narrow size distribution (generally 3–8 µm) [3]. It is thus important to control the size distribution of (Y1-xEux)10W2O21 phosphors via a high-temperature solid-state reaction. The use of flux in synthesis reactions is one of the simplest and most cost-efficient methods for increasing the crystallinity and chemical homogeneity of powder particles, as well as enhancing the
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Corresponding author. E-mail address: [email protected] (C.-S. Hwang).
http://dx.doi.org/10.1016/j.ceramint.2017.04.118 Received 6 March 2017; Received in revised form 19 April 2017; Accepted 19 April 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Chien, T.-C., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.04.118
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improving the crystallinity, particle size distribution, and luminescence properties of Eu0.5Y9.5W2O21 phosphors by investigating the effects of adding M2WO4 (M=Li, Na and K) fluxes with different melting temperatures.
2. Experimental procedure Eu0.5Y9.5W2O21 phosphor powders were synthesized via a solidstate reaction. Y2O3 (Gredmann, 99.99%), WO3 (Alfa Aesar, 99.99%), Eu2O3 (Acros, 99.99%), and tungstate M2WO4 (M=Li, Na and K) flux (Alfa Aesar, 99%) powders were used as raw materials. These were weighed in a stoichiometric ratio [flux/(Eu0.5Y9.5W2O21+flux)=10– 50 wt%] and mixed in a polyethylene jar with Y2O3-stablized ZrO2 balls using high-energy vibrating ball milling for 15 min. After mixing, the blended powders were calcined at 1400 °C for 6 h in air to form phosphors. The resultant phosphors were then ultrasonically washed three times with deionized (DI) water to remove residual flux, and dried in an oven at 100 °C. The X-ray diffraction (XRD) patterns of the samples were obtained with a Rigaku Dmax diffractometer using Cu Kα radiation (λ=0.15406 nm) with a source power of 30 kV and a current of 20 mA. The morphology of the phosphors was inspected with a scanning electron microscope (SEM, JEOL JSM 7001F). Thermogravimetric/differential thermal analysis (TG/DTA) curves were obtained with a NETZSCH STA 449 F3 instrument at a heating rate of 5 °C/min in air. The excitation and emission spectra of the phosphors were measured at room temperature using a Hitachi F-7000 fluorescence spectrophotometer using a 150 W Xe lamp as the excitation source. The thermal stability of the phosphors was measured using the heating apparatus in the F-7000 spectrophotometer.
Fig. 1. XRD patterns of Eu0.5Y9.5W2O21 powders (a) without flux and with tungstate fluxes (b) Li2WO4, (c) Na2WO4, and (d) K2WO4 calcined at 1400 °C for 6 h in air.
3.2. Powder morphology The SEM images in Figs. 3 and 4 show that the Na2WO4 and K2WO4 fluxes, respectively, accelerated the growth of the Eu0.5Y9.5W2O21 phosphor particles. The powders without flux (Figs. 3(a) and 4(a)) consisted of small particles with irregular shapes and had a strong tendency to form agglomerates. Although the particle size distribution primarily ranged from 200 to 500 nm, some agglomerates measured some dozens of micrometers. It is well known that micrometer-sized agglomerates have numerous grain boundaries and defects on the surface of phosphors; as such, more light scattering is induced and the luminescence properties are reduced. In contrast, the samples with various amounts of Na2WO4 flux exhibited clear particle growth (Fig. 3(b)–(f)). The images show a sphere-like morphology with no agglomeration and a homogenous size distribution. Moreover, the grain boundaries were eliminated because of the dissolution-recrystallization process; specifically, the Na2WO4 flux can melt, inducing the formation of a liquid phase at a lower temperature (~ 698 °C). The liquid phase promoted the reaction of raw materials and helped merge small agglomerated particles into large ones. In this manner, the average particle size increased from ~1 to ~9 µm when the amount of Na2WO4 flux added was increased from 10 to 30 wt%. However, when the amount of Na2WO4 flux exceeded 30 wt %, the morphology and particle size showed no further change. For the samples with various amounts of K2WO4 flux, uniform and nonagglomerated particles with a smooth surface were also observed; however, the particle size did not increase when the amount of K2WO4 flux added was increased. The average particle size of these samples ranged from 0.5 to 1 µm. Moreover, the results indicate that the K2WO4 flux promoted non-agglomerated particles due to its relatively high melting point of 920 °C.
3. Results and discussion 3.1. Phase characterization The effects of Li2WO4, Na2WO4, and K2WO4 fluxes (20 wt%) on the crystalline structure of Eu0.5Y9.5W2O21 phosphors calcined at 1400 °C for 6 h are shown in the XRD patterns in Fig. 1. As can be seen, the diffraction peaks of the samples with (c) Na2WO4 and (d) K2WO4 fluxes are in agreement with those of the Y10W2O21 phase (JCPDS card no. 76-1153, orthorhombic phase). In addition, the Y30W8O69 phase (JCPDS card no. 15-0559, hexagonal phase) was also found in the sample with Li2WO4 flux, as shown in Fig. 1(b). It has been reported that the cations of fluxes can diffuse into the crystalline matrix of the host during the high-temperature solid-state reaction process [7,19]. The radius of Li+ ions (r=0.92 pm at coordination number 8) is about 15% smaller than that of Y3+ ions (r=1.019 pm); as such, Li+ ions can easily replace Y3+ ions in the host lattice, leading to the formation of the Y2O3-rich phase. In contrast, the radii of Na+(r=1.18 pm) and K+(r=1.51 pm) ions are more than 15% larger than that of Y3+ ions; as a consequence, these two ions cannot easily substitute Y3+ ions. Based on the above results, the Na2WO4 and K2WO4 fluxes were chosen for the following optimal content study. The XRD patterns of yttrium-tungsten oxide with various amounts of Na2WO4 or K2WO4 fluxes (10–50 wt%) calcined at 1400 °C for 6 h and washed with DI water are shown in Fig. 2. Fig. 2(a) shows that the Na2WO4 flux amount had no perceptible influence on the crystalline structure of the resulting phosphors, which were all well crystallized phases of Eu0.5Y9.5W2O21 powders. Fig. 2(b) shows that no secondary phase existed until the amount of K2WO4 added reached 30 wt%. Some minor peaks of the Y6WO12 secondary phase were also detected when the content of K2WO4 reached 40 wt%, and this increased with increasing addition amount. 2
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Fig. 2. XRD patterns of Eu0.5Y9.5W2O21 powders with various amounts of (a) Na2WO4 and (b) K2WO4 calcined at 1400 °C for 6 h in air.
Fig. 3. SEM images of Eu0.5Y9.5W2O21 phosphors with various amounts of Na2WO4 flux calcined at 1400 °C for 6 h in air.
curves of the two samples in the temperature range of 25–400 °C are the same. The endothermic peak at 100 °C and exothermic peaks at 315 and 330 °C are associated with the evaporation of water and the burning of the organic constituents (possibly debris from the polyethylene jar), respectively. According to previous studies, the reaction of yttrium oxide and tungsten oxide to form yttrium tungstate begins at
3.3. Thermal analysis In order to elucidate the effects of the fluxes (Na2WO4, K2WO4) on the reactive process, TG/DTA analyses were conducted. Fig. 5(a) and (b) show the TG/DTA results of the mixed powders with 30 wt% Na2WO4 and K2WO4 fluxes in the raw materials, respectively. The 3
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Fig. 4. SEM images of Eu0.5Y9.5W2O21 phosphors with various amounts of K2WO4 flux calcined at 1400 °C for 6 h in air.
diffusion of yttrium oxide and tungsten oxide into each other. Moreover, due to the dissolution-recrystallization process, the grain exhibited enhanced nucleation and growth because the Na2WO4 flux could remain as an ionic liquid flux at the synthesis temperature of 1400 °C. The TG results in Fig. 5(a) suggest that there was no evaporative weight loss of Na2WO4. The melting endothermic peak of the K2WO4 flux appeared at 910 °C, as plotted in Fig. 5(b). The liquid phase of the K2WO4 flux facilitated the formation of uniform and nonagglomerated particles with a smooth surface, as shown in the SEM images (Fig. 4). As the temperature approached the synthesis temperature (1400 °C), the K2WO4 flux experienced fast evaporation or decomposition, as confirmed by the weight loss beyond 1250 °C in Fig. 5(b). Subsequently, the growth of Eu0.5Y9.5W2O21 particles proceeded mainly via the solid-state reaction; therefore, the size of the particles produced with the K2WO4 flux remained at 0.5–1 µm, much smaller than those produced with the Na2WO4 flux (~9 µm). 3.4. Optical properties The excitation spectra of the Eu0.5Y9.5W2O21 phosphors with and without flux monitored at 627 nm are shown in Fig. 6. As can be seen, the excitation spectra of all Eu0.5Y9.5W2O21 phosphors with Na2WO4 or K2WO4 flux were similar to those without any flux. The excitation spectra consisted of a broad band (250–350 nm) and several sharp peaks (350–550 nm). The broad band from 250 to 350 nm can be assigned to the charge transfer band (CTB) from the excited 2p orbitals of oxygen to the empty orbitals of the central tungsten (O2--W6+) located in the WO6 groups [22–24]. The presence of this CTB indicates that energy transferred from WO66- to Eu3+ ions in the host. The sharp peaks from 350 to 550 nm can be attributed to the intra-4f forbidden transitions of Eu3+ [25]. The dominant excitation peak lines at 392 and 465 nm are attributed to the 7F0–5L6 and 7F0–5D2 transitions, respectively. More specifically, the dominant peak at 392 nm is more intense than the peak at 465 nm, indicating that the Eu0.5Y9.5W2O21 phosphors are very suitable for near-UV LEDs (360–410 nm). Please note that magnification of the spectra at 350–400 nm is provided in the inset of Fig. 6 for better comparison of the intensity. The increase in intensity in the region of 350–400 nm was greater than that at shorter
Fig. 5. TG/DTA curves of raw material powders with (a) Na2WO4 and (b) K2WO4 fluxes prepared using vibrating ball-milling process for 15 min.
around 800 °C [20,21]. In this study, the temperature of the reaction was lowered to 700 °C because the fluxes were added at the exothermic peaks of the reactions observed at 700 and 720 °C in the samples with Na2WO4 and K2WO4 fluxes, respectively. Moreover, the melting of the Na2WO4 flux (the endothermic peak at 690 °C) can promote the 4
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Fig. 8. Dependence of integrated PL emission intensity on content of Na2WO4 or K2WO4 flux for phosphors.
Fig. 6. PL excitation spectra of Eu0.5Y9.5W2O21 phosphors with various fluxes calcined at 1400 °C for 6 h in air (λem=627 nm). Inset shows excitation spectra in range of 300– 400 nm for better intensity comparison.
the Na2WO4 flux content in the samples was increased to 30–50 wt%, the intensity of the emission increased by 170–190% compared to that of the sample with no flux added. It is well known that increases in the crystallinity and size of a phosphor usually result in a higher emission brightness due to the lower intrinsic reflection coefficient associated with larger particles [7,27–29]. In this case, the results agree with the SEM data, indicating that the average particle size grew to 9 µm, making the morphology smoother and more regular. Additionally, in order to understand the relationship between crystallinity and PL intensity, the full width at half maximum (FWHM) values of XRD data at the main diffraction peak (322) were examined. The results are shown in Table 1. The FWHM values decreased from 0.198° to 0.179° when the addition of Na2WO4 flux was increased to 50 wt% and the intensity of the emission improved by more than 170% compared to that of the sample with no flux added, which suggests a larger particle size and better crystallization with Na2WO4 flux. For the sample with K2WO4 flux, the emission intensity increased with increasing flux content. The results agree with the XRD data, with the FWHM values decreasing from 0.205° to 0.177° (Table 1) when the addition of K2WO4 flux was increased to 30 wt%; the intensity of the emission improved by 130% compared to that of the sample with no flux added. A 150% increase in intensity was obtained for the sample with 50 wt% K2WO4 flux. This was due to the existence of the Y6WO12 secondary phase in the sample with the addition of K2WO4 flux exceeding 30 wt%. In order to compare the different emission spectra of Eu0.5Y5.5WO12 and Eu0.5Y9.5W2O21 phosphors, the emission spectra of the Eu0.5Y9.5W2O21 phosphors with 30 and 50 wt% K2WO4 fluxes are shown in the inset of Fig. 7. In contrast to the results of the sample with 30 wt% K2WO4 flux, it was observed that for the sample with 50 wt%, the dominant emissions at 612 and 627 nm decreased obviously, and an emission at 603 nm appeared. According to the XRD data, a single phase, Y10W2O21, existed in the sample with 30 wt% K2WO4 flux; however, for the sample with 50 wt% K2WO4 flux, both the major Y10W2O21 phase and a minor Y6WO12 secondary phase were found. Therefore, these results suggest that the emission at 603 nm can be attributed to the Eu0.5Y5.5WO12 secondary phase. Based on the above, the K2WO4 flux enhanced the phosphor particle size and luminescence
wavelengths (CTB, < 350 nm). This might be attributed to the excessive flux inducing the formation of new defects, which would hinder the charge transfer from the host to the activator. However, the excitation of Eu3+ in the near-UV region was only slightly affected due to the shielding of the 4f configuration by the 5s25p6 electrons [26]. The emission spectra of Eu0.5Y9.5W2O21 phosphors with various fluxes under an excitation wavelength of 392 nm are shown in Fig. 7. It can be seen that the emission spectra of all samples were dominated by the emission from Eu3+, and that 5D0–7F2 forced the electric-dipole transitions, for which the highest intensities were at 627 and 612 nm. Other weak spectra were the 5D0–7F1 magnetic dipole transitions at 585 and 594 nm. The electronic dipole 5D0–7F2 transitions of the Eu0.5Y9.5W2O21 phosphors were much stronger than the magnetic 5 D0–7F1 transitions because the Eu3+ ions were located at noninversion symmetry sites. According to a previous study [21], Y3+ ions hold both seven- and eight-fold coordination in the Y10W2O21 structure. Eu3+ ions might randomly occupy these two kinds of crystallographic sites and emit two dominant emission spectra at 612 and 627 nm, respectively, because the luminescence properties of Eu3+ ions are sensitive to the local lattice symmetry. The strong 5D0–7F2 transition suggests that these two crystallographic sites had no inversion symmetry. Fig. 8 shows the integral emission relative intensities (550–650 nm) of the Eu0.5Y9.5W2O21 phosphors with different fluxes as a function of the flux content under an excitation of 392 nm. When
Table 1 FWHM values of main diffraction peak (322) for phosphors with different flux content levels. FWHM Fig. 7. PL emission spectra of Eu0.5Y9.5W2O21 phosphors with various fluxes calcined at 1400 °C for 6 h in air (λex=392 nm). Inset shows emission spectra of Eu0.5Y9.5W2O21 phosphors with 30 and 50 wt% K2WO4 flux calcined at 1400 °C for 6 h in air (λex=392 nm).
Na2WO4 flux sample K2WO4 flux sample
5
Flux content (wt%) 0
10
20
30
40
50
0.194 0.205
0.198 0.194
0.190 0.192
0.182 0.177
0.177 –
0.179 –
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Fig. 9. Temperature dependence of integrated PL emission intensity for Eu0.5Y9.5W2O21 phosphor with different fluxes.
properties. It is necessary to consider the thermal stability of phosphors because the temperature of LED chips increases during operation. The temperature-dependent luminescence intensities of the Eu0.5Y9.5W2O21 phosphors without and with 30 wt% Na2WO4 or K2WO4 flux are shown in Fig. 9. Following a previous study [30], the thermal quenching temperature (T50) is defined as the temperature at which the emission intensity is 50% of the original level. The results show that the PL intensity of all samples decreased with the working temperature, and that T50 occurred at about 120 ± 10 °C. The inferior thermal stability might stem from the electron-phonon interaction at high temperature. The excited state of the Eu3+ ions overcame the activation energy by absorbing the energy from heat, and this was released through a crossover from the 5D0 excite state to the 7Fj ground state [31]. 4. Conclusion Eu0.5Y9.5W2O21 phosphors were synthesized via a solid-state reaction with Na2WO4 and K2WO4 fluxes at 1400 °C for 6 h in air. The results show that the addition of 30 wt% Na2WO4 or K2WO4 fluxes not only enhanced the crystallinity, particle size, and morphology, but also modified the luminescence properties of Eu0.5Y9.5W2O21 phosphors. TG/DTA analysis results show that the temperature of the reaction was lowered to 700 °C with the addition of either 30 wt% Na2WO4 or K2WO4 fluxes to the sample. The Na2WO4 flux assisted the growth of phosphor particles to 9 µm and increased the PL emission intensity by 190%, as compared to that of a phosphor without any flux. Moreover, the phosphors with the K2WO4 flux showed a homogenous particle size of about 1 µm, and their PL emission intensity was 150% higher compared to that of a phosphor without any flux. The thermal quenching property measurements show that the T50 values of the samples with and without flux were both about 120 ± 10 °C. The results of the luminescence properties indicate that the red emission of the Eu0.5Y9.5W2O21 phosphor with the addition of Na2WO4 or K2WO4 flux can be effectively excited by near-UV LED chips (392 nm) and blue LED chips (465 nm). Therefore, the Eu0.5Y9.5W2O21 phosphor might potentially be used as a red phosphor for white LEDs. Acknowledgements The authors gratefully appreciate the financial support received for this project from the Ministry of Science and Technology, Taiwan (ROC) (grant NSC-97-2221-E-006-011-MY3). The authors would also like to thank the Center for Micro/Nano Science and Technology at National Cheng Kung University for providing technical support. 6
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Tungstate flux-grown Y10W2O21:Eu3+ phosphors with enhanced particle size and luminescence properties ⁎
Tzu-Chin Chiena, Chii-Shyang Hwanga, , Yung-Tang Nienb, Masahiro Yoshimuraa a b
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan Department of Materials Science and Engineering, National Formosa University, Yunlin 63201, Taiwan
A R T I C L E I N F O
A BS T RAC T
Keywords: Yttrium-tungsten oxide Photoluminescence Thermal quenching Tungstate flux Particle size
Eu0.5Y9.5W2O21 strong red emission phosphors were synthesized via a conventional solid-state reaction method with environmentally friendly tungstate flux (M2WO4, M=Li, Na, K). The growth process of the Eu0.5Y9.5W2O21 phosphors was studied using X-ray diffraction, scanning electron microscopy, and thermogravimetry/ differential thermal analysis. The photoluminescence properties, including emission and excitation spectra and thermal stability, were also investigated. The experimental results indicate that the particle size and morphology of the Eu0.5Y9.5W2O21 phosphors depend on the type and content of tungstate flux. The luminescence intensities of the phosphors were effectively enhanced by the tungstate flux. The results also show that the Eu0.5Y9.5W2O21 phosphors are very suitable for use in near-ultraviolet light-emitting diode (LED) chips (392 nm) and blue LED chips (465 nm).
1. Introduction
morphology and size distribution [4–7]. The choice of flux should be based on its ability to melt at the recrystallization temperature and avoid the formation of any secondary phases or luminescence-killing centers. The most common fluxes used for manufacturing phosphors are H3BO3, NH4Cl, NaCl, and alkali/alkaline-earth fluorides. Fluoride fluxes are commonly used in the high-temperature synthesis of siliconnitride- or oxynitride-based phosphors, and clearly promote the luminescence properties and particle size [8–10]. However, concerns remain regarding environmental pollution related to fluoride fluxes, in terms of toxicity, corrosion of kilns, and staff health [11]. Therefore, it is imperative to find a superior flux that can be applied at high temperatures. According to the literature, single-crystal calcium tungstate has been grown from M2WO4 (M=Li, Na, and K) fluxes [12,13]. Recently, single-crystal calcium tungstate phosphors were synthesized via the flux growth method [14]; the phosphor emission intensity was improved due to the high crystal quality. Other tungstate compounds, such as Na2RE(PO4)(WO4) (RE=Y, Tb-Lu) and Bi2WO6, also have higher quality single crystals when Na2WO4 flux is applied [15,16]. Some laser materials of KY(WO4)2 and KGd(WO4)2 crystals were grown via a top-seed solution growth method with K2WO4 flux [17,18]. Moreover, the tungstate flux M2WO4 (M=Li, Na and K) can dissolve easily in water. Based on these earlier reports, it is evident that little attention has been devoted to examining the influence of tungstate flux type on the particle size distribution and luminescence properties of phosphors. To address this, the present work focuses on
Tungstate compounds have been widely investigated as potential hosts for Eu3+-activated phosphors due to their high efficiency, narrow line-width emission peak centered at 615 nm, and broad excitation band in the near-UV region [1]. Previously, we studied an yttriumtungsten oxide, namely Y10W2O21, as a phosphor host candidate due to its high melting temperature [2]. The (EuxY1-x)10W2O21 phosphor was synthesized via a solid-state reaction and calcined at 1400 °C for 6 h in air. It exhibited two strong excitation peaks, one being near-UV (392 nm) and the other blue light (465 nm). The two dominant emission peaks at 612 and 627 nm were caused by the substitutions of multi-coordinate sites of Y3+(coordination numbers=7, 8) by Eu3+. Additionally, this phosphor had good thermal stability and quantum efficiency. However, the (Y1-xEux)10W2O21 particles formed agglomerations, roughly dozens of micrometers in size, which affected the photoluminescence (PL) properties of the phosphors. A fluorescent material for commercial phosphor-converted light-emitting diodes (pcLEDs) requires a high luminescence yield and a narrow size distribution (generally 3–8 µm) [3]. It is thus important to control the size distribution of (Y1-xEux)10W2O21 phosphors via a high-temperature solid-state reaction. The use of flux in synthesis reactions is one of the simplest and most cost-efficient methods for increasing the crystallinity and chemical homogeneity of powder particles, as well as enhancing the
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Corresponding author. E-mail address: [email protected] (C.-S. Hwang).
http://dx.doi.org/10.1016/j.ceramint.2017.04.118 Received 6 March 2017; Received in revised form 19 April 2017; Accepted 19 April 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Chien, T.-C., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.04.118
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improving the crystallinity, particle size distribution, and luminescence properties of Eu0.5Y9.5W2O21 phosphors by investigating the effects of adding M2WO4 (M=Li, Na and K) fluxes with different melting temperatures.
2. Experimental procedure Eu0.5Y9.5W2O21 phosphor powders were synthesized via a solidstate reaction. Y2O3 (Gredmann, 99.99%), WO3 (Alfa Aesar, 99.99%), Eu2O3 (Acros, 99.99%), and tungstate M2WO4 (M=Li, Na and K) flux (Alfa Aesar, 99%) powders were used as raw materials. These were weighed in a stoichiometric ratio [flux/(Eu0.5Y9.5W2O21+flux)=10– 50 wt%] and mixed in a polyethylene jar with Y2O3-stablized ZrO2 balls using high-energy vibrating ball milling for 15 min. After mixing, the blended powders were calcined at 1400 °C for 6 h in air to form phosphors. The resultant phosphors were then ultrasonically washed three times with deionized (DI) water to remove residual flux, and dried in an oven at 100 °C. The X-ray diffraction (XRD) patterns of the samples were obtained with a Rigaku Dmax diffractometer using Cu Kα radiation (λ=0.15406 nm) with a source power of 30 kV and a current of 20 mA. The morphology of the phosphors was inspected with a scanning electron microscope (SEM, JEOL JSM 7001F). Thermogravimetric/differential thermal analysis (TG/DTA) curves were obtained with a NETZSCH STA 449 F3 instrument at a heating rate of 5 °C/min in air. The excitation and emission spectra of the phosphors were measured at room temperature using a Hitachi F-7000 fluorescence spectrophotometer using a 150 W Xe lamp as the excitation source. The thermal stability of the phosphors was measured using the heating apparatus in the F-7000 spectrophotometer.
Fig. 1. XRD patterns of Eu0.5Y9.5W2O21 powders (a) without flux and with tungstate fluxes (b) Li2WO4, (c) Na2WO4, and (d) K2WO4 calcined at 1400 °C for 6 h in air.
3.2. Powder morphology The SEM images in Figs. 3 and 4 show that the Na2WO4 and K2WO4 fluxes, respectively, accelerated the growth of the Eu0.5Y9.5W2O21 phosphor particles. The powders without flux (Figs. 3(a) and 4(a)) consisted of small particles with irregular shapes and had a strong tendency to form agglomerates. Although the particle size distribution primarily ranged from 200 to 500 nm, some agglomerates measured some dozens of micrometers. It is well known that micrometer-sized agglomerates have numerous grain boundaries and defects on the surface of phosphors; as such, more light scattering is induced and the luminescence properties are reduced. In contrast, the samples with various amounts of Na2WO4 flux exhibited clear particle growth (Fig. 3(b)–(f)). The images show a sphere-like morphology with no agglomeration and a homogenous size distribution. Moreover, the grain boundaries were eliminated because of the dissolution-recrystallization process; specifically, the Na2WO4 flux can melt, inducing the formation of a liquid phase at a lower temperature (~ 698 °C). The liquid phase promoted the reaction of raw materials and helped merge small agglomerated particles into large ones. In this manner, the average particle size increased from ~1 to ~9 µm when the amount of Na2WO4 flux added was increased from 10 to 30 wt%. However, when the amount of Na2WO4 flux exceeded 30 wt %, the morphology and particle size showed no further change. For the samples with various amounts of K2WO4 flux, uniform and nonagglomerated particles with a smooth surface were also observed; however, the particle size did not increase when the amount of K2WO4 flux added was increased. The average particle size of these samples ranged from 0.5 to 1 µm. Moreover, the results indicate that the K2WO4 flux promoted non-agglomerated particles due to its relatively high melting point of 920 °C.
3. Results and discussion 3.1. Phase characterization The effects of Li2WO4, Na2WO4, and K2WO4 fluxes (20 wt%) on the crystalline structure of Eu0.5Y9.5W2O21 phosphors calcined at 1400 °C for 6 h are shown in the XRD patterns in Fig. 1. As can be seen, the diffraction peaks of the samples with (c) Na2WO4 and (d) K2WO4 fluxes are in agreement with those of the Y10W2O21 phase (JCPDS card no. 76-1153, orthorhombic phase). In addition, the Y30W8O69 phase (JCPDS card no. 15-0559, hexagonal phase) was also found in the sample with Li2WO4 flux, as shown in Fig. 1(b). It has been reported that the cations of fluxes can diffuse into the crystalline matrix of the host during the high-temperature solid-state reaction process [7,19]. The radius of Li+ ions (r=0.92 pm at coordination number 8) is about 15% smaller than that of Y3+ ions (r=1.019 pm); as such, Li+ ions can easily replace Y3+ ions in the host lattice, leading to the formation of the Y2O3-rich phase. In contrast, the radii of Na+(r=1.18 pm) and K+(r=1.51 pm) ions are more than 15% larger than that of Y3+ ions; as a consequence, these two ions cannot easily substitute Y3+ ions. Based on the above results, the Na2WO4 and K2WO4 fluxes were chosen for the following optimal content study. The XRD patterns of yttrium-tungsten oxide with various amounts of Na2WO4 or K2WO4 fluxes (10–50 wt%) calcined at 1400 °C for 6 h and washed with DI water are shown in Fig. 2. Fig. 2(a) shows that the Na2WO4 flux amount had no perceptible influence on the crystalline structure of the resulting phosphors, which were all well crystallized phases of Eu0.5Y9.5W2O21 powders. Fig. 2(b) shows that no secondary phase existed until the amount of K2WO4 added reached 30 wt%. Some minor peaks of the Y6WO12 secondary phase were also detected when the content of K2WO4 reached 40 wt%, and this increased with increasing addition amount. 2
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Fig. 2. XRD patterns of Eu0.5Y9.5W2O21 powders with various amounts of (a) Na2WO4 and (b) K2WO4 calcined at 1400 °C for 6 h in air.
Fig. 3. SEM images of Eu0.5Y9.5W2O21 phosphors with various amounts of Na2WO4 flux calcined at 1400 °C for 6 h in air.
curves of the two samples in the temperature range of 25–400 °C are the same. The endothermic peak at 100 °C and exothermic peaks at 315 and 330 °C are associated with the evaporation of water and the burning of the organic constituents (possibly debris from the polyethylene jar), respectively. According to previous studies, the reaction of yttrium oxide and tungsten oxide to form yttrium tungstate begins at
3.3. Thermal analysis In order to elucidate the effects of the fluxes (Na2WO4, K2WO4) on the reactive process, TG/DTA analyses were conducted. Fig. 5(a) and (b) show the TG/DTA results of the mixed powders with 30 wt% Na2WO4 and K2WO4 fluxes in the raw materials, respectively. The 3
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Fig. 4. SEM images of Eu0.5Y9.5W2O21 phosphors with various amounts of K2WO4 flux calcined at 1400 °C for 6 h in air.
diffusion of yttrium oxide and tungsten oxide into each other. Moreover, due to the dissolution-recrystallization process, the grain exhibited enhanced nucleation and growth because the Na2WO4 flux could remain as an ionic liquid flux at the synthesis temperature of 1400 °C. The TG results in Fig. 5(a) suggest that there was no evaporative weight loss of Na2WO4. The melting endothermic peak of the K2WO4 flux appeared at 910 °C, as plotted in Fig. 5(b). The liquid phase of the K2WO4 flux facilitated the formation of uniform and nonagglomerated particles with a smooth surface, as shown in the SEM images (Fig. 4). As the temperature approached the synthesis temperature (1400 °C), the K2WO4 flux experienced fast evaporation or decomposition, as confirmed by the weight loss beyond 1250 °C in Fig. 5(b). Subsequently, the growth of Eu0.5Y9.5W2O21 particles proceeded mainly via the solid-state reaction; therefore, the size of the particles produced with the K2WO4 flux remained at 0.5–1 µm, much smaller than those produced with the Na2WO4 flux (~9 µm). 3.4. Optical properties The excitation spectra of the Eu0.5Y9.5W2O21 phosphors with and without flux monitored at 627 nm are shown in Fig. 6. As can be seen, the excitation spectra of all Eu0.5Y9.5W2O21 phosphors with Na2WO4 or K2WO4 flux were similar to those without any flux. The excitation spectra consisted of a broad band (250–350 nm) and several sharp peaks (350–550 nm). The broad band from 250 to 350 nm can be assigned to the charge transfer band (CTB) from the excited 2p orbitals of oxygen to the empty orbitals of the central tungsten (O2--W6+) located in the WO6 groups [22–24]. The presence of this CTB indicates that energy transferred from WO66- to Eu3+ ions in the host. The sharp peaks from 350 to 550 nm can be attributed to the intra-4f forbidden transitions of Eu3+ [25]. The dominant excitation peak lines at 392 and 465 nm are attributed to the 7F0–5L6 and 7F0–5D2 transitions, respectively. More specifically, the dominant peak at 392 nm is more intense than the peak at 465 nm, indicating that the Eu0.5Y9.5W2O21 phosphors are very suitable for near-UV LEDs (360–410 nm). Please note that magnification of the spectra at 350–400 nm is provided in the inset of Fig. 6 for better comparison of the intensity. The increase in intensity in the region of 350–400 nm was greater than that at shorter
Fig. 5. TG/DTA curves of raw material powders with (a) Na2WO4 and (b) K2WO4 fluxes prepared using vibrating ball-milling process for 15 min.
around 800 °C [20,21]. In this study, the temperature of the reaction was lowered to 700 °C because the fluxes were added at the exothermic peaks of the reactions observed at 700 and 720 °C in the samples with Na2WO4 and K2WO4 fluxes, respectively. Moreover, the melting of the Na2WO4 flux (the endothermic peak at 690 °C) can promote the 4
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Fig. 8. Dependence of integrated PL emission intensity on content of Na2WO4 or K2WO4 flux for phosphors.
Fig. 6. PL excitation spectra of Eu0.5Y9.5W2O21 phosphors with various fluxes calcined at 1400 °C for 6 h in air (λem=627 nm). Inset shows excitation spectra in range of 300– 400 nm for better intensity comparison.
the Na2WO4 flux content in the samples was increased to 30–50 wt%, the intensity of the emission increased by 170–190% compared to that of the sample with no flux added. It is well known that increases in the crystallinity and size of a phosphor usually result in a higher emission brightness due to the lower intrinsic reflection coefficient associated with larger particles [7,27–29]. In this case, the results agree with the SEM data, indicating that the average particle size grew to 9 µm, making the morphology smoother and more regular. Additionally, in order to understand the relationship between crystallinity and PL intensity, the full width at half maximum (FWHM) values of XRD data at the main diffraction peak (322) were examined. The results are shown in Table 1. The FWHM values decreased from 0.198° to 0.179° when the addition of Na2WO4 flux was increased to 50 wt% and the intensity of the emission improved by more than 170% compared to that of the sample with no flux added, which suggests a larger particle size and better crystallization with Na2WO4 flux. For the sample with K2WO4 flux, the emission intensity increased with increasing flux content. The results agree with the XRD data, with the FWHM values decreasing from 0.205° to 0.177° (Table 1) when the addition of K2WO4 flux was increased to 30 wt%; the intensity of the emission improved by 130% compared to that of the sample with no flux added. A 150% increase in intensity was obtained for the sample with 50 wt% K2WO4 flux. This was due to the existence of the Y6WO12 secondary phase in the sample with the addition of K2WO4 flux exceeding 30 wt%. In order to compare the different emission spectra of Eu0.5Y5.5WO12 and Eu0.5Y9.5W2O21 phosphors, the emission spectra of the Eu0.5Y9.5W2O21 phosphors with 30 and 50 wt% K2WO4 fluxes are shown in the inset of Fig. 7. In contrast to the results of the sample with 30 wt% K2WO4 flux, it was observed that for the sample with 50 wt%, the dominant emissions at 612 and 627 nm decreased obviously, and an emission at 603 nm appeared. According to the XRD data, a single phase, Y10W2O21, existed in the sample with 30 wt% K2WO4 flux; however, for the sample with 50 wt% K2WO4 flux, both the major Y10W2O21 phase and a minor Y6WO12 secondary phase were found. Therefore, these results suggest that the emission at 603 nm can be attributed to the Eu0.5Y5.5WO12 secondary phase. Based on the above, the K2WO4 flux enhanced the phosphor particle size and luminescence
wavelengths (CTB, < 350 nm). This might be attributed to the excessive flux inducing the formation of new defects, which would hinder the charge transfer from the host to the activator. However, the excitation of Eu3+ in the near-UV region was only slightly affected due to the shielding of the 4f configuration by the 5s25p6 electrons [26]. The emission spectra of Eu0.5Y9.5W2O21 phosphors with various fluxes under an excitation wavelength of 392 nm are shown in Fig. 7. It can be seen that the emission spectra of all samples were dominated by the emission from Eu3+, and that 5D0–7F2 forced the electric-dipole transitions, for which the highest intensities were at 627 and 612 nm. Other weak spectra were the 5D0–7F1 magnetic dipole transitions at 585 and 594 nm. The electronic dipole 5D0–7F2 transitions of the Eu0.5Y9.5W2O21 phosphors were much stronger than the magnetic 5 D0–7F1 transitions because the Eu3+ ions were located at noninversion symmetry sites. According to a previous study [21], Y3+ ions hold both seven- and eight-fold coordination in the Y10W2O21 structure. Eu3+ ions might randomly occupy these two kinds of crystallographic sites and emit two dominant emission spectra at 612 and 627 nm, respectively, because the luminescence properties of Eu3+ ions are sensitive to the local lattice symmetry. The strong 5D0–7F2 transition suggests that these two crystallographic sites had no inversion symmetry. Fig. 8 shows the integral emission relative intensities (550–650 nm) of the Eu0.5Y9.5W2O21 phosphors with different fluxes as a function of the flux content under an excitation of 392 nm. When
Table 1 FWHM values of main diffraction peak (322) for phosphors with different flux content levels. FWHM Fig. 7. PL emission spectra of Eu0.5Y9.5W2O21 phosphors with various fluxes calcined at 1400 °C for 6 h in air (λex=392 nm). Inset shows emission spectra of Eu0.5Y9.5W2O21 phosphors with 30 and 50 wt% K2WO4 flux calcined at 1400 °C for 6 h in air (λex=392 nm).
Na2WO4 flux sample K2WO4 flux sample
5
Flux content (wt%) 0
10
20
30
40
50
0.194 0.205
0.198 0.194
0.190 0.192
0.182 0.177
0.177 –
0.179 –
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Fig. 9. Temperature dependence of integrated PL emission intensity for Eu0.5Y9.5W2O21 phosphor with different fluxes.
properties. It is necessary to consider the thermal stability of phosphors because the temperature of LED chips increases during operation. The temperature-dependent luminescence intensities of the Eu0.5Y9.5W2O21 phosphors without and with 30 wt% Na2WO4 or K2WO4 flux are shown in Fig. 9. Following a previous study [30], the thermal quenching temperature (T50) is defined as the temperature at which the emission intensity is 50% of the original level. The results show that the PL intensity of all samples decreased with the working temperature, and that T50 occurred at about 120 ± 10 °C. The inferior thermal stability might stem from the electron-phonon interaction at high temperature. The excited state of the Eu3+ ions overcame the activation energy by absorbing the energy from heat, and this was released through a crossover from the 5D0 excite state to the 7Fj ground state [31]. 4. Conclusion Eu0.5Y9.5W2O21 phosphors were synthesized via a solid-state reaction with Na2WO4 and K2WO4 fluxes at 1400 °C for 6 h in air. The results show that the addition of 30 wt% Na2WO4 or K2WO4 fluxes not only enhanced the crystallinity, particle size, and morphology, but also modified the luminescence properties of Eu0.5Y9.5W2O21 phosphors. TG/DTA analysis results show that the temperature of the reaction was lowered to 700 °C with the addition of either 30 wt% Na2WO4 or K2WO4 fluxes to the sample. The Na2WO4 flux assisted the growth of phosphor particles to 9 µm and increased the PL emission intensity by 190%, as compared to that of a phosphor without any flux. Moreover, the phosphors with the K2WO4 flux showed a homogenous particle size of about 1 µm, and their PL emission intensity was 150% higher compared to that of a phosphor without any flux. The thermal quenching property measurements show that the T50 values of the samples with and without flux were both about 120 ± 10 °C. The results of the luminescence properties indicate that the red emission of the Eu0.5Y9.5W2O21 phosphor with the addition of Na2WO4 or K2WO4 flux can be effectively excited by near-UV LED chips (392 nm) and blue LED chips (465 nm). Therefore, the Eu0.5Y9.5W2O21 phosphor might potentially be used as a red phosphor for white LEDs. Acknowledgements The authors gratefully appreciate the financial support received for this project from the Ministry of Science and Technology, Taiwan (ROC) (grant NSC-97-2221-E-006-011-MY3). The authors would also like to thank the Center for Micro/Nano Science and Technology at National Cheng Kung University for providing technical support. 6