Luminescent properties of red phosphors K2Ba(MoO4)2:Eu3+ for white light emitting diodes

JOURNAL OF RARE EARTHS, Vol. 30, No. 10, Oct. 2012, P. 990

Luminescent properties of red phosphors K2Ba(MoO4)2:Eu3+ for white light emitting diodes LI Zhaomei (ᴢ‫ܚ‬ṙ), ZHONG Yingjuan (䩳䖢࿳), GAO Shaokang (催㒡ᒋ) (College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350108, China) Received 13 February 2012; revised 25 June 2012

Abstract: K2Ba(MoO4)2:Eu3+ phosphors were synthesized by solid-state reaction. The emission and excitation spectra of K2Ba(MoO4)2:Eu3+ phosphors exhibited that the phosphors could be effectively excited by near ultraviolet (394 nm) and blue (465 nm) light, and emitted red light at 616 nm. The influence of Eu3+concentration, sintering temperature and charge compensators (K+, Na+ or Li+) on the emission intensity were investigated. The results indicated that concentration quenching of Eu3+ was not observed within 30 mol.% Eu3+, 600 ºC was a suitable sintering temperature for preparation of K2Ba(MoO4)2:Eu3+ phosphors, and K+ ions gave the best improvement to enhance the emission intensity. The CIE chromaticity coordinates of K2Ba(MoO4)2:0.05Eu3+ phosphor were calculated to be (0.68, 0.32), and color purity was 97.4%. Keywords: K2Ba(MoO4)2:Eu3+ ; red phosphor; luminescence; charge compensations; rare earths

White light emitting diodes (W-LEDs) have the potential of replacing traditional incandescent and fluorescent lamps, due to their advantages such as stability, energy saving, compactness, high efficiency, longer lifetime and environmental friendly[1–3]. Therefore, they are called the next generation of solid-state light[3,4]. At present, the commercially used red phosphor for near ultraviolet light InGaN-based LEDs is Y2O2S:Eu3+ [5,6]. Unfortunately, chemical stability and lifetime of Y2O2S:Eu3+ phosphor are inadequate and seriously affect the efficiency and lifetime of LED[7]. Hence, it is imperative to develop new alternative red phosphors with high luminance and satisfactory chemical stability that can be effectively excited by the ultraviolet (UV) or near ultraviolet light (NUV). In recent years, more and more attention is paid to the molybdates doped with Eu3+, because of its broad and intense charge-transfer (C–T) absorption bands in the NUV and effective f-f transition of Eu3+ [8]. Some Eu3+ activated molybdates have been developed as red phosphors for white LEDs[3,4,8–12]. These phosphors have excellent luminescence efficiency, color purity, and stability; especially they can be well excited by ultraviolet or bluelight, which are nicely matched with the widely applied NUV or blue LED chips[9–11]. The charge compensators can compensate the charge center and improve the luminescent properties. Liu et al.[12] prepared CaMoO4:Eu3+ red phosphors, and studied the influence of different charge compensators such as Li+, Na+, K+ on the luminescent intensity. Li et al.[13] fabricated a series of Eu3+, N+ co-activated molybdate phosphors AMoO4 (A=Ca, Sr, N=Li, Na, K) and the results showed that Na+ was a good

charge compensate reagent for Eu3+ activated molybdate phosphor. In the present work, K2Ba(MoO4)2:Eu3+ phosphors which can be efficiently excited by 394 and 465 nm were prepared by a solid state method, and their photoluminescence properties were studied. In addition, the effects of sintering temperature, Eu3+ concentration and charge compensation of K+, Na+, Li+ ions on the emission intensity were investigated. Finally, CIE chromaticity coordinates of the phosphor were calculated.

1 Experimental All powder samples were synthesized through solid state reaction. K2CO3 (A.R.), BaCO3 (A.R.) and (NH4)6Mo7O24·4H2O (A.R.) were used as starting materials. Stoichiometric amount of raw materials were thoroughly mixed and ground in an agate mortar. Then, the homogeneous mixtures were filled into an alumina crucible and sintered at 500–900 ºC for 5 h, respectively. A series of K2Ba(MoO4)2:Eu3+ phosphors were prepared with different Eu3+ concentrations. In some cases, appropriate amount of Li2CO3, Na2CO3 or K2CO3 were added as the charge compensators. X-ray powder diffraction (XRD) measurements were carried out on the Rigaku-miniflex II diffraction with Cu K radiation (=0.15406 nm) at 35 kV and 25 mA. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were preformed by using an FL/FS920 fluorescence luminous spectrometer with a 450 W xenon lamp as the excitation source. The morphology of the as-prepared phosphors was characterized by a scanning electron microscope

Foundation item: Project supported by Natural Science Foundation of Fujian Province (2011J033) Corresponding author: GAO Shaokang (E-mail: [email protected]; Tel.: +86-591-22866130) DOI: 10.1016/S1002-0721(12)60166-5

LI Zhaomei et al., Luminescent properties of red phosphors K2Ba(MoO4)2:Eu3+ for white light emitting diodes

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(FSEM, Tecnai G2 F20 S-Twin 200KV). All measurements were performed at room temperature.

2 Results and discussion 2.1 X-ray diffraction analysis The X-ray diffraction pattern of K2Ba(MoO4)2:0.05Eu3+ phosphor without charge compensators sintered at 600 ºC is shown in Fig. 1. The results indicate the diffraction peak positions and the relative intensities of the as-prepared sample match well with the standard powder diffraction file (JCPDS No. 29-0985), and Eu3+-doping in a small content has little influence on the lattice structure of the host. K2Ba(MoO4)2 belongs to the hexagonal crystal structure with space group R-3m and its lattice parameter is a=b=0.60061 nm, c= 2.13075 nm. 2.2 Luminescence of the samples As is well known, the sintering temperature has critical effect on the PL luminescence of phosphors. A series of K2Ba(MoO4)2:0.05Eu3+ powders without charge compensators was synthesized at different temperatures for 5 h, respectively. Fig. 2 shows the calcinations temperature dependence of PL spectra of K2Ba(MoO4)2:0.05Eu3+ phosphors excited by NUV radiation at 394 nm. All emission spectra consist of sharp lines with wavelengths ranging from 550 to 750 nm, which are associated with the 5D07FJ (J=1, 2, 3, and 4)

Fig. 1 XRD pattern of K2Ba(MoO4)2:0.05Eu3+ without charge compensators sintered at 600 ºC

Fig. 2 Emission spectra (ex=394 nm) of K2Ba(MoO4)2:0.05Eu3+ phosphors prepared at different calcination temperatures for 5 h

transitions from the excited levels of Eu3+ to the ground state[5]. It can be observed from Fig. 2 that the intensities of samples increased with the increase of sintering temperature and reached a maximum at 600 ºC, which may be due to the improvement of the crystallinity of the phosphors. Then the emission intensity decreased significantly as the temperature continuously increased from 700 to 900 ºC, which may be due to the temperature quenching[3,14]. This indicates that the calcinations temperature has an important influence on the crystallinity, measuring growth of the microcrystallines and emission intensity of K2Ba(MoO4)2:0.05Eu3+ phosphors. In order to investigate the effect of sintering temperature on the PL luminescence of phosphors thoroughly, the morphology of the as-prepared phosphors were measured. Fig. 3 shows the SEM images of K2Ba(MoO4)2:0.05Eu3+ powders without charge compensators sintered at different calcination temperatures for 5 h. The samples synthesized at 600 ºC exhibited maximum emission intensity show integrity and uniform of crystalline grain and the mean size of the particles with irregular shape is about 2 m, which is suitable to fabricate the solid-state lighting devices. The samples calcinated at 500 ºC appears a small amount of tiny particles, which might because the raw materials did not react completely. The powders of samples sintered at 700 ºC become inhomogeneous. It is well-known that phosphors with regular morphology and fine size can improve the packing density and make the luminescence intensity uniform on the device. Considering the emission spectra excited by 394 nm and

Fig. 3 SEM images of K2Ba(MoO4)2:0.05Eu3+ prepared at different calcination temperatures for 5 h (a) 500 ºC; (b) 600 ºC; (c) 700 ºC

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SEM images of K2Ba(MoO4)2:0.05Eu3+ phosphors sintered at different temperatures, it can be concluded that 600 ºC is the optimum sintering temperature for preparing the K2Ba(MoO4)2:Eu3+ phosphors. The excitation spectrum of K2Ba(MoO4)2:0.05Eu3+ phosphor without charge compensation sintered at 600 ºC is shown in Fig. 4. It can be seen that the excitation spectrum monitored under 616 nm composes of a wide band centered at 300 nm and several intense sharp lines. The broad band from 250 to 350 nm is attributed to the electric charge transfer (C–T) transition of O2–Mo6+ and O2–Eu3+ [15]. Several intense and sharp lines in the range of 350–550 nm originate from the intra-configurational 4f4f transitions of Eu3+ ions in the host. The peaks centered at 394 nm and 465 nm are corresponding to the 7F05L6, and 7F05D2 transitions, respectively[16,17]. The absorption of 7F05L6 at 394 nm and 7 F05D2 at 465 nm are two of the strongest absorptions among these sharp lines, which are coupled well with the characteristic emission from NUV and blue GaN-based LED chips and their intensities are much stronger than that of C–T band. Other peaks at 363, 383, 418 and 535 nm, corresponding to 7F05D4, 7F05L7, 7F05D3 and 7F05D1 transitions of Eu3+, are weak. Fig. 5 shows the emission spectra of K2Ba(MoO4)2:0.05Eu3+ phosphor without charge compensation under the excitation of 394 and 465 nm, respectively. As shown in Fig. 5, there are no observed differences for the emission band shape and position under the excitation of 394 or 465 nm except for

JOURNAL OF RARE EARTHS, Vol. 30, No. 10, Oct. 2012

luminescent intensity. Under the excitation of UV light (~394 nm) and blue-light irradiation (~465 nm), the emission spectra is composed of groups of sharp lines, which belong to the characteristic of emission of Eu3+ ion. The main emission is at 616 nm, originating from the electric dipole transitions (5D07F2) of Eu3+, which reveals that Eu3+ ions occupy the lattice sites without inversion symmetry. Other f-f transitions of Eu3+ ion, such as 590, 652 and 701 nm, which are associated with the 5D07FJ (J=1, 3, 4) transitions from the excited levels of Eu3+ to the ground state, are relatively weak[18]. This means that there is advantageous for obtaining a phosphor with good CIE chromaticity coordinates. The luminescent intensity excited by 394 nm is 1.1 times higher than that by 464 nm. 2.3 Effect of Eu3+ doping concentration on luminescent intensity The doping concentration is one of the important factors influencing the performance of luminescent materials[19]. Therefore, it is necessary to understand an optimum doping concentration. The emission features of Eu3+ on shape and peak position have no obvious changes as the doped Eu3+ concentration varies. The emission intensity (em=616 nm) of K2Ba(MoO4)2:xEu3+ samples with different Eu3+ contents (molar fraction x=0.01–0.30) excited by 394 nm is shown in Fig. 6. The emission intensity increases with the increase of Eu3+ doping concentration and concentration quenching did not occur even at a higher doping concentration 30 mol.% in the experiments. 2.4 Effect of ionic charge compensation on emission intensity In order to investigate the effect of charge compensators, Li+, Na+ and K+ ions on red emission of K2Ba(MoO4)2:0.05Eu3+, 5 mol.% Li2CO3, Na2CO3 and K2CO3 were respectively added as the charge compensators. The XRD patterns of K2Ba(MoO4)2:Eu3+ phosphors with different charge compensators (Li+, Na+ and K+) are shown in Fig. 7. The XRD patterns of the samples show that all of the phosphors are of single phase and consistent with JCPDS No. 29-0985

Fig. 4 Excitation spectra of K2Ba(MoO4)2:0.05Eu3+ phosphors (em= 616 nm)

Fig. 5 Emission spectra of K2Ba(MoO4)2:0.05Eu3+ phosphors (ex= 394, 465 nm)

Fig. 6 Emission intensity (em=616 nm) of K2Ba(MoO4)2:xEu3+ samples with different Eu3+ contents (x=0.01–0.30) excited by 394 nm

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they enhance the PL intensity. But the other charge compensators like Li+ would induce much distortion of crystal lattice of the phosphor because of much difference of the radius between Li+ and Ba2+ ion, the luminescence intensity could be evidently weakened using Li+ as charge compensator. 2.5 CIE chromaticity coordinates of K2Ba(MoO4)2:0.05Eu3+ phosphor

Fig. 7 XRD patterns of K2Ba(MoO4)2:0.05Eu3+ with different ionic charge compensations (Li+, Na+ or K+)

[K2Ba(MoO4)2]. This means that the charge compensation of Li+, Na+ and K+ did not change the crystal structure of the phosphor and no impurity phase was observed. On the assumption that when Ba2+ ions are partially substituted by Eu3+ ions and become the luminescent center of K2Ba(MoO4)2:Eu3+ phosphor. The charge balancing is necessarily required because of the unbalanced charge between Eu3+ and Ba2+ ions. To maintain the electrical neutrality, 5 mol.% alkali ions are added as charge compensator. The charge compensations mechanisms may be that two Ba2+ ions are replaced by one Eu3+ ions and one monovalent cation, 2Ba2+Eu3+ + M+, where M+ is a monovalent cation ion[20,21]. Fig. 8 shows the influence of different charge compensators (Li+, Na+ or K+, respectively) on the emission intensity of K2Ba(MoO4)2:0.05Eu3+. It can be seen that the emission intensity of the phosphor significantly increases in the case of charge compensation by monovalent ion substitutions. The luminescent intensity of phosphors with charge compensator K+ ions is much stronger than that of phosphors with other ions, which is 1.6 times stronger than that of phosphors without charge compensations. The reason could be that the radius of K+ ion is 0.138 nm, which is similar to that of Ba2+ ion (0.135 nm), while the radius of Li+ ion is 0.092 nm and the radius of Na+ ion is 0.118 nm[12]. When K+ ions are incorporated into the host, they can form solid solution with the host. Therefore, K+ ion is expected to cause less distortion in the crystal structure of the phosphors and thus

Fig. 8 Influence of different charge compensations (Li+, Na+ or K+) on emission spectra of K2Ba(MoO4)2:0.05Eu3+ (ex=394 nm)

The CIE chromaticity coordinate of the K2Ba(MoO4)2: 0.05Eu3+ phosphor are calculated to be (x=0.68, y=0.32)[22,23] according to the emission spectra (ex=394 nm), which are closer to the International Association of Lighting NTSC trichromatic coordinates specified red coordinate (x=0.67, y=0.33) than that of Y2O2S:Eu2+ (x=0.63, y=0.35)[24], and color purity is 97.4%.

3 Conclusions A series of Eu3+-activated K2Ba(MoO4)2 phosphors were synthesized with solid state method. The phosphors could be efficiently excited by 394 and 465 nm, which were well coupled with the emission of NUV LED and blue LED chips. The emission intensity increased with the increase of Eu3+ doping concentration and concentration quenching did not occur in K2Ba(MoO4)2:Eu3+ within Eu3+ doping concentration of the experiment. The charge compensation mechanisms might be modeled: 2Ba2+Eu3++M+, where M+ was a monovalent cation ionlike Li+, Na+ or K+ acting as a charge compensator. The luminescent intensity of phosphors with charge compensator Na+ and K+ ions was enhanced and K+ ions should be the best charge compensation. All the results indicated that K2Ba(MoO4)2:Eu3+ phosphors are promising red candidates for red emitting phosphors of white LED.

References: [1] Nakamura S, Fasol G. The Blue Laser Diode. Berlin: Springer, 1996. [2] Yu Q M, Liu Y F, Wu S, Lu X D, Huang X Y, Li X X. Luminescent properties of Ca2SiO4:Eu3+ red phosphor for trichromatic white light emitting diodes. J. Rare Earths, 2008, 26(6): 783. [3] Guo C F, Chen T, Luan L, Zhang W, Huang D X. Luminescent properties of R2(MoO4)3:Eu3+ (R=La, Y, Gd) phosphors prepared by sol-gel process. J. Phys. Chem. Solids, 2008, 69(8): 1905. [4] Yang Y L, Li X M, Feng W L, Li W L, Tao C Y. Co-precipitation synthesis and photoluminescence properties of (Ca1xy,Lny)MoO4:xEu3+ (Ln=Y, Gd) red phosphors. J. Alloys Compd., 2010, 505: 239. [5] Neeraj S, Kijima A K, Cheetham A K. Novel red phosphors for solid-state lighting: the system NaM(WO4)2–x(MoO4)x:Eu3+ (M=Gd, Y, Bi). Chem. Phys. Lett., 2004, 387: 23. [6] Chou T W, Mylswamy S, Liu R S, Chuang S Z. Eu substitution and particle size control of Y2O2S for the excitation by UV light emitting diodes. Solid State Commun., 2005, 136:

994 206. [7] Guo C F, Huang D X, Su Q. Methods to improve the fluorescence intensity of CaS:Eu2+ red-emitting phosphor for white LED. Mater. Sci. Eng. B, 2006, 130: 189. [8] Wang X X, Xian Y L, Wang G, Shi J X, Su Q, Gong M L. Luminescence investigation of Eu3+-Sm3+ co-doped Gd2–x–yEuxSmy(MoO4)3 phosphors as red phosphors for UV InGaN-based light-emitting diode. Opt. Mater., 2007, 30: 521. [9] Zhao X X, Wang X J, Chen B J, Meng Q Y, Di W H, Ren G Z, Yang Y M. Novel Eu3+-dope red-emitting phosphor Gd2Mo3O9 for white-light-emitting-diodes (WLEDs) application. J. Alloys Compd., 2007, 433: 352. [10] Wang X X, Wang J, Shi J X, Su Q, Gong M L. Intense redemitting phosphors for LED solid-state lighting. Mater. Res. Bull., 2007, 42: 1669. [11] Li Q X, Huang J P, Chen D H. A novel red-emitting phosphors K2Ba(MoO4)2:Eu3+, Sm3+ and improvement of luminescent properties for light emitting diodes. J. Alloys Compd., 2011, 509: 1007. [12] Liu J, Lian H Z, Shi C S. Improved optical photoluminescence by charge compensation in the phosphor system CaMoO4:Eu3+. Opt. Mater., 2007, 29(12): 1591. [13] Li X, Yang Z P, Guan L, Guo Q L, Li P L, Huai S F, Liu C. Luminescent properties of alkali metal ion and trivalent europium ion co-activated alkaline earth molybdate phosphors (in Chin.). J. Synthetic Crystals, 2007, 36(5): 1192. [14] Guo C F, Z Wei, Luan L, Chen T, Cheng H, Huang D X. A promising red-emitting phosphor for white light emitting diodes prepared by sol-gel method. Sens. Acutators B, 2008, 133: 33.

JOURNAL OF RARE EARTHS, Vol. 30, No. 10, Oct. 2012 [15] Zeng Q H, He P, Liang H B, Gong M L, Su Q. Luminescence of Eu3+-activated tetra-molybdate red phosphors and their application in near-UV InGaN-based LEDs. Mater. Chem. Phys., 2009, 118: 78. [16] Wang Z L, Liang H B, Gong M L, Su Q. Luminescence investigation of Eu3+ activated double molybdates red phosphor with scheelite structure. J. Alloys Compd., 2007, 432: 309. [17] Haque Md. M, Lee H I, Kim D K. Luminescent properties of Eu3+-activated molybdate-based novel red-emitting phosphors for LEDs. J. Alloys Compd., 2009, 481(1-2): 794. [18] Lu S Z, Zhang J S. Study on UV excitation properties of Eu3+doped rare-earth phosphates. J. Lumin., 2007, 122-123: 501. [19] Cheng L H, Zhong H Y, Sun J S, Zhang X Q, Peng Y, Yu T, Zhao X X. Flux and concentration effect on Eu3+ doped Gd2(MoO4)3 phosphor. J. Rare Earths, 2008, 26(2): 213. [20] Liu H L, HaoY Y, Wang H, Zhao J F, Huang P, Xu B S.Luminescent properties of R+ doped Sr2SiO4:Eu3+ (R+=Li+, Na+ and K+) red-emitting phosphors for white LEDs. J. Lumin., 2011, 131: 2425. [21] Choi Sungho, Moon Young-Min, Jung Ha-Kyun. Enhanced luminescence by charge compensation in red-emitting Eu3+activated Ca3Sr3(VO4)4. Mater. Res. Bull., 2010, 45: 118. [22] Xu S R, Su M Z. Luminescence and Luminescent Materials (in Chin.). Beijing: Chemical Industrial Press, 2004. 655. [23] Ferhi M, Horchani-Naifer K, Ferid M. Hydrothermal synthesis and photoluminescence of the monophosphate LaPO4:Eu (5%). J. Lumin., 2008, 128: 1781. [24] Wang Z L, Liang H B, Gong M L, Su Q. Novel red phosphor of Bi3+, Sm3+ co-activated NaEu(MoO4)2. Opt. Mater., 2007, 29: 896.