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Synthesis and luminescence characteristics of K2Bi(PO4)(MO4):Eu3+(M = Mo,W) red-emitting phosphor for white LEDs
Journal of Alloys and Compounds 492 (2010) 452–455
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and luminescence characteristics of K2 Bi(PO4 )(MO4 ): Eu3+ (M = Mo,W) red-emitting phosphor for white LEDs Xianghong He a,b,∗ , Mingyun Guan a,b , Ning Lian a,b , Jianhua Sun a,b , Tongming Shang a,b a b
School of Chemistry and Chemical Engineering, Jiangsu Teachers University of Technology, Changzhou, Jiangsu 213001, PR China Jiangsu Province Key Laboratory of Precious Metal Chemistry and Technology, Changzhou, Jiangsu 213001, PR China
a r t i c l e
i n f o
Article history: Received 15 December 2008 Received in revised form 11 November 2009 Accepted 18 November 2009 Available online 26 November 2009 Keywords: Phosphors Solid state reactions X-ray diffraction Luminescence
a b s t r a c t K2 Bi(PO4 )(MO4 ):Eu3+ (M = Mo,W) red-emitting phosphors were synthesized by solid state reaction and characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and photoluminescence (PL) spectrum. XRD and FT-IR analysis confirmed the formation of K2 Bi(PO4 )(MO4 ):Eu3+ (M = Mo,W) pure phase. Photoluminescence results showed that the series phosphors can be excited efficiently by either UV light range from 200 to 300 nm or near-UV between 375 and 410 nm, and then exhibited bright pure red emission. The concentration quenching occurs at a relatively higher concentration of europium activator, which resulted from the special structure of host lattice and efficient energy transfer within the nearest-neighbor Eu3+ ions. An enhancement of Eu3+ luminescence is observed when W6+ replaces into Mo6+ in this phosphor, which might be attributed to the changing of crystal field environment of Eu3+ resulting from the forming solid solution of host by introducing WO4 . The composition-optimized phosphor, K2 Bi(PO4 )(WO4 ):0.80Eu3+ presents the best luminescence performance, which can be used as a potential candidate for the phosphor-converted white LEDs. © 2009 Elsevier B.V. All rights reserved.
1. Introduction White light-emitting diodes (LEDs), as an emerging solid state lighting (SSL) source, offers benefits in terms of reliability, energysaving, maintenance and positive environment effects and stands a real chance of replacing the traditional light sources such as incandescent and fluorescent lamps [1–4]. Therefore, more and more interest has been focused on the phosphor-converted white LEDs which consists of a blue or near-UV radiation LEDs chip and blue/yellow or blue/green/red phosphors [2–4]. Clearly, the availability of high-quality phosphors is the single most-important requirement for better performance of such white LEDs. Hundreds of conventional phosphors are available for excitation at 254 nm, the dominant emission band of mercury lamps [4–5]. In the phosphor-converted white LEDs, however, the excitation wavelength is much longer, typically in the range of 380–480 nm, where the LEDs chip emission occurs. New high-efficiency phosphors, which can be efficiently excited at these wavelengths, are now being developed [6–8]. Whereas high-efficiency yellow phosphors are readily available (e.g., cerium-doped YAG phosphors [9–10]),
∗ Corresponding author at: Jiangsu Teachers University of Technology, School of Chemistry and Chemical Engineering, No.1801 Zhongwu Av., Changzhou, Jiangsu Province, China. Tel.: +86 519 8699 9823; fax: +86 519 8699 9825. E-mail address: [email protected] (X. He). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.11.138
the efficiency of red-emitting phosphors still lags [1]. Red-emitting phosphors are becoming the bottleneck for advancement of white LEDs innovation. Hence, there has been a widespread and growing interest in the discovery or development of novel families of redemitting phosphors with high absorption in the near-UV to blue region. The aim of this work is to demonstrate a new phosphor for white LEDs. For that purpose, the compounds formed by combination of tetrahedral anions such as MoO4 , WO4 , PO4 , and VO4 , etc., are rather good hosts for phosphors because of their excellent chemical and thermal stability [11–13]. Recently, new series phosphatetungsto-molybdates, K2 Bi(PO4 )(MO4 )(M = Mo,W) were reported [14,15]. It exhibits an original layered structure, in which the [Bi(PO4 )(MO4 )]∞ layers consist of [Bi2 M2 O18 ]∞ chains linked through single PO4 tetrahedra. The K+ cations are interleaved between these layers. Trivalent rare-earth ion (RE3+ , e.g. Y3+ and Eu3+ ) as the dopant, is expected to substitute the Bi3+ sites in K2 Bi(PO4 )(MO4 )(M = Mo,W) host lattice, because RE3+ and Bi3+ have same valence and almost identical ionic radii. Hence, doping excellent red activator Eu3+ into this original layered structure should give rise to a novel efficient red-emitting phosphor. In the present study, Eu3+ -activated K2 Bi(PO4 )(Mo1−y Wy O4 ) redemitting phosphors for white LEDs, were synthesized by a solid state reaction and their photo-luminescent properties were studied. The effects of Eu3+ -doping concentration and WO4 content on luminescence were investigated over wide composition ranges.
X. He et al. / Journal of Alloys and Compounds 492 (2010) 452–455
Fig. 1. FT-IR spectrum of KBiPMo:0.05Eu3+ phosphor.
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Fig. 2. XRD patterns of KBiPMo host, KBiPMo:xEu3+ (x = 0.05, 0.40), and KBiPMo1−y Wy :0.80Eu3+ (y = 0, 0.2, 0.4, 0.6, 0.8, 1.0) phosphors.
2. Experimental Eu3+ -doped dipotassium bismuth phosphate-tungsto-molybdate, K2 Bi1−x (PO4 ) (x = 0–1.0, y = 0–1.0) phosphors (abbreviated as (Mo1−y Wy O4 ):xEu3+ KBiPMo1−y Wy :xEu3+ hereafter) were prepared through a typical solid state reaction in air. The starting materials, K2 CO3 (99.0%), Bi(NO3 )3 ·5H2 O (99.0%), (NH4 )2 HPO4 (99.0%), WO3 (99.9%), MoO3 (99.9%) and Eu2 O3 (99.99%) were thoroughly mixed in the stoichiometric ratio by grinding in an agate mortar and pestle. A small amount of acetone was added during the grindings in order to obtain homogenous mixtures. The samples were fired at 500 ◦ C for 2 h in a muffle furnace, then calcined at 900 ◦ C for another 4 h. Finally, the samples are ground into powder for characterizations. The powder samples were characterized by X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR) and photoluminescence spectrum (PL). The XRD was carried out with a Japan Rigaku D/max-rA rotation anode X-ray diffractometer, using Ni-filtered Cu K␣ radiation. A scan rate of 0.02◦ /s was applied to record the patterns in the 2 range 10–90◦ . The FT-IR spectra were recorded with a PerkinElmer 580B spectrometer, in the spectral range 400–4000 cm−1 (compound dispersed in KBr). The excitation and emission spectra of powders were recorded using fluorescence spectrofluorometer (Varain Cary-Eclipse). All the measurements were performed at room temperature.
3. Results and discussion The body colors of as-obtained phosphor powders are white. The recorded FT-IR spectrum of K2 Bi(PO4 )(MoO4 ):Eu3+ (abbreviated as KBiPMo:Eu3+ hereafter) phosphor is shown in Fig. 1, which is in good agreement with ref. [14]. The intense band at 1051 cm−1 , may be ascribed to the asymmetric stretching vibration in PO4 tetrahedron. The band at 945 cm−1 is most likely caused by the symmetric stretching vibration of PO4 assuming the asymmetric MoO4 tetrahedra. The bands in the range of 737–900 cm−1 belong to the Mo–O stretching frequencies in the tetrahedral MoO4 : symmetric–weak 895 cm−1 , asymmetric– 859 and 816 cm−1 [16]. The 520–600 cm−1 region shows three bands expected for bending vibrations of PO4 tetrahedron: 590, 557 and 521 cm−1 . The above-mentioned characteristic bands confirm the simultaneous presence of the phosphate and molybdate groups. In order to further verify the structure of the as-prepared phosphors, the XRD patterns were examined. The XRD patterns of KBiPMo host, KBiPMo:xEu3+ (x = 0.05, 0.40), and KBiP(Mo1−y Wy ):0.80Eu3+ (y = 0, 0.2, 0.4, 0.6, 0.8, 1.0) phosphors are presented in Fig. 2. All the peaks of these compounds can be indexed to pure KBiPMo phase with orthorhombic system and consistent with that reported by Zatovsky et al. [14]. The dopants of Eu3+ do not induce a significant change of crystalline structure, which is reasonable because the ionic radii of Eu3+ (121 pm, Coordination Number [abbreviated as CN] = 8) and Bi3+
ions (131 pm, CN = 8) are close and comparable. As the W6+ content increases, the peaks show no shift. It was recognized that the almost identical ionic radius of Mo6+ (41 pm) and W6+ (42 pm) resulted in no difference in the lattice parameters of the solid solution. Fig. 3 displays the excitation spectra of pure and Eu3+ -doped KBiPMo phosphors. Under monitoring at 615 nm corresponding to 5 D → 5 F emission of Eu3+ ions, the host shows no excitation. How0 2 ever, the Eu3+ -activated compound exhibits a broad band as well as some line peaks. The broad band between 200 and 300 nm can be attributed to the charge-transfer (CT) transition of Mo–O and Eu–O [17]. The narrow lines beyond 350 nm are ascribed to the typical Eu3+ intra-4f6 transitions, including the peaks with maximum at 363 nm (7 F0 → 5 D4 ), 384 nm (7 F0 → 5 G2–4 ), 395 nm (7 F0 → 5 L6 ), 418 nm (7 F0 → 5 D3 ), 465 nm (7 F0 → 5 D2 ), 525 nm (7 F0 → 5 D1 ), and 535 nm (7 F1 → 5 D1 ), respectively. When the Eu3+ concentration is less than 60 mol% (i.e. x ≤ 0.60), the excitation peaks from Eu3+ intra-4f6 transitions are stronger than CT band. However, in higher Eu3+ concentration region, the sharp lines are comparable to CT band. Among excitation lines, the intensity of the 395, 383 and 465 nm excitation peaks is much stronger than the others, indicating that near-UV and blue LEDs are efficient pumping sources in obtaining Eu3+ emissions. When the Eu3+ concentration changed from 5 mol% to 100 mol%, the excitation spectra were similar except
Fig. 3. Excitation spectra of KBiPMo:xEu3+ with x = (a) 0, (b) 0.05, (c) 0.10, (d) 0.20, (e) 0.40, (f) 0.60, (g) 0.80, (h) 1.0 (monitoring wavelength: em = 615 nm).
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X. He et al. / Journal of Alloys and Compounds 492 (2010) 452–455
Fig. 6. Excitation spectra of KBiPMo1−y Wy :0.80Eu3+ with y = (a) 0, (b) 0.20, (c) 0.40, (d) 0.60, (e) 0.80, (f) 1.0 (em = 615 nm). Fig. 4. Emission spectra of KBiPMo:xEu3+ with x = (a) 0, (b) 0.05, (c) 0.10, (d) 0.20, (e) 0.40, (f) 0.60, (g) 0.80, (h) 1.0 (excitaion wavelength: ex = 395 nm, the inset shows the luminescence photograph of KBiPMo:Eu3+ phosphor under UV irradiation).
that the intensity of excitation peaks based on CT and f–f transitions of Eu3+ reinforced continuously. KBiPMo:Eu3+ phosphor showed an intense red photoluminescence when irradiated with UV radiation, as shown in the inset of Fig. 4. The multiband emissions of the phosphors obtained under excitation at 395 nm are shown in Fig. 4. These emission spectra consisting of lines in the orange and red spectral range exhibit exclusively the characteristic f–f transitions of Eu3+ , namely, 5 D1 → 7 F1 (537 nm), 5 D1 → 7 F2 (556 nm), 5 D0 → 7 F1 (592 nm), 5 D0 → 7 F2 (615 nm), 5 D0 → 7 F3 (650 nm), and 5 D0 → 7 F4 (702 nm), respectively. With increasing of Eu3+ concentration, all of the emission lines are enhanced significantly, reaching a maximum intensity at a concentration of 80 mol%. No distinct diversifications of the emission spectra shapes and positions occurred when the concentration of Eu3+ varied in a wide range. Yet, no emission from the host could be observed because of the strong luminescence of Eu3+ , indicating an efficient energy transfer from the host to Eu3+ in the phosphors. The luminescence intensity of phosphor materials is always dependent on the doping concentration. In the current study, concentration quenching effect was also observed. The dependence of 615 nm red emission intensity of the as-prepared phosphors on Eu3+ concentration (x) is depicted in Fig. 5. The luminescent
intensity increased with increasing x until a maximum intensity is reached at x = 0.80, and then it deceased gradually due to concentration quenching. The critical quenching concentration of Eu3+ (Xc ) is defined as the concentration at which the emission intensity begins to decrease. From Figs. 4 and 5, we can see that typical value of Xc is about 0.80 (i. e. 80 mol%). In comparison with that in other tungsto-molybdate-based luminescent materials such as ␣-Gd2 (MoO4 )3 :Eu3+ (quenching concentration, 35 mol%) [18] and AgLa(WO4 )2 :Eu3+ (quenching concentration, 40 mol%) [19] etc., the concentration quenching of Eu3+ in KBiPMo host is rather high. This can be explained as follows. In KBiPMo host compound, considering the almost identical ionic radii and same valency of Bi3+ and Eu3+ ions, Eu3+ ions prefer to occupy eight-coordinated Bi sites. The Bi–Bi distances between two nearest neighboring Bi3+ ions located at adjacent parallel zig-zig chains, is reported to be 4.05 Å [14]. This distance is quite short, thus, energy transfer between two Eu3+ ions should be very effective. Consequently, concentration quenching for KBiPMo:Eu3+ phosphor is little pronounced. The mechanism of quenching might be due to multipolar–multipolar or exchange interactions [19]. The luminescence enhancement of Eu3+ ions by WO4 was reported by Guo et al. [20]. In order to obtain a compositionoptimized red-emitting phosphor with high efficiency, Mo6+ was substituted with isovalent W6+ to form KBiPMo1−y Wy :xEu3+ series phosphors. The Eu3+ concentration was set at 80 mol%, which shows a maximum emission for PL. The excitation and emission spectra of KBiPMo1−y Wy :0.80Eu3+ are depicted in Figs. 6 and 7, respectively. The excitation and the emission spectra of the samples in this series are similar to Eu3+ -activated KBiPMo phosphors (see Figs. 3 and 4). For all these phosphors, sharp excitation peaks due to the intra-
Fig. 5. Dependence of 615 nm red emission intensity on Eu3+ concentration of phosphors (ex = 395 nm, the line is only a guide to the eyes).
Fig. 7. Emission spectra of KBiPMo1−y Wy :0.80Eu3+ with y = (a) 0, (b) 0.20, (c) 0.40, (d) 0.60, (e) 0.80, (f) 1.0 (ex = 395 nm).
X. He et al. / Journal of Alloys and Compounds 492 (2010) 452–455
4f transitions of Eu3+ along with a board excitation band ranging from 200 to 300 nm were observed. With increasing WO4 content, both of sharp excitation peaks and broad band enhanced. When all MoO4 group was replaced by WO4 , another unexpected excitation broad band between 300 and 355 nm appeared. Such a strong absorption band was beneficial to obtaining of high efficiency for this phosphor. In comparison with Fig. 4, no distinct difference of the emission spectra shapes and positions occurred when Mo6+ was substituted by isovalent W6+ in host. Under radiation of 395 nm UV light, all these phosphors showed a dominant emission peak situated at 615 nm, as shown in Fig. 7. Furthermore, the substitution of Mo6+ with W6+ resulted in an enhancement of luminescence intensity. The reason of luminescence enhancement effect might be due to the change of crystal field environment of Eu3+ resulting from the forming solid solution of host lattice by introducing of WO4 [20,21]. 4. Conclusions In conclusion, Eu3+ -activated dipotassium bismuth phosphatetungsto-molybdate, K2 Bi(PO4 )(MO4 ):Eu3+ (M = Mo,W) series phosphors were prepared by using solid state reactions and their structure and PL properties were investigated. The as-obtained phosphors have single K2 Bi(PO4 )(MoO4 ) phase. The series phosphors can be excited by UV light and near-UV range (370–400 nm), and perform pure red luminescence (615 nm) from the Eu3+ transition of 5 D0 → 7 F2 . The intensities of excitation and emission of K2 Bi(PO4 )(MO4 ):Eu3+ (M = Mo,W) phosphors considerably increased with increasing Eu3+ concentration or W6+ content. The phosphor exhibited a high quenching concentration of Eu3+ ions, which is likely due to the original layered structure of host and efficient energy transfer within the nearest-neighbor Eu3+ ions. After optimizing of the composition, it is observed that K2 Bi(PO4 )(WO4 ):0.80Eu3+ presents the best red luminescence performance in the series. The main excitation peaks of this phosphor match maximum output wavelength of near-UV LED chips, and emits red luminescence peaked at about 615 nm. Therefore, this material may be a promising candidate red-emitting phosphor for white LEDs.
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Acknowledgments The authors thank Dr. Jinping Huang of Shanghai Normal University for assistance with the XRD measurements. This work was financially supported by the Natural Science Research Project of the Jiangsu Higher Education Institutions (08KJD150014), the QingLan Project of the Jiangsu Province (2008), and the Basic Research Fund of Jiangsu Teachers University of Technology (KYY09031). References [1] E.F. d Schubert, J.K. Kim, Science 308 (2005) 1274. [2] T. Jüstel, H. Nikel, C. Ronda, Angew. Chem. Int. Ed. 37 (1998) 3084. [3] J.M. Phillips, M.E. Coltrin, M.y.H. Crawford, A.J. Fischer, M.R. Krames, R.M. Mach, G.O. Mueller, Y. i Ohno, L.E.S. Rohwer, J.A. Simmons, J.Y. Tsao, Laser Photon. Rev. 1 (4) (2007) 307. [4] X. Xu, M. Su, Luminescence and Luminescent Materials, Chemical Industry Press, Beijing, 2004, p. 321 (in Chinese). [5] S. Shionoya, W.M. Yen, et al., Phosphor Handbook, CRC Press, Boca Raton, 1999. [6] H.S. Jang, H. Yang, S.W. Kim, J.Y. Han, S.G. Lee, D.Y. Jeon, Adv. Mater. 20 (2008) 2696. [7] A.A. Setlur, W.J. Heward, Y. Gao, A.M. Srivastava, R.G. Chandran, M.V. Shankar, Chem. Mater. 18 (2006) 3314. [8] H. Zhang, T. Horikawa, H. Hanzawa, A. Hamaguchi, K. Machida, J. Electrochem. Soc. 154 (2) (2007) J59. [9] S. Nakamura, G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Lasers, Springer, Berlin, 1996, p. 216. [10] C. Lu, R. Jagannathan, Appl. Phys. Lett. 80 (19) (2002) 3608. [11] C.C. Wu, K.B. Chen, C.S. Lee, T.M. Chen, B.M. Cheng, Chem. Mater. 19 (2007) 3278. [12] V.F. Kharsika, L.N. Komissarova, A.N. Kirichenko, E.N. Murav’ev, V.P. Orlovskii, A.P. Chernyaev, Inorg. Mater. 37 (2001) 963. [13] S. Neeraj, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 387 (2004) 2. [14] I.V. Zatovsky, K.V. Terebilenko, N.S. Slobodyanik, V.N. Baumer, O.V. Shishkin, J. Solid State Chem. 179 (2006) 3550. [15] I.V. Zatovsky, K.V. Terebilenko, N.S. Slobodyanik, V.N. Baumer, O.V. Shishkin, Acta Cryst. E62 (2006) i193. [16] R.G. Brown, J. Denning, A. Hallett, S.D. Ross, Spectrochim. Acta Sect. A 26 (1970) 963. [17] G. Blasse, A.F. Corsmit, J. Solid State Chem. 6 (1973) 513. [18] X. Zhao, X. Wang, B. Chen, Q. Meng, B. Yan, W. Di, Opt. Mater. 29 (12) (2006) 1680. [19] V. Sivakumar, U.V. Varadaraju, J. Electrochem. Soc. 153 (3) (2005) H54. [20] C. Guo, B. Li, Y. Chen, J. Rare Earths 11 (3) (1993) 183. [21] J. Wang, X. Jing, C. Yan, J. Lin, F. Liao, J. Lumin. 121 (2006) 57.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and luminescence characteristics of K2 Bi(PO4 )(MO4 ): Eu3+ (M = Mo,W) red-emitting phosphor for white LEDs Xianghong He a,b,∗ , Mingyun Guan a,b , Ning Lian a,b , Jianhua Sun a,b , Tongming Shang a,b a b
School of Chemistry and Chemical Engineering, Jiangsu Teachers University of Technology, Changzhou, Jiangsu 213001, PR China Jiangsu Province Key Laboratory of Precious Metal Chemistry and Technology, Changzhou, Jiangsu 213001, PR China
a r t i c l e
i n f o
Article history: Received 15 December 2008 Received in revised form 11 November 2009 Accepted 18 November 2009 Available online 26 November 2009 Keywords: Phosphors Solid state reactions X-ray diffraction Luminescence
a b s t r a c t K2 Bi(PO4 )(MO4 ):Eu3+ (M = Mo,W) red-emitting phosphors were synthesized by solid state reaction and characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and photoluminescence (PL) spectrum. XRD and FT-IR analysis confirmed the formation of K2 Bi(PO4 )(MO4 ):Eu3+ (M = Mo,W) pure phase. Photoluminescence results showed that the series phosphors can be excited efficiently by either UV light range from 200 to 300 nm or near-UV between 375 and 410 nm, and then exhibited bright pure red emission. The concentration quenching occurs at a relatively higher concentration of europium activator, which resulted from the special structure of host lattice and efficient energy transfer within the nearest-neighbor Eu3+ ions. An enhancement of Eu3+ luminescence is observed when W6+ replaces into Mo6+ in this phosphor, which might be attributed to the changing of crystal field environment of Eu3+ resulting from the forming solid solution of host by introducing WO4 . The composition-optimized phosphor, K2 Bi(PO4 )(WO4 ):0.80Eu3+ presents the best luminescence performance, which can be used as a potential candidate for the phosphor-converted white LEDs. © 2009 Elsevier B.V. All rights reserved.
1. Introduction White light-emitting diodes (LEDs), as an emerging solid state lighting (SSL) source, offers benefits in terms of reliability, energysaving, maintenance and positive environment effects and stands a real chance of replacing the traditional light sources such as incandescent and fluorescent lamps [1–4]. Therefore, more and more interest has been focused on the phosphor-converted white LEDs which consists of a blue or near-UV radiation LEDs chip and blue/yellow or blue/green/red phosphors [2–4]. Clearly, the availability of high-quality phosphors is the single most-important requirement for better performance of such white LEDs. Hundreds of conventional phosphors are available for excitation at 254 nm, the dominant emission band of mercury lamps [4–5]. In the phosphor-converted white LEDs, however, the excitation wavelength is much longer, typically in the range of 380–480 nm, where the LEDs chip emission occurs. New high-efficiency phosphors, which can be efficiently excited at these wavelengths, are now being developed [6–8]. Whereas high-efficiency yellow phosphors are readily available (e.g., cerium-doped YAG phosphors [9–10]),
∗ Corresponding author at: Jiangsu Teachers University of Technology, School of Chemistry and Chemical Engineering, No.1801 Zhongwu Av., Changzhou, Jiangsu Province, China. Tel.: +86 519 8699 9823; fax: +86 519 8699 9825. E-mail address: [email protected] (X. He). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.11.138
the efficiency of red-emitting phosphors still lags [1]. Red-emitting phosphors are becoming the bottleneck for advancement of white LEDs innovation. Hence, there has been a widespread and growing interest in the discovery or development of novel families of redemitting phosphors with high absorption in the near-UV to blue region. The aim of this work is to demonstrate a new phosphor for white LEDs. For that purpose, the compounds formed by combination of tetrahedral anions such as MoO4 , WO4 , PO4 , and VO4 , etc., are rather good hosts for phosphors because of their excellent chemical and thermal stability [11–13]. Recently, new series phosphatetungsto-molybdates, K2 Bi(PO4 )(MO4 )(M = Mo,W) were reported [14,15]. It exhibits an original layered structure, in which the [Bi(PO4 )(MO4 )]∞ layers consist of [Bi2 M2 O18 ]∞ chains linked through single PO4 tetrahedra. The K+ cations are interleaved between these layers. Trivalent rare-earth ion (RE3+ , e.g. Y3+ and Eu3+ ) as the dopant, is expected to substitute the Bi3+ sites in K2 Bi(PO4 )(MO4 )(M = Mo,W) host lattice, because RE3+ and Bi3+ have same valence and almost identical ionic radii. Hence, doping excellent red activator Eu3+ into this original layered structure should give rise to a novel efficient red-emitting phosphor. In the present study, Eu3+ -activated K2 Bi(PO4 )(Mo1−y Wy O4 ) redemitting phosphors for white LEDs, were synthesized by a solid state reaction and their photo-luminescent properties were studied. The effects of Eu3+ -doping concentration and WO4 content on luminescence were investigated over wide composition ranges.
X. He et al. / Journal of Alloys and Compounds 492 (2010) 452–455
Fig. 1. FT-IR spectrum of KBiPMo:0.05Eu3+ phosphor.
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Fig. 2. XRD patterns of KBiPMo host, KBiPMo:xEu3+ (x = 0.05, 0.40), and KBiPMo1−y Wy :0.80Eu3+ (y = 0, 0.2, 0.4, 0.6, 0.8, 1.0) phosphors.
2. Experimental Eu3+ -doped dipotassium bismuth phosphate-tungsto-molybdate, K2 Bi1−x (PO4 ) (x = 0–1.0, y = 0–1.0) phosphors (abbreviated as (Mo1−y Wy O4 ):xEu3+ KBiPMo1−y Wy :xEu3+ hereafter) were prepared through a typical solid state reaction in air. The starting materials, K2 CO3 (99.0%), Bi(NO3 )3 ·5H2 O (99.0%), (NH4 )2 HPO4 (99.0%), WO3 (99.9%), MoO3 (99.9%) and Eu2 O3 (99.99%) were thoroughly mixed in the stoichiometric ratio by grinding in an agate mortar and pestle. A small amount of acetone was added during the grindings in order to obtain homogenous mixtures. The samples were fired at 500 ◦ C for 2 h in a muffle furnace, then calcined at 900 ◦ C for another 4 h. Finally, the samples are ground into powder for characterizations. The powder samples were characterized by X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR) and photoluminescence spectrum (PL). The XRD was carried out with a Japan Rigaku D/max-rA rotation anode X-ray diffractometer, using Ni-filtered Cu K␣ radiation. A scan rate of 0.02◦ /s was applied to record the patterns in the 2 range 10–90◦ . The FT-IR spectra were recorded with a PerkinElmer 580B spectrometer, in the spectral range 400–4000 cm−1 (compound dispersed in KBr). The excitation and emission spectra of powders were recorded using fluorescence spectrofluorometer (Varain Cary-Eclipse). All the measurements were performed at room temperature.
3. Results and discussion The body colors of as-obtained phosphor powders are white. The recorded FT-IR spectrum of K2 Bi(PO4 )(MoO4 ):Eu3+ (abbreviated as KBiPMo:Eu3+ hereafter) phosphor is shown in Fig. 1, which is in good agreement with ref. [14]. The intense band at 1051 cm−1 , may be ascribed to the asymmetric stretching vibration in PO4 tetrahedron. The band at 945 cm−1 is most likely caused by the symmetric stretching vibration of PO4 assuming the asymmetric MoO4 tetrahedra. The bands in the range of 737–900 cm−1 belong to the Mo–O stretching frequencies in the tetrahedral MoO4 : symmetric–weak 895 cm−1 , asymmetric– 859 and 816 cm−1 [16]. The 520–600 cm−1 region shows three bands expected for bending vibrations of PO4 tetrahedron: 590, 557 and 521 cm−1 . The above-mentioned characteristic bands confirm the simultaneous presence of the phosphate and molybdate groups. In order to further verify the structure of the as-prepared phosphors, the XRD patterns were examined. The XRD patterns of KBiPMo host, KBiPMo:xEu3+ (x = 0.05, 0.40), and KBiP(Mo1−y Wy ):0.80Eu3+ (y = 0, 0.2, 0.4, 0.6, 0.8, 1.0) phosphors are presented in Fig. 2. All the peaks of these compounds can be indexed to pure KBiPMo phase with orthorhombic system and consistent with that reported by Zatovsky et al. [14]. The dopants of Eu3+ do not induce a significant change of crystalline structure, which is reasonable because the ionic radii of Eu3+ (121 pm, Coordination Number [abbreviated as CN] = 8) and Bi3+
ions (131 pm, CN = 8) are close and comparable. As the W6+ content increases, the peaks show no shift. It was recognized that the almost identical ionic radius of Mo6+ (41 pm) and W6+ (42 pm) resulted in no difference in the lattice parameters of the solid solution. Fig. 3 displays the excitation spectra of pure and Eu3+ -doped KBiPMo phosphors. Under monitoring at 615 nm corresponding to 5 D → 5 F emission of Eu3+ ions, the host shows no excitation. How0 2 ever, the Eu3+ -activated compound exhibits a broad band as well as some line peaks. The broad band between 200 and 300 nm can be attributed to the charge-transfer (CT) transition of Mo–O and Eu–O [17]. The narrow lines beyond 350 nm are ascribed to the typical Eu3+ intra-4f6 transitions, including the peaks with maximum at 363 nm (7 F0 → 5 D4 ), 384 nm (7 F0 → 5 G2–4 ), 395 nm (7 F0 → 5 L6 ), 418 nm (7 F0 → 5 D3 ), 465 nm (7 F0 → 5 D2 ), 525 nm (7 F0 → 5 D1 ), and 535 nm (7 F1 → 5 D1 ), respectively. When the Eu3+ concentration is less than 60 mol% (i.e. x ≤ 0.60), the excitation peaks from Eu3+ intra-4f6 transitions are stronger than CT band. However, in higher Eu3+ concentration region, the sharp lines are comparable to CT band. Among excitation lines, the intensity of the 395, 383 and 465 nm excitation peaks is much stronger than the others, indicating that near-UV and blue LEDs are efficient pumping sources in obtaining Eu3+ emissions. When the Eu3+ concentration changed from 5 mol% to 100 mol%, the excitation spectra were similar except
Fig. 3. Excitation spectra of KBiPMo:xEu3+ with x = (a) 0, (b) 0.05, (c) 0.10, (d) 0.20, (e) 0.40, (f) 0.60, (g) 0.80, (h) 1.0 (monitoring wavelength: em = 615 nm).
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Fig. 6. Excitation spectra of KBiPMo1−y Wy :0.80Eu3+ with y = (a) 0, (b) 0.20, (c) 0.40, (d) 0.60, (e) 0.80, (f) 1.0 (em = 615 nm). Fig. 4. Emission spectra of KBiPMo:xEu3+ with x = (a) 0, (b) 0.05, (c) 0.10, (d) 0.20, (e) 0.40, (f) 0.60, (g) 0.80, (h) 1.0 (excitaion wavelength: ex = 395 nm, the inset shows the luminescence photograph of KBiPMo:Eu3+ phosphor under UV irradiation).
that the intensity of excitation peaks based on CT and f–f transitions of Eu3+ reinforced continuously. KBiPMo:Eu3+ phosphor showed an intense red photoluminescence when irradiated with UV radiation, as shown in the inset of Fig. 4. The multiband emissions of the phosphors obtained under excitation at 395 nm are shown in Fig. 4. These emission spectra consisting of lines in the orange and red spectral range exhibit exclusively the characteristic f–f transitions of Eu3+ , namely, 5 D1 → 7 F1 (537 nm), 5 D1 → 7 F2 (556 nm), 5 D0 → 7 F1 (592 nm), 5 D0 → 7 F2 (615 nm), 5 D0 → 7 F3 (650 nm), and 5 D0 → 7 F4 (702 nm), respectively. With increasing of Eu3+ concentration, all of the emission lines are enhanced significantly, reaching a maximum intensity at a concentration of 80 mol%. No distinct diversifications of the emission spectra shapes and positions occurred when the concentration of Eu3+ varied in a wide range. Yet, no emission from the host could be observed because of the strong luminescence of Eu3+ , indicating an efficient energy transfer from the host to Eu3+ in the phosphors. The luminescence intensity of phosphor materials is always dependent on the doping concentration. In the current study, concentration quenching effect was also observed. The dependence of 615 nm red emission intensity of the as-prepared phosphors on Eu3+ concentration (x) is depicted in Fig. 5. The luminescent
intensity increased with increasing x until a maximum intensity is reached at x = 0.80, and then it deceased gradually due to concentration quenching. The critical quenching concentration of Eu3+ (Xc ) is defined as the concentration at which the emission intensity begins to decrease. From Figs. 4 and 5, we can see that typical value of Xc is about 0.80 (i. e. 80 mol%). In comparison with that in other tungsto-molybdate-based luminescent materials such as ␣-Gd2 (MoO4 )3 :Eu3+ (quenching concentration, 35 mol%) [18] and AgLa(WO4 )2 :Eu3+ (quenching concentration, 40 mol%) [19] etc., the concentration quenching of Eu3+ in KBiPMo host is rather high. This can be explained as follows. In KBiPMo host compound, considering the almost identical ionic radii and same valency of Bi3+ and Eu3+ ions, Eu3+ ions prefer to occupy eight-coordinated Bi sites. The Bi–Bi distances between two nearest neighboring Bi3+ ions located at adjacent parallel zig-zig chains, is reported to be 4.05 Å [14]. This distance is quite short, thus, energy transfer between two Eu3+ ions should be very effective. Consequently, concentration quenching for KBiPMo:Eu3+ phosphor is little pronounced. The mechanism of quenching might be due to multipolar–multipolar or exchange interactions [19]. The luminescence enhancement of Eu3+ ions by WO4 was reported by Guo et al. [20]. In order to obtain a compositionoptimized red-emitting phosphor with high efficiency, Mo6+ was substituted with isovalent W6+ to form KBiPMo1−y Wy :xEu3+ series phosphors. The Eu3+ concentration was set at 80 mol%, which shows a maximum emission for PL. The excitation and emission spectra of KBiPMo1−y Wy :0.80Eu3+ are depicted in Figs. 6 and 7, respectively. The excitation and the emission spectra of the samples in this series are similar to Eu3+ -activated KBiPMo phosphors (see Figs. 3 and 4). For all these phosphors, sharp excitation peaks due to the intra-
Fig. 5. Dependence of 615 nm red emission intensity on Eu3+ concentration of phosphors (ex = 395 nm, the line is only a guide to the eyes).
Fig. 7. Emission spectra of KBiPMo1−y Wy :0.80Eu3+ with y = (a) 0, (b) 0.20, (c) 0.40, (d) 0.60, (e) 0.80, (f) 1.0 (ex = 395 nm).
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4f transitions of Eu3+ along with a board excitation band ranging from 200 to 300 nm were observed. With increasing WO4 content, both of sharp excitation peaks and broad band enhanced. When all MoO4 group was replaced by WO4 , another unexpected excitation broad band between 300 and 355 nm appeared. Such a strong absorption band was beneficial to obtaining of high efficiency for this phosphor. In comparison with Fig. 4, no distinct difference of the emission spectra shapes and positions occurred when Mo6+ was substituted by isovalent W6+ in host. Under radiation of 395 nm UV light, all these phosphors showed a dominant emission peak situated at 615 nm, as shown in Fig. 7. Furthermore, the substitution of Mo6+ with W6+ resulted in an enhancement of luminescence intensity. The reason of luminescence enhancement effect might be due to the change of crystal field environment of Eu3+ resulting from the forming solid solution of host lattice by introducing of WO4 [20,21]. 4. Conclusions In conclusion, Eu3+ -activated dipotassium bismuth phosphatetungsto-molybdate, K2 Bi(PO4 )(MO4 ):Eu3+ (M = Mo,W) series phosphors were prepared by using solid state reactions and their structure and PL properties were investigated. The as-obtained phosphors have single K2 Bi(PO4 )(MoO4 ) phase. The series phosphors can be excited by UV light and near-UV range (370–400 nm), and perform pure red luminescence (615 nm) from the Eu3+ transition of 5 D0 → 7 F2 . The intensities of excitation and emission of K2 Bi(PO4 )(MO4 ):Eu3+ (M = Mo,W) phosphors considerably increased with increasing Eu3+ concentration or W6+ content. The phosphor exhibited a high quenching concentration of Eu3+ ions, which is likely due to the original layered structure of host and efficient energy transfer within the nearest-neighbor Eu3+ ions. After optimizing of the composition, it is observed that K2 Bi(PO4 )(WO4 ):0.80Eu3+ presents the best red luminescence performance in the series. The main excitation peaks of this phosphor match maximum output wavelength of near-UV LED chips, and emits red luminescence peaked at about 615 nm. Therefore, this material may be a promising candidate red-emitting phosphor for white LEDs.
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Acknowledgments The authors thank Dr. Jinping Huang of Shanghai Normal University for assistance with the XRD measurements. This work was financially supported by the Natural Science Research Project of the Jiangsu Higher Education Institutions (08KJD150014), the QingLan Project of the Jiangsu Province (2008), and the Basic Research Fund of Jiangsu Teachers University of Technology (KYY09031). References [1] E.F. d Schubert, J.K. Kim, Science 308 (2005) 1274. [2] T. Jüstel, H. Nikel, C. Ronda, Angew. Chem. Int. Ed. 37 (1998) 3084. [3] J.M. Phillips, M.E. Coltrin, M.y.H. Crawford, A.J. Fischer, M.R. Krames, R.M. Mach, G.O. Mueller, Y. i Ohno, L.E.S. Rohwer, J.A. Simmons, J.Y. Tsao, Laser Photon. Rev. 1 (4) (2007) 307. [4] X. Xu, M. Su, Luminescence and Luminescent Materials, Chemical Industry Press, Beijing, 2004, p. 321 (in Chinese). [5] S. Shionoya, W.M. Yen, et al., Phosphor Handbook, CRC Press, Boca Raton, 1999. [6] H.S. Jang, H. Yang, S.W. Kim, J.Y. Han, S.G. Lee, D.Y. Jeon, Adv. Mater. 20 (2008) 2696. [7] A.A. Setlur, W.J. Heward, Y. Gao, A.M. Srivastava, R.G. Chandran, M.V. Shankar, Chem. Mater. 18 (2006) 3314. [8] H. Zhang, T. Horikawa, H. Hanzawa, A. Hamaguchi, K. Machida, J. Electrochem. Soc. 154 (2) (2007) J59. [9] S. Nakamura, G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Lasers, Springer, Berlin, 1996, p. 216. [10] C. Lu, R. Jagannathan, Appl. Phys. Lett. 80 (19) (2002) 3608. [11] C.C. Wu, K.B. Chen, C.S. Lee, T.M. Chen, B.M. Cheng, Chem. Mater. 19 (2007) 3278. [12] V.F. Kharsika, L.N. Komissarova, A.N. Kirichenko, E.N. Murav’ev, V.P. Orlovskii, A.P. Chernyaev, Inorg. Mater. 37 (2001) 963. [13] S. Neeraj, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 387 (2004) 2. [14] I.V. Zatovsky, K.V. Terebilenko, N.S. Slobodyanik, V.N. Baumer, O.V. Shishkin, J. Solid State Chem. 179 (2006) 3550. [15] I.V. Zatovsky, K.V. Terebilenko, N.S. Slobodyanik, V.N. Baumer, O.V. Shishkin, Acta Cryst. E62 (2006) i193. [16] R.G. Brown, J. Denning, A. Hallett, S.D. Ross, Spectrochim. Acta Sect. A 26 (1970) 963. [17] G. Blasse, A.F. Corsmit, J. Solid State Chem. 6 (1973) 513. [18] X. Zhao, X. Wang, B. Chen, Q. Meng, B. Yan, W. Di, Opt. Mater. 29 (12) (2006) 1680. [19] V. Sivakumar, U.V. Varadaraju, J. Electrochem. Soc. 153 (3) (2005) H54. [20] C. Guo, B. Li, Y. Chen, J. Rare Earths 11 (3) (1993) 183. [21] J. Wang, X. Jing, C. Yan, J. Lin, F. Liao, J. Lumin. 121 (2006) 57.