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Preparation of LaPO4:Ce,Tb phosphor with different morphologies and their fluorescence properties
Powder Technology 192 (2009) 27–32
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Powder Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p o w t e c
Preparation of LaPO4:Ce,Tb phosphor with different morphologies and their fluorescence properties Xuefang Hu, Shirun Yan ⁎, Lin Ma, Guojiang Wan, Jianguo Hu Department of Chemistry and Fudan-keheng Research Center for Luminescent Materials, Fudan University, Shanghai 200433, China
a r t i c l e
i n f o
Article history: Received 25 July 2008 Received in revised form 20 September 2008 Accepted 14 November 2008 Available online 27 November 2008 Keywords: Morphology control LaPO4,Ce,Tb Ripening condition Flux Firing temperature
a b s t r a c t LaPO4:Ce,Tb phosphor was prepared by firing the LaPO4:Ce,Tb precipitate derived from co-precipitation of aqueous solution of LaCeTb mixed rare earth nitrate with ammonium dibasic phosphate. Effects of ripening condition of precipitate, flux addition and firing temperature on the morphology and photoluminescence properties of LaPO4:Ce,Tb phosphor were investigated. It was found that ripening of LaPO4,Ce,Tb precipitate with mother liquor at pH = 1 and 130 °C for 24 h produced strip phosphor with width of 100 nm and length of 1 μm. While ripening at either 85 or 130 °C after pH adjustment with ammonia generated spherical particles of about 100 nm in size. Types of flux employed significantly influenced the morphology of the phosphor. When H3BO3 was used as flux spherical particles of ca. 300 nm were formed. In contrast, when Li2CO3 or Li3PO4 was employed as flux either separately or a component of double flux, uniform spherical or near spherical particles of ca. 5 μm were generated. Firing temperature of 1100 and 1200 °C in the presence of double flux led to the formation of smooth spherical particles with diameter of ca. 5 μm, and the phosphor prepared with double flux showed higher brightness than the commercial phosphor. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Phosphors are widely used in displays and lighting devices. Morphology of phosphors (shape and size of the powder particles) is one of the key parameters of their industrial application [1–3]. Phosphor particles should be non-aggregated spherical shape and their particle size distribution should be in a range between 1 and 8 μm depending on the application [2]. Spherical phosphor particle can optimize the optical and geometrical structure of phosphor layer. The particle size of a phosphor affects the amount of phosphor particles needed to produce an optimal coating for a particular application [1]. LaPO4:Ce,Tb (abbreviated as LAP hereinafter) is one of green phosphors of tricolor fluorescent lamps. In addition to its high quantum efficiency and stability at high temperature [4], advantage of LAP over MgAl11O19:Ce,Tb (abbreviated as MACT) is that specific gravity of LAP (5.07 g cm− 3) [5] matches well with Y2O3:Eu phosphor (5.20 g cm− 3) as compared with MACT whose specific gravity is 4.2 g cm− 3. As a result, the phosphor paint for lamp coating using LAP as a green-emitting component is more homogeneous than that made of CMAT and hence color uniformity between two ends of tubular fluorescent lamps such as T5,T8 and cold cathode fluorescent lamps is better than that using CMAT green phosphor. Extensive efforts have been paid to prepare LAP with controlled morphology and properties. Arul Dhas and Patil [6] synthesized crystalline LaPO4 by flash combustion of aqueous solutions. Fujishiro et al. [7] prepared monodispersed spherical and spindle-like ⁎ Corresponding author. Tel./fax: +86 21 65643987. E-mail address: [email protected] (S. Yan). 0032-5910/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.11.006
monazite type LaPO4 particles of 20 nm in size by hydrothermal reaction of lanthanum(III)–ethylenediamine tetraacetate complex. The monodispersed spherical particle was sintered more densely as compared with the powder prepared using the solid reaction. Heike et al. [8] prepared LAP particles, ranging in size from ca. 10 to 50 nm, and fibers with width of 5–20 nm by wet chemical synthesis with changing pH of solution. Lenggoro et al. [9] and Kang et al. [1] developed spray pyrolysis method for spherical LAP synthesis. Duault et al. [2] studied the effect of Li3PO4, Li2CO3, Na2CO3, K2CO3 flux on the morphology of LAP, quasispherical and parallelepiped-like shape LAP were prepared. Erdei et al. [10] synthesized LAP green phosphor particles by hydrolyzed colloid reaction technique at lower temperature and atmospheric pressure. To improve the chemical homogeneity of LAP, Maestro and Huguenin [11] proposed using mixed phosphate of LaCeTb instead of separated or mixed rare earth oxides as precursor to prepare LAP phosphor. In this paper, LAP phosphor was prepared by firing the LaPO4,Ce,Tb precipitate derived from co-precipitation of aqueous solution of mixed rare earth nitrate with ammonium dibasic phosphate in a reducing atmosphere. Effects of ripening condition of precipitate, flux addition and firing temperature on the morphology and photoluminescence properties of LAP phosphor were studied. 2. Experimental 2.1. Preparation of LaPO4:Ce,Tb precipitate and phosphor Appropriate amount of spectrographic grade mixed oxide (La0.51 Ce0.32Tb0.17)xOy from Shanghai Yuelong new materials Co. LTD was
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dissolved in 1:1 HNO3 aqueous solution. To ensure the complete dissolution of CeOx which contains some tetravalent species, small amount of hydrogen peroxide was added dropwisely to the solution. The resultant clear solution with rare earth concentration of 1.6 mol L− 1 was referred to as solution A. The required amount of (NH4)2HPO4 was dissolved in deionized water and thus obtained solution was referred to as solution B. To ensure the complete precipitation of the rare earth ions 5% excess of (NH4)2HPO4 was employed. The solutions A and B were added simultaneously and slowly to a beaker placed in a waterbath setting at 85 °C under stirring and the white precipitate was produced. After completion of precipitation, final pH of slurry was about 1.0. The precipitate slurry was divided into five portions and each was ripened under different conditions: (1) the slurry containing the precipitate and mother liquor was transferred to a Teflon-lined autoclave. Filling coefficient of the autoclave was equal to 80%. The autoclave was placed into an oven preheated at 130 °C and maintained for 24 h. After ripening the autoclave was cooled quickly with flowing water to room temperature. (2) pH value of the slurry was adjusted
with ammonia to 2.5 and 4.5, respectively. Subsequently, it was hydrothermally ripened at 130 °C for 24 h. (3) pH value of the slurry was adjusted with ammonia to 4.5 and 6.5 respectively and the slurry was ripened at 85 °C for 1 h. After ripening and filtration, the precipitate was washed with deionized water to neutrality and then dried at 120 °C for 8 h. Thus obtained sample was referred to as LaPO4:Ce,Tb precipitate. The LAP phosphor was prepared by firing intimate mixture of the LaPO4:Ce,Tb precipitate with flux in H2–N2 (5% of H2) reducing atmosphere at a designed temperature. The sample was allowed to cool in the reducing atmosphere to room temperature and ground for measurements. 2.2. Characterizations The X-ray diffraction (XRD) patterns were collected on a Bruker AXS D8 Advance X-ray diffractometer with Cu–Kα radiation (λ = 0.15418 nm) operated at 40 mA and 40 kV. The morphologies of
Fig. 1. SEM micrographs of LAP phosphors prepared by firing at 1200 °C for 2 h the LaPO4:Ce,Tb precipitate ripened under different conditions: (a) pH = 1, 130 °C, 24 h; (b) pH = 2.5, 130 °C, 24 h; (c) pH = 4.5, 130 °C, 24 h; (d) pH = 4.5, 85 °C, 1 h; (e) pH = 6.5, 85 °C, 1 h.
X. Hu et al. / Powder Technology 192 (2009) 27–32
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phosphors were monitored using scanning electron microscopy (SEM, Philips XL 30). Thermogravimetry and differential thermal analysis of the dried precipitate (TG-DTA) were performed on a Perkin-Elmer DTA-7 apparatus with a heating rate of 10 K/min. The photoluminescence excitation (PLE) and emission (PL) spectra were recorded using a Varian Cary Eclipse fluorescence spectrophotometer at room temperature. The CIE chromaticity coordinate of the phosphors under excitation at 254 nm was obtained on a Chuanghui CMS-2000 spectrophotocolorimeter. The relative brightness of the phosphor was measured on a JY-III phosphor brightness meter using the Chinese national standard trichromatic green phosphor 200202 (GB/T 146332002) as reference. 3. Results and discussion 3.1. Effect of ripening condition on the morphology of LAP phosphor Shown in Fig. 1 is the SEM micrographs of the LAP phosphors prepared by firing the LaPO4:Ce,Tb precipitate ripened under different conditions at 1200 °C for 2 h. It is interesting to note that LAP phosphor prepared using the precipitate hydrothermally ripened at 130 °C for 24 h in mother liquor showed strip form with width of 100 nm and length of 1 μm, similar to that reported by Heike et al. [8]. While the LAP phosphor prepared with the precipitate ripened at 130 °C and 85 °C after pH adjustment with ammonia to either 2.5, 4.5 or 6.5 all gave near spherical shape with different particle sizes. The results indicate that the strong acidic environment of the mother liquor favors growth rates along one crystallographic plane of monoclinic phase leading to the formation of strip precipitate, while the addition of ammonia inhibits the preferential growth of the precipitate resulting in the formation of spherical particles.
Fig. 3. DTA profiles of the dried LAP precipitate with different morphologies during heating from 25 to 1200 °C at a rate of 10 °C/min. (1) strip; (2) spherical.
3.2. Effect of flux on the morphology of LAP phosphor Fluxes are often used in preparation of phosphors by solid-state reaction. The role of fluxes in the growth of phosphors is well known in the literature. They are used to act as a medium to incorporate the activators, reduce the firing temperature, and improve the crystallinity of respective phosphor [12]. Boric acid and lithium salt are reported to be the suitable fluxes for LAP preparation [2,13]. Boric acid could promote the formation of trivalent rare earth ions [13], while the lithium salt (Li3PO4 or Li2CO3) leads to the formation of regular and faceted particles of phosphors of 3–4 μm [2]. In this paper, H3BO3,
Fig. 2. SEM micrographs of LAP phosphors prepared by firing at 1200 °C for 2 h the mixture of precipitate ripened at pH = 2.5, 130 °C for 24 h with different fluxes: (a) without flux; (b) 1 wt.% H3BO3 as flux; (c) 1 wt.% Li2CO3 as flux; (d) 1 wt.% H3BO3 + 1 wt.% Li2CO3 as flux.
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3.3. Effect of firing temperature on the morphology of LAP phosphor
Fig. 4. XRD patterns of the LAP phosphors fired in reducing atmosphere at different temperatures.
Li2CO3 mono-flux and H3BO3 + Li2CO3, H3BO3 + Li3PO4 concurrent double flux on the morphology and fluorescent properties of LAP were investigated. Shown in Fig. 2 is the SEM micrograph of the LAP phosphors prepared using different fluxes. It was found that the phosphor prepared without use of flux was spherical particle with mean size ca. 100 nm. The phosphor prepared using H3BO3 as flux was also spherical with particle size about 200 nm. The particle size of phosphor was increased to about 4.5–5 μm when using Li2CO3 flux or Li2CO3 and H3BO3 concurrent double flux. Moreover, the particle was near spherical when using the double flux, while it was egg-like shape when using Li2CO3 flux alone.
DTA experiment was performed to monitor the phase transformation during heating the dried LaPO4:Ce,Tb so that the suitable temperature for phosphor firing could be determined. The DTA profiles of the dried precipitates with strip and spherical shape are shown in Fig. 3. A strong exothermic peak between 700 and 1000 °C corresponding to phase transformation to monazite LaPO4:Ce,Tb was observed for both strip and spherical precipitates, indicating that the crystallization and formation of monazite LaPO4:Ce,Tb can be finished below 1000 °C. Moreover, a small endothermic peak around 270 °C which may be assigned to the evaporation of crystallized water of the phosphate appeared for the spherical precipitate. A comparison of the onset and peak temperature of the phase transformation of the two samples revealed that crystallization of the spherical particle could be realized at a temperature ca. 20 °C lower than that for the strip one. Based on the DTA results, the mixture of LaPO4,Ce,Tb precipitate with H3BO3 and Li2CO3 double flux was fired at temperatures between 900 and 1200 °C, the XRD patterns of the fresh LaPO4:Ce,Tb and those fired at designed temperatures for 2 h are shown in Fig. 4. The pattern of the fresh sample matches with 04-0635 by JCPDS fingerprint corresponding to LaPO4 structure. The patterns of the fired samples are in good agreement with the PDF 32-0493. There was no obvious difference in crystallinity between the sample fired at 900 and 1200 °C, indicating that in the presence of double flux crystallization of LAP could be completed at 900 °C within 2 h. The SEM micrographs of the LAP phosphors fired at temperature between 900 and 1200 °C for 2 h using H3BO3 and Li2CO3 double flux are shown in Fig. 5. It was found that the LaPO4:Ce,Tb fired at 900 °C took irregular shape. With increasing firing temperature to 1000 °C, the particle was quasi-spherical with coarse surface. With increasing firing temperature to 1100 °C, the spherical particle surface became smooth. With
Fig. 5. SEM micrographs of the LAP phosphors prepared by firing the intimate mixture of H3BO3 and Li2CO3 double flux with the precipitate at temperature of (a) 900 °C; (b) 1000 °C; (c) 1100 °C; (d) 1200 °C.
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further increasing firing temperature to 1200 °C the size of smooth spherical particles was increased to 4–5 μm. The evolution of particle morphologies with firing temperature may be interpreted by a liquidphase-sintering-like mechanism as proposed by Duault et al. [2]. With increasing firing temperature, the wettability of the LAP powder by the liquid, the viscosity of the liquid and the diffusivity of the cations and anions increased. As a result, mass transfer process and grain growth kinetics were enhanced. The smooth spherical particles with enlarged sizes were formed. 3.4. Effect of flux on photoluminescence properties of LAP phosphor The photoluminescence excitation (PLE) and emission (PL) properties of LAP phosphors prepared from the same LAP precipitate using different fluxes were investigated. The PLE and PL spectra are shown in Fig. 6. It was found that using either H3BO3 alone or H3BO3 + Li2CO3 double flux could improve the PLE and PL intensity of the LAP phosphor. Moreover, the double flux enhanced the excitation intensity around 300 nm. Further investigation into the effect of double flux on the relative brightness of LAP phosphor, as listed in Table 1, revealed that the LAP phosphor prepared using H3BO3 and suitable amount of Li3PO4 double flux showed the highest relative brightness among the phosphors investigated. The XRD patterns (not presented) of the LAP phosphors using H3BO3 + Li3PO4 and H3BO3 + Li2CO3 fluxes had no discernable difference. The increased brightness of the phosphor prepared using H3BO3 flux and H3BO3 + lithium salt double flux may be interpreted by the improved morphology of the phosphor. By
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Table 1 CIE chromaticity and relative brightness of the LAP phosphors prepared by using different fluxes No. 1 2 3 4 5 6 7 8
Fluxes
CIE color coordinates
Relative brightness
(wt.%)
x
y
(%)
None 1 H3BO3 + 1 Li2CO3 1 H3BO3 + 1 Li3PO4 2 H3BO3 1 H3BO3 + 0.5 Li3PO4 1 H3BO3 + 1.5 Li3PO4 1 H3BO3 + 2 Li3PO4 1 H3BO3 + 3 Li3PO4
0.3458 0.3457 0.3459 0.3448 0.3463 0.3458 0.3464 0.3466
0.5783 0.5780 0.5781 0.5781 0.5787 0.5781 0.5786 0.5782
95.7 99.2 102.4 98.8 104.3 99.1 98.1 97.9
comparing the SEM micrographs of the phosphors shown in Fig. 2 it is found that the phosphors prepared in the presence of flux had the increased particle size and improved surface smoothness, especially those using double flux. It is generally accepted that an increase in the size of a phosphor usually results in higher emission brightness because of the lower intrinsic reflection coefficient associated with the larger particles [1]. As for the more pronounced promotional effect of Li3PO4 + H3BO3 double flux than that of H3BO3 + Li2CO3, the author suppose it may be related with the fact that Li3PO4 has the same anions as (LaCeTb)PO4 and therefore Li3PO4 may compensate for the loss of PO4 group during firing in reducing atmosphere, maintaining the stoichiometric intactness of the lattice, although it was indiscernible by XRD. 4. Conclusions LaPO4:Ce,Tb green phosphor was prepared by firing the LaPO4:Ce, Tb precipitate derived from co-precipitation of aqueous solution of LaCeTb mixed rare earth nitrate with ammonium dibasic phosphate. The morphology of the phosphor could be controlled by ripening condition of the precipitate, types of flux and firing temperature. The morphology of the phosphor influenced the photoluminescence properties significantly. The phosphors prepared with the precipitate ripened after pH adjustment at 85 °C and 130 °C using H3BO3 and Li3PO4 double flux, fired at 1100–1200 °C in reducing atmosphere were spherical particle of 4–5 μm in size having the brightness higher than commercial green phosphor. Acknowledgements This work was supported by Shanghai Leading Academic Discipline Project No. B108 and Integration of Industry, Learning and Research project co-sponsored by Chinese Ministry of Education and Guangdong Province No.2006D90404025. References
Fig. 6. Photoluminescence (a) excitation spectra for the emission at 543 nm and (b) emission spectra under excitation at 254 nm of LAP phosphors fired at 1200 °C using different fluxes.
[1] Y.C. Kang, E.J. Kim, D.Y. Lee, H.D. Park, High brightness LaPO4:Ce,Tb phosphor particles with spherical shape, Journal of Alloys and Compounds 347 (2002) 266–270. [2] F. Duault, M. Junker, P. Grosseau, B. Guilhot, P. Iacconi, B. Moine, Effect of different fluxes on the morphology of the LaPO4:Ce,Tb phosphor, Powder Technology 154 (2005) 132–137. [3] S. Oshio, K. Kitamura, T. Shigeta, S. Horii, T. Matsuoka, S. Tanaka, H. Kobayashi, Firing technique for preparing a BaMgAl10O17:Eu2+ phosphor with controlled particle shape and size, Journal of The Electrochemical Society 146 (1999) 392–399. [4] W. van Schaik, S. Lizzo, W. Smit, G. Blasse, Influence of impurities on the luminescence quantum efficiency of (La,Ce,Tb)PO4, Journal of The Electrochemical Society 140 (1993) 216–222. [5] X.R. Liu, X.J. Wang, Rare earth orthophosphate green phosphor for lamp, China Light & Lighting 5 (1994) 1–8 (in Chinese). [6] N. Arul Dhas, K.C. Patil, Synthesis of A1PO4, LaPO4 and KTiOPO4 by flash combustion Journal of Alloys Compounds 202 (1993) 137. [7] Y. Fujishiro, H. Ito, T. Sato, A. Okuwaki, Synthesis of monodispersed LaPO4 particles using the hydrothermal reaction of an La(edta) chelate precursor and phosphate ions, Journal of Alloys Compounds 252 (1997) 103–109.
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[8] M. Heike, R. Karsten, K. Andrew, N. Sabine, H. Markus, Wet-chemical synthesis of doped colloidal nanomaterials: particles and fibers of LaPO4:Eu, LaPO4:Ce, and LaPO4:Ce,Tb, Advanced Materials 11 (1999) 840–844. [9] I.W. Lenggoro, B. Xia, H. Mizushima, K. Okuyama, N. Kijima, Synthesis of LaPO4:Ce,Tb phosphor particles by spray pyrolysis, Materials Letters 50 (2001) 92–96. [10] S. Erdei, F.W. Ainger, D. Ravichandran, W.B. White, L.E. Cross, Preparation of Eu3+: YVO4 red and Ce3+, Tb3+: LaPO4 green phosphors by hydrolyzed colloid reaction (HCR) technique, Materials Letters 30 (1997) 389–393.
[11] P. Maestro, D. Huguenin, Industrial applications of rare earths: which way for the end of the century? Journal of Alloys and Compounds 225 (1995) 520–528. [12] C.L. Lo, J.G. Duh, B.S. Chiou, C.C. Peng, L. Ozawa, Synthesis of Eu3+-activated yttrium oxysulfide red phosphor by flux fusion method, Materials Chemistry and Physics 71 (2001) 179–189. [13] N. Hashimoto, Y. Takada, K. Sato, S. Ibuki, Green-luminescent (LaCe)PO4:Tb phosphors for small size fluorescent lamps, Journal of Luminescence 48&49 (1991) 893–897.
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p o w t e c
Preparation of LaPO4:Ce,Tb phosphor with different morphologies and their fluorescence properties Xuefang Hu, Shirun Yan ⁎, Lin Ma, Guojiang Wan, Jianguo Hu Department of Chemistry and Fudan-keheng Research Center for Luminescent Materials, Fudan University, Shanghai 200433, China
a r t i c l e
i n f o
Article history: Received 25 July 2008 Received in revised form 20 September 2008 Accepted 14 November 2008 Available online 27 November 2008 Keywords: Morphology control LaPO4,Ce,Tb Ripening condition Flux Firing temperature
a b s t r a c t LaPO4:Ce,Tb phosphor was prepared by firing the LaPO4:Ce,Tb precipitate derived from co-precipitation of aqueous solution of LaCeTb mixed rare earth nitrate with ammonium dibasic phosphate. Effects of ripening condition of precipitate, flux addition and firing temperature on the morphology and photoluminescence properties of LaPO4:Ce,Tb phosphor were investigated. It was found that ripening of LaPO4,Ce,Tb precipitate with mother liquor at pH = 1 and 130 °C for 24 h produced strip phosphor with width of 100 nm and length of 1 μm. While ripening at either 85 or 130 °C after pH adjustment with ammonia generated spherical particles of about 100 nm in size. Types of flux employed significantly influenced the morphology of the phosphor. When H3BO3 was used as flux spherical particles of ca. 300 nm were formed. In contrast, when Li2CO3 or Li3PO4 was employed as flux either separately or a component of double flux, uniform spherical or near spherical particles of ca. 5 μm were generated. Firing temperature of 1100 and 1200 °C in the presence of double flux led to the formation of smooth spherical particles with diameter of ca. 5 μm, and the phosphor prepared with double flux showed higher brightness than the commercial phosphor. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Phosphors are widely used in displays and lighting devices. Morphology of phosphors (shape and size of the powder particles) is one of the key parameters of their industrial application [1–3]. Phosphor particles should be non-aggregated spherical shape and their particle size distribution should be in a range between 1 and 8 μm depending on the application [2]. Spherical phosphor particle can optimize the optical and geometrical structure of phosphor layer. The particle size of a phosphor affects the amount of phosphor particles needed to produce an optimal coating for a particular application [1]. LaPO4:Ce,Tb (abbreviated as LAP hereinafter) is one of green phosphors of tricolor fluorescent lamps. In addition to its high quantum efficiency and stability at high temperature [4], advantage of LAP over MgAl11O19:Ce,Tb (abbreviated as MACT) is that specific gravity of LAP (5.07 g cm− 3) [5] matches well with Y2O3:Eu phosphor (5.20 g cm− 3) as compared with MACT whose specific gravity is 4.2 g cm− 3. As a result, the phosphor paint for lamp coating using LAP as a green-emitting component is more homogeneous than that made of CMAT and hence color uniformity between two ends of tubular fluorescent lamps such as T5,T8 and cold cathode fluorescent lamps is better than that using CMAT green phosphor. Extensive efforts have been paid to prepare LAP with controlled morphology and properties. Arul Dhas and Patil [6] synthesized crystalline LaPO4 by flash combustion of aqueous solutions. Fujishiro et al. [7] prepared monodispersed spherical and spindle-like ⁎ Corresponding author. Tel./fax: +86 21 65643987. E-mail address: [email protected] (S. Yan). 0032-5910/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.11.006
monazite type LaPO4 particles of 20 nm in size by hydrothermal reaction of lanthanum(III)–ethylenediamine tetraacetate complex. The monodispersed spherical particle was sintered more densely as compared with the powder prepared using the solid reaction. Heike et al. [8] prepared LAP particles, ranging in size from ca. 10 to 50 nm, and fibers with width of 5–20 nm by wet chemical synthesis with changing pH of solution. Lenggoro et al. [9] and Kang et al. [1] developed spray pyrolysis method for spherical LAP synthesis. Duault et al. [2] studied the effect of Li3PO4, Li2CO3, Na2CO3, K2CO3 flux on the morphology of LAP, quasispherical and parallelepiped-like shape LAP were prepared. Erdei et al. [10] synthesized LAP green phosphor particles by hydrolyzed colloid reaction technique at lower temperature and atmospheric pressure. To improve the chemical homogeneity of LAP, Maestro and Huguenin [11] proposed using mixed phosphate of LaCeTb instead of separated or mixed rare earth oxides as precursor to prepare LAP phosphor. In this paper, LAP phosphor was prepared by firing the LaPO4,Ce,Tb precipitate derived from co-precipitation of aqueous solution of mixed rare earth nitrate with ammonium dibasic phosphate in a reducing atmosphere. Effects of ripening condition of precipitate, flux addition and firing temperature on the morphology and photoluminescence properties of LAP phosphor were studied. 2. Experimental 2.1. Preparation of LaPO4:Ce,Tb precipitate and phosphor Appropriate amount of spectrographic grade mixed oxide (La0.51 Ce0.32Tb0.17)xOy from Shanghai Yuelong new materials Co. LTD was
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X. Hu et al. / Powder Technology 192 (2009) 27–32
dissolved in 1:1 HNO3 aqueous solution. To ensure the complete dissolution of CeOx which contains some tetravalent species, small amount of hydrogen peroxide was added dropwisely to the solution. The resultant clear solution with rare earth concentration of 1.6 mol L− 1 was referred to as solution A. The required amount of (NH4)2HPO4 was dissolved in deionized water and thus obtained solution was referred to as solution B. To ensure the complete precipitation of the rare earth ions 5% excess of (NH4)2HPO4 was employed. The solutions A and B were added simultaneously and slowly to a beaker placed in a waterbath setting at 85 °C under stirring and the white precipitate was produced. After completion of precipitation, final pH of slurry was about 1.0. The precipitate slurry was divided into five portions and each was ripened under different conditions: (1) the slurry containing the precipitate and mother liquor was transferred to a Teflon-lined autoclave. Filling coefficient of the autoclave was equal to 80%. The autoclave was placed into an oven preheated at 130 °C and maintained for 24 h. After ripening the autoclave was cooled quickly with flowing water to room temperature. (2) pH value of the slurry was adjusted
with ammonia to 2.5 and 4.5, respectively. Subsequently, it was hydrothermally ripened at 130 °C for 24 h. (3) pH value of the slurry was adjusted with ammonia to 4.5 and 6.5 respectively and the slurry was ripened at 85 °C for 1 h. After ripening and filtration, the precipitate was washed with deionized water to neutrality and then dried at 120 °C for 8 h. Thus obtained sample was referred to as LaPO4:Ce,Tb precipitate. The LAP phosphor was prepared by firing intimate mixture of the LaPO4:Ce,Tb precipitate with flux in H2–N2 (5% of H2) reducing atmosphere at a designed temperature. The sample was allowed to cool in the reducing atmosphere to room temperature and ground for measurements. 2.2. Characterizations The X-ray diffraction (XRD) patterns were collected on a Bruker AXS D8 Advance X-ray diffractometer with Cu–Kα radiation (λ = 0.15418 nm) operated at 40 mA and 40 kV. The morphologies of
Fig. 1. SEM micrographs of LAP phosphors prepared by firing at 1200 °C for 2 h the LaPO4:Ce,Tb precipitate ripened under different conditions: (a) pH = 1, 130 °C, 24 h; (b) pH = 2.5, 130 °C, 24 h; (c) pH = 4.5, 130 °C, 24 h; (d) pH = 4.5, 85 °C, 1 h; (e) pH = 6.5, 85 °C, 1 h.
X. Hu et al. / Powder Technology 192 (2009) 27–32
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phosphors were monitored using scanning electron microscopy (SEM, Philips XL 30). Thermogravimetry and differential thermal analysis of the dried precipitate (TG-DTA) were performed on a Perkin-Elmer DTA-7 apparatus with a heating rate of 10 K/min. The photoluminescence excitation (PLE) and emission (PL) spectra were recorded using a Varian Cary Eclipse fluorescence spectrophotometer at room temperature. The CIE chromaticity coordinate of the phosphors under excitation at 254 nm was obtained on a Chuanghui CMS-2000 spectrophotocolorimeter. The relative brightness of the phosphor was measured on a JY-III phosphor brightness meter using the Chinese national standard trichromatic green phosphor 200202 (GB/T 146332002) as reference. 3. Results and discussion 3.1. Effect of ripening condition on the morphology of LAP phosphor Shown in Fig. 1 is the SEM micrographs of the LAP phosphors prepared by firing the LaPO4:Ce,Tb precipitate ripened under different conditions at 1200 °C for 2 h. It is interesting to note that LAP phosphor prepared using the precipitate hydrothermally ripened at 130 °C for 24 h in mother liquor showed strip form with width of 100 nm and length of 1 μm, similar to that reported by Heike et al. [8]. While the LAP phosphor prepared with the precipitate ripened at 130 °C and 85 °C after pH adjustment with ammonia to either 2.5, 4.5 or 6.5 all gave near spherical shape with different particle sizes. The results indicate that the strong acidic environment of the mother liquor favors growth rates along one crystallographic plane of monoclinic phase leading to the formation of strip precipitate, while the addition of ammonia inhibits the preferential growth of the precipitate resulting in the formation of spherical particles.
Fig. 3. DTA profiles of the dried LAP precipitate with different morphologies during heating from 25 to 1200 °C at a rate of 10 °C/min. (1) strip; (2) spherical.
3.2. Effect of flux on the morphology of LAP phosphor Fluxes are often used in preparation of phosphors by solid-state reaction. The role of fluxes in the growth of phosphors is well known in the literature. They are used to act as a medium to incorporate the activators, reduce the firing temperature, and improve the crystallinity of respective phosphor [12]. Boric acid and lithium salt are reported to be the suitable fluxes for LAP preparation [2,13]. Boric acid could promote the formation of trivalent rare earth ions [13], while the lithium salt (Li3PO4 or Li2CO3) leads to the formation of regular and faceted particles of phosphors of 3–4 μm [2]. In this paper, H3BO3,
Fig. 2. SEM micrographs of LAP phosphors prepared by firing at 1200 °C for 2 h the mixture of precipitate ripened at pH = 2.5, 130 °C for 24 h with different fluxes: (a) without flux; (b) 1 wt.% H3BO3 as flux; (c) 1 wt.% Li2CO3 as flux; (d) 1 wt.% H3BO3 + 1 wt.% Li2CO3 as flux.
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3.3. Effect of firing temperature on the morphology of LAP phosphor
Fig. 4. XRD patterns of the LAP phosphors fired in reducing atmosphere at different temperatures.
Li2CO3 mono-flux and H3BO3 + Li2CO3, H3BO3 + Li3PO4 concurrent double flux on the morphology and fluorescent properties of LAP were investigated. Shown in Fig. 2 is the SEM micrograph of the LAP phosphors prepared using different fluxes. It was found that the phosphor prepared without use of flux was spherical particle with mean size ca. 100 nm. The phosphor prepared using H3BO3 as flux was also spherical with particle size about 200 nm. The particle size of phosphor was increased to about 4.5–5 μm when using Li2CO3 flux or Li2CO3 and H3BO3 concurrent double flux. Moreover, the particle was near spherical when using the double flux, while it was egg-like shape when using Li2CO3 flux alone.
DTA experiment was performed to monitor the phase transformation during heating the dried LaPO4:Ce,Tb so that the suitable temperature for phosphor firing could be determined. The DTA profiles of the dried precipitates with strip and spherical shape are shown in Fig. 3. A strong exothermic peak between 700 and 1000 °C corresponding to phase transformation to monazite LaPO4:Ce,Tb was observed for both strip and spherical precipitates, indicating that the crystallization and formation of monazite LaPO4:Ce,Tb can be finished below 1000 °C. Moreover, a small endothermic peak around 270 °C which may be assigned to the evaporation of crystallized water of the phosphate appeared for the spherical precipitate. A comparison of the onset and peak temperature of the phase transformation of the two samples revealed that crystallization of the spherical particle could be realized at a temperature ca. 20 °C lower than that for the strip one. Based on the DTA results, the mixture of LaPO4,Ce,Tb precipitate with H3BO3 and Li2CO3 double flux was fired at temperatures between 900 and 1200 °C, the XRD patterns of the fresh LaPO4:Ce,Tb and those fired at designed temperatures for 2 h are shown in Fig. 4. The pattern of the fresh sample matches with 04-0635 by JCPDS fingerprint corresponding to LaPO4 structure. The patterns of the fired samples are in good agreement with the PDF 32-0493. There was no obvious difference in crystallinity between the sample fired at 900 and 1200 °C, indicating that in the presence of double flux crystallization of LAP could be completed at 900 °C within 2 h. The SEM micrographs of the LAP phosphors fired at temperature between 900 and 1200 °C for 2 h using H3BO3 and Li2CO3 double flux are shown in Fig. 5. It was found that the LaPO4:Ce,Tb fired at 900 °C took irregular shape. With increasing firing temperature to 1000 °C, the particle was quasi-spherical with coarse surface. With increasing firing temperature to 1100 °C, the spherical particle surface became smooth. With
Fig. 5. SEM micrographs of the LAP phosphors prepared by firing the intimate mixture of H3BO3 and Li2CO3 double flux with the precipitate at temperature of (a) 900 °C; (b) 1000 °C; (c) 1100 °C; (d) 1200 °C.
X. Hu et al. / Powder Technology 192 (2009) 27–32
further increasing firing temperature to 1200 °C the size of smooth spherical particles was increased to 4–5 μm. The evolution of particle morphologies with firing temperature may be interpreted by a liquidphase-sintering-like mechanism as proposed by Duault et al. [2]. With increasing firing temperature, the wettability of the LAP powder by the liquid, the viscosity of the liquid and the diffusivity of the cations and anions increased. As a result, mass transfer process and grain growth kinetics were enhanced. The smooth spherical particles with enlarged sizes were formed. 3.4. Effect of flux on photoluminescence properties of LAP phosphor The photoluminescence excitation (PLE) and emission (PL) properties of LAP phosphors prepared from the same LAP precipitate using different fluxes were investigated. The PLE and PL spectra are shown in Fig. 6. It was found that using either H3BO3 alone or H3BO3 + Li2CO3 double flux could improve the PLE and PL intensity of the LAP phosphor. Moreover, the double flux enhanced the excitation intensity around 300 nm. Further investigation into the effect of double flux on the relative brightness of LAP phosphor, as listed in Table 1, revealed that the LAP phosphor prepared using H3BO3 and suitable amount of Li3PO4 double flux showed the highest relative brightness among the phosphors investigated. The XRD patterns (not presented) of the LAP phosphors using H3BO3 + Li3PO4 and H3BO3 + Li2CO3 fluxes had no discernable difference. The increased brightness of the phosphor prepared using H3BO3 flux and H3BO3 + lithium salt double flux may be interpreted by the improved morphology of the phosphor. By
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Table 1 CIE chromaticity and relative brightness of the LAP phosphors prepared by using different fluxes No. 1 2 3 4 5 6 7 8
Fluxes
CIE color coordinates
Relative brightness
(wt.%)
x
y
(%)
None 1 H3BO3 + 1 Li2CO3 1 H3BO3 + 1 Li3PO4 2 H3BO3 1 H3BO3 + 0.5 Li3PO4 1 H3BO3 + 1.5 Li3PO4 1 H3BO3 + 2 Li3PO4 1 H3BO3 + 3 Li3PO4
0.3458 0.3457 0.3459 0.3448 0.3463 0.3458 0.3464 0.3466
0.5783 0.5780 0.5781 0.5781 0.5787 0.5781 0.5786 0.5782
95.7 99.2 102.4 98.8 104.3 99.1 98.1 97.9
comparing the SEM micrographs of the phosphors shown in Fig. 2 it is found that the phosphors prepared in the presence of flux had the increased particle size and improved surface smoothness, especially those using double flux. It is generally accepted that an increase in the size of a phosphor usually results in higher emission brightness because of the lower intrinsic reflection coefficient associated with the larger particles [1]. As for the more pronounced promotional effect of Li3PO4 + H3BO3 double flux than that of H3BO3 + Li2CO3, the author suppose it may be related with the fact that Li3PO4 has the same anions as (LaCeTb)PO4 and therefore Li3PO4 may compensate for the loss of PO4 group during firing in reducing atmosphere, maintaining the stoichiometric intactness of the lattice, although it was indiscernible by XRD. 4. Conclusions LaPO4:Ce,Tb green phosphor was prepared by firing the LaPO4:Ce, Tb precipitate derived from co-precipitation of aqueous solution of LaCeTb mixed rare earth nitrate with ammonium dibasic phosphate. The morphology of the phosphor could be controlled by ripening condition of the precipitate, types of flux and firing temperature. The morphology of the phosphor influenced the photoluminescence properties significantly. The phosphors prepared with the precipitate ripened after pH adjustment at 85 °C and 130 °C using H3BO3 and Li3PO4 double flux, fired at 1100–1200 °C in reducing atmosphere were spherical particle of 4–5 μm in size having the brightness higher than commercial green phosphor. Acknowledgements This work was supported by Shanghai Leading Academic Discipline Project No. B108 and Integration of Industry, Learning and Research project co-sponsored by Chinese Ministry of Education and Guangdong Province No.2006D90404025. References
Fig. 6. Photoluminescence (a) excitation spectra for the emission at 543 nm and (b) emission spectra under excitation at 254 nm of LAP phosphors fired at 1200 °C using different fluxes.
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