- Email: [email protected]
The effect of MgO doping on the structure and photoluminescence of YAG:Tb phosphor
Journal of Alloys and Compounds 479 (2009) 759–763
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
The effect of MgO doping on the structure and photoluminescence of YAG: Tb phosphor Hsuan-Min Lee, Yu-Shun Cheng, Chi-Yuen Huang ∗ Department of Resources Engineering, National Cheng Kung University, One University Road, Tainan 70101, Taiwan
a r t i c l e
i n f o
Article history: Received 21 October 2008 Received in revised form 7 January 2009 Accepted 18 January 2009 Available online 6 March 2009 Keywords: Phosphors Solid-state reaction Microstructure Luminescence
a b s t r a c t Y3 Al5 O12 garnet (YAG) phosphor co-doped with Terbium (Tb) and Magnesium (Mg) has been investigated. Tb3+ ions have a strong trend to transform into Tb4+ ions under an oxidizing atmosphere. Therefore, this study focused on the effects of structure and optical property of YAG co-doped with Tb4+ + Mg2+ in dodecahedral sites. Spherical phosphor particle is considered better than irregular-shaped particle to improve synthesis. Experimentally, solid reactants were mixed with pH-control and spherical particles were obtained using a spray-drying processing. YAG:Tb and YAG:(Tb, Mg) phosphors were synthesized under an attached air by this solid-state reaction method. When MgO is reached to 2.5 at.%, the MgAl2 O4 phase is formed. The cell parameters are reduced by more MgO amount and a longer soaking time. However, a substitution limit of MgO (2.5 at.%) was observed because of the retarding phenomenon at cell parameters. MgO co-doping damaged the grain growth behavior and caused weaker emitting intensities than single-doped YAG:Tb phosphors. Tb4+ ions were also induced by MgO co-doping and this paper provided an explanation about Tb4+ in PL emission spectral analysis. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Yttrium aluminum garnet (YAG) phosphors with doping rareearth elements can be excited by energy sources to emit light. Experimentally, nano-scale YAG phosphors synthesized by chemical methods were obtained at low temperatures. However, their luminescent properties may be reduced by severe agglomeration. Comparatively, YAG phosphors need a higher temperature to be synthesized by a conventional solid-state reaction, but phosphors which have uniform grains, high crystallinity, good morphology and high luminescent property can be obtained easily. YAG:Tb phosphors with Y3+ dodecahedral site replaced by Tb3+ as an activator have been reported numerously [1–10]. However, Tb3+ has an electron configuration of [Xe]4f8 and it holds stronger potential to separate an outer electron among the lanthanides. As a result, Tb4+ which has half filled 4f-orbitals, [Xe]4f7 , likely emerges from oxidizing conditions. Gramsch and Morss [11] presented the possibility of the tetravalent ions (Ln: Ce, Pr and Tb) in garnet YCaLnGa5 O12 . Difference between Pr3+ and Pr4+ doped YAG:(Pr, Mg) phosphors in absorption spectra has also been reported [12]. There are few articles on this subject, Tb4+ in garnet phosphor. In this study, YAG:Tb and YAG:(Tb, Mg) phosphors were synthesized by a solid-state reaction method. The Tb4+ was induced and
∗ Corresponding author. Tel.: +886 6 2754170; fax: +886 6 2380421. E-mail address: [email protected] (C.-Y. Huang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.01.040
obtained by incorporating Mg2+ ions into YAG host and supplying with oxidizing atmosphere. Mg2+ has a stable electron configuration of [Ne] and its radius matches the size limit of solid solution [13]. So Mg2+ ion was selected as a co-dopant. Starting materials were mixed with pH-control and this phosphor was granulated into a sphere-like shape by a spray-drying device. Structural and luminescence property measurements are presented in the paper. 2. Experimental Stoichiometric amounts of Y2 O3 (CERAC, 99.99%) and MgO (Alfa Aesar, 99.95%) were dispersed in NaOH (FULLIN, 99%) solutions (pH 12) and ball-milled (high pure 5 mm yttrium stabilized ZrO2 beads with 250 cc polypropylene vial was used and put on the rollers of tabletop ball mill at 280 r.p.m.) for 24 h independently. Half the total mill volume was loaded with 30% media and 20% mill base. Stoichiometric amounts of -Al2 O3 (Sumitomo Chemical, >99.995%) and Tb(NO3 )3 ·5H2 O (Aldrich, 99.9%) were mixed with milled Y2 O3 and MgO to form the formula (Y0.95−x Tb0.05 Mgx )3 Al5 O12 (x = 0, 0.001, 0.01, 0.025 and 0.05). The mixture (30 wt.%) was milled for 8 h to get well-mixed slurry. Particle sizes of raw materials are listed in Table 1. The spray-drying system used to prepare the spherical particles was comprised of an airbrush and an oven. The flow rate of cylinder air was 8 L/min, and the oven temperature was set at 200 ◦ C. The slurry was loaded in the airbrush, and the droplets were sprayed into the oven. After 1 h of cooling, the YAG precursors were obtained. Finally, the precursors were calcined in an oxidizing atmosphere (supplied by cylinder air) with different conditions (target temperature 1600 ◦ C at a rate of 10 ◦ C/min with soaking times 0, 2 and 4 h) to synthesize YAG phosphors. Powder X-ray diffractometer (XRD) analysis of YAG phosphors was done by a Siemens D5000 (CuK␣ radiation). The diffraction patterns were refined with the program GSAS (general structure analysis system) [14], which was designed with the Rietveld full pattern refinement method [15]. ESCA (electron spectroscopy for chemical analysis) was performed on VG Scientific ESCALAB 250. The morphology of phosphors was observed on a Hitachi S4100 ultrahigh resolution scanning
760
H.-M. Lee et al. / Journal of Alloys and Compounds 479 (2009) 759–763
Table 1 Particle sizes of raw materials. Materials
Before milling
After milling (24 h)
Y2 O3 MgO -Al2 O3
5–10 m <20 ma 20–30 nm
1–2 m 1–2 m –
a
Agglomerate.
electron microscope (SEM). The PL (photoluminescence) emission spectrum and absorption spectrum were measured by Hitachi F-4500 and Cary 100 fluorescence spectrophotometers at room temperature, respectively.
3. Results and discussion From the XRD patterns of YAG:(Tb, Mg) phosphors with different calcining terms (Fig. 1), pure cubic phase (Y0.95−x Tb0.05 Mgx )3 Al5 O12 (x = 0) phosphors synthesized at 1600 ◦ C/0 h can be detected (YAG, ICDD-PDF # 01-070-7794). However, the (1 1 0) (2 = 23.9◦ ) and (1 1 2) (2 = 34.3◦ ) peaks of the transitional YAP (yttrium aluminum perovskite) phase (ICDD-PDF # 01-089-7947) can be found in (Y0.95−x Tb0.05 Mgx )3 Al5 O12 (x = 0.01) phosphors at 1600 ◦ C/0 h, but disappeared at 1600 ◦ C/4 h. When MgO amounts increased (x = 0.025), the (4 2 2) (2 = 36.6◦ ) peak of the YAG phase was broadened at the high 2-angle side. It was caused by the accumulation of the (4 2 2) peak of YAG and the (3 1 1) (2 = 36.8◦ ) peak of the MgAl2 O4 phase (ICDD-PDF # 01-082-2424). The (2 2 0) (2 = 31.3◦ ), (4 0 0) (2 = 44.8◦ ), (5 1 1) (2 = 59.3◦ ) and (4 4 0) (2 = 65.2◦ ) peaks of the MgAl2 O4 phase were also detected in YAG phosphors, and they existed for even longer soaking conditions. Y2 O3 , ␣-Al2 O3 and MgO phases were not detected in our samples.
Fig. 2. Cell parameters of (Y0.95−x Tb0.05 Mgx )3 Al5 O12 phosphors with x = 0, 0.001, 0.01, 0.025 and 0.05 (MgO = 0, 0.1, 1, 2.5 and 5 at.%, respectively) calcined at 1600 ◦ C/0, 2 and 4 h.
The cell parameters of (Y0.95−x Tb0.05 Mgx )3 Al5 O12 (x = 0, 0.001, 0.01, 0.025 and 0.05) phosphors simulated by the GSAS program are shown in Fig. 2. The final refined Rwp (weighted profile reliability factor) and 2 (goodness of fit) values are all lower than 14% and 3%, respectively. This denotes that all of these refined results are acceptable. According to the curve, when either soaking time or MgO amounts increase, the cell parameters decrease. Generally, Tb3+ has a strong trend to transform into Tb4+ . To investigate this supposition, (Y0.95−x Tb0.05 Mgx )3 Al5 O12 (x = 0, 0.01 and 0.025) phosphors were synthesized at 1600 ◦ C/4 h and their par-
Fig. 1. XRD patterns of (Y0.95−x Tb0.05 Mgx )3 Al5 O12 phosphors with x = 0, 0.01 and 0.025 calcined at 1600 ◦ C/0 and 4 h.
H.-M. Lee et al. / Journal of Alloys and Compounds 479 (2009) 759–763
Fig. 3. Partial ESCA spectra at 375–225 eV of (Y0.95−x Tb0.05 Mgx )3 Al5 O12 phosphors with x = 0, 0.01 and 0.025 calcined at 1600 ◦ C/4 h.
tial ESCA spectra are shown in Figs. 3 and 4 (the range of spectra only includes Tb’s and Y’s). After MgO doping, the intensities of the Tb 4p3/2 peaks increased sharply and the unknown peaks showed up as well as the shift of Tb 3d peaks to a higher binding energy (Table 2) were observed [16]. The binding energy can be changed due to element kind, ionic valence, the electron layer state and the crystal structure. Even with the same elements, the shift or new peaks forming will be influenced just by the difference of valence or crystal structure [17–20]. It can be supposed that the forming of a little Tb4+ in YAG:(Tb, Mg) phosphors happens easily after a prolonged soaking time or oxidizing annealing operation. Because the outer electron of Tb4+ ([Xe]4f7 ) is more difficult to be excited than Tb3+ ([Xe]4f8 ). The higher binding energy of Tb 3d corresponded to this principle. The ionic radii of Mg2+ and Tb4+ ions are both smaller than Y3+ ions in the YAG host (in dodecahedral sites: Mg2+ = 0.89 Å, Tb3+ = 1.04 Å, Tb4+ = 0.88 Å, Y3+ = 1.02 Å). From the refined results of XRD, the decrease of cell parameters (Fig. 2) can be explained by the co-existence of Mg2+ and Tb4+ occupying some dodecahedral sites. Because of the MgAl2 O4 phase appearing (Fig. 1) and the cell parameters retarding phenomenon (Fig. 2), the substitution limit of MgO (x = 0.025) can be observed (Fig. 2).
Table 2 ESCA spectral peaks of (Y0.95−x Tb0.05 Mgx )3 Al5 O12 phosphors with x = 0, 0.01 and 0.025 calcined at 1600 ◦ C/4 h. Tb (eV)
3d3/2
3d5/2
4s
4p1/2
4p3/2
4d
5s
5p
Standard values [16]
1276
1241
396
322
285
146
45
22
x=0 x = 0.01 x = 0.025
1278 1279 1279
1242 1244 1244
396 396 396
324 324 324
285 285 285
148 148 148
45 45 45
24 24 24
761
Fig. 4. Partial ESCA spectra at 960–920 eV of (Y0.95−x Tb0.05 Mgx )3 Al5 O12 phosphors with x = 0, 0.01 and 0.025 calcined at 1600 ◦ C/4 h.
The SEM micrographs of the (Y0.95−x Tb0.05 Mgx )3 Al5 O12 (x = 0, 0.01 and 0.025) phosphors synthesized at different soaking conditions are shown in Fig. 5. The morphology of the phosphors is all worm-shaped and no complete crystalline grains at 1600 ◦ C/0 h (Fig. 5a, d and g). When soaking time increased, primary particles of sprayed powders started to sinter or reacted together and grains growth can be observed. When the MgO amount increased, the retarding of grain growth was slightly found. The diffusion of Mg2+ ions into the YAG structure is not spontaneous, and additional energy is needed to overcome the reaction enthalpies [21,22]. In the initial stage of synthesis, the diffusion behavior consumes partial energies and the rich Mg2+ in the grain boundary may interrupt the diffusion of matrix compounds. It will cause the rate of grain growth to slow down. Such phenomena have already been reported [23,24]. The PL emission spectra (ex = 275 nm) of the (Y0.95 Tb0.05 )3 Al5 O12 phosphors synthesized at different soaking conditions are shown in Fig. 6. The emission spectra can be separated into two zones: one has sharp peaks due to the 5 D → 7 F and 5 D → 7 F transitions at the blue-green light zone, 5 4 6 4 and the other has weak broad peaks due to the 5 D4 → 7 F4 and 5 D → 7 F transitions at the yellow-red light zone. The emitting 4 3 intensity increased when soaking time increased. After a prolonged soaking operation, Tb3+ was dispersed in the YAG host uniformly and the possible effect of concentration quenching was retarded. Moreover, better crystallinity and morphology can also be obtained with longer calcination time (Fig. 5c), so the emitting intensity increased. The PL emission spectra (ex = 275 nm) of the (Y0.95−x Tb0.05 Mgx )3 Al5 O12 (x = 0, 0.01 and 0.025) phosphors synthesized at 1600 ◦ C/4 h are shown in Fig. 7. When the MgO amount increased to x = 0.025, the emitting intensity decreased drastically. Fig. 8 showed the absorption spectrum of pure MgAl2 O4 . According to Fig. 8, the decrease in PL emission intensity of
762
H.-M. Lee et al. / Journal of Alloys and Compounds 479 (2009) 759–763
Fig. 5. SEM micrographs of (Y0.95−x Tb0.05 Mgx )3 Al5 O12 phosphors with (a)–(c) x = 0, (d)–(f) x = 0.01 and (g)–(i) x = 0.025 calcined at 1600 ◦ C/soaking times (a) 0 h, (b) 2 h, (c) 4 h, (d) 0 h, (e) 2 h, (f) 4 h, (g) 0 h, (h) 2 h and (i) 4 h.
(Y0.925 Tb0.05 Mg0.025 )3 Al5 O12 phosphor was not due to the residual second phase, MgAl2 O4 . In addition to the morphology effect, this paper provided another explanation for Fig. 7 PL emission spectral analysis. According to Table 2 and Figs. 2–4 results, this set of compounds (Mg2+ + Tb4+ ) replaced 2Y3+ in the Y3 Al5 O12 in order to stabilize as effectively as the Tb4+ species in the eight-coordinate site of the garnet structure. Tb4+ stabilized in the YAG:(Tb, Mg) will absorb emission energy and lower PL emission intensity of
(Y0.95−x Tb0.05 Mgx )3 Al5 O12 (x = 0.01 and 0.025) phosphors. This concept is similar to Ce4+ ([Xe]) ions existing in YAG:Ce phosphors, and Ce4+ ions will decrease the luminescence of Ce3+ ions in the visible light region. Furthermore, when co-doped with Tb4+ and Mg2+ , the YAG phosphors have a brown coloration. Nevertheless, single-doped with Tb3+ , YAG phosphor just has a white color. Tb4+ is considered a poison in the YAG:(Tb, Mg) phosphor. Additional studies are necessary to explain more about Tb4+ in this spectral
Fig. 6. PL emission spectra of (Y0.95 Tb0.05 )3 Al5 O12 phosphors calcined at 1600 ◦ C/0, 2 and 4 h.
Fig. 7. PL emission spectra of (Y0.95−x Tb0.05 Mgx )3 Al5 O12 phosphors with x = 0, 0.01 and 0.025 calcined at 1600 ◦ C/4 h.
H.-M. Lee et al. / Journal of Alloys and Compounds 479 (2009) 759–763
763
sity is supposedly caused by Tb4+ ions not MgAl2 O4 compound. Mg2+ -doped YAG:Tb phosphor prepared at an air atmosphere is unsuitable for application comparing with traditional YAG:Tb. Acknowledgement This work was sponsored by the Ministry of Economic Affairs of R.O.C. under grant No. 97-EC-17-A-08-S1-023. References
Fig. 8. Absorption spectrum of pure MgAl2 O4 powder.
region. It could be demonstrated by recording XANES (X-ray absorption near edge structure) spectra at LIII edge and ELNES (electron energy loss near edge structure) of terbium ions [20,25]. The related work with this system is under study. 4. Conclusions This study combined pH-controlled mixing and spray-dried processing to synthesize YAG:Tb and YAG:(Tb, Mg) phosphors with a solid-state reaction at an oxidizing atmosphere. The YAP phases were detected in YAG:(Tb, Mg) phosphors but disappeared after a prolonged soaking time. When MgO doping content or soaking time increased, a substitution limit of MgO can be observed and the decrease of cell parameters was due to the co-existence of Mg2+ and Tb4+ in the YAG matrix. After a prolonged soaking operation, not only can activator be uniformly dispersed but better crystallinity as well as morphology can be obtained, and the Tb3+ emitting intensity increased in YAG:Tb phosphor. The Tb4+ can be found in the YAG:(Tb, Mg) system and the decrease of Tb3+ emitting inten-
[1] K. Ohno, T. Abe, J. Electrochem. Soc: Solid-State Sci. Technol. 133 (1986) 638–643. [2] J.R. Lo, T.Y. Tseng, Mater. Chem. Phys. 57 (1998) 95–98. [3] Y.C. Kang, I.W. Lenggoro, S.B. Park, K. Okuyama, J. Phys. Chem. Solids 60 (1999) 1855–1858. [4] J.Y. Choe, D. Ravichandran, S.M. Blomquist, K.W. Kirchner, E.W. Forsythe, D.C. Morton, J. Lumines. 93 (2001) 119–128. [5] K.Y. Jung, D.Y. Lee, Y.C. Kang, Mater. Res. Bull. 40 (2005) 2212–2218. [6] X. Li, H. Liu, J. Wang, H. Cui, S. Yang, I.R. Boughton, J. Phys. Chem. Solids 66 (2005) 201–205. [7] G. Xia, S. Zhou, J. Zhang, S. Wang, J. Xu, J. Alloy. Compd. 421 (2006) 294–297. [8] S. Saxena, Mater. Lett. 60 (2006) 1315–1318. [9] H.M.H. Fadlalla, C.C. Tang, Opt. Mater. 31 (2008) 401–404. [10] X. Li, Q. Yu, Z. Zeng, X. Jing, J. Rare Earths 26 (2008) 35–39. [11] S.A. Gramsch, L.R. Morss, J. Alloy. Compd. 207–208 (1994) 432–435. [12] D. Pawlak, Z. Frukacz, Z. Mierczyk, A. Suchocki, J. Zachara, J. Alloy. Compd. 275–277 (1998) 361–364. [13] J.D. Dana, in: C. Klein, C.S. Hurlbut Jr. (Eds.), Manual of Mineralogy, John Wiley and Sons, New York, 1993, pp. 221–249. [14] A.C. Larson, R.B. Von Dreele, General Structure Analysis System, National Laboratory, Los Alamos, New Mexico, 1985. [15] H.M. Rietveld, Acta Cryst. 22 (1967) 151–152. [16] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray photoelectron spectroscopy, in: J. Chastain, R.C. King Jr. (Eds.), Physical Electronics, Eden Prairie, Minnesota, 1995, pp. 154–155. [17] G. Kalkowski, G. Kaindl, G. Wortmann, D. Lentz, S. Krause, Phys. Rev. B 37 (1988) 1376–1382. ´ [18] D.A. Pawlak, K. Wozniak, Z. Frukacz, T.L. Barr, D. Fiorentino, S. Seal, J. Phys. Chem. B 103 (1999) 1454–1461. [19] H.X. Dai, C.F. Ng, C.T. Au, J. Catal. 199 (2001) 177–192. [20] A. Martínez-Arias, A.B. Hungría, M. Fernández-García, A. Iglesias-Juez, J.C. Conesa, G.C. Mather, G. Munuera, J. Power Sources 151 (2005) 43–51. [21] L. Schuh, R. Metselaar, C.R.A. Catlow, J. Eur. Ceram. Soc. 7 (1991) 67–74. [22] M.M. Kuklja, J. Phys.: Condens. Matter 12 (2000) 2953–2967. [23] S.J. Bennison, M.P. Harmer, J. Am. Ceram. Soc. 73 (1990) 833–837. [24] K.K. Soni, A.M. Thompson, M.P. Harmer, D.B. Williams, J.M. Chabala, R. Levi-Setti, Appl. Phys. Lett. 66 (1995) 2795–2797. [25] I. Tanaka, T. Mizoguchi, T. Yamamoto, J. Am. Ceram. Soc. 88 (2005) 2013–2029.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
The effect of MgO doping on the structure and photoluminescence of YAG: Tb phosphor Hsuan-Min Lee, Yu-Shun Cheng, Chi-Yuen Huang ∗ Department of Resources Engineering, National Cheng Kung University, One University Road, Tainan 70101, Taiwan
a r t i c l e
i n f o
Article history: Received 21 October 2008 Received in revised form 7 January 2009 Accepted 18 January 2009 Available online 6 March 2009 Keywords: Phosphors Solid-state reaction Microstructure Luminescence
a b s t r a c t Y3 Al5 O12 garnet (YAG) phosphor co-doped with Terbium (Tb) and Magnesium (Mg) has been investigated. Tb3+ ions have a strong trend to transform into Tb4+ ions under an oxidizing atmosphere. Therefore, this study focused on the effects of structure and optical property of YAG co-doped with Tb4+ + Mg2+ in dodecahedral sites. Spherical phosphor particle is considered better than irregular-shaped particle to improve synthesis. Experimentally, solid reactants were mixed with pH-control and spherical particles were obtained using a spray-drying processing. YAG:Tb and YAG:(Tb, Mg) phosphors were synthesized under an attached air by this solid-state reaction method. When MgO is reached to 2.5 at.%, the MgAl2 O4 phase is formed. The cell parameters are reduced by more MgO amount and a longer soaking time. However, a substitution limit of MgO (2.5 at.%) was observed because of the retarding phenomenon at cell parameters. MgO co-doping damaged the grain growth behavior and caused weaker emitting intensities than single-doped YAG:Tb phosphors. Tb4+ ions were also induced by MgO co-doping and this paper provided an explanation about Tb4+ in PL emission spectral analysis. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Yttrium aluminum garnet (YAG) phosphors with doping rareearth elements can be excited by energy sources to emit light. Experimentally, nano-scale YAG phosphors synthesized by chemical methods were obtained at low temperatures. However, their luminescent properties may be reduced by severe agglomeration. Comparatively, YAG phosphors need a higher temperature to be synthesized by a conventional solid-state reaction, but phosphors which have uniform grains, high crystallinity, good morphology and high luminescent property can be obtained easily. YAG:Tb phosphors with Y3+ dodecahedral site replaced by Tb3+ as an activator have been reported numerously [1–10]. However, Tb3+ has an electron configuration of [Xe]4f8 and it holds stronger potential to separate an outer electron among the lanthanides. As a result, Tb4+ which has half filled 4f-orbitals, [Xe]4f7 , likely emerges from oxidizing conditions. Gramsch and Morss [11] presented the possibility of the tetravalent ions (Ln: Ce, Pr and Tb) in garnet YCaLnGa5 O12 . Difference between Pr3+ and Pr4+ doped YAG:(Pr, Mg) phosphors in absorption spectra has also been reported [12]. There are few articles on this subject, Tb4+ in garnet phosphor. In this study, YAG:Tb and YAG:(Tb, Mg) phosphors were synthesized by a solid-state reaction method. The Tb4+ was induced and
∗ Corresponding author. Tel.: +886 6 2754170; fax: +886 6 2380421. E-mail address: [email protected] (C.-Y. Huang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.01.040
obtained by incorporating Mg2+ ions into YAG host and supplying with oxidizing atmosphere. Mg2+ has a stable electron configuration of [Ne] and its radius matches the size limit of solid solution [13]. So Mg2+ ion was selected as a co-dopant. Starting materials were mixed with pH-control and this phosphor was granulated into a sphere-like shape by a spray-drying device. Structural and luminescence property measurements are presented in the paper. 2. Experimental Stoichiometric amounts of Y2 O3 (CERAC, 99.99%) and MgO (Alfa Aesar, 99.95%) were dispersed in NaOH (FULLIN, 99%) solutions (pH 12) and ball-milled (high pure 5 mm yttrium stabilized ZrO2 beads with 250 cc polypropylene vial was used and put on the rollers of tabletop ball mill at 280 r.p.m.) for 24 h independently. Half the total mill volume was loaded with 30% media and 20% mill base. Stoichiometric amounts of -Al2 O3 (Sumitomo Chemical, >99.995%) and Tb(NO3 )3 ·5H2 O (Aldrich, 99.9%) were mixed with milled Y2 O3 and MgO to form the formula (Y0.95−x Tb0.05 Mgx )3 Al5 O12 (x = 0, 0.001, 0.01, 0.025 and 0.05). The mixture (30 wt.%) was milled for 8 h to get well-mixed slurry. Particle sizes of raw materials are listed in Table 1. The spray-drying system used to prepare the spherical particles was comprised of an airbrush and an oven. The flow rate of cylinder air was 8 L/min, and the oven temperature was set at 200 ◦ C. The slurry was loaded in the airbrush, and the droplets were sprayed into the oven. After 1 h of cooling, the YAG precursors were obtained. Finally, the precursors were calcined in an oxidizing atmosphere (supplied by cylinder air) with different conditions (target temperature 1600 ◦ C at a rate of 10 ◦ C/min with soaking times 0, 2 and 4 h) to synthesize YAG phosphors. Powder X-ray diffractometer (XRD) analysis of YAG phosphors was done by a Siemens D5000 (CuK␣ radiation). The diffraction patterns were refined with the program GSAS (general structure analysis system) [14], which was designed with the Rietveld full pattern refinement method [15]. ESCA (electron spectroscopy for chemical analysis) was performed on VG Scientific ESCALAB 250. The morphology of phosphors was observed on a Hitachi S4100 ultrahigh resolution scanning
760
H.-M. Lee et al. / Journal of Alloys and Compounds 479 (2009) 759–763
Table 1 Particle sizes of raw materials. Materials
Before milling
After milling (24 h)
Y2 O3 MgO -Al2 O3
5–10 m <20 ma 20–30 nm
1–2 m 1–2 m –
a
Agglomerate.
electron microscope (SEM). The PL (photoluminescence) emission spectrum and absorption spectrum were measured by Hitachi F-4500 and Cary 100 fluorescence spectrophotometers at room temperature, respectively.
3. Results and discussion From the XRD patterns of YAG:(Tb, Mg) phosphors with different calcining terms (Fig. 1), pure cubic phase (Y0.95−x Tb0.05 Mgx )3 Al5 O12 (x = 0) phosphors synthesized at 1600 ◦ C/0 h can be detected (YAG, ICDD-PDF # 01-070-7794). However, the (1 1 0) (2 = 23.9◦ ) and (1 1 2) (2 = 34.3◦ ) peaks of the transitional YAP (yttrium aluminum perovskite) phase (ICDD-PDF # 01-089-7947) can be found in (Y0.95−x Tb0.05 Mgx )3 Al5 O12 (x = 0.01) phosphors at 1600 ◦ C/0 h, but disappeared at 1600 ◦ C/4 h. When MgO amounts increased (x = 0.025), the (4 2 2) (2 = 36.6◦ ) peak of the YAG phase was broadened at the high 2-angle side. It was caused by the accumulation of the (4 2 2) peak of YAG and the (3 1 1) (2 = 36.8◦ ) peak of the MgAl2 O4 phase (ICDD-PDF # 01-082-2424). The (2 2 0) (2 = 31.3◦ ), (4 0 0) (2 = 44.8◦ ), (5 1 1) (2 = 59.3◦ ) and (4 4 0) (2 = 65.2◦ ) peaks of the MgAl2 O4 phase were also detected in YAG phosphors, and they existed for even longer soaking conditions. Y2 O3 , ␣-Al2 O3 and MgO phases were not detected in our samples.
Fig. 2. Cell parameters of (Y0.95−x Tb0.05 Mgx )3 Al5 O12 phosphors with x = 0, 0.001, 0.01, 0.025 and 0.05 (MgO = 0, 0.1, 1, 2.5 and 5 at.%, respectively) calcined at 1600 ◦ C/0, 2 and 4 h.
The cell parameters of (Y0.95−x Tb0.05 Mgx )3 Al5 O12 (x = 0, 0.001, 0.01, 0.025 and 0.05) phosphors simulated by the GSAS program are shown in Fig. 2. The final refined Rwp (weighted profile reliability factor) and 2 (goodness of fit) values are all lower than 14% and 3%, respectively. This denotes that all of these refined results are acceptable. According to the curve, when either soaking time or MgO amounts increase, the cell parameters decrease. Generally, Tb3+ has a strong trend to transform into Tb4+ . To investigate this supposition, (Y0.95−x Tb0.05 Mgx )3 Al5 O12 (x = 0, 0.01 and 0.025) phosphors were synthesized at 1600 ◦ C/4 h and their par-
Fig. 1. XRD patterns of (Y0.95−x Tb0.05 Mgx )3 Al5 O12 phosphors with x = 0, 0.01 and 0.025 calcined at 1600 ◦ C/0 and 4 h.
H.-M. Lee et al. / Journal of Alloys and Compounds 479 (2009) 759–763
Fig. 3. Partial ESCA spectra at 375–225 eV of (Y0.95−x Tb0.05 Mgx )3 Al5 O12 phosphors with x = 0, 0.01 and 0.025 calcined at 1600 ◦ C/4 h.
tial ESCA spectra are shown in Figs. 3 and 4 (the range of spectra only includes Tb’s and Y’s). After MgO doping, the intensities of the Tb 4p3/2 peaks increased sharply and the unknown peaks showed up as well as the shift of Tb 3d peaks to a higher binding energy (Table 2) were observed [16]. The binding energy can be changed due to element kind, ionic valence, the electron layer state and the crystal structure. Even with the same elements, the shift or new peaks forming will be influenced just by the difference of valence or crystal structure [17–20]. It can be supposed that the forming of a little Tb4+ in YAG:(Tb, Mg) phosphors happens easily after a prolonged soaking time or oxidizing annealing operation. Because the outer electron of Tb4+ ([Xe]4f7 ) is more difficult to be excited than Tb3+ ([Xe]4f8 ). The higher binding energy of Tb 3d corresponded to this principle. The ionic radii of Mg2+ and Tb4+ ions are both smaller than Y3+ ions in the YAG host (in dodecahedral sites: Mg2+ = 0.89 Å, Tb3+ = 1.04 Å, Tb4+ = 0.88 Å, Y3+ = 1.02 Å). From the refined results of XRD, the decrease of cell parameters (Fig. 2) can be explained by the co-existence of Mg2+ and Tb4+ occupying some dodecahedral sites. Because of the MgAl2 O4 phase appearing (Fig. 1) and the cell parameters retarding phenomenon (Fig. 2), the substitution limit of MgO (x = 0.025) can be observed (Fig. 2).
Table 2 ESCA spectral peaks of (Y0.95−x Tb0.05 Mgx )3 Al5 O12 phosphors with x = 0, 0.01 and 0.025 calcined at 1600 ◦ C/4 h. Tb (eV)
3d3/2
3d5/2
4s
4p1/2
4p3/2
4d
5s
5p
Standard values [16]
1276
1241
396
322
285
146
45
22
x=0 x = 0.01 x = 0.025
1278 1279 1279
1242 1244 1244
396 396 396
324 324 324
285 285 285
148 148 148
45 45 45
24 24 24
761
Fig. 4. Partial ESCA spectra at 960–920 eV of (Y0.95−x Tb0.05 Mgx )3 Al5 O12 phosphors with x = 0, 0.01 and 0.025 calcined at 1600 ◦ C/4 h.
The SEM micrographs of the (Y0.95−x Tb0.05 Mgx )3 Al5 O12 (x = 0, 0.01 and 0.025) phosphors synthesized at different soaking conditions are shown in Fig. 5. The morphology of the phosphors is all worm-shaped and no complete crystalline grains at 1600 ◦ C/0 h (Fig. 5a, d and g). When soaking time increased, primary particles of sprayed powders started to sinter or reacted together and grains growth can be observed. When the MgO amount increased, the retarding of grain growth was slightly found. The diffusion of Mg2+ ions into the YAG structure is not spontaneous, and additional energy is needed to overcome the reaction enthalpies [21,22]. In the initial stage of synthesis, the diffusion behavior consumes partial energies and the rich Mg2+ in the grain boundary may interrupt the diffusion of matrix compounds. It will cause the rate of grain growth to slow down. Such phenomena have already been reported [23,24]. The PL emission spectra (ex = 275 nm) of the (Y0.95 Tb0.05 )3 Al5 O12 phosphors synthesized at different soaking conditions are shown in Fig. 6. The emission spectra can be separated into two zones: one has sharp peaks due to the 5 D → 7 F and 5 D → 7 F transitions at the blue-green light zone, 5 4 6 4 and the other has weak broad peaks due to the 5 D4 → 7 F4 and 5 D → 7 F transitions at the yellow-red light zone. The emitting 4 3 intensity increased when soaking time increased. After a prolonged soaking operation, Tb3+ was dispersed in the YAG host uniformly and the possible effect of concentration quenching was retarded. Moreover, better crystallinity and morphology can also be obtained with longer calcination time (Fig. 5c), so the emitting intensity increased. The PL emission spectra (ex = 275 nm) of the (Y0.95−x Tb0.05 Mgx )3 Al5 O12 (x = 0, 0.01 and 0.025) phosphors synthesized at 1600 ◦ C/4 h are shown in Fig. 7. When the MgO amount increased to x = 0.025, the emitting intensity decreased drastically. Fig. 8 showed the absorption spectrum of pure MgAl2 O4 . According to Fig. 8, the decrease in PL emission intensity of
762
H.-M. Lee et al. / Journal of Alloys and Compounds 479 (2009) 759–763
Fig. 5. SEM micrographs of (Y0.95−x Tb0.05 Mgx )3 Al5 O12 phosphors with (a)–(c) x = 0, (d)–(f) x = 0.01 and (g)–(i) x = 0.025 calcined at 1600 ◦ C/soaking times (a) 0 h, (b) 2 h, (c) 4 h, (d) 0 h, (e) 2 h, (f) 4 h, (g) 0 h, (h) 2 h and (i) 4 h.
(Y0.925 Tb0.05 Mg0.025 )3 Al5 O12 phosphor was not due to the residual second phase, MgAl2 O4 . In addition to the morphology effect, this paper provided another explanation for Fig. 7 PL emission spectral analysis. According to Table 2 and Figs. 2–4 results, this set of compounds (Mg2+ + Tb4+ ) replaced 2Y3+ in the Y3 Al5 O12 in order to stabilize as effectively as the Tb4+ species in the eight-coordinate site of the garnet structure. Tb4+ stabilized in the YAG:(Tb, Mg) will absorb emission energy and lower PL emission intensity of
(Y0.95−x Tb0.05 Mgx )3 Al5 O12 (x = 0.01 and 0.025) phosphors. This concept is similar to Ce4+ ([Xe]) ions existing in YAG:Ce phosphors, and Ce4+ ions will decrease the luminescence of Ce3+ ions in the visible light region. Furthermore, when co-doped with Tb4+ and Mg2+ , the YAG phosphors have a brown coloration. Nevertheless, single-doped with Tb3+ , YAG phosphor just has a white color. Tb4+ is considered a poison in the YAG:(Tb, Mg) phosphor. Additional studies are necessary to explain more about Tb4+ in this spectral
Fig. 6. PL emission spectra of (Y0.95 Tb0.05 )3 Al5 O12 phosphors calcined at 1600 ◦ C/0, 2 and 4 h.
Fig. 7. PL emission spectra of (Y0.95−x Tb0.05 Mgx )3 Al5 O12 phosphors with x = 0, 0.01 and 0.025 calcined at 1600 ◦ C/4 h.
H.-M. Lee et al. / Journal of Alloys and Compounds 479 (2009) 759–763
763
sity is supposedly caused by Tb4+ ions not MgAl2 O4 compound. Mg2+ -doped YAG:Tb phosphor prepared at an air atmosphere is unsuitable for application comparing with traditional YAG:Tb. Acknowledgement This work was sponsored by the Ministry of Economic Affairs of R.O.C. under grant No. 97-EC-17-A-08-S1-023. References
Fig. 8. Absorption spectrum of pure MgAl2 O4 powder.
region. It could be demonstrated by recording XANES (X-ray absorption near edge structure) spectra at LIII edge and ELNES (electron energy loss near edge structure) of terbium ions [20,25]. The related work with this system is under study. 4. Conclusions This study combined pH-controlled mixing and spray-dried processing to synthesize YAG:Tb and YAG:(Tb, Mg) phosphors with a solid-state reaction at an oxidizing atmosphere. The YAP phases were detected in YAG:(Tb, Mg) phosphors but disappeared after a prolonged soaking time. When MgO doping content or soaking time increased, a substitution limit of MgO can be observed and the decrease of cell parameters was due to the co-existence of Mg2+ and Tb4+ in the YAG matrix. After a prolonged soaking operation, not only can activator be uniformly dispersed but better crystallinity as well as morphology can be obtained, and the Tb3+ emitting intensity increased in YAG:Tb phosphor. The Tb4+ can be found in the YAG:(Tb, Mg) system and the decrease of Tb3+ emitting inten-
[1] K. Ohno, T. Abe, J. Electrochem. Soc: Solid-State Sci. Technol. 133 (1986) 638–643. [2] J.R. Lo, T.Y. Tseng, Mater. Chem. Phys. 57 (1998) 95–98. [3] Y.C. Kang, I.W. Lenggoro, S.B. Park, K. Okuyama, J. Phys. Chem. Solids 60 (1999) 1855–1858. [4] J.Y. Choe, D. Ravichandran, S.M. Blomquist, K.W. Kirchner, E.W. Forsythe, D.C. Morton, J. Lumines. 93 (2001) 119–128. [5] K.Y. Jung, D.Y. Lee, Y.C. Kang, Mater. Res. Bull. 40 (2005) 2212–2218. [6] X. Li, H. Liu, J. Wang, H. Cui, S. Yang, I.R. Boughton, J. Phys. Chem. Solids 66 (2005) 201–205. [7] G. Xia, S. Zhou, J. Zhang, S. Wang, J. Xu, J. Alloy. Compd. 421 (2006) 294–297. [8] S. Saxena, Mater. Lett. 60 (2006) 1315–1318. [9] H.M.H. Fadlalla, C.C. Tang, Opt. Mater. 31 (2008) 401–404. [10] X. Li, Q. Yu, Z. Zeng, X. Jing, J. Rare Earths 26 (2008) 35–39. [11] S.A. Gramsch, L.R. Morss, J. Alloy. Compd. 207–208 (1994) 432–435. [12] D. Pawlak, Z. Frukacz, Z. Mierczyk, A. Suchocki, J. Zachara, J. Alloy. Compd. 275–277 (1998) 361–364. [13] J.D. Dana, in: C. Klein, C.S. Hurlbut Jr. (Eds.), Manual of Mineralogy, John Wiley and Sons, New York, 1993, pp. 221–249. [14] A.C. Larson, R.B. Von Dreele, General Structure Analysis System, National Laboratory, Los Alamos, New Mexico, 1985. [15] H.M. Rietveld, Acta Cryst. 22 (1967) 151–152. [16] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray photoelectron spectroscopy, in: J. Chastain, R.C. King Jr. (Eds.), Physical Electronics, Eden Prairie, Minnesota, 1995, pp. 154–155. [17] G. Kalkowski, G. Kaindl, G. Wortmann, D. Lentz, S. Krause, Phys. Rev. B 37 (1988) 1376–1382. ´ [18] D.A. Pawlak, K. Wozniak, Z. Frukacz, T.L. Barr, D. Fiorentino, S. Seal, J. Phys. Chem. B 103 (1999) 1454–1461. [19] H.X. Dai, C.F. Ng, C.T. Au, J. Catal. 199 (2001) 177–192. [20] A. Martínez-Arias, A.B. Hungría, M. Fernández-García, A. Iglesias-Juez, J.C. Conesa, G.C. Mather, G. Munuera, J. Power Sources 151 (2005) 43–51. [21] L. Schuh, R. Metselaar, C.R.A. Catlow, J. Eur. Ceram. Soc. 7 (1991) 67–74. [22] M.M. Kuklja, J. Phys.: Condens. Matter 12 (2000) 2953–2967. [23] S.J. Bennison, M.P. Harmer, J. Am. Ceram. Soc. 73 (1990) 833–837. [24] K.K. Soni, A.M. Thompson, M.P. Harmer, D.B. Williams, J.M. Chabala, R. Levi-Setti, Appl. Phys. Lett. 66 (1995) 2795–2797. [25] I. Tanaka, T. Mizoguchi, T. Yamamoto, J. Am. Ceram. Soc. 88 (2005) 2013–2029.