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Morphology control and VUV photoluminescence characteristics of BaMgAl10O17:Eu2+ phosphors
ARTICLE IN PRESS
Physica B 392 (2007) 1–6 www.elsevier.com/locate/physb
Morphology control and VUV photoluminescence characteristics of BaMgAl10O17:Eu2+ phosphors Zhe Chen, Youwei Yan State Key Lab of Die and Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China Received 7 June 2006; received in revised form 22 October 2006; accepted 25 October 2006
Abstract BaMgAl10O17:Eu2+ (BAM) phosphors for plasma display panel application were successfully prepared by a novel solution combustion synthesis method. Morphology control of BAM phosphor particles in the process was attempted by using polyethylene glycol (PEG) as additive. The crystallinity, particle size, morphology and luminescent properties were characterized by XRD, FE-SEM and vacuum ultraviolet (VUV) irradiation derived synchrotron radiation, respectively. It was found that BAM phosphors resulted from PEG additive have nearly spherical morphology, narrow size distribution and strong blue emission under VUV light excitation compared to those without PEG additive. r 2006 Elsevier B.V. All rights reserved. PACS: 78.55.m; 78.55.Hx Keywords: BAM phosphors; Morphology; Luminescence; Solution combustion synthesis
1. Introduction Eu2+ activated barium magnesium hexa-aluminate, BaMgAl10O17:Eu2+ (BAM), has been widely used for the phosphor in fluorescent lights (FL), field emission displays (EED), high resolution (HR) and projection TVs (PTVs). Recently, increasing attention is being paid to BAM since it is applied in plasma display panels (PDP) as a component of the blue-emission phosphor [1–4]. For the application in PDP, the phosphor particles with small size, narrow size distribution, and spherical shape are very important for good luminescent characteristics. At present, commercial BAM phosphors are popularly prepared by conventional solid-state synthesis method, which has some disadvantages, such as process complexity, energy consuming, inhomogeneous mixing and contamination by impurities. The BAM phosphor particles produced by this method are then in big size range and irregular Corresponding author. Tel.: +86 27 87543876; fax: +86 27 87541922.
E-mail address: [email protected] (Y. Yan). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.10.036
shapes, which are inadequate for the application in HR display. As a result, many attempts are been carried out to find alternative methods for synthesis of BAM phosphors. Solution combustion synthesis (SCS) could be a promising method to prepare high-purity, small-sized phosphors because initial raw materials are homogeneously mixed in liquid phases, and low boiling point impurities are volatilized by the instantly generated energy from exothermic reaction [5–10]. In addition, particles within a narrow distribution could be obtained using a SCS method due to a short reaction time of a few seconds. Further improvement of luminance efficiency of BAM phosphors could be achieved by controlling morphology of phosphor particles using additive in a synthesis process. Experiments have proved that more small-sized spherical particles can be synthesized with additive added [11,12]. Compared to polyhedral particles, these particles could enhance optical performance due to the high packing density and also the reduction of light scattering [13]. In the present work, pure well-crystallized BAM phosphors were prepared using a facile SCS method by
ARTICLE IN PRESS 2
Z. Chen, Y. Yan / Physica B 392 (2007) 1–6
controlling the phosphor particle morphology using polyethylene glycol (PEG) (HO(C2H4O)nH) as additive in the precursor solution. The photoluminescence properties of the phosphors were investigated using XRD, FE-SEM and vacuum ultraviolet (VUV) irradiation. 2. Experimental The starting raw materials were high purity Ba(NO3)2, Mg(NO3)2 6H2O, Al(NO3)3 9H2O, and Eu(NO3)3. A stoichiometric amount of raw materials, according to the nominal composition of Ba0.85Eu0.15MgAl10O17, were dissolved in deionized water in a cylindrical Pyrex dish, and urea was then added to the solution with a 2.36:1 mole ratio of urea to nitrate based on the total oxidizing and reducing valencies of the oxidizer and the fuel (urea) [14]. Also, PEG with MW of 2000, 4000, 6000, 10000 and 12000, corresponding to PEG to BAM weight ratios of 1, 2.5, 5, 5.5 and 6.5, respectively, were added to the solutions. And then the solutions contained in dishes were heated in a muffle furnace at a temperature of 55075 1C for combustion reaction. The whole process, from solution boiling, dehydration to mixture swelling, and to burning, spent a few minutes. Subsequently, fluffy white products were synthesized from homogeneously mixing solutions. The crystal phase of the prepared BAM phosphors was characterized using a X’Pert PRO X-ray diffractometer with a Cu Ka radiation (l ¼ 1.5406 A˚). The particle size and morphology of products, coated with gold to minimize the charge problems, were evaluated using Sirion 200 field emission electron microscopy (FE-SEM). The powder densities were measured by employing a pycnometrer with xylene as the liquid medium. The BET surface area was determined by nitrogen adsorption applying a Micromeritics ASAP 2000 surface area analyzer. The size distribution of particles was measured using the laser scattering particle size distribution analyzer (Brookhaven Instruments Corporation). The excitation and emission spectra of the phosphors in VUV region were obtained by a luminescent analyzer using synchrotron radiation as a light source, which was produced from the BSRF storage ring with electron energy of 800 MeV and a beam current of 230 mA at National Synchrotron Radiation Laboratory (University of Science and Technology of China, Hefei, China). 3. Results and discussion 3.1. Crystal structure and morphology Fig. 1 shows the XRD pattern of the BAM phosphors obtained by the solution combustion method with 5% PEG (MW ¼ 10,000) and without PEG as additive. It is clear that both XRD patterns are similar and in excellent agreement with the pattern of barium magnesium aluminate (BAM) registered in the Joint Committee on Powder Diffraction Strands card (JCPDS 26-0163). All the
Fig. 1. XRD patterns of the as-prepared BAM sample with and without PEG as well as the JCPDS card 26-0163 for BAM.
diffraction lines are assigned well to BAM crystalline phase with the b-alumina structure corresponding to the space group P63 ¼ mmc and no extraneous diffraction peaks are found in the spectra, indicating that the phosphors obtained by the combustion process are monophasic BAM. Single BAM phosphors from combustion synthesis are due to a fast combustion reaction, which eliminates an intermediate formation process occurring in a conventional solid-state diffusion reaction [11]. Furthermore, the peak intensities of BAM with PEG as additive are much stronger than that without PEG, which indicates that PEG enhances the BAM crystallinity. The typical scheme of the synthesis with and without PEG is shown in Eqs. (1) and (2). Different from the reaction process without PEG, an exothermic reaction takes place in a thick absorbed layer of PEG on the BAM particle surfaces when a precursor solution with PEG burns. As a result, higher crystallinity is produced due to absorption of extra heat by BAM particles. 30AlðNO3 Þ3 þ 3ð1 xÞBaðNO3 Þ2 þ 3xEuðNO3 Þ3 þ 85COðNO2 Þ2 ! 3Bað1xÞ Eux MgAl10 O17 þ 85CO2 " þ85N2 " þ170H2 O " ,
ð1Þ
ARTICLE IN PRESS Z. Chen, Y. Yan / Physica B 392 (2007) 1–6
Fig. 2. FE-SEM micrographs of the BAM samples with different PEG concentrations: (a) 0%, (b) 1%, (c) 2.5% and (d) 5%.
Fig. 3. Size distribution of the BAM samples with different PEG concentrations: 0%, 1%, 2.5% and 5%.
3
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4
30AlðNO3 Þ3 þ 3ð1 xÞBaðNO3 Þ2 þ 3MgðNO3 Þ2 þ 3xEuðNO3 Þ3 þ 85COðNO2 Þ2 þ 2yHOðC2 H4 OÞn H þ 5ynO2 ! 3Bað1xÞ Eux MgAl10 O17 þ ð85 þ 4ynÞCO2 " þ 85N2 " þð172 þ 4yn þ 2yÞH2 O ðgasÞ " . ð2Þ 3.2. The effect of PEG concentration on BAM morphology Fig. 2 shows the FE-SEM micrographs of the asprepared BAM samples with PEG (MW ¼ 10,000) concentrations from 0% to 5%. As BaMgAl10O17 has the crystal structure of b-alumina, the particle takes the form of hexagonal platelets when the crystal freely grows [15]. Large numbers of platelet-like particles and severe conglomeration can be obviously seen in Fig. 2(a) related to a Table 1 BET surface area with different PEG concentrations (fixing MW at 10,000) PEG concentration C (%)
Powder density (g cm3)
BET surface area (m2g1)
0 1 2.5 5
3.192 3.174 3.143 3.127
53.87 57.34 61.54 63.95
sample without PEG. Its size distribution shown in Fig. 3 (PEG ¼ 0), exhibiting a wide size range with bimodal distributions, totally differs from those with PEG shown in Fig. 3 (PEG ¼ 1%, 2.5% and 5%). Obviously, the morphology (Figs. 2(b) to (d)) and size distribution (Fig. 3) of the obtained particles, depending on PEG concentration added, were greatly improved when PEG was used to change a nucleation process of crystal. The nuclei, formed in a super-saturated solution with PEG, are strongly absorbed and surrounded by PEG chains. And nearly spherical shape of particle is therefore formed because of equal strain force from these chains in all directions around the nuclei. In addition, the adsorption of PEG polymer on the particle surfaces can prevent particle–particle aggregation due to steric hindrance effect [16,17]. Platelet-like particles were almost indiscernible for a BAM sample with 1% PEG although the aggregation occurred yet (Fig. 2(b)). When the PEG concentration was 2.5%, the morphology was improved and the accumulation also decreased (Fig. 2(c)). Nearly spherical and nonagglomeration particles (Fig. 2(d)) with narrow size distribution and larger BET surface area (Fig. 3, Table 1) were obtained with 5% PEG concentration. However, further increasing PEG concentration of more than 5.5% resulted in lower emission intensity because of the effect of residual carbon in sample (see below for luminescent property). As a result,
Fig. 4. FE-SEM micrographs of the BAM samples with different PEG MW: (a) 2000, (b) 4000 and (c) 6000.
ARTICLE IN PRESS Z. Chen, Y. Yan / Physica B 392 (2007) 1–6
3.3. The effect of PEG molecular weight (MW) on BAM morphology When the MW of PEG increases, the length of the molecular chains increases accordingly, leading to the augmentation of hydrophilic –OH groups. As a result, the long PEG chains can be well absorbed on the surface of the BAM particles by some chemical bond and physical interaction, such as the Van der Waals force, polar interaction, which enhanced the surface strain force of the BAM particles. Therefore, large MW of PEG benefits the formation of nearly spherical, nonaggregation BAM phosphors. Fig. 4 show the FM-SEM images of samples, to which were added 5% PEG with MW of 2000, 4000 and 6000. With increasing MW of PEG, the particle shape and size distribution were meliorated, the extent of agglomeration was degraded, and the BET surface area increased accordingly (Table 2). When the MW of the PEG was increased to 10000, the morphology and size distribution of particles reached optimization (Fig. 3(d)). 3.4. VUV luminescence property The excitation spectra of the BAM phosphor prepared by SCS are shown in Fig. 5. The excitation spectra in VUV region from 130 to 200 nm, obtained by monitoring the blue emission at 450 nm, show a wide band from 150 to 190 nm with peak wavelength of 173 nm. This peak maximum corresponds to the band-to-band excitation of the host crystal [18], viz. the electrons are promoted from the valence band to the conduction band. This indicates that the prepared BAM is a promising blue-emitting phosphor for PDP displays. In UV region from 200 to 350 nm, the excitation spectra show two wide bands peaking at 248 and 306 nm, respectively. These peak maximums are the characteristic electronic transition of Eu2+ ions between the ground state and the crystal field splitting 5d levels. The emission spectra of the BAM phosphors shown in Fig. 6, excited by 173 nm VUV radiation, were acquired with different PEG MW. The emission spectra indicate the strongest luminescence at 450 nm for the BAM phosphors,
corresponding to the electronic transition of Eu2+ from the 4f65d excited state to the 4f7 ground state. No characteristic peaks of the Eu3+ are observed. Evidently, the Eu3+ ions initially in BaMgAl10O17 have been effectively reduced to Eu2+ ions by the reducing gases (NOx, H2O and NH3) generated from organic matter in a combustion reaction. This eliminates an addition post-treatment process of reducing Eu3+ to Eu2+ by H2/N2, which has been carried out in a conventional solid-state reaction [11]. MW effects on luminescent intensity are also shown in Fig. 6. The emission intensities of the BAM are enhanced when the MW of PEG increase with a maximum at MW ¼ 10,000, exhibiting an agreement with the change of the morphology of the phosphor particles shown in Fig. 4. Better morphologies reduce nonradiative transition probability, lower light scattering, and get higher packing densities, therefore provide higher emission intensity [19].
4000 3500 Intensity (a.u)
a 5% PEG concentration is suitable for obtaining nearly spherical nonagglomeration particles.
5
3000 2500 2000 1500 140 160 180 200 220 240 260 280 300 320 340 360 Wavelength (nm)
Fig. 5. Excitation spectra of the BAM samples.
Table 2 BET surface area with different PEG MW (fixing PEG concentration at 5%) PEG molecular weight MW
Powder density (g cm3)
BET surface area (m2g1)
2000 4000 6000 10,000 12,000
3.169 3.157 3.148 3.127 3.127
58.45 60.22 61.42 63.95 63.94
Fig. 6. Emission spectra of the BAM phosphors with different PEG molecular weights (MW).
ARTICLE IN PRESS Z. Chen, Y. Yan / Physica B 392 (2007) 1–6
6
3500
emission under VUV light excitation were acquired in optimum conditions of 5% PEG and MW of 10,000.
Reletive Intensity (a.u)
3400 3300
Acknowledgments
3200
The authors are grateful to the Natural Science Foundation of China (No. 50276023 and 50574042) for support. The authors also thank the National Synchrotron Radiation Laboratory (University of Science and Technology of China, Hefei, China) for offering VUV test.
3100 3000 2900 2800 2700 2600
References 0
1
2 4 3 5 PEG concentration (%)
6
7
Fig. 7. The relative emission intensity of the BAM samples with different PEG concentrations.
In addition, a PEG concentration in a precursor solution can affect the luminescent properties of BAM phosphors since it has influence on the particle morphology and size distribution. Fig. 7 shows the change of emission intensity of BAM with PEG concentration. The optimum PEG concentration was found to be 5%. At the higher concentration of more than 5% the emission intensity decreased probably due to residual carbon in BAM particles. Similar case was also reported by Wang et al. [16]. 4. Conclusions Monophasic BaMgAl10O17:Eu2+ (BAM) blue phosphors were synthesized using a novel solution combustion synthesis method. Polyethylene glycol (PEG) was applied as additive to control morphology in a preparing process. The effects of PEG concentration and MW of PEG on morphology and emission intensity of BAM have been studied and the properties of obtained BAM were characterized by XRD, FE-SEM and vacuum ultraviolet (VUV). Significant improvements of nearly spherical morphology, narrow size distribution and strong blue
[1] C.R. Ronda, J. Lumin. 72–74 (1997) 49. [2] S. Oshio, T. Matsuoka, S. Tanaka, H. Kobayashi, J. Electrochem. Soc. 145 (1998) 3898. [3] C.R. Ronda, J. Alloys Compounds 225 (1995) 234. [4] Y.L. Liu, C.S. Shi, Mater. Res. Bull. 36 (2001) 109. [5] P. Yang, G.Q. Yao, J.H. Lin, Inorg. Chem. Commun. 7 (2004) 389. [6] S.K. Shi, J.Y. Wang, J. Alloys Compounds 327 (2001) 82. [7] F. Gu, S.F. Wang, M.K. Lu, W.G. Zou, G.J. Zhou, D. Xu, D.R. Yuan, J. Cryst. Growth. 260 (2004) 507. [8] S. Ekambaram, K.C. Patil, M. Maaza, J. Alloys Compounds 393 (2005) 81. [9] J. McKittrick, L.E. Shea, C.F. Bacalski, E.J. Bosze, Displays 19 (1999) 169. [10] K.C. Patil, S.T. Aruna, S. Ekambaram, Curr. Opin. Solid State Mater. Sci. 2 (1997) 158. [11] S. Oshio, K. Kitamura, T. Shigeta, S. Horii, T. Matsuoka, S. Tanaka, H. Kobayashi, J. Electrochem. Soc. 146 (1999) 392. [12] A. Vecht, C. Gibbons, D. Davies, X. Jing, P. Marsh, T. Ireland, J. Silver, A. Newport, D. Barber, J. Vac. Sci. Technol. B 17 (1999) 750. [13] A.k. Albessard, N. Matsuda, M. Tamatani, S. Yokota, A. Terajima, H. hatattori, K. Akitsuki, T. Suzuk, in: Proceedings of the Seventh International Display Workshop, SID, December 2000, p. 877. [14] S.R. Jain, K.C. Adiga, V.R. Pai Verneker, Combust. Flame 40 (1981) 71. [15] D. Ravichandran, S.T. Johnson, S. Erdei, R. Roy, W.B. White, Displays 19 (1999) 197. [16] X. Wang, M. Wang, H. Song, B. Ding, Mater. Lett. 60 (2006) 2261. [17] M. Abdullah, K. Okuyama, I.W. Lenggoro, S. Taya., J. Non-Cryst. Solids 351 (2005) 697. [18] B. Howe, A.L. Diaz, J. Lumin. 109 (2004) 51. [19] Y.C. Kang, I.W. Lenggoro, S.B. Park, K. Okuyama, J. Phys. Chem. Solids 60 (1999) 1855.
Physica B 392 (2007) 1–6 www.elsevier.com/locate/physb
Morphology control and VUV photoluminescence characteristics of BaMgAl10O17:Eu2+ phosphors Zhe Chen, Youwei Yan State Key Lab of Die and Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China Received 7 June 2006; received in revised form 22 October 2006; accepted 25 October 2006
Abstract BaMgAl10O17:Eu2+ (BAM) phosphors for plasma display panel application were successfully prepared by a novel solution combustion synthesis method. Morphology control of BAM phosphor particles in the process was attempted by using polyethylene glycol (PEG) as additive. The crystallinity, particle size, morphology and luminescent properties were characterized by XRD, FE-SEM and vacuum ultraviolet (VUV) irradiation derived synchrotron radiation, respectively. It was found that BAM phosphors resulted from PEG additive have nearly spherical morphology, narrow size distribution and strong blue emission under VUV light excitation compared to those without PEG additive. r 2006 Elsevier B.V. All rights reserved. PACS: 78.55.m; 78.55.Hx Keywords: BAM phosphors; Morphology; Luminescence; Solution combustion synthesis
1. Introduction Eu2+ activated barium magnesium hexa-aluminate, BaMgAl10O17:Eu2+ (BAM), has been widely used for the phosphor in fluorescent lights (FL), field emission displays (EED), high resolution (HR) and projection TVs (PTVs). Recently, increasing attention is being paid to BAM since it is applied in plasma display panels (PDP) as a component of the blue-emission phosphor [1–4]. For the application in PDP, the phosphor particles with small size, narrow size distribution, and spherical shape are very important for good luminescent characteristics. At present, commercial BAM phosphors are popularly prepared by conventional solid-state synthesis method, which has some disadvantages, such as process complexity, energy consuming, inhomogeneous mixing and contamination by impurities. The BAM phosphor particles produced by this method are then in big size range and irregular Corresponding author. Tel.: +86 27 87543876; fax: +86 27 87541922.
E-mail address: [email protected] (Y. Yan). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.10.036
shapes, which are inadequate for the application in HR display. As a result, many attempts are been carried out to find alternative methods for synthesis of BAM phosphors. Solution combustion synthesis (SCS) could be a promising method to prepare high-purity, small-sized phosphors because initial raw materials are homogeneously mixed in liquid phases, and low boiling point impurities are volatilized by the instantly generated energy from exothermic reaction [5–10]. In addition, particles within a narrow distribution could be obtained using a SCS method due to a short reaction time of a few seconds. Further improvement of luminance efficiency of BAM phosphors could be achieved by controlling morphology of phosphor particles using additive in a synthesis process. Experiments have proved that more small-sized spherical particles can be synthesized with additive added [11,12]. Compared to polyhedral particles, these particles could enhance optical performance due to the high packing density and also the reduction of light scattering [13]. In the present work, pure well-crystallized BAM phosphors were prepared using a facile SCS method by
ARTICLE IN PRESS 2
Z. Chen, Y. Yan / Physica B 392 (2007) 1–6
controlling the phosphor particle morphology using polyethylene glycol (PEG) (HO(C2H4O)nH) as additive in the precursor solution. The photoluminescence properties of the phosphors were investigated using XRD, FE-SEM and vacuum ultraviolet (VUV) irradiation. 2. Experimental The starting raw materials were high purity Ba(NO3)2, Mg(NO3)2 6H2O, Al(NO3)3 9H2O, and Eu(NO3)3. A stoichiometric amount of raw materials, according to the nominal composition of Ba0.85Eu0.15MgAl10O17, were dissolved in deionized water in a cylindrical Pyrex dish, and urea was then added to the solution with a 2.36:1 mole ratio of urea to nitrate based on the total oxidizing and reducing valencies of the oxidizer and the fuel (urea) [14]. Also, PEG with MW of 2000, 4000, 6000, 10000 and 12000, corresponding to PEG to BAM weight ratios of 1, 2.5, 5, 5.5 and 6.5, respectively, were added to the solutions. And then the solutions contained in dishes were heated in a muffle furnace at a temperature of 55075 1C for combustion reaction. The whole process, from solution boiling, dehydration to mixture swelling, and to burning, spent a few minutes. Subsequently, fluffy white products were synthesized from homogeneously mixing solutions. The crystal phase of the prepared BAM phosphors was characterized using a X’Pert PRO X-ray diffractometer with a Cu Ka radiation (l ¼ 1.5406 A˚). The particle size and morphology of products, coated with gold to minimize the charge problems, were evaluated using Sirion 200 field emission electron microscopy (FE-SEM). The powder densities were measured by employing a pycnometrer with xylene as the liquid medium. The BET surface area was determined by nitrogen adsorption applying a Micromeritics ASAP 2000 surface area analyzer. The size distribution of particles was measured using the laser scattering particle size distribution analyzer (Brookhaven Instruments Corporation). The excitation and emission spectra of the phosphors in VUV region were obtained by a luminescent analyzer using synchrotron radiation as a light source, which was produced from the BSRF storage ring with electron energy of 800 MeV and a beam current of 230 mA at National Synchrotron Radiation Laboratory (University of Science and Technology of China, Hefei, China). 3. Results and discussion 3.1. Crystal structure and morphology Fig. 1 shows the XRD pattern of the BAM phosphors obtained by the solution combustion method with 5% PEG (MW ¼ 10,000) and without PEG as additive. It is clear that both XRD patterns are similar and in excellent agreement with the pattern of barium magnesium aluminate (BAM) registered in the Joint Committee on Powder Diffraction Strands card (JCPDS 26-0163). All the
Fig. 1. XRD patterns of the as-prepared BAM sample with and without PEG as well as the JCPDS card 26-0163 for BAM.
diffraction lines are assigned well to BAM crystalline phase with the b-alumina structure corresponding to the space group P63 ¼ mmc and no extraneous diffraction peaks are found in the spectra, indicating that the phosphors obtained by the combustion process are monophasic BAM. Single BAM phosphors from combustion synthesis are due to a fast combustion reaction, which eliminates an intermediate formation process occurring in a conventional solid-state diffusion reaction [11]. Furthermore, the peak intensities of BAM with PEG as additive are much stronger than that without PEG, which indicates that PEG enhances the BAM crystallinity. The typical scheme of the synthesis with and without PEG is shown in Eqs. (1) and (2). Different from the reaction process without PEG, an exothermic reaction takes place in a thick absorbed layer of PEG on the BAM particle surfaces when a precursor solution with PEG burns. As a result, higher crystallinity is produced due to absorption of extra heat by BAM particles. 30AlðNO3 Þ3 þ 3ð1 xÞBaðNO3 Þ2 þ 3xEuðNO3 Þ3 þ 85COðNO2 Þ2 ! 3Bað1xÞ Eux MgAl10 O17 þ 85CO2 " þ85N2 " þ170H2 O " ,
ð1Þ
ARTICLE IN PRESS Z. Chen, Y. Yan / Physica B 392 (2007) 1–6
Fig. 2. FE-SEM micrographs of the BAM samples with different PEG concentrations: (a) 0%, (b) 1%, (c) 2.5% and (d) 5%.
Fig. 3. Size distribution of the BAM samples with different PEG concentrations: 0%, 1%, 2.5% and 5%.
3
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4
30AlðNO3 Þ3 þ 3ð1 xÞBaðNO3 Þ2 þ 3MgðNO3 Þ2 þ 3xEuðNO3 Þ3 þ 85COðNO2 Þ2 þ 2yHOðC2 H4 OÞn H þ 5ynO2 ! 3Bað1xÞ Eux MgAl10 O17 þ ð85 þ 4ynÞCO2 " þ 85N2 " þð172 þ 4yn þ 2yÞH2 O ðgasÞ " . ð2Þ 3.2. The effect of PEG concentration on BAM morphology Fig. 2 shows the FE-SEM micrographs of the asprepared BAM samples with PEG (MW ¼ 10,000) concentrations from 0% to 5%. As BaMgAl10O17 has the crystal structure of b-alumina, the particle takes the form of hexagonal platelets when the crystal freely grows [15]. Large numbers of platelet-like particles and severe conglomeration can be obviously seen in Fig. 2(a) related to a Table 1 BET surface area with different PEG concentrations (fixing MW at 10,000) PEG concentration C (%)
Powder density (g cm3)
BET surface area (m2g1)
0 1 2.5 5
3.192 3.174 3.143 3.127
53.87 57.34 61.54 63.95
sample without PEG. Its size distribution shown in Fig. 3 (PEG ¼ 0), exhibiting a wide size range with bimodal distributions, totally differs from those with PEG shown in Fig. 3 (PEG ¼ 1%, 2.5% and 5%). Obviously, the morphology (Figs. 2(b) to (d)) and size distribution (Fig. 3) of the obtained particles, depending on PEG concentration added, were greatly improved when PEG was used to change a nucleation process of crystal. The nuclei, formed in a super-saturated solution with PEG, are strongly absorbed and surrounded by PEG chains. And nearly spherical shape of particle is therefore formed because of equal strain force from these chains in all directions around the nuclei. In addition, the adsorption of PEG polymer on the particle surfaces can prevent particle–particle aggregation due to steric hindrance effect [16,17]. Platelet-like particles were almost indiscernible for a BAM sample with 1% PEG although the aggregation occurred yet (Fig. 2(b)). When the PEG concentration was 2.5%, the morphology was improved and the accumulation also decreased (Fig. 2(c)). Nearly spherical and nonagglomeration particles (Fig. 2(d)) with narrow size distribution and larger BET surface area (Fig. 3, Table 1) were obtained with 5% PEG concentration. However, further increasing PEG concentration of more than 5.5% resulted in lower emission intensity because of the effect of residual carbon in sample (see below for luminescent property). As a result,
Fig. 4. FE-SEM micrographs of the BAM samples with different PEG MW: (a) 2000, (b) 4000 and (c) 6000.
ARTICLE IN PRESS Z. Chen, Y. Yan / Physica B 392 (2007) 1–6
3.3. The effect of PEG molecular weight (MW) on BAM morphology When the MW of PEG increases, the length of the molecular chains increases accordingly, leading to the augmentation of hydrophilic –OH groups. As a result, the long PEG chains can be well absorbed on the surface of the BAM particles by some chemical bond and physical interaction, such as the Van der Waals force, polar interaction, which enhanced the surface strain force of the BAM particles. Therefore, large MW of PEG benefits the formation of nearly spherical, nonaggregation BAM phosphors. Fig. 4 show the FM-SEM images of samples, to which were added 5% PEG with MW of 2000, 4000 and 6000. With increasing MW of PEG, the particle shape and size distribution were meliorated, the extent of agglomeration was degraded, and the BET surface area increased accordingly (Table 2). When the MW of the PEG was increased to 10000, the morphology and size distribution of particles reached optimization (Fig. 3(d)). 3.4. VUV luminescence property The excitation spectra of the BAM phosphor prepared by SCS are shown in Fig. 5. The excitation spectra in VUV region from 130 to 200 nm, obtained by monitoring the blue emission at 450 nm, show a wide band from 150 to 190 nm with peak wavelength of 173 nm. This peak maximum corresponds to the band-to-band excitation of the host crystal [18], viz. the electrons are promoted from the valence band to the conduction band. This indicates that the prepared BAM is a promising blue-emitting phosphor for PDP displays. In UV region from 200 to 350 nm, the excitation spectra show two wide bands peaking at 248 and 306 nm, respectively. These peak maximums are the characteristic electronic transition of Eu2+ ions between the ground state and the crystal field splitting 5d levels. The emission spectra of the BAM phosphors shown in Fig. 6, excited by 173 nm VUV radiation, were acquired with different PEG MW. The emission spectra indicate the strongest luminescence at 450 nm for the BAM phosphors,
corresponding to the electronic transition of Eu2+ from the 4f65d excited state to the 4f7 ground state. No characteristic peaks of the Eu3+ are observed. Evidently, the Eu3+ ions initially in BaMgAl10O17 have been effectively reduced to Eu2+ ions by the reducing gases (NOx, H2O and NH3) generated from organic matter in a combustion reaction. This eliminates an addition post-treatment process of reducing Eu3+ to Eu2+ by H2/N2, which has been carried out in a conventional solid-state reaction [11]. MW effects on luminescent intensity are also shown in Fig. 6. The emission intensities of the BAM are enhanced when the MW of PEG increase with a maximum at MW ¼ 10,000, exhibiting an agreement with the change of the morphology of the phosphor particles shown in Fig. 4. Better morphologies reduce nonradiative transition probability, lower light scattering, and get higher packing densities, therefore provide higher emission intensity [19].
4000 3500 Intensity (a.u)
a 5% PEG concentration is suitable for obtaining nearly spherical nonagglomeration particles.
5
3000 2500 2000 1500 140 160 180 200 220 240 260 280 300 320 340 360 Wavelength (nm)
Fig. 5. Excitation spectra of the BAM samples.
Table 2 BET surface area with different PEG MW (fixing PEG concentration at 5%) PEG molecular weight MW
Powder density (g cm3)
BET surface area (m2g1)
2000 4000 6000 10,000 12,000
3.169 3.157 3.148 3.127 3.127
58.45 60.22 61.42 63.95 63.94
Fig. 6. Emission spectra of the BAM phosphors with different PEG molecular weights (MW).
ARTICLE IN PRESS Z. Chen, Y. Yan / Physica B 392 (2007) 1–6
6
3500
emission under VUV light excitation were acquired in optimum conditions of 5% PEG and MW of 10,000.
Reletive Intensity (a.u)
3400 3300
Acknowledgments
3200
The authors are grateful to the Natural Science Foundation of China (No. 50276023 and 50574042) for support. The authors also thank the National Synchrotron Radiation Laboratory (University of Science and Technology of China, Hefei, China) for offering VUV test.
3100 3000 2900 2800 2700 2600
References 0
1
2 4 3 5 PEG concentration (%)
6
7
Fig. 7. The relative emission intensity of the BAM samples with different PEG concentrations.
In addition, a PEG concentration in a precursor solution can affect the luminescent properties of BAM phosphors since it has influence on the particle morphology and size distribution. Fig. 7 shows the change of emission intensity of BAM with PEG concentration. The optimum PEG concentration was found to be 5%. At the higher concentration of more than 5% the emission intensity decreased probably due to residual carbon in BAM particles. Similar case was also reported by Wang et al. [16]. 4. Conclusions Monophasic BaMgAl10O17:Eu2+ (BAM) blue phosphors were synthesized using a novel solution combustion synthesis method. Polyethylene glycol (PEG) was applied as additive to control morphology in a preparing process. The effects of PEG concentration and MW of PEG on morphology and emission intensity of BAM have been studied and the properties of obtained BAM were characterized by XRD, FE-SEM and vacuum ultraviolet (VUV). Significant improvements of nearly spherical morphology, narrow size distribution and strong blue
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