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Synthesis of Sr2MgSi2O7:Eu, Dy and Sr2MgSi2O7:Eu, Dy, Nd by a modified solid-state reaction and their luminescent properties
Journal of Alloys and Compounds 458 (2008) 564–568
Synthesis of Sr2MgSi2O7:Eu, Dy and Sr2MgSi2O7:Eu, Dy, Nd by a modified solid-state reaction and their luminescent properties Fenglan Song a , Chen Donghua b,∗ , Yuhong Yuan a a
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, Hubei Province, South-Central University for Nationalities, Wuhan 430074, Hubei, People’s Republic of China b College of Chemistry and Materials Science, South-Central University for Nationalities, Wuhan 430074, Hubei, People’s Republic of China Received 20 January 2007; received in revised form 12 April 2007; accepted 15 April 2007 Available online 25 April 2007
Abstract Long afterglow phosphors Sr2 MgSi2 O7 :Eu, Dy were synthesized by a modified solid-state reaction using H3 BO3 and CO(NH2 )2 as auxiliary reagents. Photoluminescence spectra showed that Sr2 MgSi2 O7 :Eu, Dy phosphors hold a better afterglow properties when the molar ratio of Eu to Dy was 2:5. When a certain amount of Nd ion was introduced and employed NH4 NO3 and citric acid instead of CO(NH2 )2 , the phosphors can emit red light. The analytical results indicated that there is a broader emission band centered at between 502 and 635 nm when excited by 340 nm, which is ascribed to the luminescent emission of the Eu3+ . The results of characterization showed that both of the phosphors have a single-phase Sr2 MgSi2 O7 structure and the particle sizes are in nanophase. © 2007 Elsevier B.V. All rights reserved. Keywords: Optical materials; Solid state reactions; Optical properties; Thermal analysis
1. Introduction Recently, alkaline earth silicates have attracted interest in the field of photoluminescence research since they are suitable hosts with high chemical stability. A new kind of long lasting phosphors Eu2+ , Dy3+ co-doped silicates M2 MgSi2 O7 (M = Ca, Sr) with afterglow time longer than 20 h has been developed. Compared with previously developed aluminate materials, the silicate phosphors have advantages on chemical stability, heat stability, and lower cost [1,2]. The emission of Eu ion is highly efficient and its emission wavelength is strongly dependent on the host lattice so that we can get different color from blue to red in theory [3–5]. However, in most hosts, the luminescence of Eu2+ occurs in blue and green regions. The Eu-doped alkaline earth sulfides yield red fluorescence and phosphorescence but lack environmental stability [6]. The luminescence of Eu3+ , in the case of a non-centrosymmetric environment, is generally dominated by the 5 D0 → 7 F2 transition yielding a red emitting phosphor suitable for lamps and displays [7]. But as far as we
∗
Corresponding author. Tel.: +86 27 6784185; fax: +86 27 67842752. E-mail address: [email protected] (C. Donghua).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.04.261
know, less attention has been concentrated on the research of Eu3+ doped silicates. In addition, many researchers synthesized rare earth ions doped Sr2 MgSi2 O7 phosphors by high temperature solid-state reaction and in a weak reductive atmosphere [8,9]. The method has been used intensively for phosphor synthesis, but this process often results in poor homogeneity and requires high calcinations temperature. Furthermore, the grain size of phosphor powders prepared by this method is in several tens of micrometers. Phosphors of small particles must be obtained by grinding the larger phosphor particles [10]. Those processes easily introduce additional defects and greatly reduce luminescence efficiency. Furthermore, phosphors are one of the materials that show promising behaviors when synthesized in nanophase [11]. Previously we synthesized CaTiO3 :Pr phosphors by a modified solid-state reaction [12]. In this paper, we developed the method. By combining the virtues of a solid-state reaction and a low-temperature combustion process, we successfully synthesized nanophosphors of Sr2 MgSi2 O7 :Eu, Dy. In order to keep europium in the bivalent state invariably which requires reducing atmosphere, urea was introduced. This enhances the safety on experiment and lower the cost of production. The other effect
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of urea is to condense reactants, which is appropriate to obtain nanoparticles. The technique has many advantages. On the processing side, the method needs little heat to start a fast reaction and requires a little or no further calcinations. Therefore, there is considerable savings in time and energy. An other advantage is that the method can be applied to prepare novel, low-temperature sol that the conventional method cannot. The products of the sol combustion are always in the form of ultrafine powders [13]. In addition, we tried to synthesize Sr2 MgSi2 O7 :Eu3+ , Dy3+ , Nd3+ by this method using ammonium nitrate and citric acid instead of urea to keep europium in the trivalent state. The luminescent properties of the phosphors and the effects of coactivated trivalent Dy ions on the luminescence properties of the phosphor were investigated. 2. Experiments
Fig. 1. TG/DTG curves of the thermal decomposition of the precursor at the heating rate of 10 ◦ C min−1 in air.
2.1. Synthesis Using ethyl silicate and nitrates as starting materials, Sr2 MgSi2 O7 :Eu2+ , Dy3+ phosphors were synthesized by a modified solid-state reaction. Stochiometric amounts of Eu2 O3 and Dy2 O3 were dissolved with HNO3 and completely converted into nitrate solution. Excess HNO3 was removed by evaporation in a fume cupboard. Sr(NO3 )2 , Mg(NO3 )2 ·6H2 O, H3 BO3 , CO(NH2 )2 and Si(OC2 H5 )4 were sufficiently ground in agate mortar at room temperature. Then nitrates solution was introduced and the mixture was carefully ground in agate mortar at room temperature for about 40 min. Subsequently the white sol was dried in a thermostatic oven at 100 ◦ C for about 10 h, and a white precursor was attained. Finally the white precursor was transferred to a muffle furnace preheated to 600 ◦ C, spontaneous ignition occurred and the entire process lasted for 3–5 min, then white powder was obtained. All chemicals used were of analytical purity, except the rare earth oxides, which were of 99.9%. The synthesis method of Sr2 MgSi2 O7 :Eu3+ , Dy3+ , Nd3+ is similar to that of Sr2 MgSi2 O7 :Eu2+ , Dy3+ except for using ammonium nitrate and citric acid instead of urea.
2.2. Characterization Thermal behaviors of the precursor were studied by thermogravimetric (TG) analysis using a TGS-2 thermal balance (Perkin-Elmer Co., USA). The crystal structures of the phosphors were characterized by X-ray diffraction analysis using a Burker D8 (Bruker Co., German) with Cu K␣ radiation. The morphology and dimension of the products were observed by transmission electron microscopy (TEM), which were taken on a Tecnai G20 (FEI Co., Holland) transmission electron microscope. Both of the samples for TEM examination were prepared by depositing an ultrasonically dispersed suspension of powder from a solution of alcohol on a carbon-coated copper grid. Excitation and emission spectra at room temperature were recorded using a Perkin-Elmer LS-55 (PerkinElmer Co., USA) luminescence spectrometer with a xenon discharge lamp. All the products obtained were ground to pass through a 100-mesh sieve and 0.015 g of materials was used for measurement. The long afterglow decay curves were measured with the ST-86LA (Pekin Normal University, China) brightness meter after the samples were irradiated with a 9 W conventional tricolor fluorescent lamp for 15 min.
of weight loss. The initial weight loss (3%) observed in the TG curve between 100 and 200 ◦ C results from desorption of the adsorbed moisture and the evaporation of organic solvents in the precursor. The second stages (60%) observed around 225 ◦ C is mainly ascribed to the elimination of structural water and the partial combustion of nitrates and urea. The weight loss of the last stage (15%) observed at about 550 ◦ C in the corresponding DTG curve can presumably be attributed to the decomposition of nitrates. The general theoretical weight loss calculated from the ratios of starting materials is in good agreement with TG result. The TG/DTG curves of the precursor of Sr2 MgSi2 O7 :Eu, Dy, Nd were not given because they are more complex than those of Sr2 MgSi2 O7 :Eu, Dy. The typical XRD patterns of calcined samples from the precursor obtained at the temperature of 600 ◦ C for about 5 min in air are shown in Fig. 2. The diffraction peaks are consistent with the standard JCPDS card No. 75-1736, which indicates that the co-doped Eu, Dy and Nd have little influence on the structure of luminescent materials, and all of the peaks are
3. Results and discussion 3.1. Phase formations The TG/DTG curves of the precursor of Sr2 MgSi2 O7 :Eu, Dy obtained by heating the precursor in air at a heating rate of 10 ◦ C min−1 are presented in Fig. 1. There are there main stages
Fig. 2. XRD patterns of Sr2 MgSi2 O7 :Eu, Dy phosphors powders.
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assigned to the phase of Sr2 MgSi2 O7 . This indicates that phasepure crystalline Sr2 MgSi2 O7 can be obtained at 600 ◦ C, a much lower temperature than in conventional solid-state reactions. The explanation can be that silicon dioxide obtained by the hydrolysis of Si(OC2 H5 )4 show high activity and nitrates can decompose at low temperature. The grinding treatment is effective in accelerating hydrolysis of Si(OC2 H5 )4 , decomposition of nitrates and solid-state reaction. Besides urea and citric acid can make the starting mixture to form homogeneous sol. From Fig. 2, it can also be seen that using ammonium nitrate and citric acid instead of urea the crystallinity of Sr2 MgSi2 O7 decreased, which also indicated, as a fuel, urea excelled to citric acid. 3.2. Photoluminescent properties of Sr2 MgSi2 O7 :Eu, Dy Fig. 3 shows the excitation and emission spectra of Sr2 MgSi2 O7 :Eu, Dy phosphors prepared by the modified solidstate reaction. The excitation spectra show a broad band from 230 to 420 nm when the emission wavelength was monitored at 460 nm. Its emission located at 460 nm (excited at 340 nm), which is attributed to the typical 4f6 5d1 –4f7 transition of Eu2+ . However, no special emission peaks of Eu3+ were observed in
Fig. 3. PL spectra of Sr2 MgSi2 O7 :Eu, Dy excitation spectra (a) and emission spectra (b).
Fig. 4. Decay curves of samples Sr2 MgSi2 O7 :Eu, Dy.
the spectra. This means that Eu3+ in the crystal matrix has been completely reduced to Eu2+ . The special Dy3+ emission peak is not present, which maybe ascribed to the function of the hole or electron traps and energy transporting of Dy3+ [14]. The doped materials are rare earth elements such as Dy3+ and Eu3+ . Therefore, for obtaining Eu2+ , a one-step-reduction process may be performed. The observations are in a good agreement with the results described in the literature [8]. The decay curve of Sr2 MgSi2 O7 :Eu, Dy phosphors is shown in Fig. 4 (the afterglow persistent time from the initial intensity to 1 mcd/m2 was determined). The luminescent properties are not as good as that of Sr2 MgSi2 O7 :Eu, Dy synthesized by traditional solid-state reaction. The reason may be that the combustion process took place rapidly and completed in a few minutes, which lead to a certain amount of defects in the inner phosphors. In order to find out the effects of these doped ions, we prepared Sr2 MgSi2 O7 :Dy phosphors. The excitation and emission spectra of the host Sr2 MgSi2 O7 :Dy areshown in Fig. 5. The excitation is broadband centered at 343 by monitoring the Dy3+ emission at 540 nm and the emission is also broadband
Fig. 5. PL excitation and emission spectra of Sr2 MgSi2 O7 :Dy.
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Fig. 6. PL excitation and emission spectra of Sr2 MgSi2 O7 :Eu, Dy, Nd.
centered at 540 nm when excited by 343 nm. The broadband emission for the host is probably due to the intrinsic defects of host. It is interesting that the host has yellowish-green afterglow after irradiation with visible light. The yellow emission peaking at 540 nm, 575 nm as well as a rather weak emission peaking at 668 nm were observed and can be assigned to the 4F9/2 –6H15/2 , 4F9/2 –6H13/2 and 4F9/2 –6H11/2 transitions of Dy3+ , respectively. However, the Dy3+ emissions in co-doped Sr2 MgSi2 O7 :Eu phosphors were greatly decreased due to competion from Eu2+ (5d–4f transition), in which the 5d states of Eu2+ may be located at near bottom of the conduction band of the host. From this it can be deduced that Dy3+ in the silicates act as traps but may meanwhile also act as luminescent centers [6]. The singly Dy3+ doped samples exhibit bright yellow afterglow. 3.3. Photoluminescence properties of Sr2 MgSi2 O7 :Eu, Dy, Nd Fig. 6 shows the excitation and emission spectra of Sr2 MgSi2 O7 :Eu, Dy, Nd phosphors prepared by a modified solid-state reaction using ammonium nitrate and citric acid as auxiliary reagents. The obtained Sr2 MgSi2 O7 :Eu, Dy, Nd phosphors showed a red emission band between 502 and 635 nm, which may be ascribed to the luminescent emission of the Eu3+ . The three sites available for incorporating Eu3+ , Dy3+ and Nd3+ in Sr2 MgSi2 O7 lattice are the Sr2+ sites, or the Mg2+ sites, ˚ and Si4+ (0.26 A) ˚ or the Si4+ sites. Ion radius of Mg2+ (0.72 A) 2+ ˚ it is equal in size to Eu3+ (1.07 A) ˚ are small, but for Sr (1.26 A) ˚ and similar to the radius of Dy3+ (1.03 A). ˚ and Nd3+ (1.12 A) Therefore, Eu3+ , Dy3+ and Nd3+ ions cannot incorporate into a tetrahedral [MgO4 ] and [SiO4 ], but only incorporate into an [SrO8 ] anion complexes in Sr2 MgSi2 O7 . The incorporation of Eu3+ , Dy3+ and Nd3+ ions into the Sr2 MgSi2 O7 crystal lattice do not cause any significant lattice distortions. By selecting the strong excitation peak at 251 nm, the phosphor exhibits broad emission at 502, 578 and 635 nm, instead of a characteris-
Fig. 7. TEM images of samples (a) Sr2 MgSi2 O7 :Eu, Dy (b) Sr2 MgSi2 O7 :Eu, Dy, Nd.
tic 460 nm emission which is the typical emission of divalent europium. Such unexpected trivalent Eu3+ emission have been observed from aluminate materials [15]. The profile of the emission band is typical of a multi phonon process, i.e. a system in which relaxation occurs by several paths, involving the participation of numerous states within the band gap of the material. The Sr2 MgSi2 O7 :Eu material does not show any afterglow to naked eyes but the Eu, Dy and Nd co-doped material does show persistent red light. The red emission observed in the Eu Dy and Nd co-doped systems may be ascribed to the combined effects of Eu3+ , Dy3+ and Nd3+ ions in host lattice Sr2 MgSi2 O7 and detailed explanations are under searched for.
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3.4. The morphology and dimension Fig. 7 represents the TEM micrographs of Sr2 MgSi2 O7 :Eu, Dy (a) and Sr2 MgSi2 O7 :Eu, Dy, Nd (b) synthesized directly by the modified solid-state reaction. They show that both of the samples are composed of nanocrystals. The grain size of the sample (a) is about 25 nm and the morphology of the sample is spherical, sample (b) is more irregular and the grain size is larger than in the former sample. So whether from crystal structure or morphology, urea excelled to citric acid as a fuel. The explanations are as follows: calculated from molar combustion heat of urea and citric acid, when they were added separately into the system with the same mass, using urea can achieve higher temperature instantly in theory, and the decomposition of ammonium nitrate (it is added into the system with citric acid) is an endothermic reaction, so the crystallinity of Sr2 MgSi2 O7 :Eu, Dy is better. Judging from combustion reaction equations of them, the density of gas released from combustion process of urea is less but the volume is larger, this is easy to form a porous structure, so the grain size of Sr2 MgSi2 O7 :Eu, Dy is smaller. 4. Conclusion 1. Long afterglow phosphors Sr2 MgSi2 O7 :Eu2+ , Dy3+ and Sr2 MgSi2 O7 :Eu3+ , Dy3+ , Nd3+ have been synthesized by a modified solid-state reaction. The process does not require external energy, and the reaction velocity is very fast. In industry, it can be employed to prepare many kinds of useful nanopowders in materials such as ceramics and green catalysts in order to save resources and time. 2. The obtained Sr2 MgSi2 O7 :Eu, Dy, Nd phosphors showed a red emission band between 502 and 635 nm, which is ascribed to the luminescent emission of Eu3+ . The effects of co-activated trivalent Dy ions on the luminescence properties of the phosphor were not only as a trap center, but also as a
luminescence center. More detailed explanations are under processing. 3. The grain size of two phosphors powders prepared by the method is in several tens of nanometers. The optical properties of this material lead us to state that it may serve as a promising material for using as lamp phosphor in the red region. Acknowledgement The financial support from the Key Natural Science Fund of Science and Technology Department of Hubei Province under grant No. 2001ABA009 for this work is greatly appreciated. References [1] Z.G. Xiao, Z.Q. Xiao, United States Patent 6,093,346. [2] Z.G. Xiao, Z.Q. Xiao, EP 972815A1. [3] B. Liu, C.S. Shi, M. Yin, L. Dong, Z.G. Xiao, J. Alloys Compd. 387 (2005) 65–69. [4] C.S. Shi, Y.B. Fu, B. Liu, G.B. Zhang, Y.H. Chen, Z.M. Qi, X.X. Luo, J. Lumin. 122–123 (2007) 11–13. [5] L. Jiang, C.K. Chang, D.L. Mao, J. Alloys Compd. 360 (2003) 193–197. [6] Z.Y. He, X.J. Wang, W.M. Yen, J. Lumin. 122–123 (2007) 381–384. [7] S.M. Wang, M.K. Lu, G.J. Zhou, H.P. Zhang, Z.S. Yang, J. Alloys Compd. 452 (2008) 432–434. [8] B. Liu, L.J. Kong, C.S. Shi, J. Lumin. 122–123 (2007) 121–124. [9] Y. Song, J.H. Zhang, X. Zhang, X.J. Wang, J. Lumin. 122–123 (2007) 914–916. [10] C.L. Zhao, D.H. Chen, Y.H. Yuan, M. Wu, Mater. Sci. Eng. B 133 (2006) 200–204. [11] H. Chander, D. Haranath, V. Shanker, P. Sharma, H. Chander, J. Cryst. Growth 271 (2004) 307–312. [12] S.Y. Yin, D.H. Chen, W.J. Tang, Y.H. Peng, Mater. Sci. Eng. B 136 (2007) 193–196. [13] Y. Wu, Y.L. Qi, Y.J. Ma, X. Li, X.J. Guo, J.Z. Gao, M. Chen, Mater. Chem. Phys. 84 (2004) 52–57. [14] A.A. Sabbagh Alvania, F. Moztarzadehb, A.A. Sarabi, J. Lumin. 114 (2005) 131–136. [15] Y.K. Song, S.K. Choi, H.S. Moon, Mater. Res. Bull. 32 (1997) 337.
Synthesis of Sr2MgSi2O7:Eu, Dy and Sr2MgSi2O7:Eu, Dy, Nd by a modified solid-state reaction and their luminescent properties Fenglan Song a , Chen Donghua b,∗ , Yuhong Yuan a a
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, Hubei Province, South-Central University for Nationalities, Wuhan 430074, Hubei, People’s Republic of China b College of Chemistry and Materials Science, South-Central University for Nationalities, Wuhan 430074, Hubei, People’s Republic of China Received 20 January 2007; received in revised form 12 April 2007; accepted 15 April 2007 Available online 25 April 2007
Abstract Long afterglow phosphors Sr2 MgSi2 O7 :Eu, Dy were synthesized by a modified solid-state reaction using H3 BO3 and CO(NH2 )2 as auxiliary reagents. Photoluminescence spectra showed that Sr2 MgSi2 O7 :Eu, Dy phosphors hold a better afterglow properties when the molar ratio of Eu to Dy was 2:5. When a certain amount of Nd ion was introduced and employed NH4 NO3 and citric acid instead of CO(NH2 )2 , the phosphors can emit red light. The analytical results indicated that there is a broader emission band centered at between 502 and 635 nm when excited by 340 nm, which is ascribed to the luminescent emission of the Eu3+ . The results of characterization showed that both of the phosphors have a single-phase Sr2 MgSi2 O7 structure and the particle sizes are in nanophase. © 2007 Elsevier B.V. All rights reserved. Keywords: Optical materials; Solid state reactions; Optical properties; Thermal analysis
1. Introduction Recently, alkaline earth silicates have attracted interest in the field of photoluminescence research since they are suitable hosts with high chemical stability. A new kind of long lasting phosphors Eu2+ , Dy3+ co-doped silicates M2 MgSi2 O7 (M = Ca, Sr) with afterglow time longer than 20 h has been developed. Compared with previously developed aluminate materials, the silicate phosphors have advantages on chemical stability, heat stability, and lower cost [1,2]. The emission of Eu ion is highly efficient and its emission wavelength is strongly dependent on the host lattice so that we can get different color from blue to red in theory [3–5]. However, in most hosts, the luminescence of Eu2+ occurs in blue and green regions. The Eu-doped alkaline earth sulfides yield red fluorescence and phosphorescence but lack environmental stability [6]. The luminescence of Eu3+ , in the case of a non-centrosymmetric environment, is generally dominated by the 5 D0 → 7 F2 transition yielding a red emitting phosphor suitable for lamps and displays [7]. But as far as we
∗
Corresponding author. Tel.: +86 27 6784185; fax: +86 27 67842752. E-mail address: [email protected] (C. Donghua).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.04.261
know, less attention has been concentrated on the research of Eu3+ doped silicates. In addition, many researchers synthesized rare earth ions doped Sr2 MgSi2 O7 phosphors by high temperature solid-state reaction and in a weak reductive atmosphere [8,9]. The method has been used intensively for phosphor synthesis, but this process often results in poor homogeneity and requires high calcinations temperature. Furthermore, the grain size of phosphor powders prepared by this method is in several tens of micrometers. Phosphors of small particles must be obtained by grinding the larger phosphor particles [10]. Those processes easily introduce additional defects and greatly reduce luminescence efficiency. Furthermore, phosphors are one of the materials that show promising behaviors when synthesized in nanophase [11]. Previously we synthesized CaTiO3 :Pr phosphors by a modified solid-state reaction [12]. In this paper, we developed the method. By combining the virtues of a solid-state reaction and a low-temperature combustion process, we successfully synthesized nanophosphors of Sr2 MgSi2 O7 :Eu, Dy. In order to keep europium in the bivalent state invariably which requires reducing atmosphere, urea was introduced. This enhances the safety on experiment and lower the cost of production. The other effect
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of urea is to condense reactants, which is appropriate to obtain nanoparticles. The technique has many advantages. On the processing side, the method needs little heat to start a fast reaction and requires a little or no further calcinations. Therefore, there is considerable savings in time and energy. An other advantage is that the method can be applied to prepare novel, low-temperature sol that the conventional method cannot. The products of the sol combustion are always in the form of ultrafine powders [13]. In addition, we tried to synthesize Sr2 MgSi2 O7 :Eu3+ , Dy3+ , Nd3+ by this method using ammonium nitrate and citric acid instead of urea to keep europium in the trivalent state. The luminescent properties of the phosphors and the effects of coactivated trivalent Dy ions on the luminescence properties of the phosphor were investigated. 2. Experiments
Fig. 1. TG/DTG curves of the thermal decomposition of the precursor at the heating rate of 10 ◦ C min−1 in air.
2.1. Synthesis Using ethyl silicate and nitrates as starting materials, Sr2 MgSi2 O7 :Eu2+ , Dy3+ phosphors were synthesized by a modified solid-state reaction. Stochiometric amounts of Eu2 O3 and Dy2 O3 were dissolved with HNO3 and completely converted into nitrate solution. Excess HNO3 was removed by evaporation in a fume cupboard. Sr(NO3 )2 , Mg(NO3 )2 ·6H2 O, H3 BO3 , CO(NH2 )2 and Si(OC2 H5 )4 were sufficiently ground in agate mortar at room temperature. Then nitrates solution was introduced and the mixture was carefully ground in agate mortar at room temperature for about 40 min. Subsequently the white sol was dried in a thermostatic oven at 100 ◦ C for about 10 h, and a white precursor was attained. Finally the white precursor was transferred to a muffle furnace preheated to 600 ◦ C, spontaneous ignition occurred and the entire process lasted for 3–5 min, then white powder was obtained. All chemicals used were of analytical purity, except the rare earth oxides, which were of 99.9%. The synthesis method of Sr2 MgSi2 O7 :Eu3+ , Dy3+ , Nd3+ is similar to that of Sr2 MgSi2 O7 :Eu2+ , Dy3+ except for using ammonium nitrate and citric acid instead of urea.
2.2. Characterization Thermal behaviors of the precursor were studied by thermogravimetric (TG) analysis using a TGS-2 thermal balance (Perkin-Elmer Co., USA). The crystal structures of the phosphors were characterized by X-ray diffraction analysis using a Burker D8 (Bruker Co., German) with Cu K␣ radiation. The morphology and dimension of the products were observed by transmission electron microscopy (TEM), which were taken on a Tecnai G20 (FEI Co., Holland) transmission electron microscope. Both of the samples for TEM examination were prepared by depositing an ultrasonically dispersed suspension of powder from a solution of alcohol on a carbon-coated copper grid. Excitation and emission spectra at room temperature were recorded using a Perkin-Elmer LS-55 (PerkinElmer Co., USA) luminescence spectrometer with a xenon discharge lamp. All the products obtained were ground to pass through a 100-mesh sieve and 0.015 g of materials was used for measurement. The long afterglow decay curves were measured with the ST-86LA (Pekin Normal University, China) brightness meter after the samples were irradiated with a 9 W conventional tricolor fluorescent lamp for 15 min.
of weight loss. The initial weight loss (3%) observed in the TG curve between 100 and 200 ◦ C results from desorption of the adsorbed moisture and the evaporation of organic solvents in the precursor. The second stages (60%) observed around 225 ◦ C is mainly ascribed to the elimination of structural water and the partial combustion of nitrates and urea. The weight loss of the last stage (15%) observed at about 550 ◦ C in the corresponding DTG curve can presumably be attributed to the decomposition of nitrates. The general theoretical weight loss calculated from the ratios of starting materials is in good agreement with TG result. The TG/DTG curves of the precursor of Sr2 MgSi2 O7 :Eu, Dy, Nd were not given because they are more complex than those of Sr2 MgSi2 O7 :Eu, Dy. The typical XRD patterns of calcined samples from the precursor obtained at the temperature of 600 ◦ C for about 5 min in air are shown in Fig. 2. The diffraction peaks are consistent with the standard JCPDS card No. 75-1736, which indicates that the co-doped Eu, Dy and Nd have little influence on the structure of luminescent materials, and all of the peaks are
3. Results and discussion 3.1. Phase formations The TG/DTG curves of the precursor of Sr2 MgSi2 O7 :Eu, Dy obtained by heating the precursor in air at a heating rate of 10 ◦ C min−1 are presented in Fig. 1. There are there main stages
Fig. 2. XRD patterns of Sr2 MgSi2 O7 :Eu, Dy phosphors powders.
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assigned to the phase of Sr2 MgSi2 O7 . This indicates that phasepure crystalline Sr2 MgSi2 O7 can be obtained at 600 ◦ C, a much lower temperature than in conventional solid-state reactions. The explanation can be that silicon dioxide obtained by the hydrolysis of Si(OC2 H5 )4 show high activity and nitrates can decompose at low temperature. The grinding treatment is effective in accelerating hydrolysis of Si(OC2 H5 )4 , decomposition of nitrates and solid-state reaction. Besides urea and citric acid can make the starting mixture to form homogeneous sol. From Fig. 2, it can also be seen that using ammonium nitrate and citric acid instead of urea the crystallinity of Sr2 MgSi2 O7 decreased, which also indicated, as a fuel, urea excelled to citric acid. 3.2. Photoluminescent properties of Sr2 MgSi2 O7 :Eu, Dy Fig. 3 shows the excitation and emission spectra of Sr2 MgSi2 O7 :Eu, Dy phosphors prepared by the modified solidstate reaction. The excitation spectra show a broad band from 230 to 420 nm when the emission wavelength was monitored at 460 nm. Its emission located at 460 nm (excited at 340 nm), which is attributed to the typical 4f6 5d1 –4f7 transition of Eu2+ . However, no special emission peaks of Eu3+ were observed in
Fig. 3. PL spectra of Sr2 MgSi2 O7 :Eu, Dy excitation spectra (a) and emission spectra (b).
Fig. 4. Decay curves of samples Sr2 MgSi2 O7 :Eu, Dy.
the spectra. This means that Eu3+ in the crystal matrix has been completely reduced to Eu2+ . The special Dy3+ emission peak is not present, which maybe ascribed to the function of the hole or electron traps and energy transporting of Dy3+ [14]. The doped materials are rare earth elements such as Dy3+ and Eu3+ . Therefore, for obtaining Eu2+ , a one-step-reduction process may be performed. The observations are in a good agreement with the results described in the literature [8]. The decay curve of Sr2 MgSi2 O7 :Eu, Dy phosphors is shown in Fig. 4 (the afterglow persistent time from the initial intensity to 1 mcd/m2 was determined). The luminescent properties are not as good as that of Sr2 MgSi2 O7 :Eu, Dy synthesized by traditional solid-state reaction. The reason may be that the combustion process took place rapidly and completed in a few minutes, which lead to a certain amount of defects in the inner phosphors. In order to find out the effects of these doped ions, we prepared Sr2 MgSi2 O7 :Dy phosphors. The excitation and emission spectra of the host Sr2 MgSi2 O7 :Dy areshown in Fig. 5. The excitation is broadband centered at 343 by monitoring the Dy3+ emission at 540 nm and the emission is also broadband
Fig. 5. PL excitation and emission spectra of Sr2 MgSi2 O7 :Dy.
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Fig. 6. PL excitation and emission spectra of Sr2 MgSi2 O7 :Eu, Dy, Nd.
centered at 540 nm when excited by 343 nm. The broadband emission for the host is probably due to the intrinsic defects of host. It is interesting that the host has yellowish-green afterglow after irradiation with visible light. The yellow emission peaking at 540 nm, 575 nm as well as a rather weak emission peaking at 668 nm were observed and can be assigned to the 4F9/2 –6H15/2 , 4F9/2 –6H13/2 and 4F9/2 –6H11/2 transitions of Dy3+ , respectively. However, the Dy3+ emissions in co-doped Sr2 MgSi2 O7 :Eu phosphors were greatly decreased due to competion from Eu2+ (5d–4f transition), in which the 5d states of Eu2+ may be located at near bottom of the conduction band of the host. From this it can be deduced that Dy3+ in the silicates act as traps but may meanwhile also act as luminescent centers [6]. The singly Dy3+ doped samples exhibit bright yellow afterglow. 3.3. Photoluminescence properties of Sr2 MgSi2 O7 :Eu, Dy, Nd Fig. 6 shows the excitation and emission spectra of Sr2 MgSi2 O7 :Eu, Dy, Nd phosphors prepared by a modified solid-state reaction using ammonium nitrate and citric acid as auxiliary reagents. The obtained Sr2 MgSi2 O7 :Eu, Dy, Nd phosphors showed a red emission band between 502 and 635 nm, which may be ascribed to the luminescent emission of the Eu3+ . The three sites available for incorporating Eu3+ , Dy3+ and Nd3+ in Sr2 MgSi2 O7 lattice are the Sr2+ sites, or the Mg2+ sites, ˚ and Si4+ (0.26 A) ˚ or the Si4+ sites. Ion radius of Mg2+ (0.72 A) 2+ ˚ it is equal in size to Eu3+ (1.07 A) ˚ are small, but for Sr (1.26 A) ˚ and similar to the radius of Dy3+ (1.03 A). ˚ and Nd3+ (1.12 A) Therefore, Eu3+ , Dy3+ and Nd3+ ions cannot incorporate into a tetrahedral [MgO4 ] and [SiO4 ], but only incorporate into an [SrO8 ] anion complexes in Sr2 MgSi2 O7 . The incorporation of Eu3+ , Dy3+ and Nd3+ ions into the Sr2 MgSi2 O7 crystal lattice do not cause any significant lattice distortions. By selecting the strong excitation peak at 251 nm, the phosphor exhibits broad emission at 502, 578 and 635 nm, instead of a characteris-
Fig. 7. TEM images of samples (a) Sr2 MgSi2 O7 :Eu, Dy (b) Sr2 MgSi2 O7 :Eu, Dy, Nd.
tic 460 nm emission which is the typical emission of divalent europium. Such unexpected trivalent Eu3+ emission have been observed from aluminate materials [15]. The profile of the emission band is typical of a multi phonon process, i.e. a system in which relaxation occurs by several paths, involving the participation of numerous states within the band gap of the material. The Sr2 MgSi2 O7 :Eu material does not show any afterglow to naked eyes but the Eu, Dy and Nd co-doped material does show persistent red light. The red emission observed in the Eu Dy and Nd co-doped systems may be ascribed to the combined effects of Eu3+ , Dy3+ and Nd3+ ions in host lattice Sr2 MgSi2 O7 and detailed explanations are under searched for.
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3.4. The morphology and dimension Fig. 7 represents the TEM micrographs of Sr2 MgSi2 O7 :Eu, Dy (a) and Sr2 MgSi2 O7 :Eu, Dy, Nd (b) synthesized directly by the modified solid-state reaction. They show that both of the samples are composed of nanocrystals. The grain size of the sample (a) is about 25 nm and the morphology of the sample is spherical, sample (b) is more irregular and the grain size is larger than in the former sample. So whether from crystal structure or morphology, urea excelled to citric acid as a fuel. The explanations are as follows: calculated from molar combustion heat of urea and citric acid, when they were added separately into the system with the same mass, using urea can achieve higher temperature instantly in theory, and the decomposition of ammonium nitrate (it is added into the system with citric acid) is an endothermic reaction, so the crystallinity of Sr2 MgSi2 O7 :Eu, Dy is better. Judging from combustion reaction equations of them, the density of gas released from combustion process of urea is less but the volume is larger, this is easy to form a porous structure, so the grain size of Sr2 MgSi2 O7 :Eu, Dy is smaller. 4. Conclusion 1. Long afterglow phosphors Sr2 MgSi2 O7 :Eu2+ , Dy3+ and Sr2 MgSi2 O7 :Eu3+ , Dy3+ , Nd3+ have been synthesized by a modified solid-state reaction. The process does not require external energy, and the reaction velocity is very fast. In industry, it can be employed to prepare many kinds of useful nanopowders in materials such as ceramics and green catalysts in order to save resources and time. 2. The obtained Sr2 MgSi2 O7 :Eu, Dy, Nd phosphors showed a red emission band between 502 and 635 nm, which is ascribed to the luminescent emission of Eu3+ . The effects of co-activated trivalent Dy ions on the luminescence properties of the phosphor were not only as a trap center, but also as a
luminescence center. More detailed explanations are under processing. 3. The grain size of two phosphors powders prepared by the method is in several tens of nanometers. The optical properties of this material lead us to state that it may serve as a promising material for using as lamp phosphor in the red region. Acknowledgement The financial support from the Key Natural Science Fund of Science and Technology Department of Hubei Province under grant No. 2001ABA009 for this work is greatly appreciated. References [1] Z.G. Xiao, Z.Q. Xiao, United States Patent 6,093,346. [2] Z.G. Xiao, Z.Q. Xiao, EP 972815A1. [3] B. Liu, C.S. Shi, M. Yin, L. Dong, Z.G. Xiao, J. Alloys Compd. 387 (2005) 65–69. [4] C.S. Shi, Y.B. Fu, B. Liu, G.B. Zhang, Y.H. Chen, Z.M. Qi, X.X. Luo, J. Lumin. 122–123 (2007) 11–13. [5] L. Jiang, C.K. Chang, D.L. Mao, J. Alloys Compd. 360 (2003) 193–197. [6] Z.Y. He, X.J. Wang, W.M. Yen, J. Lumin. 122–123 (2007) 381–384. [7] S.M. Wang, M.K. Lu, G.J. Zhou, H.P. Zhang, Z.S. Yang, J. Alloys Compd. 452 (2008) 432–434. [8] B. Liu, L.J. Kong, C.S. Shi, J. Lumin. 122–123 (2007) 121–124. [9] Y. Song, J.H. Zhang, X. Zhang, X.J. Wang, J. Lumin. 122–123 (2007) 914–916. [10] C.L. Zhao, D.H. Chen, Y.H. Yuan, M. Wu, Mater. Sci. Eng. B 133 (2006) 200–204. [11] H. Chander, D. Haranath, V. Shanker, P. Sharma, H. Chander, J. Cryst. Growth 271 (2004) 307–312. [12] S.Y. Yin, D.H. Chen, W.J. Tang, Y.H. Peng, Mater. Sci. Eng. B 136 (2007) 193–196. [13] Y. Wu, Y.L. Qi, Y.J. Ma, X. Li, X.J. Guo, J.Z. Gao, M. Chen, Mater. Chem. Phys. 84 (2004) 52–57. [14] A.A. Sabbagh Alvania, F. Moztarzadehb, A.A. Sarabi, J. Lumin. 114 (2005) 131–136. [15] Y.K. Song, S.K. Choi, H.S. Moon, Mater. Res. Bull. 32 (1997) 337.