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Microwave-assisted synthesis of BaWO4 nanoparticles and its photoluminescence properties
Materials Letters 65 (2011) 2956–2958
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Microwave-assisted synthesis of BaWO4 nanoparticles and its photoluminescence properties Yanhua Shen a, Wen Li b, Taohai Li a, c,⁎ a
College of Chemistry, Key Lab of Environment Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan, 411105, China Key Laboratory of Low Dimensional Materials and Application Technology (Ministry of Education) and Faculty of Materials and Optoelectronic Physics, Xiangtan University, Xiangtan, 411105, China c State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, China b
a r t i c l e
i n f o
Article history: Received 20 May 2011 Accepted 8 June 2011 Available online 15 June 2011 Keywords: BaWO4 Microwave-assisted Nanoparticles Optical materials and properties
a b s t r a c t Single-crystal BaWO4 nanoparticles have been successfully synthesized under microwave irradiation. The results show that nearly monodisperse BaWO4 nanoparticles have been successfully prepared without using surfactants. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and photoluminescence (PL). The XRD results indicated that the BaWO4 nanoparticles obtained had a tetragonal unit cell (a = 0.5612, c = 1.2706 nm). The TEM images show that the as-prepared BaWO4 have good narrow particle-sized distributions containing a number of nanoparticles with uniform sizes. The products show a strong photoluminescence peak at 432–436 nm with the excitation at 365 nm. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, inorganic nanomaterials based on the scheelitetype structure, especially BaWO4 has attracted much attention because of its strong luminescence and potential applications in stimulated Raman scattering [1], nuclear spin optical hole burning hosts [2], photocatalysts [3], scintillating medium and electro-optic applications [4]. As we know, the properties and applications of nanomaterials depend not only on their composition, but also on their structure, shape and size distribution. Thus, morphology-controlled synthesis of nanomaterials with well-defined shapes is an important goal in the field of modern nanomaterials. The traditional preparation methods of BaWO4 include flux method [5], solid-state reaction [6], and hydrothermal–electrochemical method et al. [7]. Most of traditional methods adopt conventional convective heating to provide the required energy and drive the reaction. Conventional thermal technique will cause sharp thermal gradients throughout the bulk solution and inefficient, nonuniform reaction conditions, leading to high reaction temperature or long reaction time. It's more important that the nonuniform reaction conditions will introduce impurities into final products, and affect the uniform nucleation and growth rates of nanomaterials. Microwave heating methods can avoid the problem of heating inhomogeneity. In fact, it has been demonstrated that microwave ⁎ Corresponding author at: College of Chemistry, Key Lab of Environment Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan, 411105, China. Tel.: + 86 731 58292202; fax: + 86 731 58292251. E-mail address: [email protected] (T. Li). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.06.033
irradiation can enhance reaction rates, selectivity, and product yields [8–12], which owes to the efficient transformation of energy and the homogeneity of temperature distribution in the reaction vessel. Li et al. have synthesized single crystal BaWO4 nanosheets and nanobelts through microwave irradiation [13]. Compared with those reported preparation methods, it was confirmed that the preparation of BaWO4 with microwave heating method shows a reduction of reaction time and temperature. However, the PVP templates are necessary in their preparation. In this paper, we adopt a simple and rapid microwaveassisted synthesis method obtaining nearly monodisperse BaWO4 nanoparticles successfully. In this method, we don't use any surfactants as templates, and the products exhibited good photoluminescence (PL) properties. It should be emphasized that the reaction medium must have an adequately high dielectric constant (ε) for microwave absorption. Water is a polar solvent, which makes it absorb more microwave irradiation and be the best reaction medium for microwave irradiation. Thus, we choose water as reaction medium here. 2. Experimental 2.1. Synthesis of BaWO4 NaWO4·2H2O (0.3500 g) and BaCl2 (0.2600 g) were added into 10 ml H2O. By microwave irradiation, the mixture was heated for different reaction time. After cooling to room temperature naturally, the white precipitation was collected by centrifugation and washed several times with deionized water and absolute ethanol. Then the washed precipitation was dried in vacuum oven at 60 °C for 12 h.
Y. Shen et al. / Materials Letters 65 (2011) 2956–2958
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3. Results and discussion 3.1. XRD patterns analysis Fig. 1 shows typical XRD patterns of as-products heated for 10 and 15 min by microwave irradiation at 180 °C. The results indicated that the BaWO4 nanomaterial had a tetragonal unit cell (a = 0.5612, c = 1.2706 nm). All diffraction peaks are in good agreement with those in the JCPDS card no. 43-0646. No impurities have been detected. Furthermore, the sharp diffraction peaks indicated good crystallization of BaWO4. The relative peak intensity of the two nanostructures varies significantly. It is found that the crystallization of BaWO4 crystal can be controlled by changing the reaction time when reaction temperature keeps at 180 °C. 3.2. TEM micrographs Fig. 1. XRD patterns of BaWO4 nanostructures prepared at 180 °C for 10 min (a) and 15 min (b).
2.2. Characterization The sample was synthesized by microwave instrument (Initiator 8 Exp). Powder X-ray diffraction (XRD) analysis was carried out on a MiniFlex II diffractometer with Cu Kα radiation (λ = 0.15406 nm). The transmission electron microscope (TEM) images were recorded on a JEM-2010 microscope at an accelerating voltage of 200 kV. The fluorescence spectral analysis was conducted on a PerkineElmer LS55 luminescence spectrometer.
Fig. 2 displays TEM images of products synthesized at 180 °C for 10 min (a) and 15 min (b) (samples A and B). The average size of sample A and B is 31 and 37 nm in length, respectively (Fig. 2a and b). The results indicate strong influence of reaction time on the morphology. The selected area electron diffraction (SAED) pattern of BaWO4 marked by the white dots exhibits a set of diffraction spots, demonstrating the single crystalline nature of the BaWO4 (Fig. 2c). A HRTEM image shows the lattice planes spacing of about 0.25 nm which corresponds to the (202) planes (Fig. 2d). Furthermore, it could be concluded that BaWO4 particles were assembled by means of oriented attachment along the [101] direction. The typical HRTEM (Fig. 2b) was selected to explore the possible growth pathways. It was possible that the BaWO4 nanobricks self-assembled through oriented attachment [14,15]. The TEM images show the morphologies of
Fig. 2. TEM morphologies of the BaWO4 nanostructure at different reaction time, 10 min (a) and 15 min (b–d).
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Y. Shen et al. / Materials Letters 65 (2011) 2956–2958
Cavalcante [8] believes that microwave heating is able to promote the formation of defects and distortions on [WO4] tetrahedrons. The disorder caused by the system is a favorable condition to generate intense PL band [24]. Above all, although Chen et al. [23] attributed the PL emission to the charge-transfer transitions between the O2p orbits and the empty d orbits of the central W 6+ ions in WO42− complex, we believe that the PL properties of the BaWO4 nanoparticles are also strongly dependent on its surface defects and distortions. 4. Conclusions In this work, nearly monodisperse BaWO4 nanoparticles have been synthesized under microwave irradiation in the pure water system without using surfactants. The PL emission peaks of the BaWO4 are in the spectral region at 432–436 nm. Fig. 3. Room temperature photoluminescence spectra of samples formed at the same temperature of 180 °C for different time; 10 min (a) and 15 min (b).
Acknowledgments samples A and B. Both samples have good narrow particle-sized distributions containing a number of nanoparticles with uniform sizes, which improves their luminescent properties [16].
The authors thank the financial support of the Key Laboratory of Quantum Engineering and Micro-Nano Energy Technology, Department of Education of Hunan Province, China (no. 09QNET07).
3.3. PL spectra
References
The PL spectra of the as-prepared BaWO4 were studied at room temperature. The luminescent property of BaWO4 is a result of its scheelite-type structure. In our study, with excitation at 365 nm, BaWO4 nanostructures show the intrinsic emission peaks with their surrounding at 432–436 nm (Fig. 3). This broad band suggests that the emission process is a typical multiphonon or multilevel process. The emission peaks were different from those reported BaWO4 nanostructures [17–19], which mainly attributed to the charge-transfer transitions within the WO42− complexes [20,21]. In addition, the intensity of PL emission of BaWO4 heated for 15 min is noticeably stronger than that of BaWO4 heated for 10 min. The PL intensity is controlled by the number of charged transfers and surface defects [22]. Moreover, the emission peaks obviously shift to the region of long wavelength with reaction time increase, which may be due to the particle-forming effect and the increased size of nanoparticles. Although there are different opinions explaining the origin of the emission bands and the nature of the optical transition is unclear, the WO42− complex and the slight deviation from a perfect crystal structure are believed to be responsible for the emission bands. The luminescent properties are mainly determined by charge-transfer transitions between the O2p orbits and the empty d orbits of the central W 6+ ions in WO42− complex [23]. Furthermore, the PL is related to the structural disorder in the lattice. The tungsten atoms and water molecules are good microwave absorbers. In this case, the interaction between microwave radiation and tungsten atoms resulted in a rapid heating process on the [WO4] tetrahedron groups.
[1] Cerny P, Jelinkova H, Zverev PG, Basiev TT. Prog Quantum Electron 2004;28: 113–43. [2] Caprez A, Meyer P, Hulliger J. Mater Res Bull 1997;32:1045–54. [3] Afanasiev P. Mater Lett 2007;61:4622–6. [4] Nikl M, Bohacek P, Mihokova E, Kobayashi M, Ishii M, Usuki Y, et al. J Lumin 2000;87:1136–9. [5] Roy BN, Roy MR. Cryst Res Technol 1981;16:1267–71. [6] Fujita T, Yamaoka S, Fukunaga O. Mater Res Bull 1974;9:141–6. [7] Cho WS, Yoshimura M. Jpn J Appl Phys 1997;36:1216–9. [8] (a) Tsuji M, Hashimoto M, Nishizawa Y, Kubokawa M, Tsuji T. J Chem Eur 2005;11:440–52. (b) Cavalcante LS, Sczancoski JC, Lima LF, Espinosa Jr JWM, Pizani PS. Cryst Growth Des 2005;9:1002–12. [9] Wu L, Bi JH, Li ZH, Wang XX, Fu XZ. Catal Today 2008;131:15–20. [10] Luo ZJ, Li HM, Shu HM, Wang K, Xia JX, Yan YS. Mater Chem Phys 2008;110:17–20. [11] Chang M, Chung CC, Deka JR, Lin CH, Chung TW. Nanotechnology 2008;19:025710. [12] Hu HY, Yang H, Huang P, Cui DX, Zhang JC, Lu FY, et al. Chem Commun 2010;46: 3866–8. [13] Luo ZJ, Li HM, Xia JX, Zhu WS, Gao JX, Zhang BB. Mater Lett 2007;61:1845–8. [14] Penn RL, Banfield JF. Science 1998;281:969–71. [15] Wang M, Huang QL, Zhong HX, Chen XT, Xue ZL, You XZ. Cryst Growth Des 2007;7: 2106–11. [16] Thongtem T, Kungwankunakorn S, Phuruangrat A. J Alloy Compd 2010;506: 475–81. [17] Yin YK, Gan ZB, Sun YZ, Zhou BB, Zhang X, Zhang DW, et al. Mater Lett 2010;64: 789–92. [18] Wang R, Liu C, Zeng J, Li KW, Wang H. J Solid State Chem 2009;182:677–84. [19] Tyagi M, Sangeeta, Sabharwal SC. J Lumin 2008;128:1528–32. [20] Groeninl JA, Hakfoort C, Blasse G. Phys Status Solidi A 1979;54:329–36. [21] Ryu J, Yoon J, Lim C, Shim K. J Alloy Compd 2005;390:245–9. [22] Thongtem T, Kaowphong S, Thongtem S. Appl Surf Sci 2008;254:7765–9. [23] Chen SJ, Zhou JH, Chen XT. Chem Phys Lett 2003;375:85–90. [24] Lima RC, Anicete-Santos M, Orhan E, Maurera MAMA, Souza AG, Pizani PS, et al. J Lumin 2007;126:741–6.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Microwave-assisted synthesis of BaWO4 nanoparticles and its photoluminescence properties Yanhua Shen a, Wen Li b, Taohai Li a, c,⁎ a
College of Chemistry, Key Lab of Environment Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan, 411105, China Key Laboratory of Low Dimensional Materials and Application Technology (Ministry of Education) and Faculty of Materials and Optoelectronic Physics, Xiangtan University, Xiangtan, 411105, China c State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, China b
a r t i c l e
i n f o
Article history: Received 20 May 2011 Accepted 8 June 2011 Available online 15 June 2011 Keywords: BaWO4 Microwave-assisted Nanoparticles Optical materials and properties
a b s t r a c t Single-crystal BaWO4 nanoparticles have been successfully synthesized under microwave irradiation. The results show that nearly monodisperse BaWO4 nanoparticles have been successfully prepared without using surfactants. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and photoluminescence (PL). The XRD results indicated that the BaWO4 nanoparticles obtained had a tetragonal unit cell (a = 0.5612, c = 1.2706 nm). The TEM images show that the as-prepared BaWO4 have good narrow particle-sized distributions containing a number of nanoparticles with uniform sizes. The products show a strong photoluminescence peak at 432–436 nm with the excitation at 365 nm. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, inorganic nanomaterials based on the scheelitetype structure, especially BaWO4 has attracted much attention because of its strong luminescence and potential applications in stimulated Raman scattering [1], nuclear spin optical hole burning hosts [2], photocatalysts [3], scintillating medium and electro-optic applications [4]. As we know, the properties and applications of nanomaterials depend not only on their composition, but also on their structure, shape and size distribution. Thus, morphology-controlled synthesis of nanomaterials with well-defined shapes is an important goal in the field of modern nanomaterials. The traditional preparation methods of BaWO4 include flux method [5], solid-state reaction [6], and hydrothermal–electrochemical method et al. [7]. Most of traditional methods adopt conventional convective heating to provide the required energy and drive the reaction. Conventional thermal technique will cause sharp thermal gradients throughout the bulk solution and inefficient, nonuniform reaction conditions, leading to high reaction temperature or long reaction time. It's more important that the nonuniform reaction conditions will introduce impurities into final products, and affect the uniform nucleation and growth rates of nanomaterials. Microwave heating methods can avoid the problem of heating inhomogeneity. In fact, it has been demonstrated that microwave ⁎ Corresponding author at: College of Chemistry, Key Lab of Environment Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan, 411105, China. Tel.: + 86 731 58292202; fax: + 86 731 58292251. E-mail address: [email protected] (T. Li). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.06.033
irradiation can enhance reaction rates, selectivity, and product yields [8–12], which owes to the efficient transformation of energy and the homogeneity of temperature distribution in the reaction vessel. Li et al. have synthesized single crystal BaWO4 nanosheets and nanobelts through microwave irradiation [13]. Compared with those reported preparation methods, it was confirmed that the preparation of BaWO4 with microwave heating method shows a reduction of reaction time and temperature. However, the PVP templates are necessary in their preparation. In this paper, we adopt a simple and rapid microwaveassisted synthesis method obtaining nearly monodisperse BaWO4 nanoparticles successfully. In this method, we don't use any surfactants as templates, and the products exhibited good photoluminescence (PL) properties. It should be emphasized that the reaction medium must have an adequately high dielectric constant (ε) for microwave absorption. Water is a polar solvent, which makes it absorb more microwave irradiation and be the best reaction medium for microwave irradiation. Thus, we choose water as reaction medium here. 2. Experimental 2.1. Synthesis of BaWO4 NaWO4·2H2O (0.3500 g) and BaCl2 (0.2600 g) were added into 10 ml H2O. By microwave irradiation, the mixture was heated for different reaction time. After cooling to room temperature naturally, the white precipitation was collected by centrifugation and washed several times with deionized water and absolute ethanol. Then the washed precipitation was dried in vacuum oven at 60 °C for 12 h.
Y. Shen et al. / Materials Letters 65 (2011) 2956–2958
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3. Results and discussion 3.1. XRD patterns analysis Fig. 1 shows typical XRD patterns of as-products heated for 10 and 15 min by microwave irradiation at 180 °C. The results indicated that the BaWO4 nanomaterial had a tetragonal unit cell (a = 0.5612, c = 1.2706 nm). All diffraction peaks are in good agreement with those in the JCPDS card no. 43-0646. No impurities have been detected. Furthermore, the sharp diffraction peaks indicated good crystallization of BaWO4. The relative peak intensity of the two nanostructures varies significantly. It is found that the crystallization of BaWO4 crystal can be controlled by changing the reaction time when reaction temperature keeps at 180 °C. 3.2. TEM micrographs Fig. 1. XRD patterns of BaWO4 nanostructures prepared at 180 °C for 10 min (a) and 15 min (b).
2.2. Characterization The sample was synthesized by microwave instrument (Initiator 8 Exp). Powder X-ray diffraction (XRD) analysis was carried out on a MiniFlex II diffractometer with Cu Kα radiation (λ = 0.15406 nm). The transmission electron microscope (TEM) images were recorded on a JEM-2010 microscope at an accelerating voltage of 200 kV. The fluorescence spectral analysis was conducted on a PerkineElmer LS55 luminescence spectrometer.
Fig. 2 displays TEM images of products synthesized at 180 °C for 10 min (a) and 15 min (b) (samples A and B). The average size of sample A and B is 31 and 37 nm in length, respectively (Fig. 2a and b). The results indicate strong influence of reaction time on the morphology. The selected area electron diffraction (SAED) pattern of BaWO4 marked by the white dots exhibits a set of diffraction spots, demonstrating the single crystalline nature of the BaWO4 (Fig. 2c). A HRTEM image shows the lattice planes spacing of about 0.25 nm which corresponds to the (202) planes (Fig. 2d). Furthermore, it could be concluded that BaWO4 particles were assembled by means of oriented attachment along the [101] direction. The typical HRTEM (Fig. 2b) was selected to explore the possible growth pathways. It was possible that the BaWO4 nanobricks self-assembled through oriented attachment [14,15]. The TEM images show the morphologies of
Fig. 2. TEM morphologies of the BaWO4 nanostructure at different reaction time, 10 min (a) and 15 min (b–d).
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Y. Shen et al. / Materials Letters 65 (2011) 2956–2958
Cavalcante [8] believes that microwave heating is able to promote the formation of defects and distortions on [WO4] tetrahedrons. The disorder caused by the system is a favorable condition to generate intense PL band [24]. Above all, although Chen et al. [23] attributed the PL emission to the charge-transfer transitions between the O2p orbits and the empty d orbits of the central W 6+ ions in WO42− complex, we believe that the PL properties of the BaWO4 nanoparticles are also strongly dependent on its surface defects and distortions. 4. Conclusions In this work, nearly monodisperse BaWO4 nanoparticles have been synthesized under microwave irradiation in the pure water system without using surfactants. The PL emission peaks of the BaWO4 are in the spectral region at 432–436 nm. Fig. 3. Room temperature photoluminescence spectra of samples formed at the same temperature of 180 °C for different time; 10 min (a) and 15 min (b).
Acknowledgments samples A and B. Both samples have good narrow particle-sized distributions containing a number of nanoparticles with uniform sizes, which improves their luminescent properties [16].
The authors thank the financial support of the Key Laboratory of Quantum Engineering and Micro-Nano Energy Technology, Department of Education of Hunan Province, China (no. 09QNET07).
3.3. PL spectra
References
The PL spectra of the as-prepared BaWO4 were studied at room temperature. The luminescent property of BaWO4 is a result of its scheelite-type structure. In our study, with excitation at 365 nm, BaWO4 nanostructures show the intrinsic emission peaks with their surrounding at 432–436 nm (Fig. 3). This broad band suggests that the emission process is a typical multiphonon or multilevel process. The emission peaks were different from those reported BaWO4 nanostructures [17–19], which mainly attributed to the charge-transfer transitions within the WO42− complexes [20,21]. In addition, the intensity of PL emission of BaWO4 heated for 15 min is noticeably stronger than that of BaWO4 heated for 10 min. The PL intensity is controlled by the number of charged transfers and surface defects [22]. Moreover, the emission peaks obviously shift to the region of long wavelength with reaction time increase, which may be due to the particle-forming effect and the increased size of nanoparticles. Although there are different opinions explaining the origin of the emission bands and the nature of the optical transition is unclear, the WO42− complex and the slight deviation from a perfect crystal structure are believed to be responsible for the emission bands. The luminescent properties are mainly determined by charge-transfer transitions between the O2p orbits and the empty d orbits of the central W 6+ ions in WO42− complex [23]. Furthermore, the PL is related to the structural disorder in the lattice. The tungsten atoms and water molecules are good microwave absorbers. In this case, the interaction between microwave radiation and tungsten atoms resulted in a rapid heating process on the [WO4] tetrahedron groups.
[1] Cerny P, Jelinkova H, Zverev PG, Basiev TT. Prog Quantum Electron 2004;28: 113–43. [2] Caprez A, Meyer P, Hulliger J. Mater Res Bull 1997;32:1045–54. [3] Afanasiev P. Mater Lett 2007;61:4622–6. [4] Nikl M, Bohacek P, Mihokova E, Kobayashi M, Ishii M, Usuki Y, et al. J Lumin 2000;87:1136–9. [5] Roy BN, Roy MR. Cryst Res Technol 1981;16:1267–71. [6] Fujita T, Yamaoka S, Fukunaga O. Mater Res Bull 1974;9:141–6. [7] Cho WS, Yoshimura M. Jpn J Appl Phys 1997;36:1216–9. [8] (a) Tsuji M, Hashimoto M, Nishizawa Y, Kubokawa M, Tsuji T. J Chem Eur 2005;11:440–52. (b) Cavalcante LS, Sczancoski JC, Lima LF, Espinosa Jr JWM, Pizani PS. Cryst Growth Des 2005;9:1002–12. [9] Wu L, Bi JH, Li ZH, Wang XX, Fu XZ. Catal Today 2008;131:15–20. [10] Luo ZJ, Li HM, Shu HM, Wang K, Xia JX, Yan YS. Mater Chem Phys 2008;110:17–20. [11] Chang M, Chung CC, Deka JR, Lin CH, Chung TW. Nanotechnology 2008;19:025710. [12] Hu HY, Yang H, Huang P, Cui DX, Zhang JC, Lu FY, et al. Chem Commun 2010;46: 3866–8. [13] Luo ZJ, Li HM, Xia JX, Zhu WS, Gao JX, Zhang BB. Mater Lett 2007;61:1845–8. [14] Penn RL, Banfield JF. Science 1998;281:969–71. [15] Wang M, Huang QL, Zhong HX, Chen XT, Xue ZL, You XZ. Cryst Growth Des 2007;7: 2106–11. [16] Thongtem T, Kungwankunakorn S, Phuruangrat A. J Alloy Compd 2010;506: 475–81. [17] Yin YK, Gan ZB, Sun YZ, Zhou BB, Zhang X, Zhang DW, et al. Mater Lett 2010;64: 789–92. [18] Wang R, Liu C, Zeng J, Li KW, Wang H. J Solid State Chem 2009;182:677–84. [19] Tyagi M, Sangeeta, Sabharwal SC. J Lumin 2008;128:1528–32. [20] Groeninl JA, Hakfoort C, Blasse G. Phys Status Solidi A 1979;54:329–36. [21] Ryu J, Yoon J, Lim C, Shim K. J Alloy Compd 2005;390:245–9. [22] Thongtem T, Kaowphong S, Thongtem S. Appl Surf Sci 2008;254:7765–9. [23] Chen SJ, Zhou JH, Chen XT. Chem Phys Lett 2003;375:85–90. [24] Lima RC, Anicete-Santos M, Orhan E, Maurera MAMA, Souza AG, Pizani PS, et al. J Lumin 2007;126:741–6.