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The synthesis and photoluminescent properties of one-dimensional ZnMoO4:Eu3+ nanocrystals
Materials Letters 64 (2010) 1644–1646
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
The synthesis and photoluminescent properties of one-dimensional ZnMoO4:Eu3+ nanocrystals Lixin Yu a,⁎, Masayuki Nogami b a b
Department of Materials Science and Technology, Nanchang University, 999 Xuefu Road, Nanchang, 330031, PR China Department of Materials Science and Engineering, Nagoya Institute of Technology, Showa Nagoya, 466-8555, Japan
a r t i c l e
i n f o
Article history: Received 12 February 2010 Accepted 8 April 2010 Available online 14 April 2010 Keywords: Luminescence Nanomaterials
a b s t r a c t ZnMoO4:Eu3+ nanocrystals were synthesized by a mild and simple hydrothermal method. The results indicate that the pH value of the precursor solution plays a crucial role in controlling the morphology, size and structures. The leaflike, nanorods and nanowires were obtained by modulating the pH value of the precursor solution. The crystalline structure of ZnMoO4:Eu3+ nanocrystals are affected by the precursor solution. The excitation and emission spectra were studied. The results indicate that relative intensity of f–f transitions to charge transfer (CT) absorption and 5D0-7F2 to 5D0-7F1 transitions greatly change in different samples. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The nanosized materials have been found to exhibit rich and unique physical and chemical characteristics. The structure and shape of nanocrystals can affect physical properties. For morphology control, the self-assemble method (such as hydrothermal) has attracted many interests due to easy and low-cost. Rare earth (RE) compounds were extensively applied in luminescence and display, such as lighting, field emission display (FED), cathode ray tubes (CRT) and plasma display panel (PDP) [1–5]. It is expected that nanosized RE compounds can increase luminescent quantum yield, fluorescent intensity and display resolution [6–10], especially one-dimensional (1D) nanocrystals. In addition, because Eu3+ ions are hypersensitive to the local environment, they can be used as fluorescent probe to research the microstructures of hosts. In previous research, we found that the luminescent properties of Eu 3+ doped nanocrystals strongly depended on the morphology and size [6–8]. Thus we controled the morphology of ZnMoO4 doped with Eu3+ ions by changing the pH value of the precursor solution. The excitation and emission spectra were also studied. Molybdate hosts doped with RE ions have attracted much attention due to their excellent luminescent properties. They can be used as luminescence device, optic communications and scintillators [11–15]. CaMoO4, BaMoO4 and SrMoO4 micrometer powders doped with Eu3+ or Sm3+ have been reported [16–18]. In this paper, we reported the dependence of structure, size, shape and photolumines-
⁎ Corresponding author. Tel.: + 86 0791 3969329. E-mail address: [email protected] (L. Yu). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.04.016
cent properties of ZnMoO4:Eu3+ on the pH value of the precursor solution. 2. Experimental 2.1. Preparation All samples were prepared by a hydrothermal method. (NH4)6Mo7O24 (AR), Zn(NO3)2⋅6H2O (AR) and Eu(NO3)3⋅6H2O (5 N) were used as starting materials. A typical synthesis, 1.6848 g Zn (NO3)2⋅6H2O and 0.0505 g Eu(NO3)3⋅6H2O (5% mol ratio) were dissolved in 50 ml de-ionized water and stirred for 2 h (solution A). 1.0201 g (NH4)6Mo7O24 was dissolved in 90 ml de-ionized water and also stirred for 2 h (solution B). Then solution A was slowly added to solution B drop by drop with stirring. The pH value of mixed solution was about 7. The final pH value was adjusted to 8, 10 and 12 with NaOH solution (1 M), respectively. The milky colloid solution was obtained and poured into closed Teflon-lined autoclave and subsequently heated at 160 °C for 12 h. The obtained suspension was centrifuged at 10,000 rpm for 30 min and supernatant was discarded. Then, the precipitate was washed with de-ionized water for three times. The final sample was dried at 120 °C. The corresponding samples were labeled with M8 (pH = 8), M10 (pH = 10) and M12 (pH = 12), respectively. 2.2. Characterization Crystal structure, morphology and size were obtained by X-ray diffraction(XRD) using Cu target radiation resource (CuK α = 1.54078 Å), field emission electron micrographs (FEM) utilizing JSM5500 electron microscope. The excitation and emission spectra at
L. Yu, M. Nogami / Materials Letters 64 (2010) 1644–1646
1645
Fig. 1. The FEM photos of ZnMoO4:Eu3+ nanocrystals. a: pH = 8, b: pH = 10; c: pH = 12.
room temperature were measured with a Hitachi F-4500 fluorescence spectrometer. 3. Results and discussion Fig. 1 shows the FEM photos. According to Fig. 1, the morphology is strongly dependent on the pH value of the precursor solution. As the pH value is about 8, the shape of samples is the thin leaflike. When pH value is 10, the morphology is short nanorods, whose length is about 2 μm and diameter is about 200 nm. As the pH value is 12, the nanowires are obtained. The length is about 500 nm and diameter is about 50 nm. It is obvious that the pH value of the precursor solution strongly affects the morphology and size of samples. The similar phenomena were observed in LaPO4 nanocrystals [6–8]. Fig. 2 shows the XRD patterns. The structure of M8 is assigned to the monoclinic phase (PDF 25-1025). M10 is associated with the mixture phase of anorthic (PDF 25-1203) and hexagonal (PDF 20-0682). M12 is pure anorthic phase (PDF 25-1023). Fig. 3 is the excitation spectra of ZnMoO4:Eu3+ nanocrystals monitoring 612 nm emission. The excitation spectra are composed of a broad band absorption ranging from 220 to 330 nm and some sharp lines. The broad band is assigned to the combination of the CT transition of O2− → Mo6+ and O2− → Eu3+[16]. The weak absorption lines locating at the low-energy side are from the f–f transition absorption of Eu3+ ions. From M8 to M12, the intensity of absorption hardly changes. The relative proportion of CT absorption to f–f transition absorption greatly changes from M8 to M12. In M8, the relative intensity of f–f transition in comparison with CT is higher than that in M12. This result indicates that the local microstructures around Eu3+ ions greatly changed in different samples. Fig. 4 is the emission spectra of ZnMoO4:Eu3+ at 260 nm excitation. The orange emissions at about 590 nm are from 5D0-7F1 transitions of
Fig. 2. The XRD patterns of ZnMoO4:Eu3+ nanocrystals.
Eu3+, while red emissions at 612 nm from 5D0-7F2 transitions [16]. It is clear that the relative proportion of 5D0-7F2 to 5D0-7F1 transitions from sample M8 to M12 decreases, indicating that the chromaticity reduces from M8 to M12. As is well known, the 5D0-7F1 lines originate from magnetic dipole transition, while the 5D0-7F2 lines from electric dipole one. In terms of the Judd–Ofelt theory [19,20], the magnetic dipole transition is permitted. The electric dipole transition is allowed only on condition that the europium ion occupies a site without an inversion center and is sensitive to local symmetry. Subsequently, when Eu3+ ions occupy inversion center sites, the 5D0-7F1 transitions should be relative strong. Otherwise, the 5D0-7F2 transitions should be relative weak. The results above indicate that from M8 to M12, more Eu3+ ions occupy inversion center sites.
Fig. 3. The excitation spectra of ZnMoO4:Eu3+ nanocrystals (λem = 612 nm).
Fig. 4. The emission spectra of ZnMoO4:Eu3+ nanocrystals (λex = 260 nm).
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L. Yu, M. Nogami / Materials Letters 64 (2010) 1644–1646
4. Conclusions ZnMoO4:Eu3+ nanocrystals were prepared by an easy hydrothermal method. The results indicate that the morphology and structures strongly depend on the pH value of the precursor solution. For pH value of 8, the thin leaflike microcrystals form. Nanorods for pH value of 10 and nanowires for pH value of 12 are obtained. The crystalline structure of ZnMoO4:Eu3+ nanocrystals is affected by the precursor solution. The excitation and emission spectra were also studied. The results indicate that relative intensity of f–f transition absorption to CT absorption and 5D0-7F2 to 5D0-7F1 transitions greatly change in different samples, indicating that the local microstructures around Eu3+ greatly change in three samples with different shapes and structures. References [1] Dhanaraj J, Jagannathan R, Kutty T, Lu C. J Phys Chem B 2001;105:11098–105. [2] Wei Z, Sun L, Liao C, Yan C. Appl Phys Lett 2002;80:1447–9. [3] Meltzer R, Feofilov S, Tissue B. Phys Rev B 1999;60:R14012.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Williams D, Bihari B, Tissue B, McHale J. J Phys Chem B 1998;102:916–20. Hong K, Meltzer R, Bihari B, Williams D, Tissue B. J Lumin 1998;76&77:234–7. Yu L, Song H, Lu S, Liu Z, Yang L, Kong X. J Phys Chem B 2004;108:16697–702. Yu L, Song H, Liu Z, Yang L, Lu S, Zheng Z. J Phys Chem B 2005;109:11450–5. Song H, Yu L, Lu S, Wang T, Liu Z, Yang L. Appl Phys Lett 2004;85:470–2. Yu L, Song H, Lu S, Liu Z, Yang L. Chem Phys Lett 2004;399:384–8. Yu L, Song H, Liu Z, Yang L, Lu S. Phys Chem Chem Phys 2006;8:303–8. Qi T, Takagi K, Fukazawa T. Appl Phys Lett 1980;36:278–80. Wang H, Medina F, Zhou Y, Zhang Q. Phys Rev B 1992;45:10356–62. Tanaka K, Miyajima T, Shirai N, Zhuang Q, Nakana R. J Appl Phys 1995;77:6581–7. Liao H, Wang Y, Liu X, Li Y, Qian Y. Chem Mater 2000;12:2819–21. Kwan S, Kim F, Akana J, Yang P. Chem Commun 2001;5:447–8. Jin Y, Zhang J, Lu S, Zhao H, Zhang X, Wang X. J Phys Chem C 2008;112:5860–4. Hu Y, Zhuang W, Ye H, Wang D, Zhang S, Huang X. J Alloy Comp 2005;390:226–9. Wang Z, Liang H, Wang J, Gong M, Su Q. Appl Phys Lett 2006;89:071921. Judd B. Phys Rev 1962;127:750–61. Ofelt G. J Chem Phys 1962;37:511–20.
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
The synthesis and photoluminescent properties of one-dimensional ZnMoO4:Eu3+ nanocrystals Lixin Yu a,⁎, Masayuki Nogami b a b
Department of Materials Science and Technology, Nanchang University, 999 Xuefu Road, Nanchang, 330031, PR China Department of Materials Science and Engineering, Nagoya Institute of Technology, Showa Nagoya, 466-8555, Japan
a r t i c l e
i n f o
Article history: Received 12 February 2010 Accepted 8 April 2010 Available online 14 April 2010 Keywords: Luminescence Nanomaterials
a b s t r a c t ZnMoO4:Eu3+ nanocrystals were synthesized by a mild and simple hydrothermal method. The results indicate that the pH value of the precursor solution plays a crucial role in controlling the morphology, size and structures. The leaflike, nanorods and nanowires were obtained by modulating the pH value of the precursor solution. The crystalline structure of ZnMoO4:Eu3+ nanocrystals are affected by the precursor solution. The excitation and emission spectra were studied. The results indicate that relative intensity of f–f transitions to charge transfer (CT) absorption and 5D0-7F2 to 5D0-7F1 transitions greatly change in different samples. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The nanosized materials have been found to exhibit rich and unique physical and chemical characteristics. The structure and shape of nanocrystals can affect physical properties. For morphology control, the self-assemble method (such as hydrothermal) has attracted many interests due to easy and low-cost. Rare earth (RE) compounds were extensively applied in luminescence and display, such as lighting, field emission display (FED), cathode ray tubes (CRT) and plasma display panel (PDP) [1–5]. It is expected that nanosized RE compounds can increase luminescent quantum yield, fluorescent intensity and display resolution [6–10], especially one-dimensional (1D) nanocrystals. In addition, because Eu3+ ions are hypersensitive to the local environment, they can be used as fluorescent probe to research the microstructures of hosts. In previous research, we found that the luminescent properties of Eu 3+ doped nanocrystals strongly depended on the morphology and size [6–8]. Thus we controled the morphology of ZnMoO4 doped with Eu3+ ions by changing the pH value of the precursor solution. The excitation and emission spectra were also studied. Molybdate hosts doped with RE ions have attracted much attention due to their excellent luminescent properties. They can be used as luminescence device, optic communications and scintillators [11–15]. CaMoO4, BaMoO4 and SrMoO4 micrometer powders doped with Eu3+ or Sm3+ have been reported [16–18]. In this paper, we reported the dependence of structure, size, shape and photolumines-
⁎ Corresponding author. Tel.: + 86 0791 3969329. E-mail address: [email protected] (L. Yu). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.04.016
cent properties of ZnMoO4:Eu3+ on the pH value of the precursor solution. 2. Experimental 2.1. Preparation All samples were prepared by a hydrothermal method. (NH4)6Mo7O24 (AR), Zn(NO3)2⋅6H2O (AR) and Eu(NO3)3⋅6H2O (5 N) were used as starting materials. A typical synthesis, 1.6848 g Zn (NO3)2⋅6H2O and 0.0505 g Eu(NO3)3⋅6H2O (5% mol ratio) were dissolved in 50 ml de-ionized water and stirred for 2 h (solution A). 1.0201 g (NH4)6Mo7O24 was dissolved in 90 ml de-ionized water and also stirred for 2 h (solution B). Then solution A was slowly added to solution B drop by drop with stirring. The pH value of mixed solution was about 7. The final pH value was adjusted to 8, 10 and 12 with NaOH solution (1 M), respectively. The milky colloid solution was obtained and poured into closed Teflon-lined autoclave and subsequently heated at 160 °C for 12 h. The obtained suspension was centrifuged at 10,000 rpm for 30 min and supernatant was discarded. Then, the precipitate was washed with de-ionized water for three times. The final sample was dried at 120 °C. The corresponding samples were labeled with M8 (pH = 8), M10 (pH = 10) and M12 (pH = 12), respectively. 2.2. Characterization Crystal structure, morphology and size were obtained by X-ray diffraction(XRD) using Cu target radiation resource (CuK α = 1.54078 Å), field emission electron micrographs (FEM) utilizing JSM5500 electron microscope. The excitation and emission spectra at
L. Yu, M. Nogami / Materials Letters 64 (2010) 1644–1646
1645
Fig. 1. The FEM photos of ZnMoO4:Eu3+ nanocrystals. a: pH = 8, b: pH = 10; c: pH = 12.
room temperature were measured with a Hitachi F-4500 fluorescence spectrometer. 3. Results and discussion Fig. 1 shows the FEM photos. According to Fig. 1, the morphology is strongly dependent on the pH value of the precursor solution. As the pH value is about 8, the shape of samples is the thin leaflike. When pH value is 10, the morphology is short nanorods, whose length is about 2 μm and diameter is about 200 nm. As the pH value is 12, the nanowires are obtained. The length is about 500 nm and diameter is about 50 nm. It is obvious that the pH value of the precursor solution strongly affects the morphology and size of samples. The similar phenomena were observed in LaPO4 nanocrystals [6–8]. Fig. 2 shows the XRD patterns. The structure of M8 is assigned to the monoclinic phase (PDF 25-1025). M10 is associated with the mixture phase of anorthic (PDF 25-1203) and hexagonal (PDF 20-0682). M12 is pure anorthic phase (PDF 25-1023). Fig. 3 is the excitation spectra of ZnMoO4:Eu3+ nanocrystals monitoring 612 nm emission. The excitation spectra are composed of a broad band absorption ranging from 220 to 330 nm and some sharp lines. The broad band is assigned to the combination of the CT transition of O2− → Mo6+ and O2− → Eu3+[16]. The weak absorption lines locating at the low-energy side are from the f–f transition absorption of Eu3+ ions. From M8 to M12, the intensity of absorption hardly changes. The relative proportion of CT absorption to f–f transition absorption greatly changes from M8 to M12. In M8, the relative intensity of f–f transition in comparison with CT is higher than that in M12. This result indicates that the local microstructures around Eu3+ ions greatly changed in different samples. Fig. 4 is the emission spectra of ZnMoO4:Eu3+ at 260 nm excitation. The orange emissions at about 590 nm are from 5D0-7F1 transitions of
Fig. 2. The XRD patterns of ZnMoO4:Eu3+ nanocrystals.
Eu3+, while red emissions at 612 nm from 5D0-7F2 transitions [16]. It is clear that the relative proportion of 5D0-7F2 to 5D0-7F1 transitions from sample M8 to M12 decreases, indicating that the chromaticity reduces from M8 to M12. As is well known, the 5D0-7F1 lines originate from magnetic dipole transition, while the 5D0-7F2 lines from electric dipole one. In terms of the Judd–Ofelt theory [19,20], the magnetic dipole transition is permitted. The electric dipole transition is allowed only on condition that the europium ion occupies a site without an inversion center and is sensitive to local symmetry. Subsequently, when Eu3+ ions occupy inversion center sites, the 5D0-7F1 transitions should be relative strong. Otherwise, the 5D0-7F2 transitions should be relative weak. The results above indicate that from M8 to M12, more Eu3+ ions occupy inversion center sites.
Fig. 3. The excitation spectra of ZnMoO4:Eu3+ nanocrystals (λem = 612 nm).
Fig. 4. The emission spectra of ZnMoO4:Eu3+ nanocrystals (λex = 260 nm).
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L. Yu, M. Nogami / Materials Letters 64 (2010) 1644–1646
4. Conclusions ZnMoO4:Eu3+ nanocrystals were prepared by an easy hydrothermal method. The results indicate that the morphology and structures strongly depend on the pH value of the precursor solution. For pH value of 8, the thin leaflike microcrystals form. Nanorods for pH value of 10 and nanowires for pH value of 12 are obtained. The crystalline structure of ZnMoO4:Eu3+ nanocrystals is affected by the precursor solution. The excitation and emission spectra were also studied. The results indicate that relative intensity of f–f transition absorption to CT absorption and 5D0-7F2 to 5D0-7F1 transitions greatly change in different samples, indicating that the local microstructures around Eu3+ greatly change in three samples with different shapes and structures. References [1] Dhanaraj J, Jagannathan R, Kutty T, Lu C. J Phys Chem B 2001;105:11098–105. [2] Wei Z, Sun L, Liao C, Yan C. Appl Phys Lett 2002;80:1447–9. [3] Meltzer R, Feofilov S, Tissue B. Phys Rev B 1999;60:R14012.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Williams D, Bihari B, Tissue B, McHale J. J Phys Chem B 1998;102:916–20. Hong K, Meltzer R, Bihari B, Williams D, Tissue B. J Lumin 1998;76&77:234–7. Yu L, Song H, Lu S, Liu Z, Yang L, Kong X. J Phys Chem B 2004;108:16697–702. Yu L, Song H, Liu Z, Yang L, Lu S, Zheng Z. J Phys Chem B 2005;109:11450–5. Song H, Yu L, Lu S, Wang T, Liu Z, Yang L. Appl Phys Lett 2004;85:470–2. Yu L, Song H, Lu S, Liu Z, Yang L. Chem Phys Lett 2004;399:384–8. Yu L, Song H, Liu Z, Yang L, Lu S. Phys Chem Chem Phys 2006;8:303–8. Qi T, Takagi K, Fukazawa T. Appl Phys Lett 1980;36:278–80. Wang H, Medina F, Zhou Y, Zhang Q. Phys Rev B 1992;45:10356–62. Tanaka K, Miyajima T, Shirai N, Zhuang Q, Nakana R. J Appl Phys 1995;77:6581–7. Liao H, Wang Y, Liu X, Li Y, Qian Y. Chem Mater 2000;12:2819–21. Kwan S, Kim F, Akana J, Yang P. Chem Commun 2001;5:447–8. Jin Y, Zhang J, Lu S, Zhao H, Zhang X, Wang X. J Phys Chem C 2008;112:5860–4. Hu Y, Zhuang W, Ye H, Wang D, Zhang S, Huang X. J Alloy Comp 2005;390:226–9. Wang Z, Liang H, Wang J, Gong M, Su Q. Appl Phys Lett 2006;89:071921. Judd B. Phys Rev 1962;127:750–61. Ofelt G. J Chem Phys 1962;37:511–20.