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Combustion synthesis and luminescence properties of Dy3+-doped MgO nanocrystals
ARTICLE IN PRESS
Journal of Crystal Growth 260 (2004) 507–510
Combustion synthesis and luminescence properties of Dy3+-doped MgO nanocrystals Feng Gu, Shu Fen Wang, Meng Kai Lu*, . Wen Guo Zou, Guang Jun Zhou, Dong Xu, Duo Rong Yuan State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China Received 4 August 2003; accepted 27 August 2003 Communicated by M. Schieber
Abstract MgO nanocrystals doped with Dy3+ have been synthesized by a combustion method. The synthesized sample is characterized by X-ray diffraction, transmission electron micrograph, Fourier transform infrared, and photoluminescence spectroscopy. The as-prepared MgO nanocrystals appear to be single cubic crystalline phase and the diameter is in the range of 20–25 nm. The hypersensitive transition (4F9/2-6H13/2 of Dy3+) emission is prominent in the emission spectra resulting from the low-symmetry local site at which Dy3+ ions locate. In addition, the dependence of the luminescence intensity on Dy3+ concentration is also discussed. r 2003 Elsevier B.V. All rights reserved. PACS: 78.55.Hx; 78.67.Bf; 81.07.Wx; 81.20.Ka Keywords: A1. Doping; B1. Oxides; B1. Nanomaterials
1. Introduction Recent progress in optical devices, such as lasers and optical amplifiers, based on electronic transitions of rare earth ions, has inspired a lot of work in different materials doped with these ions [1,2]. Dysprosium 3+ ions are well-known as activator dopants for many different inorganic lattices producing white light emission by suitably adjusting the yellow and blue emission [3]. The yellow *Corresponding author. Tel.: +86-5318564591; fax: +865318565403. E-mail address: [email protected] (M.K. Lu). .
color is due to the electric dipole transitions of electrons from the 4F9/2 level to the 6H13/2 level. However, the emission is not entirely controlled by the Dy3+ ions, since the host lattice also plays an important role. If a dysprosium site has no inversion symmetry the strong yellow emission is prominent, whereas, if the dysprosium site has inversion symmetry then the emission line would be expected to present blue [4]. Over the past few years, much interest has been focused on photoluminescent properties of metal oxide such as ZnO [5], TiO2 [6], SnO2 [7,8], and ZrO2 [9], which have been regarded as potential candidates used, for example, as nanoscopic
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.08.044
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F. Gu et al. / Journal of Crystal Growth 260 (2004) 507–510
optical storage elements or as probes in living systems [10]. Magnesium oxide (MgO), a very important wide bandgap insulator, has been attracting both fundamental and application studies for use in catalysis [11], toxic waste remediation [12], or as additives in refractory, paint, and superconductor products [13–15]. The most conventional method for synthesis of MgO is the decomposition of various magnesium salts or magnesium hydroxide [13]. In recent years, sol–gel technique has been used to synthesize MgO nanoparticles and the exploration of their novel properties has also been carried out [16]. And the synthesis of MgO with one-dimensional (1D) nanostructures has also been reported [17]. Although the photoluminescent properties of a large variety of host matrices for Dy3+, such as borate, niobate, and phosphate has drawn some attention [18,19], no paper has been reported, to our knowledge, on the preparation of Dy3+-doped MgO nanoparticles, and the influence of MgO host lattices on the luminescence characteristics of Dy3+ ions. In the present paper, a combustion method for the synthesis of Dy3+-doped nanocrystalline MgO is reported, and its luminescence properties dependent on the MgO host are investigated.
2. Experimental procedure The starting materials were magnesium nitrate (Mg(NO3)2 6H2O, (A.R.)), urea (NH2CONH2, (A.R.)), and dysprosium nitrate. The experimental process was performed by dissolving the above reagents together in a minimum amount of deionized water and the concentration of the dysprosium ions was varied in the range 0–2.5 at% in relation to Mg content. After mixing the starting materials placed in a crucible was then introduced into a muffle furnace at 550 C for 20 min. As the ignition occurred, the reaction went on vigorously for a few seconds. Fluffy product was obtained after the combustion reaction. The crystalline structure of the material was analyzed by X-ray diffraction (XRD) using a Japan RigaKu D/MAX 2200PC diffractometer with Cu-Ka radiation (l ¼ 0:15418 nm) and
graphite monochromator. Transmission electron micrograph (TEM) images were taken with a JEM-100CX transmission electron microscope. The Fourier transform infrared (FTIR) spectra of the samples were collected using a 5DX FTIR spectrometer. The photoluminescence (PL) spectra were measured with a Hitachi M-850 fluorescence spectrometer.
3. Results and discussion Fig. 1 gives the XRD pattern of the MgO particles obtained by the combustion method. The XRD pattern of the as-prepared MgO showed the presence of very broad peaks. The broad peaks indicate either particles of very small crystalline size, or particles are semicrystalline in nature [20]. All the diffraction lines are assigned well to cubic crystalline phases of magnesium oxide. The XRD pattern is in excellent agreement with a reference pattern (JCPDS 4-829) of magnesium oxide. A TEM image of the as-prepared MgO particles, shown in Fig. 2, reveals the MgO nanocrystals exhibit cuboid-like morphology and the grain size was distributed in the range of 20–25 nm. Fig. 3 shows the FTIR spectra of the prepared MgO samples. From this spectrum, it can be observed apparently that strong band at 416 cm1associated with the characteristic vibrational mode of symmetric MgO6 octahedra of MgO. The absorption at 3421 cm1 indicates that the presence of hydroxy, which is probably due to the fact that the spectra were not recorded in situ and some readsorption water from the ambient
Fig. 1. XRD pattern of the Dy3+-doped MgO particles (CDy ¼ 1:5% in this case).
ARTICLE IN PRESS F. Gu et al. / Journal of Crystal Growth 260 (2004) 507–510
509
Fig. 2. TEM image of the as-prepared MgO nanocrystals.
Fig. 4. Emission spectrum of Dy3+-doped MgO nanocrystals (lex ¼ 260 nm, CDy ¼ 1:5%).
Fig. 3. FTIR spectrum for MgO nanocrystals.
atmosphere has occurred. The bands in the range of 1300–1700 cm1 may be related to hydroxyl groups of molecular water at 1621 cm1 and to NH3 at 1448 cm1. These bands further confirm that the MgO phase can be indeed formed after combustion at 550 C. The luminescence spectrum of Dy3+-doped MgO nanocrystals prepared by the combustion method is shown in Fig. 4. The excitation wavelength was 260 nm. The emission spectrum has similar shape for all as-prepared samples, the one shown here with the concentration of Dy3+ is 1.5%. The spectral peaks at 480 nm and 575 nm correspond to 4F9/2–6H15/2 and 4F9/2–6H13/2 transitions of Dy3+. With respect to dysprosium, the 4 F9/2–6H15/2 transition is mainly magnetically allowed and hardly varies with the crystal field strength around the dysprosium ion. On the other hand, the 4F9/2–6H13/2 transition is a forced electric dipole transition being allowed only at low symmetries with no inversion center. The ratio
between the electric dipole and the magnetic dipole is a measure of the site symmetry in which the dysprosium situated. When the Dy3+ ion is located at a low-symmetry local site (without an inversion center), this emission transition is often prominent in its emission spectra [21]. As the ( is larger than that of radius of Dy3+ (0.91 A) ( ion, it is difficult for Dy3+ ion to Mg2+ (0.72 A) substitute for Mg2+ ion. Thus, by introducing Dy3+ ions into the MgO host, it is suggested that only a minor fraction of the total amount of Dy3+ goes into the Mg substitutional positions, most of the Dy3+ ions may well be precipitated into MgO: Dy3+ clusters, or even into a separate dysprosium oxide phase. The excess Dy3+ phases will likely reside on the surface or on the grain boundaries of the nanocrystals to yield optimum strain relief. Similar condition occurs in Sb-doped colloidal SnO2 nanocrystals [22]. Therefore, situated at such low-symmetry local sites for Dy3+ ions, the 4 F9/2–6H13/2 transition emission may show prominent in the emission spectra. Because the increase in Dy3+ concentration can result in cross-relaxation processes in close
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affected by both the local site symmetry around Dy3+ ions and concentration of Dy3+ in the MgO host.
References
Fig. 5. Dy3+ concentration dependence of relative luminescence intensity at 575 nm of the MgO samples.
Dy3+–Dy3+ pairs, the quenching of Dy3+ luminescence often occurs at low concentration [23]. The emission intensity at 575 nm of Dy3+ ions has been studied as a function of Dy3+ concentration in MgO nanocrystals (shown in Fig. 5). It can be found that the PL emission intensity of Dy3+ increases with the increase of the concentration, reaching a maximum value at 1.5% for Dy3+, and then decreases with increasing the concentration because of concentration quenching. As mentioned above, with introducing more Dy3+ ions into the MgO host, only a minor portion of Dy3+ ions would go into the Mg substitutional positions, most may be absorbed at the surface of the nanocrystalline MgO or in form of a second phase, which offers much more opportunity for the crossrelaxation processes.
4. Conclusion In conclusion, a combustion method has been employed for the synthesis of ultrafine Dy3+doped MgO nanocrystals. Luminescence of the assynthesized Dy3+-doped MgO nanocrystals was observed and investigated. Experimental results reveal that the luminescence processes may be
[1] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994. [2] R. Scheps, Prog. Quantum Electron. 20 (1996) 271. [3] J.L. Sommerdijk, A. Bril, J. Electrochem. Soc. 122 (1975) 952. [4] M. Yu, J. Lin, Z. Zhang, J. Fu, S. Wang, H.J. Zhang, Y.C. Han, Chem. Mater. 14 (2002) 2224. [5] K. Vanheusden, W.L. Warren, C.H. Seaeger, D.R. Jallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983. [6] Y.J. Liu, R.O. Claus, J. Am. Chem. Soc. 119 (22) (1997) 5273. [7] G.S. Pang, S.G. Chen, Y. Koltypin, A. Zaban, S.H. Feng, A. Gedanken, Nano. Lett. 1 (2001) 723. [8] F. Gu, S.F. Wang, C.F. Song, M.K. Lu, . Y.X. Qi, G.J. Zhou, D. Xu, D.R. Yuan, Chem. Phys. Lett. 372 (2003) 451. [9] J.H. Liang, Z.X. Deng, X. Jiang, F.L. Li, Y.D. Li, Inorg. Chem. 41 (2002) 3602. [10] L.A. Peyser, A.E. Vinson, A.P. Bartko, R.M. Dickson, Science 291 (2001) 103. [11] H. Tsuji, F. Yagi, H. Hattori, H. Kita, J. Catal. 148 (1994) 759. [12] A.N. Copp, Am. Ceram. Soc. Bull. 74 (1995) 135. [13] A. Bhargava, J.A. Alarco, I.D.R. Mackinnon, D. Page, A. Ilyushechkin, Mater. Lett. 34 (1998) 133. [14] Y.S. Yuan, M.S. Wong, S.S. Wang, J. Mater. Res. 11 (1996) 8. [15] P.D. Yang, C.M. Lieber, Science 273 (1996) 1836. [16] J.A. Wang, O. Novaro, X. Bokhimi, T. Lopez, R. Gomez, J. Navarrete, M.E. Llanos, E. Lopez-Salinas, J. Phys. Chem. B 101 (1997) 7448. [17] J. Zhang, L.D. Zhang, Chem. Phys. Lett. 363 (2002) 293. [18] J.L. Sommerdijk, A. Bril, J. Electrochem. Soc. 122 (1975) 952. [19] H. Xiao-Hua, G. Feng-Yu, Z. Feng-Mei, Proceedings of the Third International Conference on Rare Earths, North-Holland, Amsterdam, 1979, p. 226. [20] H. Klug, L. Alexander (Eds.), X-ray Diffraction Procedure, Willey, New York, 1962, p. 125. [21] M. Yu, J. Lin, Z. Zhang, J. Fu, S. Wang, H.J. Zhang, Y.C. Han, Chem. Mater. 14 (2002) 2224. [22] T. Nutz, M. Haase, J. Phys. Chem. B 104 (2000) 8430. [23] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994 (Chapter 4).
Journal of Crystal Growth 260 (2004) 507–510
Combustion synthesis and luminescence properties of Dy3+-doped MgO nanocrystals Feng Gu, Shu Fen Wang, Meng Kai Lu*, . Wen Guo Zou, Guang Jun Zhou, Dong Xu, Duo Rong Yuan State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China Received 4 August 2003; accepted 27 August 2003 Communicated by M. Schieber
Abstract MgO nanocrystals doped with Dy3+ have been synthesized by a combustion method. The synthesized sample is characterized by X-ray diffraction, transmission electron micrograph, Fourier transform infrared, and photoluminescence spectroscopy. The as-prepared MgO nanocrystals appear to be single cubic crystalline phase and the diameter is in the range of 20–25 nm. The hypersensitive transition (4F9/2-6H13/2 of Dy3+) emission is prominent in the emission spectra resulting from the low-symmetry local site at which Dy3+ ions locate. In addition, the dependence of the luminescence intensity on Dy3+ concentration is also discussed. r 2003 Elsevier B.V. All rights reserved. PACS: 78.55.Hx; 78.67.Bf; 81.07.Wx; 81.20.Ka Keywords: A1. Doping; B1. Oxides; B1. Nanomaterials
1. Introduction Recent progress in optical devices, such as lasers and optical amplifiers, based on electronic transitions of rare earth ions, has inspired a lot of work in different materials doped with these ions [1,2]. Dysprosium 3+ ions are well-known as activator dopants for many different inorganic lattices producing white light emission by suitably adjusting the yellow and blue emission [3]. The yellow *Corresponding author. Tel.: +86-5318564591; fax: +865318565403. E-mail address: [email protected] (M.K. Lu). .
color is due to the electric dipole transitions of electrons from the 4F9/2 level to the 6H13/2 level. However, the emission is not entirely controlled by the Dy3+ ions, since the host lattice also plays an important role. If a dysprosium site has no inversion symmetry the strong yellow emission is prominent, whereas, if the dysprosium site has inversion symmetry then the emission line would be expected to present blue [4]. Over the past few years, much interest has been focused on photoluminescent properties of metal oxide such as ZnO [5], TiO2 [6], SnO2 [7,8], and ZrO2 [9], which have been regarded as potential candidates used, for example, as nanoscopic
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.08.044
ARTICLE IN PRESS 508
F. Gu et al. / Journal of Crystal Growth 260 (2004) 507–510
optical storage elements or as probes in living systems [10]. Magnesium oxide (MgO), a very important wide bandgap insulator, has been attracting both fundamental and application studies for use in catalysis [11], toxic waste remediation [12], or as additives in refractory, paint, and superconductor products [13–15]. The most conventional method for synthesis of MgO is the decomposition of various magnesium salts or magnesium hydroxide [13]. In recent years, sol–gel technique has been used to synthesize MgO nanoparticles and the exploration of their novel properties has also been carried out [16]. And the synthesis of MgO with one-dimensional (1D) nanostructures has also been reported [17]. Although the photoluminescent properties of a large variety of host matrices for Dy3+, such as borate, niobate, and phosphate has drawn some attention [18,19], no paper has been reported, to our knowledge, on the preparation of Dy3+-doped MgO nanoparticles, and the influence of MgO host lattices on the luminescence characteristics of Dy3+ ions. In the present paper, a combustion method for the synthesis of Dy3+-doped nanocrystalline MgO is reported, and its luminescence properties dependent on the MgO host are investigated.
2. Experimental procedure The starting materials were magnesium nitrate (Mg(NO3)2 6H2O, (A.R.)), urea (NH2CONH2, (A.R.)), and dysprosium nitrate. The experimental process was performed by dissolving the above reagents together in a minimum amount of deionized water and the concentration of the dysprosium ions was varied in the range 0–2.5 at% in relation to Mg content. After mixing the starting materials placed in a crucible was then introduced into a muffle furnace at 550 C for 20 min. As the ignition occurred, the reaction went on vigorously for a few seconds. Fluffy product was obtained after the combustion reaction. The crystalline structure of the material was analyzed by X-ray diffraction (XRD) using a Japan RigaKu D/MAX 2200PC diffractometer with Cu-Ka radiation (l ¼ 0:15418 nm) and
graphite monochromator. Transmission electron micrograph (TEM) images were taken with a JEM-100CX transmission electron microscope. The Fourier transform infrared (FTIR) spectra of the samples were collected using a 5DX FTIR spectrometer. The photoluminescence (PL) spectra were measured with a Hitachi M-850 fluorescence spectrometer.
3. Results and discussion Fig. 1 gives the XRD pattern of the MgO particles obtained by the combustion method. The XRD pattern of the as-prepared MgO showed the presence of very broad peaks. The broad peaks indicate either particles of very small crystalline size, or particles are semicrystalline in nature [20]. All the diffraction lines are assigned well to cubic crystalline phases of magnesium oxide. The XRD pattern is in excellent agreement with a reference pattern (JCPDS 4-829) of magnesium oxide. A TEM image of the as-prepared MgO particles, shown in Fig. 2, reveals the MgO nanocrystals exhibit cuboid-like morphology and the grain size was distributed in the range of 20–25 nm. Fig. 3 shows the FTIR spectra of the prepared MgO samples. From this spectrum, it can be observed apparently that strong band at 416 cm1associated with the characteristic vibrational mode of symmetric MgO6 octahedra of MgO. The absorption at 3421 cm1 indicates that the presence of hydroxy, which is probably due to the fact that the spectra were not recorded in situ and some readsorption water from the ambient
Fig. 1. XRD pattern of the Dy3+-doped MgO particles (CDy ¼ 1:5% in this case).
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509
Fig. 2. TEM image of the as-prepared MgO nanocrystals.
Fig. 4. Emission spectrum of Dy3+-doped MgO nanocrystals (lex ¼ 260 nm, CDy ¼ 1:5%).
Fig. 3. FTIR spectrum for MgO nanocrystals.
atmosphere has occurred. The bands in the range of 1300–1700 cm1 may be related to hydroxyl groups of molecular water at 1621 cm1 and to NH3 at 1448 cm1. These bands further confirm that the MgO phase can be indeed formed after combustion at 550 C. The luminescence spectrum of Dy3+-doped MgO nanocrystals prepared by the combustion method is shown in Fig. 4. The excitation wavelength was 260 nm. The emission spectrum has similar shape for all as-prepared samples, the one shown here with the concentration of Dy3+ is 1.5%. The spectral peaks at 480 nm and 575 nm correspond to 4F9/2–6H15/2 and 4F9/2–6H13/2 transitions of Dy3+. With respect to dysprosium, the 4 F9/2–6H15/2 transition is mainly magnetically allowed and hardly varies with the crystal field strength around the dysprosium ion. On the other hand, the 4F9/2–6H13/2 transition is a forced electric dipole transition being allowed only at low symmetries with no inversion center. The ratio
between the electric dipole and the magnetic dipole is a measure of the site symmetry in which the dysprosium situated. When the Dy3+ ion is located at a low-symmetry local site (without an inversion center), this emission transition is often prominent in its emission spectra [21]. As the ( is larger than that of radius of Dy3+ (0.91 A) ( ion, it is difficult for Dy3+ ion to Mg2+ (0.72 A) substitute for Mg2+ ion. Thus, by introducing Dy3+ ions into the MgO host, it is suggested that only a minor fraction of the total amount of Dy3+ goes into the Mg substitutional positions, most of the Dy3+ ions may well be precipitated into MgO: Dy3+ clusters, or even into a separate dysprosium oxide phase. The excess Dy3+ phases will likely reside on the surface or on the grain boundaries of the nanocrystals to yield optimum strain relief. Similar condition occurs in Sb-doped colloidal SnO2 nanocrystals [22]. Therefore, situated at such low-symmetry local sites for Dy3+ ions, the 4 F9/2–6H13/2 transition emission may show prominent in the emission spectra. Because the increase in Dy3+ concentration can result in cross-relaxation processes in close
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affected by both the local site symmetry around Dy3+ ions and concentration of Dy3+ in the MgO host.
References
Fig. 5. Dy3+ concentration dependence of relative luminescence intensity at 575 nm of the MgO samples.
Dy3+–Dy3+ pairs, the quenching of Dy3+ luminescence often occurs at low concentration [23]. The emission intensity at 575 nm of Dy3+ ions has been studied as a function of Dy3+ concentration in MgO nanocrystals (shown in Fig. 5). It can be found that the PL emission intensity of Dy3+ increases with the increase of the concentration, reaching a maximum value at 1.5% for Dy3+, and then decreases with increasing the concentration because of concentration quenching. As mentioned above, with introducing more Dy3+ ions into the MgO host, only a minor portion of Dy3+ ions would go into the Mg substitutional positions, most may be absorbed at the surface of the nanocrystalline MgO or in form of a second phase, which offers much more opportunity for the crossrelaxation processes.
4. Conclusion In conclusion, a combustion method has been employed for the synthesis of ultrafine Dy3+doped MgO nanocrystals. Luminescence of the assynthesized Dy3+-doped MgO nanocrystals was observed and investigated. Experimental results reveal that the luminescence processes may be
[1] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994. [2] R. Scheps, Prog. Quantum Electron. 20 (1996) 271. [3] J.L. Sommerdijk, A. Bril, J. Electrochem. Soc. 122 (1975) 952. [4] M. Yu, J. Lin, Z. Zhang, J. Fu, S. Wang, H.J. Zhang, Y.C. Han, Chem. Mater. 14 (2002) 2224. [5] K. Vanheusden, W.L. Warren, C.H. Seaeger, D.R. Jallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983. [6] Y.J. Liu, R.O. Claus, J. Am. Chem. Soc. 119 (22) (1997) 5273. [7] G.S. Pang, S.G. Chen, Y. Koltypin, A. Zaban, S.H. Feng, A. Gedanken, Nano. Lett. 1 (2001) 723. [8] F. Gu, S.F. Wang, C.F. Song, M.K. Lu, . Y.X. Qi, G.J. Zhou, D. Xu, D.R. Yuan, Chem. Phys. Lett. 372 (2003) 451. [9] J.H. Liang, Z.X. Deng, X. Jiang, F.L. Li, Y.D. Li, Inorg. Chem. 41 (2002) 3602. [10] L.A. Peyser, A.E. Vinson, A.P. Bartko, R.M. Dickson, Science 291 (2001) 103. [11] H. Tsuji, F. Yagi, H. Hattori, H. Kita, J. Catal. 148 (1994) 759. [12] A.N. Copp, Am. Ceram. Soc. Bull. 74 (1995) 135. [13] A. Bhargava, J.A. Alarco, I.D.R. Mackinnon, D. Page, A. Ilyushechkin, Mater. Lett. 34 (1998) 133. [14] Y.S. Yuan, M.S. Wong, S.S. Wang, J. Mater. Res. 11 (1996) 8. [15] P.D. Yang, C.M. Lieber, Science 273 (1996) 1836. [16] J.A. Wang, O. Novaro, X. Bokhimi, T. Lopez, R. Gomez, J. Navarrete, M.E. Llanos, E. Lopez-Salinas, J. Phys. Chem. B 101 (1997) 7448. [17] J. Zhang, L.D. Zhang, Chem. Phys. Lett. 363 (2002) 293. [18] J.L. Sommerdijk, A. Bril, J. Electrochem. Soc. 122 (1975) 952. [19] H. Xiao-Hua, G. Feng-Yu, Z. Feng-Mei, Proceedings of the Third International Conference on Rare Earths, North-Holland, Amsterdam, 1979, p. 226. [20] H. Klug, L. Alexander (Eds.), X-ray Diffraction Procedure, Willey, New York, 1962, p. 125. [21] M. Yu, J. Lin, Z. Zhang, J. Fu, S. Wang, H.J. Zhang, Y.C. Han, Chem. Mater. 14 (2002) 2224. [22] T. Nutz, M. Haase, J. Phys. Chem. B 104 (2000) 8430. [23] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994 (Chapter 4).