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Combustion synthesis and photoluminescence of MgO:Eu3+ nanocrystals with Li+ addition
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Journal of Crystal Growth 289 (2006) 400–404 www.elsevier.com/locate/jcrysgro
Combustion synthesis and photoluminescence of MgO : Eu3þ nanocrystals with Liþ addition Feng Gu, Chun Zhong Li, Hai Bo Jiang Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China Received 25 September 2005; received in revised form 8 November 2005; accepted 15 November 2005 Available online 3 February 2006 Communicated by R. Kern
Abstract Well-crystalline MgO nanocrystals have been synthesized successfully via a facile combustion method with lithium addition. After Liþ doping into the MgO:Eu3þ host, the crystallinity of the sample becomes better, resulting in enhancement of luminescence intensity of the characteristic emission of Eu3þ ions. This approach provides economically viable route for large-scale synthesis of this kind of nanomaterials. r 2006 Elsevier B.V. All rights reserved. PACS: 61.46; 78.55; 81.10 Keywords: A1. Low-dimensional structures; A1. X-ray diffraction; B1. Nanomaterials; B2. Phosphors
1. Introduction Recent progress in optical devices, such as lasers and optical amplifiers, based on electronic transitions of rareearth ions, has inspired a lot of work in different materials doped with these ions [1,2]. Of the many rare-earth ions, Eu3þ ions have been shown to behave as potential as an efficient probe for the microstructure of material because the fluorescence from Eu3þ ions originates from 5d to 4f transition, and the 5d state is easily affected by the outer crystal field [3]. As a type of promising oxide material, magnesium oxide (MgO) has found applications in a number of technologies, including catalysis [4], toxic waste remediation [5], or as additives in refractory, paint, and superconductor products [6–9]. Recently, many investigations have been carried out on the optical and electrical properties of MgO nanocrystals [9]. For many applications, rare-earth ions doped MgO nanocrystals have been reported, exhibiting well optical properties. Since the f–f absorption transitions in rare-earth ions are parity Corresponding authors. Tel.: +86 21 64252055; fax: +86 21 64250624.
E-mail address: [email protected] (F. Gu). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.11.116
forbidden, the number of carriers excited through f–f transitions is much less than that excited through excitation of host. Moreover, the luminescence efficiency of these rare-earth doped oxide materials is a key factor influencing their practical applications. With respect to luminescent materials, one solution to this problem is to find a way to enhance the energy transfer from host to doped rare-earth ions and decrease the influence of defects on the optical properties. It has been reported that, in some oxide matrix, the addition of coactive ions or charge compensators, such as lithium or nitrogen ions, could facilitate energy transfer from the host to doped rare-earth ions, resulting in the enhancement of luminescence intensity. In this paper, lithium ions are chosen as co-dopant into MgO:Eu3þ host to investigate the influence on the photoluminescence(PL) properties. Until now, diverse methods have been employed to synthesize nanocrystalline MgO, such as thermal evaporation, vapor-phase transport, sol–gel, and hydrothermal methods [9–13]. Generally, the above-mentioned methods are rather sophisticated or constrained by expense or apparatus. Compared with the conventional synthesis methods, combustion method has an advantage for
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preparing ultrafine materials due to its low cost, high yield, and good ability to achieve high purity in making single or multiphase complex oxide powders at the as-synthesized state [14,15]. Herein, we report a novel and facile combustion synthesis of MgO:Eu3þ nanocrystals having size of 20–25 nm and the effect of Liþ ions on the photoluminescence properties has been studied systematically. The experimental results show that the crystallinity of the MgO crystals becomes better after Liþ doping, resulting in the remarkable enhancement of PL intensity. 2. Experimental procedure All of the reactants are analytical-grade and used without any further purification. The raw materials are magnesium nitrate ðMgðNO3 Þ2 6H2 O), europium nitrate ðEuðNO3 Þ3 6H2 OÞ and lithium chloride ðLiCl 6H2 OÞ. Urea ðNH2 CONH2 Þ was used as fuel for the combustion. The mixture of reagents was dissolved in a minimum amount of deionized water to obtain a homogeneous solution and the concentrations of europium and lithium are 1% and 10%. After stirring, precursor solution was transferred to a muffle furnace at 550 C for 20 min. As the ignition occurred, the reaction went on vigorously for a few seconds. Then, the fluffy products were collected for characterization. X-ray powder diffraction (XRD) data were recorded on a Japan Rigaku D/max-rA X-ray diffractometer system with graphite monochromatized CuKa radiation ðl ¼ 0:15418 nmÞ. 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 Nicolet Magna-550 infrared spectrometer. The PL spectra were measured with a Hitachi M-850 Fluorescence Spectrometer. 3. Results and discussion During the combustion process, the reaction temperature rose very rapidly from 550 C to about 1500 C in a very short time along with vigorous vapor gas. The MgO vapor precipitated and formed the resultant products in few seconds. XRD was used to examine the crystal structure and phase purity of the products, and the typical XRD patterns of the Eu3þ -doped MgO and Eu3þ , Liþ codoped MgO samples are shown in Fig. 1. For comparison, the diffraction pattern of commercial MgO is also included. The broadening of the diffraction peaks of the samples indicates that the particle sizes are in the nano-scale range. All the diffraction lines are assigned to cubic crystalline phase of magnesium oxide (JCPDS 4-829). Without Liþ co-doping, the crystallinity of MgO:Eu3þ is poor as indicated by relatively weak and broad diffraction peaks, whereas the diffraction intensity of Liþ -doped MgO:Eu3þ is remarkably increased. It is likely that much better crystallinity of the present MgO:Eu3þ , Liþ samples can be
Fig. 1. XRD patterns of the as-prepared MgO nanocrystals: (a) MgO:Eu3þ , (b) MgO:Eu3þ , Liþ . Inset shows the enlarged diffraction peak of (2 0 0) of these two samples. For comparison, the pattern of commercial MgO is also included (c).
Fig. 2. TEM images of the as-prepared MgO nanocrystals: (a) pristine MgO, (b) MgO:Eu3þ , (c) MgO:Eu3þ , Liþ . The bar is 100 nm. Insets show the corresponding ED patterns of these three samples.
obtained even under similar combustion condition. The full-width at half-maximum (FWHM) of the (2 0 0) peak of Liþ -doped MgO:Eu3þ is much smaller than that of MgO:Eu3þ without Liþ doping (shown in inset of Fig. 1). Moreover, the (2 0 0) peak position of MgO : Eu3þ , Liþ shifts compared with that of MgO:Eu3þ , indicating that there exists a change with respect to the lattice parameter after Liþ doping, which may affect the optical properties of doped Eu3þ ions. TEM images of the as-prepared MgO nanocrystals are shown in Fig. 2. It is obvious that the pristine MgO crystals exhibit cuboid-like morphology and the particle size is distributed uniformly in the range of 20–25 nm (Fig. 2a). Introduction of Eu3þ into the host hardly affects the morphology and size of the sample, whereas a size value of about 40–60 nm was found for Eu3þ , Liþ co-doped sample, which is well in accordance with the XRD results (Fig. 2b and c). The ED patterns of the three samples are given in the insets of the corresponding figure. The diffraction rings could be assigned to 200, 220, and 222 planes of cubic MgO from the centermost ring, respectively. Therefore, the Liþ addition should facilitate the crystallization of MgO, which can also be observed in some hosts such as Y2 O3 and Gd2 O3 [16,17]. Fig. 3 shows the FTIR spectra of the prepared MgO samples with and without lithium addition. The strong band at 417 cm1 can be observed, associated with the
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Fig. 3. FTIR spectra of the as-prepared MgO samples before (a) and after (b) lithium addition.
characteristic vibrational mode of symmetric MgO6 octahedra of MgO. The absorption at 3430 cm1 indicates the presence of hydroxyl-groups, which is probably due to the fact that the spectra were not recorded in situ and some water readsorption from the ambient atmosphere has occurred. After introducing lithium ions into the host, the absorption in the range of 130021800 cm1 related to hydroxyl groups of molecular water at 1638 cm1 and to NH3 at 1439 cm1 decreased sharply, implying that better crystallinity can be obtained after combustion at 550 C. PL spectra were recorded at room temperature on both the pristine and doped MgO nanocrystals prepared by combustion method. Under excitation at 294 nm, the PL spectrum of the pristine MgO sample exhibits a strong luminescence band centered at 382 nm (not shown here). With respect to MgO, especially at the surface, there exist a number of reasonably well-defined surface defect structures such as low coordinated ions and/or vacancies [18]. Therefore, in the past decade, many researchers have paid much interest for MgO which has become one of the preferred targets of surface structural and catalytic research. Zhang et al. reported a similar band at 383 nm in the PL spectrum of MgO nanobelts [9], which has been proposed originating from the contribution of oxygen vacancies in the MgO host. However, the other two emission bands at 508 and 721 nm have not be observed in the present case, which might attribute to the different morphology and experimental conditions. In the present case, due to the rapid combustion process, oxygen vacancies should also be generated because of partially incomplete crystallization. In addition, the MgO nanocrystals with large surface-to-volume ratio should also favor the existence of large quantities of oxygen vacancies. The energy levels of the f–f transitions in Eu3þ are wellknown, the 5 D0 ! 7 F1 transition is mainly magnetically allowed and independent on the site symmetry in which europium situated, while the 5 D0 ! 7 F2 is a forced electric dipole transition being allowed only at low symmetries with no inversion center. The ratio between the electric
Fig. 4. Emission spectra of the MgO:Eu3þ nanocrystals under excitation at 395 and 330 nm, respectively.
dipole and the magnetic dipole is a measure of the site symmetry in which europium is situated. Following the direct excitation of the 7 F0 ! 5 L6 transition of Eu3þ ion with 395 nm, two dominant emission bands at 590 and 614 nm were observed in MgO:Eu3þ , which can be assigned to 5 D0 27 FJ ; J ¼ 1; 2, transitions, respectively (shown in Fig. 4). The asymmetry ratio of Eu3þ -doped MgO nanocrystals is calculated and has a value of 1.6. However, under excitation at 330 nm, a 2 times stronger emission band has been observed with an asymmetry ratio of 4.1. The change of the asymmetry ratio is due to the different sites in which europium ions are situated. In the current MgO, the significant difference in the ionic radius bet˚ and Mg2þ ð0:72 AÞ ˚ results in difficulty ween Eu3þ ð1:07 AÞ 3þ for Eu to substitute for Mg2þ ion extensively. Thus, when introducing Eu3þ ions into the MgO host, it is suggested that Eu3þ ions will likely reside on the surface or on the grain boundaries of the nanocrystals to yield optimum strain relief. Following the direct excitation of the 7 F0 ! 5 L6 transition, the smaller asymmetry ratio
ARTICLE IN PRESS F. Gu et al. / Journal of Crystal Growth 289 (2006) 400–404
indicates the average site asymmetry of the dopant europium ions. However, upon photoexcitation of the Eu3þ -doped MgO samples at 330 nm, as the f–f absorption transitions of Eu3þ are parity forbidden, the number of carriers excited through f–f transitions in Eu3þ is much less. On the other hand, as indicated in the emission spectrum of the pristine MgO nanocrystals, a rather strong emission band centered at 382 nm originating from the contribution of oxygen vacancies formed during the combustion process, which is justly favored for the 7 F0 ! 5 L6 transition of Eu3þ ion. Therefore, the enhanced emission comes mainly from radiative recombination of the large amount of trapped carriers excited from MgO. It can be assumed that the oxygen vacancies formed during the combustion process might act as sensitizer for the energy transfer to the rare-earth ions at the surface, due to the strong mixing of charge transfer states resulting in the highly enhanced luminescence. Fig. 5 shows the photoluminescence spectra of the Eu3þ doped and Eu3þ , Liþ co-doped MgO nanocrystals under excitation at 395 nm. It has been observed that the emission intensity at 614 nm has been remarkably enhanced about 1.5 times after Liþ doping. The mechanism of the effect of Liþ on the optical properties is not very clear at this stage. It is generally accepted that, in bulk phosphor materials, a strong luminescence can be obtained with the presence of
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co-activators such as Liþ , even in very small quantities, owing to the facilitated energy transfer from the host to doped rare-earth ions. Lithium ion has been introduced into different phosphor host lattices such as Y2 O3 :Eu3þ , Gd2 O3 :Eu3þ , ZnS:Tm3þ , and ZnO:Dy3þ [15,16,19,20], acting as a co-activator and charge compensator, and the enhanced luminescence has been observed. On the other hand, in some publications, the effect of Liþ substitution on the luminescence properties has been investigated such as in SrTiO3 :Pr3þ and Gd2x Yx O3 :Eu3þ [21,22], which reveals that the Liþ addition remarkably affects the morphology of particles as well as the photo- and cathodoluminescent efficiency of phosphors. Generally, the phosphor particles of higher crystallinity have improved brightness. In the present case, it is assumed that the codopant Liþ may serve as a self-promoter for the crystallinity of MgO:Eu3þ nanocrystals, as it follows from both XRD and TEM images. Moreover, the ð5 D0 27 F2 Þ= ð5 D0 27 F1 Þ intensity ratio (known also as the asymmetry ratio) of Eu3þ -doped MgO nanocrystals has a calculated value of 1.6, while for Liþ -doped MgO:Eu3þ nanocrystals has a value of 3.0. When introducing Liþ into the host, as ˚ is similar to that of Mg2þ ion the radius of Liþ ion ð0:76 AÞ ˚ ð0:72 AÞ, it can be assumed that the Liþ ion can substitute for Mg2þ ion extensively, leaving somewhere in the lattice in the form of oxygen vacancy to compensate the negative charge of the Li0Mg . As shown in the inset of Fig. 1, the (2 0 0) diffraction peak shifts toward higher angle, so implying a reduction in the lattice parameter. The shrinking of the lattice will not favor the substitution of Eu3þ for Mg2þ , and hence it is reasonable that most of the doped Eu3þ ions will reside on the surface or on the grain boundaries of the MgO nanocrystals. The increment of the asymmetry ratio can further prove that there exists a change of symmetry and of vibrational modes around the luminescent Eu3þ centers owing to the Liþ doping. 4. Conclusions In summary, Eu3þ - and Eu3þ , Liþ -doped MgO nanocrystals were successfully synthesized by a facile combustion method. The MgO nanocrystals prepared have been characterized using microstructure and luminescence measurements. Room-temperature photoluminescence measurements under excitation at 330 nm of the MgO:Eu3þ sample show different and stronger emission bands in comparison with that excited at 395 nm. Finally, the luminescence intensity has been greatly enhanced after introducing Liþ into the host, acting as self-promoter for the crystallinity of the MgO:Eu3þ nanocrystals. Acknowledgments
3þ
Fig. 5. Emission spectra of MgO:Eu under excitation at 395 nm.
3þ
þ
and MgO:Eu , Li nanocrystals
This work was supported by the National Natural Science Foundation of China (20236020), the Shanghai Municipal Science and Technology Commission
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(04DZ14002, 04DZ05622) and the pre-973 project of China (2002CCA2200). References [1] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994. [2] R. Scheps, Pros. Quantum Electron 20 (1996) 271. [3] S. Ram, S.K. Sinha, J. Solid State Chem. 66 (1987) 225. [4] H. Tsuji, F. Yagi, H. Hattori, H. Kita, J. Catalysis 148 (1994) 759. [5] A.N. Copp, Am. Ceram. Soc. Bull. 74 (1995) 135. [6] A. Bhargava, J.A. Alarco, I.D.R. Mackinnon, D. Page, A. Ilyushechkin, Mater. Lett. 34 (1998) 133. [7] Y.S. Yuan, M.S. Wong, S.S. Wang, J. Mater. Res. 11 (1996) 8. [8] P.D. Yang, C.M. Lieber, Science 273 (1996) 1836. [9] J. Zhang, L.D. Zhang, Chem. Phys. Lett. 363 (2002) 293. [10] J. Zhan, Y. Bando, J. Hu, D. Golberg, Inrog. Chem. 43 (2004) 2462. [11] Y. Yin, G. Zhang, Y. Xia, Adv. Funct. Mater. 12 (2002) 293. [12] R. Ma, Y. Bando, Chem. Phys. Lett. 370 (2002) 770.
[13] Y. Ding, G. Zhang, H. Wu, B. Hai, L. Wang, Y. Qian, Chem. Mater. 13 (2001) 435. [14] R. Garcia, G.A. Hirata, J. McKittrick, J. Mater. Res. 16 (2001) 1059. [15] F. Gu, S.F. Wang, M.K. Lu, G.J. Zhou, D. Xu, D.R. Yuan, Langmuir 20 (2004) 3528. [16] L.D. Sun, C. Qian, C.S. Liao, X.L. Wang, C.H. Yan, Solid State Commun. 119 (2001) 393. [17] J.C. Park, H.K. Moon, D.K. Kim, S.H. Byeon, B.C. Kim, K.S. Suh, Appl. Phys. Lett. 77 (2000) 2162. [18] D.M. Murphy, R.D. Farley, I.J. Purnell, C.C. Rowlands, A.R. Yacob, M.C. Paganini, E. Giamello, J. Phys. Chem. B 103 (1999) 1944. [19] S.M. Yeh, C.S. Su, Mater. Sci. Eng. B 38 (1996) 245. [20] S.H. Sohn, D.G. Hyun, A. Yamada, Y. Hamakawa, Appl. Phys. Lett. 62 (1993) 991. [21] K.A. Hyeon, S.H. Byeon, J.C. Park, D.K. Kim, K.S. Suh, Solid State Commun. 115 (2000) 99. [22] S.H. Byeon, M.G. Ko, J.C. Park, D.K. Kim, Chem. Mater. 14 (2002) 603.
Journal of Crystal Growth 289 (2006) 400–404 www.elsevier.com/locate/jcrysgro
Combustion synthesis and photoluminescence of MgO : Eu3þ nanocrystals with Liþ addition Feng Gu, Chun Zhong Li, Hai Bo Jiang Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China Received 25 September 2005; received in revised form 8 November 2005; accepted 15 November 2005 Available online 3 February 2006 Communicated by R. Kern
Abstract Well-crystalline MgO nanocrystals have been synthesized successfully via a facile combustion method with lithium addition. After Liþ doping into the MgO:Eu3þ host, the crystallinity of the sample becomes better, resulting in enhancement of luminescence intensity of the characteristic emission of Eu3þ ions. This approach provides economically viable route for large-scale synthesis of this kind of nanomaterials. r 2006 Elsevier B.V. All rights reserved. PACS: 61.46; 78.55; 81.10 Keywords: A1. Low-dimensional structures; A1. X-ray diffraction; B1. Nanomaterials; B2. Phosphors
1. Introduction Recent progress in optical devices, such as lasers and optical amplifiers, based on electronic transitions of rareearth ions, has inspired a lot of work in different materials doped with these ions [1,2]. Of the many rare-earth ions, Eu3þ ions have been shown to behave as potential as an efficient probe for the microstructure of material because the fluorescence from Eu3þ ions originates from 5d to 4f transition, and the 5d state is easily affected by the outer crystal field [3]. As a type of promising oxide material, magnesium oxide (MgO) has found applications in a number of technologies, including catalysis [4], toxic waste remediation [5], or as additives in refractory, paint, and superconductor products [6–9]. Recently, many investigations have been carried out on the optical and electrical properties of MgO nanocrystals [9]. For many applications, rare-earth ions doped MgO nanocrystals have been reported, exhibiting well optical properties. Since the f–f absorption transitions in rare-earth ions are parity Corresponding authors. Tel.: +86 21 64252055; fax: +86 21 64250624.
E-mail address: [email protected] (F. Gu). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.11.116
forbidden, the number of carriers excited through f–f transitions is much less than that excited through excitation of host. Moreover, the luminescence efficiency of these rare-earth doped oxide materials is a key factor influencing their practical applications. With respect to luminescent materials, one solution to this problem is to find a way to enhance the energy transfer from host to doped rare-earth ions and decrease the influence of defects on the optical properties. It has been reported that, in some oxide matrix, the addition of coactive ions or charge compensators, such as lithium or nitrogen ions, could facilitate energy transfer from the host to doped rare-earth ions, resulting in the enhancement of luminescence intensity. In this paper, lithium ions are chosen as co-dopant into MgO:Eu3þ host to investigate the influence on the photoluminescence(PL) properties. Until now, diverse methods have been employed to synthesize nanocrystalline MgO, such as thermal evaporation, vapor-phase transport, sol–gel, and hydrothermal methods [9–13]. Generally, the above-mentioned methods are rather sophisticated or constrained by expense or apparatus. Compared with the conventional synthesis methods, combustion method has an advantage for
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preparing ultrafine materials due to its low cost, high yield, and good ability to achieve high purity in making single or multiphase complex oxide powders at the as-synthesized state [14,15]. Herein, we report a novel and facile combustion synthesis of MgO:Eu3þ nanocrystals having size of 20–25 nm and the effect of Liþ ions on the photoluminescence properties has been studied systematically. The experimental results show that the crystallinity of the MgO crystals becomes better after Liþ doping, resulting in the remarkable enhancement of PL intensity. 2. Experimental procedure All of the reactants are analytical-grade and used without any further purification. The raw materials are magnesium nitrate ðMgðNO3 Þ2 6H2 O), europium nitrate ðEuðNO3 Þ3 6H2 OÞ and lithium chloride ðLiCl 6H2 OÞ. Urea ðNH2 CONH2 Þ was used as fuel for the combustion. The mixture of reagents was dissolved in a minimum amount of deionized water to obtain a homogeneous solution and the concentrations of europium and lithium are 1% and 10%. After stirring, precursor solution was transferred to a muffle furnace at 550 C for 20 min. As the ignition occurred, the reaction went on vigorously for a few seconds. Then, the fluffy products were collected for characterization. X-ray powder diffraction (XRD) data were recorded on a Japan Rigaku D/max-rA X-ray diffractometer system with graphite monochromatized CuKa radiation ðl ¼ 0:15418 nmÞ. 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 Nicolet Magna-550 infrared spectrometer. The PL spectra were measured with a Hitachi M-850 Fluorescence Spectrometer. 3. Results and discussion During the combustion process, the reaction temperature rose very rapidly from 550 C to about 1500 C in a very short time along with vigorous vapor gas. The MgO vapor precipitated and formed the resultant products in few seconds. XRD was used to examine the crystal structure and phase purity of the products, and the typical XRD patterns of the Eu3þ -doped MgO and Eu3þ , Liþ codoped MgO samples are shown in Fig. 1. For comparison, the diffraction pattern of commercial MgO is also included. The broadening of the diffraction peaks of the samples indicates that the particle sizes are in the nano-scale range. All the diffraction lines are assigned to cubic crystalline phase of magnesium oxide (JCPDS 4-829). Without Liþ co-doping, the crystallinity of MgO:Eu3þ is poor as indicated by relatively weak and broad diffraction peaks, whereas the diffraction intensity of Liþ -doped MgO:Eu3þ is remarkably increased. It is likely that much better crystallinity of the present MgO:Eu3þ , Liþ samples can be
Fig. 1. XRD patterns of the as-prepared MgO nanocrystals: (a) MgO:Eu3þ , (b) MgO:Eu3þ , Liþ . Inset shows the enlarged diffraction peak of (2 0 0) of these two samples. For comparison, the pattern of commercial MgO is also included (c).
Fig. 2. TEM images of the as-prepared MgO nanocrystals: (a) pristine MgO, (b) MgO:Eu3þ , (c) MgO:Eu3þ , Liþ . The bar is 100 nm. Insets show the corresponding ED patterns of these three samples.
obtained even under similar combustion condition. The full-width at half-maximum (FWHM) of the (2 0 0) peak of Liþ -doped MgO:Eu3þ is much smaller than that of MgO:Eu3þ without Liþ doping (shown in inset of Fig. 1). Moreover, the (2 0 0) peak position of MgO : Eu3þ , Liþ shifts compared with that of MgO:Eu3þ , indicating that there exists a change with respect to the lattice parameter after Liþ doping, which may affect the optical properties of doped Eu3þ ions. TEM images of the as-prepared MgO nanocrystals are shown in Fig. 2. It is obvious that the pristine MgO crystals exhibit cuboid-like morphology and the particle size is distributed uniformly in the range of 20–25 nm (Fig. 2a). Introduction of Eu3þ into the host hardly affects the morphology and size of the sample, whereas a size value of about 40–60 nm was found for Eu3þ , Liþ co-doped sample, which is well in accordance with the XRD results (Fig. 2b and c). The ED patterns of the three samples are given in the insets of the corresponding figure. The diffraction rings could be assigned to 200, 220, and 222 planes of cubic MgO from the centermost ring, respectively. Therefore, the Liþ addition should facilitate the crystallization of MgO, which can also be observed in some hosts such as Y2 O3 and Gd2 O3 [16,17]. Fig. 3 shows the FTIR spectra of the prepared MgO samples with and without lithium addition. The strong band at 417 cm1 can be observed, associated with the
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Fig. 3. FTIR spectra of the as-prepared MgO samples before (a) and after (b) lithium addition.
characteristic vibrational mode of symmetric MgO6 octahedra of MgO. The absorption at 3430 cm1 indicates the presence of hydroxyl-groups, which is probably due to the fact that the spectra were not recorded in situ and some water readsorption from the ambient atmosphere has occurred. After introducing lithium ions into the host, the absorption in the range of 130021800 cm1 related to hydroxyl groups of molecular water at 1638 cm1 and to NH3 at 1439 cm1 decreased sharply, implying that better crystallinity can be obtained after combustion at 550 C. PL spectra were recorded at room temperature on both the pristine and doped MgO nanocrystals prepared by combustion method. Under excitation at 294 nm, the PL spectrum of the pristine MgO sample exhibits a strong luminescence band centered at 382 nm (not shown here). With respect to MgO, especially at the surface, there exist a number of reasonably well-defined surface defect structures such as low coordinated ions and/or vacancies [18]. Therefore, in the past decade, many researchers have paid much interest for MgO which has become one of the preferred targets of surface structural and catalytic research. Zhang et al. reported a similar band at 383 nm in the PL spectrum of MgO nanobelts [9], which has been proposed originating from the contribution of oxygen vacancies in the MgO host. However, the other two emission bands at 508 and 721 nm have not be observed in the present case, which might attribute to the different morphology and experimental conditions. In the present case, due to the rapid combustion process, oxygen vacancies should also be generated because of partially incomplete crystallization. In addition, the MgO nanocrystals with large surface-to-volume ratio should also favor the existence of large quantities of oxygen vacancies. The energy levels of the f–f transitions in Eu3þ are wellknown, the 5 D0 ! 7 F1 transition is mainly magnetically allowed and independent on the site symmetry in which europium situated, while the 5 D0 ! 7 F2 is a forced electric dipole transition being allowed only at low symmetries with no inversion center. The ratio between the electric
Fig. 4. Emission spectra of the MgO:Eu3þ nanocrystals under excitation at 395 and 330 nm, respectively.
dipole and the magnetic dipole is a measure of the site symmetry in which europium is situated. Following the direct excitation of the 7 F0 ! 5 L6 transition of Eu3þ ion with 395 nm, two dominant emission bands at 590 and 614 nm were observed in MgO:Eu3þ , which can be assigned to 5 D0 27 FJ ; J ¼ 1; 2, transitions, respectively (shown in Fig. 4). The asymmetry ratio of Eu3þ -doped MgO nanocrystals is calculated and has a value of 1.6. However, under excitation at 330 nm, a 2 times stronger emission band has been observed with an asymmetry ratio of 4.1. The change of the asymmetry ratio is due to the different sites in which europium ions are situated. In the current MgO, the significant difference in the ionic radius bet˚ and Mg2þ ð0:72 AÞ ˚ results in difficulty ween Eu3þ ð1:07 AÞ 3þ for Eu to substitute for Mg2þ ion extensively. Thus, when introducing Eu3þ ions into the MgO host, it is suggested that Eu3þ ions will likely reside on the surface or on the grain boundaries of the nanocrystals to yield optimum strain relief. Following the direct excitation of the 7 F0 ! 5 L6 transition, the smaller asymmetry ratio
ARTICLE IN PRESS F. Gu et al. / Journal of Crystal Growth 289 (2006) 400–404
indicates the average site asymmetry of the dopant europium ions. However, upon photoexcitation of the Eu3þ -doped MgO samples at 330 nm, as the f–f absorption transitions of Eu3þ are parity forbidden, the number of carriers excited through f–f transitions in Eu3þ is much less. On the other hand, as indicated in the emission spectrum of the pristine MgO nanocrystals, a rather strong emission band centered at 382 nm originating from the contribution of oxygen vacancies formed during the combustion process, which is justly favored for the 7 F0 ! 5 L6 transition of Eu3þ ion. Therefore, the enhanced emission comes mainly from radiative recombination of the large amount of trapped carriers excited from MgO. It can be assumed that the oxygen vacancies formed during the combustion process might act as sensitizer for the energy transfer to the rare-earth ions at the surface, due to the strong mixing of charge transfer states resulting in the highly enhanced luminescence. Fig. 5 shows the photoluminescence spectra of the Eu3þ doped and Eu3þ , Liþ co-doped MgO nanocrystals under excitation at 395 nm. It has been observed that the emission intensity at 614 nm has been remarkably enhanced about 1.5 times after Liþ doping. The mechanism of the effect of Liþ on the optical properties is not very clear at this stage. It is generally accepted that, in bulk phosphor materials, a strong luminescence can be obtained with the presence of
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co-activators such as Liþ , even in very small quantities, owing to the facilitated energy transfer from the host to doped rare-earth ions. Lithium ion has been introduced into different phosphor host lattices such as Y2 O3 :Eu3þ , Gd2 O3 :Eu3þ , ZnS:Tm3þ , and ZnO:Dy3þ [15,16,19,20], acting as a co-activator and charge compensator, and the enhanced luminescence has been observed. On the other hand, in some publications, the effect of Liþ substitution on the luminescence properties has been investigated such as in SrTiO3 :Pr3þ and Gd2x Yx O3 :Eu3þ [21,22], which reveals that the Liþ addition remarkably affects the morphology of particles as well as the photo- and cathodoluminescent efficiency of phosphors. Generally, the phosphor particles of higher crystallinity have improved brightness. In the present case, it is assumed that the codopant Liþ may serve as a self-promoter for the crystallinity of MgO:Eu3þ nanocrystals, as it follows from both XRD and TEM images. Moreover, the ð5 D0 27 F2 Þ= ð5 D0 27 F1 Þ intensity ratio (known also as the asymmetry ratio) of Eu3þ -doped MgO nanocrystals has a calculated value of 1.6, while for Liþ -doped MgO:Eu3þ nanocrystals has a value of 3.0. When introducing Liþ into the host, as ˚ is similar to that of Mg2þ ion the radius of Liþ ion ð0:76 AÞ ˚ ð0:72 AÞ, it can be assumed that the Liþ ion can substitute for Mg2þ ion extensively, leaving somewhere in the lattice in the form of oxygen vacancy to compensate the negative charge of the Li0Mg . As shown in the inset of Fig. 1, the (2 0 0) diffraction peak shifts toward higher angle, so implying a reduction in the lattice parameter. The shrinking of the lattice will not favor the substitution of Eu3þ for Mg2þ , and hence it is reasonable that most of the doped Eu3þ ions will reside on the surface or on the grain boundaries of the MgO nanocrystals. The increment of the asymmetry ratio can further prove that there exists a change of symmetry and of vibrational modes around the luminescent Eu3þ centers owing to the Liþ doping. 4. Conclusions In summary, Eu3þ - and Eu3þ , Liþ -doped MgO nanocrystals were successfully synthesized by a facile combustion method. The MgO nanocrystals prepared have been characterized using microstructure and luminescence measurements. Room-temperature photoluminescence measurements under excitation at 330 nm of the MgO:Eu3þ sample show different and stronger emission bands in comparison with that excited at 395 nm. Finally, the luminescence intensity has been greatly enhanced after introducing Liþ into the host, acting as self-promoter for the crystallinity of the MgO:Eu3þ nanocrystals. Acknowledgments
3þ
Fig. 5. Emission spectra of MgO:Eu under excitation at 395 nm.
3þ
þ
and MgO:Eu , Li nanocrystals
This work was supported by the National Natural Science Foundation of China (20236020), the Shanghai Municipal Science and Technology Commission
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F. Gu et al. / Journal of Crystal Growth 289 (2006) 400–404
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