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Y2O3:Eu3+ core–shell submicrospheres
Materials Letters 60 (2006) 737 – 740 www.elsevier.com/locate/matlet
Synthesis and characterization of monodispersed SiO2/Y2O3:Eu 3+ core–shell submicrospheres Hua-Jun Feng a,b , Yuan Chen a,b , Fang-Qiong Tang a,⁎, Jun Ren a a
b
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101, PR China Graduate School of the Chinese Academy of Science, Chinese Academy of Sciences, Beijing 100000, PR China Received 23 December 2004; accepted 3 October 2005 Available online 11 November 2005
Abstract Monodispersed SiO2/Y2O3:Eu3+ core–shell submicrospheres were prepared through a simply homogeneous precipitation method. SEM, TEM, and Zetasizer analysis indicated the regular microstructures and uniform size distributions of obtained submicrospheres. FTIR spectra showed that the Y2O3:Eu3+ shell had linked to the silica surface by forming a Si–O–Y bond. XRD patterns characterized the crystal structures of deposited Y2O3:Eu3+ shell after heat treatment. Photoluminescence studies showed that the thickness of Y2O3:Eu3+ shell had greatly affected the luminescent property of coated particles. © 2005 Elsevier B.V. All rights reserved. Keywords: Rare earth oxides; Core–shell submicrospheres; Luminescence; Precipitation
1. Introduction It is well known that much attention has been paid on europium-doped Y2O3 phosphors (Y2O3:Eu3+) because it is widely used in lighting and cathode ray tubes [1] as a red-emitting phosphor. Recent researches have indicated that it is also the promising candidate for field emission display (FED) devices [2]. For the FED applications, it is highly desirable to offer spherical phosphors with controllable diameters and narrow size distributions [3]. For that purpose, wet chemical techniques (e.g. sol–gel [4], hydrothermal [5], chemical precipitation [6– 8], emulsion [9], etc.) have received considerable attention during the last few years because they offer the possibilities for controlling homogeneity, purity of phase, size distribution, surface area and microstructures uniformity of the powder. However, the prices of rare earth oxides are very expensive and this has limited their applications to a great extent. Recently, core–shell particles have attracted a great deal of interest because of their difference from those of single-component materials [10,11]. The core–shell structures of these particles make it easy to tailor or combine their magnetic, optical, ⁎ Corresponding author. Tel.: +86 10 64888064; fax: +86 10 64862951. E-mail address: [email protected] (F.-Q. Tang). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.10.022
electrical, mechanical or catalytic properties in a controllable way [12]. Especially, coating relative inexpensive cores with the expensive shell materials can lower the cost of materials [13]. So combining the excellent luminescence properties of Y2O3:Eu3+ as a deposited layer on the inertia cores is a potential way to fabricate the low-cost phosphors. Silica submicrospheres prepared by Stober method are the ideal core materials because of their inexpensiveness, easiness to get spherical particles with narrow size distribution, chemical inertness, and optical transparency. In this paper, we, for the first time, reported a new monodispersed SiO2/Y2O3:Eu3+ core– shell submicrospheres through coprecipitation method, which was easy to scale up. By controlling the hydrolysis of urea, the obtained particles had spherical, monodispersed morphology and uniform shell thickness. Although with the existence of silica cores, the luminescent efficiency was still high enough. The resulting photoluminescence properties, core–shell structure and morphology of obtained products were presented. 2. Experimental work Monodispersed silica spheres were prepared with the procedure originally described by Stober et al. [14], i.e., hydrolysis of tetraethyl orthosilicate in an ethanol solution containing
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Fig. 1. TEM photograph of monodispersed silica spheres with diameters of about 250 nm.
water and ammonia. In a typical experiment, a 5-mL ethanol solution of tetraethyl orthosilicate was added to a 35-mL ethanol solution of water and ammonia. The 40-mL mixture containing 0.3 mol/L tetraethyl orthosilicate, 2.3 mol/L H2O, and 1 mol/L NH3 was stirred at 30 °C for 5 h. The resulting silica spheres were centrifugally separated from the suspension and ultrasonically washed with ethanol. For analysis, the silica spheres were further washed with water. In this paper, we use 250 nm silica spheres as core materials, and the addition of silica core in the following experiments is 0.4 g/100 mL aqueous solution unless especially explained. Y2O3 and Eu2O3 powders were, respectively, dissolved in excessive nitric acid to form Y(NO3)3 or Eu(NO3)3 solutions. Then the solutions were heated under 60 °C to remove the excessive nitric acid. At the same time, Y(NO3)3 or Eu(NO3)3 crystals were deposited with the evaporation of solvents. Until the solvents were exhausted, these deposited crystals were collected and dissolved again in distilled water to form the aqueous solution of Y(NO3)3 or Eu(NO3)3. A certain quantity of silica spheres was ultrasonically dispersed in 100 mL aqueous solution containing yttrium nitrate (0.02 mol/L), europium nitrate (8 × 10− 4 mol/L) and urea (2.4 g). The dispersions were then aged at 85–90 °C under continuous stirring for 2 h. The resulting precursors were separated by centrifugation, washed by ethanol and distilled water for several times, and then dried at 70 °C. At last, the samples were heated at 700 °C in air for 3 h to produce the final particles.
Fig. 3. Particle size distribution of SiO2 (a) and SiO2/Y2O3:Eu3+ (b) measured by Malvern Zetasizer.
The morphologies of SiO2/Y2O3:Eu3+ core–shell submicrospheres produced were observed using a transmission electron microscope (JEM-200CX electron microscope) and a scanning electron microscope (Hitachi). The size distribution of particles was measured by Malvern Zetasizer 3000HS. Particles' structure and surface bonding between core and shell were detected by a X-ray diffractometer and a Fourier transform infrared spectrometer. The emission spectra of samples were obtained using a Hitachi F-4500 FL Spectrophotometer excited at 250 nm with a 150-W xenon lamp. 3. Results and discussion Fig. 1 shows the TEM photograph of monodispersed SiO2 spheres of about 250 nm prepared by Stober method. In this work, we choose monodispersed 250-nm silica spheres as core materials. Monodispersed silica spheres can offer a good base to prepare monodispersed core–shell luminescent spheres because of silica's chemical inertness, optical transparency, and inexpensiveness. Fig. 2 shows the SEM photographs of SiO2 (a) and SiO2/Y2O3:Eu3+ (b) samples. The picture quality of Fig. 2a is very blur because of the discharge phenomena of bare SiO2 materials under the SEM circumstances. Oppositely, the clear quality of Fig. 2b (SiO2/Y2O3:Eu3+) shows that the luminescent shell has coated on the surface of silica spheres resulting in the disappearance of blur phenomena of bare silica. The polydispersity of SiO2 samples is 0.05 with the diameter of about 250 nm. And the polydispersity of SiO2/Y2O3:Eu3+ samples is 0.12
Fig. 2. SEM photographs of SiO2 (a) and SiO2/Y2O3:Eu3+ (b) samples.
H.-J. Feng et al. / Materials Letters 60 (2006) 737–740
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Fig. 4. TEM photographs of SiO2/Y2O3:Eu3+ samples (a, b) and their electronic diffraction pattern (c).
with the diameter of about 350 nm. Both Figs. 2 and 3 indicate a monodispersed size distribution. Fig. 4b is the magnification image of a single sphere in Fig. 4a, and Fig. 4c is their electronic diffraction pattern. TEM photographs of Fig. 4a and b show the uniform size distribution and clear core–shell structure of SiO2/Y2O3:Eu3+ particles. From Fig. 4b, we can see that the thickness of Y2O3:Eu3+ shell is about 50 nm. Fig. 4c characterizes the crystal structure of coated phosphors. In Fig. 4c, clearly exhibited rings demonstrate the polycrystalline nature of luminescent shell. FTIR spectra of both SiO2/Y2O3:Eu3+ samples (a), and bare silica spheres (b) are shown in Fig. 5. The infrared absorption peaks near 3500 cm− 1 are due to internal silanols and hydrogen bond surface silanols. A decrease in the relative intensity of this band is observed in Fig. 5a. The peaks near 1100 cm− 1 are assigned to the asymmetric stretching vibration modes of the Si–O–Si bridge of the siloxane link [15]. The peaks near 801 cm− 1 (Fig. 5b) correspond to the symmetric stretching of the Si–O–Si group and shift to 825 cm− 1 for SiO2/Y2O3: Eu3+ samples (Fig. 5a). Also a decrease in the relative intensity of these two peaks is observed which indicates that the coating of Y2O3:Eu3+ shell has decreased and even broken the Si–O–Si group. The band near 950 cm− 1 (Si–OH) for SiO2 (Fig. 5b) is almost non-existent in Fig. 5a. All these changes in FTIR spectra indicate the breaking of the surface Si–O–Si bonds of SiO2 and the formation of Si–O–Y bonds at the interface of SiO2 core and Y2O3:Eu3+ shell. The Si–O–Y bond would appear below 400 cm− 1, so we cannot see it in Fig. 5. X-ray diffraction patterns of both SiO2/Y2O3:Eu3+ core–shell samples (a), and bare silica spheres (b) are shown in Fig. 6. The bare silica is amorphous, as evident from the presence of a broader hump in Fig. 6b. In Fig. 6a, SiO2/Y2O3:Eu3+ core–shell samples present two sharp peaks assigned as (222) and (440) reflection lines, which indicates that Y2O3:Eu3+ crystal grows well. The emission spectra of SiO2/Y2O3:Eu3+ samples are shown in Fig. 7. All samples yield intense red light around 611 nm when excited at 250 nm. The photoluminescence emission is attributed to the electron
Fig. 5. FTIR spectra of (a) SiO2/Y2O3:Eu3+ and (b) SiO2 samples.
transition of Eu3+ in the S6 and C2 symmetry sites of Y2O3 from 5D0 to 7 F1a. From Fig. 7, it can be seen that the photoluminescent intensities of all three samples are affected by the addition of SiO2 spheres. With the increase of silica spheres, the PL intensities become weaker and weaker. The max value appears at 0.4 g silica spheres into 100 mL aqueous solution (Fig. 7a), and the thickness of luminescent shell is about 50 nm. Compared with Fig. 7a, the relative PL intensity of Fig. 7b is about 70%, and Fig. 7c just under 50%. It is well known that interface effect is one of the most important factors to influence the PL intensities of phosphors, because there are many defects in the interface such as hanging bonds, unexpected impurities, and imperfect crystals [16]. These defects destroy the periodical arrangement of atoms, and many nonradiative centers are then formed in the interface. The enhancement of nonradiative relaxation will result in the obvious decrease of PL intensities. In this work, with the increase of the amount of silica cores, the shell thickness is decreased, which means that the volume percentage of the interface is increased. So more
Fig. 6. XRD patterns of (a) SiO2/Y2O3:Eu3+ and (b) SiO2 samples.
Fig. 7. Emission spectra of SiO2/Y2O3:Eu3+ samples with different silica cores of (a) 0.4 g, (b) 0.6 g, and (c) 0.8 g SiO2. All samples were excited at 250 nm.
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interface percentage will lead to the increase of defects, and the decrease of the PL intensities. We have also synthesized Y2O3:Eu3+ shell with the thickness more than 50 nm. But the luminescent properties of them are almost the same as that of 50 nm shell. It can be explained that, when the shell is more than a critical thickness, the volume percentage of the interface varies so little that it cannot obviously influence the luminescent property.
4. Conclusions A simple precipitation method is used to prepare monodispersed SiO2/Y2O3:Eu3+ core–shell submicrospheres with obvious luminescence. SEM, TEM, and Zetasizer indicate their narrow size distribution and clear core–shell microstructure. FTIR spectra show that luminescent shell has linked to the surface of silica spheres by the formation of Si–O–Y bond. XRD patterns characterize that the Y2O3:Eu3+ is crystallized well after heat treatment. Photoluminescence studies show that the amount of silica spheres has an obvious influence on the luminescence property of Y2O3:Eu3+ shell. Acknowledgment This work has been supported by the Hi-Tech Research and Development Program of China (863) (2004AA302012) and
the National Natural Science Foundation of China (60372009, 20301015). References [1] T. Ye, Z. Guiwen, Z. Weiping, X. Shangda, Mater. Res. Bull. 32 (1977) 501. [2] V.A. Bolchouchine, E.T. Goldburt, B.N. Levonovitch, V.N. Litchmanova, N.P. Sochtine, J. Lumin. 87/89 (2000) 1277. [3] A. Vecht, C. Gibbons, D. Davies, X. Jing, P. Marsh, T. Ireland, J. Silver, A. Newport, D. Barber, J. Vac. Sci. Technol., B 17 (1999) 750. [4] Xiaoheng Liu, Juan Yang, Ling Wang, Xujie Yang, Lude Lu, Xin Wang, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 289 (2000) 241. [5] Seokwoo Jeon, Paul V. Braun, Chem. Mater. 15 (2003) 1256. [6] D. Sordelet, M. Akinc, J. Colloid Interface Sci. 122 (1988) 47. [7] N. Kawahashi, E. Matijevic, J. Colloid Interface Sci. 138 (1990) 534. [8] H. Giesche, E. Matijevic, J. Mater. Res. 9 (1994) 436. [9] X. Fu, Syed Qutubuddin, Colloid Surf. 179 (2001) 65. [10] R.C. Plaza, J.D.G. Duran, A. Quirantes, J. Colloid Interface Sci. 194 (1997) 398. [11] A. Garg, E. Matijevic, J. Colloid Interface Sci. 126 (1988) 243. [12] A.S. Nair, R.T. Tom, V. Suryanarayanan, T. Pradeep, J. Mater. Chem. 13 (2003) 297. [13] Y. Yin, Y. Liu, B. Gates, Y. Xia, Chem. Mater. 13 (2001) 1146. [14] W. Stober, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62. [15] S. Ramesh, Y. Koltypin, R. Prozorov, A. Gedanken, Chem. Mater. 9 (1997) 546. [16] R.N. Bhargava, D. Gallagher, T. Wekler, J. Lumin. 60/61 (1994) 275.
Synthesis and characterization of monodispersed SiO2/Y2O3:Eu 3+ core–shell submicrospheres Hua-Jun Feng a,b , Yuan Chen a,b , Fang-Qiong Tang a,⁎, Jun Ren a a
b
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101, PR China Graduate School of the Chinese Academy of Science, Chinese Academy of Sciences, Beijing 100000, PR China Received 23 December 2004; accepted 3 October 2005 Available online 11 November 2005
Abstract Monodispersed SiO2/Y2O3:Eu3+ core–shell submicrospheres were prepared through a simply homogeneous precipitation method. SEM, TEM, and Zetasizer analysis indicated the regular microstructures and uniform size distributions of obtained submicrospheres. FTIR spectra showed that the Y2O3:Eu3+ shell had linked to the silica surface by forming a Si–O–Y bond. XRD patterns characterized the crystal structures of deposited Y2O3:Eu3+ shell after heat treatment. Photoluminescence studies showed that the thickness of Y2O3:Eu3+ shell had greatly affected the luminescent property of coated particles. © 2005 Elsevier B.V. All rights reserved. Keywords: Rare earth oxides; Core–shell submicrospheres; Luminescence; Precipitation
1. Introduction It is well known that much attention has been paid on europium-doped Y2O3 phosphors (Y2O3:Eu3+) because it is widely used in lighting and cathode ray tubes [1] as a red-emitting phosphor. Recent researches have indicated that it is also the promising candidate for field emission display (FED) devices [2]. For the FED applications, it is highly desirable to offer spherical phosphors with controllable diameters and narrow size distributions [3]. For that purpose, wet chemical techniques (e.g. sol–gel [4], hydrothermal [5], chemical precipitation [6– 8], emulsion [9], etc.) have received considerable attention during the last few years because they offer the possibilities for controlling homogeneity, purity of phase, size distribution, surface area and microstructures uniformity of the powder. However, the prices of rare earth oxides are very expensive and this has limited their applications to a great extent. Recently, core–shell particles have attracted a great deal of interest because of their difference from those of single-component materials [10,11]. The core–shell structures of these particles make it easy to tailor or combine their magnetic, optical, ⁎ Corresponding author. Tel.: +86 10 64888064; fax: +86 10 64862951. E-mail address: [email protected] (F.-Q. Tang). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.10.022
electrical, mechanical or catalytic properties in a controllable way [12]. Especially, coating relative inexpensive cores with the expensive shell materials can lower the cost of materials [13]. So combining the excellent luminescence properties of Y2O3:Eu3+ as a deposited layer on the inertia cores is a potential way to fabricate the low-cost phosphors. Silica submicrospheres prepared by Stober method are the ideal core materials because of their inexpensiveness, easiness to get spherical particles with narrow size distribution, chemical inertness, and optical transparency. In this paper, we, for the first time, reported a new monodispersed SiO2/Y2O3:Eu3+ core– shell submicrospheres through coprecipitation method, which was easy to scale up. By controlling the hydrolysis of urea, the obtained particles had spherical, monodispersed morphology and uniform shell thickness. Although with the existence of silica cores, the luminescent efficiency was still high enough. The resulting photoluminescence properties, core–shell structure and morphology of obtained products were presented. 2. Experimental work Monodispersed silica spheres were prepared with the procedure originally described by Stober et al. [14], i.e., hydrolysis of tetraethyl orthosilicate in an ethanol solution containing
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H.-J. Feng et al. / Materials Letters 60 (2006) 737–740
Fig. 1. TEM photograph of monodispersed silica spheres with diameters of about 250 nm.
water and ammonia. In a typical experiment, a 5-mL ethanol solution of tetraethyl orthosilicate was added to a 35-mL ethanol solution of water and ammonia. The 40-mL mixture containing 0.3 mol/L tetraethyl orthosilicate, 2.3 mol/L H2O, and 1 mol/L NH3 was stirred at 30 °C for 5 h. The resulting silica spheres were centrifugally separated from the suspension and ultrasonically washed with ethanol. For analysis, the silica spheres were further washed with water. In this paper, we use 250 nm silica spheres as core materials, and the addition of silica core in the following experiments is 0.4 g/100 mL aqueous solution unless especially explained. Y2O3 and Eu2O3 powders were, respectively, dissolved in excessive nitric acid to form Y(NO3)3 or Eu(NO3)3 solutions. Then the solutions were heated under 60 °C to remove the excessive nitric acid. At the same time, Y(NO3)3 or Eu(NO3)3 crystals were deposited with the evaporation of solvents. Until the solvents were exhausted, these deposited crystals were collected and dissolved again in distilled water to form the aqueous solution of Y(NO3)3 or Eu(NO3)3. A certain quantity of silica spheres was ultrasonically dispersed in 100 mL aqueous solution containing yttrium nitrate (0.02 mol/L), europium nitrate (8 × 10− 4 mol/L) and urea (2.4 g). The dispersions were then aged at 85–90 °C under continuous stirring for 2 h. The resulting precursors were separated by centrifugation, washed by ethanol and distilled water for several times, and then dried at 70 °C. At last, the samples were heated at 700 °C in air for 3 h to produce the final particles.
Fig. 3. Particle size distribution of SiO2 (a) and SiO2/Y2O3:Eu3+ (b) measured by Malvern Zetasizer.
The morphologies of SiO2/Y2O3:Eu3+ core–shell submicrospheres produced were observed using a transmission electron microscope (JEM-200CX electron microscope) and a scanning electron microscope (Hitachi). The size distribution of particles was measured by Malvern Zetasizer 3000HS. Particles' structure and surface bonding between core and shell were detected by a X-ray diffractometer and a Fourier transform infrared spectrometer. The emission spectra of samples were obtained using a Hitachi F-4500 FL Spectrophotometer excited at 250 nm with a 150-W xenon lamp. 3. Results and discussion Fig. 1 shows the TEM photograph of monodispersed SiO2 spheres of about 250 nm prepared by Stober method. In this work, we choose monodispersed 250-nm silica spheres as core materials. Monodispersed silica spheres can offer a good base to prepare monodispersed core–shell luminescent spheres because of silica's chemical inertness, optical transparency, and inexpensiveness. Fig. 2 shows the SEM photographs of SiO2 (a) and SiO2/Y2O3:Eu3+ (b) samples. The picture quality of Fig. 2a is very blur because of the discharge phenomena of bare SiO2 materials under the SEM circumstances. Oppositely, the clear quality of Fig. 2b (SiO2/Y2O3:Eu3+) shows that the luminescent shell has coated on the surface of silica spheres resulting in the disappearance of blur phenomena of bare silica. The polydispersity of SiO2 samples is 0.05 with the diameter of about 250 nm. And the polydispersity of SiO2/Y2O3:Eu3+ samples is 0.12
Fig. 2. SEM photographs of SiO2 (a) and SiO2/Y2O3:Eu3+ (b) samples.
H.-J. Feng et al. / Materials Letters 60 (2006) 737–740
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Fig. 4. TEM photographs of SiO2/Y2O3:Eu3+ samples (a, b) and their electronic diffraction pattern (c).
with the diameter of about 350 nm. Both Figs. 2 and 3 indicate a monodispersed size distribution. Fig. 4b is the magnification image of a single sphere in Fig. 4a, and Fig. 4c is their electronic diffraction pattern. TEM photographs of Fig. 4a and b show the uniform size distribution and clear core–shell structure of SiO2/Y2O3:Eu3+ particles. From Fig. 4b, we can see that the thickness of Y2O3:Eu3+ shell is about 50 nm. Fig. 4c characterizes the crystal structure of coated phosphors. In Fig. 4c, clearly exhibited rings demonstrate the polycrystalline nature of luminescent shell. FTIR spectra of both SiO2/Y2O3:Eu3+ samples (a), and bare silica spheres (b) are shown in Fig. 5. The infrared absorption peaks near 3500 cm− 1 are due to internal silanols and hydrogen bond surface silanols. A decrease in the relative intensity of this band is observed in Fig. 5a. The peaks near 1100 cm− 1 are assigned to the asymmetric stretching vibration modes of the Si–O–Si bridge of the siloxane link [15]. The peaks near 801 cm− 1 (Fig. 5b) correspond to the symmetric stretching of the Si–O–Si group and shift to 825 cm− 1 for SiO2/Y2O3: Eu3+ samples (Fig. 5a). Also a decrease in the relative intensity of these two peaks is observed which indicates that the coating of Y2O3:Eu3+ shell has decreased and even broken the Si–O–Si group. The band near 950 cm− 1 (Si–OH) for SiO2 (Fig. 5b) is almost non-existent in Fig. 5a. All these changes in FTIR spectra indicate the breaking of the surface Si–O–Si bonds of SiO2 and the formation of Si–O–Y bonds at the interface of SiO2 core and Y2O3:Eu3+ shell. The Si–O–Y bond would appear below 400 cm− 1, so we cannot see it in Fig. 5. X-ray diffraction patterns of both SiO2/Y2O3:Eu3+ core–shell samples (a), and bare silica spheres (b) are shown in Fig. 6. The bare silica is amorphous, as evident from the presence of a broader hump in Fig. 6b. In Fig. 6a, SiO2/Y2O3:Eu3+ core–shell samples present two sharp peaks assigned as (222) and (440) reflection lines, which indicates that Y2O3:Eu3+ crystal grows well. The emission spectra of SiO2/Y2O3:Eu3+ samples are shown in Fig. 7. All samples yield intense red light around 611 nm when excited at 250 nm. The photoluminescence emission is attributed to the electron
Fig. 5. FTIR spectra of (a) SiO2/Y2O3:Eu3+ and (b) SiO2 samples.
transition of Eu3+ in the S6 and C2 symmetry sites of Y2O3 from 5D0 to 7 F1a. From Fig. 7, it can be seen that the photoluminescent intensities of all three samples are affected by the addition of SiO2 spheres. With the increase of silica spheres, the PL intensities become weaker and weaker. The max value appears at 0.4 g silica spheres into 100 mL aqueous solution (Fig. 7a), and the thickness of luminescent shell is about 50 nm. Compared with Fig. 7a, the relative PL intensity of Fig. 7b is about 70%, and Fig. 7c just under 50%. It is well known that interface effect is one of the most important factors to influence the PL intensities of phosphors, because there are many defects in the interface such as hanging bonds, unexpected impurities, and imperfect crystals [16]. These defects destroy the periodical arrangement of atoms, and many nonradiative centers are then formed in the interface. The enhancement of nonradiative relaxation will result in the obvious decrease of PL intensities. In this work, with the increase of the amount of silica cores, the shell thickness is decreased, which means that the volume percentage of the interface is increased. So more
Fig. 6. XRD patterns of (a) SiO2/Y2O3:Eu3+ and (b) SiO2 samples.
Fig. 7. Emission spectra of SiO2/Y2O3:Eu3+ samples with different silica cores of (a) 0.4 g, (b) 0.6 g, and (c) 0.8 g SiO2. All samples were excited at 250 nm.
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H.-J. Feng et al. / Materials Letters 60 (2006) 737–740
interface percentage will lead to the increase of defects, and the decrease of the PL intensities. We have also synthesized Y2O3:Eu3+ shell with the thickness more than 50 nm. But the luminescent properties of them are almost the same as that of 50 nm shell. It can be explained that, when the shell is more than a critical thickness, the volume percentage of the interface varies so little that it cannot obviously influence the luminescent property.
4. Conclusions A simple precipitation method is used to prepare monodispersed SiO2/Y2O3:Eu3+ core–shell submicrospheres with obvious luminescence. SEM, TEM, and Zetasizer indicate their narrow size distribution and clear core–shell microstructure. FTIR spectra show that luminescent shell has linked to the surface of silica spheres by the formation of Si–O–Y bond. XRD patterns characterize that the Y2O3:Eu3+ is crystallized well after heat treatment. Photoluminescence studies show that the amount of silica spheres has an obvious influence on the luminescence property of Y2O3:Eu3+ shell. Acknowledgment This work has been supported by the Hi-Tech Research and Development Program of China (863) (2004AA302012) and
the National Natural Science Foundation of China (60372009, 20301015). References [1] T. Ye, Z. Guiwen, Z. Weiping, X. Shangda, Mater. Res. Bull. 32 (1977) 501. [2] V.A. Bolchouchine, E.T. Goldburt, B.N. Levonovitch, V.N. Litchmanova, N.P. Sochtine, J. Lumin. 87/89 (2000) 1277. [3] A. Vecht, C. Gibbons, D. Davies, X. Jing, P. Marsh, T. Ireland, J. Silver, A. Newport, D. Barber, J. Vac. Sci. Technol., B 17 (1999) 750. [4] Xiaoheng Liu, Juan Yang, Ling Wang, Xujie Yang, Lude Lu, Xin Wang, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 289 (2000) 241. [5] Seokwoo Jeon, Paul V. Braun, Chem. Mater. 15 (2003) 1256. [6] D. Sordelet, M. Akinc, J. Colloid Interface Sci. 122 (1988) 47. [7] N. Kawahashi, E. Matijevic, J. Colloid Interface Sci. 138 (1990) 534. [8] H. Giesche, E. Matijevic, J. Mater. Res. 9 (1994) 436. [9] X. Fu, Syed Qutubuddin, Colloid Surf. 179 (2001) 65. [10] R.C. Plaza, J.D.G. Duran, A. Quirantes, J. Colloid Interface Sci. 194 (1997) 398. [11] A. Garg, E. Matijevic, J. Colloid Interface Sci. 126 (1988) 243. [12] A.S. Nair, R.T. Tom, V. Suryanarayanan, T. Pradeep, J. Mater. Chem. 13 (2003) 297. [13] Y. Yin, Y. Liu, B. Gates, Y. Xia, Chem. Mater. 13 (2001) 1146. [14] W. Stober, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62. [15] S. Ramesh, Y. Koltypin, R. Prozorov, A. Gedanken, Chem. Mater. 9 (1997) 546. [16] R.N. Bhargava, D. Gallagher, T. Wekler, J. Lumin. 60/61 (1994) 275.