- Email: [email protected]
Preparation of YAG:Ce spheroidal phase-pure particles by solvo-thermal method and their photoluminescence
Journal of Alloys and Compounds 468 (2009) 571–574
Preparation of YAG:Ce spheroidal phase-pure particles by solvo-thermal method and their photoluminescence Zuogui Wu a , Xudong Zhang a,∗ , Wen He a , Yuanwei Du a , Naitao Jia a , Guogang Xu b a
Department of Materials Science and Engineering, Shandong Institute of Light Industry, Jinan, Shandong 250353, PR China b State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, PR China Received 29 October 2007; received in revised form 3 January 2008; accepted 11 January 2008 Available online 7 March 2008
Abstract Cerium-doped yttrium aluminum garnet (YAG:Ce) powder was synthesized by a solvo-thermal method under mild conditions with inexpensive aluminum and yttrium nitrates as the starting materials and the ethylenediamine (EDA) solution as the solvent. The formation of YAG:Ce was determined by X-ray diffraction (XRD) techniques and FT-IR spectra. The pure YAG crystalline phase was formed after solvo-thermal treatment at 200 ◦ C for 5 h and no intermediate phase was observed. Transmission electronic microscope (TEM) and scanning electronic microscope (SEM) images showed that the resultant YAG:Ce powders were basically spherical and well dispersed. The sizes of the grains were in the range of 200–260 nm. The photoluminescence spectrum of the crystalline YAG:Ce phosphors showed the green-yellow emission with 5d → 4f transition was the most prominent, and the excitation and emission intensities enhanced with prolonging the holding time. © 2008 Elsevier B.V. All rights reserved. Keywords: Ceramics; Chemical synthesis; Luminescence; X-ray diffraction
1. Introduction Yttrium aluminum garnet (Y3 Al5 O12 , YAG) doped with a small amount of impurities such as Eu, Cr and Tb is now widely used as optical host materials for its good optical properties in cathode-ray tubes (CRTs), field emission displays (FED), and scintillation and electro-luminescent applications [1–5]. Recently, polycrystalline fluorescence material has received much attention since the quality of YAG:Ce phosphor has been improved greatly [6,7]. Owing to such a wide and diverse application potential for YAG-based materials, new methods of synthesis of pure and homogeneously doped yttrium aluminum garnet are highly desirable. There have been many methods used for preparation of YAG-based materials. YAG powders are easy to form by the traditional solid-state method. However, it requires a high temperature more than 1100 ◦ C for eliminating several byproducts such as Y4 Al2 O9 (monoclinic yttrium aluminate, YAM) and YAlO3 (yttrium aluminum perovskite, YAP) [8,9]. YAG can
∗
Corresponding author. Tel.: +86 531 8963 1066; fax: +86 531 8861 9798. E-mail address: [email protected] (X. Zhang).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.01.064
directly crystallize from the amorphous precursor at the lower temperature of 900 ◦ C when co-precipitation method is used [4–6]. Potdevin et al. [10] have reported the sol–gel method for synthesizing of YAG particles at low temperature of 900 ◦ C. Kasuya et al. [11] synthesized YAG powders below 400 ◦ C by the glycothermal method using alkoxides as the starting materials. However, alkoxides are too expensive and the powders synthesized by this method with agglomeration of irregular grains, which are not beneficial for the application of YAG powders. In this study, inexpensive metal nitrates and ethylenediamine (EDA) solution were used as the starting materials and the reaction solvent, respectively. Well-dispersed spherical YAG phosphors were synthesized at the temperature as low as 200 ◦ C. 2. Experimental The yttrium, aluminum and cerium sources for YAG:Ce synthesis were yttrium oxide (99.99%), Al(NO3 )3 ·9H2 O (analytical grade) and Ce(NO3 )3 ·6H2 O (analytical grade), respectively. The ammonium hydrogen carbonate (AHC) was analytical grade. Aluminum nitrate solution and cerium nitrate solution were prepared by dissolving Al(NO3 )3 ·9H2 O and Ce(NO3 )3 ·6H2 O in distilled water, respectively. Yttrium nitrate solution was obtained by dissolving high purity Y2 O3 in HNO3 under stirring and heating. Subsequently, the above three salt solutions (the molar ratio of the Y3+ , Ce3+ and Al3+ was maintained as 3 − x:x:5, x = 0.04) were mixed homogeneously under
572
Z. Wu et al. / Journal of Alloys and Compounds 468 (2009) 571–574
mild stirring, which was used as the mother salt solution. 2 mol L−1 solution of AHC was used as the precipitant, and appropriate amount of ammonium sulfate was added to the solution. The precursor precipitate was prepared by adding 50 ml of the mother salt solution at a speed of 2 ml min−1 into 50 ml of the above ACH mixture solution contained in a beaker under vigorous agitation at room temperature. The final pH of the suspensions was 8.2. After aging for 24 h, the precipitates were repeatedly washed with distilled water to remove the residual nitric and ammonia ions. The precursor precipitates were dispersed in an aqueous solution of EDA, and then placed in an autoclave. Finally, the resulting suspension was heated to the desired temperature at a heating rate 2 ◦ C min−1 and kept at that temperature for varying times (1, 5 and 10 h). During the reaction period, the pressure gradually increased up to 10 MPa. When the reaction was finished, it was cooled to room temperature. The resulting powders were centrifugated and repeatedly washed with distilled water, and then dried in air at 100 ◦ C for 2 h. Phase development was observed using X-ray diffraction on a D8 advance (Switzerland, Bruker). X-ray diffractometer with Cu K␣ (λ = 0.15417 nm) radiation and a scanning speed of 0.2◦ /min. Fourier transform infrared spectroscopy (FT-IR) studies with crystalline KBr were performed (Model Tensor 27, Switzerland, Bruker). The morphology and dispersion of the powders were observed using a transmission electron microscope (TEM; Hitachi model H-800) and a scanning electron microscope (SEM; Quanta200ESEM FElco-Holland). Photoluminescence (PL) spectra were measured by a Hitachi F-4500 spectrometer with a 1.0 cm−1 resolution equipped with a 150 W Xe lamp at room temperature. The excitation and emission slit widths are 1.0 nm.
3. Results and discussion Fig. 1 presents the XRD patterns for YAG:Ce powders selected holding time from 1 to 10 h and a theoretical one for comparison. Due to no obvious diffraction peaks were observed in sample a, it can be concluded that the precursor remained amorphous up to 200 ◦ C for 1 h. After holding for more than 5 h, some obvious peaks were found in sample b. The obtained diffraction pattern corresponded well to the standard sample (JCPDS Card 33-40), which indicated that the single phase of YAG was successfully prepared at the temperature as low as 200 ◦ C for 5 h. When the holding time was prolonged to 10 h, more distinct peaks with stronger intensities were observed and no other phases were detected, which indi-
Fig. 1. XRD patterns of the products obtained for different holding time at 200 ◦ C (a–c) and corresponding to the JCPDS ([33-40]) indexing (d).
Fig. 2. FT-IR spectra of the powders synthesized for different holding time at 200 ◦ C.
cated the growth of YAG crystallite with prolonging the holding time. Fig. 2 shows the FT-IR spectra of YAG:Ce phosphors obtained for different holding time at 200 ◦ C, respectively. FTIR analysis of the synthesized samples is important both for the reaction process and the properties of YAG:Ce phosphors. The broad band at 3406 cm−1 should be attributed to moisture absorbed on the surface of the sample. There is an absorption band at 1520 cm−1 , which is stretching vibration of the CO3 − vibration [12]. For inorganic nitrates, the bands in the range 1400–1370 and 840–820 cm−1 can be assigned to nitrate group [13]. Thus, the band at 1384 cm−1 is associated with NO3 − . The band at 1066 cm−1 corresponds to SO4 2− . The band at about 790 cm−1 represents the characteristics of Al–O vibrations, while the bands at about 727, 572 and 477 cm−1 represent the characteristics of Y–O vibrations [14–16]. As mentioned above, intensity of absorption bands for various organic groups decreases and gradually pyrolyzes with the holding time increase. From the IR patterns, it can be seen clearly that after holding for 5 h, the characteristic absorption bands of the sample related to metal–oxygen vibrations are detected. This indicates that YAG phase has formed, which is consistent with the above results of XRD. Generally, the luminescence properties of phosphor depend on the characteristics of the particles such as sizes, morphologies, crystallinities and number of defects. The TEM and SEM images of YAG:Ce powders obtained at 200 ◦ C for different holding time are shown in Fig. 3. It can be seen that the sizes of most of the YAG particles synthesized with a holding time of 5 h are in the range of 200–260 nm. And the particles are almost spherical (Fig. 3a, c and d). Importantly, the particles are not aggregated from each other, which will be beneficial for phosphor applications [17]. When the holding time is prolonged to 10 h, the YAG particles grow and the sizes increase slightly. Additionally, the formation of agglomerates occurs (Fig. 3b). This phenomenon is mainly caused by the dissolution of smaller YAG particles and the growth of some larger ones. With the growth of YAG particles, the attraction force between them will increase, which will cause the aggre-
Z. Wu et al. / Journal of Alloys and Compounds 468 (2009) 571–574
573
Fig. 3. TEM photographs of YAG:Ce particles obtained at 200 ◦ C: (a) 5 h, (b) 10 h and SEM photographs (c) and (d) 5 h.
gation among the particles. Therefore, a holding time of 5 h should be suitable for the synthesis of YAG powder with good properties. Fig. 4 shows the excitation (a) and emission (b) spectra of phosphors prepared at 200 ◦ C for different holding time. The excitation bands in all samples are located at 343 and 460 nm, respectively. These excitation bands are due to the electron transition from ground state of Ce3+ (2 F5/2, 2 F7/2 ) to the different crystal field splitting components of excited 5d state [18]. The emission spectrum for Ce3+ in sample 1 consists of a broad band with a maximum at 530 nm, while the emission spectrum for
samples 2 and 3 have a maximum at 525 nm. The Ce3+ has a 4f1 configuration and its excited configuration is 4f0 5d1 state. The 5d electron of the excited 4f0 5d1 configuration forms a 2 D term split into 2 D3/2 and 2 D5/2 states, and the 4f electron of the ground 4f1 configuration forms 2 F7/2 and 2 F5/2 states by spin–orbit coupling [19]. In YAG structure, the lowest 5d level (2 D3/2 ) is exceptionally low, i.e. at 22,000 cm−1 [18]. This energy corresponds to blue light. Following the prolonging of holding time, excitation and emission intensities increase. The increasing of intensity results from the enhancement of crystallization degree and the decrease of surface defects [20]. Despite synthesized for different holding time, the maximum excitation wavelength is hardly changed. Nano-sized particles usually show blue shift of the emission spectra [19]. Following the holding time increasing, the red shift phenomenon appears which maybe caused by the growth of the particle sizes. This is in line with the fact indicated by the emission spectrum in Fig. 4b. 4. Conclusions
Fig. 4. Excitation (a) and emission (b) spectra of the sample synthesized at 200 ◦ C for different holding time.
The YAG:Ce phosphors were prepared by the solvo-thermal method under mild conditions with ethylenediamine solution as the solvent. The amorphous precursor transforms directly to pure phase YAG:Ce at the temperature of 200 ◦ C for 5 h. At the same time, the degree of crystallization of YAG:Ce phase increased with the prolonging of holding time. TEM and SEM images showed that well-dispersed, spherical and submicronsized YAG:Ce particles with a relatively narrow grain size distribution can be synthesized. The obtained YAG:Ce parti-
574
Z. Wu et al. / Journal of Alloys and Compounds 468 (2009) 571–574
cles with spherical morphology and no aggregation have good photoluminescence characteristics. Acknowledgements This work was financially supported by the Natural Science Foundation of China of Shandong Province (Grant No. Y2003F02). The authors would like to thank the Analytical Center of Shandong Institute of Light Industry for sample characterization. References [1] S.M. Sim, K.A. Keller, J. Mater. Sci. 35 (2000) 713–716. [2] X.D. Zhang, H. Liu, W. He, J.Y. Wang, X. Li, R.I. Boughton, J. Alloys Compd. 372 (2004) 300–303. [3] Y.H. Zhou, J. Lin, M. Yu, S.M. Han, S.B. Wang, H.J. Zhang, Mater. Res. Bull. 38 (2003) 1289–1299. [4] X. Li, H. Liu, J.Y. Wang, H.M. Cui, X.D. Zhang, F. Han, Mater. Sci. Eng. A 379 (2004) 347–350. [5] R.A. Rodriguez, E. De la Rosa, L.A. Diaz-Torres, P. Salas, R. Melendrez, M. Barboza-Flores, Opt. Mater. 27 (2004) 293–299.
[6] G.G. Xu, X.D. Zhang, W. He, H. Liu, H. Li, R.I. Boughton, Mater. Lett. 60 (2006) 962–965. [7] G.D. Xia, S.M. Zhou, J.J. Zhang, J. Xu, J. Cryst. Growth 279 (2005) 357–362. [8] E. Zych, C. Brecher, A.J. Wojtowicz, H. Lingertat, J. Lumin. 75 (1997) 193–230. [9] S.H. Zhou, Z.L. Fu, J.J. Zhang, S.Y. Zhang, J. Lumin. 118 (2006) 179– 185. [10] A. Potdevin, G. Chadeyron, D. Boyer, R. Mahiou, J. Non-Cryst. Solids 352 (2006) 2510–2514. [11] R. Kasuya, T. Isobe, H. Kuma, J. Alloys Compd. 408 (2006) 820–823. [12] X. Li, H. Liu, J.Y. Wang, H.M. Cui, S.L. Yang, I.R. Boughton, J. Phys. Chem. Solids 66 (2005) 201–205. [13] W.S. Peng, G.K. Liu, Infrared Spectra of Minerals, Science Press, Beijing, 1982, in Chinese. [14] P. Apte, H. Burke, H.J. Pickup, J. Mater. Res. 7 (1992) 706–711. [15] R.S. Hay, J. Mater. Res. 8 (1993) 578–604. [16] P. Colomban, J. Mater. Sci. 24 (1989) 3002–3010. [17] Y.C. Kang, S.B. Park, I.W. Lenggoro, K. Okuyama, J. Phys. Chem. Solids 379–384 (60) (1999) 3002–3010. [18] Ho Seong Jang, Won Bin Im, Dong Chin Lee, Duk Young Jeon, Shi Surk Kim, J. Lumin. 126 (2007) 371–377. [19] Z.G. Wei, L.D. Sun, C.S. Liao, X.C. Jiang, C.H. Yana, J. Appl. Phys. 93 (2003) 9783–9787. [20] A. Bol Ageeth, A. Meijerink, J. Phys. Chem. B. 105 (2001) 10197–10202.
Preparation of YAG:Ce spheroidal phase-pure particles by solvo-thermal method and their photoluminescence Zuogui Wu a , Xudong Zhang a,∗ , Wen He a , Yuanwei Du a , Naitao Jia a , Guogang Xu b a
Department of Materials Science and Engineering, Shandong Institute of Light Industry, Jinan, Shandong 250353, PR China b State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, PR China Received 29 October 2007; received in revised form 3 January 2008; accepted 11 January 2008 Available online 7 March 2008
Abstract Cerium-doped yttrium aluminum garnet (YAG:Ce) powder was synthesized by a solvo-thermal method under mild conditions with inexpensive aluminum and yttrium nitrates as the starting materials and the ethylenediamine (EDA) solution as the solvent. The formation of YAG:Ce was determined by X-ray diffraction (XRD) techniques and FT-IR spectra. The pure YAG crystalline phase was formed after solvo-thermal treatment at 200 ◦ C for 5 h and no intermediate phase was observed. Transmission electronic microscope (TEM) and scanning electronic microscope (SEM) images showed that the resultant YAG:Ce powders were basically spherical and well dispersed. The sizes of the grains were in the range of 200–260 nm. The photoluminescence spectrum of the crystalline YAG:Ce phosphors showed the green-yellow emission with 5d → 4f transition was the most prominent, and the excitation and emission intensities enhanced with prolonging the holding time. © 2008 Elsevier B.V. All rights reserved. Keywords: Ceramics; Chemical synthesis; Luminescence; X-ray diffraction
1. Introduction Yttrium aluminum garnet (Y3 Al5 O12 , YAG) doped with a small amount of impurities such as Eu, Cr and Tb is now widely used as optical host materials for its good optical properties in cathode-ray tubes (CRTs), field emission displays (FED), and scintillation and electro-luminescent applications [1–5]. Recently, polycrystalline fluorescence material has received much attention since the quality of YAG:Ce phosphor has been improved greatly [6,7]. Owing to such a wide and diverse application potential for YAG-based materials, new methods of synthesis of pure and homogeneously doped yttrium aluminum garnet are highly desirable. There have been many methods used for preparation of YAG-based materials. YAG powders are easy to form by the traditional solid-state method. However, it requires a high temperature more than 1100 ◦ C for eliminating several byproducts such as Y4 Al2 O9 (monoclinic yttrium aluminate, YAM) and YAlO3 (yttrium aluminum perovskite, YAP) [8,9]. YAG can
∗
Corresponding author. Tel.: +86 531 8963 1066; fax: +86 531 8861 9798. E-mail address: [email protected] (X. Zhang).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.01.064
directly crystallize from the amorphous precursor at the lower temperature of 900 ◦ C when co-precipitation method is used [4–6]. Potdevin et al. [10] have reported the sol–gel method for synthesizing of YAG particles at low temperature of 900 ◦ C. Kasuya et al. [11] synthesized YAG powders below 400 ◦ C by the glycothermal method using alkoxides as the starting materials. However, alkoxides are too expensive and the powders synthesized by this method with agglomeration of irregular grains, which are not beneficial for the application of YAG powders. In this study, inexpensive metal nitrates and ethylenediamine (EDA) solution were used as the starting materials and the reaction solvent, respectively. Well-dispersed spherical YAG phosphors were synthesized at the temperature as low as 200 ◦ C. 2. Experimental The yttrium, aluminum and cerium sources for YAG:Ce synthesis were yttrium oxide (99.99%), Al(NO3 )3 ·9H2 O (analytical grade) and Ce(NO3 )3 ·6H2 O (analytical grade), respectively. The ammonium hydrogen carbonate (AHC) was analytical grade. Aluminum nitrate solution and cerium nitrate solution were prepared by dissolving Al(NO3 )3 ·9H2 O and Ce(NO3 )3 ·6H2 O in distilled water, respectively. Yttrium nitrate solution was obtained by dissolving high purity Y2 O3 in HNO3 under stirring and heating. Subsequently, the above three salt solutions (the molar ratio of the Y3+ , Ce3+ and Al3+ was maintained as 3 − x:x:5, x = 0.04) were mixed homogeneously under
572
Z. Wu et al. / Journal of Alloys and Compounds 468 (2009) 571–574
mild stirring, which was used as the mother salt solution. 2 mol L−1 solution of AHC was used as the precipitant, and appropriate amount of ammonium sulfate was added to the solution. The precursor precipitate was prepared by adding 50 ml of the mother salt solution at a speed of 2 ml min−1 into 50 ml of the above ACH mixture solution contained in a beaker under vigorous agitation at room temperature. The final pH of the suspensions was 8.2. After aging for 24 h, the precipitates were repeatedly washed with distilled water to remove the residual nitric and ammonia ions. The precursor precipitates were dispersed in an aqueous solution of EDA, and then placed in an autoclave. Finally, the resulting suspension was heated to the desired temperature at a heating rate 2 ◦ C min−1 and kept at that temperature for varying times (1, 5 and 10 h). During the reaction period, the pressure gradually increased up to 10 MPa. When the reaction was finished, it was cooled to room temperature. The resulting powders were centrifugated and repeatedly washed with distilled water, and then dried in air at 100 ◦ C for 2 h. Phase development was observed using X-ray diffraction on a D8 advance (Switzerland, Bruker). X-ray diffractometer with Cu K␣ (λ = 0.15417 nm) radiation and a scanning speed of 0.2◦ /min. Fourier transform infrared spectroscopy (FT-IR) studies with crystalline KBr were performed (Model Tensor 27, Switzerland, Bruker). The morphology and dispersion of the powders were observed using a transmission electron microscope (TEM; Hitachi model H-800) and a scanning electron microscope (SEM; Quanta200ESEM FElco-Holland). Photoluminescence (PL) spectra were measured by a Hitachi F-4500 spectrometer with a 1.0 cm−1 resolution equipped with a 150 W Xe lamp at room temperature. The excitation and emission slit widths are 1.0 nm.
3. Results and discussion Fig. 1 presents the XRD patterns for YAG:Ce powders selected holding time from 1 to 10 h and a theoretical one for comparison. Due to no obvious diffraction peaks were observed in sample a, it can be concluded that the precursor remained amorphous up to 200 ◦ C for 1 h. After holding for more than 5 h, some obvious peaks were found in sample b. The obtained diffraction pattern corresponded well to the standard sample (JCPDS Card 33-40), which indicated that the single phase of YAG was successfully prepared at the temperature as low as 200 ◦ C for 5 h. When the holding time was prolonged to 10 h, more distinct peaks with stronger intensities were observed and no other phases were detected, which indi-
Fig. 1. XRD patterns of the products obtained for different holding time at 200 ◦ C (a–c) and corresponding to the JCPDS ([33-40]) indexing (d).
Fig. 2. FT-IR spectra of the powders synthesized for different holding time at 200 ◦ C.
cated the growth of YAG crystallite with prolonging the holding time. Fig. 2 shows the FT-IR spectra of YAG:Ce phosphors obtained for different holding time at 200 ◦ C, respectively. FTIR analysis of the synthesized samples is important both for the reaction process and the properties of YAG:Ce phosphors. The broad band at 3406 cm−1 should be attributed to moisture absorbed on the surface of the sample. There is an absorption band at 1520 cm−1 , which is stretching vibration of the CO3 − vibration [12]. For inorganic nitrates, the bands in the range 1400–1370 and 840–820 cm−1 can be assigned to nitrate group [13]. Thus, the band at 1384 cm−1 is associated with NO3 − . The band at 1066 cm−1 corresponds to SO4 2− . The band at about 790 cm−1 represents the characteristics of Al–O vibrations, while the bands at about 727, 572 and 477 cm−1 represent the characteristics of Y–O vibrations [14–16]. As mentioned above, intensity of absorption bands for various organic groups decreases and gradually pyrolyzes with the holding time increase. From the IR patterns, it can be seen clearly that after holding for 5 h, the characteristic absorption bands of the sample related to metal–oxygen vibrations are detected. This indicates that YAG phase has formed, which is consistent with the above results of XRD. Generally, the luminescence properties of phosphor depend on the characteristics of the particles such as sizes, morphologies, crystallinities and number of defects. The TEM and SEM images of YAG:Ce powders obtained at 200 ◦ C for different holding time are shown in Fig. 3. It can be seen that the sizes of most of the YAG particles synthesized with a holding time of 5 h are in the range of 200–260 nm. And the particles are almost spherical (Fig. 3a, c and d). Importantly, the particles are not aggregated from each other, which will be beneficial for phosphor applications [17]. When the holding time is prolonged to 10 h, the YAG particles grow and the sizes increase slightly. Additionally, the formation of agglomerates occurs (Fig. 3b). This phenomenon is mainly caused by the dissolution of smaller YAG particles and the growth of some larger ones. With the growth of YAG particles, the attraction force between them will increase, which will cause the aggre-
Z. Wu et al. / Journal of Alloys and Compounds 468 (2009) 571–574
573
Fig. 3. TEM photographs of YAG:Ce particles obtained at 200 ◦ C: (a) 5 h, (b) 10 h and SEM photographs (c) and (d) 5 h.
gation among the particles. Therefore, a holding time of 5 h should be suitable for the synthesis of YAG powder with good properties. Fig. 4 shows the excitation (a) and emission (b) spectra of phosphors prepared at 200 ◦ C for different holding time. The excitation bands in all samples are located at 343 and 460 nm, respectively. These excitation bands are due to the electron transition from ground state of Ce3+ (2 F5/2, 2 F7/2 ) to the different crystal field splitting components of excited 5d state [18]. The emission spectrum for Ce3+ in sample 1 consists of a broad band with a maximum at 530 nm, while the emission spectrum for
samples 2 and 3 have a maximum at 525 nm. The Ce3+ has a 4f1 configuration and its excited configuration is 4f0 5d1 state. The 5d electron of the excited 4f0 5d1 configuration forms a 2 D term split into 2 D3/2 and 2 D5/2 states, and the 4f electron of the ground 4f1 configuration forms 2 F7/2 and 2 F5/2 states by spin–orbit coupling [19]. In YAG structure, the lowest 5d level (2 D3/2 ) is exceptionally low, i.e. at 22,000 cm−1 [18]. This energy corresponds to blue light. Following the prolonging of holding time, excitation and emission intensities increase. The increasing of intensity results from the enhancement of crystallization degree and the decrease of surface defects [20]. Despite synthesized for different holding time, the maximum excitation wavelength is hardly changed. Nano-sized particles usually show blue shift of the emission spectra [19]. Following the holding time increasing, the red shift phenomenon appears which maybe caused by the growth of the particle sizes. This is in line with the fact indicated by the emission spectrum in Fig. 4b. 4. Conclusions
Fig. 4. Excitation (a) and emission (b) spectra of the sample synthesized at 200 ◦ C for different holding time.
The YAG:Ce phosphors were prepared by the solvo-thermal method under mild conditions with ethylenediamine solution as the solvent. The amorphous precursor transforms directly to pure phase YAG:Ce at the temperature of 200 ◦ C for 5 h. At the same time, the degree of crystallization of YAG:Ce phase increased with the prolonging of holding time. TEM and SEM images showed that well-dispersed, spherical and submicronsized YAG:Ce particles with a relatively narrow grain size distribution can be synthesized. The obtained YAG:Ce parti-
574
Z. Wu et al. / Journal of Alloys and Compounds 468 (2009) 571–574
cles with spherical morphology and no aggregation have good photoluminescence characteristics. Acknowledgements This work was financially supported by the Natural Science Foundation of China of Shandong Province (Grant No. Y2003F02). The authors would like to thank the Analytical Center of Shandong Institute of Light Industry for sample characterization. References [1] S.M. Sim, K.A. Keller, J. Mater. Sci. 35 (2000) 713–716. [2] X.D. Zhang, H. Liu, W. He, J.Y. Wang, X. Li, R.I. Boughton, J. Alloys Compd. 372 (2004) 300–303. [3] Y.H. Zhou, J. Lin, M. Yu, S.M. Han, S.B. Wang, H.J. Zhang, Mater. Res. Bull. 38 (2003) 1289–1299. [4] X. Li, H. Liu, J.Y. Wang, H.M. Cui, X.D. Zhang, F. Han, Mater. Sci. Eng. A 379 (2004) 347–350. [5] R.A. Rodriguez, E. De la Rosa, L.A. Diaz-Torres, P. Salas, R. Melendrez, M. Barboza-Flores, Opt. Mater. 27 (2004) 293–299.
[6] G.G. Xu, X.D. Zhang, W. He, H. Liu, H. Li, R.I. Boughton, Mater. Lett. 60 (2006) 962–965. [7] G.D. Xia, S.M. Zhou, J.J. Zhang, J. Xu, J. Cryst. Growth 279 (2005) 357–362. [8] E. Zych, C. Brecher, A.J. Wojtowicz, H. Lingertat, J. Lumin. 75 (1997) 193–230. [9] S.H. Zhou, Z.L. Fu, J.J. Zhang, S.Y. Zhang, J. Lumin. 118 (2006) 179– 185. [10] A. Potdevin, G. Chadeyron, D. Boyer, R. Mahiou, J. Non-Cryst. Solids 352 (2006) 2510–2514. [11] R. Kasuya, T. Isobe, H. Kuma, J. Alloys Compd. 408 (2006) 820–823. [12] X. Li, H. Liu, J.Y. Wang, H.M. Cui, S.L. Yang, I.R. Boughton, J. Phys. Chem. Solids 66 (2005) 201–205. [13] W.S. Peng, G.K. Liu, Infrared Spectra of Minerals, Science Press, Beijing, 1982, in Chinese. [14] P. Apte, H. Burke, H.J. Pickup, J. Mater. Res. 7 (1992) 706–711. [15] R.S. Hay, J. Mater. Res. 8 (1993) 578–604. [16] P. Colomban, J. Mater. Sci. 24 (1989) 3002–3010. [17] Y.C. Kang, S.B. Park, I.W. Lenggoro, K. Okuyama, J. Phys. Chem. Solids 379–384 (60) (1999) 3002–3010. [18] Ho Seong Jang, Won Bin Im, Dong Chin Lee, Duk Young Jeon, Shi Surk Kim, J. Lumin. 126 (2007) 371–377. [19] Z.G. Wei, L.D. Sun, C.S. Liao, X.C. Jiang, C.H. Yana, J. Appl. Phys. 93 (2003) 9783–9787. [20] A. Bol Ageeth, A. Meijerink, J. Phys. Chem. B. 105 (2001) 10197–10202.