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Different alkali carbonates on the microstructure and photoluminescence properties of BaY2ZnO5:Tb3+ phosphors prepared using the solid-state method
Journal of Physics and Chemistry of Solids 74 (2013) 344–347
Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Different alkali carbonates on the microstructure and photoluminescence properties of BaY2ZnO5:Tb3 þ phosphors prepared using the solid-state method Huang-Yu Chen a, Ru-Yuan Yang b,n, Shoou-Jinn Chang a a
Institute of Microelectronics, Department of Electrical Engineering, Advanced Optoelectronic Technology Center, Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan b Graduate Institute of Materials Engineering, National Pingtung University of Science and Technology, Pingtung County 912, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 September 2012 Received in revised form 11 October 2012 Accepted 18 October 2012 Available online 29 October 2012
For this study, different alkali carbonates (Li2CO3, Na2CO3, and K2CO3) were added to a Tb3 þ activated BaY2ZnO5 phosphor synthesized using the solid-state reaction, and the morphology and photoluminescence properties of BaY2ZnO5:Tb3 þ phosphors are investigated and discussed. When BaY2ZnO5:Tb3 þ phosphors to which alkali carbonates were added were sintered at 1250 1C for 12 h, X-ray powder diffraction analysis showed that the unreacted Y2O3 raw material decreased, and scanning electron microscopy (SEM) showed that large particle sizes and necking shapes were obtained. Emission intensities decreased, and the wavelengths of the excitation peaks changed when alkali carbonates were added to BaY2ZnO5:Tb3 þ phosphors. The excitation peaks of phosphors prepared with alkali carbonates were shifted to a short wavelength because the addition of alkali ions may increase the oxygen vacancy in BaY2ZnO5:Tb3 þ phosphors and change the photoluminescence properties. The CIE chromaticity (x,y) is (0.35,0.55), and does not change with the addition of alkali carbonates. & 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides B. Crystal growth C. X-ray diffraction D. Luminescence
1. Introduction Phosphors are essential materials used in imaging, display, and lighting applications. In recent years, rare-earth-doped oxide-based phosphors have been potential materials for use in these applications because of their advantages of superior color richness and good chemical and thermal stabilities compared to non-oxide materials [1]. Good phosphor should have strong absorption and excitation in the region from ultraviolet to blue, and emit the desired visible light. Moreover, the shape and size of the phosphor are found to affect photoluminescence (PL) intensity and device efficiency [2]. For industry applications, phosphor particles should ideally be non-aggregated and of uniform particle-size distribution. In the past, alkali metal ions such as Li þ , Na þ , and K þ have readily been used for charge compensation to increase the crystallinity and enhance the emission intensity of phosphors [3]. Lithium metal compounds have also been revealed to promote the crystal growth of phosphor and improve phosphor luminance by reducing the crystal strain [4]. Moreover, alkali carbonates also act as a flux to increase the reaction rate and to modify the shape of crystals [5]. The flux-assisted solid-state reaction method was excellent for obtaining phosphor powders with desirable phases and properties. The flux also plays an important role in the control
n
Corresponding author. E-mail address: [email protected] (R.-Y. Yang).
0022-3697/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2012.10.010
of the mean size, size distribution, and the shape of phosphor particles, which are used to improve the brightness of regularly shaped phosphor particles of micrometer size [6]. Several researches about BaLn2ZnO5 phosphors, such as BaGd2ZnO5:Dy3 þ phosphor [7], BaGd2ZnO5:Eu3 þ phosphor [8], red BaY2ZnO5:Eu3 þ phosphor [9], green BaY2ZnO5:Tb3 þ phosphor [10], and near-white light BaY2ZnO5:Dy3 þ phosphor [11], have been conducted. BaY2ZnO5, having an orthorhombic structure with a space group of Pbnm, is a luminescent host with a stable crystalline structure and high thermal stability [12]. The basic structure of BaY2ZnO5 consists of YO7, BaO11, and ZnO5 polyhedra. In previous work, we prepared the BaY2ZnO5:Tb3 þ phosphor by microwaveassisted sintering. However, the effects of alkali ions on the structure and the photoluminescence properties of BaY2ZnO5:Tb3 þ phosphors have not been investigated before. In this work, we synthesized the BaY2ZnO5:0.04Tb3 þ phosphor with different alkali carbonates by using the solid-state method, and investigated the microstructure and photoluminescence properties by X-ray diffraction (XRD), SEM, and photoluminescence measurement (PL).
2. Experimental procedure BaCO3, ZnO, Y2O3, and Tb4O7 with a purity of 99.9% were used as starting materials. Alkali carbonates such as Li2CO3, Na2CO3,
H.-Y. Chen et al. / Journal of Physics and Chemistry of Solids 74 (2013) 344–347
and K2CO3 were used as additives. BaCO3, Y2O3, ZnO, and Tb4O7 were mixed together in 1:0.98:1:0.01 mol ratio, and the introduced quantity of alkali ions was fixed to 0.0125 mol of the whole BaY2ZnO5:Tb3 þ phosphor. The raw materials were mixed in a ball mill and ground for 1 h with zirconia balls. After drying, BaY2ZnO5:Tb3 þ , A þ (A þ ¼Li þ , Na þ , and K þ ) phosphors were synthesized by using the solid-state reaction method. The samples were transferred into a sintering furnace for heating under an air atmosphere. The temperature was increased up to 1250 1C at a heating rate of 10 1C/min, and maintained at this temperature for 12 h to form the target phosphors. Finally, the samples were cooled to room temperature at a cooling rate of 10 1C/min, and we then proceeded to undertake various identifications of properties. To determine the effect of the sintering process on phosphor crystallization, the crystalline phases of the phosphors were identified by XRD (Bruker D8 Advance) analysis with a CuKa radiation of l ¼1.5406 A˚ using a Ni filter, and with a secondary graphite monochromator. A scan range of 2y ¼20–801 with a step of 0.031 and 0.4 s as the count time per step were used. Particle morphology was observed using SEM (HORIBA EX-200). The excitation and emission spectra were obtained using a spectrofluorimeter (PL, JASCO FP-6000) equipped with a 150 W xenon lamp as the light source. For accuracy of data, the specimens were measured within the same sample holder to ensure consistent amounts of phosphor materials in all samples.
3. Results and discussion Fig. 1 shows the XRD patterns of BaY2ZnO5:0.04Tb3 þ , 0.0125A þ (A þ ¼Li þ , Na þ , and K þ ) phosphors prepared by the solid-state reaction at 1250 1C for 12 h. The main peaks are attributed to the orthorhombic BaY2ZnO5 phase, which are in excellent agreement with the standard card (PDF#49-0516). The phosphors synthesized without alkali carbonates exhibiting weak peaks appeared at 2y of approximately 29.11, 33.81, and 57.61, which are indexed to the Y2O3 phase. However, the raw material Y2O3 phase is decreased as the alkali ion A þ (A þ ¼Li þ , Na þ , and K þ ) is added, indicating that the addition of alkali carbonates acts as a flux to improve the sintering of the BaY2ZnO5:Tb3 þ phosphor effectively. Moreover, no diffraction peaks shifted, regardless of which alkali ion
Fig. 1. XRD patterns of BaY2ZnO5:0.04Tb3 þ phosphors with different alkali ions addition.
345
was added, which indicated that alkali ions do not substitute the Ba2 þ site or Y3þ site, and not do influence the host structure. Fig. 2 shows SEM micrographs of BaY2ZnO5:0.04Tb3 þ phosphors obtained with the flux of alkali carbonates Li2CO3, Na2CO3, and K2CO3. The alkali carbonate quantity in each case is 0.625 mol%, that is, 1.25 mol% of alkali ion. Another phosphor sample is synthesized with the same conditions without alkali carbonates for comparison. The BaY2ZnO5:0.04Tb3 þ phosphors prepared from the solid-state reaction without alkali carbonates had irregular morphologies and an aggregated structure with many flawed particles, as shown in Fig. 2(a). Because BaY2ZnO5:0.04Tb3 þ was hard, the particle morphology of the phosphor was virtually destroyed during grinding. Conversely, the phosphor powders obtained with alkali carbonates revealed smooth, aggregated, larger particles, and a high connection within particles, as shown in Fig. 2(b)–(d). The results could indicate that a liquid phase caused a strong fluxing action, which enabled an increase in process flexibility and accelerated the reaction with the addition of alkali carbonates. Among alkali carbonates, the addition of K2CO3 increases the particle size significantly, which is approximately 5–8 mm. Fig. 3 shows the excitation spectra of BaY2ZnO5:Tb3 þ phosphors with different alkali ions sintered at 1250 1C for 12 h, and the spectral region is from 200 to 500 nm. The spectrum of BaY2ZnO5:Tb3 þ phosphors exhibits two broad bands in the UV region centered at approximately 237 and 290 nm. The broad band with maxima peaks at 237 nm and 290 nm would be attributed to the absorption of BaY2ZnO5 matrix and the 4f–5d transition bands in the Tb3 þ center lying in the band gap region of the host matrix, respectively. These broad excitation bands cannot be assigned to Tb3 þ –O2 charge-transfer (CT) transition because the CT states have much higher energy ( 60,000 cm 1) than do 5d states of Tb3 þ [13]. According to the absorption spectra of BaY2ZnO5 powders investigated by Liang et al. [11], the absorption peak located at 237 nm can be assigned to the absorption of BaY2ZnO5 matrix. The second broad peak located at 290 nm should correspond to 4f8–4f75d1 transition of Tb3 þ ion [14,15]. For Tb3 þ ions with 4f8 electronic configuration, the ground states are 7FJ; 4f7 half-filled configurations are stable electronic configurations. Hence, the low energy is believed to be easily absorbed for 4f8–4f75d1 transitions. In addition, the wavelength of 4f8-4f75d1 absorption bands shifted slightly to a shorter wavelength with the increasing ionic radii of added alkali ions, as shown in Fig. 4, in which the cation radii of Li þ , Na þ , and K þ are 0.068 nm, 0.097 nm, and 0.133 nm, respectively. As we can see in Fig. 1, the BaY2ZnO5:Tb3 þ phosphors synthesized without alkali carbonates exhibit few Y2O3. Therefore, some Tb3 þ may occupy Y3 þ in the Y2O3 host. When alkali carbonates were added, the ratio of Y2O3 component in the products decreases remarkably, which may result in the decrease of spectral component of Tb3 þ in the Y2O3. Therefore, the blueshift was observed. Fig. 5 shows the emission spectra of BaY2ZnO5:Tb3 þ phosphors with different alkali ions prepared by solid-state reactions at 1250 1C for 12 h, under excitation at 290 nm. When lex ¼290 nm, the emission wavelengths for BaY2ZnO5:Tb3 þ phosphors are located at 491, 546, 589, and 625 nm, corresponding to the Tb3 þ intra-4f transition from the excited levels to lower levels, which are 5D4-7FJ (J¼6,5,4,3) transitions [16], respectively. The strongest emission peak is located at 546 nm, corresponding to the typical hypersensitive transition 5D4-7F5 for Tb3 þ ion. When the Tb3 þ ion absorbed the ultraviolet light energy, the Tb3 þ ion was excited by the energy transfer from the absorption energy, and then mainly nonradiatively relaxed to the 5D4 levels of the Tb3 þ ion. Finally, the transition occurs from the 5D4 to 7FJ level of Tb3 þ .
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H.-Y. Chen et al. / Journal of Physics and Chemistry of Solids 74 (2013) 344–347
Fig. 2. SEM images of BaY2ZnO5:0.04Tb3 þ phosphors prepared (a) without alkali ions and with 1.25 mol% of (b) Li þ , (c) Na þ , and (d) K þ .
Fig. 3. Photoluminescence excitation spectra of BaY2ZnO5:Tb3 þ phosphors with different alkali ions prepared by the solid-state reaction (lem ¼ 546 nm).
Fig. 5. Photoluminescence emission spectra of BaY2ZnO5:Tb3 þ phosphors with different alkali ions prepared by the solid-state reaction (lex ¼290 nm).
Fig. 4. Peak location of 4f8-4f75d1 absorption bands without alkali ions and with 1.25 mol% of Li þ , Na þ , and K þ ions added.
The intensities of the emission peaks can be found to decrease with the increasing ionic radii of added alkali ions. The photoluminescence property was expected to improve as alkali carbonates were added because alkali carbonates can act as a flux to improve the sintering process and reduce the remaining raw material (as shown in Fig. 1), but the optical performance of the BaY2ZnO5:Tb3 þ phosphor worsened instead. The addition of alkali ions is suggested to increase the concentrations of oxygen vacancy [17], and disturbs the charge valence balance because no charge compensation problem exists when Tb3 þ ions substitute Y3 þ ions. The excessive oxygen vacancy and the added alkali ions existing in the BaY2ZnO5 matrix can be a poison or a quenching center. When the excited electrons relaxed to the trap state of the oxygen vacancy, energy was released by lattice vibrations. Therefore, when the alkali ions were introduced into the BaY2ZnO5:Tb3 þ phosphor, the trap states appeared and then reduced the intensity of the BaY2ZnO5:Tb3 þ phosphor. Larger ionic radii of added alkali ions are indicative of a greater concentration of generated oxygen vacancy. Finally, we found
H.-Y. Chen et al. / Journal of Physics and Chemistry of Solids 74 (2013) 344–347
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oxygen vacancy in BaY2ZnO5:0.04Tb phosphors with the increasing ionic radii of added alkali ions and reduces PL intensity. The excitation and emission spectra of BaY2ZnO5:0.04Tb3 þ , 0.0125A þ (A¼Li, Na, and K) phosphors indicate that it can be effectively excited by a UV LED, and that they emit 546 nm green light.
Acknowledgments The authors would like to thank the National Science Council and Bureau of Energy, Ministry of Economic Affairs of Taiwan, ROC., for the financial support under Contract nos. NSC 100-2221E-006-040-MY2, NSC 101-2622-E-020-001-CC2, and 100-D0204-6, and the LED Lighting Research Center of NCKU, National Nano Device Laboratories, and the Precision Instrument Center of National Pingtung University of Science and Technology for providing the experimental equipment. References
Fig. 6. CIE 1931 chromaticity coordinates of BaY2ZnO5:0.04Tb3 þ under excitation at 290 nm.
that the chromaticity (x,y) is (0.35,0.55), which did not change as different alkali ions were added because the emission wavelengths were the same, as shown in Fig. 6.
4. Conclusion This paper investigated the characteristics of BaY2ZnO5: 0.04Tb3 þ phosphors with different alkali ions prepared using the solid-state reaction. The alkali carbonates also act as flux to improve the sintering process and to reduce the unreacted raw materials so that the particle size becomes larger and aggregation is observed. However, the addition of alkali ions increases the
[1] S.D. Cheng, C.H. Kam, S. Buddhudu, Mater. Res. Bull. 36 (2001) 1131. [2] J.S. Kim, P.E. Jeon, J.C. Choi, H.L. Park, Solid State Commun. 133 (2005) 187–190. [3] Y. Su, L. Li, G. Li, Chem. Mater. 20 (2008) 6060–6067. [4] Y. Shimomura, N. Kijima, J. Electrochem. Soc. 151 (2004) H86–H92. [5] F. Duault, M. Junker, P. Grosseau, B. Guilhot, P. Iacconi, B. Moine, Powder Technol. 154 (2005) 132–137. [6] D. Haranath, P. Sharma, H. Chander, A. Ali, N. Bhalla, S.K. Halder, Mater. Chem. Phys. 101 (2007) 163–169. [7] B. Tian, B. Chen, Y. Tian, J. Sun, X. Li, J. Zhang, H. Zhong, L. Cheng, R. Hua, J. Phys. Chem. Solids 73 (2012) 1314–1319. [8] B. Tian, B. Chen, Y. Tianb, J. Sun, X. Li, J. Zhang, H. Zhong, L. Cheng, Z. Wu, R. Hua, Ceram. Int. 38 (2012) 3537–3540. [9] C.H. Liang, Y.C. Chang, Y.S. Chang, Appl. Phys. Lett. 93 (2008) 211902. [10] H.Y. Chen, M.H. Weng, R.Y. Yang, S.J. Chang, Ceram. Int. 37 (2011) 1521–1524. [11] C.H. Liang, L.G. Teoh, K.T. Liu, Y.S. Chang, J. Alloys Compd. 517 (2012) 9–13. [12] J.A. Kaduk, W. Wong-Ng, W. Greenwood, J. Dillingham, B.H. Toby, J. Res. Natl. Inst. Stand. Technol. 104 (1999) 147. [13] L.J. Nugent, R.D. Baybarz, J.L. Burnett, J.L. Ryan, J. Phys. Chem. 77 (1973) 1528. [14] Y. Tian, B. Chen, H. Yu, R. Hua, X. Li, J. Sun, L. Cheng, H. Zhong, J. Zhang, Y. Zheng, T. Yu, L. Huang, J. Colloid Interface Sci. 360 (2011) 586–592. [15] Y. Tian, B. Chen, B. Tian, J. Sun, X. Li, J. Zhang, L. Cheng, H. Zhong, H. Zhong, Q. Meng, Ruinian Hua, Physica B 407 (2012) 2556–2559. [16] Y.S. Chang, H.J. Lin, Y.C. Li, Y.L. Chai, Y.Y. Tsai, J. Solid State Chem. 180 (2007) 3076. [17] K.L. Liou, Y.F. W, I.G. Chen, J. Taiwan Vac. Soc. 23 (2010) 41–48.
Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Different alkali carbonates on the microstructure and photoluminescence properties of BaY2ZnO5:Tb3 þ phosphors prepared using the solid-state method Huang-Yu Chen a, Ru-Yuan Yang b,n, Shoou-Jinn Chang a a
Institute of Microelectronics, Department of Electrical Engineering, Advanced Optoelectronic Technology Center, Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan b Graduate Institute of Materials Engineering, National Pingtung University of Science and Technology, Pingtung County 912, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 September 2012 Received in revised form 11 October 2012 Accepted 18 October 2012 Available online 29 October 2012
For this study, different alkali carbonates (Li2CO3, Na2CO3, and K2CO3) were added to a Tb3 þ activated BaY2ZnO5 phosphor synthesized using the solid-state reaction, and the morphology and photoluminescence properties of BaY2ZnO5:Tb3 þ phosphors are investigated and discussed. When BaY2ZnO5:Tb3 þ phosphors to which alkali carbonates were added were sintered at 1250 1C for 12 h, X-ray powder diffraction analysis showed that the unreacted Y2O3 raw material decreased, and scanning electron microscopy (SEM) showed that large particle sizes and necking shapes were obtained. Emission intensities decreased, and the wavelengths of the excitation peaks changed when alkali carbonates were added to BaY2ZnO5:Tb3 þ phosphors. The excitation peaks of phosphors prepared with alkali carbonates were shifted to a short wavelength because the addition of alkali ions may increase the oxygen vacancy in BaY2ZnO5:Tb3 þ phosphors and change the photoluminescence properties. The CIE chromaticity (x,y) is (0.35,0.55), and does not change with the addition of alkali carbonates. & 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides B. Crystal growth C. X-ray diffraction D. Luminescence
1. Introduction Phosphors are essential materials used in imaging, display, and lighting applications. In recent years, rare-earth-doped oxide-based phosphors have been potential materials for use in these applications because of their advantages of superior color richness and good chemical and thermal stabilities compared to non-oxide materials [1]. Good phosphor should have strong absorption and excitation in the region from ultraviolet to blue, and emit the desired visible light. Moreover, the shape and size of the phosphor are found to affect photoluminescence (PL) intensity and device efficiency [2]. For industry applications, phosphor particles should ideally be non-aggregated and of uniform particle-size distribution. In the past, alkali metal ions such as Li þ , Na þ , and K þ have readily been used for charge compensation to increase the crystallinity and enhance the emission intensity of phosphors [3]. Lithium metal compounds have also been revealed to promote the crystal growth of phosphor and improve phosphor luminance by reducing the crystal strain [4]. Moreover, alkali carbonates also act as a flux to increase the reaction rate and to modify the shape of crystals [5]. The flux-assisted solid-state reaction method was excellent for obtaining phosphor powders with desirable phases and properties. The flux also plays an important role in the control
n
Corresponding author. E-mail address: [email protected] (R.-Y. Yang).
0022-3697/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2012.10.010
of the mean size, size distribution, and the shape of phosphor particles, which are used to improve the brightness of regularly shaped phosphor particles of micrometer size [6]. Several researches about BaLn2ZnO5 phosphors, such as BaGd2ZnO5:Dy3 þ phosphor [7], BaGd2ZnO5:Eu3 þ phosphor [8], red BaY2ZnO5:Eu3 þ phosphor [9], green BaY2ZnO5:Tb3 þ phosphor [10], and near-white light BaY2ZnO5:Dy3 þ phosphor [11], have been conducted. BaY2ZnO5, having an orthorhombic structure with a space group of Pbnm, is a luminescent host with a stable crystalline structure and high thermal stability [12]. The basic structure of BaY2ZnO5 consists of YO7, BaO11, and ZnO5 polyhedra. In previous work, we prepared the BaY2ZnO5:Tb3 þ phosphor by microwaveassisted sintering. However, the effects of alkali ions on the structure and the photoluminescence properties of BaY2ZnO5:Tb3 þ phosphors have not been investigated before. In this work, we synthesized the BaY2ZnO5:0.04Tb3 þ phosphor with different alkali carbonates by using the solid-state method, and investigated the microstructure and photoluminescence properties by X-ray diffraction (XRD), SEM, and photoluminescence measurement (PL).
2. Experimental procedure BaCO3, ZnO, Y2O3, and Tb4O7 with a purity of 99.9% were used as starting materials. Alkali carbonates such as Li2CO3, Na2CO3,
H.-Y. Chen et al. / Journal of Physics and Chemistry of Solids 74 (2013) 344–347
and K2CO3 were used as additives. BaCO3, Y2O3, ZnO, and Tb4O7 were mixed together in 1:0.98:1:0.01 mol ratio, and the introduced quantity of alkali ions was fixed to 0.0125 mol of the whole BaY2ZnO5:Tb3 þ phosphor. The raw materials were mixed in a ball mill and ground for 1 h with zirconia balls. After drying, BaY2ZnO5:Tb3 þ , A þ (A þ ¼Li þ , Na þ , and K þ ) phosphors were synthesized by using the solid-state reaction method. The samples were transferred into a sintering furnace for heating under an air atmosphere. The temperature was increased up to 1250 1C at a heating rate of 10 1C/min, and maintained at this temperature for 12 h to form the target phosphors. Finally, the samples were cooled to room temperature at a cooling rate of 10 1C/min, and we then proceeded to undertake various identifications of properties. To determine the effect of the sintering process on phosphor crystallization, the crystalline phases of the phosphors were identified by XRD (Bruker D8 Advance) analysis with a CuKa radiation of l ¼1.5406 A˚ using a Ni filter, and with a secondary graphite monochromator. A scan range of 2y ¼20–801 with a step of 0.031 and 0.4 s as the count time per step were used. Particle morphology was observed using SEM (HORIBA EX-200). The excitation and emission spectra were obtained using a spectrofluorimeter (PL, JASCO FP-6000) equipped with a 150 W xenon lamp as the light source. For accuracy of data, the specimens were measured within the same sample holder to ensure consistent amounts of phosphor materials in all samples.
3. Results and discussion Fig. 1 shows the XRD patterns of BaY2ZnO5:0.04Tb3 þ , 0.0125A þ (A þ ¼Li þ , Na þ , and K þ ) phosphors prepared by the solid-state reaction at 1250 1C for 12 h. The main peaks are attributed to the orthorhombic BaY2ZnO5 phase, which are in excellent agreement with the standard card (PDF#49-0516). The phosphors synthesized without alkali carbonates exhibiting weak peaks appeared at 2y of approximately 29.11, 33.81, and 57.61, which are indexed to the Y2O3 phase. However, the raw material Y2O3 phase is decreased as the alkali ion A þ (A þ ¼Li þ , Na þ , and K þ ) is added, indicating that the addition of alkali carbonates acts as a flux to improve the sintering of the BaY2ZnO5:Tb3 þ phosphor effectively. Moreover, no diffraction peaks shifted, regardless of which alkali ion
Fig. 1. XRD patterns of BaY2ZnO5:0.04Tb3 þ phosphors with different alkali ions addition.
345
was added, which indicated that alkali ions do not substitute the Ba2 þ site or Y3þ site, and not do influence the host structure. Fig. 2 shows SEM micrographs of BaY2ZnO5:0.04Tb3 þ phosphors obtained with the flux of alkali carbonates Li2CO3, Na2CO3, and K2CO3. The alkali carbonate quantity in each case is 0.625 mol%, that is, 1.25 mol% of alkali ion. Another phosphor sample is synthesized with the same conditions without alkali carbonates for comparison. The BaY2ZnO5:0.04Tb3 þ phosphors prepared from the solid-state reaction without alkali carbonates had irregular morphologies and an aggregated structure with many flawed particles, as shown in Fig. 2(a). Because BaY2ZnO5:0.04Tb3 þ was hard, the particle morphology of the phosphor was virtually destroyed during grinding. Conversely, the phosphor powders obtained with alkali carbonates revealed smooth, aggregated, larger particles, and a high connection within particles, as shown in Fig. 2(b)–(d). The results could indicate that a liquid phase caused a strong fluxing action, which enabled an increase in process flexibility and accelerated the reaction with the addition of alkali carbonates. Among alkali carbonates, the addition of K2CO3 increases the particle size significantly, which is approximately 5–8 mm. Fig. 3 shows the excitation spectra of BaY2ZnO5:Tb3 þ phosphors with different alkali ions sintered at 1250 1C for 12 h, and the spectral region is from 200 to 500 nm. The spectrum of BaY2ZnO5:Tb3 þ phosphors exhibits two broad bands in the UV region centered at approximately 237 and 290 nm. The broad band with maxima peaks at 237 nm and 290 nm would be attributed to the absorption of BaY2ZnO5 matrix and the 4f–5d transition bands in the Tb3 þ center lying in the band gap region of the host matrix, respectively. These broad excitation bands cannot be assigned to Tb3 þ –O2 charge-transfer (CT) transition because the CT states have much higher energy ( 60,000 cm 1) than do 5d states of Tb3 þ [13]. According to the absorption spectra of BaY2ZnO5 powders investigated by Liang et al. [11], the absorption peak located at 237 nm can be assigned to the absorption of BaY2ZnO5 matrix. The second broad peak located at 290 nm should correspond to 4f8–4f75d1 transition of Tb3 þ ion [14,15]. For Tb3 þ ions with 4f8 electronic configuration, the ground states are 7FJ; 4f7 half-filled configurations are stable electronic configurations. Hence, the low energy is believed to be easily absorbed for 4f8–4f75d1 transitions. In addition, the wavelength of 4f8-4f75d1 absorption bands shifted slightly to a shorter wavelength with the increasing ionic radii of added alkali ions, as shown in Fig. 4, in which the cation radii of Li þ , Na þ , and K þ are 0.068 nm, 0.097 nm, and 0.133 nm, respectively. As we can see in Fig. 1, the BaY2ZnO5:Tb3 þ phosphors synthesized without alkali carbonates exhibit few Y2O3. Therefore, some Tb3 þ may occupy Y3 þ in the Y2O3 host. When alkali carbonates were added, the ratio of Y2O3 component in the products decreases remarkably, which may result in the decrease of spectral component of Tb3 þ in the Y2O3. Therefore, the blueshift was observed. Fig. 5 shows the emission spectra of BaY2ZnO5:Tb3 þ phosphors with different alkali ions prepared by solid-state reactions at 1250 1C for 12 h, under excitation at 290 nm. When lex ¼290 nm, the emission wavelengths for BaY2ZnO5:Tb3 þ phosphors are located at 491, 546, 589, and 625 nm, corresponding to the Tb3 þ intra-4f transition from the excited levels to lower levels, which are 5D4-7FJ (J¼6,5,4,3) transitions [16], respectively. The strongest emission peak is located at 546 nm, corresponding to the typical hypersensitive transition 5D4-7F5 for Tb3 þ ion. When the Tb3 þ ion absorbed the ultraviolet light energy, the Tb3 þ ion was excited by the energy transfer from the absorption energy, and then mainly nonradiatively relaxed to the 5D4 levels of the Tb3 þ ion. Finally, the transition occurs from the 5D4 to 7FJ level of Tb3 þ .
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H.-Y. Chen et al. / Journal of Physics and Chemistry of Solids 74 (2013) 344–347
Fig. 2. SEM images of BaY2ZnO5:0.04Tb3 þ phosphors prepared (a) without alkali ions and with 1.25 mol% of (b) Li þ , (c) Na þ , and (d) K þ .
Fig. 3. Photoluminescence excitation spectra of BaY2ZnO5:Tb3 þ phosphors with different alkali ions prepared by the solid-state reaction (lem ¼ 546 nm).
Fig. 5. Photoluminescence emission spectra of BaY2ZnO5:Tb3 þ phosphors with different alkali ions prepared by the solid-state reaction (lex ¼290 nm).
Fig. 4. Peak location of 4f8-4f75d1 absorption bands without alkali ions and with 1.25 mol% of Li þ , Na þ , and K þ ions added.
The intensities of the emission peaks can be found to decrease with the increasing ionic radii of added alkali ions. The photoluminescence property was expected to improve as alkali carbonates were added because alkali carbonates can act as a flux to improve the sintering process and reduce the remaining raw material (as shown in Fig. 1), but the optical performance of the BaY2ZnO5:Tb3 þ phosphor worsened instead. The addition of alkali ions is suggested to increase the concentrations of oxygen vacancy [17], and disturbs the charge valence balance because no charge compensation problem exists when Tb3 þ ions substitute Y3 þ ions. The excessive oxygen vacancy and the added alkali ions existing in the BaY2ZnO5 matrix can be a poison or a quenching center. When the excited electrons relaxed to the trap state of the oxygen vacancy, energy was released by lattice vibrations. Therefore, when the alkali ions were introduced into the BaY2ZnO5:Tb3 þ phosphor, the trap states appeared and then reduced the intensity of the BaY2ZnO5:Tb3 þ phosphor. Larger ionic radii of added alkali ions are indicative of a greater concentration of generated oxygen vacancy. Finally, we found
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oxygen vacancy in BaY2ZnO5:0.04Tb phosphors with the increasing ionic radii of added alkali ions and reduces PL intensity. The excitation and emission spectra of BaY2ZnO5:0.04Tb3 þ , 0.0125A þ (A¼Li, Na, and K) phosphors indicate that it can be effectively excited by a UV LED, and that they emit 546 nm green light.
Acknowledgments The authors would like to thank the National Science Council and Bureau of Energy, Ministry of Economic Affairs of Taiwan, ROC., for the financial support under Contract nos. NSC 100-2221E-006-040-MY2, NSC 101-2622-E-020-001-CC2, and 100-D0204-6, and the LED Lighting Research Center of NCKU, National Nano Device Laboratories, and the Precision Instrument Center of National Pingtung University of Science and Technology for providing the experimental equipment. References
Fig. 6. CIE 1931 chromaticity coordinates of BaY2ZnO5:0.04Tb3 þ under excitation at 290 nm.
that the chromaticity (x,y) is (0.35,0.55), which did not change as different alkali ions were added because the emission wavelengths were the same, as shown in Fig. 6.
4. Conclusion This paper investigated the characteristics of BaY2ZnO5: 0.04Tb3 þ phosphors with different alkali ions prepared using the solid-state reaction. The alkali carbonates also act as flux to improve the sintering process and to reduce the unreacted raw materials so that the particle size becomes larger and aggregation is observed. However, the addition of alkali ions increases the
[1] S.D. Cheng, C.H. Kam, S. Buddhudu, Mater. Res. Bull. 36 (2001) 1131. [2] J.S. Kim, P.E. Jeon, J.C. Choi, H.L. Park, Solid State Commun. 133 (2005) 187–190. [3] Y. Su, L. Li, G. Li, Chem. Mater. 20 (2008) 6060–6067. [4] Y. Shimomura, N. Kijima, J. Electrochem. Soc. 151 (2004) H86–H92. [5] F. Duault, M. Junker, P. Grosseau, B. Guilhot, P. Iacconi, B. Moine, Powder Technol. 154 (2005) 132–137. [6] D. Haranath, P. Sharma, H. Chander, A. Ali, N. Bhalla, S.K. Halder, Mater. Chem. Phys. 101 (2007) 163–169. [7] B. Tian, B. Chen, Y. Tian, J. Sun, X. Li, J. Zhang, H. Zhong, L. Cheng, R. Hua, J. Phys. Chem. Solids 73 (2012) 1314–1319. [8] B. Tian, B. Chen, Y. Tianb, J. Sun, X. Li, J. Zhang, H. Zhong, L. Cheng, Z. Wu, R. Hua, Ceram. Int. 38 (2012) 3537–3540. [9] C.H. Liang, Y.C. Chang, Y.S. Chang, Appl. Phys. Lett. 93 (2008) 211902. [10] H.Y. Chen, M.H. Weng, R.Y. Yang, S.J. Chang, Ceram. Int. 37 (2011) 1521–1524. [11] C.H. Liang, L.G. Teoh, K.T. Liu, Y.S. Chang, J. Alloys Compd. 517 (2012) 9–13. [12] J.A. Kaduk, W. Wong-Ng, W. Greenwood, J. Dillingham, B.H. Toby, J. Res. Natl. Inst. Stand. Technol. 104 (1999) 147. [13] L.J. Nugent, R.D. Baybarz, J.L. Burnett, J.L. Ryan, J. Phys. Chem. 77 (1973) 1528. [14] Y. Tian, B. Chen, H. Yu, R. Hua, X. Li, J. Sun, L. Cheng, H. Zhong, J. Zhang, Y. Zheng, T. Yu, L. Huang, J. Colloid Interface Sci. 360 (2011) 586–592. [15] Y. Tian, B. Chen, B. Tian, J. Sun, X. Li, J. Zhang, L. Cheng, H. Zhong, H. Zhong, Q. Meng, Ruinian Hua, Physica B 407 (2012) 2556–2559. [16] Y.S. Chang, H.J. Lin, Y.C. Li, Y.L. Chai, Y.Y. Tsai, J. Solid State Chem. 180 (2007) 3076. [17] K.L. Liou, Y.F. W, I.G. Chen, J. Taiwan Vac. Soc. 23 (2010) 41–48.