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Effects of Na2CO3 flux addition on the structure and photoluminescence properties for Eu3+-doped YVO4 phosphor
Journal of Physics and Chemistry of Solids 72 (2011) 1117–1121
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Effects of Na2CO3 flux addition on the structure and photoluminescence properties for Eu3 þ -doped YVO4 phosphor Yee-Shin Chang a, Feng-Ming Huang a, Hao-Long Chen b, Yeou-Yih Tsai b,n a b
Department of Electronic Engineering, National Formosa University, Huwei, Yunlin 632, Taiwan Department of Electronic Engineering, Kao Yuan University, Lujhu, Kaohsiung 821, Taiwan
a r t i c l e i n f o
abstract
Article history: Received 18 November 2010 Received in revised form 17 April 2011 Accepted 3 June 2011 Available online 14 July 2011
The luminescence properties of (Y0.9Eu0.1)VO4 phosphor with Na2CO3 flux prepared using the solidstate reaction were investigated. The XRD patterns show that all of the peaks are attributed to the YVO4 phase. The best crystallinity was obtained with 2 wt% Na2CO3 flux addition. The surface morphology of (Y0.9Eu0.1)VO4 phosphor changed from fluffy to a bar shape structure after Na2CO3 flux addition due to the tetragonal crystal system of YVO4. The calcined powders emit bright red luminescence centered at 618 nm due to the 5D0-7F2 electric dipole transition under an excitation wavelength of 318 nm; its intensity was increased about 15% with 2 wt% Na2CO3 flux addition. Red shift behavior was observed for the charge transfer state (CTS) absorption, which was due to the grain size of (Y0.9Eu0.1)VO4 phosphor increasing with increasing flux content. For 2 wt% Na2CO3 flux addition, the red emission of the (Y0.9Eu0.1)VO4 phosphor had CIE chromaticity coordinates of (0.66, 0.34), which are very close to the NTSC system standard red chromaticity coordinates of (0.67, 0.33). & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Optical materials A. Oxides C. X-ray diffraction D. Luminescence D. Optical properties
1. Introduction It is highly desirable to develop low-voltage phosphors with high efficiency and chemical stability under electron beam bombardment in a high-vacuum system for next generation displays and solid-state lighting [1–5]. The morphology of phosphor particles with a suitable shape and size is a key parameter in industrial applications [6,7]. The size and shape of phosphor particles affect the emission intensity and the efficiency of a device [8]. The size of phosphor grains should be as homogeneous as possible without any aggregates or agglomerates. Moreover, an optimal compactness of the powder enhances the brightness of a display and lowers production costs. The phosphor particle surface should also be as smooth as possible and have a high crystallization degree to improve efficiency. A flux is a material that melts at temperatures below the solidstate reaction temperature, dissolves one or more of the components, and allows material transport to the reaction zone, without participating in the solid-state reaction [9]. Preferably, the end-product should be insoluble in the flux. The addition of a flux reduces the calcination temperature by increasing diffusion coefficients and improves the shape of phosphor powders. Flux materials include Na2CO3, LiCl, and KF [10–12].
Yttrium vanadium oxide (YVO4) has a tetragonal-type oxide structure and has excellent electro-optical and acoustic-optical properties, making it suitable for various parts of optical devices such as solid-state laser hosts [13], polarizers [14,15], and phosphors [16]. Some previous studies investigated the effect of the size, morphology of particles, and calcination temperature on the photoluminescence properties of YVO4:Eu3 þ nanocrystalline phosphors [17–20]. The results indicated that the emission intensities of Eu3 þ ion f–f transitions increase with increasing calcination temperature. However, few studies have examined the effects of flux reagents on the structure and photoluminescence properties of YVO4:Eu3 þ phosphors. According to our previous study [21], the saturation of the emission intensity excited by the charge transfer state (CTS) was obtained when the Eu3 þ concentration was 10 mol%. Therefore, in this paper, the Na2CO3 used as a flux and added into the (Y0.9Eu0.1)VO4 phosphor was synthesized using the solid-state reaction. The influences of the Na2CO3 flux reagent concentration on the resulting structure, particle distribution, and the photoluminescence properties including excitation and emission behavior of YVO4:Eu3 þ phosphors were also investigated.
2. Experimental procedure 2.1. Preparation of samples
n
Corresponding author. Tel.: þ886 7 6077002; fax: þ 886 7 6077000. E-mail address: [email protected] (Y.-Y. Tsai).
0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.06.008
The compounds were synthesized using the solid-state reaction. The starting materials were Y2O3, V2O5, Eu2O3, and Na2CO3 with
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Y.-S. Chang et al. / Journal of Physics and Chemistry of Solids 72 (2011) 1117–1121
a purity of 99.99% obtained from Aldrich Chemical Company and Acros Organics. The raw powders were based on the formula of (Y0.9Eu0.1)VO4:(Na2CO3)x and the addition of Na2CO3 flux was varied from x¼ 1 to 20 wt%. First, the starting materials of Y2O3, V2O5, and Eu2O3 powders were mixed in ethanol by ball milling with zirconia balls in polyethylene jars for 24 hr. In order to obtain the columbite precursor for the (Y0.9Eu0.1)VO4 powders, the mixture was dried at 120 1C for 24 hr and calcined at 800 1C for 3 h in air. The (Y0.9Eu0.1)VO4:(Na2CO3)x powders were then prepared by mixing the (Y0.9Eu0.1)VO4 and Na2CO3 powders using a planetary ball mill solid-state reaction. After mechanical mixing by planetary ball milling for 30 min at a speed of 150 rpm with zirconia balls in a polyethylene jar, the mixtures were heat treated at 1000 1C for 3 hr in a programmable furnace. 2.2. Characterizations The crystal structure of the obtained powders was analyzed with an X-ray diffractometer (XRD, Rigaku Dmax-33 X-ray diffractometer) using Cu Ka radiation to identify the possible phases formed after heat treatment. The surface morphology of powders was examined using high-resolution scanning electron microscopy (HR-SEM, S4200, Hitachi). A Hitachi U-3010 UV visible spectrophotometer was used to measure the optical absorption behavior of the (Y0.9Eu0.1)VO4:(Na2CO3) phosphors, which were placed inside a closed quartz glass and measured from 200 to 700 nm at room temperature. Both excitation and luminescence spectra of the phosphors were analyzed with a Hitachi F-4500 fluorescence spectrophotometer using a 150 W xenon arc lamp as the excitation source at room temperature.
3. Results and discussion 3.1. Phases in the samples Fig. 1 shows the X-ray powder diffraction pattern of (Y0.9Eu0.1)VO4 phosphor doped with various Na2CO3 concentrations calcined at 1000 1C for 3 h in air. For low Na2CO3 content, all the peaks are attributed to the YVO4 phase [JCPD No. 72-0861]. No distinct XRD peaks corresponding to Na2CO3 phase were observed, which indicates that most of the Na2CO3 flux was removed after powder calcining. In the conventional solid-state reaction method, the repeated milling process for obtaining the particles causes many surface defects, degrading the photoluminescence properties of the phosphor. Because Na2CO3 has a low melting point of about 851 1C, it acted as a flux reagent in this study, reducing the calcination temperature and
Fig. 1. XRD pattern of (Y0.9Eu0.1)VO4 phosphor added with various Na2CO3 flux concentrations.
surface defects, which increased the crystallinity of the phosphor particles [22]. For the (Y0.9Eu0.1)VO4 phosphor diffraction peaks, the intensity increased and the full width of half maximum (FWHM) decreased when Na2CO3 was added. The maximum intensities for diffraction peaks occurred when the Na2CO3 content was 2 wt%. When the Na2CO3 content was increased further, the intensities of the diffraction peaks decreased and a second phase formed. 3.2. Microstructures Fig. 2 shows the SEM micrographs of (Y0.9Eu0.1)VO4 phosphor added with various Na2CO3 concentrations calcined at 1000 1C for 3 hr in air. For the flux-free phosphor, the particles are very small, and the structure of the powders appears fluffy. The particle size increased with increasing Na2CO3 content. The results show that the particle size of the phosphor increases and the shape becomes regular with increasing Na2CO3 flux concentration. It is known that the surface tension of a liquid helps particles to coagulate when fluxes melt. The melt also makes it easier for particles to slide and rotate. This effect provides opportunities for particle– particle contact and promotes particle growth [23]. When fluxes sufficiently melt, the liquid helps orient the aggregation of metal oxide particles in one dimension, which results in a transformation of some particle morphology into a bar shape due to the tetragonal crystal system of YVO4. When the Na2CO3 flux concentration was increased further, the (Y0.9Eu0.1)VO4 powders congregated together (see Fig. 2 (e)). This is due to a large amount of flux melting and producing a liquid interface between the particles, resulting in the formation of larger particles [24]. 3.3. Optical properties Fig. 3 shows the optical absorption spectra for YVO4, flux-free (Y0.9Eu0.1)VO4 and (Y0.9Eu0.1)VO4 phosphor added with 2 wt% Na2CO3 flux measured at room temperature. All of the compounds exhibited two obvious broad bands peaking at 266 and 350 nm, respectively, which are attributed to the charge transfer from the oxygen ligands to the central vanadium atom inside the VO3 4 anionic group in the host lattice [25]. After europium was added to YVO4, a strong absorption peak appeared between 250 and 350 nm caused by the CTS for electrons transferred from the oxygen 2p orbital to the empty 4f orbital of europium ions, which indicates that the host absorption is very efficient. The weak peaks in the range of 320–580 nm are associated with the typical f–f transitions of the Eu3 þ ions that appear at 396 and 467 nm, which are attributed to the 7F0-5L6 and 7F0-5D2 transitions, respectively. With 2 wt% Na2CO3 in the (Y0.9Eu0.1)VO4 phosphor, the intensity of CTS transition was larger than that for the flux-free (Y0.9Eu0.1)VO4 phosphor. The best crystallinity of (Y0.9Eu0.1)VO4 was obtained with 2 wt% Na2CO3 addition. Fig. 4 shows the excitation spectra (lem ¼618 nm) for flux-free (Y0.9Eu0.1)VO4 and (Y0.9Eu0.1)VO4 phosphor added with 2 wt% Na2CO3 flux. A strong wide band centered at 318 nm corresponds to the CTS of the host lattice due to the oxygen-to-europium interaction. The electrons move from completely-filled 2p orbitals of O2 to partially-filled 4f6 levels of Eu3 þ ion [26]. A series of peaks between 350 and 550 nm that appear at 363, 383, 396, 418, 467, and 537 nm, which are associated with the typical f–f transitions of the Eu3 þ ions, correspond to the 7F0-5D4, 7 F0-5L7, 7F0-5L6, 7F0-5D3, 7F0-5D2, and 7F0-5D1 transitions, respectively. The intensities of CTS increase with increasing Na2CO3 flux content, reaching a maximum value at 2 wt%, and then decrease with increasing Na2CO3 flux content. In addition, the location of the CTS center shifts slightly to the long wavelength region with flux addition. Table 1 shows the average grain sizes of (Y0.9Eu0.1)VO4 phosphor with various Na2CO3 flux
Y.-S. Chang et al. / Journal of Physics and Chemistry of Solids 72 (2011) 1117–1121
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Fig. 2. (Y0.9Eu0.1)VO4 phosphor added with (a) 0, (b) 1, (c) 2, (d) 5, and (f) 20 wt% Na2CO3 flux.
concentrations calculated using Scherrer’s equation [27]. It shows that the grain size increases with increasing flux content, which causes a red shift of CTS [28]. Fig. 5 shows the photoluminescence emission spectra under an excitation wavelength of 318 nm for (Y0.9Eu0.1)VO4 phosphor added with various Na2CO3 flux concentrations calcined at 1000 1C for 3 h in air. The major emission peak at about 618 nm is due to the 5D0-7F2 electric dipole transition, for which the intensity is hyper-sensitive to the variation of the local structure environment. A weak peak at 594 nm is the 5D0-7F1 magneticdipole allowed transition; its intensity hardly changes with the local structure symmetry of Eu3 þ ions [8]. In addition, a weak emission peak appears at 537 nm, corresponding to the 5D0-7F0 transition. In the (Y0.9Eu0.1)VO4:Na2CO3 system, Na2CO3 addition did not change the shape of curves, but it did change the
intensities of the emission spectra. The relationship between relative photoluminescence intensity and Na2CO3 content is shown in Fig. 6. As can be seen, the relative photoluminescence intensity of samples with Na2CO3 flux is higher than that of the sample without Na2CO3 flux. The intensity of the 5D0-7F2 transition for (Y0.9Eu0.1)VO4 phosphor increased about 15% for 2 wt% Na2CO3 flux addition. This is due to the sodium compounds being particularly effective in promoting crystal growth and the diffusion of activators of Eu3 þ ions into the YVO4 lattice. The second phase forms when the Na2CO3 flux concentration is increased further, decreasing the intensity of the 5D0-7F2 transition. The Commission Internationale de I’Eclairage (CIE) color coordinates for various color tones for (Y0.9Eu0.1)VO4 phosphor added with various Na2CO3 flux concentrations excited at 318 nm are
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Y.-S. Chang et al. / Journal of Physics and Chemistry of Solids 72 (2011) 1117–1121
Fig. 5. Emission spectra of (Y0.9Eu0.1)VO4 phosphor added with (a) 0, (b) 1, (c) 2, (d) 5, and (e) 20 wt% Na2CO3 flux.
Fig. 3. Absorption spectra of YVO4 and (Y0.9Eu0.1)VO4 phosphor added with 0 and 2 wt% Na2CO3 flux, respectively.
Fig. 6. Emission intensity of 5D0-7F2 transition for (Y0.9Eu0.1)VO4 phosphor added with various Na2CO3 flux concentrations.
Fig. 4. Excitation spectra of (Y0.9Eu0.1)VO4 phosphor added with various Na2CO3 flux concentrations.
Table 1 Average grain sizes for (Y0.9Eu0.1)VO4 phosphor added with various Na2CO3 flux concentrations calculated using Scherrer’s equation. Na2CO3 content (wt%)
Average grain sizes (nm)
0 1 2 5
29.28 30.40 34.28 35.79
shown in Fig. 7 and listed in Table 2. The CIE color coordinates for the (Y0.9Eu0.1)VO4:Na2CO3 system vary slightly. For all samples, the CIE color coordinates are in the bright red color region. With
2 wt% Na2CO3 flux addition, the CIE color coordinates are (0.66, 0.34), which is very close to the NTSC system standard red chromaticity coordinates of (0.67, 0.33).
4. Conclusions The luminescence properties of (Y0.9Eu0.1)VO4 phosphor with Na2CO3 flux prepared using the solid-state reaction were investigated. The XRD pattern shows that all of the peaks are attributed to the YVO4 phase. The best crystallinity was obtained with 2 wt% Na2CO3 flux addition. The SEM results indicate that bar-shaped particles formed for the (Y0.9Eu0.1)VO4 phosphor with the Na2CO3 flux addition corresponding to YVO4 had the tetragonal crystal structure. The calcined powders emit bright red luminescence centered at 618 nm due to the 5D0-7F2 electric dipole transition under an excitation wavelength of 318 nm; its intensity is
Y.-S. Chang et al. / Journal of Physics and Chemistry of Solids 72 (2011) 1117–1121
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ions into the YVO4 lattice. Red shift behavior was observed for CTS absorption, which was due to the grain size of (Y0.9Eu0.1)VO4 phosphor increasing with increasing flux content. In addition, the bright red emission of the (Y0.9Eu0.1)VO4 phosphor has CIE chromaticity coordinates of (0.66, 0.34) with 2 wt% Na2CO3 flux addition, and which is very close to the NTSC system standard red chromaticity coordinates of (0.67, 0.33).
Acknowledgments The authors gratefully acknowledge the National Science Council of the Republic of China for financially supporting this research under the Grant NSC 98-2622-E-150-065-CC3. References
Fig. 7. CIE chromaticity diagram for (Y0.9Eu0.1)VO4 phosphor added with various Na2CO3 flux concentrations. Data points 1, 2, 3, and 4 are assigned to Na2CO3 concentrations of 0, 1, 2, and 5 wt%, respectively.
Table 2 CIE color coordinates of (Y0.9Eu0.1)VO4 phosphor added with various Na2CO3 flux concentrations. Na2CO3 flux content (wt%)
0 1 2 5
kex ¼ 318 nm x
y
0.649 0.651 0.660 0.653
0.348 0.347 0.340 0.345
increased by about 15% with 2 wt% of Na2CO3 flux addition. This is due to the sodium compounds being particularly effective in promoting crystal growth and the diffusion of activators of Eu3 þ
[1] X.Q. Su, B. Yan, J. Non-Cryst. Solids 351 (2005) 3542. [2] S. Ekambaram, J. Alloys Compd. 390 (2005) L1. [3] S.D. Han, S.P. Khatkar, V.B. Taxak, G. Sharma, D. Kumar, Mater. Sci. Eng. B 129 (2006) 126. [4] K. Riwotzki, M. Haase, J. Phys. Chem. B 102 (1998) 10129. [5] B. Yan, X.Q. Su, Mater. Sci. Eng. B 119 (2005) 196. [6] S. Oshion, K. Kitamura, T. Shigeta, S. Horii, T. Matsuoka, S. Tanaka, H. Kobayashi, J. Electrochem. Soc. 146 (1999) 392. [7] Y.C. Kang, E.J. Kim, D.Y. Lee, H.D. Park, J. Alloys Compd. 347 (2002) 266. [8] S. Shionoya, W.M. Yen, Phosphor Handbook, , 1998. ¨ [9] W. Dorr, H. Assmann, G. Maier, J. Steven, J. Nucl. Mater. 81 (1979) 135. [10] X.X. Zhao, X.J. Wang, B.J. Chen, Q.Y. Meng, W.H. Di, G.Z. Ren, Y.M. Yang, J. Rare Earths 25 (2007) 15. [11] Y.C. Kang, H.S. Roh, S.B. Park, H.D. Park, J. Eur. Ceram. Soc. 2002 (22, 1661). [12] T. Shishido, S. Nojima, M. Tanaka, H. Horiuchi, T. Fukuda, J. Alloys Compd. 227 (1995) 175. [13] J.R. O’Connor, Appl. Phys. Lett. 9 (1966) 407. [14] E.A. Maunders, L.G. Deshazer, J. Opt. Soc. Am. B 61 (1971) 684. [15] M. Bass, IEEE J. Quantum Electron. QE-11 (1975) 938. [16] A.K. Levine, F.C. Palilla, Appl. Phys. Lett. 5 (1964) 118. [17] Y.H. Li, G.Y. Hong, J. Solid State Chem. 178 (2005) 645. [18] H.W. Zhang, X.Y. Fu, S.Y. Niu, G.Q. Sun, Q. Xin, J. Solid State Chem. 177 (2004) 2649. [19] W.J. Park, M.K. Jung, T. Masaki, S.J. Im, D.H. Yoon, Mater. Sci. Eng. B 146 (2008) 95. [20] L.H. Tian, S.I. Mho, J. Lumin. 122-123 (2007) 99. [21] Y.S. Chang, F.M. Huang, Y.Y. Tsai, L.G. Teoh, J. Lumin. 129 (2009) 1181. [22] X. Yi, H. Chen, W. Cao, M. Zhao, D. Yang, G. Ma, C. Yang, J. Han, J. Cryst. Growth 281 (2005) 365. [23] W.M. Yen, S. Shionoya, H. Yamamoto, Phosphor Handbook, CRC Press, 2006. [24] J. Niittykoski, T. Aitasalo, J. Holsa, H. Jungner, M. Lastusaari, M. Parkkinen, M. Tukia, J. Alloys Compd. 374 (2004) 108. [25] H. Zhang, X. Fu, S. Niu, G. Sun, Q. Xin, Solid State Commun. 132 (2004) 527. [26] M. Yin, W. Zhang, S. Xia, J.-C. Krupa, J. Lumin. 68 (1996) 335. [27] B.D. Cullity, Elements of X-ray Diffraction, 2nd ed., Addison-Wesley Publishing Company, Inc., Reading, MA, 1978. [28] Q. Li, L. Gao, D. Yan, Mater. Chem. Phys. 64 (2000) 41.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Effects of Na2CO3 flux addition on the structure and photoluminescence properties for Eu3 þ -doped YVO4 phosphor Yee-Shin Chang a, Feng-Ming Huang a, Hao-Long Chen b, Yeou-Yih Tsai b,n a b
Department of Electronic Engineering, National Formosa University, Huwei, Yunlin 632, Taiwan Department of Electronic Engineering, Kao Yuan University, Lujhu, Kaohsiung 821, Taiwan
a r t i c l e i n f o
abstract
Article history: Received 18 November 2010 Received in revised form 17 April 2011 Accepted 3 June 2011 Available online 14 July 2011
The luminescence properties of (Y0.9Eu0.1)VO4 phosphor with Na2CO3 flux prepared using the solidstate reaction were investigated. The XRD patterns show that all of the peaks are attributed to the YVO4 phase. The best crystallinity was obtained with 2 wt% Na2CO3 flux addition. The surface morphology of (Y0.9Eu0.1)VO4 phosphor changed from fluffy to a bar shape structure after Na2CO3 flux addition due to the tetragonal crystal system of YVO4. The calcined powders emit bright red luminescence centered at 618 nm due to the 5D0-7F2 electric dipole transition under an excitation wavelength of 318 nm; its intensity was increased about 15% with 2 wt% Na2CO3 flux addition. Red shift behavior was observed for the charge transfer state (CTS) absorption, which was due to the grain size of (Y0.9Eu0.1)VO4 phosphor increasing with increasing flux content. For 2 wt% Na2CO3 flux addition, the red emission of the (Y0.9Eu0.1)VO4 phosphor had CIE chromaticity coordinates of (0.66, 0.34), which are very close to the NTSC system standard red chromaticity coordinates of (0.67, 0.33). & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Optical materials A. Oxides C. X-ray diffraction D. Luminescence D. Optical properties
1. Introduction It is highly desirable to develop low-voltage phosphors with high efficiency and chemical stability under electron beam bombardment in a high-vacuum system for next generation displays and solid-state lighting [1–5]. The morphology of phosphor particles with a suitable shape and size is a key parameter in industrial applications [6,7]. The size and shape of phosphor particles affect the emission intensity and the efficiency of a device [8]. The size of phosphor grains should be as homogeneous as possible without any aggregates or agglomerates. Moreover, an optimal compactness of the powder enhances the brightness of a display and lowers production costs. The phosphor particle surface should also be as smooth as possible and have a high crystallization degree to improve efficiency. A flux is a material that melts at temperatures below the solidstate reaction temperature, dissolves one or more of the components, and allows material transport to the reaction zone, without participating in the solid-state reaction [9]. Preferably, the end-product should be insoluble in the flux. The addition of a flux reduces the calcination temperature by increasing diffusion coefficients and improves the shape of phosphor powders. Flux materials include Na2CO3, LiCl, and KF [10–12].
Yttrium vanadium oxide (YVO4) has a tetragonal-type oxide structure and has excellent electro-optical and acoustic-optical properties, making it suitable for various parts of optical devices such as solid-state laser hosts [13], polarizers [14,15], and phosphors [16]. Some previous studies investigated the effect of the size, morphology of particles, and calcination temperature on the photoluminescence properties of YVO4:Eu3 þ nanocrystalline phosphors [17–20]. The results indicated that the emission intensities of Eu3 þ ion f–f transitions increase with increasing calcination temperature. However, few studies have examined the effects of flux reagents on the structure and photoluminescence properties of YVO4:Eu3 þ phosphors. According to our previous study [21], the saturation of the emission intensity excited by the charge transfer state (CTS) was obtained when the Eu3 þ concentration was 10 mol%. Therefore, in this paper, the Na2CO3 used as a flux and added into the (Y0.9Eu0.1)VO4 phosphor was synthesized using the solid-state reaction. The influences of the Na2CO3 flux reagent concentration on the resulting structure, particle distribution, and the photoluminescence properties including excitation and emission behavior of YVO4:Eu3 þ phosphors were also investigated.
2. Experimental procedure 2.1. Preparation of samples
n
Corresponding author. Tel.: þ886 7 6077002; fax: þ 886 7 6077000. E-mail address: [email protected] (Y.-Y. Tsai).
0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.06.008
The compounds were synthesized using the solid-state reaction. The starting materials were Y2O3, V2O5, Eu2O3, and Na2CO3 with
1118
Y.-S. Chang et al. / Journal of Physics and Chemistry of Solids 72 (2011) 1117–1121
a purity of 99.99% obtained from Aldrich Chemical Company and Acros Organics. The raw powders were based on the formula of (Y0.9Eu0.1)VO4:(Na2CO3)x and the addition of Na2CO3 flux was varied from x¼ 1 to 20 wt%. First, the starting materials of Y2O3, V2O5, and Eu2O3 powders were mixed in ethanol by ball milling with zirconia balls in polyethylene jars for 24 hr. In order to obtain the columbite precursor for the (Y0.9Eu0.1)VO4 powders, the mixture was dried at 120 1C for 24 hr and calcined at 800 1C for 3 h in air. The (Y0.9Eu0.1)VO4:(Na2CO3)x powders were then prepared by mixing the (Y0.9Eu0.1)VO4 and Na2CO3 powders using a planetary ball mill solid-state reaction. After mechanical mixing by planetary ball milling for 30 min at a speed of 150 rpm with zirconia balls in a polyethylene jar, the mixtures were heat treated at 1000 1C for 3 hr in a programmable furnace. 2.2. Characterizations The crystal structure of the obtained powders was analyzed with an X-ray diffractometer (XRD, Rigaku Dmax-33 X-ray diffractometer) using Cu Ka radiation to identify the possible phases formed after heat treatment. The surface morphology of powders was examined using high-resolution scanning electron microscopy (HR-SEM, S4200, Hitachi). A Hitachi U-3010 UV visible spectrophotometer was used to measure the optical absorption behavior of the (Y0.9Eu0.1)VO4:(Na2CO3) phosphors, which were placed inside a closed quartz glass and measured from 200 to 700 nm at room temperature. Both excitation and luminescence spectra of the phosphors were analyzed with a Hitachi F-4500 fluorescence spectrophotometer using a 150 W xenon arc lamp as the excitation source at room temperature.
3. Results and discussion 3.1. Phases in the samples Fig. 1 shows the X-ray powder diffraction pattern of (Y0.9Eu0.1)VO4 phosphor doped with various Na2CO3 concentrations calcined at 1000 1C for 3 h in air. For low Na2CO3 content, all the peaks are attributed to the YVO4 phase [JCPD No. 72-0861]. No distinct XRD peaks corresponding to Na2CO3 phase were observed, which indicates that most of the Na2CO3 flux was removed after powder calcining. In the conventional solid-state reaction method, the repeated milling process for obtaining the particles causes many surface defects, degrading the photoluminescence properties of the phosphor. Because Na2CO3 has a low melting point of about 851 1C, it acted as a flux reagent in this study, reducing the calcination temperature and
Fig. 1. XRD pattern of (Y0.9Eu0.1)VO4 phosphor added with various Na2CO3 flux concentrations.
surface defects, which increased the crystallinity of the phosphor particles [22]. For the (Y0.9Eu0.1)VO4 phosphor diffraction peaks, the intensity increased and the full width of half maximum (FWHM) decreased when Na2CO3 was added. The maximum intensities for diffraction peaks occurred when the Na2CO3 content was 2 wt%. When the Na2CO3 content was increased further, the intensities of the diffraction peaks decreased and a second phase formed. 3.2. Microstructures Fig. 2 shows the SEM micrographs of (Y0.9Eu0.1)VO4 phosphor added with various Na2CO3 concentrations calcined at 1000 1C for 3 hr in air. For the flux-free phosphor, the particles are very small, and the structure of the powders appears fluffy. The particle size increased with increasing Na2CO3 content. The results show that the particle size of the phosphor increases and the shape becomes regular with increasing Na2CO3 flux concentration. It is known that the surface tension of a liquid helps particles to coagulate when fluxes melt. The melt also makes it easier for particles to slide and rotate. This effect provides opportunities for particle– particle contact and promotes particle growth [23]. When fluxes sufficiently melt, the liquid helps orient the aggregation of metal oxide particles in one dimension, which results in a transformation of some particle morphology into a bar shape due to the tetragonal crystal system of YVO4. When the Na2CO3 flux concentration was increased further, the (Y0.9Eu0.1)VO4 powders congregated together (see Fig. 2 (e)). This is due to a large amount of flux melting and producing a liquid interface between the particles, resulting in the formation of larger particles [24]. 3.3. Optical properties Fig. 3 shows the optical absorption spectra for YVO4, flux-free (Y0.9Eu0.1)VO4 and (Y0.9Eu0.1)VO4 phosphor added with 2 wt% Na2CO3 flux measured at room temperature. All of the compounds exhibited two obvious broad bands peaking at 266 and 350 nm, respectively, which are attributed to the charge transfer from the oxygen ligands to the central vanadium atom inside the VO3 4 anionic group in the host lattice [25]. After europium was added to YVO4, a strong absorption peak appeared between 250 and 350 nm caused by the CTS for electrons transferred from the oxygen 2p orbital to the empty 4f orbital of europium ions, which indicates that the host absorption is very efficient. The weak peaks in the range of 320–580 nm are associated with the typical f–f transitions of the Eu3 þ ions that appear at 396 and 467 nm, which are attributed to the 7F0-5L6 and 7F0-5D2 transitions, respectively. With 2 wt% Na2CO3 in the (Y0.9Eu0.1)VO4 phosphor, the intensity of CTS transition was larger than that for the flux-free (Y0.9Eu0.1)VO4 phosphor. The best crystallinity of (Y0.9Eu0.1)VO4 was obtained with 2 wt% Na2CO3 addition. Fig. 4 shows the excitation spectra (lem ¼618 nm) for flux-free (Y0.9Eu0.1)VO4 and (Y0.9Eu0.1)VO4 phosphor added with 2 wt% Na2CO3 flux. A strong wide band centered at 318 nm corresponds to the CTS of the host lattice due to the oxygen-to-europium interaction. The electrons move from completely-filled 2p orbitals of O2 to partially-filled 4f6 levels of Eu3 þ ion [26]. A series of peaks between 350 and 550 nm that appear at 363, 383, 396, 418, 467, and 537 nm, which are associated with the typical f–f transitions of the Eu3 þ ions, correspond to the 7F0-5D4, 7 F0-5L7, 7F0-5L6, 7F0-5D3, 7F0-5D2, and 7F0-5D1 transitions, respectively. The intensities of CTS increase with increasing Na2CO3 flux content, reaching a maximum value at 2 wt%, and then decrease with increasing Na2CO3 flux content. In addition, the location of the CTS center shifts slightly to the long wavelength region with flux addition. Table 1 shows the average grain sizes of (Y0.9Eu0.1)VO4 phosphor with various Na2CO3 flux
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Fig. 2. (Y0.9Eu0.1)VO4 phosphor added with (a) 0, (b) 1, (c) 2, (d) 5, and (f) 20 wt% Na2CO3 flux.
concentrations calculated using Scherrer’s equation [27]. It shows that the grain size increases with increasing flux content, which causes a red shift of CTS [28]. Fig. 5 shows the photoluminescence emission spectra under an excitation wavelength of 318 nm for (Y0.9Eu0.1)VO4 phosphor added with various Na2CO3 flux concentrations calcined at 1000 1C for 3 h in air. The major emission peak at about 618 nm is due to the 5D0-7F2 electric dipole transition, for which the intensity is hyper-sensitive to the variation of the local structure environment. A weak peak at 594 nm is the 5D0-7F1 magneticdipole allowed transition; its intensity hardly changes with the local structure symmetry of Eu3 þ ions [8]. In addition, a weak emission peak appears at 537 nm, corresponding to the 5D0-7F0 transition. In the (Y0.9Eu0.1)VO4:Na2CO3 system, Na2CO3 addition did not change the shape of curves, but it did change the
intensities of the emission spectra. The relationship between relative photoluminescence intensity and Na2CO3 content is shown in Fig. 6. As can be seen, the relative photoluminescence intensity of samples with Na2CO3 flux is higher than that of the sample without Na2CO3 flux. The intensity of the 5D0-7F2 transition for (Y0.9Eu0.1)VO4 phosphor increased about 15% for 2 wt% Na2CO3 flux addition. This is due to the sodium compounds being particularly effective in promoting crystal growth and the diffusion of activators of Eu3 þ ions into the YVO4 lattice. The second phase forms when the Na2CO3 flux concentration is increased further, decreasing the intensity of the 5D0-7F2 transition. The Commission Internationale de I’Eclairage (CIE) color coordinates for various color tones for (Y0.9Eu0.1)VO4 phosphor added with various Na2CO3 flux concentrations excited at 318 nm are
1120
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Fig. 5. Emission spectra of (Y0.9Eu0.1)VO4 phosphor added with (a) 0, (b) 1, (c) 2, (d) 5, and (e) 20 wt% Na2CO3 flux.
Fig. 3. Absorption spectra of YVO4 and (Y0.9Eu0.1)VO4 phosphor added with 0 and 2 wt% Na2CO3 flux, respectively.
Fig. 6. Emission intensity of 5D0-7F2 transition for (Y0.9Eu0.1)VO4 phosphor added with various Na2CO3 flux concentrations.
Fig. 4. Excitation spectra of (Y0.9Eu0.1)VO4 phosphor added with various Na2CO3 flux concentrations.
Table 1 Average grain sizes for (Y0.9Eu0.1)VO4 phosphor added with various Na2CO3 flux concentrations calculated using Scherrer’s equation. Na2CO3 content (wt%)
Average grain sizes (nm)
0 1 2 5
29.28 30.40 34.28 35.79
shown in Fig. 7 and listed in Table 2. The CIE color coordinates for the (Y0.9Eu0.1)VO4:Na2CO3 system vary slightly. For all samples, the CIE color coordinates are in the bright red color region. With
2 wt% Na2CO3 flux addition, the CIE color coordinates are (0.66, 0.34), which is very close to the NTSC system standard red chromaticity coordinates of (0.67, 0.33).
4. Conclusions The luminescence properties of (Y0.9Eu0.1)VO4 phosphor with Na2CO3 flux prepared using the solid-state reaction were investigated. The XRD pattern shows that all of the peaks are attributed to the YVO4 phase. The best crystallinity was obtained with 2 wt% Na2CO3 flux addition. The SEM results indicate that bar-shaped particles formed for the (Y0.9Eu0.1)VO4 phosphor with the Na2CO3 flux addition corresponding to YVO4 had the tetragonal crystal structure. The calcined powders emit bright red luminescence centered at 618 nm due to the 5D0-7F2 electric dipole transition under an excitation wavelength of 318 nm; its intensity is
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ions into the YVO4 lattice. Red shift behavior was observed for CTS absorption, which was due to the grain size of (Y0.9Eu0.1)VO4 phosphor increasing with increasing flux content. In addition, the bright red emission of the (Y0.9Eu0.1)VO4 phosphor has CIE chromaticity coordinates of (0.66, 0.34) with 2 wt% Na2CO3 flux addition, and which is very close to the NTSC system standard red chromaticity coordinates of (0.67, 0.33).
Acknowledgments The authors gratefully acknowledge the National Science Council of the Republic of China for financially supporting this research under the Grant NSC 98-2622-E-150-065-CC3. References
Fig. 7. CIE chromaticity diagram for (Y0.9Eu0.1)VO4 phosphor added with various Na2CO3 flux concentrations. Data points 1, 2, 3, and 4 are assigned to Na2CO3 concentrations of 0, 1, 2, and 5 wt%, respectively.
Table 2 CIE color coordinates of (Y0.9Eu0.1)VO4 phosphor added with various Na2CO3 flux concentrations. Na2CO3 flux content (wt%)
0 1 2 5
kex ¼ 318 nm x
y
0.649 0.651 0.660 0.653
0.348 0.347 0.340 0.345
increased by about 15% with 2 wt% of Na2CO3 flux addition. This is due to the sodium compounds being particularly effective in promoting crystal growth and the diffusion of activators of Eu3 þ
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