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Eu3+:CdWO4 single-phased white-light-emitting phosphor: pH controlled luminescence properties
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Eu3 þ :CdWO4 single-phased white-light-emitting phosphor: pH controlled luminescence properties Mingjiang You, Jiayue Xun, Zhijie Zhangn, Yu Zhou Institute of Crystal Growth, School of Materials Science and Engineering, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai 201418, PR China Received 27 January 2015; received in revised form 17 February 2015; accepted 17 February 2015
Abstract Eu3 þ :CdWO4 single-phased phosphor for white-light-emitting diode (W-LED) was prepared by a hydrothermal method. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) as well as photoluminescence (PL) spectra. The main luminescence center changed between WO24 groups and Eu3 þ ions at different pH values, which led to different light-emitting properties of the samples. Under the excitation of 296 nm UV light, stable and independent white emission could be achieved when the pH value of the precursor was 7. The CIE chromaticity coordinates of the Eu3 þ :CdWO4 phosphor synthesized at pH ¼7 were x ¼ 0.323 and y ¼0.317, which were close to the values of standard chromaticity (x ¼ 0.33 and y ¼ 0.33) for an NTSC system. The CIE chromaticity calculation demonstrated its potential application in white light emission devices based on UV chips. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Single-phased; Eu3 þ :CdWO4 phosphor; W-LED; UV-excitation; Photoluminescence
1. Introduction For the superior advantages such as high brightness, high efficiency, long lifetime, substantial energy savings, etc [1–4], white light-emitting diodes (W-LEDs) are considered as a potential replacement for the traditional illumination devices and have attracted much attention [5–7]. At present, most of the commercial W-LED products are fabricated by combining Y3Al5O12:Ce3 þ (YAG:Ce3 þ ) [8–10] yellow-emitting phosphor with a blue LED (440–465 nm, GaN-based) chip. However, this kind of white light shows poor color rendering index (CRI) due to the color deficiency in the red light region and unsatisfactory high color temperature [11–15]. The commonly alternative way is to employ an ultraviolet (UV) LED chip as the excitation light source with red/green/blue (RGB) tri-color phosphors [16,17]. Nevertheless, these multi-phased phosphors have different decay times and reabsorption of emission colors, which may lead to a decreased luminous n
Corresponding authors. Tel.: þ86 21 6087 3581; fax: þ 86 21 6087 3439. E-mail addresses: [email protected] (J. Xu), [email protected] (Z. Zhang).
efficiency, and its application is restricted. Recently, singlephased white-emitting phosphor has attracted much interest [18], which includes singly doping Eu3 þ ion into an appropriate host lattice [19]. Due to the europium is more stable in an alkaline earth host and more easily diffuses into the lattice sites [20]. What is more with an abundant energy levels in a wide range of wavelengths [21], the Eu3 þ emissions which yield the blue, green and red emissions simultaneously, the combination of comparable multi-color emissions can produce white light. Metal tungstates have been widely studied as competitive host materials in the optical field due to their promising applications for phosphors, scintillators, lasers, catalysts and optical fibers [22–25] and their luminescence characteristics can be tuned by doping different active cations according to their ionic electronegativities [26]. Cadmium tungstate (CdWO4) with a monoclinic wolframite structure is an important family among metal tungstates that have potential applications in a variety of fields. For example, CdWO4 has already been used as an XCT detector material for its good scintillation properties [27,28]. Meanwhile, it is a popular functional material because of its low-radiation damage, high
http://dx.doi.org/10.1016/j.ceramint.2015.02.099 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: M. You, et al., Eu3 þ :CdWO4 single-phased white-light-emitting phosphor: pH controlled luminescence properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.02.099
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average refractive index and high X-ray absorption coefficient [29]. As a self-activating phosphor, CdWO4 exhibits a broad, intrinsic blue–green emission band under deep UV excitation. Considering the high chemical stability, short decay time and non-toxicity of tungstate, Eu3 þ -doped CdWO4 may be an ideal candidate to generate white light by combining the blue– green emission from tungstate group with the red emission from Eu3 þ . To the best of our knowledge, the luminescence property and mechanism of Eu3 þ -doped CdWO4 phosphor for white light emission under UV light excitation has seldom been reported before. In this article, single-phased Eu3 þ :CdWO4 white phosphor with excellent photoluminescence (PL) properties was synthesized by a hydrothermal method. The effect of the pH values of the precursors on the luminescence properties of the phosphors was investigated. When the pH value of the precursor was 7, white light emission with high color purity and high luminous intensity could be achieved under the excitation of 296 nm UV light. 2. Experimental 2.1. Synthesis Eu3 þ -doped CdWO4 white light phosphors were synthesized by the hydrothermal method. All chemical reagents used were analytic grade without further purification or other treatments. The preparation process was as follows: 1 mmol of Cd(Ac)2 2H2O was dissolved in 20 mL of deionized water, and appropriate amount of Eu2O3 was dissolved into HNO3 solution and was then added to the former solution. The mixed solution was stirred for 30 min to get solution A. Meanwhile, 1 mmol of Na2WO4 2H2O was dissolved in 20 mL of deionized water to obtain solution B. Then, solution B was added dropwise into solution A with magnetic stirring to obtain a white suspension. Afterwards, the pH value of the suspension was adjusted by the addition of HNO3 or
NH3 H2O. After being stirred for 1 h, the suspension was transferred into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was heated at 180 1C for 20 h under autogenous pressure, and then cooled to room temperature naturally. The resulting products were separated by filtration, washed with deionized water and absolute alcohol for several times, and then dried at 60 1C for 12 h. In this work, the atomic ratios of Eu3 þ to Cd2 þ were 0.5%, and the pH values of the precursors were 3, 5, 7, 9 and 11. 2.2. Characterization The composition and phase of the Eu3 þ :CdWO4 phosphors for W-LED were characterized by using powder X-ray diffraction (XRD) analysis on a Bruker D8 Focus diffractometer with Cu Kα radiation operated at 40 kV and 40 mA, respectively. The photoluminescence excitation (PLE) and photoluminescence (PL) spectra were analyzed using a FluoroMax-4 fluorescence spectrometer with a 450 W Xenon lamp as light source at room temperature (parameters of excitation and emission slit widths were set to be 1.5 nm). The morphology of the samples was characterized by a JEOL JEM-2100 transmission electron microscope (TEM) at an accelerating voltage of 200 kV. The colorimetry parameters were measured on a PMS-50 Plus UV–Vis-near IR spectrophotocolorimeter (Everfine, China). 3. Results and discussion 3.1. X-ray diffraction (XRD) analysis Fig. 1a showed the XRD patterns of the CdWO4 (pH ¼ 7) and Eu3 þ :CdWO4 samples prepared at different pH values (pH ¼ 3, 5, 7, 9 and 11). The patterns showed a monoclinic structure of CdWO4 with lattice parameters of α ¼ γ ¼ 901 and β¼ 91.681,which were close to the standard data of α ¼ γ ¼ 901 and β ¼ 91.471. All the diffraction peaks in the patterns could be indexed to cadmium tungstate phase (JCPDS no. 14-0676)
Fig. 1. (a) XRD patterns of CdWO4 (pH¼7) and CdWO4:0.5% Eu3 þ prepared at different pH values. (b) Diffraction peak positions of CdWO4:0.5% Eu3 þ in the range of 2θ¼ 28–301. Please cite this article as: M. You, et al., Eu3 þ :CdWO4 single-phased white-light-emitting phosphor: pH controlled luminescence properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.02.099
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and not any other diffraction peaks belonging to impurity phase were observed. This indicated that the products were pure phase and the small amount of Eu3 þ (0.5%) did not change the lattice structure. The ( 111) and (111) diffraction peak positions of Eu3 þ : CdWO4 prepared at different pH values were compared carefully with pure CdWO4 in the range of 2θ¼ 28–30.51, as shown in Fig. 1b. It could be seen that the diffraction peaks of Eu3 þ -doped CdWO4 shifted slightly to the lower angle compared to pure CdWO4, while the pH value did not have obvious influence on the peak positions. According to Bragg's law [30]: dðhklÞ ¼ λ=ð2 sin θÞ d(hkl): the distance between crystal planes of (hkl) λ: the X-ray wavelength θ: the diffraction angle of the crystal plane (hkl)
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The larger the lattice parameters, the smaller the 2θ value. Because the ionic radius of Eu3 þ (1.07 Å) is larger than that of Cd2 þ (0.97 Å), the substitution of Cd2 þ by Eu3 þ could lead to larger lattice parameters, and the diffraction peak shifted toward lower angles. 3.2. Morphology Fig. 2 showed the morphologies and microstructures of pure CdWO4 and Eu3 þ :CdWO4 samples which were tested by transmission electron microscopy (TEM). The TEM image in Fig. 2a showed that pure CdWO4 exhibited uniform nanorod structure. The 0.5% Eu3 þ :CdWO4 prepared at different pH values were also nanorod structure, which indicated that low concentration doping of Eu3 þ cannot change the morphology of CdWO4, particularly for its thermodynamic growth habit
Fig. 2. Typical TEM images of (a) pure CdWO4 and CdWO4:0.5% Eu3 þ synthesized at different pH values; (b) HRTEM images of pure CdWO4 and CdWO4:0.5% Eu3 þ prepared at pH¼ 7. Please cite this article as: M. You, et al., Eu3 þ :CdWO4 single-phased white-light-emitting phosphor: pH controlled luminescence properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.02.099
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[31]. While the size distribution of these nanorods were different with the change of the pH values due to their kinetic control effect [32–34]. When the pH value of the precursors was 5 or 7, the nanorods were uniform in size with ca. 20 nm in width and 100 nm in length. However, the width of the nanorods became larger to ca. 50 nm when the pH value of the precursors was 3 or 11. For the sample prepared at pH=9, besides the nanorods, some nanoparticles could also be observed. According to the TEM images, Eu3 þ :CdWO4 prepared at pH=5–7 might be a good candidate to form phosphor layer with higher packing density and lower surface scattering for manufacturing W-LEDs. The representative HRTEM images of pure CdWO4 and Eu3 þ :CdWO4 prepared at pH=7 were shown in Fig. 2b. The HRTEM images showed clear lattice fringes with interlayer spacing of d ¼ 0.301 nm and d ¼ 0.327 nm, which corresponded to the (111) plane of pure CdWO4 and Eu3 þ :CdWO4, respectively. This result was in accordance with that of the XRD, which indicated that doping Eu3 þ ions could enlarge the lattice of CdWO4. 3.3. Photoluminescence (PL) properties of Eu3 þ :CdWO4 phosphors The photoluminescence excitation (PLE) spectra of Eu3 þ : CdWO4 phosphors prepared at different pH values monitored at 614 nm were shown in Fig. 3a. It could be seen that pure CdWO4 exhibited only one broad PLE peak at 296 nm, which was due to the charge transfer between the empty d orbitals of the central W ion and the O 2p orbitals. For the Eu3 þ -doped CdWO4, however, not only the broad peak but also some sharp lines at wavelengths of 362, 378, 380, 395, 416 and 468 nm could be observed, which could be assigned to the 7F0–5D4, 7 F0–5GJ, 7F0–5L7, 7F0–5L6, 7F0–5D3 and 7F0–5D2 transitions of Eu3 þ , respectively [35]. Moreover, the PLE intensities at 296 nm and 464 nm were higher than the other peaks, which
indicated that the UV and blue LED chips were suitable to excite Eu3 þ :CdWO4 phosphors. Fig. 3b showed the PLE intensity at 296 nm and 464 nm with λem ¼ 614 nm as a function of pH values. As can be seen from Fig. 3b, UV light was the best light source for the samples prepared at pH ¼ 3, 5, 7 and 9. However, for the Eu3 þ :CdWO4 phosphor prepared at pH ¼ 11, the blue light became more suitable than UV light to excite the sample. This phenomenon could be explained by the Laporte selection rule [36]: The electric-dipole 7F0–5D2 transition of Eu3 þ with an approximate wavenumber of 21.5 103 cm 1 ( 464 nm) is a forbidden transition. However, the forbidden transition become partial allowed due to the spin–orbit coupling at the excited state as well as the noncentrosymmetric crystal field. On the other hand, the magnetic-dipole 7F0–5L6 transition is suppressed. Therefore, for Eu3 þ :CdWO4 prepared at pH ¼ 11, the J–J coupling approximation would be more suitable, while l–s coupling approximation is applied to Eu3 þ in CdWO4 prepared at other pH values. The strong excitation peak related to the 7F0–5D2 hypersensitive transition of Eu3 þ enables Eu3 þ :CdWO4 phosphor prepared at pH ¼ 11 match well with the output wavelength of commercial GaN-based blue LED to generate red emission. This result has been demonstrated in our previous research [37]. Fig. 4a showed the PL emission spectra of Eu3 þ :CdWO4 phosphors prepared at different pH values excited by 296 nm UV light. As can be seen from the spectra, there was only one broad peak centered at 468 nm for pure CdWO4 sample, which was due to the 1A1-3T1 transition within the WO66 complex. For the Eu3 þ :CdWO4 phosphors, besides the typical emission of WO24 , some sharp lines at 578, 587, 592, 614, 625, 654 and 702 nm (as shown in the inset of Fig. 4a) could also be observed, which could be assigned to 5D0–7FJ (J ¼ 0–4) transitions of Eu3 þ cation [38]. The peak at 614 nm (corresponding to 5D0–7F2 electric dipole transitions of Eu3 þ ) was the strongest among the emissions, which indicated that Eu3 þ
Fig. 3. (a) The photoluminescence excitation (PLE) spectra of CdWO4:0.5% Eu3 þ prepared at different pH values (3, 5, 7, 9, and 11) at room temperature monitored at 614 nm. (b) The PLE intensity at 296 nm and 464 nm with λem ¼ 614 nm as a function of pH values. Please cite this article as: M. You, et al., Eu3 þ :CdWO4 single-phased white-light-emitting phosphor: pH controlled luminescence properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.02.099
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Fig. 4. (a) The photoluminescence (PL) emission spectra of CdWO4:0.5% Eu3 þ prepared at different pH values at room temperature with λexc ¼296 nm (the inset shows the emission spectra scanning from 550 nm to 710 nm, the left top corner is the intensity of Eu3 þ emission peaks). (b) The comparison of PL intensity at 614 nm and 468 nm as a function of pH values by λexc ¼ 296 nm.
ions have occupied the non-symmetry site in the CdWO4 host lattice. The intensity of main emission peaks at 468 nm and 614 nm as a function of pH values were compared carefully in Fig. 4b. It could be seen that the intensity of WO66 emission at 468 nm decreased gradually with the increase of the pH value, while the intensity of Eu3 þ emission at 614 nm increased. This result indicated that there was energy transfer from WO24 groups to Eu3 þ and suggested effective doping of Eu3 þ into the CdWO4 lattice. Therefore, the emission center could be changed between WO24 groups and Eu3 þ ions by adjusting the pH values of the precursor. In other words, by properly tuning the pH values, white-light-emitting Eu3 þ :CdWO4 phosphors could be achieved. 3.4. CIE chromaticity coordinates of Eu3 þ :CdWO4 phosphor Fig. 5 showed the color coordinates of Eu3 þ :CdWO4 phosphors prepared at different pH values. As can be seen from the figure, the emission color of pure CdWO4 was blue (point 1). With the increase of the pH value, the Eu3 þ :CdWO4 phosphors exhibited different light-emitting properties from blue to orange region. Especially, when the pH value of the precursor was 7, white light emission could be obtained (point 4). It was worth mentioning that the CIE chromaticity coordinates of the Eu3 þ : CdWO4 phosphor synthesized at pH¼ 7 were x¼ 0.323 and y¼ 0.317, which were close to the values of standard chromaticity (x¼ 0.33 and y¼ 0.33) for the National Television Standards Committee (NTSC) system. This result demonstrated that Eu3 þ : CdWO4 prepared at neutral environment could be an excellent and independent white-light-emitting phosphor in illuminating and display devices excited by UV light.
Fig. 5. The 1931 CIE chromaticity diagram with points (1–6) indicating coordinates for the CdWO4 phosphors prepared at different pH values. Point 1: pure CdWO4 (pH¼7), point 2–6: CdWO4:0.5% Eu3 þ (pH ¼3, 5, 7, 9 and 11, respectively) excited by 296 nm UV light. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
4. Conclusions In this article, Eu3 þ :CdWO4 single-phased phosphors with different emission colors have been synthesized by a hydrothermal method. Tunable color from blue through white to orange could be obtained by adjusting the pH values of the precursors. Especially, when the pH value of the precursor was 7, stable and independent white emission with CIE chromaticity coordinates of
Please cite this article as: M. You, et al., Eu3 þ :CdWO4 single-phased white-light-emitting phosphor: pH controlled luminescence properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.02.099
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x¼ 0.323 and y¼ 0.317 could be achieved, which were very close to the values of standard chromaticity (x¼ 0.33 and y¼ 0.33) for the NTSC system. Our work demonstrated that Eu3 þ :CdWO4 could be an excellent white-light-emitting phosphor and have potential applications in illuminating and display devices based on UV chips. Acknowledgment This work was supported by the National Natural Science Foundation of China (51342007) and the Shanghai Science and Technology Committee (11JC1412400). References [1] Z.G. Cui, R.G. Ye, D.G. Deng, Y.J. Hua, S.L. Zhao, G.H. Jia, C.X. Li, S.Q. Xu, Eu2 þ /Sm3 þ ions co-doped white light luminescence SrSiO3 glass-ceramics phosphor for white LED, J. Alloy. Compd. 509 (2011) 3553–3558. [2] Q.Q. Chen, N.L. Dai, Z.J. Liu, Y.B. Chu, White light luminous properties and energy transfer mechanism of rare earth ions in Ce3 þ /Tb3 þ /Sm3 þ co-doped glasses, Appl. Phys. A 115 (2014) 1159–1161. [3] H.Y. Chen, M.H. Weng, S.J. Chang, R.Y. Yang, Preparation of Sr2SiO4: Eu3 þ phosphors by microwave-assisted sintering and their luminescent properties, Ceram. Int. 38 (2012) 125–130. [4] J. Sun, J. Lai, J. Zhu, Z. Xia, H. Du, Luminescence properties and energy transfer investigations of Sr2B2O5:Ce3 þ , Tb3 þ phosphors, Ceram. Int. 38 (2012) 5341–5345. [5] X.J. Wang, D.D. Jia, W.M. Yen, Mn2 þ activated green, yellow, and red long persistent phosphor, J. Lumin. 102 (2003) 34–37. [6] Q.T. Zhang, L. Zhang, P.D. Han, Y. Chen, H. Yang, L.X. Wang, Light converting inorganic phosphors for white light-emitting diodes, Prog. Chem. 23 (2011) 1108–1122. [7] W.B. Im, N. George, J. Kurzman, S. Brinkley, A. Mikhailovsky, J. Hu, B.F. Chmelka, S.P. Denbaars, R. Seshadri, Efficient and color-tunable oxyfluoride solid solution phosphors for solid-state white lighting, Adv. Mater. 23 (2011) 2300–2305. [8] X.F. Wang, X.H. Yan, Y.Y. Bu, J. Zhen, Y. Xuan, Fabrication, photoluminescence, and potential application in white light emitting diode of Dy3 þ –Tm3 þ doped transparent glass ceramics containing GdSr2F7 nanocrystals, Appl. Phys. A 112 (2013) 317–322. [9] W.Q. Chen, D.S. Jo, Y.H. Song, T. Masaki, D.H. Yoon, Synthesis and photoluminescence properties of YAG:Ce3 þ phosphor using a liquidphase precursor method, J. Lumin. 147 (2014) 304–309. [10] S. Nakamura, G. Fasol, in: The Blue Laser Diode: GaN Based Light Emitters and Lasers, Springer, Berlin, 1997. [11] C. Ronda, Luminescence:From Theory to Applications, Weinheim, Germany, 2008. [12] M.J. Lee, Y.H. Song, Y.L. Song, G.S. Han, H.S. Jung, D.H. Yoon, Enhanced luminous efficiency of deep red emitting K2SiF6:Mn4 þ phosphor dependent on KF ratio for warm-white LED, Mater. Lett. 141 (2015) 27–30. [13] C.F. Guo, D.X. Huang, Q. Su, Methods to improve the fluorescence intensity of CaS:Eu2 þ red-emitting phosphor for white LED, Mater. Sci. Eng.: B 130 (2006) 189–193. [14] J. Merikhi, C. Feldmann, Homogeneous coatings of nanosized Fe2O3 particles on Y2O2S:Eu3 þ , J. Mater. Sci. 35 (2000) 3959–3961. [15] H.S. Jang, Y.H. Won, D.Y. Jeon, Improvement of electroluminescent property of blue LED coated with highly luminescent yellow-emitting phosphors, Appl. Phys. B 95 (2009) 715–720.
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Please cite this article as: M. You, et al., Eu3 þ :CdWO4 single-phased white-light-emitting phosphor: pH controlled luminescence properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.02.099
CERAMICS INTERNATIONAL
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Eu3 þ :CdWO4 single-phased white-light-emitting phosphor: pH controlled luminescence properties Mingjiang You, Jiayue Xun, Zhijie Zhangn, Yu Zhou Institute of Crystal Growth, School of Materials Science and Engineering, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai 201418, PR China Received 27 January 2015; received in revised form 17 February 2015; accepted 17 February 2015
Abstract Eu3 þ :CdWO4 single-phased phosphor for white-light-emitting diode (W-LED) was prepared by a hydrothermal method. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) as well as photoluminescence (PL) spectra. The main luminescence center changed between WO24 groups and Eu3 þ ions at different pH values, which led to different light-emitting properties of the samples. Under the excitation of 296 nm UV light, stable and independent white emission could be achieved when the pH value of the precursor was 7. The CIE chromaticity coordinates of the Eu3 þ :CdWO4 phosphor synthesized at pH ¼7 were x ¼ 0.323 and y ¼0.317, which were close to the values of standard chromaticity (x ¼ 0.33 and y ¼ 0.33) for an NTSC system. The CIE chromaticity calculation demonstrated its potential application in white light emission devices based on UV chips. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Single-phased; Eu3 þ :CdWO4 phosphor; W-LED; UV-excitation; Photoluminescence
1. Introduction For the superior advantages such as high brightness, high efficiency, long lifetime, substantial energy savings, etc [1–4], white light-emitting diodes (W-LEDs) are considered as a potential replacement for the traditional illumination devices and have attracted much attention [5–7]. At present, most of the commercial W-LED products are fabricated by combining Y3Al5O12:Ce3 þ (YAG:Ce3 þ ) [8–10] yellow-emitting phosphor with a blue LED (440–465 nm, GaN-based) chip. However, this kind of white light shows poor color rendering index (CRI) due to the color deficiency in the red light region and unsatisfactory high color temperature [11–15]. The commonly alternative way is to employ an ultraviolet (UV) LED chip as the excitation light source with red/green/blue (RGB) tri-color phosphors [16,17]. Nevertheless, these multi-phased phosphors have different decay times and reabsorption of emission colors, which may lead to a decreased luminous n
Corresponding authors. Tel.: þ86 21 6087 3581; fax: þ 86 21 6087 3439. E-mail addresses: [email protected] (J. Xu), [email protected] (Z. Zhang).
efficiency, and its application is restricted. Recently, singlephased white-emitting phosphor has attracted much interest [18], which includes singly doping Eu3 þ ion into an appropriate host lattice [19]. Due to the europium is more stable in an alkaline earth host and more easily diffuses into the lattice sites [20]. What is more with an abundant energy levels in a wide range of wavelengths [21], the Eu3 þ emissions which yield the blue, green and red emissions simultaneously, the combination of comparable multi-color emissions can produce white light. Metal tungstates have been widely studied as competitive host materials in the optical field due to their promising applications for phosphors, scintillators, lasers, catalysts and optical fibers [22–25] and their luminescence characteristics can be tuned by doping different active cations according to their ionic electronegativities [26]. Cadmium tungstate (CdWO4) with a monoclinic wolframite structure is an important family among metal tungstates that have potential applications in a variety of fields. For example, CdWO4 has already been used as an XCT detector material for its good scintillation properties [27,28]. Meanwhile, it is a popular functional material because of its low-radiation damage, high
http://dx.doi.org/10.1016/j.ceramint.2015.02.099 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: M. You, et al., Eu3 þ :CdWO4 single-phased white-light-emitting phosphor: pH controlled luminescence properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.02.099
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average refractive index and high X-ray absorption coefficient [29]. As a self-activating phosphor, CdWO4 exhibits a broad, intrinsic blue–green emission band under deep UV excitation. Considering the high chemical stability, short decay time and non-toxicity of tungstate, Eu3 þ -doped CdWO4 may be an ideal candidate to generate white light by combining the blue– green emission from tungstate group with the red emission from Eu3 þ . To the best of our knowledge, the luminescence property and mechanism of Eu3 þ -doped CdWO4 phosphor for white light emission under UV light excitation has seldom been reported before. In this article, single-phased Eu3 þ :CdWO4 white phosphor with excellent photoluminescence (PL) properties was synthesized by a hydrothermal method. The effect of the pH values of the precursors on the luminescence properties of the phosphors was investigated. When the pH value of the precursor was 7, white light emission with high color purity and high luminous intensity could be achieved under the excitation of 296 nm UV light. 2. Experimental 2.1. Synthesis Eu3 þ -doped CdWO4 white light phosphors were synthesized by the hydrothermal method. All chemical reagents used were analytic grade without further purification or other treatments. The preparation process was as follows: 1 mmol of Cd(Ac)2 2H2O was dissolved in 20 mL of deionized water, and appropriate amount of Eu2O3 was dissolved into HNO3 solution and was then added to the former solution. The mixed solution was stirred for 30 min to get solution A. Meanwhile, 1 mmol of Na2WO4 2H2O was dissolved in 20 mL of deionized water to obtain solution B. Then, solution B was added dropwise into solution A with magnetic stirring to obtain a white suspension. Afterwards, the pH value of the suspension was adjusted by the addition of HNO3 or
NH3 H2O. After being stirred for 1 h, the suspension was transferred into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was heated at 180 1C for 20 h under autogenous pressure, and then cooled to room temperature naturally. The resulting products were separated by filtration, washed with deionized water and absolute alcohol for several times, and then dried at 60 1C for 12 h. In this work, the atomic ratios of Eu3 þ to Cd2 þ were 0.5%, and the pH values of the precursors were 3, 5, 7, 9 and 11. 2.2. Characterization The composition and phase of the Eu3 þ :CdWO4 phosphors for W-LED were characterized by using powder X-ray diffraction (XRD) analysis on a Bruker D8 Focus diffractometer with Cu Kα radiation operated at 40 kV and 40 mA, respectively. The photoluminescence excitation (PLE) and photoluminescence (PL) spectra were analyzed using a FluoroMax-4 fluorescence spectrometer with a 450 W Xenon lamp as light source at room temperature (parameters of excitation and emission slit widths were set to be 1.5 nm). The morphology of the samples was characterized by a JEOL JEM-2100 transmission electron microscope (TEM) at an accelerating voltage of 200 kV. The colorimetry parameters were measured on a PMS-50 Plus UV–Vis-near IR spectrophotocolorimeter (Everfine, China). 3. Results and discussion 3.1. X-ray diffraction (XRD) analysis Fig. 1a showed the XRD patterns of the CdWO4 (pH ¼ 7) and Eu3 þ :CdWO4 samples prepared at different pH values (pH ¼ 3, 5, 7, 9 and 11). The patterns showed a monoclinic structure of CdWO4 with lattice parameters of α ¼ γ ¼ 901 and β¼ 91.681,which were close to the standard data of α ¼ γ ¼ 901 and β ¼ 91.471. All the diffraction peaks in the patterns could be indexed to cadmium tungstate phase (JCPDS no. 14-0676)
Fig. 1. (a) XRD patterns of CdWO4 (pH¼7) and CdWO4:0.5% Eu3 þ prepared at different pH values. (b) Diffraction peak positions of CdWO4:0.5% Eu3 þ in the range of 2θ¼ 28–301. Please cite this article as: M. You, et al., Eu3 þ :CdWO4 single-phased white-light-emitting phosphor: pH controlled luminescence properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.02.099
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and not any other diffraction peaks belonging to impurity phase were observed. This indicated that the products were pure phase and the small amount of Eu3 þ (0.5%) did not change the lattice structure. The ( 111) and (111) diffraction peak positions of Eu3 þ : CdWO4 prepared at different pH values were compared carefully with pure CdWO4 in the range of 2θ¼ 28–30.51, as shown in Fig. 1b. It could be seen that the diffraction peaks of Eu3 þ -doped CdWO4 shifted slightly to the lower angle compared to pure CdWO4, while the pH value did not have obvious influence on the peak positions. According to Bragg's law [30]: dðhklÞ ¼ λ=ð2 sin θÞ d(hkl): the distance between crystal planes of (hkl) λ: the X-ray wavelength θ: the diffraction angle of the crystal plane (hkl)
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The larger the lattice parameters, the smaller the 2θ value. Because the ionic radius of Eu3 þ (1.07 Å) is larger than that of Cd2 þ (0.97 Å), the substitution of Cd2 þ by Eu3 þ could lead to larger lattice parameters, and the diffraction peak shifted toward lower angles. 3.2. Morphology Fig. 2 showed the morphologies and microstructures of pure CdWO4 and Eu3 þ :CdWO4 samples which were tested by transmission electron microscopy (TEM). The TEM image in Fig. 2a showed that pure CdWO4 exhibited uniform nanorod structure. The 0.5% Eu3 þ :CdWO4 prepared at different pH values were also nanorod structure, which indicated that low concentration doping of Eu3 þ cannot change the morphology of CdWO4, particularly for its thermodynamic growth habit
Fig. 2. Typical TEM images of (a) pure CdWO4 and CdWO4:0.5% Eu3 þ synthesized at different pH values; (b) HRTEM images of pure CdWO4 and CdWO4:0.5% Eu3 þ prepared at pH¼ 7. Please cite this article as: M. You, et al., Eu3 þ :CdWO4 single-phased white-light-emitting phosphor: pH controlled luminescence properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.02.099
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[31]. While the size distribution of these nanorods were different with the change of the pH values due to their kinetic control effect [32–34]. When the pH value of the precursors was 5 or 7, the nanorods were uniform in size with ca. 20 nm in width and 100 nm in length. However, the width of the nanorods became larger to ca. 50 nm when the pH value of the precursors was 3 or 11. For the sample prepared at pH=9, besides the nanorods, some nanoparticles could also be observed. According to the TEM images, Eu3 þ :CdWO4 prepared at pH=5–7 might be a good candidate to form phosphor layer with higher packing density and lower surface scattering for manufacturing W-LEDs. The representative HRTEM images of pure CdWO4 and Eu3 þ :CdWO4 prepared at pH=7 were shown in Fig. 2b. The HRTEM images showed clear lattice fringes with interlayer spacing of d ¼ 0.301 nm and d ¼ 0.327 nm, which corresponded to the (111) plane of pure CdWO4 and Eu3 þ :CdWO4, respectively. This result was in accordance with that of the XRD, which indicated that doping Eu3 þ ions could enlarge the lattice of CdWO4. 3.3. Photoluminescence (PL) properties of Eu3 þ :CdWO4 phosphors The photoluminescence excitation (PLE) spectra of Eu3 þ : CdWO4 phosphors prepared at different pH values monitored at 614 nm were shown in Fig. 3a. It could be seen that pure CdWO4 exhibited only one broad PLE peak at 296 nm, which was due to the charge transfer between the empty d orbitals of the central W ion and the O 2p orbitals. For the Eu3 þ -doped CdWO4, however, not only the broad peak but also some sharp lines at wavelengths of 362, 378, 380, 395, 416 and 468 nm could be observed, which could be assigned to the 7F0–5D4, 7 F0–5GJ, 7F0–5L7, 7F0–5L6, 7F0–5D3 and 7F0–5D2 transitions of Eu3 þ , respectively [35]. Moreover, the PLE intensities at 296 nm and 464 nm were higher than the other peaks, which
indicated that the UV and blue LED chips were suitable to excite Eu3 þ :CdWO4 phosphors. Fig. 3b showed the PLE intensity at 296 nm and 464 nm with λem ¼ 614 nm as a function of pH values. As can be seen from Fig. 3b, UV light was the best light source for the samples prepared at pH ¼ 3, 5, 7 and 9. However, for the Eu3 þ :CdWO4 phosphor prepared at pH ¼ 11, the blue light became more suitable than UV light to excite the sample. This phenomenon could be explained by the Laporte selection rule [36]: The electric-dipole 7F0–5D2 transition of Eu3 þ with an approximate wavenumber of 21.5 103 cm 1 ( 464 nm) is a forbidden transition. However, the forbidden transition become partial allowed due to the spin–orbit coupling at the excited state as well as the noncentrosymmetric crystal field. On the other hand, the magnetic-dipole 7F0–5L6 transition is suppressed. Therefore, for Eu3 þ :CdWO4 prepared at pH ¼ 11, the J–J coupling approximation would be more suitable, while l–s coupling approximation is applied to Eu3 þ in CdWO4 prepared at other pH values. The strong excitation peak related to the 7F0–5D2 hypersensitive transition of Eu3 þ enables Eu3 þ :CdWO4 phosphor prepared at pH ¼ 11 match well with the output wavelength of commercial GaN-based blue LED to generate red emission. This result has been demonstrated in our previous research [37]. Fig. 4a showed the PL emission spectra of Eu3 þ :CdWO4 phosphors prepared at different pH values excited by 296 nm UV light. As can be seen from the spectra, there was only one broad peak centered at 468 nm for pure CdWO4 sample, which was due to the 1A1-3T1 transition within the WO66 complex. For the Eu3 þ :CdWO4 phosphors, besides the typical emission of WO24 , some sharp lines at 578, 587, 592, 614, 625, 654 and 702 nm (as shown in the inset of Fig. 4a) could also be observed, which could be assigned to 5D0–7FJ (J ¼ 0–4) transitions of Eu3 þ cation [38]. The peak at 614 nm (corresponding to 5D0–7F2 electric dipole transitions of Eu3 þ ) was the strongest among the emissions, which indicated that Eu3 þ
Fig. 3. (a) The photoluminescence excitation (PLE) spectra of CdWO4:0.5% Eu3 þ prepared at different pH values (3, 5, 7, 9, and 11) at room temperature monitored at 614 nm. (b) The PLE intensity at 296 nm and 464 nm with λem ¼ 614 nm as a function of pH values. Please cite this article as: M. You, et al., Eu3 þ :CdWO4 single-phased white-light-emitting phosphor: pH controlled luminescence properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.02.099
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Fig. 4. (a) The photoluminescence (PL) emission spectra of CdWO4:0.5% Eu3 þ prepared at different pH values at room temperature with λexc ¼296 nm (the inset shows the emission spectra scanning from 550 nm to 710 nm, the left top corner is the intensity of Eu3 þ emission peaks). (b) The comparison of PL intensity at 614 nm and 468 nm as a function of pH values by λexc ¼ 296 nm.
ions have occupied the non-symmetry site in the CdWO4 host lattice. The intensity of main emission peaks at 468 nm and 614 nm as a function of pH values were compared carefully in Fig. 4b. It could be seen that the intensity of WO66 emission at 468 nm decreased gradually with the increase of the pH value, while the intensity of Eu3 þ emission at 614 nm increased. This result indicated that there was energy transfer from WO24 groups to Eu3 þ and suggested effective doping of Eu3 þ into the CdWO4 lattice. Therefore, the emission center could be changed between WO24 groups and Eu3 þ ions by adjusting the pH values of the precursor. In other words, by properly tuning the pH values, white-light-emitting Eu3 þ :CdWO4 phosphors could be achieved. 3.4. CIE chromaticity coordinates of Eu3 þ :CdWO4 phosphor Fig. 5 showed the color coordinates of Eu3 þ :CdWO4 phosphors prepared at different pH values. As can be seen from the figure, the emission color of pure CdWO4 was blue (point 1). With the increase of the pH value, the Eu3 þ :CdWO4 phosphors exhibited different light-emitting properties from blue to orange region. Especially, when the pH value of the precursor was 7, white light emission could be obtained (point 4). It was worth mentioning that the CIE chromaticity coordinates of the Eu3 þ : CdWO4 phosphor synthesized at pH¼ 7 were x¼ 0.323 and y¼ 0.317, which were close to the values of standard chromaticity (x¼ 0.33 and y¼ 0.33) for the National Television Standards Committee (NTSC) system. This result demonstrated that Eu3 þ : CdWO4 prepared at neutral environment could be an excellent and independent white-light-emitting phosphor in illuminating and display devices excited by UV light.
Fig. 5. The 1931 CIE chromaticity diagram with points (1–6) indicating coordinates for the CdWO4 phosphors prepared at different pH values. Point 1: pure CdWO4 (pH¼7), point 2–6: CdWO4:0.5% Eu3 þ (pH ¼3, 5, 7, 9 and 11, respectively) excited by 296 nm UV light. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
4. Conclusions In this article, Eu3 þ :CdWO4 single-phased phosphors with different emission colors have been synthesized by a hydrothermal method. Tunable color from blue through white to orange could be obtained by adjusting the pH values of the precursors. Especially, when the pH value of the precursor was 7, stable and independent white emission with CIE chromaticity coordinates of
Please cite this article as: M. You, et al., Eu3 þ :CdWO4 single-phased white-light-emitting phosphor: pH controlled luminescence properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.02.099
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x¼ 0.323 and y¼ 0.317 could be achieved, which were very close to the values of standard chromaticity (x¼ 0.33 and y¼ 0.33) for the NTSC system. Our work demonstrated that Eu3 þ :CdWO4 could be an excellent white-light-emitting phosphor and have potential applications in illuminating and display devices based on UV chips. Acknowledgment This work was supported by the National Natural Science Foundation of China (51342007) and the Shanghai Science and Technology Committee (11JC1412400). References [1] Z.G. Cui, R.G. Ye, D.G. Deng, Y.J. Hua, S.L. Zhao, G.H. Jia, C.X. Li, S.Q. Xu, Eu2 þ /Sm3 þ ions co-doped white light luminescence SrSiO3 glass-ceramics phosphor for white LED, J. Alloy. Compd. 509 (2011) 3553–3558. [2] Q.Q. Chen, N.L. Dai, Z.J. Liu, Y.B. Chu, White light luminous properties and energy transfer mechanism of rare earth ions in Ce3 þ /Tb3 þ /Sm3 þ co-doped glasses, Appl. Phys. A 115 (2014) 1159–1161. [3] H.Y. Chen, M.H. Weng, S.J. Chang, R.Y. Yang, Preparation of Sr2SiO4: Eu3 þ phosphors by microwave-assisted sintering and their luminescent properties, Ceram. Int. 38 (2012) 125–130. [4] J. Sun, J. Lai, J. Zhu, Z. Xia, H. Du, Luminescence properties and energy transfer investigations of Sr2B2O5:Ce3 þ , Tb3 þ phosphors, Ceram. Int. 38 (2012) 5341–5345. [5] X.J. Wang, D.D. Jia, W.M. Yen, Mn2 þ activated green, yellow, and red long persistent phosphor, J. Lumin. 102 (2003) 34–37. [6] Q.T. Zhang, L. Zhang, P.D. Han, Y. Chen, H. Yang, L.X. Wang, Light converting inorganic phosphors for white light-emitting diodes, Prog. Chem. 23 (2011) 1108–1122. [7] W.B. Im, N. George, J. Kurzman, S. Brinkley, A. Mikhailovsky, J. Hu, B.F. Chmelka, S.P. Denbaars, R. Seshadri, Efficient and color-tunable oxyfluoride solid solution phosphors for solid-state white lighting, Adv. Mater. 23 (2011) 2300–2305. [8] X.F. Wang, X.H. Yan, Y.Y. Bu, J. Zhen, Y. Xuan, Fabrication, photoluminescence, and potential application in white light emitting diode of Dy3 þ –Tm3 þ doped transparent glass ceramics containing GdSr2F7 nanocrystals, Appl. Phys. A 112 (2013) 317–322. [9] W.Q. Chen, D.S. Jo, Y.H. Song, T. Masaki, D.H. Yoon, Synthesis and photoluminescence properties of YAG:Ce3 þ phosphor using a liquidphase precursor method, J. Lumin. 147 (2014) 304–309. [10] S. Nakamura, G. Fasol, in: The Blue Laser Diode: GaN Based Light Emitters and Lasers, Springer, Berlin, 1997. [11] C. Ronda, Luminescence:From Theory to Applications, Weinheim, Germany, 2008. [12] M.J. Lee, Y.H. Song, Y.L. Song, G.S. Han, H.S. Jung, D.H. Yoon, Enhanced luminous efficiency of deep red emitting K2SiF6:Mn4 þ phosphor dependent on KF ratio for warm-white LED, Mater. Lett. 141 (2015) 27–30. [13] C.F. Guo, D.X. Huang, Q. Su, Methods to improve the fluorescence intensity of CaS:Eu2 þ red-emitting phosphor for white LED, Mater. Sci. Eng.: B 130 (2006) 189–193. [14] J. Merikhi, C. Feldmann, Homogeneous coatings of nanosized Fe2O3 particles on Y2O2S:Eu3 þ , J. Mater. Sci. 35 (2000) 3959–3961. [15] H.S. Jang, Y.H. Won, D.Y. Jeon, Improvement of electroluminescent property of blue LED coated with highly luminescent yellow-emitting phosphors, Appl. Phys. B 95 (2009) 715–720.
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Please cite this article as: M. You, et al., Eu3 þ :CdWO4 single-phased white-light-emitting phosphor: pH controlled luminescence properties, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.02.099