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Preparation and luminescence properties of Eu2O3 doped glass-ceramics containing NaY(MoO4)2
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
Preparation and luminescence properties of Eu2O3 doped glass-ceramics containing NaY(MoO4)2 Tong Wang, Siying Wang, Yulin Wei, Xiangyu Zou*, Hongbo Zhang*, Liying Wang, Zhaohua Guo, Huimin Lv School of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun 130022, China
A R T I C LE I N FO
A B S T R A C T
Keywords: NaY(MoO4)2 Eu2O3 doped Glass-ceramics Luminescence properties
Eu2O3 doped transparent glass-ceramics containing NaY(MoO4)2 crystalline phase were prepared via meltingcrystallization. The optimum heat treatment condition (660℃/3h) was determined by DSC, XRD, SEM and transmittance curves. The transmittance of glass-ceramic can reach 80 % in the visible region. The emission spectra of Eu2O3 doped glass-ceramics consist of Eu3+ ions characteristic emission peaks at 591nm (5D0→7F1) and 614nm (5D0→7F2). The optimal doping concentration of Eu2O3 in the glass-ceramics is 0.9 mol%, and fluorescence lifetime is 1.37042ms. The change of the ratio of red emission intensity to orange emission intensity leads to the shift of chromaticity coordinates from orange to red region, and the chromaticity coordinate (0.6337, 0.3635) of 0.9 mol% Eu2O3 doped glass-ceramic is closest to the standard red light coordinate. The results show that this kind of glass-ceramic is expected to be good red emission material.
1. Introduction White light-emitting diodes (WLEDs) have received widespread attention due to their superior performance in terms of energy saving, long lifetime, low maintenance requirements, color creation and illumination [1]. There are usually three different ways to obtain white light based on LEDs: (i) by mixing red, green and blue (RGB) LEDs, (ii) by using ultraviolet (UV) LED to excite RGB phosphors, (iii) by using blue LED to pump yellow phosphor or green and red phosphors [2–4]. At present, commercial WLEDs have the problem of low color rendering index and high correlated color temperature due to lacking of red component [5–7]. To solve this problem, researchers have done a series of studies on phosphors, and rare earth doped glass-ceramics also provide a new solution. Eu3+ ions are considered as an important red emission activator. It can be excited by ultraviolet and blue light to emit red light, and is widely used in phosphors, LEDs, lasers, etc. [8,9]. The dimolybdate ARE (MoO4)2 (A = Li, Na, K; RE = Y, La, Gd, Lu) belongs to the scheelite structure, which is chemical stability and low phonon energy. Dimolybdate is deemed as the ideal luminescent matrix material due to its structure characteristics [10,11]. Lin Bai et al. successfully synthesized Eu3+ doped NaY(MoO4)2 phosphor by molten salt method. They studied the luminescent properties, fluorescence lifetime and optimum doping concentration of the phosphor [12]. Yuntong Li et al. prepared
⁎
NaY0.9(MoO4)2:0.1Eu3+ phosphors by microwave assisted hydrothermal method at low temperature. It was discussed that the crystallinity and photoluminescence of phosphors depend on reaction time and temperature. The luminescence properties of NaY0.9(MoO4)2:0.1Eu3+ phosphors were measured at different heating time and temperatures [13]. Park Sung Wook et al. prepared intense red-emitting NaY(MoO4)2:Eu3+ phosphors by hydrothermal method. The crystal structure and photoluminescence properties of molybdate were studied, and the important influence of pH on particle size and morphology distribution was discussed [14]. All the above studies indicate that NaY(MoO4)2:Eu3+ phosphor has potential application in the field of WLEDs. So far, NaY(MoO4)2:Eu3+ has been widely studied in phosphors, and there have been no report on Eu3+ doped glass-ceramics containing NaY(MoO4)2 crystalline phase. Compared with phosphor powder, glass-ceramics have the advantages of thermal stability, chemical stability, light stability and low coefficient of thermal expansion. In this work, we have been prepared Eu2O3 doped transparent glassceramics containing NaY(MoO4)2 crystalline phase for the first time. The optimum heat treatment condition and the optimum doping concentration of Eu2O3 were determined. The luminescence properties and structure of glass-ceramics were discussed in detail. The obtained information will be helpful for further studying the red emission materials.
Corresponding authors. E-mail address: [email protected] (X. Zou).
https://doi.org/10.1016/j.jeurceramsoc.2019.11.034 Received 16 September 2019; Accepted 11 November 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Tong Wang, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.11.034
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
T. Wang, et al.
2. Experimental
However, when the heat treatment temperature is increased to 650℃, 660℃ and 670℃, there are obvious diffraction peaks. Compared with the standard PDF card, it can be confirmed that the crystalline phase precipitated in the sample is NaY(MoO4)2. The positions of original peaks do not change with increasing temperature and no new peak appeared, it can be concluded that no new crystalline phase formed and pure NaY(MoO4)2 crystalline phase precipitated in the sample. By comparing the diffraction peak intensity of the sample at 650℃ and 660℃, it can be seen that the intensity of the main peak is similar, but the secondary peaks of 660℃ is stronger. Therefore, 660℃ is selected as the best heat treatment temperature for the precursor glass. The XRD patterns of glass-ceramics at different heat treatment time are shown in Fig. 2(b). It can be seen that the intensity of diffraction peaks increases gradually with the increase of heat treatment time, which indicates that more and more grains are precipitated and the size of grains is larger and larger. The positions of the diffraction peaks have not change, so the heat treatment time will not influence the crystalline phase. Fig. 3 shows the crystal structure of NaY(MoO4)2 for observing the coordination environment of Na/Y and Mo atoms, the unit cell parameters a = b = 0.51989 nm, c = 1.13299 nm. It can be observed that the unit of NaY(MoO4)2 contains isolated MoO4 tetrahedra and (Na/Y) O8 dodecahedron. In the structure centered on a molybdenum atom, the distance between molybdenum and four oxygen atoms around which is 0.1739 nm. In the structure centered on sodium or yttrium atmos, four oxygen atoms are 0.2442 nm away from the central atom and the rest are 0.2500 nm. The effective radii of Na+ and Y3+ in the (Na/Y)O8 are 0.118 nm and 0.1019 nm, respectively [15]. Considering that the Eu3+ radius is 0.095 nm, which is similar to the Y3+ radius and the valence is same, there is a possibility that Eu3+ replaces the Y3+ site in the lattice. In order to further study the relationship between heat treatment condition and grain size, the micro-morphology of glass-ceramics was observed by scanning electron microscopy (SEM). The SEM pictures are shown in Fig. 4. It can be seen that the grain size is small and the quantity is few after heat treatment at 660 C for 2 h. When the heat treatment time is 3 h, the quantity of grains increase obviously and the distribution is uniform. At this time, the grain size is about 384 nm. When the time is extended to 4 h, the adjacent grain edges begin to contact and appear agglomeration phenomenon. The longer the holding time, the more serious this agglomeration phenomenon is. Agglomeration can cause serious light loss and decrease the transmittance of the sample. Fig. 5 shows the transmittance curves of precursor glass and glassceramic. It can be clearly observed that the transmittance of glassceramic is weaker than precursor glass, because glass-ceramic has grains precipitation. When incident light through glass-ceramic, refraction and reflection occur on the surface of grains, and multiple scattering and absorption occur in the grains interior. In addition, the increase of grain size and boundary result in the uneven distribution of grains in the residual glass phase. All these factors will cause light loss and transmittance reduction. However, the transmittance of glassceramic can still reach 80 % in the visible range, this is because average grain size is smaller than the incident wavelength. The absorption peaks at 394 nm, 465 nm and 540 nm are attributed to 7F0→5L6, 7F0→5D2 and 7 F0→5D1 transition of Eu3+ ions. Considering XRD, DSC and SEM, the optimum heat treatment condition for the precursor glass is 660℃ for 3 h.
2.1. Materials and synthesis A series of transparent glass-ceramics with molar percentage of 8.7Na2CO3-4.4Y2O3-4.2MoO3-38.4SiO2-30.2H3BO3-13.8NaF-0.3Sb2O3xEu2O3 (x = 0.1, 0.3, 0.5, 0.7, 0.9, 1.1 mol%) were prepared by melt crystallization. The purity of Eu2O3 and Y2O3 is 5 N, and the other raw materials are chemical pure. The raw materials were weighed 20 g according to the proportion and transferred to a mortar for full grinding. The mixture was placed in heating furnace, which temperature was increased to 1100℃ at the rate of 5℃/min, and then to 1470℃ at the rate of 3℃/min for 1.5 h. The melt was poured onto the preheated copper plate and then pressed by another plate to get glass samples. Finally, the glass samples were annealed in 450℃ muffle furnace for 1 h. The precursor glass naturally lowered to room temperature in muffle furnace was labeled as PG, and the glass-ceramic sample obtained after heat treatment of precursor glass was labeled as GC. 2.2. Characterizations Differential scanning calorimetry (DSC) of precursor glass was carried out by STA-449F3 synchronous thermal analyzer at the temperature range of 200℃～900℃ with the heating rate of 10℃/min, and Al2O3 was used as reference material. X-ray diffraction data of glassceramics was performed using 2500PC X-ray diffractometer with CuKα1 radiation over the angular range 10° ≤ 2θ ≤ 80°, the operating voltage was 30 kV, current was 20 mA. The micro-morphology of glassceramics was observed by JSM-7610 F scanning electron microscope (SEM) and the working voltage was 10 kV. The transmittance curve was measured by UVmini-1240 ultraviolet-visible spectrophotometer. The photoluminescence (PL) excitation and emission spectra of glass-ceramics were observed on Ex OPO fluorescence spectrometer, with the measurement range from 200 to 800 nm. 3. Results and discussion 3.1. Determination of heat treatment condition Fig. 1 shows the DSC curve of 0.1 mol% Eu2O3 doped precursor glass. It can be seen from the curve that initial crystallization temperature (TX) is around 645℃, and the crystallization peak locates at 665℃(TC). Thus, the crystallization temperature range is 640℃～680℃. The precursor glass samples were heat-treated for 2 h at 640℃, 650℃, 660℃ and 670℃ to obtain glass-ceramics, respectively. Fig. 2 shows the XRD patterns of 0.1 mol% Eu2O3 doped glassceramics under different heat treatment conditions. As can be seen from Fig. 2(a), there is no obvious diffraction peak under 640℃ for 2 h.
3.2. Fluorescence spectroscopy Fig. 6 shows the excitation spectrum of 0.1 mol% Eu2O3 doped glass-ceramic under the monitoring wavelength at 614 nm. The excitation spectrum consists of a broad excitation band in the range of 225−350 nm and several sharp excitation peaks in the range of 350−475 nm. The broad band can be attributed to two aspects:(i) the 1 A1 − 3T2 transition of MoO42− group involving the charge transfer state (CTS) from O2- to Mo6+ [11], (ii)the charge transfer transition
Fig. 1. DSC curve of 0.1 mol% Eu2O3 doped PG. 2
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Fig. 2. XRD patterns of 0.1 mol% Eu2O3 doped GCs under different heat treatment conditions. (a) Different heat treatment temperature (b) Different holding time under 660℃.
Fig. 3. Crystal structure of NaY(MoO4)2.
between Eu3+ and O2- [16]. The other characteristic sharp peaks located at 363 nm, 382 nm, 394 nm, 415 nm and 465 nm can attribute from ground state 7F0 to the excited states 5D4, 5L7, 5L6, 5D3 and 5D2 of Eu3+ ions, respectively. Fig. 7 shows the emission spectra of xEu2O3 (x = 0.1, 0.3, 0.5, 0.7, 0.9, 1.1 mol%) doped glass-ceramics. It is clearly seen that there are two emission peaks at 591 nm and 614 nm, which are matching with the 5D0→7F1 magnetic-dipole transition and the 5D0→7F2 electric-dipole transition of Eu3+ ions, respectively. According to the general rule of Eu3+ electron transition, when 5D0→7F1 magnetic-dipole transition is dominant, it mainly emits orange light. At this time, Eu3 + ions have strict inversion center in the lattice. When 5D0→7F2 electric-dipole transition is dominant, the red emission is stronger than orange emission, and Eu3+ ions have not the inversion center [17]. As can be seen from the spectra, the red emission at 614 nm is always stronger than the orange emission at 591 nm, which indicates that Eu3+ ions are mainly in the lattice without the inversion center. With the increase of Eu2O3 doping concentration, the intensity of the emission peak at 614 nm is gradually enhanced. When Eu2O3 doping concentration up to 0.9 mol%, the peak intensity is the strongest and the peak intensity decreases as the concentration further increasing. The reason for this phenomenon is that the distance between Eu3+ ions will begin to shorten when the concentration exceeds 0.9 mol %, resulting in concentration quenching. In order to study the concentration quenching mechanism, it is necessary to calculate the critical distance between Eu3+ ions. The critical distance of Eu3+ ions can be
roughly estimated according to Formula (1) [18]. 1/3 3V ⎤ Rc = 2 ⎡ ⎣ 4πXcN ⎦
(1)
Where XC is the critical concentration, N is the quantity of cationic sites per unit cell, V is the unit cell volume. In this paper, N = 2; V = 0.30623 nm3; XC = 0.018, the value of RC is calculated as 2.53 nm. Generally, there are two common mechanisms for non-radiative energy transfer between ions viz. exchange interaction and electric multipolar interaction. For exchange interaction model, the critical distance between ions should be shorter than 0.5 nm. Therefore, electro multipolar interaction is the main reason for concentration quenching. According to Fig. 7(a) and (b), it is found that the red emission is stronger when the excitation wavelength is 394 nm, so 394 nm is chosen as the optimum excitation wavelength. The decay curves of xEu2O3 doped GCs excited at 394 nm and monitored at 614 nm are measured and shown in Fig.8. The decay curves are well fitted with a single exponential function by the following (2) [6].
t I(t) = I0 exp ⎛− ⎞ ⎝ τ⎠
(2)
Using this formula, the average fluorescence lifetime (τ) of GCs is calculated and shown in the inset. From the inset, the fluorescence lifetime increased from 1.17024 ms to 1.37042 ms and then decreased to 1.34272 ms, which further proves the existence of concentration 3
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Fig. 4. SEM pictures of GCs at 660℃ for different time.
Fig. 5. Transmittance curves of PG and GC.
Fig. 7. Emission spectra of xEu2O3 (x = 0.1, 0.3, 0.5, 0.7, 0.9, 1.1 mol%) doped GCs. (a: λex = 394 nm, b: λex = 465 nm). Fig. 6. Excitation spectrum of 0.1 mol% Eu2O3 doped GC.
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Fig. 10. Chromaticity coordinates of xEu2O3 (x = 0.1, 0.3, 0.5, 0.7, 0.9, 1.1 mol%) doped GCs. Fig. 8. The decay curves of xEu2O3(x = 0.1,0.3,0.5,0.7,0.9,1.1 mol%) doped GCs.
Table 1 Chromaticity coordinate data of GCs. No
Eu2O3/mol%
λex /nm
CIEx
CIEy
1 2 3 4 5 6
0.1 0.3 0.5 0.7 0.9 1.1
394 394 394 394 394 394
0.5937 0.5993 0.6048 0.6176 0.6337 0.6253
0.398 0.3938 0.3889 0.3781 0.3635 0.3716
Fig. 9. Comparison of luminescence intensity between PG and GCs at 614 nm.
quenching. The luminescence intensity comparison of precursor glass and glassceramics at 614 nm is shown in Fig. 9. The luminescence intensity of glass-ceramics is stronger than the corresponding precursor glass. This is because Eu3 + ions are in the silica glass matrix before heat treatment. After heat treatment, with the precipitation of NaY (MoO4)2 crystalline phase, some Eu3+ ions replace the lattices of Y3+ and enter crystalline phase, and others are in the environment containing molybdate. The change of chemical environment decreases the maximum phonon energy around Eu3 + ions and increases the radiation transition efficiency and luminescence intensity.
Fig. 11. Ratios of red emission intensity to orange emission intensity.
the relative intensity of each emitted light. The ratio change line of the luminescence intensity at 614 nm and 591 nm is shown in Fig. 11. This line shows an upward trend with the increase of Eu2O3 doping concentration, and reaches the highest point when the doping concentration is 0.9 mol%. At this time, the red emission has the largest proportion and the chromaticity coordinate is closest to the standard red region.
3.3. Chromaticity coordinate analysis The Chromaticity coordinates of glass-ceramics doping xEu2O3 (x = 0.1, 0.3, 0.5, 0.7, 0.9, 1.1 mol%) are depicted in Fig. 10 and corresponding chromaticity coordinate data is listed in Table 1. It can be seen from the figure that with the increasing of Eu2O3 doping concentration the chromaticity coordinates shift from the orange region to the red region. When the doping concentration is 0.9 mol%, the coordinate (0.6337, 0.3635) is closest to the standard red coordinate. The shift of the chromaticity coordinates may be caused by the change of
4. Conclusions Eu2O3 doped transparent glass-ceramics containing NaY(MoO4)2 crystalline phase were prepared successfully for the first time. The optimum heat treatment condition of precursor glass is 660℃ for 3 h. Under this condition, the grain size in glass-ceramics is about 384 nm, and the transmittance reached 80 % in the visible range. The emission 5
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peaks at 591 and 614 nm belong to the 5D0 →7F1 magnetic-dipole transition and 5D0→7F2 electric-dipole transition of Eu3 + ions, in which the electric-dipole transition dominates and shows intense red emission. The optimum doping concentration of Eu2O3 in the glassceramic is 0.9 mol%. At this time, the critical distance between Eu3+ ions is about 2.53 nm and fluorescence lifetime is 1.37042 ms. Continue to increase the concentration of Eu2O3, the electro-multipolar interaction will occur resulting in concentration quenching, which will reduce the luminescence intensity. Under the excitation of 394 nm, the chromaticity coordinates shift from the orange region to the red region with increasing the concentration of Eu2O3. When the doping concentration is 0.9 mol%, the chromaticity coordinate is closest to the standard red coordinate. Results show that Eu2O3 doped transparent glass-ceramics containing NaY(MoO4)2 crystalline phase are good red emission materials, which have potential application in WLEDs.
Dy3+ phosphor, J. Lumin. 206 (2019) 380–385. [5] C.C. Lin, A. Meijerink, R.S. Liu, Critical red components for next-generation white LEDs, J. Phys. Chem. Lett. 7 (2016) 495–503. [6] Y.H. Xia, X.Y. Zou, H.B. Zhang, M.J. Zhao, S. Meng, C.H. Su, J. Shao, Luminescence properties of Eu3+ doped BaMoO4 transparent glass ceramics, J. Non-Crytall. Solids 500 (2018) 243–248. [7] C.Y. Xu, H.X. Guan, Y.H. Song, Z.C. An, X.T. Zhang, X.Q. Zhou, Z. Shi, Y. Sheng, H.F. Zou, Novel highly efficient single-component multi-peak emitting aluminosilicate phosphors co-activated with Ce3+, Tb3+ and Eu2+: luminescence properties, tunable color, and thermal properties, Phys. Chem. Chem. Phys. 20 (2018) 1591–1607. [8] J. Wang, Z.J. Zhang, J.T. Zhao, Luminescence properties of Eu3+-doped new scheelite-type compounds, J. Rare Earths 33 (2015) 1241–1245. [9] R.K. Tamrakar, K. Upadhyay, I.P. Sahu, D.P. Bisen, Tuning of photoluminescence emission properties of Eu3+ doped Gd2O3 by different excitations, Optik 135 (2017) 281–289. [10] Z.L. Wang, H.B. Liang, M.L. Gong, Q. Su, Novel red phosphor of Bi3+, Sm3+ coactivated NaEu(MoO4)2, Opt. Mater. 29 (2007) 896–900. [11] P.K. Tawalarea, V.B. Bhatkarb, R.A. Talewarcd, C.P. Joshid, S.V. Moharile, Host sensitized NIR emission in rare-earth doped NaY(MoO4)2 phosphors, J. Alloys. Compd. 732 (2018) 64–69. [12] L. Bai, Q.Y. Meng, M.X. Huo, Y. Sui, Optical properties of NaY(MoO4)2:Eu3+ nanophosphors prepared bymolten salt method, J. Rare (2019). [13] Y.T. Li, X.H. Liu. Photoluminescence properties of NaY(MoO4)2:Eu3+ phosphor synthesized by microwave assisted hydrothermal method, Mater. Sci. Eng. B 188 (2014) 20–25. [14] S.W. Park, H.M. Noh, B.K. Moon, B.C. Choi, J.H. Jeong, H.K. Yang, Hydrothermal synthesis and photoluminescence investigation of NaY(MoO4)2:Eu3+ nanophosphor, J. Nanosci. Nanotechnol. 14 (2014) 8724–8728. [15] R.A. Talewarab, V.M. Gaikwadc, P.K. Tawalared, S.V. Moharild, Sensitization of Er3+/Ho3+ visible and NIR emission in NaY(MoO4)2 phosphors, Opt. Laser Technol. 115 (2019) 215–221. [16] H.Y. Chen, M.H. Weng, S.J. Chang, R. Yang, Preparation of Sr2SiO4:Eu3+ phosphors by microwave-assisted sintering and their luminescent properties, Ceram. Int. 38 (2012) 125–130. [17] L.H. Tian, S.J. Kim, H.L. Park, S. Mho, Variation of the photoluminescence and vacuum ultraviolet excitation characteristics of BaZr(BO3)2:Eu3+ by the incorporation of Al3+, La3+, or Y3+ into the lattice, Mater. Res. Bull. 41 (2005) 29–37. [18] G. Blasse, Energy transfer in oxidic phosphors, Phys. Lett. A 28 (1968) 444–445.
Acknowledgment This work was supported by the Project of Jilin Province Science and Technology Department (20190802014ZG). References [1] Y.K. Choi, P. Halappa, C. Shivakumara, V. Dubey, V. Singh, Blue emitting Ce3+doped CaYAl3O7 phosphors prepared by combustion route, Optik 181 (2019) 1113–1121. [2] Z.G. Xia, Q.L. Liu, Progress in discovery and structural design of color conversion phosphors for LEDs, Prog. Mater. Sci. 84 (2016) 59–117. [3] H.M. Zhang, H.R. Zhang, W.R. Liu, Y.L. Liu, B.F. Lei, J.K. Deng, J.Y. Zhang, S.Y. Yan, H.B. Kuang, J.D. Zang, Photoluminescence properties and energy transfer between activators at different crystallographic sites in Ce3+ doped Sr2MgAl22O36, Ceram. Int. 42 (2016) 16659–16665. [4] K. Dev, A. Selot, G.B. Nair, V.L. Barai, F.Z. Haque, M. Aynyas, S.J. Dhoble, Energy transfer from Ce3+to Dy3+ ions for white light emission in Sr2MgAl22O36:Ce3+,
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Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
Preparation and luminescence properties of Eu2O3 doped glass-ceramics containing NaY(MoO4)2 Tong Wang, Siying Wang, Yulin Wei, Xiangyu Zou*, Hongbo Zhang*, Liying Wang, Zhaohua Guo, Huimin Lv School of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun 130022, China
A R T I C LE I N FO
A B S T R A C T
Keywords: NaY(MoO4)2 Eu2O3 doped Glass-ceramics Luminescence properties
Eu2O3 doped transparent glass-ceramics containing NaY(MoO4)2 crystalline phase were prepared via meltingcrystallization. The optimum heat treatment condition (660℃/3h) was determined by DSC, XRD, SEM and transmittance curves. The transmittance of glass-ceramic can reach 80 % in the visible region. The emission spectra of Eu2O3 doped glass-ceramics consist of Eu3+ ions characteristic emission peaks at 591nm (5D0→7F1) and 614nm (5D0→7F2). The optimal doping concentration of Eu2O3 in the glass-ceramics is 0.9 mol%, and fluorescence lifetime is 1.37042ms. The change of the ratio of red emission intensity to orange emission intensity leads to the shift of chromaticity coordinates from orange to red region, and the chromaticity coordinate (0.6337, 0.3635) of 0.9 mol% Eu2O3 doped glass-ceramic is closest to the standard red light coordinate. The results show that this kind of glass-ceramic is expected to be good red emission material.
1. Introduction White light-emitting diodes (WLEDs) have received widespread attention due to their superior performance in terms of energy saving, long lifetime, low maintenance requirements, color creation and illumination [1]. There are usually three different ways to obtain white light based on LEDs: (i) by mixing red, green and blue (RGB) LEDs, (ii) by using ultraviolet (UV) LED to excite RGB phosphors, (iii) by using blue LED to pump yellow phosphor or green and red phosphors [2–4]. At present, commercial WLEDs have the problem of low color rendering index and high correlated color temperature due to lacking of red component [5–7]. To solve this problem, researchers have done a series of studies on phosphors, and rare earth doped glass-ceramics also provide a new solution. Eu3+ ions are considered as an important red emission activator. It can be excited by ultraviolet and blue light to emit red light, and is widely used in phosphors, LEDs, lasers, etc. [8,9]. The dimolybdate ARE (MoO4)2 (A = Li, Na, K; RE = Y, La, Gd, Lu) belongs to the scheelite structure, which is chemical stability and low phonon energy. Dimolybdate is deemed as the ideal luminescent matrix material due to its structure characteristics [10,11]. Lin Bai et al. successfully synthesized Eu3+ doped NaY(MoO4)2 phosphor by molten salt method. They studied the luminescent properties, fluorescence lifetime and optimum doping concentration of the phosphor [12]. Yuntong Li et al. prepared
⁎
NaY0.9(MoO4)2:0.1Eu3+ phosphors by microwave assisted hydrothermal method at low temperature. It was discussed that the crystallinity and photoluminescence of phosphors depend on reaction time and temperature. The luminescence properties of NaY0.9(MoO4)2:0.1Eu3+ phosphors were measured at different heating time and temperatures [13]. Park Sung Wook et al. prepared intense red-emitting NaY(MoO4)2:Eu3+ phosphors by hydrothermal method. The crystal structure and photoluminescence properties of molybdate were studied, and the important influence of pH on particle size and morphology distribution was discussed [14]. All the above studies indicate that NaY(MoO4)2:Eu3+ phosphor has potential application in the field of WLEDs. So far, NaY(MoO4)2:Eu3+ has been widely studied in phosphors, and there have been no report on Eu3+ doped glass-ceramics containing NaY(MoO4)2 crystalline phase. Compared with phosphor powder, glass-ceramics have the advantages of thermal stability, chemical stability, light stability and low coefficient of thermal expansion. In this work, we have been prepared Eu2O3 doped transparent glassceramics containing NaY(MoO4)2 crystalline phase for the first time. The optimum heat treatment condition and the optimum doping concentration of Eu2O3 were determined. The luminescence properties and structure of glass-ceramics were discussed in detail. The obtained information will be helpful for further studying the red emission materials.
Corresponding authors. E-mail address: [email protected] (X. Zou).
https://doi.org/10.1016/j.jeurceramsoc.2019.11.034 Received 16 September 2019; Accepted 11 November 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Tong Wang, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.11.034
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
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2. Experimental
However, when the heat treatment temperature is increased to 650℃, 660℃ and 670℃, there are obvious diffraction peaks. Compared with the standard PDF card, it can be confirmed that the crystalline phase precipitated in the sample is NaY(MoO4)2. The positions of original peaks do not change with increasing temperature and no new peak appeared, it can be concluded that no new crystalline phase formed and pure NaY(MoO4)2 crystalline phase precipitated in the sample. By comparing the diffraction peak intensity of the sample at 650℃ and 660℃, it can be seen that the intensity of the main peak is similar, but the secondary peaks of 660℃ is stronger. Therefore, 660℃ is selected as the best heat treatment temperature for the precursor glass. The XRD patterns of glass-ceramics at different heat treatment time are shown in Fig. 2(b). It can be seen that the intensity of diffraction peaks increases gradually with the increase of heat treatment time, which indicates that more and more grains are precipitated and the size of grains is larger and larger. The positions of the diffraction peaks have not change, so the heat treatment time will not influence the crystalline phase. Fig. 3 shows the crystal structure of NaY(MoO4)2 for observing the coordination environment of Na/Y and Mo atoms, the unit cell parameters a = b = 0.51989 nm, c = 1.13299 nm. It can be observed that the unit of NaY(MoO4)2 contains isolated MoO4 tetrahedra and (Na/Y) O8 dodecahedron. In the structure centered on a molybdenum atom, the distance between molybdenum and four oxygen atoms around which is 0.1739 nm. In the structure centered on sodium or yttrium atmos, four oxygen atoms are 0.2442 nm away from the central atom and the rest are 0.2500 nm. The effective radii of Na+ and Y3+ in the (Na/Y)O8 are 0.118 nm and 0.1019 nm, respectively [15]. Considering that the Eu3+ radius is 0.095 nm, which is similar to the Y3+ radius and the valence is same, there is a possibility that Eu3+ replaces the Y3+ site in the lattice. In order to further study the relationship between heat treatment condition and grain size, the micro-morphology of glass-ceramics was observed by scanning electron microscopy (SEM). The SEM pictures are shown in Fig. 4. It can be seen that the grain size is small and the quantity is few after heat treatment at 660 C for 2 h. When the heat treatment time is 3 h, the quantity of grains increase obviously and the distribution is uniform. At this time, the grain size is about 384 nm. When the time is extended to 4 h, the adjacent grain edges begin to contact and appear agglomeration phenomenon. The longer the holding time, the more serious this agglomeration phenomenon is. Agglomeration can cause serious light loss and decrease the transmittance of the sample. Fig. 5 shows the transmittance curves of precursor glass and glassceramic. It can be clearly observed that the transmittance of glassceramic is weaker than precursor glass, because glass-ceramic has grains precipitation. When incident light through glass-ceramic, refraction and reflection occur on the surface of grains, and multiple scattering and absorption occur in the grains interior. In addition, the increase of grain size and boundary result in the uneven distribution of grains in the residual glass phase. All these factors will cause light loss and transmittance reduction. However, the transmittance of glassceramic can still reach 80 % in the visible range, this is because average grain size is smaller than the incident wavelength. The absorption peaks at 394 nm, 465 nm and 540 nm are attributed to 7F0→5L6, 7F0→5D2 and 7 F0→5D1 transition of Eu3+ ions. Considering XRD, DSC and SEM, the optimum heat treatment condition for the precursor glass is 660℃ for 3 h.
2.1. Materials and synthesis A series of transparent glass-ceramics with molar percentage of 8.7Na2CO3-4.4Y2O3-4.2MoO3-38.4SiO2-30.2H3BO3-13.8NaF-0.3Sb2O3xEu2O3 (x = 0.1, 0.3, 0.5, 0.7, 0.9, 1.1 mol%) were prepared by melt crystallization. The purity of Eu2O3 and Y2O3 is 5 N, and the other raw materials are chemical pure. The raw materials were weighed 20 g according to the proportion and transferred to a mortar for full grinding. The mixture was placed in heating furnace, which temperature was increased to 1100℃ at the rate of 5℃/min, and then to 1470℃ at the rate of 3℃/min for 1.5 h. The melt was poured onto the preheated copper plate and then pressed by another plate to get glass samples. Finally, the glass samples were annealed in 450℃ muffle furnace for 1 h. The precursor glass naturally lowered to room temperature in muffle furnace was labeled as PG, and the glass-ceramic sample obtained after heat treatment of precursor glass was labeled as GC. 2.2. Characterizations Differential scanning calorimetry (DSC) of precursor glass was carried out by STA-449F3 synchronous thermal analyzer at the temperature range of 200℃～900℃ with the heating rate of 10℃/min, and Al2O3 was used as reference material. X-ray diffraction data of glassceramics was performed using 2500PC X-ray diffractometer with CuKα1 radiation over the angular range 10° ≤ 2θ ≤ 80°, the operating voltage was 30 kV, current was 20 mA. The micro-morphology of glassceramics was observed by JSM-7610 F scanning electron microscope (SEM) and the working voltage was 10 kV. The transmittance curve was measured by UVmini-1240 ultraviolet-visible spectrophotometer. The photoluminescence (PL) excitation and emission spectra of glass-ceramics were observed on Ex OPO fluorescence spectrometer, with the measurement range from 200 to 800 nm. 3. Results and discussion 3.1. Determination of heat treatment condition Fig. 1 shows the DSC curve of 0.1 mol% Eu2O3 doped precursor glass. It can be seen from the curve that initial crystallization temperature (TX) is around 645℃, and the crystallization peak locates at 665℃(TC). Thus, the crystallization temperature range is 640℃～680℃. The precursor glass samples were heat-treated for 2 h at 640℃, 650℃, 660℃ and 670℃ to obtain glass-ceramics, respectively. Fig. 2 shows the XRD patterns of 0.1 mol% Eu2O3 doped glassceramics under different heat treatment conditions. As can be seen from Fig. 2(a), there is no obvious diffraction peak under 640℃ for 2 h.
3.2. Fluorescence spectroscopy Fig. 6 shows the excitation spectrum of 0.1 mol% Eu2O3 doped glass-ceramic under the monitoring wavelength at 614 nm. The excitation spectrum consists of a broad excitation band in the range of 225−350 nm and several sharp excitation peaks in the range of 350−475 nm. The broad band can be attributed to two aspects:(i) the 1 A1 − 3T2 transition of MoO42− group involving the charge transfer state (CTS) from O2- to Mo6+ [11], (ii)the charge transfer transition
Fig. 1. DSC curve of 0.1 mol% Eu2O3 doped PG. 2
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Fig. 2. XRD patterns of 0.1 mol% Eu2O3 doped GCs under different heat treatment conditions. (a) Different heat treatment temperature (b) Different holding time under 660℃.
Fig. 3. Crystal structure of NaY(MoO4)2.
between Eu3+ and O2- [16]. The other characteristic sharp peaks located at 363 nm, 382 nm, 394 nm, 415 nm and 465 nm can attribute from ground state 7F0 to the excited states 5D4, 5L7, 5L6, 5D3 and 5D2 of Eu3+ ions, respectively. Fig. 7 shows the emission spectra of xEu2O3 (x = 0.1, 0.3, 0.5, 0.7, 0.9, 1.1 mol%) doped glass-ceramics. It is clearly seen that there are two emission peaks at 591 nm and 614 nm, which are matching with the 5D0→7F1 magnetic-dipole transition and the 5D0→7F2 electric-dipole transition of Eu3+ ions, respectively. According to the general rule of Eu3+ electron transition, when 5D0→7F1 magnetic-dipole transition is dominant, it mainly emits orange light. At this time, Eu3 + ions have strict inversion center in the lattice. When 5D0→7F2 electric-dipole transition is dominant, the red emission is stronger than orange emission, and Eu3+ ions have not the inversion center [17]. As can be seen from the spectra, the red emission at 614 nm is always stronger than the orange emission at 591 nm, which indicates that Eu3+ ions are mainly in the lattice without the inversion center. With the increase of Eu2O3 doping concentration, the intensity of the emission peak at 614 nm is gradually enhanced. When Eu2O3 doping concentration up to 0.9 mol%, the peak intensity is the strongest and the peak intensity decreases as the concentration further increasing. The reason for this phenomenon is that the distance between Eu3+ ions will begin to shorten when the concentration exceeds 0.9 mol %, resulting in concentration quenching. In order to study the concentration quenching mechanism, it is necessary to calculate the critical distance between Eu3+ ions. The critical distance of Eu3+ ions can be
roughly estimated according to Formula (1) [18]. 1/3 3V ⎤ Rc = 2 ⎡ ⎣ 4πXcN ⎦
(1)
Where XC is the critical concentration, N is the quantity of cationic sites per unit cell, V is the unit cell volume. In this paper, N = 2; V = 0.30623 nm3; XC = 0.018, the value of RC is calculated as 2.53 nm. Generally, there are two common mechanisms for non-radiative energy transfer between ions viz. exchange interaction and electric multipolar interaction. For exchange interaction model, the critical distance between ions should be shorter than 0.5 nm. Therefore, electro multipolar interaction is the main reason for concentration quenching. According to Fig. 7(a) and (b), it is found that the red emission is stronger when the excitation wavelength is 394 nm, so 394 nm is chosen as the optimum excitation wavelength. The decay curves of xEu2O3 doped GCs excited at 394 nm and monitored at 614 nm are measured and shown in Fig.8. The decay curves are well fitted with a single exponential function by the following (2) [6].
t I(t) = I0 exp ⎛− ⎞ ⎝ τ⎠
(2)
Using this formula, the average fluorescence lifetime (τ) of GCs is calculated and shown in the inset. From the inset, the fluorescence lifetime increased from 1.17024 ms to 1.37042 ms and then decreased to 1.34272 ms, which further proves the existence of concentration 3
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Fig. 4. SEM pictures of GCs at 660℃ for different time.
Fig. 5. Transmittance curves of PG and GC.
Fig. 7. Emission spectra of xEu2O3 (x = 0.1, 0.3, 0.5, 0.7, 0.9, 1.1 mol%) doped GCs. (a: λex = 394 nm, b: λex = 465 nm). Fig. 6. Excitation spectrum of 0.1 mol% Eu2O3 doped GC.
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Fig. 10. Chromaticity coordinates of xEu2O3 (x = 0.1, 0.3, 0.5, 0.7, 0.9, 1.1 mol%) doped GCs. Fig. 8. The decay curves of xEu2O3(x = 0.1,0.3,0.5,0.7,0.9,1.1 mol%) doped GCs.
Table 1 Chromaticity coordinate data of GCs. No
Eu2O3/mol%
λex /nm
CIEx
CIEy
1 2 3 4 5 6
0.1 0.3 0.5 0.7 0.9 1.1
394 394 394 394 394 394
0.5937 0.5993 0.6048 0.6176 0.6337 0.6253
0.398 0.3938 0.3889 0.3781 0.3635 0.3716
Fig. 9. Comparison of luminescence intensity between PG and GCs at 614 nm.
quenching. The luminescence intensity comparison of precursor glass and glassceramics at 614 nm is shown in Fig. 9. The luminescence intensity of glass-ceramics is stronger than the corresponding precursor glass. This is because Eu3 + ions are in the silica glass matrix before heat treatment. After heat treatment, with the precipitation of NaY (MoO4)2 crystalline phase, some Eu3+ ions replace the lattices of Y3+ and enter crystalline phase, and others are in the environment containing molybdate. The change of chemical environment decreases the maximum phonon energy around Eu3 + ions and increases the radiation transition efficiency and luminescence intensity.
Fig. 11. Ratios of red emission intensity to orange emission intensity.
the relative intensity of each emitted light. The ratio change line of the luminescence intensity at 614 nm and 591 nm is shown in Fig. 11. This line shows an upward trend with the increase of Eu2O3 doping concentration, and reaches the highest point when the doping concentration is 0.9 mol%. At this time, the red emission has the largest proportion and the chromaticity coordinate is closest to the standard red region.
3.3. Chromaticity coordinate analysis The Chromaticity coordinates of glass-ceramics doping xEu2O3 (x = 0.1, 0.3, 0.5, 0.7, 0.9, 1.1 mol%) are depicted in Fig. 10 and corresponding chromaticity coordinate data is listed in Table 1. It can be seen from the figure that with the increasing of Eu2O3 doping concentration the chromaticity coordinates shift from the orange region to the red region. When the doping concentration is 0.9 mol%, the coordinate (0.6337, 0.3635) is closest to the standard red coordinate. The shift of the chromaticity coordinates may be caused by the change of
4. Conclusions Eu2O3 doped transparent glass-ceramics containing NaY(MoO4)2 crystalline phase were prepared successfully for the first time. The optimum heat treatment condition of precursor glass is 660℃ for 3 h. Under this condition, the grain size in glass-ceramics is about 384 nm, and the transmittance reached 80 % in the visible range. The emission 5
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peaks at 591 and 614 nm belong to the 5D0 →7F1 magnetic-dipole transition and 5D0→7F2 electric-dipole transition of Eu3 + ions, in which the electric-dipole transition dominates and shows intense red emission. The optimum doping concentration of Eu2O3 in the glassceramic is 0.9 mol%. At this time, the critical distance between Eu3+ ions is about 2.53 nm and fluorescence lifetime is 1.37042 ms. Continue to increase the concentration of Eu2O3, the electro-multipolar interaction will occur resulting in concentration quenching, which will reduce the luminescence intensity. Under the excitation of 394 nm, the chromaticity coordinates shift from the orange region to the red region with increasing the concentration of Eu2O3. When the doping concentration is 0.9 mol%, the chromaticity coordinate is closest to the standard red coordinate. Results show that Eu2O3 doped transparent glass-ceramics containing NaY(MoO4)2 crystalline phase are good red emission materials, which have potential application in WLEDs.
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