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Tb3+ co-doped ZnAl amorphous materials and their annealed samples
Journal of Rare Earths xxx (2018) 1e9
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Multi-color luminescence in Eu3þ/Tb3þ co-doped ZnAl amorphous materials and their annealed samples* Yufeng Chen a, *, Jiwan Zhang a, Yanmei Hu a, Xiaoqing Wang a, Li Wang b a b
College of Chemistry, Nanchang University, Nanchang 330031, China College of Materials Science and Engineering, Nanchang University, Nanchang 330031, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 November 2017 Received in revised form 5 January 2018 Accepted 9 January 2018 Available online xxx
Multi-color luminescence basing on amorphous Eu3þ/Tb3þ co-doped Zn-Al hydroxides and their annealed samples were studied in detail. Results suggest that excellent red emissions due to Eu3þ and green emissions attributed to Tb3þ appear under the excitation of favorable wavelength for all the asprepared amorphous samples. Moreover, the emission intensity depends on the Eu3þ/Tb3þ molar ratio. The samples annealed at 300, 500, and 700 C still exhibit amorphous state, and multi-color luminescence kept in the samples annealed at 300 C, while luminescence quenched for the samples annealed at 500 and 700 C. However, a broad emission ranging from 450 to 650 nm occurs in some samples annealed at 900 C. Further, the fluorescence decay and lifetimes for the as-prepared samples and the samples annealed at 300 C were investigated. It is found that all the decay curves of emissions due to Tb3þ and Eu3þ present characteristic double exponential function despite their different lifetimes. The present work may be a good example for developing new multi-color even white light emitting materials. © 2018 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Keywords: Eu3þ/Tb3þ -dopant Multi-color luminescence Fluorescence decay Amorphous solid Rare earths
1. Introduction Recently, phosphors with multi-color luminescent property are attractive because of their potential application in white-lightemitting devices. Some potential white-light-emitting materials, including organic polymers,1e4 lanthanide organic complexes or inorganic-organic hybrids,5e9 and inorganic multi-crystalline powders,10e13 etc., have been paid attention. Although the organic polymers, lanthanide organic complexes, and inorganic-organic hybrids often have high photoluminescence efficiency and longlived luminescence,14,15 their thermal stability is inferior to that of the inorganic materials. For this reason, the inorganic phosphors with multi-color or full color luminescence deserve attention. However, some phosphors with multi-color luminescent property have been in the focus of materials science researchers.16e19 Moreover, most of phosphors with multi-color luminescence have been obtained by co-doping rare earth ions in various hosts, such as Eu3þ/Tb3þ co-doped materials,20e26 Tb3þ/Sm3þ co-doped
* Foundation item: Project supported by the National Natural Science Foundation of China (61564007). * Corresponding author. E-mail address: [email protected] (Y. Chen).
materials,27,28 Ce3þ/Tb3þ co-doped materials,29e31 and some other rare earth ions co-doped organic complex or polymers,32,33 etc. In consideration of a wide variety of hosts as well as their variable compositions, the phosphors with multi-color or full color luminescence have broad application.34e40 However, most of above phosphors were synthesized with expensive experimental instruments, complex experimental conditions, and high reaction temperatures, etc. Therefore, it is necessary to develop a simple and alternative route to preparing new phosphors with multi-color or full color luminescence. As is well known, Eu3þ and Tb3þ ions are good red emission and green emission dopants, respectively. In theory, Eu3þ/Tb3þ co-doped agents may produce red emission, green emissions, or even white light. The present work is consequently to prepare Eu3þ/Tb3þ co-doped Zn-Al-hydroxides amorphous phase with cheap and less toxic zinc halt and aluminum salts in order to obtain multi-color luminescent materials. The amorphous phase exhibited excellent multi-color luminescence range from green to red emissions, and their samples annealed at 900 C exhibited white light emissions covering 450e650 nm, indicating the as-prepared amorphous Eu3þ/Tb3þ co-doped Zn-Al-hydroxide phase and their samples annealed at 900 C may have potential application in multi-color or white light emitting devices.
https://doi.org/10.1016/j.jre.2018.01.024 1002-0721/© 2018 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Chen Y, et al., Multi-color luminescence in Eu3þ/Tb3þ co-doped ZnAl amorphous materials and their annealed samples, Journal of Rare Earths (2018), https://doi.org/10.1016/j.jre.2018.01.024
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Y. Chen et al. / Journal of Rare Earths xxx (2018) 1e9
2. Experimental Zinc nitrate hexahydrate (99.99%), aluminium nitrate ninehydrate (99.99%), europium oxide (Eu2O3 99.99%), and terbium oxide (Tb4O7, 99.99%) were purchased from Sinopharm Group Co., Ltd., China. Tb4O7 was dissolved in concentrated HNO3 and H2O2 solution to transform to Tb3þ ions. Eu2O3 was dissolved using HNO3. Then the Eu3þ/Tb3þ mixed solutions with Eu3þ/Tb3þ molar ratios of 1/4, 1/1, and 4/1 were obtained by mixing Eu3þ and Tb3þ solution. A certain amounts of Zn(NO3)2$6H2O and Al(NO3)3$9H2O solid were dissolved in ultrapure water, and the Eu3þ/Tb3þ mixed solution was dropped in and kept at Zn2þ/Al3þ molar ratio of 2/0.5, Al3þ/ (Eu3þþTb3þ) molar ratio of 1/1, along with Eu3þ/Tb3þ molar ratio of 1/4, 1/1, and 4/1, respectively. After concentrated ammonia (~25%) water was dropped in above mixed system, continuously stirred, the slurries (pH ¼ 8e9) formed. The Eu3/Tb3þ co-doped Zn-Al amorphous phases with Eu3/Tb3þ molar ratios of 1/4, 1/1, and 4/1 have been obtained, and labeled as ZnAl-1Eu4Tb, ZnAl-1Eu1Tb and ZnAl-4Eu1Tb, respectively after being aged at 40 C for 2 h, filtrated, washed, and dried at 70 C for 12 h. Further, the as-prepared ZnAl1Eu4Tb (2.0 g), ZnAl-1Eu1Tb (2.0 g), and ZnAl-4Eu1Tb (2.0 g) were annealed at 300, 500, 700, and 900 C for 2 h in air, respectively. Compositional analysis was performed with inductively coupled plasma atomic emission spectroscopy (ICP-AES) and CHN elemental analyzer (Elementar Vario EL II). X-ray powder diffraction (XRD) pattern data were collected on an XD-2/XD-3 diffractometer (Cu Ka, Beijing Puxi Tongyong Yiqi Ltd., China). The scanning was in the 2q range of 5 e60 at a scan rate of 2 ( )/min. Fourier transform infrared (FT-IR) spectra were obtained with a Shimadzu IR spectrometer (Prestige-21, Japan) in the range of 400e4000 cm1 region. A KBr pellet was used to obtain the FT-IR spectra. The photoluminescence (excitation and emission spectra) was measured at room temperature on a F-7000 FL spectrophotometer, attached to a phosphorimeter equipped with a Xe-arc lamp (450 W) as the excitation source. Photoluminescence decay was measured with a Quantaurus-tau (C11367&U11487, Japan) spectrometer. All the measurements were carried out at room temperature. 3. Results and discussion The chemical compositions of the Eu3þ/Tb3þ co-doped ZnAl amorphous phases with Eu3þ/Tb3þ molar ratios of 1/4, 1/1, and 4/1 measured by ICP and CHN elemental analyses are listed in Table 1, which shows the elemental mass percent of zinc (Zn), aluminum (Al), europium (Eu), terbium (Tb), hydrogen (H), nitrogen (N), and
Fig. 1. XRD patterns of as-prepared sample (1) and their samples annealed at 300 C (2), 500 C (3), 700 C (4), and 900 C (5) for ZnAl-1Eu4Tb (a), ZnAl-1Eu1Tb (b), and ZnAl-4Eu1Tb (c).
Table 1 Chemical composition of the as-prepared ZnAl-amorphous phases. Samples
a
Zn
Al
Eu
As-prepared ZnAl-1Eu4Tb
33.62 (33.60) Chemical formula 3.51 (3.53) Chemical formula 33.60 (33.61) Chemical formula 3.72 (3.70) Chemical formula 34.06 (34.04 Chemical formula 3.50 (3.53) Chemical formula
3.60 (3.61)
4.76 (4.75) 15.36 (15.38) Zn1.98Al0.51(Eu0.12Tb0.37)(OH)5.96NO3 2.46 (2.48) 10.31 (10.28) ZnAl29.5O47.1:0.3Eu:1.2 Tb 10.02 (10.04) 10.06 (10.03) Zn1.96Al0.50(Eu0.25Tb0.24)(OH)5.89NO3 2.61 (2.64) 2.72 (2.70) ZnAl30.8O47.5:0.3Eu:0.3 Tb 15.13 (15.15) 4.58 (4.56) Zn2.0Al0.48(Eu0.38Tb0.11)(OH)5.97NO3 9.83 (9.82) 2.58 (2.55) ZnAl29.8O47.2:1.2Eu:0.3 Tb
ZnAl-1Eu4Tb calcined at 900 C As-prepared ZnAl-1Eu1Tb ZnAl-1Eu1Tb calcined at 900 C As-prepared ZnAl-4Eu1Tb ZnAl-4Eu1Tb calcined at 900 C
b
Content for Cald. (Found) (wt%)
43.02 (43.04) 3.56 (3.55) 47.52 (47.49) 3.40 (3.39) 43.38 (43.36)
Tb
c
Content (wt%)
Content (wt%)
H
N
O
1.56 (1.59)
3.66 (3.65)
37.44 (37.42)
e
e
40.70 (40.67)
1.55 (1.57)
3.70 (3.68)
37.51 (37.52)
e
e
43.43 (43.47)
1.56 (1.60)
3.67 (3.65)
37.60 (37.61)
e
e
40.71 (40.74)
Cald. values: calculated from chemical formula. Found values: aanalyzed by ICP, banalyzed by CHN elemental analyses, and cobtained by subtraction.
Please cite this article in press as: Chen Y, et al., Multi-color luminescence in Eu3þ/Tb3þ co-doped ZnAl amorphous materials and their annealed samples, Journal of Rare Earths (2018), https://doi.org/10.1016/j.jre.2018.01.024
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Table 2 Structural symmetry and cell parameters of samples annealed at 900 C. Samples
Symmetry
Cell parameters (nm)
ZnAl-1Eu1Tb annealed at 900 C ZnAl1-Eu4Tb annealed at 900 C ZnAl-4Eu1Tb C Zn3Al94O144 phase Ref. PDF# 23-1490
Monoclinic Monoclinic Monoclinic Monoclinic
a a a a
¼ ¼ ¼ ¼
0.9266 (9), b ¼ 0.5592 (7), c ¼ 1.2042 (8), b ¼ 114.08 (8) 0.9227 (7), b ¼ 0.5565 (6), c ¼ 1.209 (4), b ¼ 114.04 (4) 0.9288 (7), b ¼ 0.5583 (3), c ¼ 1.211 (0.1), b ¼ 113.78 (6) 0.93, b ¼ 0.563, c ¼ 1.21, b ¼ 100.82
oxygen (O). The found values of oxygen in all the samples have been obtained by subtraction method, namely using total mass to subtract all other elemental mass because the oxygen element could not be detected by ICP and CHN elemental analyzer. The chemical formulas of samples were estimated basing on the results of ICP and CHN elemental analysis, along with charge balance principle. According to Table 1, the found values of Zn, Al, Eu, Tb, H, and N were close to the values calculated from the chemical formulas, suggesting the reasonable chemical formula. The XRD patterns of the samples showed that the as-prepared samples and the samples annealed at 300 C exhibited amorphous state (seen in Fig. 1). After being annealed at 500 and 700 C, respectively, some extremely weak diffractions emerged. Then strong diffractions matched to Zn3Al94O144 phase appeared with temperature up to 900 C. Based on the XRD data of the samples annealed at 900 C and [PDF# 231490], all the diffractions attributed to Zn3Al94O144 phase could be indexed, and their structure symmetry and cell parameters are refined in Table 2. The present structure symmetry and cell parameters were in agreement with that of the previous report [PDF# 23-1490]. The b value of the present samples has large difference compared with that of the literature, which was possibly due to
Fig. 2. FT-IR spectra of ZnAl-1Eu1Tb (a), ZnAl-1Eu4Tb (b), ZnAl-4Eu1Tb (c), and their samples annealed at various temperatures.
Fig. 3. Excitation spectra of ZnAl-1Eu4Tb and their samples annealed at various temperatures under the excitation of 545 nm (a) and 618 nm (b).
Please cite this article in press as: Chen Y, et al., Multi-color luminescence in Eu3þ/Tb3þ co-doped ZnAl amorphous materials and their annealed samples, Journal of Rare Earths (2018), https://doi.org/10.1016/j.jre.2018.01.024
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Fig. 5. Excitation spectra of ZnAl-1Eu1Tb and their samples annealed at various temperatures under the excitation of 545 nm (a) and 618 nm (b).
Fig. 4. Emission spectra of ZnAl-1Eu4Tb and their samples annealed at various temperatures under the excitation of 350 (a), 370 (b), 380 (c), and 395 (d) nm.
large lattice distortion or different crystallinity. The lattice distortion and different crystallinity are often caused by preparing process or reaction conditions. Meanwhile, some weak diffractions attributed to Tb2O3 phase (marked as *) [PDF# 65-6985] and Eu2O3 phase [PDF# 43-1009] (marked as ;) were found in all the samples annealed at 900 C. In view of similarity of lanthanide elements, some diffraction lines ascribed to Tb2O3 and Eu2O3 phases were overlapped. FT-IR spectra of ZnAl-1Eu4Tb, ZnAl-1Eu1Tb, ZnAl-4Eu1Tb, and their samples annealed at various temperatures are given in Fig. 2. The broad strong bands at 3440 and 1637 cm1 were ascribed to OeH stretching and bending vibration of layers and physically adsorbed water, respectively. The strong sharp band at 1385 cm1 was attributed to vibration mode of NO3.41e43 The extremely weak bands at 615 and 1072 cm1 may be due to MetaleO and MetaleOeMetal stretching vibrations, respectively.44,45 After being annealed at 300, 500, 700, and 900 C, respectively, the band at 1385 cm1 greatly reduced because most of NO3 ions were removed at higher temperature. Moreover, all the bands present in the samples hardly shifted except for the weak band at 615 cm1. The band at 615 cm1 exhibiting in all amorphous phases obviously shifted to 663 cm1 after the samples were annealed at 900 C. The possible reason is that the amorphous ZnAl-hydroxides were prepared by co-precipitation method at room temperature, and the chemical bond between metal and oxygen may be weak. Then the chemical bond between metal and oxygen became stronger when the crystallinity of samples annealed at 900 C obviously became better in view of the result of XRD. Accordingly, the band at 615 cm1 present amorphous phase shifted to 663 cm1 after being annealed at 900 C. In addition, a strong band at around 2360 cm1
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ascribed to ethanol appeared in all the samples, which resulted from cleaning with ethanol during IR measurement. Fig. 3 shows the excitation spectra by monitoring the emissions at 545 nm attributed to 5D4/7F5 transition of Tb3þ and 618 nm ascribed to 5D0/7F2 transition of Tb3þ for the as-prepared ZnAl1Eu4Tb and their samples annealed at various temperatures. For the as-prepared ZnAl-1Eu4Tb, four excitation bands centered at 340, 350, 370, and 380 nm owing to 7F6/5L8, 7F6/5D1, 7F6/5D2, and 7F6/5D3 transitions of Tb3þ, respectively, appeared as monitored at 545 nm.35 Meanwhile five excitation bands centered at 353, 362, 368, 380, and 395 nm corresponding to the 7F0/5L10, 7 F0/5D4, 7F0/5L9, 7F0/5L7, and 7F0/5L6 transitions of Eu3þ, respectively, emerged while monitored at 618 nm.35e37 However, the excitation bands centered at 340, 362, 353 and 368 nm obtained for emission monitored at 618 nm that corresponds to 5 D0/7F2 transition in Eu3þ also possibly resulted from Tb3þ / Eu3þ energy transfer.46 After being annealed at 300 C for 2 h, all excitation bands markedly decreased. Further, the excitation bands almost vanished as temperature increased to 500, 700, and 900 C. Under the excitation with favorable excitation wavelengths of 350, 370, 380, and 395 nm, the emission spectra of the ZnAl1Eu4Tb and their samples annealed at various temperatures are presented in Fig. 4. Strong emissions at 545 and 490 nm ascribed to 5 D4/7F5 and 5D4/7F6 transitions of Tb3þ and weak emissions at 593 and 618 nm owed to 5D0/7F1 and 5D0/7F2 transitions of Eu3þ were observed under the excitation at 350, 370, and 380 nm. Only emissions due to 5D0/7FJ (J ¼ 1, 2, 3, 4) transitions of Eu3þ exhibited under the excitation at 395 nm, which was in accordance with that of the excitation spectra. When the ZnAl-1Eu4Tb was annealed at 300 C, all emission peaks greatly decreased. Then the
Fig. 6. Emission spectra of ZnAl-1Eu1Tb and their samples annealed at various temperatures under the excitation of 350 (a), 370 (b), 380 (c), and 395 (d) nm.
Fig. 7. Excitation spectra of ZnAl-4Eu1Tb and their samples annealed at various temperatures under the excitation of 545 nm (a) and 618 nm (b).
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emissions almost disappeared with temperature up to 500, 700, and 900 C. The change of excitation spectra and emission spectra depending on temperature was possibly due to host transformation from amorphous to multi-crystalline phase. With regard to the as-prepared ZnAl-1Eu1Tb, as expected, the excitation bands centered at 340, 350, 370, and 380 nm attributed to Tb3þ along with the bands centered at 362, 380 and 395 nm ascribed to Eu3þ were observed (shown in Fig. 5). While the asprepared sample was annealed at 300 C for 2 h, the excitation bands due to Tb3þ greatly decreased, and the bands owing to Eu3þ slightly reduced. Then the excitation bands almost disappeared as temperature varied from 500 to 700 C. With temperature up to 900 C, a broad excitation band ranging from 330 to 390 nm emerged as monitored at 545 nm, which may be due to host material. The emission spectra of the as-prepared sample and the sample annealed at 300 C under excitation at 350, 370, 380, and 395 nm showed a series of narrow emission lines corresponding to the 5D4/7FJ (J ¼ 5, 6) transitions of Tb3þ and 5D0/7FJ (J ¼ 1, 2) transitions of Eu3þ (shown in Fig. 6). Moreover, the intensity of emissions attributed to Eu3þ with favorable excitation wavelength of 395 nm was obviously stronger than that of the emissions ascribed to Tb3þ under favorable excitation at 350, 370, and 380 nm, suggesting the present host was more favorable for Eu3þ emission. However, strong emissions attributed to Tb3þ and Eu3þ simultaneously appeared under excitation at 380 nm for the as-prepared ZnAl-1Eu1Tb, which was different from that of the as-prepared ZnAl-1Eu4Tb. The similar intensity of emissions attributed to Tb3þ and Eu3þ may be due to the similar contents of Tb3þ and Eu3þ
Fig. 8. Emission spectra of ZnAl-4Eu1Tb and their samples annealed at various temperatures under the excitation of 350 (a), 370 (b), 380 (c), and 395 (d) nm.
Fig. 9. Decay curves of emissions at 545 and 490 nm of Tb3þ as well as emissions at 593 and 618 nm of Eu3þ for as-prepared ZnAl-1Eu4Tb and its sample annealed at 300 C under the excitation of 365 nm (a) and 405 nm (b).
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incorporated in the ZnAl-1Eu1Tb. When the as-prepared sample was annealed at 500 or 700 C, the narrow emissions attributed to Tb3þ and Eu3þ almost quenched. Meanwhile a broad emission in the range of 450e650 nm (white light) was observed under excitation at 350, 370, and 380 nm for the sample annealed at 900 C. This broad emission band possibly resulted from the synergistic effects of Tb3þ, Eu3þ, and host material. Fig. 7 displays excitation spectra of the as-prepared ZnAl4Eu1Tb and their samples annealed at various temperatures. The excitation spectrum monitored at 618 nm due to Eu3þ has similar intensity as that of the excitation spectrum monitored at 545 nm due to Tb3þ for the as-prepared sample. After being annealed at 300 C, the excitation band attributed to Tb3þ markedly decreased and the excitation band ascribed to Eu3þ greatly increased, which may be due to energy transfer from Tb3þ to Eu3þ.20,46 As temperature rose to 500 and 700 C, the excitation bands due to Tb3þ and Eu3þ disappeared. Then a weak broad band ranging from 300 to 390 nm occurred with temperature up to 900 C as monitored at 545 nm, which may be due to host phase transition. Under excitation at 350, 370, and 380 nm, emissions owing to 5D0/7F1 and 5 D0/7F2 transitions of Eu3þ and 5D4/7F5 and 5D4/7F6 transitions of Tb3þ were found (shown in Fig. 8). Only emissions ascribed to 5 D0/7F1 and 5D0/7F2 transitions of Eu3þ emerged under excitation at 395 nm. After the as-prepared ZnAl-4Eu1Tb was annealed at 300 C, the emissions attributed to Eu3þ markedly increased and the emissions owed to Tb3þ obviously reduced, which was due to energy transfer from Tb3þ to Eu3þ.16,42 Then both Tb3þ and Eu3þ emissions almost vanished with temperature increasing to 500 and
700 C. As temperature reached to 900 C, a broad emission ranging from 450 to 650 nm occurred under excitation at 350 and 370 nm. The broad emission was possibly due to the synergistic effects of Tb3þ, Eu3þ, and host material. However, the broad emission obviously appeared in the ZnAl-1Eu1Tb sample annealed at 900 C and the ZnAl-4Eu1Tb sample annealed at 900 C but not in the ZnAl1Eu4Tb sample annealed at 900 C. This may be owing to more Tb4þ incorporated in the ZnAl-1Eu4Tb sample annealed at 900 C because the content of Tb incorporated in it is obviously higher than those of in the ZnAl-1Eu1Tb and ZnAl-4Eu1Tb samples annealed at 900 C. Luminescence decay profiles of the dominated emission lines for Eu3þ and Tb3þ incorporated in the as-prepared ZnAl-1Eu4Tb, ZnAl-1Eu1Tb, ZnAl-4Eu1Tb, and their samples annealed at 300 C are presented in Figs. 9e11, respectively. The decay curves of emissions at 490 and 545 nm excited by 365 nm as well as emissions at 593 and 618 nm excited by 405 nm exhibited characteristic double exponential function,37,38,47,48 which was very different from that of the mono-exponential character of the profiles.35,39,46 In addition, the lifetimes of the luminescence attributed to Tb3þ gradually decreased and the lifetimes of the luminescence ascribed to Eu3þ gradually increased as the Eu3þ/Tb3þ molar ratio varied from 1/4, 1/1, to 4/1 (shown in Table 3), which may be due to energy transfer from Tb3þ to Eu3þ.20,46,49 Moreover, the present lifetimes of emissions ascribed to Tb3þ or Eu3þ are only nanosecond orders, which is greatly smaller than that of some literature (microsecond orders).37,49 The possible reason is that the present as-prepared samples and their samples annealed at 300 C were amorphous
Fig. 10. Decay curves of emissions at 545 and 490 nm of Tb3þ as well as emissions at 593 and 618 nm of Eu3þ for as-prepared ZnAl-1Eu1Tb and its sample annealed at 300 C under the excitation of 365 nm (a) and 405 nm (b).
Fig. 11. Decay curves of emissions at 545 nm of Tb3þ as well as emissions at 593 and 618 nm of Eu3þ for as-prepared ZnAl-4Eu1Tb and its sample annealed at 300 C under the excitation of 365 nm (a) and 405 nm (b).
Please cite this article in press as: Chen Y, et al., Multi-color luminescence in Eu3þ/Tb3þ co-doped ZnAl amorphous materials and their annealed samples, Journal of Rare Earths (2018), https://doi.org/10.1016/j.jre.2018.01.024
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Table 3 Lifetimes of the luminescence centers incorporated in as-prepared samples and samples annealed at 300 C (ns). Samples
365 nm excitation
405 nm excitation
Emission due to 5D4/7F5 of Tb3þ Emission due to 5D4/7F6 of Tb3þ Emission due to 5D0/7F2 of Eu3þ Emission due to 5D0/7F1 of Eu3þ As-prepared ZnAl-1Eu4Tb 10.66 As-prepared ZnAl-1Eu1Tb 7.23 As-prepared ZnAl-4Eu1Tb 5.90
6.83 5.42 e
state and may have more defect energy levels that can make the electrons have more opportunity to move from higher energy levels to lower energy levels, accordingly cause shorter lifetimes.49 The photoluminescent property of the present as-prepared samples and their samples annealed at 300 and 900 C is different from that of the previous Eu3þ/Tb3þ co-doped MgAl amorphous phase and their annealed samples as well as other Eu3þ/Tb3þ co-doped phosphors,31,44 which may be due to different host materials. 4. Conclusions The Eu3þ/Tb3þ co-doped ZnAl amorphous solid with different Eu /Tb3þ molar ratios of 1/4, 1/1, and 4/1 were prepared. The asprepared samples and their samples annealed at 300 C are favorable for the Eu3þ and Tb3þ emissions, and can simultaneously emit red and green emissions under favorable excitation. Moreover, a broad emission at 450e650 nm emerges in the ZnAl-1Eu1Tb and ZnAl-4Eu1Tb annealed at 900 C under excitation at 350, 370, and 380 nm. These results provide a practical basis for the design of white light illumination sources by blending the present powders samples with an appropriate UV emitter (340e400 nm), such as UV LEDs. 3þ
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4.42 5.37 11.90
4.30 4.83 7.85
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44. Rojas Delgado R, De Pauli CP, Barriga Carrasco C, Avena MJ. Influence of MII/MIII ratio in surface-charging behavior of ZneAl layered double hydroxides. Appl Clay Sci. 2008;40:27. 45. Yu DY, Wen SG, Yang JX, Wang JH, Chen Y, Luo J, et al. RGO modified ZnAl-LDH as epoxy nanostructure filler: a novel synthetic approach to anticorrosive waterborne coating. Surf Coat Technol. 2017;326:207. 46. Xie MB, Zhu GX, Li DY, Pan RK, Fu XH. Eu2þ/Tb3þ/Eu3þ energy transfer in Ca6La2Na2(PO4)6F2:Eu,Tb phosphors. RSC Adv. 2016;6:33990. 47. Lin ZX, Huang R, Wang HP, Wang Y, Zhang Y, Guo YQ, et al. Dense nanosized europium silicate clusters induced light emission enhancement in Eu-doped silicon oxycarbide films. J Alloys Compd. 2017;694:946. 48. Guckan V, Altunal V, Nur N, Depci T, Ozdemir A, Kurt K, et al. Studying CaSO4: Eu as an OSL phosphor. Nucl Instr Methods Phys Res B. 2017;407:145. 49. Xiong JH, Meng QY, Sun WJ. Luminescent properties and energy transfer mechanism from Tb3þ to Eu3þ in CaMoO4:Tb3þ,Eu3þ phosphors. J Rare Earths. 2016;34:251.
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Multi-color luminescence in Eu3þ/Tb3þ co-doped ZnAl amorphous materials and their annealed samples* Yufeng Chen a, *, Jiwan Zhang a, Yanmei Hu a, Xiaoqing Wang a, Li Wang b a b
College of Chemistry, Nanchang University, Nanchang 330031, China College of Materials Science and Engineering, Nanchang University, Nanchang 330031, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 November 2017 Received in revised form 5 January 2018 Accepted 9 January 2018 Available online xxx
Multi-color luminescence basing on amorphous Eu3þ/Tb3þ co-doped Zn-Al hydroxides and their annealed samples were studied in detail. Results suggest that excellent red emissions due to Eu3þ and green emissions attributed to Tb3þ appear under the excitation of favorable wavelength for all the asprepared amorphous samples. Moreover, the emission intensity depends on the Eu3þ/Tb3þ molar ratio. The samples annealed at 300, 500, and 700 C still exhibit amorphous state, and multi-color luminescence kept in the samples annealed at 300 C, while luminescence quenched for the samples annealed at 500 and 700 C. However, a broad emission ranging from 450 to 650 nm occurs in some samples annealed at 900 C. Further, the fluorescence decay and lifetimes for the as-prepared samples and the samples annealed at 300 C were investigated. It is found that all the decay curves of emissions due to Tb3þ and Eu3þ present characteristic double exponential function despite their different lifetimes. The present work may be a good example for developing new multi-color even white light emitting materials. © 2018 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Keywords: Eu3þ/Tb3þ -dopant Multi-color luminescence Fluorescence decay Amorphous solid Rare earths
1. Introduction Recently, phosphors with multi-color luminescent property are attractive because of their potential application in white-lightemitting devices. Some potential white-light-emitting materials, including organic polymers,1e4 lanthanide organic complexes or inorganic-organic hybrids,5e9 and inorganic multi-crystalline powders,10e13 etc., have been paid attention. Although the organic polymers, lanthanide organic complexes, and inorganic-organic hybrids often have high photoluminescence efficiency and longlived luminescence,14,15 their thermal stability is inferior to that of the inorganic materials. For this reason, the inorganic phosphors with multi-color or full color luminescence deserve attention. However, some phosphors with multi-color luminescent property have been in the focus of materials science researchers.16e19 Moreover, most of phosphors with multi-color luminescence have been obtained by co-doping rare earth ions in various hosts, such as Eu3þ/Tb3þ co-doped materials,20e26 Tb3þ/Sm3þ co-doped
* Foundation item: Project supported by the National Natural Science Foundation of China (61564007). * Corresponding author. E-mail address: [email protected] (Y. Chen).
materials,27,28 Ce3þ/Tb3þ co-doped materials,29e31 and some other rare earth ions co-doped organic complex or polymers,32,33 etc. In consideration of a wide variety of hosts as well as their variable compositions, the phosphors with multi-color or full color luminescence have broad application.34e40 However, most of above phosphors were synthesized with expensive experimental instruments, complex experimental conditions, and high reaction temperatures, etc. Therefore, it is necessary to develop a simple and alternative route to preparing new phosphors with multi-color or full color luminescence. As is well known, Eu3þ and Tb3þ ions are good red emission and green emission dopants, respectively. In theory, Eu3þ/Tb3þ co-doped agents may produce red emission, green emissions, or even white light. The present work is consequently to prepare Eu3þ/Tb3þ co-doped Zn-Al-hydroxides amorphous phase with cheap and less toxic zinc halt and aluminum salts in order to obtain multi-color luminescent materials. The amorphous phase exhibited excellent multi-color luminescence range from green to red emissions, and their samples annealed at 900 C exhibited white light emissions covering 450e650 nm, indicating the as-prepared amorphous Eu3þ/Tb3þ co-doped Zn-Al-hydroxide phase and their samples annealed at 900 C may have potential application in multi-color or white light emitting devices.
https://doi.org/10.1016/j.jre.2018.01.024 1002-0721/© 2018 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Chen Y, et al., Multi-color luminescence in Eu3þ/Tb3þ co-doped ZnAl amorphous materials and their annealed samples, Journal of Rare Earths (2018), https://doi.org/10.1016/j.jre.2018.01.024
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2. Experimental Zinc nitrate hexahydrate (99.99%), aluminium nitrate ninehydrate (99.99%), europium oxide (Eu2O3 99.99%), and terbium oxide (Tb4O7, 99.99%) were purchased from Sinopharm Group Co., Ltd., China. Tb4O7 was dissolved in concentrated HNO3 and H2O2 solution to transform to Tb3þ ions. Eu2O3 was dissolved using HNO3. Then the Eu3þ/Tb3þ mixed solutions with Eu3þ/Tb3þ molar ratios of 1/4, 1/1, and 4/1 were obtained by mixing Eu3þ and Tb3þ solution. A certain amounts of Zn(NO3)2$6H2O and Al(NO3)3$9H2O solid were dissolved in ultrapure water, and the Eu3þ/Tb3þ mixed solution was dropped in and kept at Zn2þ/Al3þ molar ratio of 2/0.5, Al3þ/ (Eu3þþTb3þ) molar ratio of 1/1, along with Eu3þ/Tb3þ molar ratio of 1/4, 1/1, and 4/1, respectively. After concentrated ammonia (~25%) water was dropped in above mixed system, continuously stirred, the slurries (pH ¼ 8e9) formed. The Eu3/Tb3þ co-doped Zn-Al amorphous phases with Eu3/Tb3þ molar ratios of 1/4, 1/1, and 4/1 have been obtained, and labeled as ZnAl-1Eu4Tb, ZnAl-1Eu1Tb and ZnAl-4Eu1Tb, respectively after being aged at 40 C for 2 h, filtrated, washed, and dried at 70 C for 12 h. Further, the as-prepared ZnAl1Eu4Tb (2.0 g), ZnAl-1Eu1Tb (2.0 g), and ZnAl-4Eu1Tb (2.0 g) were annealed at 300, 500, 700, and 900 C for 2 h in air, respectively. Compositional analysis was performed with inductively coupled plasma atomic emission spectroscopy (ICP-AES) and CHN elemental analyzer (Elementar Vario EL II). X-ray powder diffraction (XRD) pattern data were collected on an XD-2/XD-3 diffractometer (Cu Ka, Beijing Puxi Tongyong Yiqi Ltd., China). The scanning was in the 2q range of 5 e60 at a scan rate of 2 ( )/min. Fourier transform infrared (FT-IR) spectra were obtained with a Shimadzu IR spectrometer (Prestige-21, Japan) in the range of 400e4000 cm1 region. A KBr pellet was used to obtain the FT-IR spectra. The photoluminescence (excitation and emission spectra) was measured at room temperature on a F-7000 FL spectrophotometer, attached to a phosphorimeter equipped with a Xe-arc lamp (450 W) as the excitation source. Photoluminescence decay was measured with a Quantaurus-tau (C11367&U11487, Japan) spectrometer. All the measurements were carried out at room temperature. 3. Results and discussion The chemical compositions of the Eu3þ/Tb3þ co-doped ZnAl amorphous phases with Eu3þ/Tb3þ molar ratios of 1/4, 1/1, and 4/1 measured by ICP and CHN elemental analyses are listed in Table 1, which shows the elemental mass percent of zinc (Zn), aluminum (Al), europium (Eu), terbium (Tb), hydrogen (H), nitrogen (N), and
Fig. 1. XRD patterns of as-prepared sample (1) and their samples annealed at 300 C (2), 500 C (3), 700 C (4), and 900 C (5) for ZnAl-1Eu4Tb (a), ZnAl-1Eu1Tb (b), and ZnAl-4Eu1Tb (c).
Table 1 Chemical composition of the as-prepared ZnAl-amorphous phases. Samples
a
Zn
Al
Eu
As-prepared ZnAl-1Eu4Tb
33.62 (33.60) Chemical formula 3.51 (3.53) Chemical formula 33.60 (33.61) Chemical formula 3.72 (3.70) Chemical formula 34.06 (34.04 Chemical formula 3.50 (3.53) Chemical formula
3.60 (3.61)
4.76 (4.75) 15.36 (15.38) Zn1.98Al0.51(Eu0.12Tb0.37)(OH)5.96NO3 2.46 (2.48) 10.31 (10.28) ZnAl29.5O47.1:0.3Eu:1.2 Tb 10.02 (10.04) 10.06 (10.03) Zn1.96Al0.50(Eu0.25Tb0.24)(OH)5.89NO3 2.61 (2.64) 2.72 (2.70) ZnAl30.8O47.5:0.3Eu:0.3 Tb 15.13 (15.15) 4.58 (4.56) Zn2.0Al0.48(Eu0.38Tb0.11)(OH)5.97NO3 9.83 (9.82) 2.58 (2.55) ZnAl29.8O47.2:1.2Eu:0.3 Tb
ZnAl-1Eu4Tb calcined at 900 C As-prepared ZnAl-1Eu1Tb ZnAl-1Eu1Tb calcined at 900 C As-prepared ZnAl-4Eu1Tb ZnAl-4Eu1Tb calcined at 900 C
b
Content for Cald. (Found) (wt%)
43.02 (43.04) 3.56 (3.55) 47.52 (47.49) 3.40 (3.39) 43.38 (43.36)
Tb
c
Content (wt%)
Content (wt%)
H
N
O
1.56 (1.59)
3.66 (3.65)
37.44 (37.42)
e
e
40.70 (40.67)
1.55 (1.57)
3.70 (3.68)
37.51 (37.52)
e
e
43.43 (43.47)
1.56 (1.60)
3.67 (3.65)
37.60 (37.61)
e
e
40.71 (40.74)
Cald. values: calculated from chemical formula. Found values: aanalyzed by ICP, banalyzed by CHN elemental analyses, and cobtained by subtraction.
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Table 2 Structural symmetry and cell parameters of samples annealed at 900 C. Samples
Symmetry
Cell parameters (nm)
ZnAl-1Eu1Tb annealed at 900 C ZnAl1-Eu4Tb annealed at 900 C ZnAl-4Eu1Tb C Zn3Al94O144 phase Ref. PDF# 23-1490
Monoclinic Monoclinic Monoclinic Monoclinic
a a a a
¼ ¼ ¼ ¼
0.9266 (9), b ¼ 0.5592 (7), c ¼ 1.2042 (8), b ¼ 114.08 (8) 0.9227 (7), b ¼ 0.5565 (6), c ¼ 1.209 (4), b ¼ 114.04 (4) 0.9288 (7), b ¼ 0.5583 (3), c ¼ 1.211 (0.1), b ¼ 113.78 (6) 0.93, b ¼ 0.563, c ¼ 1.21, b ¼ 100.82
oxygen (O). The found values of oxygen in all the samples have been obtained by subtraction method, namely using total mass to subtract all other elemental mass because the oxygen element could not be detected by ICP and CHN elemental analyzer. The chemical formulas of samples were estimated basing on the results of ICP and CHN elemental analysis, along with charge balance principle. According to Table 1, the found values of Zn, Al, Eu, Tb, H, and N were close to the values calculated from the chemical formulas, suggesting the reasonable chemical formula. The XRD patterns of the samples showed that the as-prepared samples and the samples annealed at 300 C exhibited amorphous state (seen in Fig. 1). After being annealed at 500 and 700 C, respectively, some extremely weak diffractions emerged. Then strong diffractions matched to Zn3Al94O144 phase appeared with temperature up to 900 C. Based on the XRD data of the samples annealed at 900 C and [PDF# 231490], all the diffractions attributed to Zn3Al94O144 phase could be indexed, and their structure symmetry and cell parameters are refined in Table 2. The present structure symmetry and cell parameters were in agreement with that of the previous report [PDF# 23-1490]. The b value of the present samples has large difference compared with that of the literature, which was possibly due to
Fig. 2. FT-IR spectra of ZnAl-1Eu1Tb (a), ZnAl-1Eu4Tb (b), ZnAl-4Eu1Tb (c), and their samples annealed at various temperatures.
Fig. 3. Excitation spectra of ZnAl-1Eu4Tb and their samples annealed at various temperatures under the excitation of 545 nm (a) and 618 nm (b).
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Fig. 5. Excitation spectra of ZnAl-1Eu1Tb and their samples annealed at various temperatures under the excitation of 545 nm (a) and 618 nm (b).
Fig. 4. Emission spectra of ZnAl-1Eu4Tb and their samples annealed at various temperatures under the excitation of 350 (a), 370 (b), 380 (c), and 395 (d) nm.
large lattice distortion or different crystallinity. The lattice distortion and different crystallinity are often caused by preparing process or reaction conditions. Meanwhile, some weak diffractions attributed to Tb2O3 phase (marked as *) [PDF# 65-6985] and Eu2O3 phase [PDF# 43-1009] (marked as ;) were found in all the samples annealed at 900 C. In view of similarity of lanthanide elements, some diffraction lines ascribed to Tb2O3 and Eu2O3 phases were overlapped. FT-IR spectra of ZnAl-1Eu4Tb, ZnAl-1Eu1Tb, ZnAl-4Eu1Tb, and their samples annealed at various temperatures are given in Fig. 2. The broad strong bands at 3440 and 1637 cm1 were ascribed to OeH stretching and bending vibration of layers and physically adsorbed water, respectively. The strong sharp band at 1385 cm1 was attributed to vibration mode of NO3.41e43 The extremely weak bands at 615 and 1072 cm1 may be due to MetaleO and MetaleOeMetal stretching vibrations, respectively.44,45 After being annealed at 300, 500, 700, and 900 C, respectively, the band at 1385 cm1 greatly reduced because most of NO3 ions were removed at higher temperature. Moreover, all the bands present in the samples hardly shifted except for the weak band at 615 cm1. The band at 615 cm1 exhibiting in all amorphous phases obviously shifted to 663 cm1 after the samples were annealed at 900 C. The possible reason is that the amorphous ZnAl-hydroxides were prepared by co-precipitation method at room temperature, and the chemical bond between metal and oxygen may be weak. Then the chemical bond between metal and oxygen became stronger when the crystallinity of samples annealed at 900 C obviously became better in view of the result of XRD. Accordingly, the band at 615 cm1 present amorphous phase shifted to 663 cm1 after being annealed at 900 C. In addition, a strong band at around 2360 cm1
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ascribed to ethanol appeared in all the samples, which resulted from cleaning with ethanol during IR measurement. Fig. 3 shows the excitation spectra by monitoring the emissions at 545 nm attributed to 5D4/7F5 transition of Tb3þ and 618 nm ascribed to 5D0/7F2 transition of Tb3þ for the as-prepared ZnAl1Eu4Tb and their samples annealed at various temperatures. For the as-prepared ZnAl-1Eu4Tb, four excitation bands centered at 340, 350, 370, and 380 nm owing to 7F6/5L8, 7F6/5D1, 7F6/5D2, and 7F6/5D3 transitions of Tb3þ, respectively, appeared as monitored at 545 nm.35 Meanwhile five excitation bands centered at 353, 362, 368, 380, and 395 nm corresponding to the 7F0/5L10, 7 F0/5D4, 7F0/5L9, 7F0/5L7, and 7F0/5L6 transitions of Eu3þ, respectively, emerged while monitored at 618 nm.35e37 However, the excitation bands centered at 340, 362, 353 and 368 nm obtained for emission monitored at 618 nm that corresponds to 5 D0/7F2 transition in Eu3þ also possibly resulted from Tb3þ / Eu3þ energy transfer.46 After being annealed at 300 C for 2 h, all excitation bands markedly decreased. Further, the excitation bands almost vanished as temperature increased to 500, 700, and 900 C. Under the excitation with favorable excitation wavelengths of 350, 370, 380, and 395 nm, the emission spectra of the ZnAl1Eu4Tb and their samples annealed at various temperatures are presented in Fig. 4. Strong emissions at 545 and 490 nm ascribed to 5 D4/7F5 and 5D4/7F6 transitions of Tb3þ and weak emissions at 593 and 618 nm owed to 5D0/7F1 and 5D0/7F2 transitions of Eu3þ were observed under the excitation at 350, 370, and 380 nm. Only emissions due to 5D0/7FJ (J ¼ 1, 2, 3, 4) transitions of Eu3þ exhibited under the excitation at 395 nm, which was in accordance with that of the excitation spectra. When the ZnAl-1Eu4Tb was annealed at 300 C, all emission peaks greatly decreased. Then the
Fig. 6. Emission spectra of ZnAl-1Eu1Tb and their samples annealed at various temperatures under the excitation of 350 (a), 370 (b), 380 (c), and 395 (d) nm.
Fig. 7. Excitation spectra of ZnAl-4Eu1Tb and their samples annealed at various temperatures under the excitation of 545 nm (a) and 618 nm (b).
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emissions almost disappeared with temperature up to 500, 700, and 900 C. The change of excitation spectra and emission spectra depending on temperature was possibly due to host transformation from amorphous to multi-crystalline phase. With regard to the as-prepared ZnAl-1Eu1Tb, as expected, the excitation bands centered at 340, 350, 370, and 380 nm attributed to Tb3þ along with the bands centered at 362, 380 and 395 nm ascribed to Eu3þ were observed (shown in Fig. 5). While the asprepared sample was annealed at 300 C for 2 h, the excitation bands due to Tb3þ greatly decreased, and the bands owing to Eu3þ slightly reduced. Then the excitation bands almost disappeared as temperature varied from 500 to 700 C. With temperature up to 900 C, a broad excitation band ranging from 330 to 390 nm emerged as monitored at 545 nm, which may be due to host material. The emission spectra of the as-prepared sample and the sample annealed at 300 C under excitation at 350, 370, 380, and 395 nm showed a series of narrow emission lines corresponding to the 5D4/7FJ (J ¼ 5, 6) transitions of Tb3þ and 5D0/7FJ (J ¼ 1, 2) transitions of Eu3þ (shown in Fig. 6). Moreover, the intensity of emissions attributed to Eu3þ with favorable excitation wavelength of 395 nm was obviously stronger than that of the emissions ascribed to Tb3þ under favorable excitation at 350, 370, and 380 nm, suggesting the present host was more favorable for Eu3þ emission. However, strong emissions attributed to Tb3þ and Eu3þ simultaneously appeared under excitation at 380 nm for the as-prepared ZnAl-1Eu1Tb, which was different from that of the as-prepared ZnAl-1Eu4Tb. The similar intensity of emissions attributed to Tb3þ and Eu3þ may be due to the similar contents of Tb3þ and Eu3þ
Fig. 8. Emission spectra of ZnAl-4Eu1Tb and their samples annealed at various temperatures under the excitation of 350 (a), 370 (b), 380 (c), and 395 (d) nm.
Fig. 9. Decay curves of emissions at 545 and 490 nm of Tb3þ as well as emissions at 593 and 618 nm of Eu3þ for as-prepared ZnAl-1Eu4Tb and its sample annealed at 300 C under the excitation of 365 nm (a) and 405 nm (b).
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Y. Chen et al. / Journal of Rare Earths xxx (2018) 1e9
7
incorporated in the ZnAl-1Eu1Tb. When the as-prepared sample was annealed at 500 or 700 C, the narrow emissions attributed to Tb3þ and Eu3þ almost quenched. Meanwhile a broad emission in the range of 450e650 nm (white light) was observed under excitation at 350, 370, and 380 nm for the sample annealed at 900 C. This broad emission band possibly resulted from the synergistic effects of Tb3þ, Eu3þ, and host material. Fig. 7 displays excitation spectra of the as-prepared ZnAl4Eu1Tb and their samples annealed at various temperatures. The excitation spectrum monitored at 618 nm due to Eu3þ has similar intensity as that of the excitation spectrum monitored at 545 nm due to Tb3þ for the as-prepared sample. After being annealed at 300 C, the excitation band attributed to Tb3þ markedly decreased and the excitation band ascribed to Eu3þ greatly increased, which may be due to energy transfer from Tb3þ to Eu3þ.20,46 As temperature rose to 500 and 700 C, the excitation bands due to Tb3þ and Eu3þ disappeared. Then a weak broad band ranging from 300 to 390 nm occurred with temperature up to 900 C as monitored at 545 nm, which may be due to host phase transition. Under excitation at 350, 370, and 380 nm, emissions owing to 5D0/7F1 and 5 D0/7F2 transitions of Eu3þ and 5D4/7F5 and 5D4/7F6 transitions of Tb3þ were found (shown in Fig. 8). Only emissions ascribed to 5 D0/7F1 and 5D0/7F2 transitions of Eu3þ emerged under excitation at 395 nm. After the as-prepared ZnAl-4Eu1Tb was annealed at 300 C, the emissions attributed to Eu3þ markedly increased and the emissions owed to Tb3þ obviously reduced, which was due to energy transfer from Tb3þ to Eu3þ.16,42 Then both Tb3þ and Eu3þ emissions almost vanished with temperature increasing to 500 and
700 C. As temperature reached to 900 C, a broad emission ranging from 450 to 650 nm occurred under excitation at 350 and 370 nm. The broad emission was possibly due to the synergistic effects of Tb3þ, Eu3þ, and host material. However, the broad emission obviously appeared in the ZnAl-1Eu1Tb sample annealed at 900 C and the ZnAl-4Eu1Tb sample annealed at 900 C but not in the ZnAl1Eu4Tb sample annealed at 900 C. This may be owing to more Tb4þ incorporated in the ZnAl-1Eu4Tb sample annealed at 900 C because the content of Tb incorporated in it is obviously higher than those of in the ZnAl-1Eu1Tb and ZnAl-4Eu1Tb samples annealed at 900 C. Luminescence decay profiles of the dominated emission lines for Eu3þ and Tb3þ incorporated in the as-prepared ZnAl-1Eu4Tb, ZnAl-1Eu1Tb, ZnAl-4Eu1Tb, and their samples annealed at 300 C are presented in Figs. 9e11, respectively. The decay curves of emissions at 490 and 545 nm excited by 365 nm as well as emissions at 593 and 618 nm excited by 405 nm exhibited characteristic double exponential function,37,38,47,48 which was very different from that of the mono-exponential character of the profiles.35,39,46 In addition, the lifetimes of the luminescence attributed to Tb3þ gradually decreased and the lifetimes of the luminescence ascribed to Eu3þ gradually increased as the Eu3þ/Tb3þ molar ratio varied from 1/4, 1/1, to 4/1 (shown in Table 3), which may be due to energy transfer from Tb3þ to Eu3þ.20,46,49 Moreover, the present lifetimes of emissions ascribed to Tb3þ or Eu3þ are only nanosecond orders, which is greatly smaller than that of some literature (microsecond orders).37,49 The possible reason is that the present as-prepared samples and their samples annealed at 300 C were amorphous
Fig. 10. Decay curves of emissions at 545 and 490 nm of Tb3þ as well as emissions at 593 and 618 nm of Eu3þ for as-prepared ZnAl-1Eu1Tb and its sample annealed at 300 C under the excitation of 365 nm (a) and 405 nm (b).
Fig. 11. Decay curves of emissions at 545 nm of Tb3þ as well as emissions at 593 and 618 nm of Eu3þ for as-prepared ZnAl-4Eu1Tb and its sample annealed at 300 C under the excitation of 365 nm (a) and 405 nm (b).
Please cite this article in press as: Chen Y, et al., Multi-color luminescence in Eu3þ/Tb3þ co-doped ZnAl amorphous materials and their annealed samples, Journal of Rare Earths (2018), https://doi.org/10.1016/j.jre.2018.01.024
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Y. Chen et al. / Journal of Rare Earths xxx (2018) 1e9
Table 3 Lifetimes of the luminescence centers incorporated in as-prepared samples and samples annealed at 300 C (ns). Samples
365 nm excitation
405 nm excitation
Emission due to 5D4/7F5 of Tb3þ Emission due to 5D4/7F6 of Tb3þ Emission due to 5D0/7F2 of Eu3þ Emission due to 5D0/7F1 of Eu3þ As-prepared ZnAl-1Eu4Tb 10.66 As-prepared ZnAl-1Eu1Tb 7.23 As-prepared ZnAl-4Eu1Tb 5.90
6.83 5.42 e
state and may have more defect energy levels that can make the electrons have more opportunity to move from higher energy levels to lower energy levels, accordingly cause shorter lifetimes.49 The photoluminescent property of the present as-prepared samples and their samples annealed at 300 and 900 C is different from that of the previous Eu3þ/Tb3þ co-doped MgAl amorphous phase and their annealed samples as well as other Eu3þ/Tb3þ co-doped phosphors,31,44 which may be due to different host materials. 4. Conclusions The Eu3þ/Tb3þ co-doped ZnAl amorphous solid with different Eu /Tb3þ molar ratios of 1/4, 1/1, and 4/1 were prepared. The asprepared samples and their samples annealed at 300 C are favorable for the Eu3þ and Tb3þ emissions, and can simultaneously emit red and green emissions under favorable excitation. Moreover, a broad emission at 450e650 nm emerges in the ZnAl-1Eu1Tb and ZnAl-4Eu1Tb annealed at 900 C under excitation at 350, 370, and 380 nm. These results provide a practical basis for the design of white light illumination sources by blending the present powders samples with an appropriate UV emitter (340e400 nm), such as UV LEDs. 3þ
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Please cite this article in press as: Chen Y, et al., Multi-color luminescence in Eu3þ/Tb3þ co-doped ZnAl amorphous materials and their annealed samples, Journal of Rare Earths (2018), https://doi.org/10.1016/j.jre.2018.01.024