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Tb3 + co-doped MgAl-hydroxide salts depending on phase transition
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Tunable light emission of amorphous Eu3 +/Tb3 + co-doped MgAl-hydroxide salts depending on phase transition Yufeng Chen⁎, Kunlei Zhang, Hongqin Wang, Xiaodan Ren, Xiaoqing Wang College of Chemistry, Nanchang University, Nanchang 330031, China
A R T I C L E I N F O
A B S T R A C T
Keywords: MgAl-hydroxide salt Eu3 + and Tb3 + emissions Tunable light emissions
Two of Eu3 +/Tb3 + co-doped amorphous Mg-Al double hydroxide salts, with Eu3 +/Tb3 + molar ratios of 1/4 and 4/1, were prepared and their photoluminescence dependence on phase transition has been investigated. Our results revealed that the co-doped amorphous samples exhibited excellent green emissions due to 5D4 → 7FJ (J = 5, 6) transitions of Tb3 + under the excitation of 350, 370 and 380 nm, and red emissions due to 5D0 → 7FJ (J = 1, 2) transitions of Eu3 + under the excitation of 395 and 468 nm. Moreover, the emission intensity is dependent on the Eu3 +/Tb3 + molar ratio. When the amorphous Eu3 +/Tb3 + co-doped samples were annealed at 200 and 300 °C, the phase transition was unobservable. After the samples were annealed at 500 °C, the samples began to transform. Further, the multi-crystalline phases formed after the samples were annealed at 700 and 900 °C. Moreover, the emissions attributed to Eu3 + and Tb3 + incorporated in the amorphous host are greatly stronger than that of in the multi-crystalline host. These results indicated the amorphous phase was more favorable for the emissions of Eu3 + and Tb3 + compared with that of the multi-crystalline phases.
1. Introduction Recently, studies of white-light-emitting materials are on the rise because they have potential applications in the lighting field. Some potential white-light-emitting materials, such as inorganic multi-crystalline powders [1–4], organic polymers [5–8], and inorganic-organic hybrids [9,10], lanthanide–organic complexes [11–13], etc. have been paid attention. In general, organic polymers, inorganic-organic hybrids, and lanthanide–organic complexes are the ideal candidates to use as white-light-emitting materials due to their high photoluminescence efficiency, long-lived luminescence, and various emission spectra [14,15], but their thermal stability is not well compared with that of the inorganic materials. Therefore, the inorganic white-light-emitting materials are still attractive. At present, most of studies on inorganic white-light-emitting materials are mainly focused on multi-crystalline powders [16–28], and very limit study involves inorganic glass or amorphous solid [29–31]. However, some research results revealed that the inorganic glasses or amorphous solid have better luminescent property than that of some inorganic multi-crystalline materials [32–34]. For the reason, the present work is to prepare amorphous codoped Eu3 +/Tb3 + Mg-Al-hydroxide salts and to investigate its photoluminescence depending on phase transition. Our results suggested that the amorphous as-prepared co-doped Eu3 +/Tb3 + Mg-Al-hydroxide salts exhibited excellent tunable emissions range from green to red
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emissions, and the present Eu3 + and Tb3 + emissions in the amorphous phase are better than those of in the multi-crystalline phase. These results indicated that the multicolor light-emitting amorphous co-doped Eu3 +/Tb3 + Mg-Al-hydroxide salt would be potential application in white-light-emitting LEDs. 2. Experimental procedure The analytical reagents Mg(NO3)2·6H2O, Al(NO3)3·9H2O, Eu2O3, Tb2O3 were purchased from Sinopharm Group Co. Ltd. of China. All source materials were used without further purification. Eu2O3 and Tb2O3 with Eu3 +/Tb3 + molar ratio of 1/4 and 4/1 were dissolved in concentrated HNO3 and H2O2 mixed solution, respectively, and formed into Eu3 +/Tb3 + mixed solution. A certain amounts of Mg(NO3)2·6H2O and Al(NO3)3·9H2O with Mg2 +/Al3 + molar ratio of 2/0.5 are dissolved in above Eu3 +/Tb3 + mixed solution, putting in ultrapure water and keeping molar ratios of M2 +/M3 + = 2/1, Al3 +/(Eu3 + + Tb3 +) = 0.5/0.5 and Eu3 +/Tb3 + = 1/4 or 4/1, vigorously stirred. Next, concentrated ammonia water was dropped to above system, continuously stirred, and slurry formed (pH = 8–9). After being aged at 40 °C for 2 h, filtrated, washed, and dried at 70 °C for 12 h, the Eu3/ Tb3 + co-doped Mg-Al hydroxide salts with Eu3/Tb3 + molar ratios of 1/ 4 and 4/1 were obtained, respectively (labeled as MgAl-1Eu4Tb and MgAl-4Eu1Tb). Then the as-prepared MgAl-1Eu4Tb (2.0 g) and MgAl-
Corresponding author. E-mail address: [email protected] (Y. Chen).
http://dx.doi.org/10.1016/j.jnoncrysol.2017.09.054 Received 7 August 2017; Received in revised form 13 September 2017; Accepted 28 September 2017 0022-3093/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Chen, Y., Journal of Non-Crystalline Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.09.054
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Table 1 Chemical composition of the as-prepared MgAl-1Eu4Tb and MgAl-4Eu1Tb. Samples
MgAl-1Eu4Tb
MgAl-4Eu1Tb
a b c
Content (wt%) for found (Cald)a
Content (wt%)b
Content (wt%)c
Mg
Al
Eu
H
O
16.18 (16.19) Chemical formula 15.86 (15.84) Chemical formula
4.26 (4.25)
5.39 20.43 2.03 4.48 (5.37) (20.43) (1.99) (4.50) Mg2.1Al0.49(Eu0.11Tb0.40)(OH)6.2NO3
4.58 (4.59)
18.74 6.37 2.04 4.65 47.76 (18.75) (6.36) (1.99) (4.67) (47.80) Mg1.98Al0.51(Eu0.37Tb0.12)(OH)5.96NO3
Tb
N
47.23 (47.27)
Analyzed by ICP. Analyzed by CHN elemental analyses. Obtained by difference with subtraction.
Fig. 1. XRD patterns of MgAl-1Eu4Tb and its samples annealed at various temperatures.
Fig. 2. XRD patterns of MgAl-4Eu1Tb and its samples annealed at various temperatures.
Fig. 3. SEM images of (a) as-prepared MgAl-1Eu4Tb and its samples annealed at (b) 500 °C, and (c) 900 °C.
4Eu1Tb (2.0 g) were annealed at 200, 300, 500, 700, and 900 °C for 2 h in air, respectively. X-Ray powder diffraction (XRD) patterns were obtained using a XD2/XD-3 diffractometer (Cu-Ka, Beijing Puxi Tongyong Yiqi Ltd. China). All the samples were scanned in the 2θ range of 5–60° at a scan rate of 2°/min. Compositional analyses were done by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and CHN Elemental Analyzer (Elementar Vario EL II). Samples for scanning electron microscopy (SEM, JSM6701F) measurement were prepared by pasting powder samples with an electric tape, then flatted and coated with a Pt film. Next, the coated samples were subject to the SEM measurement.
The photoluminescence (excitation and emission spectra) was characterized by F-7000 FL spectrophotometer, attached to a phosphorimeter equipped with a Xe-arc lamp (450 W) as the excitation source. All the measurements were carried out at room temperature. 3. Results Basing on the results of ICP and CHN elemental analyses, along with
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Fig. 5. IR spectra of the as-prepared MgAl-1Eu4Tb and its samples annealed at various temperatures (I) as well as the as-prepared MgAl-4Eu1Tb and its samples annealed at various temperatures (II).
respectively, no reflections appeared, suggesting these annealed samples are still amorphous. As temperature increased to 500 °C, several extremely weak reflections occurred, indicating phase transition began. With temperature up to 700 °C, some reflections attributed to Mg-Al-O magnesium aluminium oxide phase [PDF# 10-0238] emerged, and the phase is monoclinic with cell parameters of a = 9.305, b = 5.64, c = 12.0 Å, and β = 110.8°. As temperature rose up to 900 °C, the reflections attributed to Mg-Al-O oxide disappeared, and some reflections ascribed to tetragonal MgAl26O40 phase [PDF#20–0660] and cubic Tb2O3 phase (signed as *) [PDF#65-3180] were found. With regard to the MgAl-4Eu1Tb (seen in Fig. 2), below 500 °C, no reflections were observable. Then Mg-Al-O magnesium aluminium oxide emerged with temperature up to 500 °C, which was different from that of the MgAl1Eu4Tb. As temperature went up to 700 °C, some reflection ascribed to monoclinic Eu2O3 phase [PDF#34-0072] and orthorhombic TbO1.75 (signed as ▼) [PDF#35-0102] were observed. Further the tetragonal MgAl26O40 phase [PDF#20-0660] and cubic Tb2O3 phase (signed as *) [PDF#65-3180] appeared with temperature rising to 900 °C. SEM images of the as-prepared MgAl-1Eu4Tb, as-prepared MgAl4Eu1Tb, and their annealing samples displayed in Figs. 3 and 4. The as-
Fig. 4. SEM images of (a) as-prepared MgAl-4Eu1Tb and its samples annealed at (b) 500 °C, and (c) 900 °C.
charge balance principle, the chemical formulas of the as-prepared MgAl1Eu4Tb and MgAl-4Eu1Tb are estimated to be Mg2.1Al0.49(Eu0.11Tb0.40) (OH)6.2NO3 and Mg1.98Al0.51(Eu0.37Tb0.12)(OH)5.96NO3, respectively (seen in Table 1). X-Ray diffraction patterns of the as-prepared MgAl1Eu4Tb and its annealed samples display in Fig. 1. As shown in Fig. 1, the as-prepared MgAl-1Eu4Tb exhibited amorphous state. After it was thermal treatment at 200 and 300 °C in air atmosphere for 2 h,
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set at 545 and 620 nm. With the favorable excitation wavelengths of 350, 370, 380, 395, and 468 nm, the emission spectra of the as-prepared MgAl-1Eu4Tb and its samples annealed at various temperatures are given in Fig. 7. The intensity of emissions due to 5D4 → 7FJ(J = 5, 6) transitions of Tb3 + and 5D0 → 7FJ(J = 1, 2) transitions of Eu3 + depending on annealing temperature was given Fig. 8. The excitation spectra of the as-prepared MgAl-4Eu1Tb and its samples annealed at various temperatures are shown in Fig. 9. Under the excitation of favorable wavelengths of 350, 370, 380, 395, and 468 nm, the emission spectra of the as-prepared MgAl-4Eu1Tb and its samples annealed various temperatures are represented in Fig. 10. The intensity of emissions due to 5D4 → 7FJ(J = 5, 6) transitions of Tb3 + and 5 D0 → 7FJ(J = 1, 2) transitions of Eu3 + depending on annealing temperature was given Fig. 11. 4. Discussion Compositional analyses revealed that all the elemental contents calculated from the chemical formula and found values from measurement were similar (seen in Table 1). Meanwhile, the M2 +/M3 + and Eu3 +/Tb3 + molar ratios present in the as-prepared MgAl-1Eu4Tb and MgAl-4Eu1Tb were close to that of their initial reactants, suggesting the reasonable chemical formula. In addition, the larger errors for the H and O elements between the calculated and found values may be due to physically adsorbed water. Structural analysis indicated that the asMgAl-1Eu4Tb and MgAl-4Eu1Tb present amorphous. After they were annealed at 200 and 300 °C for 2 h, respectively, they were still amorphous. As temperature increased to 500 °C, amorphous phase transforming to multi-crystalline phase began. With temperature up to 700 and 900 °C, different multi-crystalline phases formed. The excitation spectrum of the as-prepared MgAl-1Eu4Tb showed two different excitation bands, namely the typical excitation band monitored at 545 nm of Tb3 + and excitation band minored at 620 nm f Eu3 + (shown in Fig. 6). The excitation spectrum monitored at 545 nm due to 5D4 → 7F5 transition of Tb3 + displayed strong bands centered at 319, 342, 353, 370, and 380 nm, which were associated with transitions from the 7F6 ground state to the 5D2, 5D4, 5L9, 5L10, and 5G6 excited states of Tb3 +, respectively [30,41,42]. The excitation band was similar to that of the Tb3 +-doped LDHs or other Eu3 +/Tb3 +-codoped materials [36,43–45]. While the excitation spectrum monitored at 620 nm (where in Tb3 + does not emit) due to5D1 → 7F2 transition of Eu3 + exhibited well-known excitation peaks associated with the transitions of 7F0 → 5D4 (360 nm), 7F0 → 5 L7 (375 nm), 7F0 → 5G2 (382 nm), 7F0 → 5 L6(395 nm), and 7F0 → 5D2 (468 nm) [22,29]. The strong excitation peaks for the as-prepared MgAl-1Eu4Tb were found at 353, 370, and 380 nm monitored at 545 nm of Tb3 +, along with 395 and 468 nm monitored at 620 nm of Eu3 +. Moreover, the excitation peak attributed to Tb3 + is greatly stronger than that of the Eu3 + since more Tb3 + ions were included in the as-prepared sample compared with that of the Eu3 +. After the as-prepared MgAl-1Eu4Tb was thermally treated at 200 °C for 2 h, the excitation peaks attributed to Tb3 + markedly decreased and the excitation peaks ascribed to Eu3 + greatly enhanced, which may be due to energy transfer from Tb3 + to Eu3 + [42,45–50]. As the temperature rose to 300 °C, the excitation bands monitoring at both 545 and 620 nm were weaken. Above 300 °C, the excitation bands were unobservable, which was possibly due to phase transformation. The emission spectrum of the as-prepared MgAl-1Eu4Tb exhibited typical strong emissions owed to 5D4 → 7FJ (J = 5, 6) transitions of Tb3 + under the excitation of 350, 370, and 380 nm, and strong emissions ascribed to 5D0 → 7FJ (J = 1, 2) transitions of Eu3 + under the excitation of 395 and 468 nm at which Tb3 + did not emit (seen in Fig. 7). What is more, the photoluminescence of Tb3 + and Eu3 +
Fig. 6. Excitation spectra of the as-prepared MgAl-1Eu4Tb and its samples annealed at various temperatures obtained by monitoring the Tb3 + emission at 545 nm and Eu3 + emission at 620 nm.
prepared samples exhibited tabular shape which was similar to that of the previous Eu-doped MgAl-Eu LDH and Tb-doped MgAl-LDH [35,36]. After they were annealed at 500 °C for 2 h, the tabular samples fused together and changed into fuse link. With temperature up to 900 °C, the fuse link was broken and emerged small bulky. These evidences further supported that both the as-prepared MgAl-1Eu4Tb and MgAl-4Eu1Tb happened phase transition. The IR spectra of the as-prepared MgAl1Eu4Tb, as-prepared MgAl-4Eu1Tb, and their annealing samples are given in Fig. 5. There were similar spectral profiles present in the MgAl1Eu4Tb and MgAl-4Eu1Tb. A strong broad band at 3438 cm− 1 ascribed to OeH stretching mode of physically adsorbed water, interlayer water and hydroxyl of layers, along with a sharp peak at 1635 cm− 1 owed to OeH bend mode [37]. The band at 1386 cm− 1 was ascribed to stretching vibrations of NO3− [38,39]. The extremely weak bands at 840 and 709 cm− 1 were due to stretching vibration of MeO and MeOeM, respectively [40]. After thermal treatment at 500 and 900 °C for 2 h, respectively, the band at 1386 cm− 1 obviously reduced, which is due to the removal of some NO3−. The rest of other bands, including 3438, 1635, 840, and 709 cm− 1, have no much changes. This is because that some physically adsorbed water is unavoidable during measuring, and the MeO and MeOeM bond were not broken during thermal treatment. These results are in agreement with the XRD results of samples. Fig. 6 shows the excitation spectra obtained for the as-prepared MgAl-1Eu4Tb and its annealing samples when the detection has been
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Fig. 7. Emission spectra of the as-prepared MgAl-1Eu4Tb and its samples annealed at various temperatures.
370 nm and 5D0 → 7FJ (J = 1, 2) transitions of Eu3 + under the excitation of 395 nm dependence on annealing temperatures are showed in Fig. 8. According to Fig. 8, the Tb3 + emission is stronger than that of the Eu3 + due to more Tb3 + and less Eu3 + incorporated in the asprepared MgAl-1Eu4Tb. After annealing at 200 °C, the energy transfer from Tb3 + to Eu3 + led to the increase in the Tb3 + emission and the decrease in the Eu3 + emission. As temperature increased to 300 °C, the highest peak intensity of Tb3 + and Eu3 + with favorable excitation wavelength has different changes, which may be due to nature of lanthanide ions. Up to 500, 700, and 900 °C, the emissions attributed to Tb3 + and Eu3 + ions almost quenched, which may be owing to phase
incorporated in the amorphous Mg-Al hydroxide was independent and not interactional. In addition, the Tb3 + emissions were obviously stronger than that of the Eu3 + because moreTb3 + and less Eu3 + involved in the as-prepared sample. When the as-prepared MgAl-1Eu4Tb was thermal treatment at 200 °C for 2 h, the Tb3 + emissions markedly diminished and the Eu3 + emissions greatly enhanced, owing to the energy transfer from Tb3 + to Eu3 + [42,45–50]. With temperature up to 300 °C, the Eu3 + and Tb3 + emissions are weaker than that of the asprepared sample. Above 300 °C, the emissions almost vanished, which was possibly owing to phase transition. The intensity of emissions owed to 5D4 → 7FJ (J = 5, 6) transitions of Tb3 + under the excitation of
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Fig. 8. The intensity of emissions due to 5D4 → 7FJ(J = 5, 6) of Tb3 + under the excitation of 370 nm and 5D0 → 7FJ(J = 1, 2) of Eu3 + under the excitation of 395 nm depending on annealing temperature for the MgAl-1Eu4Tb.
Fig. 9. Excitation spectra of the as-prepared MgAl-4Eu1Tb and its samples annealed at various temperatures.
transition. According to the XRD results of the as-prepared MgAl1Eu4Tb and its annealed samples, amorphous phase appeared between room temperature and 300 °C. At 500 °C, some extremely weak reflections began to emerge. Above 500 °C, multi-crystalline phases formed. These results indicated that the amorphous phase was more favorable for the photoluminescence of Eu3 + and Tb3 + compared with that of the multi-crystalline phase. With regard to the as-prepared MgAl-4Eu1Tb, different photoluminescent property was found. The excitation spectrum of the asprepared MgAl-4Eu1Tb showed extremely weak excitation band monitored at 545 nm attributed to 5D4 → 7F5 transition of Tb3 + and strong excitation band monitored at 620 nm ascribed to 5D0 → 7F2 transition of Eu3 + (seen in Fig. 9) because more Eu3 + ions were incorporated in host than that of the Tb3 +. The strong bands centered at 360, 375, 382, 395, and 468 nm were ascribed to 7F0 → 5D4, 7F0 → 5 L7, 7F0 → 5G2, 7 F0 → 5 L6, and 7F0 → 5D2 transitions of Eu3 +, respectively [29,38]. While the as-prepared MgAl-4Eu1Tb was thermal treatment at 200 °C for 2 h, the excitation bands attributed to Tb3 + markedly enhanced and the excitation bands ascribed to Eu3 + greatly decreased, which was contrary to that of the MgAl-1Eu4Tb annealed at 200 °C. This abnormal phenomenon is likely due to the changes of the chemical surroundings of Tb3 + and Eu3 +. As temperature rose to 300 °C, the excitation peak attributed to Tb3 + greatly reduced and the excitation peak ascribed to Eu3 + markedly enhanced, owing to energy transfer from Tb3 + to Eu3 + [45–50]. Up to 500, 700, and 900 °C, the excitation peaks attributed to
Tb3 + and Eu3 + disappeared due to phase transition. As expected, the extremely weak Tb3 + emissions and strong Eu3 + emissions were observable for the as-prepared MgAl-4Eu1Tb because more Eu3 + and less Tb3 + were doped in the host (seen in Fig. 10). After thermal treatment at 200 °C, the Tb3 + emissions markedly enhanced and the Eu3 + emissions greatly decreased, likely due to phase transition which changed the micro chemical surroundings of Tb3 + and Eu3 +..Up to 300 °C, the Tb3 + emissions markedly decreased and the Eu3 + emissions greatly increased since energy transferred from Tb3 + to Eu3 + [42,45–50]. Going up to 500, 700, and 900 °C, both the Tb3 + and Eu3 + emissions were unobservable. In view of the XRD results of the asprepared MgAl-4Eu1Tb and its annealed samples, the amorphous phases were present in the temperature range from room temperature to 300 °C. With temperature up to 500, 700, and 900 °C, the multicrystalline phases emerged. These results further supported that the amorphous host phase was more favorable for the emissions of Eu3 + and Tb3 + ions than that of the multi-crystalline phase. In addition, the photoluminescence of the as-prepared MgAl-1Eu4Tb and its samples annealed at 200, 300 °C is different from that of the as-prepared MgAl4Eu1Tb and its samples annealed at 200, 300 °C, which was possibly due to the different Eu3 +/Tb3 + molar ratio and micro chemical surroundings of Eu3 + and Tb3 + ions. As it is well known, the Tb3 + emission attributed to 5D4-7F5 transition and the Eu3 + emission ascribed to 5D0-7F2 transition are hypersensitive to their chemical environments (or hosts). Meanwhile the Tb3 + and Eu3 + emissions were
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Fig. 10. Emission spectra of the as-prepared MgAl-4Eu1Tb and its samples annealed at various temperatures.
depending on their contents (namely Eu3 +/Tb3 + molar ratio) [35,36]. Therefore, it is important to study the emissions of samples dependence on the Eu3 +/Tb3 + molar ratios and phase transition in order to define the optimal Eu3 +/Tb3 + molar ratio and their favorable host to obtain excellent multi-color phosphors.
emissions attributed to Tb3 + or Eu3 + were observable in multi-crystalline host, including MgAl26O40 spinel structure and Mg-Al-O metastable intermediate phase of the spinel structure. These results indicate that some amorphous phases are favorable for the emissions of Tb3 + and Eu3 + and the amorphous Eu3 +/Tb3 + co-doped MgAl hydroxide salt may be potential application in luminescence field.
5. Conclusions Acknowledgments
Amorphous Eu3 +/Tb3 + co-doped Mg-Al double hydroxide salts, with different Eu3 +/Tb3 + molar ratios, were prepared. Photoluminescence dependence on host structure has been investigated. Excellent tunable emissions ascribed to Eu3 + and Tb3 + present in the amorphous hosts. No
The Project supported by National Natural Science Foundation of China (No. 51162021).
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[13]
[14]
[15] [16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26] Fig. 11. The intensity of emissions due to 5D4 → 7FJ(J = 5, 6) of Tb3 + under the excitation of 370 nm and 5D0 → 7FJ(J = 1, 2) of Eu3 + under the excitation of 395 nm depending on annealing temperature for the MgAl-4Eu1Tb.
[27]
[28]
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Tunable light emission of amorphous Eu3 +/Tb3 + co-doped MgAl-hydroxide salts depending on phase transition Yufeng Chen⁎, Kunlei Zhang, Hongqin Wang, Xiaodan Ren, Xiaoqing Wang College of Chemistry, Nanchang University, Nanchang 330031, China
A R T I C L E I N F O
A B S T R A C T
Keywords: MgAl-hydroxide salt Eu3 + and Tb3 + emissions Tunable light emissions
Two of Eu3 +/Tb3 + co-doped amorphous Mg-Al double hydroxide salts, with Eu3 +/Tb3 + molar ratios of 1/4 and 4/1, were prepared and their photoluminescence dependence on phase transition has been investigated. Our results revealed that the co-doped amorphous samples exhibited excellent green emissions due to 5D4 → 7FJ (J = 5, 6) transitions of Tb3 + under the excitation of 350, 370 and 380 nm, and red emissions due to 5D0 → 7FJ (J = 1, 2) transitions of Eu3 + under the excitation of 395 and 468 nm. Moreover, the emission intensity is dependent on the Eu3 +/Tb3 + molar ratio. When the amorphous Eu3 +/Tb3 + co-doped samples were annealed at 200 and 300 °C, the phase transition was unobservable. After the samples were annealed at 500 °C, the samples began to transform. Further, the multi-crystalline phases formed after the samples were annealed at 700 and 900 °C. Moreover, the emissions attributed to Eu3 + and Tb3 + incorporated in the amorphous host are greatly stronger than that of in the multi-crystalline host. These results indicated the amorphous phase was more favorable for the emissions of Eu3 + and Tb3 + compared with that of the multi-crystalline phases.
1. Introduction Recently, studies of white-light-emitting materials are on the rise because they have potential applications in the lighting field. Some potential white-light-emitting materials, such as inorganic multi-crystalline powders [1–4], organic polymers [5–8], and inorganic-organic hybrids [9,10], lanthanide–organic complexes [11–13], etc. have been paid attention. In general, organic polymers, inorganic-organic hybrids, and lanthanide–organic complexes are the ideal candidates to use as white-light-emitting materials due to their high photoluminescence efficiency, long-lived luminescence, and various emission spectra [14,15], but their thermal stability is not well compared with that of the inorganic materials. Therefore, the inorganic white-light-emitting materials are still attractive. At present, most of studies on inorganic white-light-emitting materials are mainly focused on multi-crystalline powders [16–28], and very limit study involves inorganic glass or amorphous solid [29–31]. However, some research results revealed that the inorganic glasses or amorphous solid have better luminescent property than that of some inorganic multi-crystalline materials [32–34]. For the reason, the present work is to prepare amorphous codoped Eu3 +/Tb3 + Mg-Al-hydroxide salts and to investigate its photoluminescence depending on phase transition. Our results suggested that the amorphous as-prepared co-doped Eu3 +/Tb3 + Mg-Al-hydroxide salts exhibited excellent tunable emissions range from green to red
⁎
emissions, and the present Eu3 + and Tb3 + emissions in the amorphous phase are better than those of in the multi-crystalline phase. These results indicated that the multicolor light-emitting amorphous co-doped Eu3 +/Tb3 + Mg-Al-hydroxide salt would be potential application in white-light-emitting LEDs. 2. Experimental procedure The analytical reagents Mg(NO3)2·6H2O, Al(NO3)3·9H2O, Eu2O3, Tb2O3 were purchased from Sinopharm Group Co. Ltd. of China. All source materials were used without further purification. Eu2O3 and Tb2O3 with Eu3 +/Tb3 + molar ratio of 1/4 and 4/1 were dissolved in concentrated HNO3 and H2O2 mixed solution, respectively, and formed into Eu3 +/Tb3 + mixed solution. A certain amounts of Mg(NO3)2·6H2O and Al(NO3)3·9H2O with Mg2 +/Al3 + molar ratio of 2/0.5 are dissolved in above Eu3 +/Tb3 + mixed solution, putting in ultrapure water and keeping molar ratios of M2 +/M3 + = 2/1, Al3 +/(Eu3 + + Tb3 +) = 0.5/0.5 and Eu3 +/Tb3 + = 1/4 or 4/1, vigorously stirred. Next, concentrated ammonia water was dropped to above system, continuously stirred, and slurry formed (pH = 8–9). After being aged at 40 °C for 2 h, filtrated, washed, and dried at 70 °C for 12 h, the Eu3/ Tb3 + co-doped Mg-Al hydroxide salts with Eu3/Tb3 + molar ratios of 1/ 4 and 4/1 were obtained, respectively (labeled as MgAl-1Eu4Tb and MgAl-4Eu1Tb). Then the as-prepared MgAl-1Eu4Tb (2.0 g) and MgAl-
Corresponding author. E-mail address: [email protected] (Y. Chen).
http://dx.doi.org/10.1016/j.jnoncrysol.2017.09.054 Received 7 August 2017; Received in revised form 13 September 2017; Accepted 28 September 2017 0022-3093/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Chen, Y., Journal of Non-Crystalline Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.09.054
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Table 1 Chemical composition of the as-prepared MgAl-1Eu4Tb and MgAl-4Eu1Tb. Samples
MgAl-1Eu4Tb
MgAl-4Eu1Tb
a b c
Content (wt%) for found (Cald)a
Content (wt%)b
Content (wt%)c
Mg
Al
Eu
H
O
16.18 (16.19) Chemical formula 15.86 (15.84) Chemical formula
4.26 (4.25)
5.39 20.43 2.03 4.48 (5.37) (20.43) (1.99) (4.50) Mg2.1Al0.49(Eu0.11Tb0.40)(OH)6.2NO3
4.58 (4.59)
18.74 6.37 2.04 4.65 47.76 (18.75) (6.36) (1.99) (4.67) (47.80) Mg1.98Al0.51(Eu0.37Tb0.12)(OH)5.96NO3
Tb
N
47.23 (47.27)
Analyzed by ICP. Analyzed by CHN elemental analyses. Obtained by difference with subtraction.
Fig. 1. XRD patterns of MgAl-1Eu4Tb and its samples annealed at various temperatures.
Fig. 2. XRD patterns of MgAl-4Eu1Tb and its samples annealed at various temperatures.
Fig. 3. SEM images of (a) as-prepared MgAl-1Eu4Tb and its samples annealed at (b) 500 °C, and (c) 900 °C.
4Eu1Tb (2.0 g) were annealed at 200, 300, 500, 700, and 900 °C for 2 h in air, respectively. X-Ray powder diffraction (XRD) patterns were obtained using a XD2/XD-3 diffractometer (Cu-Ka, Beijing Puxi Tongyong Yiqi Ltd. China). All the samples were scanned in the 2θ range of 5–60° at a scan rate of 2°/min. Compositional analyses were done by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and CHN Elemental Analyzer (Elementar Vario EL II). Samples for scanning electron microscopy (SEM, JSM6701F) measurement were prepared by pasting powder samples with an electric tape, then flatted and coated with a Pt film. Next, the coated samples were subject to the SEM measurement.
The photoluminescence (excitation and emission spectra) was characterized by F-7000 FL spectrophotometer, attached to a phosphorimeter equipped with a Xe-arc lamp (450 W) as the excitation source. All the measurements were carried out at room temperature. 3. Results Basing on the results of ICP and CHN elemental analyses, along with
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Fig. 5. IR spectra of the as-prepared MgAl-1Eu4Tb and its samples annealed at various temperatures (I) as well as the as-prepared MgAl-4Eu1Tb and its samples annealed at various temperatures (II).
respectively, no reflections appeared, suggesting these annealed samples are still amorphous. As temperature increased to 500 °C, several extremely weak reflections occurred, indicating phase transition began. With temperature up to 700 °C, some reflections attributed to Mg-Al-O magnesium aluminium oxide phase [PDF# 10-0238] emerged, and the phase is monoclinic with cell parameters of a = 9.305, b = 5.64, c = 12.0 Å, and β = 110.8°. As temperature rose up to 900 °C, the reflections attributed to Mg-Al-O oxide disappeared, and some reflections ascribed to tetragonal MgAl26O40 phase [PDF#20–0660] and cubic Tb2O3 phase (signed as *) [PDF#65-3180] were found. With regard to the MgAl-4Eu1Tb (seen in Fig. 2), below 500 °C, no reflections were observable. Then Mg-Al-O magnesium aluminium oxide emerged with temperature up to 500 °C, which was different from that of the MgAl1Eu4Tb. As temperature went up to 700 °C, some reflection ascribed to monoclinic Eu2O3 phase [PDF#34-0072] and orthorhombic TbO1.75 (signed as ▼) [PDF#35-0102] were observed. Further the tetragonal MgAl26O40 phase [PDF#20-0660] and cubic Tb2O3 phase (signed as *) [PDF#65-3180] appeared with temperature rising to 900 °C. SEM images of the as-prepared MgAl-1Eu4Tb, as-prepared MgAl4Eu1Tb, and their annealing samples displayed in Figs. 3 and 4. The as-
Fig. 4. SEM images of (a) as-prepared MgAl-4Eu1Tb and its samples annealed at (b) 500 °C, and (c) 900 °C.
charge balance principle, the chemical formulas of the as-prepared MgAl1Eu4Tb and MgAl-4Eu1Tb are estimated to be Mg2.1Al0.49(Eu0.11Tb0.40) (OH)6.2NO3 and Mg1.98Al0.51(Eu0.37Tb0.12)(OH)5.96NO3, respectively (seen in Table 1). X-Ray diffraction patterns of the as-prepared MgAl1Eu4Tb and its annealed samples display in Fig. 1. As shown in Fig. 1, the as-prepared MgAl-1Eu4Tb exhibited amorphous state. After it was thermal treatment at 200 and 300 °C in air atmosphere for 2 h,
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set at 545 and 620 nm. With the favorable excitation wavelengths of 350, 370, 380, 395, and 468 nm, the emission spectra of the as-prepared MgAl-1Eu4Tb and its samples annealed at various temperatures are given in Fig. 7. The intensity of emissions due to 5D4 → 7FJ(J = 5, 6) transitions of Tb3 + and 5D0 → 7FJ(J = 1, 2) transitions of Eu3 + depending on annealing temperature was given Fig. 8. The excitation spectra of the as-prepared MgAl-4Eu1Tb and its samples annealed at various temperatures are shown in Fig. 9. Under the excitation of favorable wavelengths of 350, 370, 380, 395, and 468 nm, the emission spectra of the as-prepared MgAl-4Eu1Tb and its samples annealed various temperatures are represented in Fig. 10. The intensity of emissions due to 5D4 → 7FJ(J = 5, 6) transitions of Tb3 + and 5 D0 → 7FJ(J = 1, 2) transitions of Eu3 + depending on annealing temperature was given Fig. 11. 4. Discussion Compositional analyses revealed that all the elemental contents calculated from the chemical formula and found values from measurement were similar (seen in Table 1). Meanwhile, the M2 +/M3 + and Eu3 +/Tb3 + molar ratios present in the as-prepared MgAl-1Eu4Tb and MgAl-4Eu1Tb were close to that of their initial reactants, suggesting the reasonable chemical formula. In addition, the larger errors for the H and O elements between the calculated and found values may be due to physically adsorbed water. Structural analysis indicated that the asMgAl-1Eu4Tb and MgAl-4Eu1Tb present amorphous. After they were annealed at 200 and 300 °C for 2 h, respectively, they were still amorphous. As temperature increased to 500 °C, amorphous phase transforming to multi-crystalline phase began. With temperature up to 700 and 900 °C, different multi-crystalline phases formed. The excitation spectrum of the as-prepared MgAl-1Eu4Tb showed two different excitation bands, namely the typical excitation band monitored at 545 nm of Tb3 + and excitation band minored at 620 nm f Eu3 + (shown in Fig. 6). The excitation spectrum monitored at 545 nm due to 5D4 → 7F5 transition of Tb3 + displayed strong bands centered at 319, 342, 353, 370, and 380 nm, which were associated with transitions from the 7F6 ground state to the 5D2, 5D4, 5L9, 5L10, and 5G6 excited states of Tb3 +, respectively [30,41,42]. The excitation band was similar to that of the Tb3 +-doped LDHs or other Eu3 +/Tb3 +-codoped materials [36,43–45]. While the excitation spectrum monitored at 620 nm (where in Tb3 + does not emit) due to5D1 → 7F2 transition of Eu3 + exhibited well-known excitation peaks associated with the transitions of 7F0 → 5D4 (360 nm), 7F0 → 5 L7 (375 nm), 7F0 → 5G2 (382 nm), 7F0 → 5 L6(395 nm), and 7F0 → 5D2 (468 nm) [22,29]. The strong excitation peaks for the as-prepared MgAl-1Eu4Tb were found at 353, 370, and 380 nm monitored at 545 nm of Tb3 +, along with 395 and 468 nm monitored at 620 nm of Eu3 +. Moreover, the excitation peak attributed to Tb3 + is greatly stronger than that of the Eu3 + since more Tb3 + ions were included in the as-prepared sample compared with that of the Eu3 +. After the as-prepared MgAl-1Eu4Tb was thermally treated at 200 °C for 2 h, the excitation peaks attributed to Tb3 + markedly decreased and the excitation peaks ascribed to Eu3 + greatly enhanced, which may be due to energy transfer from Tb3 + to Eu3 + [42,45–50]. As the temperature rose to 300 °C, the excitation bands monitoring at both 545 and 620 nm were weaken. Above 300 °C, the excitation bands were unobservable, which was possibly due to phase transformation. The emission spectrum of the as-prepared MgAl-1Eu4Tb exhibited typical strong emissions owed to 5D4 → 7FJ (J = 5, 6) transitions of Tb3 + under the excitation of 350, 370, and 380 nm, and strong emissions ascribed to 5D0 → 7FJ (J = 1, 2) transitions of Eu3 + under the excitation of 395 and 468 nm at which Tb3 + did not emit (seen in Fig. 7). What is more, the photoluminescence of Tb3 + and Eu3 +
Fig. 6. Excitation spectra of the as-prepared MgAl-1Eu4Tb and its samples annealed at various temperatures obtained by monitoring the Tb3 + emission at 545 nm and Eu3 + emission at 620 nm.
prepared samples exhibited tabular shape which was similar to that of the previous Eu-doped MgAl-Eu LDH and Tb-doped MgAl-LDH [35,36]. After they were annealed at 500 °C for 2 h, the tabular samples fused together and changed into fuse link. With temperature up to 900 °C, the fuse link was broken and emerged small bulky. These evidences further supported that both the as-prepared MgAl-1Eu4Tb and MgAl-4Eu1Tb happened phase transition. The IR spectra of the as-prepared MgAl1Eu4Tb, as-prepared MgAl-4Eu1Tb, and their annealing samples are given in Fig. 5. There were similar spectral profiles present in the MgAl1Eu4Tb and MgAl-4Eu1Tb. A strong broad band at 3438 cm− 1 ascribed to OeH stretching mode of physically adsorbed water, interlayer water and hydroxyl of layers, along with a sharp peak at 1635 cm− 1 owed to OeH bend mode [37]. The band at 1386 cm− 1 was ascribed to stretching vibrations of NO3− [38,39]. The extremely weak bands at 840 and 709 cm− 1 were due to stretching vibration of MeO and MeOeM, respectively [40]. After thermal treatment at 500 and 900 °C for 2 h, respectively, the band at 1386 cm− 1 obviously reduced, which is due to the removal of some NO3−. The rest of other bands, including 3438, 1635, 840, and 709 cm− 1, have no much changes. This is because that some physically adsorbed water is unavoidable during measuring, and the MeO and MeOeM bond were not broken during thermal treatment. These results are in agreement with the XRD results of samples. Fig. 6 shows the excitation spectra obtained for the as-prepared MgAl-1Eu4Tb and its annealing samples when the detection has been
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Fig. 7. Emission spectra of the as-prepared MgAl-1Eu4Tb and its samples annealed at various temperatures.
370 nm and 5D0 → 7FJ (J = 1, 2) transitions of Eu3 + under the excitation of 395 nm dependence on annealing temperatures are showed in Fig. 8. According to Fig. 8, the Tb3 + emission is stronger than that of the Eu3 + due to more Tb3 + and less Eu3 + incorporated in the asprepared MgAl-1Eu4Tb. After annealing at 200 °C, the energy transfer from Tb3 + to Eu3 + led to the increase in the Tb3 + emission and the decrease in the Eu3 + emission. As temperature increased to 300 °C, the highest peak intensity of Tb3 + and Eu3 + with favorable excitation wavelength has different changes, which may be due to nature of lanthanide ions. Up to 500, 700, and 900 °C, the emissions attributed to Tb3 + and Eu3 + ions almost quenched, which may be owing to phase
incorporated in the amorphous Mg-Al hydroxide was independent and not interactional. In addition, the Tb3 + emissions were obviously stronger than that of the Eu3 + because moreTb3 + and less Eu3 + involved in the as-prepared sample. When the as-prepared MgAl-1Eu4Tb was thermal treatment at 200 °C for 2 h, the Tb3 + emissions markedly diminished and the Eu3 + emissions greatly enhanced, owing to the energy transfer from Tb3 + to Eu3 + [42,45–50]. With temperature up to 300 °C, the Eu3 + and Tb3 + emissions are weaker than that of the asprepared sample. Above 300 °C, the emissions almost vanished, which was possibly owing to phase transition. The intensity of emissions owed to 5D4 → 7FJ (J = 5, 6) transitions of Tb3 + under the excitation of
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Fig. 8. The intensity of emissions due to 5D4 → 7FJ(J = 5, 6) of Tb3 + under the excitation of 370 nm and 5D0 → 7FJ(J = 1, 2) of Eu3 + under the excitation of 395 nm depending on annealing temperature for the MgAl-1Eu4Tb.
Fig. 9. Excitation spectra of the as-prepared MgAl-4Eu1Tb and its samples annealed at various temperatures.
transition. According to the XRD results of the as-prepared MgAl1Eu4Tb and its annealed samples, amorphous phase appeared between room temperature and 300 °C. At 500 °C, some extremely weak reflections began to emerge. Above 500 °C, multi-crystalline phases formed. These results indicated that the amorphous phase was more favorable for the photoluminescence of Eu3 + and Tb3 + compared with that of the multi-crystalline phase. With regard to the as-prepared MgAl-4Eu1Tb, different photoluminescent property was found. The excitation spectrum of the asprepared MgAl-4Eu1Tb showed extremely weak excitation band monitored at 545 nm attributed to 5D4 → 7F5 transition of Tb3 + and strong excitation band monitored at 620 nm ascribed to 5D0 → 7F2 transition of Eu3 + (seen in Fig. 9) because more Eu3 + ions were incorporated in host than that of the Tb3 +. The strong bands centered at 360, 375, 382, 395, and 468 nm were ascribed to 7F0 → 5D4, 7F0 → 5 L7, 7F0 → 5G2, 7 F0 → 5 L6, and 7F0 → 5D2 transitions of Eu3 +, respectively [29,38]. While the as-prepared MgAl-4Eu1Tb was thermal treatment at 200 °C for 2 h, the excitation bands attributed to Tb3 + markedly enhanced and the excitation bands ascribed to Eu3 + greatly decreased, which was contrary to that of the MgAl-1Eu4Tb annealed at 200 °C. This abnormal phenomenon is likely due to the changes of the chemical surroundings of Tb3 + and Eu3 +. As temperature rose to 300 °C, the excitation peak attributed to Tb3 + greatly reduced and the excitation peak ascribed to Eu3 + markedly enhanced, owing to energy transfer from Tb3 + to Eu3 + [45–50]. Up to 500, 700, and 900 °C, the excitation peaks attributed to
Tb3 + and Eu3 + disappeared due to phase transition. As expected, the extremely weak Tb3 + emissions and strong Eu3 + emissions were observable for the as-prepared MgAl-4Eu1Tb because more Eu3 + and less Tb3 + were doped in the host (seen in Fig. 10). After thermal treatment at 200 °C, the Tb3 + emissions markedly enhanced and the Eu3 + emissions greatly decreased, likely due to phase transition which changed the micro chemical surroundings of Tb3 + and Eu3 +..Up to 300 °C, the Tb3 + emissions markedly decreased and the Eu3 + emissions greatly increased since energy transferred from Tb3 + to Eu3 + [42,45–50]. Going up to 500, 700, and 900 °C, both the Tb3 + and Eu3 + emissions were unobservable. In view of the XRD results of the asprepared MgAl-4Eu1Tb and its annealed samples, the amorphous phases were present in the temperature range from room temperature to 300 °C. With temperature up to 500, 700, and 900 °C, the multicrystalline phases emerged. These results further supported that the amorphous host phase was more favorable for the emissions of Eu3 + and Tb3 + ions than that of the multi-crystalline phase. In addition, the photoluminescence of the as-prepared MgAl-1Eu4Tb and its samples annealed at 200, 300 °C is different from that of the as-prepared MgAl4Eu1Tb and its samples annealed at 200, 300 °C, which was possibly due to the different Eu3 +/Tb3 + molar ratio and micro chemical surroundings of Eu3 + and Tb3 + ions. As it is well known, the Tb3 + emission attributed to 5D4-7F5 transition and the Eu3 + emission ascribed to 5D0-7F2 transition are hypersensitive to their chemical environments (or hosts). Meanwhile the Tb3 + and Eu3 + emissions were
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Fig. 10. Emission spectra of the as-prepared MgAl-4Eu1Tb and its samples annealed at various temperatures.
depending on their contents (namely Eu3 +/Tb3 + molar ratio) [35,36]. Therefore, it is important to study the emissions of samples dependence on the Eu3 +/Tb3 + molar ratios and phase transition in order to define the optimal Eu3 +/Tb3 + molar ratio and their favorable host to obtain excellent multi-color phosphors.
emissions attributed to Tb3 + or Eu3 + were observable in multi-crystalline host, including MgAl26O40 spinel structure and Mg-Al-O metastable intermediate phase of the spinel structure. These results indicate that some amorphous phases are favorable for the emissions of Tb3 + and Eu3 + and the amorphous Eu3 +/Tb3 + co-doped MgAl hydroxide salt may be potential application in luminescence field.
5. Conclusions Acknowledgments
Amorphous Eu3 +/Tb3 + co-doped Mg-Al double hydroxide salts, with different Eu3 +/Tb3 + molar ratios, were prepared. Photoluminescence dependence on host structure has been investigated. Excellent tunable emissions ascribed to Eu3 + and Tb3 + present in the amorphous hosts. No
The Project supported by National Natural Science Foundation of China (No. 51162021).
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[26] Fig. 11. The intensity of emissions due to 5D4 → 7FJ(J = 5, 6) of Tb3 + under the excitation of 370 nm and 5D0 → 7FJ(J = 1, 2) of Eu3 + under the excitation of 395 nm depending on annealing temperature for the MgAl-4Eu1Tb.
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