White-Light Ba13Al22Si10O66:Eu Phosphor with

Journal of The Electrochemical Society, 157 共8兲 J271-J274 共2010兲

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0013-4651/2010/157共8兲/J271/4/$28.00 © The Electrochemical Society

White-Light Ba13Al22Si10O66:Eu Phosphor with Bluish Long Afterglow via Spontaneous Reduction of Eu3+ Yezhou Li,a Yuhua Wang,a,b,z Zhaofeng Wang,a Xuhui Xu,a Yanqin Li,a and Yu Gonga a Department of Materials Science, School of Physical Science and Technology, and bKey Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, People’s Republic of China

A white-emitting light associated with bluish long afterglow phosphor, Ba13Al22Si10O66 共BASO兲:Eu, was synthesized by conventional solid-state reaction in air condition. The luminescent properties of the phosphor, including the emission and afterglow spectra, afterglow decay, and thermoluminescence curves, are depicted. Under 365 nm excitation, the intense characteristic 5d–4f emission of Eu2+ was observed, denoting that spontaneous reduction of Eu3+ to Eu2+ occurred in the host lattice. After removing the irradiation source, the phosphor showed a bluish afterglow with the optimum durative time of 43.5 min, owing to the trapping level at 332 K. The possible mechanism of this long-lasting phosphorescence is proposed and discussed. All the results indicate that BASO:Eu prepared in air could be a promising long-lasting phosphor. © 2010 The Electrochemical Society. 关DOI: 10.1149/1.3436647兴 All rights reserved. Manuscript submitted January 6, 2010; revised manuscript received April 26, 2010. Published June 3, 2010.

Long-persistent phosphor is a special category of luminescent materials, which continues to emit light in darkness for a long time even after removal of the excitation source. In virtue of this intrinsic merit, they could be implemented in various fields, such as safety indication, emergency lighting, road signs, billboards, graphic arts, and interior decoration. At present, the long-lasting phosphorescence has been observed from Eu2+-doped alkaline earth aluminates as well as Ce3+, Tb3+, Pr3+, Tm3+, Ti4+, and Mn2+ in other various oxysalt materials.1 Therein, the best performers of long afterglow were alkaline earth aluminates doped with Eu2+ and rare-earth R3+ ions, MAl2O4:Eu2+,R3+ 共M = Ca, Sr, and Ba兲,2-4 which remain visible well over hours when irradiated by UV light and without radioactive raw materials. Presently, a consuming incentive was dedicated to aluminosilicate on account of its long afterglow lifetime, high luminosity, and great chemical stability.5,6 In the early studies of persistent luminescence, Eu2+ ions were used frequently in various host lattices because it was assumed to act both as a luminescent center and as an electron trap.7 Generally speaking, the acquirement of Eu2+ is realized in reducing atmosphere 共H2 /N2, Ar/N2, CO, etc.兲 because the starting material of the europium source is Eu2O3 as a rule. However, if Eu3+ can be reduced to Eu2+ in air condition, it could consumedly decrease the cost and enhance the security during the fabrication process. Up to now, the spontaneous reduction of Eu3+ without reducing atmosphere has been realized in some limited host compounds, such as sulfates 共BaSO4:Eu兲, phosphates 关Ba3共PO4兲2:Eu兴, borophosphates MBPO5:Eu 共M = Ca, Sr, Ba兲, borates 共SrB4O7:Eu, SrB6O10:Eu, Sr2B5O9Cl:Eu, and BaB8O13:Eu兲, and aluminates 共Sr4Al14O25:Eu and BaAl2O4:Eu兲.8-13 However, few reports were focused on aluminosilicate compounds except MAl2Si2O8 共M = Ca, Sr, Ba兲, in which the spontaneous europium reduction could be found in air at high temperature.14 Hence, it is one purpose of this work to find more such aluminosilicate compounds. Because Ba13Al22Si10O66 共BASO兲 is viewed as a derivative of the BaAl2O4 structure, its crystal was built upon a tridimensional framework with 32 corner-shared 关TO4兴 tetrahedra 共T = Al, Si兲 per unit cell, forming hexagonal and rectangular six-tetrahedral ring cavities with Ba in 7-, 8-, and 9-fold coordination.15 On the basis of the aforementioned clues, there could be a great possibility to generate long afterglow in BASO by making use of the spontaneous reduction of Eu3+. Therefore, in this paper, we first synthesize BASO:Eu series in air condition and explore the reduction phenomenon of Eu3+

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→ Eu2+ in these host compounds. Furthermore, the afterglow properties of the air-prepared samples are characterized and discussed. Experimental Synthesis.— Barium carbonate 共BaCO3, 99.0%兲, aluminum oxide 共Al2O3, 98.9%兲, silicic acid 共H2SiO3, 99.5%兲, and europium oxide 共Eu2O3, 99.99%兲 were used as raw materials. The silica excess compositions with the Al/Si molar ratio of 1.83 共22/12.0兲 were adopted because the single phase of Ba13Al22Si10O66 only formed under a Si-excess deviation from the stoichiometric ratio of Al and Si when Eu was undoped and slightly doped.15 All the other raw materials were weighed according to the stoichiometric amounts. The reagents with the composition of BASO:xEu 共x = 0.1, 0.25, 0.5, 0.75, 1 mol %兲 were, respectively, commixed in an agate mortar with an appropriate amount of ethanol and then sintered at 1425°C in air condition for 4 h. Characterization.— The crystal phase of the obtained materials was determined by powder X-ray diffraction 共XRD, Rigaku D/MAX-2400 X-ray diffractometer兲 with Ni-filtered Cu K␣ radiation at a scanning step of 0.02° in the 2␪ range from 10 to 80°. The emission spectra were measured by an FLS920T fluorescence spectrophotometer. The decay curve was measured with a PR305 long afterglow instrument after the sample was irradiated by artificial light 共1000 ⫾ 5% lx兲 for 10 min. The thermoluminescence 共TL兲 curve was measured on an FJ-427A TL meter 共Beijing Nuclear Instrument Factory兲. The sample weight was kept constant 共0.002 g兲. Before the measurements, the powder sample was first exposed for 20 min to standard artificial daylight 共1000 lx兲, then kept in a dark room for 3 h, and finally heated from room temperature to 673 K at a rate of 1 K/s. Results and Discussion Figure 1 presents the XRD patterns of the Eu-doped BASO series prepared in air condition at the Al/Si ratio of 1.83. It was mentioned by Denis et al.15 that the single phase of Ba13Al22Si10O66 only formed under a Si-excess deviation from the stoichiometric ratio of Al and Si when Eu was undoped and slightly doped. This is because the replacement of 关AlO4兴5+ by a 关SiO4兴4+ tetrahedron generates cationic vacancies, which excludes the long-range Si/Alordering model but a disordered one. To ascertain the suitable Al/Si molar ratio for the particles with Eu content lower than 1%, we prepared the particles at different Al/Si ratios: 1.91 共22/11.5兲, 1.83 共22/12兲, and 1.76 共22/12.5兲. However, the single phase of BASO is only formed at the ratio value of 1.83, which affirmed the thesis given by Denis et al. All the diffraction peaks of the doped samples could be well indexed to the simulated XRD pattern of

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Figure 1. 共Color online兲 XRD patterns of the air-prepared BASO:Eu series at the Al/Si ratio of 1.83.

Ba13Al22Si10O66 with a hexagonal symmetry, and no obvious impurity phase was detected. Hence, it could be inferred that the doping of Eu did not cause the breakage of the lattice structure. The photoluminescence 共PL兲 spectrum of the representative BASO:0.75% Eu sample prepared in air is expressed by Fig. 2. Upon excitation with 365 nm UV light, a broad intense peak in the range of 370–700 nm is observed. The spectral line nearly covered the whole visible region, combining to a bright white light visible by the naked eye, as shown in the inset of Fig. 2. Because the blank sample of BASO does not produce any light and no trail of line spectrum of Eu3+ appeared, the broad emission band must be due to the typical 4f65d1共t2g兲–4f7共 8S7/2兲 transition of Eu2+. Therefore, phenomenally, Eu3+ has been spontaneously reduced to Eu2+ when sintered in air condition. When those Eu2+ ions are incorporated into the crystal lattice of BASO, they may substitute at all cationic sites Ba2+, Al3+, and Si4+. Considering their respective ionic radii and allowed oxygencoordination number, it is difficult for Eu2+ to substitute for Al3+ or Si4+.16 Thus, it is evident and reasonable to deduce that Eu2+ ions only substitute for Ba2+ ions. The emission spectra of a

Figure 2. 共Color online兲 PL spectrum 共blue solid line兲 and the fitting of the Gaussian curves 共red dashed lines兲 of the BASO:0.75 mol % Eu sample. The inset in the top right corner is the photograph of its luminescence under UV lamp with 365 nm excitation.

Figure 3. 共Color online兲 Afterglow spectra of BASO:0.75 mol % Eu sample at different intervals after removing the irradiation source. The inset in the top right corner is the photograph of its phosphorescence at the time of 30 s.

BASO:0.75% Eu sample could be fitted by three Gaussian curves positioned at 2.92 共426 nm兲, 2.64 共471 nm兲, and 2.29 eV 共543 nm兲, as shown by the red dashed line in Fig. 2. This illuminates that Eu2+ is distributed over the three different Ba2+ sites 共8-, 7-, and 9-coordinated Ba sites兲, corresponding to blue 关Ba共8兲 + Ba共7兲兴 and green 关Ba 共9兲兴 parts in the PL spectrum, respectively.15 The other finding in this work is that bluish long afterglow is presented by BASO:Eu samples prepared in air. After switching off the UV irradiation of 365 nm at different intervals 共30 s and 1, 3, 5, and 10 min兲, the persistent luminescent spectra of the typical BASO:0.75 mol % Eu are illustrated in Fig. 3. The emission peak of the phosphorescence locates at 433 nm and fades gradually with the extension of time. Meanwhile, the shapes of the afterglow curves are distinctly not consistent with the PL recorded in Fig. 2. Compared with the intensity of the green emission part, the blue part becomes dominant. Thus, the integrated spectra exhibit slight bluish white light for the eye, as shown in the inset noted 30 s after the irradiation. This phenomenon hints that the 7- and 8-coordinated Ba sites have the principal contribution to engender the afterglow in airprepared BASO:Eu. After irradiation by artificial light 共1000 ⫾ 5% lx兲 for 10 min, the air-prepared series with different content of Eu showed a persistent luminescent phenomenon. Figure 4 characterizes the afterglow

Figure 4. 共Color online兲 The afterglow decay curves of the air-prepared BASO:xEu 共x = 0.1, 0.25, 0.5, 0.75, 1 mol %兲 series.



Figure 5. TL curve of the typical air-prepared BASO:0.75 mol % Eu sample.

decay curves of the five samples. Every one of the five decay patterns consists of a fast decay and a consequent slow decay with a long decay tail, implying the existence of various trap depths. From the figure, the optimum persistent time is 43.5 min; that is, the BASO:0.75 mol % Eu sample possesses the best long-lasting property. Generally, as we know, lattice defects as trapping centers play a significant role in the formation of persistent luminescence.17 To probe into the defect states in air-prepared BASO:Eu2+, the TL technique was carried out because of its sensitivity to the trapping property of defects, as represented in Fig. 5. The majority of the curve distribution is presented despite the measurement limitation. One asymmetric peak at 332 K could be found in the TL curve, confirming that various trapping centers are formed in the sample. The traps corresponding to the TL in the range of 323–383 K are the most important ones responsible for the afterglow process at room temperature.3 Hence, the air-prepared BASO:Eu is an appropriate phosphor to engender long afterglow. We now consider the process of the spontaneous reduction and the phosphorescence. The reduction of Eu3+ to Eu2+ in air-prepared BASO could be elucidated by the charge compensation model proposed by Peng et al.18 As trivalent Eu ions are introduced, they would replace the divalent Ba2+ ions. To maintain the charge balance, two Eu3+ ions would be required to substitute for three Ba2+ ions 共the total charge value of two trivalent Eu ions is equal to that of three Ba2+ ions兲. Meanwhile, one vacancy defect of V⬙Ba with two negative charges and two positive defects of Eu•Ba would be generated by each substitution of every two Eu3+ ions in the compound. The vacancy V⬙Ba would represent a donor of electrons, and the two Eu•Ba defects act as acceptors of the electrons. As a result, by thermal stimulation, the negative charges in the vacancy defects of V⬙Ba would be transferred to the Eu3+ sites and make Eu3+ reduce to Eu2+. The whole process can be expressed by the following equations • ⬙ + 2EuBa 3Ba2+ + 2Eu3+ → VBa x ⬙ → VBa VBa + 2e • x + 2e → 2EuBa 2EuBa

In general, the compounds in which the spontaneous reduction of Eu3+ to Eu2+ occurs in air condition possess some features in common. Four terms are prerequisite to realize the reduction process:18 共i兲 No oxidizing ions exist in the host compounds; 共ii兲 the divalent cations are replaced by the dopant trivalent Eu3+ ion in the host; 共iii兲 the substituted cation has a similar radius to the divalent Eu2+ ion;

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and 共iv兲 the host compound has an appropriate crystal structure, which is composed of the tetrahedral anion groups 共BO4, SO4, PO4, SiO4, or AlO4兲. The case of BASO completely accords with the four terms mentioned above. Besides that, the tetrahedral framework structure of BASO is formed from interlinked O-sharing AlO4 and SiO4 tetrahedra, consisting of hexagonal and rectangular sixtetrahedral ring cavities, and this makes BASO a rigid tridimensional network structure. The cavities of channels are occupied by Ba2+ ions. As Eu3+ ions are substituted for Ba2+ ions and reduced to Eu2+, they are closely surrounded by AlO4 and SiO4 tetrahedra. This could effectively shield the impact of oxygen on Eu2+ ions, so Eu2+ can retain the divalent state in air even at high temperature. Theoretically speaking, the inbeing of the traps and the mechanisms for them to capture energy are intricate, and the correlative details are largely indecipherable. The most acceptable explanation for long-lasting phosphorescence in the crystals is a model involving electron–hole recombination at trap centers, which emerge over the thermally activated process.19 At room temperature, the electrons 共or holes兲 captured at superficial traps in the host lattice could be easily released by thermal energy and subsequently recombined with holes 共or electrons兲 captured by deep traps. The released energy derived from the recombination of electrons and holes transfers to the Eu2+ distributed over the Ba sites, resulting in the characteristic emission of Eu2+. Considering the defects introduced by the spontaneous reduction process mentioned above, we suggest that the electron and hole traps in air-prepared BASO may be a part of the positive de•• and Eu•Ba兲 and negative Ba vacancies 共V⬙Ba兲, respectively. fects 共VO Meanwhile, the holes produced at Eu2+ in BASO are assumed to be self-trapped at AlO4 tetrahedra, the same as AlO4 tetrahedra in Ca2Al2SiO7 and Eu2+ in Ba2SiO4 and Ba3SiO5.20,21 However, further studies utilizing different experimental spectroscopic and other techniques are necessary to clarify the details of the proposed mechanism. Besides, why the Eu2+ ions in lower coordinated Ba sites impact the lasting time still needs to be excogitated. Conclusion Ba13Al22Si10O66:Eu phosphor has been successfully prepared by solid-state reaction in air condition. Because the broad emission of the Eu2+ characteristic transition was detected, the spontaneous reduction of Eu3+ → Eu2+ was proved to occur in this host lattice, which has been detailedly interpreted by a charge compensation model. The air-prepared samples exhibited white light under UV excitation and exhibited bluish long afterglow, which lasted about 43.5 min after removing the irradiation source. The findings could diminish the waste energy and enrich the color of existing longlasting phosphors. Acknowledgment This work is supported by the National Natural Science Foundation of China 共no. 10874061兲, the Research Fund for the Doctoral Program of Higher Education 共no. 200807300010兲, and the National Science Foundation for Distinguished Young Scholars 共no. 50925206兲. Lanzhou University assisted in meeting the publication costs of this article.

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