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Laboratory Experiment pubs.acs.org/jchemeduc

Preparing, Characterizing, and Investigating Luminescent Properties of a Series of Long-Lasting Phosphors in a SrO−Al2O3 System: An Integrated and Inquiry-Based Experiment in Solid State Chemistry for the Undergraduate Laboratory Yan-Zi Ma,* Li Jia, Kai-Guo Ma, Hai-Hong Wang, and Xi-Ping Jing Experimental Chemical Center, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: An integrated and inquiry-based experiment on solid state chemistry is applied to an inorganic chemistry lab course to provide insight into the characteristics of the solid phase reaction. In this experiment, students have the opportunity to synthesize long-lasting phosphors with formula xSrO·yAl2O3:Eu2+, Dy3+, by means of precipitation and high temperature methods, and to characterize them by powder X-ray diffractometry. The luminescent properties of the phosphors are also suitable for investigation by student research teams with the assistance of solid fluorescence analysis. With a change in the Sr/Al ratio of strontium aluminates, the phosphors show distinct emission bands, and with a change in the content of Dy in a certain range, the different data of luminescence intensity and decay time can be obtained. By analyzing the crystal structures and photoluminescence mechanism of products, students can better understand abstract concepts of luminescent solid materials, including nonstoichiometric compounds, extrinsic point defects, crystalline fields in the solid state, band levels, and hole and electron transfer. KEYWORDS: Second-Year Undergraduate, Inorganic Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Inquiry-Based/Discovery Learning, Crystal Field/Ligand Field Theory, Fluorescence Spectroscopy, Materials Science, Phases/Phase Transitions/Diagrams, Solid State Chemistry

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occupied by activator ions of Eu2+ and auxiliary activator ions of Dy3+, where the radius of lanthanide ions is similar to that of Sr2+. It is worth noting that, unlike Ln3+, the main electron configuration of the lowest excited state for Eu2+is 4f65d1. There is no 5s25p6 shielding for a 5d electron, and the absorption and emission properties of Eu2+ in substrate are influenced largely by the chemical environment around Eu2+. So, different phases of phosphors using Eu2+ as an activator will exhibit unique luminescent colors under UV light.5 In addition, the auxiliary activator Dy3+ in the substrate does not emit, but will affect, the luminance and the afterglow time of the phosphors, which can be well-explained by the hole transfer model.6 Because of the attractive nature of this kind of inorganic material, the authors of ref 3a have already used them [(Sr/Ca)Al2O4:Eu, Dy] for introducing condensed matter chemistry to high school students and first- and second-year undergraduate level students.3a Compared with the laboratory experiment in ref 3a that focuses on synthesis, this current experiment focuses on an

he chemistry of the solid state is an exciting area of research in inorganic chemistry, with many of its technological applications being challenging to understand.1 In the past few decades, a common view of chemical educators held that solid state chemistry was supposed to be an integral part of the undergraduate curriculum.2 A series of teaching experiments on solid materials with various properties and synthesis techniques were introduced in this Journal.3 However, a dearth of material on teaching experimental solid state chemistry meant that, in many universities worldwide, including ours, students new to inorganic chemistry found basic concepts and theories of solid state chemistry hard to comprehend. To improve this situation, in 2011 we developed a solid state chemistry experiment on long-lasting phosphors of Eu2+and Dy3+ codoped strontium aluminates that has been an integral part of the inorganic chemistry lab curriculum ever since. Eu2+, Dy3+ codoped strontium aluminates are among the most widely used, environmentally friendly commercial longafterglow phosphors. They can absorb the energy of sunlight or artificial light and then gradually emit visible light over 10 h after the light source is removed.4 The long-afterglow phosphorescence is caused by artificial defects in the oxide substrate in which a small portion of lattice sites of Sr2+ are © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: March 20, 2016 Revised: June 1, 2017

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DOI: 10.1021/acs.jchemed.6b00211 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Figure 1. Preparation process of the strontium aluminates doped with lanthanide atoms by precipitation and high temperature methods. Inset: the pictures of the dry white precursor powder and the precursor covered with short carbon rods in an alumina crucible.

integrated research process consisting of three main didactical objectives for students: (i) understanding the key points of soft chemistry methods and high temperature solid phase synthesis; (ii) gaining hands-on experience with conventional instrumentation used to characterize solid materials; (iii) improving the ability to deal with the experimental results. Specifically, in this laboratory experiment, precipitation and high temperature methods are used to prepare the phosphors in order to introduce the concepts of solid state diffusion and phase.7 The precipitation method has the advantages of lower processing temperature, higher homogeneity, less harmful gas generation, and shorter reaction time.8 Furthermore, students can use XRD patterns as a tool to identify the main and impurity phases of their products, and why and how impurities result from the process of preparing phosphors. Students can observe the longlasting phosphorescence by the naked eye and also quantify the phosphorescence intensity and the long decay time of their products by solid fluorescence analysis and the corresponding data processing. On the basis of the same experimental procedure and data analysis, students can attempt to change the raw ratio within a certain range to obtain a different product ratio, allowing structured, inquiry-based learning to be introduced into the experimental laboratory. The instructor does not give the answer to students but instead encourages students to find explanations based on the experimental results by consulting the literature, independent thinking, and team communication. We hope this approach stimulates students to think about the relationship between electronic structures and luminescent properties of long-lasting phosphors. This experiment is taught annually in the spring semester with 14 groups of students (12 students in each group) mainly composed of second-year undergraduate students. The experiment takes 3.5 h, including the precursor preparation (2 h) and the characterization (1.5 h), but not including the wait time of high temperature solid phase synthesis and discussion section (2.5 h). Students work individually, but can freely decide to join with other classmates to form a research team and share their results with each other. Through this integrated and inquirybased research,9 students tend to master experimental techniques of solid state chemistry stemming from real scientific problems and build a deeper understanding of abstract concepts of luminescent solid materials, such as nonstoichiometric compounds, extrinsic point defects, crystalline fields in solid state, band levels, and hole and electron transfer. In this paper, we describe a series of experiments with varying raw ratios of Sr/Al and Dy content. On the basis of the analysis of experimental results, we hope to demonstrate the

depth and scope of solid state chemistry scientific problems available for students to investigate during this experiment.

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EXPERIMENTAL DETAILS Analytical reagent-grade chemicals and deionized water were used throughout. All reagents formed into solutions were prepared by teaching assistants. The preparation process of the phosphors is given in Figure 1. Take the preparation of 0.01 mol of Sr0.96Al2O4:Eu2+0.01, Dy3+0.02 (nonstoichiometric) as an example. Two mixture solutions were prepared. Solution A consisted of 48.0 mL of 0.20 mol L−1 Sr(NO3)2, 50.0 mL of 0.40 mol L−1 Al(NO3)3· 9H2O, 5.0 mL of 0.020 mol L−1 Eu(NO3)3, and 10.0 mL of 0.020 mol L−1 Dy(NO3)3. Solution B consisted of a 50 mL portion of 40 g L−1 (NH4)2C2O4, and 5.5 mL concentrated ammonia. When metal nitrate solution A was stirred and heated to near boiling, the NH4C2O4 base solution B employed as precipitator was added in droplets to produce a white precursor. After adjusting the pH to 7.5, the reaction mixture was vacuum filtered. The filter cake was then transferred into an evaporating dish and added with 10.0 mL of 0.10 mol L−1 H3BO3/EtOH flux solution; the mixture was then mixed to be as homogeneous as possible. The resulting emulsion was then ignited, and the burning alcohol took the moisture away, leaving behind a dry white precursor powder. The powder was transferred into an alumina crucible and covered with high purity carbon rods (graphite) of short size that could produce carbon monoxide, reducing the atmosphere above 700 °C in the closed crucible.10 There were 12 crucibles with the precursors separated into two mullite pots that were then put into the muffle furnace together. The final phosphors were obtained by calcination at 1300 °C for 100 min. After the muffle furnace was cooled to 1000 °C, the mullite pots were taken out to avoid potential oxidation of Eu2+ during the slow cooling process. This high temperature operation was demonstrated by the instructor, and students could observe the typical phenomenon of blackbody radiation. The granular product was ground to a powder that was finer than 120 mesh in an agate mortar. The sample holders for XRD and solid fluorescence were finished with the compaction of the powder. The XRD spectra were recorded with SHIMADZU XRD-6000 powder X-ray diffractometer. The diffraction angle 2θ was scanned from 19.5° to 32.5° by a step-length of 0.01°and scanning rate of 5°/min. Students operated this instrument under the guidance of the instructor and obtained the XRD patterns in class. The emission and fluorescence kinetics spectrum were recorded with a NanoLog (Horiba B




stoichiometries, SrAl12O19, SrAl4O7, Sr4Al14O25, SrAl2O4, and Sr3Al2O6, arranged according to increasing molar ratio of Sr/ Al.14 Due to limitations of the preparation method, we recommend students select their target product from SrAl12O19, Sr4Al14O25, and SrAl2O4. Students select one of the candidates and calculate the amount of nonstoichiometric reagents during the prelab period. In particular, the double precipitator containing C2O42− and OH− was used for preparation of the precursor, which mainly consisted of SrC2O4, Al(OH)3, Eu2(C2O4)3, and Dy2(C2O4)3 determined by the principle of solubility. Thus, it is important for students to control the amount of C2O42− and pH of the solution to avoid impurity generation resulting from incomplete precipitation. After the high-temperature reaction, three phosphors of varying brightness and increasing raw ratio of Sr/Al were obtained, which emitted light varying from purple, to blue, to green under the UV lamp, suggesting the formation of new distinct phases. The crystalline phase structures of these powders were determined by XRD patterns, as shown in Figure 2.

Jobin Yvon) spectrometer. For reduction of the influence of stray light on different sizes of solid particles in quantitative analysis, a large excitation slit width was used. The long-lasting luminescence property of the samples should be taken into consideration, so samples were illuminated for 150 s (saturated excitation) uniformly before scanning the emission spectra.11 Conversely, a narrow excitation slit width was chosen for the fluorescence kinetics spectrum so that the intensity variation could be better observed. The excitation wavelength and emission slit width were determined by the teaching assistant according to the phase information on the students’ samples.

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HAZARDS Lab coats and goggles should be worn throughout the experiment. Reactions involving concentrated ammonia and the burning alcohol must be performed with gloves and in a fume hood. We advise using a dust mask to avoid powder inhalation. To prevent potential carbon monoxide poisoning during the high temperature operation, the muffle furnace needs to be placed in a well-ventilated laboratory or in a fume hood. Potential damage from UV light should mitigated by avoiding exposure to eyes and skin under direct irradiation. In addition, gloves should be worn during thermal operations such as holding the heated beakers or evaporating dishes. For extremely hot crucibles and mullite pots, crucible tongs need to be used. When the instructor demonstrates handling the samples under high temperature, the heat insulation face shield and aluminized heat resistant gloves should be worn, and students must stand at least three meters away or wear high temperature protection.

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RESULTS AND DISCUSSION

Formation of Phosphors and Phase Analysis

Solid state reactions are a typical kind of heterogeneous reaction that only reacts on the solid−solid interface. The rate of solid state reactions depends on the diffusion rate of ions in the lattices of the reactants, which is always extremely slow under the condition of normal heating, especially for the strontium aluminates.12 However, students can complete preparations within the class time provided, for the following reasons: • Using high temperatures accelerates the ions’ diffusion process • Preparing precursors by precipitation improves the homogeneity, which shortens the distance of ion diffusion; in addition, oxalates have good activity during high temperature decomposition • Mixing the fluxing agent H3BO3 in the precursors improves the rate of ion diffusion on the surface of the solids by forming a liquid film between the reactants • Carrying out formation of the new phase and reduction of Eu2+ simultaneously saves time Compared with other methods used for preparing the phosphors,13 this combined method is easier for second-year undergraduate students to accomplish, and is more environmentally friendly because the filtration removes most of the NH4+ and NO3− in the precursor, which would otherwise release significant amounts of harmful gas under high temperature. As the two-component system phase diagram of SrO−Al2O3 shows, strontium aluminates always occur in the five

Figure 2. (a) Representative samples started with a different mole ratio of Sr/Al under the irradiation of UV light (365 nm, 254 nm, 30 W), where sample A started with 1:12 of nSr/nAl, sample B started with 2:7 of nSr/nAl, and sample C started with 1:2 of nSr/nAl. (b) The peak assignments of powder XRD patterns of sample A (above), sample B (middle), and sample C (bottom) determined by standard powder diffraction of α-Al2O3, SrAl12O19, Sr4Al14O25, and SrAl2O4.

Due to time constraints, the XRD patterns of students’ samples were just a part of the whole patterns, ranging from 19.5° to 32.5° scanning within 3 min; however, it was sufficient for students to record only the essential information to confirm the main and the impurity phases. Because the crystalline phase structures of the solids belong to different crystal systems, there are obvious differences between the patterns of the three. Compared with a series of standard powder diffraction patterns of possible strontium aluminates and oxides provided in the Supporting Information, the XRD patterns in Figure 2b show that the main phases of the samples were identified as SrAl12O19:Eu2+, Dy3+ for sample A; Sr4Al14O25:Eu2+, Dy3+ for sample B; and SrAl2O4:Eu2+, Dy3+ for sample C. The impurity C




phases were identified as α-Al2O3, SrAl12O19, and Sr4Al14O25 for sample A, sample B, and sample C, respectively. By comparing the strength of the impurity peaks and main peaks, students can assess their preparation procedure. Using the described preparation method, students could unfortunately not obtain the phase of SrAl4O7 due to the combined effect of the reaction temperature and the heating rate.15 Students also could not obtain the product of bright red phosphors of Sr3Al2O6:Eu2+, Dy3+ under the irradiation, because the ions of Eu3+ in the compact Sr3Al2O6 could not be reduced adequately when using the reducing atmosphere produced by the incomplete combustion of carbon. This could perhaps be performed in the atmosphere of 70% H2/30% N2.5 This part of the experiment is also suitable for a thermodynamics/kinetics course for physical chemistry.

ligands’ bond covalency increases with the reducing number of the electron withdrawing group (AlO4), resulting in the barycenter downshifting of 5d orbital energy level of Eu2+; (ii) the coordination number and bond distance of Sr−O (Eu− O), which determine that the strength of the crystal field around Eu2+, are larger in Al-rich Sr-aluminates, resulting in the enhancement of crystal field splitting.5 The phase diagram and different fluorescence spectra drawn and analysis by instructors above are not given in class. Regarding the crystal structures of the strontium aluminates provided in the Supporting Information, students are encouraged to integrate the data of classmates and summarize in their reports the impact of composition and structures on luminescent behavior of phosphors. In addition, the phosphor doped Dy in substrates SrAl2O4 and Sr4Al14O25 have long-afterglow properties that can be monitored by the fluorescence kinetics spectrum. When the representative samples of Sr0.99−1.5xAl2O4:Eu0.01, Dyx (0 ≤ x ≤ 0.03) were irradiated by UV at 350 nm continuously, the electrons of Eu2+ in the lattice sites of Sr2+ were excited from the ground state (8S7/2, narrow band level) to the excited state (4f65d1, broad band level). Some of the electrons underwent the radiative transition back to the ground state, while the rest were transmitted into the conduction band under thermal perturbation. Because of the formation of the positively charged holes, Dy•Sr, that originated from occupying the lattice sites of Sr2+ by Dy3+, the electrons in the conduction band were easily captured by the defect energy level of Dy•Sr, and electric neutrality defects of Dy×Sr were generated. At the same time, the captive electrons could be released into the conduction band also under the thermal perturbation, and go back to the excited state of Eu2+. This electron/energy storage−release process is demonstrated by the curves of the fluorescence emission signals at 520 nm, which gradually rise until they plateau, as shown in Figure 4a. Meanwhile, Figure 4b shows that, with increasing content of Dy, the fluorescence intensity decreased following the Stern−Volmer plot (I0/I − 1 vs [Q], where [Q] is the content ratio of Dy), suggesting there were more electrons transferring into the trap energy level. Conversely, after switching off the excitation illumination, the fluorescence signals decayed gradually, except for the sudden change in the curve of the sample without Dy3+, as shown in

Solid Fluorescence Analysis of the Phosphors

The emission spectra of three kinds of phosphors are shown in Figure 3. Broadband spectra can be explained by electron

Figure 3. Emission spectra of three kinds of phosphors where Eu2+ ions are in different strontium aluminates. The excitation wavelength (λ ex ) was 275 nm for SrAl 12 O 19 :Eu 2+ , Dy 3+ , 360 nm for Sr4Al14O25:Eu2+, Dy3+, and 350 nm for SrAl2O4:Eu2+, Dy3+.

energy level splitting occurring on the 5d electron of Eu2+. With decreasing content of Al in the series of Sr-aluminates, a red shift of the phosphor’s emission peak can be observed, which is primarily attributed to two factors: (i) the Eu2+ and the anion

Figure 4. (a) Representative data for the fluorescence kinetics spectrum of Sr0.99−1.5xAl2O4:Eu2+0.01, Dy3+x, by UV irradiation at 350 nm for 50 s followed with closing the shutter. Inset: Decay curve of Sr0.945Al2O4:Eu2+0.01, Dy3+0.03 fitted using eq 1. (b) Representative data for emission spectra of Sr0.99−1.5xAl2O4:Eu2+0.01, Dy3+x. Inset: The relationship between the value of I0/Ix − 1 and the content ratio of Dy by linear fitting, where Ix is the fluorescence intensity of Sr0.99−1.5xAl2O4:Eu2+0.01, Dy3+x, at 520 nm after saturated excitation. I0 is the fluorescence intensity of Sr0.99Al2O4:Eu2+0.01 at 520 nm after saturated excitation. D




Figure 4a, exhibiting the releasing process of captive electrons from Dy×Sr. The decay times of the long-lasting phosphors are given in Table 1 by two-term exponential curves fit using eq 116 I = I0 + A1 exp( −t /τ1) + A 2 exp(−t /τ2)

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(1)

Table 1. Decay Times of Representative Student Samples Sample 1 2 3 4

Composition 2+

Sr0.99Al2O4:Eu 0.01 Sr0.975Al2O4:Eu2+0.01, Dy3+0.01 Sr0.96Al2O4:Eu2+0.01, Dy3+0.02 Sr0.945Al2O4:Eu2+0.01, Dy3+0.03

AUTHOR INFORMATION

Corresponding Author

τ1/s

τ2/s

*E-mail: [email protected].

0.2 0.3 0.5 0.8

0.2 7.3 10.3 12.7

ORCID

Yan-Zi Ma: 0000-0003-4447-995X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Shu-Jian Tian and Yan Guan for their pioneering work. In addition, we are grateful to Horiba Jobin Yvon Company for their technical support on fluorescence kinetics spectrum.

I is the phosphorescence intensity at any time t after removing the light source, and A1 and A2 are coefficients. τ1 and τ2 are the decay time for the intrinsic lifetime of Eu2+, and the long decay time of the afterglow, respectively. It can be seen that with increasing content of Dy, longer decay times in two types were acquired, suggesting that more electrons had been stored both in shallow and deeper traps after the hole of Dy•Sr reached saturation.16b For students willing to investigate the impact of Dy content on the kinetics of luminescence, they can make a detailed plan and discuss it as a team during the prelab period, and use other students’ data for processing. Through rationalizing their own discoveries on the hole transfer model mechanism of longlasting luminescence, students will likely better understand important concepts of energy bands, energy levels, and electron transfer.

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REFERENCES

(1) Atkins, P. W.; Overton, T. L.; Rourke, J. P.; Weller, M. T.; Armstrong, F. A. Shriver and Atkins’ Inorganic Chemistry, 5th ed.; Oxford University Press: Oxford, Great Britain, 2010; pp 601. (2) Ellis, A. B. Pimentel Award: Elements of Curriculum Reform: Putting Solids in the Foundation. J. Chem. Educ. 1997, 74 (9), 1033− 1039. (3) (a) Zitoun, D.; Bernaud, L.; Manteghetti, A.; Filhol, J. S. Microwave Synthesis of a Long-Lasting Phosphor. J. Chem. Educ. 2009, 86 (1), 72−75. (b) Leyral, G.; Bernaud, L.; Manteghetti, A.; Filhol, J. S. Microwave Synthesis of a Fluorescent Ruby Powder. J. Chem. Educ. 2013, 90 (10), 1380−1383. (c) Boffa, V.; Yue, Y. Z.; He, W. Sol−Gel Synthesis of a Biotemplated Inorganic Photocatalyst: A Simple Experiment for Introducing Undergraduate Students to Materials Chemistry. J. Chem. Educ. 2012, 89 (11), 1466−1469. (d) Milán, G. A.; Millier, B.; Ritchie, A.; Bryan, C.; Vinette, S.; Wielens, B.; White, M. A. Bismuth Crystals: Preparation and Measurement of Thermal and Electrical Properties. J. Chem. Educ. 2013, 90 (12), 1675−1680. (4) Suriyamurthy, N.; Panigrahi, B. S. Effects of Non-Stoichiometry and Substitution on Photoluminescence and Afterglow Luminescence of Sr4Al14O25:Eu2+, Dy3+ Phosphor. J. Lumin. 2008, 128 (11), 1809− 1814. (5) Dutczak, D.; Jü s tel, T.; Ronda, C.; Meijerink, A. Eu 2+ Luminescence in Strontium Aluminates. Phys. Chem. Chem. Phys. 2015, 17 (23), 15236−15249. (6) (a) Matsuzawa, T.; Aoki, Y.; Takeuchi, N.; Murayama, Y. A New Long Phosphorescent Phosphor with High Brightness SrAl2O4:Eu2+, Dy3+. J. Electrochem. Soc. 1996, 143 (8), 2670−2673. (b) Yamamoto, H.; Matsuzawa, T. Mechanism of Long Phosphorescence of SrAl2O4:Eu2+, Dy3+ and CaAl2O4:Eu2+, Nd3+. J. Lumin. 1997, 72−74, 287−289. (7) (a) McCarthy, G. J. An Experiment in High Temperature Solid State Chemistry. J. Chem. Educ. 1972, 49 (3), 209−211. (b) Cogdell, C. D.; Wayment, D. G.; Casadonte, D. J., Jr.; Kubat-Martin, K. A. A Convenient, One-Step Synthesis of YBaCu3O7‑x Superconductors: An Undergraduate Inorganic/Materials Laboratory Experiment. J. Chem. Educ. 1995, 72 (9), 840−841. (8) Chang, C. K.; Yuan, Z. X.; Mao, D. L. Eu2+ Activated Long Persistent Strontium Aluminate Nano Scaled Phosphor Prepared by Precipitation Method. J. Alloys Compd. 2006, 415 (1−2), 220−224. (9) Hartings, M. R.; Fox, D. M.; Miller, A. E.; Muratore, K. E. A Hybrid Integrated Laboratory and Inquiry-Based Research Experience: Replacing Traditional Laboratory Instruction with a Sustainable Student-Led Research Project. J. Chem. Educ. 2015, 92 (6), 1016− 1023.

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CONCLUSION We designed an integrated and inquiry-based experiment on long-lasting phosphors to guide students in the area of solid state chemistry. In this laboratory experiment, students can prepare three kinds of phosphors by means of precipitation and high temperature methods, with strontium aluminates as substrates. Students can also determine the phase composition, fluorescence intensity, and the duration of the phosphors by powder X-ray diffractometry and fluorescence analysis. Through analysis of typical results by changing the raw ratio of Sr/Al and the content of Dy, the relationship between the electronic structures and the luminescent properties of the phosphors can be revealed, allowing student research teams to explore and summarize these structure−property relationships. By investigating through inquiry-based learning, students can better comprehend many abstract solid state chemistry concepts. Additional influencing factors on formation of the solid phase and luminescent properties of these phosphors, such as the content of Eu, or Dy in wide range, the ratio of Eu/ Dy, and so on, and the link between the two decay times and microscopic phenomena are not conclusive yet; we will continue to investigate them as this lab course continues to develop.

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Student handout and appendix, including reference data on standard powder diffraction, phase diagram, and crystal structures of the strontium aluminates (PDF, DOCX) Material and equipment information (PDF, DOCX)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00211. Instructor notes (PDF, DOCX) E




(10) Ellingham, H. J. T. Reducibility of Oxides and Sulphides in Metallurgical Processes. J. Soc. Chem. Ind. 1944, 63 (5), 125−133. (11) Guan, Y.; Ma, Y. Z.; Jia, L.; Wang, H. H.; Tian, S. J. An Improvement of Spectral Characterization for Long Afterglow Phosphors SrAl2O4:Eu2+, Dy3+. Res. Explor. Lab. 2014, 33 (5), 38−40. (12) (a) Chen, R.; Wang, Y. H.; Hu, Y. H.; Hu, Z. F.; Liu, C. Modification on Luminescent Properties of SrAl2O4:Eu2+, Dy3+ Phosphor by Yb3+ Ions Doping. J. Lumin. 2008, 128 (7), 1180− 1184. (b) Meléndrez, R.; Arellano-Tánori, O.; Pedroza-Montero, M.; Yen, W. M.; Barboza-Flores, M. Temperature Dependence of Persistent Luminescence in β-Irradiated SrAl2O4:Eu2+, Dy3+ Phosphor. J. Lumin. 2009, 129 (12), 679−685. (13) (a) Ishigaki, T.; Mizushina, H.; Uematsu, K.; Matsushita, N.; Yoshimura, M.; Toda, K.; Sato, M. Microwave Synthesis Technique for Long Phosphorescence Phosphor SrAl2O4:Eu2+, Dy3+ Using Carbon Reduction. Mater. Sci. Eng., B 2010, 173 (1−3), 109−112. (b) Qiu, Z. F.; Zhou, Y. Y.; Lü, M. K.; Zhang, A. Y.; Ma, Q. Combustion Synthesis of Long-Persistent Luminescent MAl2O4: Eu2+, R3+ (M = Sr, Ba, Ca, R = Dy, Nd and La) Nanoparticles and Luminescence Mechanism Research. Acta Mater. 2007, 55 (8), 2615−2620. (c) Hsu, C. H.; Lu, C. H. Influence of pH on the Formation and Luminescence Properties of the Sol-Gel Derived SrAl2O4:Eu2+, Dy3+ Phosphors. Adv. Appl. Ceram. 2009, 108 (3), 149−154. (d) Lu, C. H.; Chen, S. Y.; Hsu, C. H. Nanosized Strontium Aluminate Phosphors Prepared via a Reverse Microemulsion Route. Mater. Sci. Eng., B 2007, 140 (3), 218−221. (14) (a) Massazza, F. The System SrO-Al2O3. Chim. Ind. (Milan, Italy) 1959, 41 (2), 108−115. (b) Ye, X. Y.; Zhuang, W. D.; Wang, J. F.; Yuan, W. X.; Qiao, Z. Y. Thermodynamic Description of SrOAl2O3 System and Comparison with Similar Systems. J. Phase Equilib. Diffus. 2007, 28 (4), 362−368. (15) Douy, A.; Capron, M. Crystallisation of Spray-Dried Amorphous Precursors in the SrO−Al2O3 System: A DSC Study. J. Eur. Ceram. Soc. 2003, 23 (12), 2075−2081. (16) (a) Xiao, L. Y.; Xiao, Q.; Liu, Y. L.; Ai, P. F.; Li, Y. D.; Wang, H. J. A Transparent Surface-Crystallized Eu2+, Dy3+ Co-Doped Strontium Aluminate Long-Lasting Phosphorescent Glass-Ceramic. J. Alloys Compd. 2010, 495 (1), 72−75. (b) Chander, H.; Haranath, D.; Shanker, V.; Sharma, P. Synthesis of Nanocrystals of Long Persisting Phosphor by Modified Combustion Technique. J. Cryst. Growth 2004, 271 (1−2), 307−312.

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