Eu 2+ activated long persistent strontium aluminate nano scaled phosphor prepared by precipitation method

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1 Journal of Alloys and Compounds ) Eu 2+ activated long persistent strontium aluminate nano scaled phosphor prepared by precipitation method Chengkang Chang, Zhaoxin Yuan, Dali Mao State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiaotong University, 1954 Huashan Road, Shanghai , PR China Received 29 November 2004; accepted 13 April 2005 Available online 11 October 2005 Abstract This paper reports the detailed preparation process of Eu 2+ activated long lasting Sr 4 Al 14 O 25 nano sized phosphor by precipitation method. Scanning electron microscopy SEM), simultaneous DSC TG, X-ray diffraction XRD), photo luminescent spectroscope PLS) and thermal luminescent TL) spectroscope were employed to characterize the phosphor. SrAl 2 O 4 :Eu 2+ Dy 3+ and Sr 4 Al 14 O 25 :Eu 2+ Dy 3+ phosphors in nano scale were obtained by calcinating the precipitated precursor at 1200 and 1300 C, respectively. Both the low-temperature product SrAl 2 O 4 :Eu 2+ Dy 3+ and the high-temperature product Sr 4 Al 14 O 25 :Eu 2+ Dy 3+ showed photoluminescence upon UV illumination with an emission peak at 480 and 505 nm, respectively. When compared to the powder obtained in conventional method, the nano sized powders revealed a blue shift in emission spectrum due to the decrease in grain size. Those two phosphors showed long persistent afterglow, and the Sr 4 Al 14 O 25 :Eu 2+ Dy 3+ phosphor exhibited better afterglow property than SrAl 2 O 4 :Eu 2+ Dy 3+ phosphor due to a deeper trap level and a higher trap concentration formed in the host material Elsevier B.V. All rights reserved. Keywords: Strontium aluminate; Afterglow; Phosphor; Luminescence 1. Introduction Phosphors with long afterglow are the materials that can absorb light energy and then gradually emit visible light after the light source is removed. It has been a long history since people began to study the phosphorescent materials. In the early time, Co and Cu doped zinc sulphide ZnS: Cu, Co) was considered a main kind of phosphorescent materials. However, it can only maintain phosphorescence no more than a few hours and its chemical stability is not satisfying. Therefore, it has been substituted by the strontium aluminates presently, SrAl 2 O 4 [1], for example, since they have excellent properties, such as high quantum efficiency [2], long persistence of phosphorescence and good stability [3,4]. As luminescent materials, the phosphorescent properties are greatly affected by the grain size, when the grain size reaches nano scale, many new properties can be obtained [5]. There have been some methods for preparation of fine powders in nano size, including sol gel method, hydrothermal synthesis, chem- Corresponding author. Tel.: ; fax: address: ckchang@sjtu.edu.cn C. Chang). ical precipitation and so on. Among these methods, chemical precipitation exhibits some advantages, such as low processing temperature, high homogeneity and purity of the products and so on. By this method, nano sized particles that are uniformly distributed could be prepared. In recent years, Sr 4 Al 14 O 25 doped with europium, which was regarded as another excellent phosphor host, has attracted the researches intensely [6,7]. Wang et al. reported the chemical reaction process using solid-state reaction method when preparing Sr 4 Al 14 O 25 :Eu 2+ Dy 3+ phosphor [8]. However, until now, there have been no detailed reports about preparation process and related photoluminescent properties of Sr 4 Al 14 O 25 :Eu 2+ Dy 3+ by chemical precipitation method. This paper reports the process of preparing method of Sr 4 Al 14 O 25 :Eu 2+ Dy 3+ nano scaled phosphor by chemical precipitation. Phase transformation during the calcination process was studied in details as well as the afterglow properties. 2. Experimental procedures AlNO 3 ) 3 9H 2 O, SrNO 3 ) 2, NH 4 ) 2 CO 3, EuNO 3 ) 3 and DyNO 3 ) 3, all in analytical purity, were employed as the starting materials. AlNO 3 ) 3 9H 2 O, SrNO 3 ) 2 and NH 4 ) 2 CO 3 were dissolved in distilled water to form 1 M solutions /$ see front matter 2005 Elsevier B.V. All rights reserved. doi: /j.jallcom

2 C. Chang et al. / Journal of Alloys and Compounds ) while EuNO 3 ) 3 and DyNO 3 ) 3 solutions of 0.1 M were prepared by dissolving the nitrates into distilled water, respectively. One hundred and forty millilitres of the prepared AlNO 3 ) 3 solution, mixed with 38.4 ml SrNO 3 ) 2, 8 ml EuNO 3 ) 3 and 8 ml DyNO 3 ) 3, were stirred under an agitator to form homogeneous nitrate solution according to the nominal composition of Sr 3.84 Eu 0.08 Dy 0.08 Al 14 O 25. The 1 M NH 4 ) 2 CO 3 solution was employed as the precipitating agency. The mixed nitrate solution was dumped into a highly-revolving agitator and the NH 4 ) 2 CO 3 solution was added in droplets to produce a white precursor. An excessive amount of 5% precipitator was added to the mixed nitrate solution to make sure that all the ions were precipitated. The obtained precursor was held on for 24 h and then washed with water and ethanol for several times. After dried at 120 C for 24 h, the final luminescent powders were obtained by calcinating the dried precursor at different temperature from 1200 to 1300 C in a reducing environment of 5%H %N 2. The morphologies of the phosphors were observed by employing scanning electron microscopy SEM). Phase identification of the obtained powders was performed by X-ray diffraction XRD) analysis. The experiments were conducted on a Bruker-Axs D8 Advance X-ray diffractometer running at 40 kv and 40 ma, with a scan speed of 0.2 s 1 and a step of 0.02 using Cu K X-rays. The thermal reaction processing of synthesis was investigated by NETZSCH STA 449C simultaneous DSC TG thermoanalyzer between 30 and 1350 C with a heating rate of 10 C min 1. The photoluminescent properties were investigated by measuring the excitation and emission spectra employing a photoluminescent spectrum LS55B). Afterglow properties were also collected by the instrument after the samples were fully activated under UV light for 10 min. A thermally stimulated spectrometer FJ427-A1) was used to analyze the trap level and the trap concentration. 3. Results and discussion 3.1. Reaction process during the preparation of the luminescent phosphors To investigate the reaction that took place in the process of phosphor preparation, a simultaneous DSC TG test was conducted from 30 to 1350 C, as shown in Fig. 1. Four endothermic peaks and two exothermic peaks are evident in the DSC curve, implying a complicated chemical process during the formation of the phosphors. Theoretically, the precursor mainly consists of SrCO 3 and AlOH) 3 assured according to the following chemical reaction equation: SrNO 3 ) 2 + NH 4 ) 2 CO 3 SrCO 3 + 2NH 4 NO 3 1) 2AlNO 3 ) 3 + 3NH 4 ) 2 CO 3 + 3H 2 O 6NH 4 NO 3 + 3CO 2 + 2AlOH) 3 2) Therefore, the endothermic peaks A and B in DSC curve can be assigned to the dehydration of AlOH) 3 and the other two endothermic peaks C and D can be assigned to the decomposition of SrCO 3, since the dehydration usually occurs at a relatively low-temperature. The TG curve also shows a good accordance with the DSC result. It can be seen from the TG curve that the weight loss WL) after the dehydration is 20.6%, very close to the theoretical value 22.5%. And the weight loss after the decomposition of the carbonate is 10.2%, which is also very close to the theoretical value 10.5%. The phenomena that both the dehydration and decomposition process reveal two endothermic peaks implies that these processes take two steps with a complicate mechanism, which is not the topic of this paper and therefore not discussed here. It is obvious that the TG curve reveals no weight loss after 1100 C, indicating that the two exothermic peaks E and F may result from the chemical reaction between the calcined oxides at high-temperature. A detailed investigation on the phase transition was conducted by XRD. The precursor was calcined at 1200, 1250 and 1300 C, respectively, for 4 h to investigate reaction process during the preparing of Sr 4 Al 14 O 25 phosphor. Fig. 2 shows the XRD patterns of the original precursor and the phosphors obtained at different calcination temperature. The XRD pattern of the precursor as shown in Fig. 2a) only revealed a diffraction pattern of SrCO 3 phase, no diffraction peak from AlOH) 3 was identified, implying that the precipitated AlOH) 3 exists in the precipitate in the form of amorphous phase or with a very low crystallinity that cannot be identified by XRD method. XRD pattern of the obtained phosphor calcined at 1200 C revealed an SrAl 2 O 4 structure, which can be indexed to PDF , rather than the expected orthorhombic Sr 4 Al 14 O 25 phase. The result indicates that the formation of the orthorhombic aluminate could take several steps, and the low-temperature products after the dehydration and decomposition, say Al 2 O 3 and SrO, will react spontaneously to form Fig. 1. DSC and TG profiles of precursors, showing the reaction process during calcinations. Fig. 2. XRD patterns of the precursors calcined at different temperature: a) raw powder, b) 1200 C, c) 1250 C and d) 1300 C, indicating the phase transformation during the calcination.

3 222 C. Chang et al. / Journal of Alloys and Compounds ) the monoclinic SrAl 2 O 4 around 1200 C. When the calcination temperature increased to 1250 C, some peaks of new phases, SrAl 12 O 19 and Sr 4 Al 14 O 25, are identified from the XRD pattern, showing the complex of the phase transition in this temperature region. However, with the calcination temperature increasing to 1300 C, almost all the diffraction peaks can be indexed to the orthorhombic Sr 4 Al 14 O 25 phase when referring to PDF , implying that SrAl 2 O 4 and SrAl 12 O 19 can be viewed as the intermediate phases during the process of Sr 4 Al 14 O 25 :Eu 2+ Dy 3+ preparation. So, it is clear that the synthesis of Sr 4 Al 14 O 25 involves complex reaction process. The precursor undergoes dehydration and decomposition process before 1000 C followed by a solidreaction at higher temperature without weight loss. Sr 4 Al 14 O 25 is obtained as the final product while SrAl 2 O 4 and SrAl 12 O 19 are formed as the intermediate phases. The synthesis process can be expressed by the following chemical reactions: AlOH) 3 SrCO 3 SrO + Al 2 O C Al 2 O 3 + H 2 O 3) C SrO + CO 2 4) SrAl 2 O 4 + 5Al 2 O 3 17SrAl 2 O 4 + 3SrAl 12 O C SrAl 2 O 4 5) 1250 C SrAl 12 O 19 6) 1300 C 5Sr 4 Al 14 O 25 7) 3.2. SEM micrographs of the prepared nano scaled phosphors Although three kinds of strontium aluminates were generated in the experiment, only SrAl 2 O 4 and Sr 4 Al 14 O 25 were reported to exhibit long persistence. Therefore, the phosphors obtained at 1200 and 1300 C were investigated in detail. The reason why the sample calcinated at 1250 C was not studied comes from the fact that the powder consists of several aluminate phases, not a pure material. Fig. 3 shows the micrographs of SrAl 2 O 4 and Sr 4 Al 14 O 25 phosphors with size around 100 nm, illustrating that nano scaled phosphors were obtained by precipitation method. To our surprise, the obtained phosphor exhibits spherical morphology rather than the conventional aggregated morphology, which is very common for powder obtained through precipitation method. The reason why spherical powder was produced is still under instigation and will be discussed elsewhere. It can be seen from the SEM micrograph that the crystal size is about nm for SrAl 2 O 4 and nm for Sr 4 Al 14 O 25, showing an increment of the crystal size with the increase in calcination temperature Luminescent properties of the obtained phosphors The photoluminescent curves of the strontium aluminate phosphors obtained at 1200 and 1300 C are shown in Fig. 4a and b). Curve 1 is the excitation spectrum and curve 2 represents the emission spectrum after excitation at 350 nm. The SrAl 2 O 4 :Eu,Dy phosphor reveals an emission peaking at 505 nm while Sr 4 Al 14 O 25 :Eu,Dy phosphor reveals a peak at 480 nm. When compared to the data reported by the other researchers 520 nm for SrAl 2 O 4 :Eu,Dy [9] and 490 nm for Sr 4 Al 14 O 25 :Eu,Dy [7]), the emission of the nano scaled phosphors exhibit a shift to short wavelength, namely blue shift. The reason why the nano sized SrAl 2 O 4 :Eu 2+ Dy 3+ and Sr 4 Al 14 O 25 :Eu 2+ Dy 3+ phosphors we obtained have shorter emission wavelength than those prepared by solid-reaction method results from the smaller size of phosphor grains. As we know, the luminescence of strontium aluminate phosphor is contributed from the substitution of Eu 2+ for Sr 2+ to form a solid solution phase [10]. The addition of Dy 3+ can make produce electron traps to prolong the afterglow time. When the grain size reaches a nano scale, the atom structure and crystal field around Eu 2+ will be changed due to the increase in surface energy, resulting in the change of luminescent property of the doped Eu 2+ ion that leads to the wavelength shift in emission spectra. On the other hand, the percentage of Eu 2+ on the crystal surface will also increase with the decrease in crystal size. Since the Eu 2+ on the surface is not as densely shielded as the inner Eu 2+, the emission from this center will probably show different emission behavior, and therefore cause the shift in emission peaks Afterglow properties of the nano scaled SrAl 2 O 4 and Sr 4 Al 14 O 25 phosphors The comparisons of luminescent decay curve between the two phosphors obtained by co-precipitation method were shown in Fig. 5. It can be seen from the curves that both phosphors showed a long persistence. The phosphors firstly showed a rapid decay and then long lasting phosphorescence afterward. It was Fig. 3. SEM micrographs of a) SrAl 2 O 4, calcined at 1200 C and b) Sr 4 Al 14 O 25, calcined at 1300 C.

4 C. Chang et al. / Journal of Alloys and Compounds ) Fig. 4. Excitation and emission spectra: a) SrAl 2 O 4 :Eu,Dy and b) Sr 4 Al 14 O 25 :Eu,Dy. Fig. 5. Afterglow property of the nano scaled phosphors. obvious that the materials have different long persistent properties. Both the initial intensity after excitation and the afterglow intensity of the Sr 4 Al 14 O 25 :Eu,Dy phosphors were higher than SrAl 2 O 4 :Eu,Dy phosphor, indicating that the difference in host material probably has great effect on the afterglow property of the obtained phosphors. It is revealed by some authors [11] that the decay file of long lasting phosphors can be expressed by an empirical equation as follows: I = A 1 exp t ) + A 2 exp t ) + A 3 exp t ) 8) τ 1 τ 2 τ 3 The results shown in Table 1 reveal that the values of the decay times of Sr 4 Al 14 O 25 :Eu 2+ Dy 3+ phosphor are all bigger than that of SrAl 2 O 4 :Eu 2+ Dy 3+ phosphor, illustrating that the Sr 4 Al 14 O 25 :Eu 2+ Dy 3+ phosphor hold a better afterglow property than the SrAl 2 O 4 :Eu 2+ Dy 3+ phosphor. A study on thermally stimulated luminescence TSL) for the trap level and trap density within the phosphors, shown in Fig. 6, revealed the important reason why the phosphors showed different afterglow characteristics. The TSL parameters such as thermal activation energy E associated with the trap depth), frequency factor s and order of kinetics l, were obtained by fitting experimental data to the theoretical expression. Generally, the TSL intensity I as a function of temperature T is given by [12,13]: IT ) = sn 0 exp E ) k B T [ l 1)s T exp E ) l/l 1) β T 0 k B T dt + 1] 9) where n 0 is the concentration of trapped charges at t =0, E the energy level of the trap, k B the Boltzmann s constant = JK 1 ) and β is the heating rate = 1 Cs 1 ). The parameters E and n 0 are the most important parameters that where I is the phosphorescence intensity, A 1, A 2 and A 3 the constants, t the time and τ 1, τ 2 and τ 3 are the decay times for the exponential components, respectively. Using the software of Origin 7.0, the values of τ 1, τ 2 and τ 3 of the two phosphors can be obtained, as listed in Table 1. Table 1 Decay times of the phosphors Host materials τ 1 min) τ 2 min) τ 3 min) SrAl 2 O 4 :Eu 2+ Dy Sr 4 Al 14 O 25 :Eu 2+ Dy Fig. 6. TSL curve for nano scaled SrAl 2 O 4 :Eu,Dy and Sr 4 Al 14 O 25 :Eu,Dy phosphors.

5 224 C. Chang et al. / Journal of Alloys and Compounds ) Table 2 TSL parameters of the phosphors Phosphors Glow-peak T m K) Glow-peak shape parameters TSL parameters τ K) δ K) ω K) µ g E ev) n 0 cm 3 ) SrAl 2 O 4 :Eu 2+ Dy Sr 4 Al 14 O 25 :Eu 2+ Dy describe the physical properties of the traps generated by the codoping of the rare earth ion Dy 3+. E and n 0 can be simply calculated from the glow-peak shape parameters by the following equations [13]: kb T 2 ) m E = [ µ g 0.42)] 2k B T m 10) ω ωi m n 0 = 11) β[ µ g 0.42)] where ω, the FWHM, is known as the shape parameter and defined as ω = δ + τ, with δ being the high-temperature half width and τ the low-temperature half width. The asymmetric glowpeak shape is defined by the asymmetry parameter µ g = δ/ω. I m is the TSL intensity of the glow peaks and T m is the temperature of the glow peaks. In all of the parameters shown in Table 2, E and n 0 are the parameters influencing the afterglow characteristics directly. It is obvious that a higher trap concentration will produce a longer afterglow since a larger amount of traps will capture more free electrons generated by the UV illumination. Moreover, another important factor for long lasting phosphor is to produce a suitable energy level within the host. If the energy level is too low, the electron in the trap can return to the energy level of the excited state easily, thus resulting in short afterglow lifespan. On the other hand, if the energy level is too deep, the energy necessary for the electron to return to the excited state level is so difficult to obtain, that the electron cannot return, also resulting in poor afterglow property. According to Sakai s report [14], ev is suitable trap depth for long afterglow. In our case, both the energy level and the concentration of the trap for SrAl 2 O 4 :Eu,Dy and Sr 4 Al 14 O 25 :Eu,Dy phosphors are quite different. The deep trap level and the higher trap concentration are the two main factors that caused the better afterglow property of Sr 4 Al 14 O 25 :Eu,Dy phosphor. 4. Conclusions Long afterglow Sr 4 Al 14 O 25 :Eu 2+ Dy 3+ phosphor in nano scale were synthesized by chemical precipitated method. The chemical process of the synthesis is much more complicated, with the dehydration of AlOH) 3, decomposition of SrCO 3 and the phase transition between the aluminates. Both the low-temperature product SrAl 2 O 4 :Eu 2+ Dy 3+ and the hightemperature product Sr 4 Al 14 O 25 :Eu 2+ Dy 3+ show photoluminescence peaking at 480 and 505 nm, respectively. When compared to the powder obtained in conventional method, the nano sized powders revealed a blue shift in emission spectrum due to the decrease in grain size. Both phosphors showed long persistent afterglow, and the Sr 4 Al 14 O 25 :Eu 2+ Dy 3+ phosphor exhibited better afterglow property than the SrAl 2 O 4 :Eu 2+ Dy 3+ phosphor due to a deeper trap level and a higher trap concentration formed in the host material. Acknowledgements This work was financially supported by Natural Science Foundation of Shanghai contract no. 02ZE14055) and Shanghai Nanotechnology Promotion Center contract no. 0252nm018). References [1] T. Matsuzawa, Y. Aoki, T. Takeuchi, Y. Murayama, J. Electrochem. Soc ) [2] B. Smets, J. Rutten, G. Hoeks, J. Electrochem. Soc ) 1989) [3] P.C. Palilla, A.K. Levine, M.R. Tomkus, J. Electrochem. Soc ) 642. [4] W.Y. Jia, H.B. Yuan, W.M. Yen, J. Lumin ) 424. [5] Z. Tang, F. Zhang, Z. Zhang, C. Huang, Y. Lin, J. Eur. Ceram. Soc ) [6] H. Yamamoto, T. Matsuzawa, J. Lumin ) 287. [7] Y. Lin, Z. Yang, Z. Zhang, Mater. Lett ) 14. [8] M. Wang, D. Wang, G. Lu, Mater. Sci. Eng. B ) 18. [9] T. Katsumata, R. Sakai, S. Komuro, T. Morikawa, H. Kimura, J. Cryst. Growth ) 869. [10] R. Jagannanthan, R.P. Rao, T.R.N. Kutty, Mater. Res. Bull. 27 4) 1992) 459. [11] T. Matasuzawa, T. Nabae, T. Katsumata, J. Electrochem. Soc ) L243. [12] D.W. Cooke, B.L. Bennett, E.H. Farnum, W.L. Hults, Appl. Phys. Lett ) [13] E.W. Forsythe, D.C. Morton, C.W. Tang, Y.L. Gao, Appl. Phys. Lett ) [14] R. Sakai, T. Katsumata, S. Komuro, T. Morikawa, J. Lumin ) 149.