Preparation and characterization of Sr4Al2O7:Eu3+, Eu2+ phosphors

Materials Science and Engineering B 176 (2011) 1521–1525

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Short communication

Preparation and characterization of Sr4 Al2 O7 :Eu3+ , Eu2+ phosphors Sue Jin Kim a , Hyung Il Won a,∗ , Nersisyan Hayk a , Chang Whan Won a , Duk Young Jeon b , Artavazd G. Kirakosyan b a b

RASOM, Chungnam National University, Yuseong, Daejeon 305-764, South Korea Department of Materials Science and Engineering, KAIST, Yuseong, Daejeon, South Korea

a r t i c l e

i n f o

Article history: Received 16 March 2011 Received in revised form 30 June 2011 Accepted 4 September 2011 Available online 17 September 2011 Keywords: Phosphor Sr4 Al2 O7 Luminescent property

a b s t r a c t A new composition of red strontium aluminate phosphor (Sr4 Al2 O7 :Eu3+ , Eu2+ ) is synthesized using a solid state reaction method in air and in a reducing atmosphere. The investigation of firing temperature indicates that a single phase of Sr4 Al2 O7 is formed when the firing temperature is higher than 1300 ◦ C and that a Sr3 Al2 O6 phase is formed as the main peak below 1300 ◦ C. The effects of firing temperature and doping concentration on luminescent properties are investigated. Sr4 Al2 O7 phosphors exhibit the typical red luminescent properties of Eu3+ and Eu2+ . A comparison photoluminescence study with Sr3 Al2 O6 phosphor shows that Sr4 Al2 O7 has higher emission intensity than Sr3 Al2 O6 as a result of the higher optimum doping concentration of Sr4 Al2 O7 phosphor. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Phosphors activated with rare earth metal have been widely investigated in the past few decades on account of their technological importance [1]. Strontium aluminate phosphors are one of phosphor groups which have received great attention and extensively studied because they exhibit a high chemical stability and good luminescent characteristics [2–4]. In SrO–Al2 O3 system, there are many different phases which can emit various colors with different activators [1–4]. Sr2 Al6 O11 :Eu2+ , Sr4 Al14 O25 :Eu2+ , Dy3+ , Sr4 Al14 O25 :Eu2+ and SrAl2 O4 :Eu2+ , Dy3+ are efficient blue and green-emitting phosphor [5–8]. It is believed that when the ratio of Al/Sr is equal to 2, an emission peak lies at about 520 nm, and when the ratio is above 2, a blue emission peak at approximately 495 nm appears [9]. In the case of red emitting strontium aluminate, Sr3 Al2 O6 :Eu3+ ,Eu2+ and Sr5 Al2 O8 :Eu3+ are reported, and the Al/Sr ratio of these phosphors are below 1 [10–13]. These red phosphors have wide excitation band so that they can excited in the UV (320–380 nm) and blue range (450–460 nm). According to the SrO–Al2 O3 phase diagram proposed by Massazza [14], it is shown that Sr3 Al2 O6 and Sr5 Al2 O8 phases are stable so that these phases start to form at low temperature (800 ◦ C). Besides these two phases, Sr4 Al2 O7 phase of which ratio of Al/Sr is below 1 has also a potential to have red emission. However, red luminescent behavior of Eu ions in a pure Sr4 Al2 O7 phase has not

∗ Corresponding author. Tel.: +82 42 821 6587; fax: +82 42 822 9401. E-mail address: [email protected] (H.I. Won). 0921-5107/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2011.09.014

yet been investigated because the area of forming Sr4 Al2 O7 phase is small in the phase diagram and this phase can be synthesized at the relatively higher temperature than synthesizing temperature of Sr3 Al2 O6 and Sr5 Al2 O8 phases. In this study, a red strontium aluminate phosphor (Sr4 Al2 O7 :Eu2+ , Eu3+ ) was prepared that exhibits strong excitation in wavelength ranges of 250–350 nm and 450–460 nm using a conventional solid state reaction. Furthermore, the photoluminescent properties of the Sr4 Al2 O7 :Eu2+ , Eu3+ powders resulting from variations of such synthesis conditions as temperature, activator concentration, and synthesis atmosphere were investigated. 2. Experimental The sample prepared in the present work was designed to have an overall composition of Sr4 Al2 O7 . The main starting materials for the Sr4 Al2 O7 :Eu phosphors were strontium carbonate (SrCO3 , Daejung Chemical and Metals, Korea), alumina (Al2 O3 , Junsei, Japan), and europium oxide (Eu2 O3 , Metal Rare Earth Lim., China). Every starting material was of greater than 99.9% purity. All starting materials were weighed according to the stoichiometric ratio and mixed in an aqueous medium for several hours using the ball–mill technique. After drying, the powder mixture was loosely compacted into an alumina crucible. After closing the cover, the set of crucibles was loaded into a laboratory box furnace and heated to the desired temperature (1000–1500 ◦ C) at the rate of 300 ◦ C/h. The sample was kept at the set temperature for 3 h then cooled down naturally. Two types of heating atmospheres were investigated in this work, namely, air and a reducing CO atmosphere.

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For the reducing atmosphere, a mixture-containing alumina crucible was inserted into a relatively large alumina crucible that was partially filled with small pieces of charcoal to create a reducing CO atmosphere for the Eu3+ → Eu2+ transition. The powder diffraction patterns were recorded by an X-ray diffractometer with Cu K␣ radiation (Siemens D5000, Germany). The powder morphology was studied using a scanning electron microscope (SEM; JSM 5410, JEOL, Japan). The relative PL emission intensities of the samples were measured by a fluorescence spectrophotometer (F-7000, Hitachi, Japan) using a Xe lamp with excitation wavelengths of 325 nm and 450 nm at room temperature. 3. Results and discussion 3.1. Synthesis of Sr4 Al2 O7 To determine the optimal synthesis temperature, several firing temperatures were investigated. Fig. 1 presents the XRD patterns of the final products obtained at different firing temperatures. The atmosphere of the synthesis was air. From the figure, it can be determined that the noticeable changes of phase composition in the final products occur above 1100 ◦ C. When the temperature reached approximately 1000 ◦ C, the reaction between the starting materials was initiated; the Sr3 Al2 O6 phase appeared at 1100 ◦ C and became a main peak at 1200 ◦ C. As the temperature increased, Sr3 Al2 O6 was transformed into Sr4 Al2 O7 and then Sr4 Al2 O7 , became a single phase at 1400 ◦ C. Based on these results, it can be concluded that the Sr3 Al2 O6 phase is stable in the range of 1000–1200 ◦ C, even though the starting composition was fixed to Sr4 Al2 O7 . Moreover, according to the phase diagram of SrO–Al2 O3 , the ␤-phase of Sr4 Al2 O7 is stable from 1130 to 1450 ◦ C [15]; however, the formation of the ␤-phase in this temperature range was not observed in the temperature investigation. A similar result indicating that Sr3 Al2 O6 is a stable phase in the range of 1000–1200 ◦ C was found in the solid state reaction of SrAlO4 [16]. In the case of SrAlO4 synthesis, the Sr3 Al2 O6 phase was stable up to 1250 ◦ C. Considering the above results, the reaction pathway of Sr4 Al2 O7 can be written as follows: SrCO3 → SrO + CO2

< 1200 ◦ C

3SrO + Al2 O3 → Sr3 Al2 O6

1000–1200 ◦ C

(1) (2)

Fig. 1. XRD patterns of final products obtained at different firing temperatures: (a) 1000 ◦ C, (b) 1100 ◦ C, (c) 1200 ◦ C, (d) 1300 ◦ C, (e) 1400 ◦ C and (f) 1500 ◦ C.

SrO + Sr3 Al2 O6 → Sr4 Al2 O7

> 1200 ◦ C

(3)

The decomposition of SrCO3 begins at approximately 1000 ◦ C despite its inherent decomposition temperature (1290 ◦ C), and simultaneously, the Sr3 Al2 O6 phase is formed from the reaction between SrO and Al2 O3 . When the temperature exceeds 1200 ◦ C, phase transformation from Sr3 Al2 O6 to Sr4 Al2 O7 occurs as a result of a reaction with the residues of SrO. It is conjectured that the cause of the lowering of the SrCO3 decomposition temperature is an interaction between SrCO3 and Al2 O3 . Fig. 2 presents SEM images of the products obtained at different temperatures. As can be seen, no particle size change was found with increasing firing temperature; except for the product obtained at 1100 ◦ C, every powder obtained at different temperature is similar in that the products consist of small and large particles that look like fragments chipped off from a melted substance. It is supposed that this similarity in particle morphology and product size is attributable to the melting point of Sr4 Al2 O7 . The Sr4 Al2 O7 phase is stable at high temperatures (>1200 ◦ C), and it has a relatively

Fig. 2. SEM images of the products obtained at different firing temperatures: (a) 1000 ◦ C, (b) 1100 ◦ C, (c) 1200 ◦ C, (d) 1300 ◦ C and (e) 1400 ◦ C.

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Intensity(a.u.)

(a) (b) (c) (d)

400

450

500

550

600

650

700

Wavelength(nm) Fig. 4. Excitation and emission spectra of Sr4 Al2 O7 :Eu2+ obtained from different firing temperatures: (a) 1100 ◦ C, (b) 1200 ◦ C, (c) 1300 ◦ C, (d) 1400 ◦ C and (e) 1500 ◦ C.

low melting point in comparison with other strontium aluminates. According to our experiments, the products obtained at firing temperatures above 1500 ◦ C were mostly melted so that it was difficult to obtain the products in powder form. 3.2. Luminescent properties of Sr4 Al2 O7 Fig. 3(a) presents the excitation spectra of the products prepared in the air atmosphere at different firing temperatures. The excitation of all samples was monitored at 619 nm. The excitation spectra show an intense broad band from 230 to 350 nm with the maximum at 305 nm, which is due to the charge energy transfer from europium–oxygen interaction. Besides the broad band, many sharp peaks are observed at 362, 382, 393, 401, 406, and 415 nm; these peaks are assigned to the higher energy level (f–f) transitions of Eu3+ ions. The emission spectra of the Sr4 Al2 O7 :Eu3+ powders under UV-light excitation at 305 nm are shown in Fig. 3(b). As can be seen, the product prepared at 1200 ◦ C exhibited weak emission intensity, but a remarkable increase of emission intensity was observed when the firing temperature was above 1300 ◦ C. The measured emission spectra of all the products are similar in that the emission is in the region of the red spectrum (570–720 nm) with two main peaks at 590 nm and 619 nm. These main peaks correspond to 5 D0 → 7 F1 and 5 D → 7 F transitions, which are typical red emissions of Eu3+ [17]. 0 2 Fig. 4 presents excitation and emission spectra of Sr4 Al2 O7 prepared in a reducing atmosphere at different firing temperatures. Similar to the Eu3+ -doped products, the products doped with Eu2+ exhibited emission intensities that increased from a firing temper-

3+

Sr4Al2O7:Eu

Intensity(a.u.)

Fig. 3. Emission spectra of Sr4 Al2 O7 :Eu3+ obtained at different firing temperatures: (a) 1200 ◦ C, (b) 1300 ◦ C, (c) 1400 ◦ C and (d) 1500 ◦ C.

ature of 1300 ◦ C under an excitation of 450 nm, as shown in Fig. 4. The highest emission intensity was observed from the product fired at 1500 ◦ C. The excitation spectra show that the powders exhibit a strong excitation band from 425 nm to 500 nm, which is assigned to the europium–oxygen charge transfer band. With the excitation wavelength at 450 nm, the powders exhibited a red emission spectrum in the range of 550–700 nm, with the main emission peak at 607 nm. This red emission has a broadband feature belonging to the emission of the 4f6 5d1 → 4f7 (8S7/2 ) transition of Eu2+ ions [17]. Sr4 Al2 O7 :Eu2+ powder excited under 450-nm light shows an internal quantum efficiency of 0.25 and an external quantum efficiency of 0.19. Considering that the main composition of the product fired at 1200 ◦ C is Sr3 Al2 O6 , it can be concluded that the emission spectra of Sr4 Al2 O7 and Sr3 Al2 O6 are very similar in Eu3+ and Eu2+ . In order to make a more careful comparison, Sr4 Al2 O7 and Sr3 Al2 O6 were prepared under the same conditions, and their X-ray diffraction patterns were obtained, as shown in Fig. 5. It can be observed that the single phases of both Sr4 Al2 O7 and Sr3 Al2 O6 are well formed. Fig. 6 provides a comparison of the emission spectra of Sr4 Al2 O7 :Eu3+ and Sr3 Al2 O6 :Eu3+ . The samples were prepared at

2+

Sr4Al2O7:Eu

3+

Sr3Al2O6:Eu

2+

Sr3Al2O6:Eu

20

30

40

50

60

70

80

2θ Fig. 5. X-ray diffraction patterns of Sr3 Al2 O6 and Sr4 Al2 O7 prepared at 1500 ◦ C for 3 h.

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Sr4Al2O7

Intensity(a.u)

Sr3Al2O6

560

580

600

620

640

660

680

Wavelength(nm) Fig. 6. Emission spectra of Sr4 Al2 O7 :Eu3+ and Sr3 Al2 O6 :Eu3+ prepared under the same conditions.

1500 ◦ C for 3 h in an air atmosphere, and a 10 mol% concentration of Eu was doped into both Sr4 Al2 O7 and Sr3 Al2 O6 . The two main peaks are similar; however, upon closer inspection, two small peaks at 649 nm and 656 nm are evident with the Sr4 Al2 O7 :Eu3+ , and there was a remarkable difference in the intensity of the main peaks. Fig. 7 provides a comparison of the emission spectra of Sr4 Al2 O7 :Eu2+ and Sr3 Al2 O6 :Eu2+ , which were prepared at 1500 ◦ C for 3 h in a reducing atmosphere. The Eu concentration of both samples was 10 mol%. The peaks of Sr4 Al2 O7 :Eu2+ and Sr3 Al2 O6 :Eu2+ are similar in that the emission spectra are in the range of 550–700 nm, with the maximum emission peak at 607 nm, but the emission intensity of Sr4 Al2 O7 :Eu2+ is much higher than that of Sr3 Al2 O6 :Eu2+ . From the data presented in Figs. 6 and 7, it is clear that Sr4 Al2 O7 exhibits a higher intensity than does Sr3 Al2 O6 . We conjecture that the reason for the superior emission intensity of Sr4 Al2 O7 is related to the emission quenching behavior of Sr3 Al2 O6 and Sr4 Al2 O7 according to Eu concentration. Fig. 8 presents the dependence of the emission intensity of Sr3 Al2 O6 and Sr4 Al2 O7 on Eu2+ concentration (x). In the case of Sr4 Al2 O7 , a higher Eu2+ doping concentration in the host lattice leads to a higher emission intensity of the phosphor powder, provided that the Eu2+ doping concentration (x) is below 0.3. However,

Fig. 8. Dependence of the emission intensity of (Sr3−x Eux )Al2 O6 and (Sr4−x Eux )Al2 O7 on Eu2+ concentration (x).

when the Eu concentration (x) is above 0.3, the emission intensity begins to decrease with increasing Eu2+ concentration; this indicates that the emission center is quenched as a result of the high Eu2+ concentration. In the case of Sr3 Al2 O6 , though the curvature of the emission quenching is similar to that of Sr4 Al2 O7 , the starting Eu concentration of emission quenching and the intensity at this concentration (x = 0.15) are lower than those of Sr4 Al2 O7 . These differences indicate that the lattice of Sr4 Al2 O7 has more sites – which can be substituted by Eu with maintaining maximum intensity – than does Sr3 Al2 O6 . Most rare earth ion-activated phosphors have their own optimum doping concentration for producing the highest emission intensity, and this optimum doping concentration is mainly influenced by a critical distance between activators, which can prohibit non-radiative decay [18]. If the doping concentration of the activator is higher than the optimum doping concentration, the distance between activators will be shorter than the critical distance; thus, the possibility of non-radiative decay will increase causing emission intensity to decrease. On the other hand, if the host material has more sites for doping without a distance between activators that is decreased to less than the critical distance, the optimum doping concentration will be increased. In addition, the critical energy transfer distance of Sr3 Al2 O6 and Sr4 Al2 O7 phases were calculated to better understand the quenching behavior of these compounds. According to Blasse’s theory on the energy transfer mechanism in oxide phosphors [19], the critical energy transfer distance (Rc ) can be calculated from the concentration quenching data using the following equation:

 3V 1/3

Rc ≈ 2

Fig. 7. Emission spectra of Sr4 Al2 O7 :Eu2+ and Sr3 Al2 O6 :Eu2+ prepared under the same conditions.

2xc N

(4)

where xc is the critical concentration, N is the number of host cations in the unit cell, and V is the volume of the unit cell. The ˚ calculated Rc of Sr3 Al2 O6 and Sr4 Al2 O7 is about 11.3 and 8.5 A, respectively, and this calculation is consistent with the hypothesis that the lattice of Sr4 Al2 O7 has more sites for Eu substitution than Sr3 Al2 O6 . Considering the results from Fig. 8 and the critical energy transfer distance calculation, it can therefore be supposed that Sr4 Al2 O7 has more sites for doping in the lattice than does Sr3 Al2 O6 , and this is why Sr4 Al2 O7 exhibits higher emission intensity at the optimum doping concentration.

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4. Conclusions Eu-doped Sr4 Al2 O7 phosphors were synthesized using a solidstate method. The firing temperature necessary for the synthesis of single-phase Sr4 Al2 O7 is greater than 1300 ◦ C, and below 1300 ◦ C, the Sr3 Al2 O6 phase is dominant. The prepared single phase Sr4 Al2 O7 powder consists of small and large particles that look like fragments chipped off from a melted substance. The luminescent properties of Sr4 Al2 O7 indicate that it has typical red spectra, and the emission intensity of Sr4 Al2 O7 increases with increasing firing temperature. A comparison of the luminescent properties of Sr4 Al2 O7 and Sr3 Al2 O6 shows that Sr4 Al2 O7 has higher emission intensity, which is a result of its higher optimum doping concentration. References [1] S.H. Poort, W.P. Blokpoel, G. Blasse, Chem. Mater. 7 (1995) 1547–1551. [2] K. Yokota, S.X. Zhang, K. Kimura, J. Lumin. 92 (2001) 223–227.

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