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Calcining Temperatures of Sr1-3xEuxDy2xAl2O4 (x = 0−0.12) Phosphors Prepared Using the Potassium Carbonate Coprecipitation method
Journal of Alloys and Compounds 669 (2016) 38e45
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Calcining Temperatures of Sr1-3xEuxDy2xAl2O4 (x ¼ 00.12) Phosphors Prepared Using the Potassium Carbonate Coprecipitation method Chen-Jui Liang*, Hao-Yi Siao Department of Materials Science and Engineering, Feng Chia University, Taichung 40724, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 November 2015 Received in revised form 26 January 2016 Accepted 29 January 2016 Available online 1 February 2016
This study investigated the optimal calcining temperatures of Sr1-3xEuxDy2xAl2O4 (x ¼ 00.12) phosphors prepared by a new coprecipitation method using potassium carbonate as the precipitant. The results show that the potassium carbonate coprecipitation method was useful and successful, and avoids the problems of residual precipitant presence and the elution of small metal ions. The optimal calcining temperatures of the phosphors ranged from 920 to 950 C, which were lower than those obtained using other methods. An empirical model was established to examine the calcining temperatures of prepared precursors. The average absolute relative error between the starting temperatures obtained from thermogravimetric analysis and temperatures estimated by the empirical model was 0.23%. The emission intensity increment rate of the phosphors produced using the potassium carbonate coprecipitation method between the doping amounts x of 0.01 and 0.06 is approximately 10-fold higher than that produced using the solidestate reaction method. Emission intensities of the prepared phosphors were increased approximately 23.5-fold when the doping amount was increased from 0.01 to 0.06. Sr0.64Eu0.12Dy0.24Al2O4 phosphor has moderate surface characteristics and the highest surface Dy/Eu ratio of all phosphors; therefore, its emission intensity is greater than those of other prepared phosphors. © 2016 Elsevier B.V. All rights reserved.
Keywords: Luminescence Optical materials Chemical synthesis Calcium aluminates
1. Introduction Phosphor materials have considerable potential in several applications such as optical brighteners, copy and product protections, security labeling, conversion of high-energy radiation, markers for analysis, lithography, photochemistry, bioimaging detection, and medicine. Numerous studies have focused on enhancing the properties of long-persistent [1e3], long-afterglow [4e6], sunlight-activated [7e10], and elastic, mechano-, thermo-, and lyoluminescences [11e15] to form high-performance phosphors. Synthesizing efficient phosphors requires not only the best in high-temperature chemistry but also careful precursor preparation, proper handling, and high purity of starting materials. Phosphor materials can be prepared using the following techniques: solidestate reaction, solegel calcination, spray drying and calcination, ultrasonic atomization and combustion, selfpropagating high-temperature synthesis, and coprecipitation. The solidestate reaction method has been used widely for preparing
* Corresponding author. E-mail address: [email protected] (C.-J. Liang). http://dx.doi.org/10.1016/j.jallcom.2016.01.227 0925-8388/© 2016 Elsevier B.V. All rights reserved.
polycrystalline solids from a mixture of solid starting materials. This method is necessary to heat solid mixtures to markedly higher temperatures (10001500 C) for the reaction to occur at an appreciable rate [16e18]. The solegel synthesis of ceramic oxides demonstrates advantages such as high purity and favorable homogeneity. Dry gel particles are prefired at 8501000 C to remove the organic groups, followed by calcining at high temperature (12001450 C) to crystallize and form luminescent centers [19,20]. The spray drying and calcination method involves heating the powder at approximately 750 C for 3 h, and then calcined at 1300 C for 3 h [21]. In the ultrasonic atomization and combustion method, a solution created by ultrasonic atomization passes through a tubular furnace with a temperature of 1500 C [22]. The synthesis of phosphors by self-propagating high-temperature synthesis is carried out in a reactor containing static argon gas at a pressure of 0.5 MPa and temperature of >1800 C [23]. In coprecipitation methods, precipitants such as (NH4)2CO3, NH4OH, CO(NH2)2 and oxalic acid are generally used [24e27]. A mixed nitrate solution is dumped into a highly revolving agitator and an alkaline precipitant solution of approximately 1M is added in droplets to produce a precursor. After being dried and washed, the final powders are obtained by calcining the dried precursor at
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different temperatures from 1000 to 1300 C. As mentioned, the final calcining temperatures of most preparation methods are extremely high, thus resulting in high energy expenditure. Although the final calcining temperatures of the coprecipitation methods are lower than those of other methods, two problems must be avoided: residual precipitant influences the chemical homogeneity of mixed metal oxides when the precursors are calcined, and small metal ions elute from the precursors when washed with distilled water. Additionally, the crystal structure of phosphors depends on the calcining temperature, as observed in
39
at an optimal concentration for synthesizing Eu and Dy codoping SrO,Al2O3 phosphors. Theoretically, the precursor mainly consists of SrCO3 and M(III)(OH)3 (M(III) ¼ Al, Eu, Dy), according to the following chemical reaction equations [24]: Sr(NO3)2 þ K2CO3 / SrCO3)2 þ K2(NO3)2
(2)
2M(III)(NO3)3 þ 3K2CO3 þ 3H2O / 2M(III)(OH þ 3C þ 6N
(3)
The calcination proceeds according to the following equation:
calcining
ð1 3xÞSrCO3ðsÞ þxEuðOHÞ3ðsÞ þ2xDyðOHÞ3ðsÞ þ2AlðOHÞ3ðsÞ !Sr13x Eux Dy2x Al2 O3ðsÞ þ ð3 þ 4:5xÞH2 O þ ð1 3xÞCO2 þ 0:75xO2 (4)
rare-earth-doped strontium aluminate systems [21]. However, reducing energy use for industrial production and environmental protection is crucial. This work introduces a new coprecipitation method that involves using K2CO3 solution as the precipitant to prepare phosphors that are efficient, have low-energy requirements, have high purity, and demonstrate favorable homogeneity. The results show a direct relationship between the optimal calcining temperature and the composition of the starting materials. 2. Experimental Sr1-3xEuxDy2xAl2O4 (x ¼ 00.12) phosphors were prepared using the potassium carbonate coprecipitation method with 800 mL of 0.18 M K2CO3 solution (pH ¼ 10.98) as the precipitant and a stirring rate of 500 rpm. Subsequently, 100 mL of metal nitrate solution, contained 0.1280.20 M Sr(NO3)2, 00.048 M Eu(NO3)3, 00.096 M Dy(NO3)3, and 0.40 M Al(NO3)3, was added to the potassium carbonate solution at an approximate rate of 10.0 mL min1. The precipitates were filtered out repeatedly, washed with distilled water, and then dried in a vacuum oven at 110 C for 2 h. Next, the precursor of the phosphors was calcined in a reducing environment of 10% H2 þ 90% N2 with the calcination process shown in Eq. (1):
1h
2h
6h
Infrared spectra of the precipitates were obtained using Fourier transform infrared (FTIR) spectroscopy (Perkin Elmer, FT-IR Spectrometer Frontier). The crystalline phases of the prepared phosphors were identified using X-ray diffraction analysis (Bruker, D8 SSS) with Cu Ka radiation of 1.54 Å between 10 and 70 at a scan rate of 0.5 min1. The morphologies and surface atomic percentages of the phosphors were observed using a variable vacuum scanning electron microscope and an energy dispersive spectrometer (Hitachi, S3000eN VVSEM). The BrunauereEmmetteTeller (BET) surface properties were acquired using a highresolution surface area and porosimetry analyzer (Micromeritics, ASAP 2020). The photoluminescent properties between wavelengths of 200 and 1500 nm were obtained by employing a cryogenic cathodoluminescence system (JEOL, JSM7001F) at an accelerating voltage (VII) of 20 kV. Afterglow properties were collected using fluorescence spectrometry (Agilent, Cary Eclipse FLR) after the samples were fully activated under a select excitation spectrum. 3. Results and discussion 3.1. Preparation The precursors were prepared and calcined as mentioned.
cooling
room temp,! 400 C!max, calcined temperature!max, calcined temperature!room temp,
The calcination process involved heating the coprecipitates from room temperature to 400 C, which was maintained for 2 h for maximum transformation of metal hydroxides to metal oxides. The temperature was then raised to the maximum calcining temperature, which was maintained for an additional 6 h to complete the transformation of metal oxides to phosphors. The maximum calcination temperature of each precursor was determined using Simultaneous DSC-TGA (TA Instrument, SDT2960) between 50 and 1200 C at a heating rate of 5 C min1. An equilibrium pH of 9.16 was measured in the coprecipitates by using 0.18 M K2CO3 solution as the precipitant. Between pH 4 and 10, aluminum, Al(OH)3 is nearly insoluble. Hydroxide compounds of Sr are somewhat soluble at pH 9.16, but their carbonate compounds are insoluble. Therefore, the loss of Sr ions in solution can be minimized through this method. The 0.18 M K2CO3 solution was
(1)
Fig. 1 shows the FTIR spectra of the precipitates obtained using the potassium carbonate coprecipitation method. The FTIR spectra of the precipitates were characterized by a broad band between 2700 and 3700 cm1 (OeH stretching), and three shaper bands at 2483 cm1 (OeH bending), 1646 cm1 (OeH bending), and1168 cm1 (OeH stretching), all of which correspond to structural M(III)(OH)3. The area of the broad band between 2700 and 3700 cm1 increases by increasing Eu and Dy doping amounts. This demonstrates that the major precipitates of Eu and Dy metals are Eu and Dy hydrates. SrCO3 presence was also confirmed by the appearance of a shoulder on the two asymmetric stretching CO3 bands at 1470 (main band) and 855 cm1. In addition, three shaper symmetric stretching bands were located at 1774, 1068, and 744 cm1. The main asymmetric stretching CO3 bands at 1470 cm1 separated into two bands, 1518 and
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Fig. 1. FTIR spectra of the prepared precursors.
1401 cm1, when x 0.08. The results show that the potassium carbonate coprecipitation method was useful and successful, and avoids the problems of residual precipitant presence and the elution of small metal ions. DSC and TG curves of the prepared precursors are shown in Fig. 2a and b, respectively. The calcining process can be expressed by three main reactions: the dehydration of M(III)(OH)3 to form M(III)2O3, the decomposition of SrCO3 to form SrO, and the recombination of metal oxides to form alkaline earth metal aluminates. Three endothermic peaks are evident in the DSC curves of the sample precursors (Fig. 2a). Their TG curves also show favorable agreement with the DSC results. The two endothermic peaks lower than 500 C can be assigned to the dehydration of M(III)(OH)3 to form M(III)2O3. Because the dehydration of Eu(OH)3, Dy(OH)3, and Al(OH)3 simultaneously occurred before 500 C, the patterns of DSC and TG varied considerably. The final peaks at 700e930 C can be assigned to the decomposition of metal carbonates. The samples underwent dehydration and decomposition reactions followed by a recombination of metal
oxides at higher temperature without weight loss to form alkaline earth metals aluminates. The results show that the starting temperature of the recombination of metal oxides decreases linearly when the amount of Eu and Dy doping was increased (Fig. 2). Table 1 presents the starting temperatures of the recombination of the prepared precursors, which are shown in Fig. 2. The maximum calcining temperature of the precursors is equal to or greater than this starting temperature. For each phosphor, a temperature was selected as the maximum calcining temperature that was higher than its starting temperature of 20 C. In this study, the maximum calcining temperature ranged from 920 to 960 C to form efficient phosphors. Because the starting temperature of the recombination of metal oxides decreases linearly by increasing the doping amount of Eu and Dy, a multiple regression analysis was performed using the data of the prepared concentrations of metals in solution (Table 1). The empirical equation resulting from the multiple regression analysis is Eq. (5). The experimental results of the calcining temperatures of Sr1-xEuxDy2xAl2O4 (x ¼ 01.2) phosphors through this
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41
Fig. 2. The DSC and TG patterns of Sr1-3xEuxDy2xAl2O4 (x ¼ 0 0.12) precursors.
empirical model are presented in Table 1. The average absolute relative error between the starting temperatures obtained from Fig. 2 and the temperatures estimated using Eq. (5) was 0.23%. The results demonstrate that the empirical equation can provide a quantification of the calcining temperature of Eu and Dy codoping strontium aluminate phosphors, thereby enabling the synthesis process to operate under optimal conditions for reducing energy use. Tcalcined temp. ¼ 2857.1 3040.7[Al] 3462.8[Sr] 3014.0[Eu] 4103.2[Dy] (5)
3.2. Structure
phosphor in this study was higher than its starting temperature of 20 C. The X-ray diffraction (XRD) patterns of Sr1-3xEuxDy2xAl2O4 (x ¼ 00.12) phosphors are shown in Fig. 3. In undoped strontium aluminates (x ¼ 0), SrAl2O4 is the main phase. The relative amounts of each phase are in the following order: SrAl2O4 > SrAl4O7 >> Sr4Al14O25. Twenty-seven characteristic XRD peaks of the SrAl2O4 phase were observed in the Sr0.94Eu0.02Dy0.04Al2O4 phosphor curve, and four small peaks of the SrAl4O7 phase occurred at 19.6 , 25.1, 32.1, and 32.6 2q. One shaper and two trace peaks of the Sr4Al14O25 phase appeared at 23.8 , 34.3 , and 62.9 2q. The XRD patterns of Sr1-3xEuxDy2xAl2O4 phosphors with a doping amount of x 0.04 were similar, and had amounts similar to those of the SrAl4O7 and Sr4Al14O25 phases. However, the amounts of the EuAlO3, EuAl12O19, and Dy2O3 phases slightly increased by increasing the doping amount. The diffraction peaks of the EuAl12O19 and Dy2O3 phases began to appear at x ¼ 0.04, and the diffraction peaks of the EuAlO3 phase began to appear at
As mentioned, the maximum calcining temperature of each
Table 1 Summary of data for multiple regression and the estimated results. Structures
SrAl2O3 Sr0.97Eu0.01Dy0.02Al2O4 Sr0.94Eu0.02Dy0.04Al2O4 Sr0.88Eu0.04Dy0.08Al2O4 Sr0.82Eu0.06Dy0.12Al2O4 Sr0.76Eu0.08Dy0.16Al2O4 Sr0.70Eu0.10Dy0.20Al2O4 Sr0.84Eu0.02Dy0.14Al2O4c Sr0.64Eu0.12Dy0.24Al2O4 Mean error Gross error a b c
Prepared concentrations of metals in solution, M [Al]
[Sr]
[Eu]
[Dy]
0.4032 0.4058 0.4057 0.4041 0.4059 0.4034 0.4056 0.4030 0.4073 2.1 C 0.23%
0.2000 0.1941 0.1681 0.1881 0.1760 0.1640 0.1520 0.1403 0.1283
e 0.0019 0.0042 0.0041 0.0081 0.0121 0.0163 0.0204 0.0246
e 0.0039 0.0283 0.0081 0.0161 0.0241 0.0320 0.0418 0.0484
The starting temperatures obtained from Fig. 2. The temperatures estimated by Eq. (1). An additional structure was using to facilitate the multiple regression.
Trecombin.a
Testimateb
938 930 928 927 926 920 915 912 898
938.6 929.4 931.7 923.3 927.2 916.7 913.2 912.2 901.7
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Fig. 3. The XRD patterns of Sr1-3xEuxDy2xAl2O4 (x ¼ 0 0.12) phosphors.
x ¼ 0.08. Nevertheless, the peaks of these phases were extremely small. It can be concluded that the points at which Eu and Dy begin to become liberated from the strontium aluminate crystal are x ¼ 0.04 and x ¼ 0.08, respectively. Therefore, a doping amount that is too high does not enhance the formation of Eu emission centers.
3.3. Surface characterization The SEM images of Sr1-3xEuxDy2xAl2O4 (x ¼ 00.12) phosphors are shown in Fig. 4. The surface of undoped strontium aluminate (Fig. 4a) was sparsely covered with a fine grain of approximately 10 nm. These fine grains may be the nanocrystallites of the SrAl4O7 phase compared with its XRD pattern (Fig. 3). Irregular morphological grains of varying sizes were observed in the Eu and Dy codoped strontium aluminate phosphors (Fig. 4beh). The particle size is distributed over a wide range, and the grain boundary is not clean. The average grain sizes of Sr1-3xEuxDy2xAl2O4 phosphors with the doping amounts of x ¼ 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, and 0.12 were approximately 200, 170, 140, 80, 120, 180, and 120 nm (Fig. 4beh), respectively. As shown in Fig. 4e, the morphologies of Sr0.82Eu0.06Dy0.12Al2O4 phosphors with a lower grain size show uniform 1D nanostructures with
lengths of several hundreds of nanometers. The differences among the microstructures may be attributed to the effect of recombination of metal oxides. Europium and dysprosium hydroxides play a crucial role in the recombination of the calcining process. Table 2 presents the surface properties of prepared phosphors. The surface properties and atomic percentage of each phosphor were measured using a BET technique with N2 adsorption and SEM-EDS, respectively. The ranges of the specific surface area, pore volume, and average diameter for prepared phosphors were 5.77e18.91 m2 g1, 0.0136e0.0364 cm3 g1, and 7.59e11.03 nm, respectively. The Sr0.82Eu0.06Dy0.12Al2O4 phosphor has a specific surface area, pore volume, and average diameter close to the averages, which were 13.28 m2 g1, 0.0299 cm3 g1, and 9.24 nm, respectively. Clearly, there is an effect of Eu and Dy codoping on surface properties of the phosphors. The ratio of surface Sr, Al, and O atoms of the undoped strontium aluminate was 1:1.23:1.41. The amounts of surface Al and O atoms were lower than the stoichiometric molar ratio of SrAl2O4. In may refer to SrAl4O7 and Sr4Al14O25 phases in the bulk of the undoped strontium aluminate (Fig. 3). The Al/Sr atom ratios of the prepared phosphors when 0.02 x 0.10 ranged from 1.66 to 2.35, which was lower than their stoichiometric molar
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Fig. 4. SEM photographs and EDX surface element compositions of Sr1-3xEuxDy2xAl2O4 (x ¼ 0 0.12) phosphors. The subfigures from (a) to (h) in sequence are x ¼ 0, 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, and 0.12, respectively.
ratios but higher than the ratio of undoped strontium aluminate. The Al/Sr atom ratio of higher Eu and Dy codoped strontium aluminates (x ¼ 0.12) was 7.30. The reason can be ascribed to the formation of the EuAlO3, EuAl12O19, and Dy2O3 phases on the surface. The range of Dy/Eu atom ratios of the Eu and Dy codoped phosphors was 1.42e1.87, with the minimum at x ¼ 0.06. These
results demonstrate that the precursors and phosphors of uniform molar stoichiometry are easily formed when uses the potassium carbonate coprecipitation method. However, the liberation of Eu and Dy from the strontium aluminate crystal occurs when x 0.08.
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Table 2 Surface characterizations for Sr1-3xEuxDy2xAl2O4 (x ¼ 0 0.12) phosphors. Doping amount, x
0 0.02 0.04 0.06 0.08 0.10 0.12 a b c d
BET surface area (m2 g1)a
5.078 5.873 18.33 13.27 15.03 16.47 18.91
Pore volume (cm3 g1)b
0.0136 0.0159 0.0356 0.0264 0.0289 0.0364 0.0522
Average pore diameter (nm)c
10.74 10.81 7.59 7.97 7.68 8.83 11.03
Surface atomic percentage (%)d Sr
Al
Eu
Dy
O
27.46 11.12 11.03 11.94 13.75 13.79 4.33
33.85 21.74 22.72 21.45 22.76 32.41 31.60
e 0.37 0.49 0.81 1.72 3.63 3.37
e 0.68 0.90 1.15 3.05 6.61 6.31
38.69 66.09 64.86 64.65 58.72 43.56 54.39
Surface areas were measured by a BET technique using N2 adsorption. Single point total pore volume of pores less than 1360 Å diameter at P/Po ¼ 0.9860. Average pore diameter for 4V/A by BET. Surface atomic percentage were measurement by SEM-EDX.
3.4. Luminescence Fig. 5 shows the emission spectra of the phosphors prepared by employing a cryogenic cathodoluminescence system at room temperature. The emission intensity of the prepared phosphors varies according to the following sequence: Sr0.82Eu0.06Dy0.12Al2O4 > Sr0.88Eu0.04Dy0.08Al2O4 > Sr0.76Eu0.08Dy0.16Al2O4 > Sr0.70Eu0.10Dy0.20Al2O4 > Sr0.96Eu0.02Dy0.04Al2O4 > Sr0.64Eu0.12Dy0.24Al2O4. In the emission spectra of the prepared phosphors, the main and minor emission bands are observed at 523.4 nm and 573.0 nm, respectively. This suggests that the emission of approximately 540 nm originates from the Eu2þ 4f65 d1 / 4f7(8S7/ 2þ ions, which are randomly 2) transitions associated with the Eu 2þ distributed at the three Sr sites. One weak emission band of the prepared phosphors, with both x ¼ 0.01 and 0.02, is observed at
610.3 nm. Because the EuAlO3, EuAl12O19, and Dy2O3 phases occur in the structure of Sr0.64Eu0.12Dy0.24Al2O4, the other types of emission centers emit two spectra of 425.0 and 481.3 nm. The peaks at 840.4 and 1037.6 nm are caused by wavelength doubling and scattering of peaks at 425.0 and 523.4 nm, respectively, because of the second harmonic. As mentioned, Sr0.64Eu0.12Dy0.24Al2O4 phosphor has moderate surface characteristics and the highest surface Dy/Eu ratio; hence, its emission intensity is greater than those of others. The emission intensities of the prepared phosphors are increased approximately 23.5-fold when the doping amount is increased from 0.01 to 0.06 (Figs. 5 and 6). Furthermore, it can be observed that the emission intensities of the prepared phosphors with doping amounts ranging from 0.01 to 0.06 increase linearly and rapidly (Fig. 6). The emission intensity increment rate of the phosphors produced using the potassium carbonate coprecipitation method between doping amounts of 0.01 and 0.06 is approximately 10-fold higher than that produced using the solidestate reaction method [28] (Fig. 6). 4. Conclusions
Fig. 5. CL spectra of Sr1-3xEuxDy2xAl2O4 (x ¼ 0 0.12) phoshors.
Synthesis using the potassium carbonate coprecipitation method is simple, rapid, and successful. The method demonstrates many advantages such as low energy requirements, simple equipment, high purity, favorable homogeneity, and short time for preparing mixed metal oxides. The precipitant of potassium carbonate provides optimal hydroxide and carbonate ions under appropriate pH conditions for the coprecipitation reaction of metals to form a well-mixed precursor. This reduces the energy required for the recombination of metal oxides, thereby reducing the final calcining temperature of the crystallization of Eu and Dy codoped strontium aluminates. The optimal calcining temperature can be estimated accurately by using an equation to form an efficient phosphor. The optimal calcining temperature decreases by increasing the doping amount of Eu and Dy metals. The emission intensity increment rate of the phosphors produced using the potassium carbonate coprecipitation method between the doping amounts of 0.01 and 0.06 is approximately 10-fold higher than that produced using the solidestate reaction method. The emission intensities of the prepared phosphors are approximately 23.5-fold higher when the doping amount is increased from 0.01 to 0.06. Equation (5) is crucial for proper design and field operation of a synthesis reactor, particularly in reducing energy use and environmental protection.
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Fig. 6. Emission intensities dependence of Eu2þ concentration x for the present study (CL, VII ¼ 20 kV) and reported by Wang et al. (PL, lex ¼ 254 nm) [28].
Acknowledgments The authors would like to thank the Ministry of Science and Technology of the Republic of China, Taiwan, for financially supporting this research under Contract No. MOST-104-2218-E-035008. References [1] M. Allix, S. Chenu, E. Veron, T. Poumeyrol, E.A.K. Boudjelthia, S. Alahrache, F. Porcher, D. Massiot, F. Fayon, Considerable improvement of long-persistent luminescence in germanium and tin substituted ZnGa2O4, Chem. Mater. 25 (2013) 1600. [2] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, A new long phosphorescent phosphor with high brightness, SrAl2O4:Eu2þDy3þ, J. Electrochem. Soc. 143 (1996) 2670. [3] K.V. den Eeckhout, D. Poelman, P.F. Smet, Persistent luminescence in nonEu2þ- Doped compounds: a review, Mater. 6 (2013) 2789. [4] S.H. Choi, N.H. Kim, Y.H. Yun, S.C. Choi, properties of SrAl2O4 and CaAl2O4 long-phosphorescent phosphors synthesized by an oxalate coprecipitation method, J. Ceram. Process. Res. 7 (2006) 62. [5] G.M. Qiu, Y.J. Chen, J.Q. Cui, X.J. Geng, H. Wang, D. Song, Synthesis of long afterglow phosphors doped B SrAl2O4: Eu2þ, Dy3þ and its luminescent properties, J. Rare Earth 25 (2007) 86. [6] J.Y. Kuang, Y.L. Liu, Trapping effects in CdSiO3:In3þ long afterglow phosphor, Chin. Phys. Lett. 23 (2006) 204. [7] A. Kumar, G. Kedawat, P. Kumar, J. Dwivedi, B.K. Gupta, Sunlight-activated Eu2þ/Dy3þ doped SrAl2O4 water resistant phosphorescent layer for optical displays and defence applications, New J. Chem. 39 (2015) 3380. [8] G. Fang, X. Zhaoxian, X. Hao, L. Yongxi, Improved performance of strontium aluminate luminous coating on the ceramic surface, J. Phys. Conf. Ser. 152 (2009) 012082. re, S.K. Sharma, N. Basavaraju, K.R. Priolkar, L. Binet, B. Viana, [9] A. Bessie A.J.J. Bos, T. Maldiney, C. Richard, D. Scherman, D. Gourier, Storage of visible light for long-lasting phosphorescence in chromiumdoped zinc gallate, Chem. Mater 26 (2014) 1365. [10] Z. Pan, Y.Y. Lu, F. Liu, Sunlight-activated long-persistent luminescence in the near-infrared from Cr3þ- doped zinc gallogermanates, Nat. Mater 11 (2011) 58. [11] Y. Imai, R. Momoda, ChN. Xu, Elasticoluminescence of europium-doped strontium aluminate spherical particles dispersed in polymeric matrices, Mater. Lett. 61 (2007) 4124. [12] S. Kamimura, H. Yamada, C.N. Xu, Development of new elasticoluminescent
material SrMg2(PO4)2: Eu, J. Lumin 132 (2012) 526. [13] S. Aman, J. Tomas, Mechanoluminescence of quartz particles during grinding in a stirred media mill, Powder Technol. 146 (2004) 147. [14] N. Brahme, A. Gupta, D.P. Bisen, R.S. Kher, S.J. Dhoble, Thermoluminescence and mechanoluminescence of Eu doped Y2O3 nanophosphors, Phys. Procedia 29 (2012) 97. [15] P.M. Bhujbal, S.J. Dhoble, Correlation between mechanoluminescence and lyoluminescence in gamma-irradiated KBr:Ce3þ phosphor for radiation dosimetry, Radiat. Meas. 47 (2012) 512. [16] V. Singha, T.K. Gundu Raob, J.J. Zhua, Preparation, luminescence and defect studies of Eu2þ-activated strontium hexa-aluminate phosphor prepared via combustion method, J. Solid State Chem. 179 (2006) 2589. [17] L. Chen, Y. Zhang, F. Liu, A. Luo, Z. Chen, Y. Jiang, S. Chen, R.S. Liu, A new green phosphor of SrAl2O4:Eu2þ,Ce3þ,Liþ for alternating current driven lightemitting diodes, Mater. Res. Bull. 47 (2012) 4071. [18] S.D. Han, K.C. Singh, T.Y. Cho, H.S. Lee, D. Jakhar, J.P. Hulme, C.H. Han, J.D. Kim, I.S. Chun, J. Gwak, Preparation and characterization of long persistence strontium aluminate phosphor, J. Lumin 128 (2008) 301. [19] S.Y. Kaya, E. Karacaoglu, B. Karasu, Effect of Al/Sr ratio on the luminescence properties of SrAl2O4:Eu2þ,Dy3þ phosphors, Ceram. Int. 38 (2012) 3701. [20] H. Ping, C. Cai-E, W. Sen, Synthesis and characterization of Sr3Al2O6:Eu2þ, Dy3þ phosphors prepared by sol-gel-combustion processing, Chin. Phys. B 18 (2009) 4524. [21] A. Douy, M. Capron, Crystallisation of spray-dried amorphous precursors in the SrOeAl2O3 system: a DSC study, J. Eur. Ceram. Soc. 23 (2003) 2075. [22] Y. Imai, R. Momoda, C.N. Xu, Elasticoluminescence of europium-doped strontium aluminate spherical particles dispersed in polymeric matrices, Mater. Lett. 61 (2007) 4124. [23] T. Sathaporn, S. Niyomwas, Synthesis and characterization of MAl2O4 (M ¼ Ba, Ca, Sr) phosphor by self-propagating high temperature synthesis, Energ. Procedia 9 (2011) 410. [24] C. Chang, Z. Yuan, D. Mao, Eu2þ activated long persistent strontium aluminate nano scaled phosphor prepared by precipitation method, J. Alloy. Compd. 415 (2006) 220. [25] I. Halikia, L. Zoumpoulakis, E. Christodoulou, D. Prattis, Kinetic study of the thermal decomposition of calcium carbonate by isothermal methods of analysis, Eur. J. Min. Process. Environ. Prot. 1 (2001) 89. [26] S.A. Morozov, A.A. Malkov, A.A. Malygin, Synthesis of porous magnesium oxide by thermal decomposition of basic magnesium carbonate, Russ. J. Gen. Chem. 73 (2003) 37. [27] J. Chen, F. Gu, C. Li, Influence of precalcination and boron-Doping on the initial photoluminescent properties of SrAl2O4:Eu,Dy phosphors, Cryst. Growth Des. 8 (2008) 3175. [28] D. Wang, Q. Yin, Y. La, M. Wang, Concentration quenching of Eu2þ in SrO,Al2O3:Eu2þ phosphor, J. Lumin 97 (2002) 1.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Calcining Temperatures of Sr1-3xEuxDy2xAl2O4 (x ¼ 00.12) Phosphors Prepared Using the Potassium Carbonate Coprecipitation method Chen-Jui Liang*, Hao-Yi Siao Department of Materials Science and Engineering, Feng Chia University, Taichung 40724, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 November 2015 Received in revised form 26 January 2016 Accepted 29 January 2016 Available online 1 February 2016
This study investigated the optimal calcining temperatures of Sr1-3xEuxDy2xAl2O4 (x ¼ 00.12) phosphors prepared by a new coprecipitation method using potassium carbonate as the precipitant. The results show that the potassium carbonate coprecipitation method was useful and successful, and avoids the problems of residual precipitant presence and the elution of small metal ions. The optimal calcining temperatures of the phosphors ranged from 920 to 950 C, which were lower than those obtained using other methods. An empirical model was established to examine the calcining temperatures of prepared precursors. The average absolute relative error between the starting temperatures obtained from thermogravimetric analysis and temperatures estimated by the empirical model was 0.23%. The emission intensity increment rate of the phosphors produced using the potassium carbonate coprecipitation method between the doping amounts x of 0.01 and 0.06 is approximately 10-fold higher than that produced using the solidestate reaction method. Emission intensities of the prepared phosphors were increased approximately 23.5-fold when the doping amount was increased from 0.01 to 0.06. Sr0.64Eu0.12Dy0.24Al2O4 phosphor has moderate surface characteristics and the highest surface Dy/Eu ratio of all phosphors; therefore, its emission intensity is greater than those of other prepared phosphors. © 2016 Elsevier B.V. All rights reserved.
Keywords: Luminescence Optical materials Chemical synthesis Calcium aluminates
1. Introduction Phosphor materials have considerable potential in several applications such as optical brighteners, copy and product protections, security labeling, conversion of high-energy radiation, markers for analysis, lithography, photochemistry, bioimaging detection, and medicine. Numerous studies have focused on enhancing the properties of long-persistent [1e3], long-afterglow [4e6], sunlight-activated [7e10], and elastic, mechano-, thermo-, and lyoluminescences [11e15] to form high-performance phosphors. Synthesizing efficient phosphors requires not only the best in high-temperature chemistry but also careful precursor preparation, proper handling, and high purity of starting materials. Phosphor materials can be prepared using the following techniques: solidestate reaction, solegel calcination, spray drying and calcination, ultrasonic atomization and combustion, selfpropagating high-temperature synthesis, and coprecipitation. The solidestate reaction method has been used widely for preparing
* Corresponding author. E-mail address: [email protected] (C.-J. Liang). http://dx.doi.org/10.1016/j.jallcom.2016.01.227 0925-8388/© 2016 Elsevier B.V. All rights reserved.
polycrystalline solids from a mixture of solid starting materials. This method is necessary to heat solid mixtures to markedly higher temperatures (10001500 C) for the reaction to occur at an appreciable rate [16e18]. The solegel synthesis of ceramic oxides demonstrates advantages such as high purity and favorable homogeneity. Dry gel particles are prefired at 8501000 C to remove the organic groups, followed by calcining at high temperature (12001450 C) to crystallize and form luminescent centers [19,20]. The spray drying and calcination method involves heating the powder at approximately 750 C for 3 h, and then calcined at 1300 C for 3 h [21]. In the ultrasonic atomization and combustion method, a solution created by ultrasonic atomization passes through a tubular furnace with a temperature of 1500 C [22]. The synthesis of phosphors by self-propagating high-temperature synthesis is carried out in a reactor containing static argon gas at a pressure of 0.5 MPa and temperature of >1800 C [23]. In coprecipitation methods, precipitants such as (NH4)2CO3, NH4OH, CO(NH2)2 and oxalic acid are generally used [24e27]. A mixed nitrate solution is dumped into a highly revolving agitator and an alkaline precipitant solution of approximately 1M is added in droplets to produce a precursor. After being dried and washed, the final powders are obtained by calcining the dried precursor at
C.-J. Liang, H.-Y. Siao / Journal of Alloys and Compounds 669 (2016) 38e45
different temperatures from 1000 to 1300 C. As mentioned, the final calcining temperatures of most preparation methods are extremely high, thus resulting in high energy expenditure. Although the final calcining temperatures of the coprecipitation methods are lower than those of other methods, two problems must be avoided: residual precipitant influences the chemical homogeneity of mixed metal oxides when the precursors are calcined, and small metal ions elute from the precursors when washed with distilled water. Additionally, the crystal structure of phosphors depends on the calcining temperature, as observed in
39
at an optimal concentration for synthesizing Eu and Dy codoping SrO,Al2O3 phosphors. Theoretically, the precursor mainly consists of SrCO3 and M(III)(OH)3 (M(III) ¼ Al, Eu, Dy), according to the following chemical reaction equations [24]: Sr(NO3)2 þ K2CO3 / SrCO3)2 þ K2(NO3)2
(2)
2M(III)(NO3)3 þ 3K2CO3 þ 3H2O / 2M(III)(OH þ 3C þ 6N
(3)
The calcination proceeds according to the following equation:
calcining
ð1 3xÞSrCO3ðsÞ þxEuðOHÞ3ðsÞ þ2xDyðOHÞ3ðsÞ þ2AlðOHÞ3ðsÞ !Sr13x Eux Dy2x Al2 O3ðsÞ þ ð3 þ 4:5xÞH2 O þ ð1 3xÞCO2 þ 0:75xO2 (4)
rare-earth-doped strontium aluminate systems [21]. However, reducing energy use for industrial production and environmental protection is crucial. This work introduces a new coprecipitation method that involves using K2CO3 solution as the precipitant to prepare phosphors that are efficient, have low-energy requirements, have high purity, and demonstrate favorable homogeneity. The results show a direct relationship between the optimal calcining temperature and the composition of the starting materials. 2. Experimental Sr1-3xEuxDy2xAl2O4 (x ¼ 00.12) phosphors were prepared using the potassium carbonate coprecipitation method with 800 mL of 0.18 M K2CO3 solution (pH ¼ 10.98) as the precipitant and a stirring rate of 500 rpm. Subsequently, 100 mL of metal nitrate solution, contained 0.1280.20 M Sr(NO3)2, 00.048 M Eu(NO3)3, 00.096 M Dy(NO3)3, and 0.40 M Al(NO3)3, was added to the potassium carbonate solution at an approximate rate of 10.0 mL min1. The precipitates were filtered out repeatedly, washed with distilled water, and then dried in a vacuum oven at 110 C for 2 h. Next, the precursor of the phosphors was calcined in a reducing environment of 10% H2 þ 90% N2 with the calcination process shown in Eq. (1):
1h
2h
6h
Infrared spectra of the precipitates were obtained using Fourier transform infrared (FTIR) spectroscopy (Perkin Elmer, FT-IR Spectrometer Frontier). The crystalline phases of the prepared phosphors were identified using X-ray diffraction analysis (Bruker, D8 SSS) with Cu Ka radiation of 1.54 Å between 10 and 70 at a scan rate of 0.5 min1. The morphologies and surface atomic percentages of the phosphors were observed using a variable vacuum scanning electron microscope and an energy dispersive spectrometer (Hitachi, S3000eN VVSEM). The BrunauereEmmetteTeller (BET) surface properties were acquired using a highresolution surface area and porosimetry analyzer (Micromeritics, ASAP 2020). The photoluminescent properties between wavelengths of 200 and 1500 nm were obtained by employing a cryogenic cathodoluminescence system (JEOL, JSM7001F) at an accelerating voltage (VII) of 20 kV. Afterglow properties were collected using fluorescence spectrometry (Agilent, Cary Eclipse FLR) after the samples were fully activated under a select excitation spectrum. 3. Results and discussion 3.1. Preparation The precursors were prepared and calcined as mentioned.
cooling
room temp,! 400 C!max, calcined temperature!max, calcined temperature!room temp,
The calcination process involved heating the coprecipitates from room temperature to 400 C, which was maintained for 2 h for maximum transformation of metal hydroxides to metal oxides. The temperature was then raised to the maximum calcining temperature, which was maintained for an additional 6 h to complete the transformation of metal oxides to phosphors. The maximum calcination temperature of each precursor was determined using Simultaneous DSC-TGA (TA Instrument, SDT2960) between 50 and 1200 C at a heating rate of 5 C min1. An equilibrium pH of 9.16 was measured in the coprecipitates by using 0.18 M K2CO3 solution as the precipitant. Between pH 4 and 10, aluminum, Al(OH)3 is nearly insoluble. Hydroxide compounds of Sr are somewhat soluble at pH 9.16, but their carbonate compounds are insoluble. Therefore, the loss of Sr ions in solution can be minimized through this method. The 0.18 M K2CO3 solution was
(1)
Fig. 1 shows the FTIR spectra of the precipitates obtained using the potassium carbonate coprecipitation method. The FTIR spectra of the precipitates were characterized by a broad band between 2700 and 3700 cm1 (OeH stretching), and three shaper bands at 2483 cm1 (OeH bending), 1646 cm1 (OeH bending), and1168 cm1 (OeH stretching), all of which correspond to structural M(III)(OH)3. The area of the broad band between 2700 and 3700 cm1 increases by increasing Eu and Dy doping amounts. This demonstrates that the major precipitates of Eu and Dy metals are Eu and Dy hydrates. SrCO3 presence was also confirmed by the appearance of a shoulder on the two asymmetric stretching CO3 bands at 1470 (main band) and 855 cm1. In addition, three shaper symmetric stretching bands were located at 1774, 1068, and 744 cm1. The main asymmetric stretching CO3 bands at 1470 cm1 separated into two bands, 1518 and
40
C.-J. Liang, H.-Y. Siao / Journal of Alloys and Compounds 669 (2016) 38e45
Fig. 1. FTIR spectra of the prepared precursors.
1401 cm1, when x 0.08. The results show that the potassium carbonate coprecipitation method was useful and successful, and avoids the problems of residual precipitant presence and the elution of small metal ions. DSC and TG curves of the prepared precursors are shown in Fig. 2a and b, respectively. The calcining process can be expressed by three main reactions: the dehydration of M(III)(OH)3 to form M(III)2O3, the decomposition of SrCO3 to form SrO, and the recombination of metal oxides to form alkaline earth metal aluminates. Three endothermic peaks are evident in the DSC curves of the sample precursors (Fig. 2a). Their TG curves also show favorable agreement with the DSC results. The two endothermic peaks lower than 500 C can be assigned to the dehydration of M(III)(OH)3 to form M(III)2O3. Because the dehydration of Eu(OH)3, Dy(OH)3, and Al(OH)3 simultaneously occurred before 500 C, the patterns of DSC and TG varied considerably. The final peaks at 700e930 C can be assigned to the decomposition of metal carbonates. The samples underwent dehydration and decomposition reactions followed by a recombination of metal
oxides at higher temperature without weight loss to form alkaline earth metals aluminates. The results show that the starting temperature of the recombination of metal oxides decreases linearly when the amount of Eu and Dy doping was increased (Fig. 2). Table 1 presents the starting temperatures of the recombination of the prepared precursors, which are shown in Fig. 2. The maximum calcining temperature of the precursors is equal to or greater than this starting temperature. For each phosphor, a temperature was selected as the maximum calcining temperature that was higher than its starting temperature of 20 C. In this study, the maximum calcining temperature ranged from 920 to 960 C to form efficient phosphors. Because the starting temperature of the recombination of metal oxides decreases linearly by increasing the doping amount of Eu and Dy, a multiple regression analysis was performed using the data of the prepared concentrations of metals in solution (Table 1). The empirical equation resulting from the multiple regression analysis is Eq. (5). The experimental results of the calcining temperatures of Sr1-xEuxDy2xAl2O4 (x ¼ 01.2) phosphors through this
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41
Fig. 2. The DSC and TG patterns of Sr1-3xEuxDy2xAl2O4 (x ¼ 0 0.12) precursors.
empirical model are presented in Table 1. The average absolute relative error between the starting temperatures obtained from Fig. 2 and the temperatures estimated using Eq. (5) was 0.23%. The results demonstrate that the empirical equation can provide a quantification of the calcining temperature of Eu and Dy codoping strontium aluminate phosphors, thereby enabling the synthesis process to operate under optimal conditions for reducing energy use. Tcalcined temp. ¼ 2857.1 3040.7[Al] 3462.8[Sr] 3014.0[Eu] 4103.2[Dy] (5)
3.2. Structure
phosphor in this study was higher than its starting temperature of 20 C. The X-ray diffraction (XRD) patterns of Sr1-3xEuxDy2xAl2O4 (x ¼ 00.12) phosphors are shown in Fig. 3. In undoped strontium aluminates (x ¼ 0), SrAl2O4 is the main phase. The relative amounts of each phase are in the following order: SrAl2O4 > SrAl4O7 >> Sr4Al14O25. Twenty-seven characteristic XRD peaks of the SrAl2O4 phase were observed in the Sr0.94Eu0.02Dy0.04Al2O4 phosphor curve, and four small peaks of the SrAl4O7 phase occurred at 19.6 , 25.1, 32.1, and 32.6 2q. One shaper and two trace peaks of the Sr4Al14O25 phase appeared at 23.8 , 34.3 , and 62.9 2q. The XRD patterns of Sr1-3xEuxDy2xAl2O4 phosphors with a doping amount of x 0.04 were similar, and had amounts similar to those of the SrAl4O7 and Sr4Al14O25 phases. However, the amounts of the EuAlO3, EuAl12O19, and Dy2O3 phases slightly increased by increasing the doping amount. The diffraction peaks of the EuAl12O19 and Dy2O3 phases began to appear at x ¼ 0.04, and the diffraction peaks of the EuAlO3 phase began to appear at
As mentioned, the maximum calcining temperature of each
Table 1 Summary of data for multiple regression and the estimated results. Structures
SrAl2O3 Sr0.97Eu0.01Dy0.02Al2O4 Sr0.94Eu0.02Dy0.04Al2O4 Sr0.88Eu0.04Dy0.08Al2O4 Sr0.82Eu0.06Dy0.12Al2O4 Sr0.76Eu0.08Dy0.16Al2O4 Sr0.70Eu0.10Dy0.20Al2O4 Sr0.84Eu0.02Dy0.14Al2O4c Sr0.64Eu0.12Dy0.24Al2O4 Mean error Gross error a b c
Prepared concentrations of metals in solution, M [Al]
[Sr]
[Eu]
[Dy]
0.4032 0.4058 0.4057 0.4041 0.4059 0.4034 0.4056 0.4030 0.4073 2.1 C 0.23%
0.2000 0.1941 0.1681 0.1881 0.1760 0.1640 0.1520 0.1403 0.1283
e 0.0019 0.0042 0.0041 0.0081 0.0121 0.0163 0.0204 0.0246
e 0.0039 0.0283 0.0081 0.0161 0.0241 0.0320 0.0418 0.0484
The starting temperatures obtained from Fig. 2. The temperatures estimated by Eq. (1). An additional structure was using to facilitate the multiple regression.
Trecombin.a
Testimateb
938 930 928 927 926 920 915 912 898
938.6 929.4 931.7 923.3 927.2 916.7 913.2 912.2 901.7
42
C.-J. Liang, H.-Y. Siao / Journal of Alloys and Compounds 669 (2016) 38e45
Fig. 3. The XRD patterns of Sr1-3xEuxDy2xAl2O4 (x ¼ 0 0.12) phosphors.
x ¼ 0.08. Nevertheless, the peaks of these phases were extremely small. It can be concluded that the points at which Eu and Dy begin to become liberated from the strontium aluminate crystal are x ¼ 0.04 and x ¼ 0.08, respectively. Therefore, a doping amount that is too high does not enhance the formation of Eu emission centers.
3.3. Surface characterization The SEM images of Sr1-3xEuxDy2xAl2O4 (x ¼ 00.12) phosphors are shown in Fig. 4. The surface of undoped strontium aluminate (Fig. 4a) was sparsely covered with a fine grain of approximately 10 nm. These fine grains may be the nanocrystallites of the SrAl4O7 phase compared with its XRD pattern (Fig. 3). Irregular morphological grains of varying sizes were observed in the Eu and Dy codoped strontium aluminate phosphors (Fig. 4beh). The particle size is distributed over a wide range, and the grain boundary is not clean. The average grain sizes of Sr1-3xEuxDy2xAl2O4 phosphors with the doping amounts of x ¼ 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, and 0.12 were approximately 200, 170, 140, 80, 120, 180, and 120 nm (Fig. 4beh), respectively. As shown in Fig. 4e, the morphologies of Sr0.82Eu0.06Dy0.12Al2O4 phosphors with a lower grain size show uniform 1D nanostructures with
lengths of several hundreds of nanometers. The differences among the microstructures may be attributed to the effect of recombination of metal oxides. Europium and dysprosium hydroxides play a crucial role in the recombination of the calcining process. Table 2 presents the surface properties of prepared phosphors. The surface properties and atomic percentage of each phosphor were measured using a BET technique with N2 adsorption and SEM-EDS, respectively. The ranges of the specific surface area, pore volume, and average diameter for prepared phosphors were 5.77e18.91 m2 g1, 0.0136e0.0364 cm3 g1, and 7.59e11.03 nm, respectively. The Sr0.82Eu0.06Dy0.12Al2O4 phosphor has a specific surface area, pore volume, and average diameter close to the averages, which were 13.28 m2 g1, 0.0299 cm3 g1, and 9.24 nm, respectively. Clearly, there is an effect of Eu and Dy codoping on surface properties of the phosphors. The ratio of surface Sr, Al, and O atoms of the undoped strontium aluminate was 1:1.23:1.41. The amounts of surface Al and O atoms were lower than the stoichiometric molar ratio of SrAl2O4. In may refer to SrAl4O7 and Sr4Al14O25 phases in the bulk of the undoped strontium aluminate (Fig. 3). The Al/Sr atom ratios of the prepared phosphors when 0.02 x 0.10 ranged from 1.66 to 2.35, which was lower than their stoichiometric molar
C.-J. Liang, H.-Y. Siao / Journal of Alloys and Compounds 669 (2016) 38e45
43
Fig. 4. SEM photographs and EDX surface element compositions of Sr1-3xEuxDy2xAl2O4 (x ¼ 0 0.12) phosphors. The subfigures from (a) to (h) in sequence are x ¼ 0, 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, and 0.12, respectively.
ratios but higher than the ratio of undoped strontium aluminate. The Al/Sr atom ratio of higher Eu and Dy codoped strontium aluminates (x ¼ 0.12) was 7.30. The reason can be ascribed to the formation of the EuAlO3, EuAl12O19, and Dy2O3 phases on the surface. The range of Dy/Eu atom ratios of the Eu and Dy codoped phosphors was 1.42e1.87, with the minimum at x ¼ 0.06. These
results demonstrate that the precursors and phosphors of uniform molar stoichiometry are easily formed when uses the potassium carbonate coprecipitation method. However, the liberation of Eu and Dy from the strontium aluminate crystal occurs when x 0.08.
44
C.-J. Liang, H.-Y. Siao / Journal of Alloys and Compounds 669 (2016) 38e45
Table 2 Surface characterizations for Sr1-3xEuxDy2xAl2O4 (x ¼ 0 0.12) phosphors. Doping amount, x
0 0.02 0.04 0.06 0.08 0.10 0.12 a b c d
BET surface area (m2 g1)a
5.078 5.873 18.33 13.27 15.03 16.47 18.91
Pore volume (cm3 g1)b
0.0136 0.0159 0.0356 0.0264 0.0289 0.0364 0.0522
Average pore diameter (nm)c
10.74 10.81 7.59 7.97 7.68 8.83 11.03
Surface atomic percentage (%)d Sr
Al
Eu
Dy
O
27.46 11.12 11.03 11.94 13.75 13.79 4.33
33.85 21.74 22.72 21.45 22.76 32.41 31.60
e 0.37 0.49 0.81 1.72 3.63 3.37
e 0.68 0.90 1.15 3.05 6.61 6.31
38.69 66.09 64.86 64.65 58.72 43.56 54.39
Surface areas were measured by a BET technique using N2 adsorption. Single point total pore volume of pores less than 1360 Å diameter at P/Po ¼ 0.9860. Average pore diameter for 4V/A by BET. Surface atomic percentage were measurement by SEM-EDX.
3.4. Luminescence Fig. 5 shows the emission spectra of the phosphors prepared by employing a cryogenic cathodoluminescence system at room temperature. The emission intensity of the prepared phosphors varies according to the following sequence: Sr0.82Eu0.06Dy0.12Al2O4 > Sr0.88Eu0.04Dy0.08Al2O4 > Sr0.76Eu0.08Dy0.16Al2O4 > Sr0.70Eu0.10Dy0.20Al2O4 > Sr0.96Eu0.02Dy0.04Al2O4 > Sr0.64Eu0.12Dy0.24Al2O4. In the emission spectra of the prepared phosphors, the main and minor emission bands are observed at 523.4 nm and 573.0 nm, respectively. This suggests that the emission of approximately 540 nm originates from the Eu2þ 4f65 d1 / 4f7(8S7/ 2þ ions, which are randomly 2) transitions associated with the Eu 2þ distributed at the three Sr sites. One weak emission band of the prepared phosphors, with both x ¼ 0.01 and 0.02, is observed at
610.3 nm. Because the EuAlO3, EuAl12O19, and Dy2O3 phases occur in the structure of Sr0.64Eu0.12Dy0.24Al2O4, the other types of emission centers emit two spectra of 425.0 and 481.3 nm. The peaks at 840.4 and 1037.6 nm are caused by wavelength doubling and scattering of peaks at 425.0 and 523.4 nm, respectively, because of the second harmonic. As mentioned, Sr0.64Eu0.12Dy0.24Al2O4 phosphor has moderate surface characteristics and the highest surface Dy/Eu ratio; hence, its emission intensity is greater than those of others. The emission intensities of the prepared phosphors are increased approximately 23.5-fold when the doping amount is increased from 0.01 to 0.06 (Figs. 5 and 6). Furthermore, it can be observed that the emission intensities of the prepared phosphors with doping amounts ranging from 0.01 to 0.06 increase linearly and rapidly (Fig. 6). The emission intensity increment rate of the phosphors produced using the potassium carbonate coprecipitation method between doping amounts of 0.01 and 0.06 is approximately 10-fold higher than that produced using the solidestate reaction method [28] (Fig. 6). 4. Conclusions
Fig. 5. CL spectra of Sr1-3xEuxDy2xAl2O4 (x ¼ 0 0.12) phoshors.
Synthesis using the potassium carbonate coprecipitation method is simple, rapid, and successful. The method demonstrates many advantages such as low energy requirements, simple equipment, high purity, favorable homogeneity, and short time for preparing mixed metal oxides. The precipitant of potassium carbonate provides optimal hydroxide and carbonate ions under appropriate pH conditions for the coprecipitation reaction of metals to form a well-mixed precursor. This reduces the energy required for the recombination of metal oxides, thereby reducing the final calcining temperature of the crystallization of Eu and Dy codoped strontium aluminates. The optimal calcining temperature can be estimated accurately by using an equation to form an efficient phosphor. The optimal calcining temperature decreases by increasing the doping amount of Eu and Dy metals. The emission intensity increment rate of the phosphors produced using the potassium carbonate coprecipitation method between the doping amounts of 0.01 and 0.06 is approximately 10-fold higher than that produced using the solidestate reaction method. The emission intensities of the prepared phosphors are approximately 23.5-fold higher when the doping amount is increased from 0.01 to 0.06. Equation (5) is crucial for proper design and field operation of a synthesis reactor, particularly in reducing energy use and environmental protection.
C.-J. Liang, H.-Y. Siao / Journal of Alloys and Compounds 669 (2016) 38e45
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Fig. 6. Emission intensities dependence of Eu2þ concentration x for the present study (CL, VII ¼ 20 kV) and reported by Wang et al. (PL, lex ¼ 254 nm) [28].
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