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Foundation item: Project supported by the Fundamental Research Funds for the Central Universities (JUSRP51723B), National Natural Science. Foundation of ...
JOURNAL OF RARE EARTHS, Vol. 35, No. 7, Jul. 2017, P. 652

Structural characterization and optical properties of long-lasting CaAl2O4:Eu2+,Nd3+ phosphors synthesized by microwave-assisted chemical co-precipitation YU Yuan (余 媛), WANG Jian (王 建), WANG Jidong (王纪冬), LI Jing (李 婧), ZHU Yanan (朱亚楠), LI Xiaoqiang (李晓强), SONG Xiaolei (宋晓蕾), GE Mingqiao (葛明桥)* (Key Laboratory of Eco-Textile of Ministry of Education, College of Textiles and Clothing, Jiangnan University, Wuxi 214122, China) Received 28 September 2016; revised 3 March 2017

Abstract: The present paper reported the structural and luminescent properties of Eu2+ and Nd3+ doped CaAl2O4 phosphor. The samples were prepared by microwave-assisted chemical co-precipitation (MA-CCP), a synthesis technique which is suitable for small and uniform particle that could be used directly without grinding. The effects of different microwave temperatures on structure and photoluminescence behavior were studied. Formation of a phosphor and phase purity were confirmed by X-ray diffraction technique (XRD) with variable microwave temperatures. XRD analysis showed that the phosphors prepared by MA-CCP method at the temperature of 750, 900 ºC, respectively and solid-state reaction (SSR) method at 1300 ºC consisted of impurities. Commission Internationale de L'Eclairage (CIE) color coordinates of CaAl2O4:Eu2+,Nd3+ were suitable as blue light emitting phosphor. Excitation and emission peaks of the samples prepared by different methods in this study were almost the same. The images of SEM showed that the size of the phosphors prepared by MA-CCP method reached a submicrometer. Keywords: CaAl2O4:Eu2+,Nd3+; microwave; co-precipitation; optical properties; structural; rare earths

Long persistent phosphors based on alkaline earth aluminates can emit light in visible region for a long time after irradiation with light source[1,2]. MAl2O4:Eu2+,R3+ (M=Sr, Ba, Ca, R=Dy, Nd and La) with strong photo-luminescence (PL) has attracted much interest in recent years due to their good stability and strong PL properties at the blue-green visible region[3] and many researchers have made extensive studies. Therefore these materials are widely used in various applications, e.g., luminous paints for roads, fibers, fabrics, display technology, lighting, etc[4]. In particular, CaAl2O4:Eu2+,Nd3+ has been considered as an excellent phosphor, which possesses a good PL performance in the blue region[5]. And many researches have been studied on the methods to make high-performance CaAl2O4:Eu2+,Nd3+ phosphor. However, CaAl2O4:Eu2+,Nd3+ is mainly synthesized by the conventional solid-state reaction (SSR) method[6]. This process requires high temperature and a long period of time, also should be in a reductive atmosphere[7]. Moreover, phosphor powders have the grain size and the asymmetrical distribution of components. Phosphors of small particles are difficult to obtain by crushing the hard

blocks, which will reduce luminescence intensity and introduce additional defects. To overcome these disadvantages, wet chemical methods including co-precipitation[8], sol-gel[9–11], microemulsion[12], detonation method[13], hydrothermal treatment[14] and combustion synthesis method[15,16] have been applied to prepare phosphors. Each of these methods has its own advantages and disadvantages. Among the most used methods, the co-precipitation method is the simplest with low cost. It can be accurately controlled and uniformly mixed. The disadvantages of the co-precipitation, however, contained the agglomerated particles with low crystallinity[17]. One method that can greatly increase the reaction rate and at the same time overcome the disadvantage of the co-precipitation method is microwave irradiation. In this work, a microwave-assisted chemical co-precipitation (MA-CCP) method[18] was firstly applied to synthesize the CaAl2O4:Eu2+,Nd3+ particles. Compared with the SSR method, the MA-CCP process is very facile, safe and energy saving. And the effects of microwave irradiation temperatures on the final submicrometer phosphors were investigated systematically.

Foundation item: Project supported by the Fundamental Research Funds for the Central Universities (JUSRP51723B), National Natural Science Foundation of China (51503083), Jiangsu Province Ordinary University Academic Degree Graduate Student Scientific Research Innovation Projects (KYLX16_0798), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and Production, Education & Research Cooperative Innovation Fund Project of Jiangsu Province (BY2015057-23) * Corresponding author: GE Mingqiao (E-mail: [email protected]; Tel.: +86-510-85912329) DOI: 10.1016/S1002-0721(17)60959-1

YU Yuan et al., Structural characterization and optical properties of long-lasting CaAl2O4:Eu2+,Nd3+ phosphors …

1 Experimental 1.1 Sample preparation The samples were prepared by MA-CCP method. The CaAl2O4:Eu2+,Nd3+ phosphor was initially prepared by Ca(NO3)2 (A.R.), Al(NO3)3 (A.R.), Eu2O3 (99.99%), Nd2O3 (99. 99%), then a certain amount of sodium dodecyl sulfate (SDS) and ammonium carbonate ((NH4)2CO3) were added. Firstly, Eu2O3 and Nd2O3 were dissolved in nitric acid, and then mixed with the solution of Ca(NO3)2, Al(NO3)3 and a certain amount of SDS as surfactant. Following that, the mixed solution was stirred by a magnetic heated stirrer and the (NH4)2CO3 solution was added dropwise to precipitate the nitrates. The previous experimental results[19] showed that the particles agglomerated easily and the particle size was not uniform in acidic conditions. While the morphology of products was excellent in neutral or alkaline conditions. So the optimum condition was in the statement of PH=7 in the experimental period. After Filtering, washing and drying, white powder was obtained. Afterwards, we put this white powder into an alumina crucible with an alumina crucible filled with carbon particles. Then, the alumina crucibles were placed in the microwave furnace (Chengyue CY-F1700-2IT), and the samples were performed at 750, 900, 1100, 1250 ºC, respectively for 1h. Finally, the samples were obtained. The synthesis process of CaAl2O4:Eu2+,Nd3+ by MA-CCP is summarized in Fig. 1. In order to compare the properties such as luminescence property and surface morphology of the products, a coarse CaAl2O4:Eu2+,Nd3+ phosphor was also prepared by SSR method. The phosphor of SSR was prepared by CaCO3, Al2O3, H3BO3 and rare-earth oxides Eu2O3, Nd2O3. According to the same molar stoichiometry of MA-CCP, appropriate amounts of raw materials were thoroughly mixed, then heated at 1300 ºC for 3 h under 95%N2+5%H2 atmosphere[20,21]. The sample was obtained after cooling. 1.2 Characterization The X-ray diffraction (D8 Advance X-ray diffractormeter, Bruker AXS, Germany) was used to measure the

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phase composition of the luminescent materials by a Cu Kα radiation at a voltage of 40 kV, and a current of 30 mA, of which the diffraction angle 2θ ranges from 10° to 70° and the scan speed is 4(°)/min at room temperature. The afterglow of samples was tested by a PR-305 long afterglow phosphors tester. Excitation and emission spectra of the samples were measured by a spectrophotometer ((HITACHI 650-60, Japan). With the help of scanning electron microscopy (HITACHI SU-1510, Japan), the structure and morphology of the phosphor particles were determined.

2 Results and discussion 2.1 XRD phase analysis In order to determine the crystal structure, powder XRD analysis was carried out. The XRD patterns of luminescent powders prepared by MA-CCP and SSR methods are shown in Fig. 2. The results reveal a complex reaction process during the formation of CaAl2O4:Eu2+,Nd3+ phosphor. Comparing with JCPDS card (No. 70-0134), the major peaks of the samples prepared by MA-CCP at the temperatures of 1100 and 1250 ºC are indexed to the crystal phase CaAl2O4. The space group is p21/n(14), the cell constants of a=0.87 nm, b=0.8092 nm, c=1.5191 nm, β=90.17º. With the rise of the microwave temperature, the degree of CaAl2O4 crystallization phase increases as well. By the Debye-Scherrer formula and D=kλ/(βcosθ), the average grain size of the samples can be calculated. k=0.89, λ=0.1541 nm is diffraction wavelength of X-ray. By calculation, we can get the point that with the increase of microwave temperature, grain size of the samples increases gradually[22]. Although the main phase CaAl2O4 exists, there are still strong peaks at around 2θ=25.43° and 2θ=32.5° belonging to the diffraction peaks of CaAl4O7 (JCPDS card No. 76-0706) when the temperatures are only 750 and 900 ºC. And a few XRD lines are present in doublet, which demonstrates that the lattice has two dissimilar lattice parameters, thus this suggests that the phase is changing from hexagonal phase to monoclinic phase at 1050 ºC which had been reported before[23,24]. The phosphor pre-

Fig. 1 Manufacturing technique of MA-CCP method

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Fig. 2 XRD patterns of CaAl2O4:Eu2+,Nd3+ phosphors prepared by MA-CCP and SSR methods

pared by SSR method also has a mixed crystalline phase of CaAl2O4 and CaAl4O7. It indicates that the phosphors prepared by MA-CCP method at the temperature of 750, 900 ºC respectively and SSR method at 1300 ºC consist impurities. The reason is that the samples can fully crystallize only when the temperature is above 1100 ºC by the method of MA-CCP, while the required temperature of SSR is higher than MA-CCP. The product does not crystallize completely even at 1300 ºC for 3 h. Furthermore, it is easier to generate a variety of crystal phase and can not get a single crystal phase in the process of SSR method. 2.2 Afterglow intensity profile The afterglow decay curves of luminescent phosphors made by MA-CCP in different temperatures and SSR method are shown in Fig. 3. From the graph, we know that the afterglow decay process of the MA-CCP method shows the similar characteristics to that of SSR method. The afterglow decay process of samples consists of a rapid decay and then a long-lasting decay. In the quick process, the afterglow intensity decreases rapidly in tens of seconds. While in the slow decay process, the intensity

Fig. 3 Decay curves of CaAl2O4:Eu2+,Nd3+ phosphors prepared by MA-CCP and SSR methods a: SSR method; b: MA-CCP (b1: 750 ºC, b2: 900 ºC, b3: 1100 ºC, b4: 1250 ºC)

JOURNAL OF RARE EARTHS, Vol. 35, No. 7, Jul. 2017

decreases slowly and persists from several hours to ten hours[25,26]. From Fig. 3, we know that the initial afterglow intensity increases with the rise of microwave temperature by the method of MA-CCP. From XRD phase analysis, we can find that when the temperatures at exact 750 and 900 ºC, the main phase CaAl2O4 forms basically, while the reaction is inadequate and other impurity phases are existing. Therefore, the initial afterglow intensity is relatively low. At the temperature of 1100 ºC, there is almost no impurity. While the particles are small, which could influence the initial luminous intensity. Although the phosphor powder made by SSR at 1300 ºC contained other material phases, the initial afterglow intensity is still higher than the samples made by MA-CCP. The reason is that the particles prepared by SSR without grinding are much larger than MA-CCP and the degree of crystallinity is also higher than MA-CCP. It is suggested that, long duration afterglow process is dominated by the recombination process of the electrons, which are thermally released from the traps. And the schematic PL mechanism of CaAl2O4:Eu2+,Nd3+ phosphor is shown in Fig. 4. The electrons in the ground state can jump to excitation state and then be captured in the trap energy levels due to the thermal disturbance. Then the excited electrons can move to the ground state and form luminescence. Electrons jump back and forth between the excitation state 4f65d1 and ground state 4f7(8S7/2) with the help of thermal disturbance[27], so the long afterglow phenomenon is formed. The doping of Nd3+ can not only increase density of trap but also introduce deeper electron trap centers[25,28]. The trap energy levels can capture more electrons and extend the average time[29]. So CaAl2O4:Eu2+,Nd3+ phosphor has higher luminescence intensity and longer decay time compared with CaAl2O4:Eu2+ phosphor. 2.3 Excitation spectral analysis Fig. 5 describes the excitation spectra of the phosphors prepared by MA-CCP and SSR methods. The excitation spectrum is a 240–360 nm broad band. The samples prepared by different methods contain two main excitation

Fig. 4 Schematic PL mechanism of CaAl2O4:Eu2+,Nd3+ phosphor

YU Yuan et al., Structural characterization and optical properties of long-lasting CaAl2O4:Eu2+,Nd3+ phosphors …

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Table 1 Chromaticity coordinates of the CaAl2O4:Eu2+,Nd3+ phosphors prepared by MA-CCP and SSR methods Methods

MA-CCP

SSR

Fig. 5 Excitation spectra of CaAl2O4:Eu2+,Nd3+ phosphors prepared by MA-CCP and SSR methods a: SSR method; b: MA-CCP (b1: 750 ºC, b2: 900 ºC, b3: 1100 ºC, b4: 1250 ºC)

peaks and the peaks are slightly different. As we all know, the absorption peaks are attributed to the Eu2+ due to the 4f7(8S7/2)→4f65d1 transition[30]. The slight difference of excitation peaks can be attributed to the reason that different preparation methods should lead to the differences of surface morphology and internal lattice[31]. The CaAl2O4:Eu,Nd phosphor prepared by microwave method is a kind of sub-micron particles formed from a large number of nanoparticles. There are a large amount of voids among the particles. However, the sample prepared by SSR method is micron scale particles, which will influence the excitation wavelength. And we can observe that the excitation peaks of the samples prepared by different methods in this study are almost the same. Fig. 6 shows CIE chromaticity coordinates and emission spectrum of luminous materials prepared by MA-CCP and SSR methods. The wavelength of excitation light is 320 nm. The CaAl2O4:Eu2+,Nd3+ phosphor obtained from SSR method reveals emitting peak at

CIE

T/ºC

x

y

b1

750

0.1729

0.0698

b2

900

0.1746

0.0746

b3

1100

0.1689

0.0723

b4

1250

0.1695

0.0806

a

1300

0.1645

0.0850

442.5 nm, while the phosphors obtained by MA-CCP method reveal emitting peak at 441.8 nm. There are few differences of emission between MA-CCP method and SSR method. The slightly band shift can be ascribed as the size of particles made by MA-CCP method is smaller than SSR method, and with the decrease of the particle size, the surface energy of the particles can be dramatically increased[32] and crystalline field around Eu2+ can be changed, therefore lead to slight difference in the emission spectrum. 2.4 SEM analysis The morphologies of obtained phosphors observed by SEM are shown in Fig. 7. Compared with phosphor prepared by SSR method, which normally has an average diameter size about 5–20 μm, the size of the phosphor prepared by MA-CCP method reached a submicrometer level. It can not only avoid the damage of crystal structure, but also ensure no introduction of other impurities during the process of grinding. The reason for different morphologies could be explained as the powders are reunited together due to the capillary contraction in the high-temperature calcination process. However, the "burst" mechanism exists in the microwave heating process, at the same time the internal water is excluded as gaseous form during the process, which caused the agglomeration strength of CaAl2O4:Eu2+,Nd3+ phosphor weakened and the structure of the phosphor is loose. It can be observed that the large particles prepared by microwave-assisted chemical co-precipitation method are agglomerated by a large amount of small particles. The sodium dodecyl sulfate (SDS) is a kind of surfactant and excellent dispersant, which can reduce surface tension of the precursor solution effectively, therefore can reduce the degree of agglomeration and increase specific surface area of powders[33].

3 Conclusions

Fig. 6 Luminous character to the light of color as well as emission spectrum of the CaAl2O4:Eu2+,Nd3+ phosphors prepared by MA-CCP and SSR methods

Long afterglow phosphor CaAl2O4:Eu2+,Nd3+ was prepared by microwave-assisted chemical co-precipitation (MA-CCP), a synthesis technique at different microwave temperatures and the properties of the samples were studied. The results showed that when the microwave

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JOURNAL OF RARE EARTHS, Vol. 35, No. 7, Jul. 2017

Fig. 7 SEM micrographs of CaAl2O4:Eu2+,Nd3+ phosphors prepared by MA-CCP and SSR methods a: SSR method; b: MA-CCP (b1: 750 ºC, b2: 900 ºC, b3: 1100 ºC, b4: 1250 ºC)

temperature was below 1100 ºC, the main phase CaA12O4 could not fully crystallize. While the required temperature of SSR method was higher than MA-CCP method and the crystallization was not complete even at 1300 ºC. The samples had strong initial brightness and long afterglow time, and the color of the afterglow was blue-violet. The phosphor exhibited two absorption bands attributed to the Eu2+ due to the 4f7(8S7/2)→4f65d1 transition. Excitation and emission peaks of the samples prepared by different methods in this study were almost the same. This size of the phosphor prepared by MA-CCP method reached a submicrometer level that can be used directly without grinding.

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