J. Phys. Chem. C 2009, 113, 2685–2689
2685
Novel Necklace-like MAl2O4:Eu2+, Dy3+ (M ) Sr, Ba, Ca) Phosphors via a CTAB-Assisted Solution-Phase Synthesis and Postannealing Approach Xiang Ying Chen,*,†,‡ Chao Ma,† Xiao Xuan Li,†,‡ Cheng Wu Shi,†,‡ Xue Liang Li,†,‡ and Dao Rong Lu†,‡ School of Chemical Engineering, Hefei UniVersity of Technology, Hefei, Anhui 230009, People’s Republic of China, and Anhui Key Laboratory of Controllable Chemistry Reaction & Material Chemical Engineering, Hefei, Anhui 230009, People’s Republic of China ReceiVed: July 19, 2008; ReVised Manuscript ReceiVed: September 27, 2008
We herein present a facile but efficient CTAB-assisted solution-phase synthesis and postannealing approach for preparing a series of necklace-like phosphors, including SrAl2O4:Eu2+, Dy3+, CaAl2O4:Eu2+, Dy3+, and BaAl2O4:Eu2+, Dy3+. The as-prepared samples were characterized by means of XRPD, FESEM, TEM, and PL techniques. The experimental results indicate that CTAB plays the crucial role in the formation of needlelike precursors in solution, which are further converted into the final phosphors in a reducing atmosphere of H2/Ar (15% + 85%). To investigate the optical characteristics of the aluminate phosphors, we conducted the excitation, emission, and afterglow decaying measurements at room temperature. It is noteworthy that the aluminate phosphors are so floppy after being calcined at 1300 °C for 5 h, which effectively avoids the traditional crushing of hard phosphor blocks into small particles. 1. Introduction Long-lasting rare earth doped inorganic phosphors have attracted much interest because of their excellent luminescent properties and applications in the wide range of lamp industry, radiation dosimetry, X-ray imaging, and color display.1-3 For example, Eu2+-doped alkaline earth aluminates (MAl2O4:Eu2+, M ) Sr, Ba, Ca) exhibit intense broadband emissions, which are generated from the electronic transitions between the ground state of 4f7 and the excited state of 4f65d1 of Eu2+ ions. Compared with the traditionally used sulfide-based phosphors, this kind of aluminates possess several advantages such as the lack of radioactive elements, lower chemical toxicity, and higher chemical and moisture stability. On the other hand, to enhance the initial intensity of luminescence and afterglow of Eu2+-doped aluminates, one usually introduces a certain amount of Ln3+ coactivators (most commonly as Nd, Dy), which result in a more than 10-fold increase on them.4 As we know, the SrAl2O4:Eu2+, Dy3+, BaAl2O4:Eu2+, Dy3+, and CaAl2O4:Eu2+, Dy3+ samples are most promising phosphors, which respectively display the intense green, green, and blue colors with long afterglow characteristics. To date, much effort has been devoted to preparing these phosphors through various routes. For example, SrAl2O4:Eu2+, Dy3+ phosphors were prepared by the sol-gel route,5 coprecipation method,6 solidstate method,7 combustion route,8 etc. BaAl2O4:Eu2+, Dy3+ phosphors were synthesized through the solid-state method,9,10 combustion route,11 etc. CaAl2O4:Eu2+, Dy3+ phosphors were prepared by the coprecipation method,12 solid-state method,13,14 combustion route,15 etc. On the other hand, nano- or microscale materials have received continuous attention owing to their physical and chemical properties as well as their use as building blocks for * Corresponding author. Phone: +86-551-2901450. Fax: +86-5512901450. E-mail:
[email protected]. † Hefei University of Technology. ‡ Anhui Key Laboratory of Controllable Chemistry Reaction & Material Chemical Engineering.
nanodevices, which commonly differ from those of bulky materials.16,17 However, as far as we know, there are few reports on the synthesis of necklace-like Eu2+-doped aluminate phosphors. Herein, we developed the CTAB-assisted solution-phase synthesis and postannealing approach to prepare aluminate phosphors of SrAl2O4:Eu2+, Dy3+, BaAl2O4:Eu2+, Dy3+, and CaAl2O4:Eu2+, Dy3+. It is found that the surfactant of CTAB acts as the shape-directed agent to prepare the needle-like precursors in solution. These precursors were further converted into the phosphors in a reducing atmosphere. The optical properties of these phosphors were also investigated in brief. 2. Experimental Section All the chemicals are of analytical grade and were used as received without further purification. 2.1. Typical Solution-Phase Method for Preparing the Precursor of SrAl2O4:Eu2+, Dy3+ Phosphor. Sr(NO3)2 (4mmol), Al(NO3)3 · 9H2O (8mmol), CO(NH2)2 (100 mmol), and cetyl trimethyl ammonium bromide (CTAB, 4 mmol) were dissolved in distilled H2O (80 mL) in a beaker under magnetic stirring. After being stirred for 30 min, the resulting white suspension was transferred into a 100 mL Teflon-lined stainless steel autoclave, which was then sealed and kept at 100 °C. After 12 h, the resulting fluffy white product was filtered off, washed with distilled water and absolute ethanol several times, and then dried under vacuum at 60 °C for 6 h. 2.2. Typical Postannealing Approach for preparing the SrAl2O4:Eu2+, Dy3+ Phosphor. The final SrAl2O4:Eu2+, Dy3+ phosphor was obtained in an electric furnace by postannealing the precursor in a reducing atmosphere of H2/Ar (15% + 85%) at 1300 °C for 5 h. Similarly, CaAl2O4:Eu2+, Dy3+ and BaAl2O4:Eu2+, Dy3+ samples were prepared by respectively substituting the starting material of Sr(NO3)2 as BaCl2 · 2H2O (4 mmol) and CaCl2 · 2H2O (4 mmol) while keeping other reaction parameters unchanged. 2.3. Characterization. X-ray powder diffraction (XRPD) patterns were obtained on a Rigaku Max-2200 with Cu KR
10.1021/jp806375p CCC: $40.75 2009 American Chemical Society Published on Web 01/27/2009
2686 J. Phys. Chem. C, Vol. 113, No. 7, 2009
Figure 1. XRPD patterns of (a) the precursor obtained by hydrothermal treatment at 100 °C for 12 h and (b) nanophosphors obtained by calcining the precursor in a reducing atmosphere of H2/Ar (15% + 85%) at 1300 °C for 5 h, using Sr(NO3)2, Al(NO3)3 · 9H2O, CO(NH2)2, and CTAB as the starting materials: (/) orthorhombic NH4Al(OH)2CO3 (JCPDS Card 71-1314); (#) orthorhombic SrCO3 (JCPDS Card 050418); and (§) monoclinic SrAl2O4 (JCPDS Card 34-0379).
Chen et al.
Figure 2. XRPD patterns of (a) the precursor obtained by hydrothermal treatment at 100 °C for 12 h and (b) nanophosphors obtained by calcining the precursor in a reducing atmosphere of H2/Ar (15% + 85%) at 1300 °C for 5 h, using CaCl2 · 2H2O, Al(NO3)3 · 9H2O, CO(NH2)2, and CTAB as the starting materials: (/) orthorhombic NH4Al(OH)2CO3 (JCPDS Card 71-1314); (#) monoclinic CaCO3 (JCPDS Card 29-0305); (§) monoclinic CaAl2O4 (JCPDS Card 700134); ($) monoclinic CaAl4O7 (JCPDS Card 23-1037); and (&) cubic Ca12Al14O33 (JCPDS Card 09-0413).
radiation. Field emission scanning electron microscopy (FESEM) images were taken with a Hitachi S-4800 scanning electron microscope. Transmission electron microscopy (TEM) images were taken on a JEOL 2010 instrument performed at 200 kV. Photoluminescent (PL) analysis was conducted on a Hitachi F-4500 spectrophotometer with Xe lamp at room temperature. 3. Results and Discussion The X-ray powder diffraction (XRPD) technique is an effective tool to determine the phase, crystallinity, and purity of samples prepared under various conditions. We first investigated the reaction system containing Sr(NO3)2, Al(NO3)3 · 9H2O, CO(NH2)2, and CTAB as the starting materials. With hydrothermal treatment of the above starting materials at 100 °C for 12 h, we obtain white fluffy products in an autoclave. The corresponding XRPD pattern is shown in Figure 1a, which indicates that the precursor is composed of orthorhombic NH4Al(OH)2CO3 (JCPDS Card 71-1314) and orthorhombic SrCO3 (JCPDS Card 05-0418). Next, pure monoclinic SrAl2O4 (JCPDS Card 34-0379) samples were obtained by calcining the precursor in a reducing atmosphere of H2/Ar (15% + 85%) at 1300 °C for 5 h, shown in Figure 1b. Similarly, when we hydrothermally treated the reaction system containing CaCl2 · 2H2O, Al(NO3)3 · 9H2O, CO(NH2)2, and CTAB as the starting materials at 100 °C for 12 h, white fluffy products appeared in the autoclave. The typical XRPD pattern in Figure 2a demonstrates the resulting precursor consists of orthorhombic NH4Al(OH)2CO3 (JCPDS Card 71-1314) and monoclinic CaCO3 (JCPDS Card 29-0305). However, when further calcining the precursor in a reducing atmosphere of H2/ Ar (15% + 85%) at 1300 °C for 5 h, the mixture of monoclinic CaAl2O4 (JCPDS Card 70-0134), monoclinic CaAl4O7 (JCPDS Card 23-1037), and cubic Ca12Al14O33 (JCPDS Card 09-0413) occurred, as shown in Figure 2b. This kind of result in Figure 2b was also reported by Kim and co-workers.13 When the reaction system containing BaCl2 · 2H2O, Al(NO3)3 · 9H2O, CO(NH2)2, and CTAB as the starting materials was hydrothermally treated at 100 °C for 12 h, we can also obtain white fluffy products in the autoclave. The typical XRPD pattern in Figure 3a reveals the resulting precursor consists of orthorhombic NH4Al(OH)2CO3 (JCPDS Card 71-1314) and orthorhombic BaCO3 (JCPDS Card 05-0378). When further
Figure 3. XRPD patterns of (a) the precursor obtained by hydrothermal treatment at 100 °C for 12 h and (b) nanophosphors obtained by calcining the precursor in a reducing atmosphere of H2/Ar (15% + 85%) at 1300 °C for 5 h, using BaCl2 · 2H2O, Al(NO3)3 · 9H2O, CO(NH2)2, and CTAB as the starting materials: (/) orthorhombic NH4Al(OH)2CO3 (JCPDS Card 71-1314); (#) orthorhombic BaCO3 (JCPDS Card 05-0378); and (§) hexagonal BaAl2O4 (JCPDS Card 170306).
calcining the precursor in a reducing atmosphere of H2/Ar (15% + 85%) at 1300 °C for 5 h, pure hexagonal BaAl2O4 (JCPDS Card 17-0306) samples were obtained, shown in Figure 3b. The FESEM technique has been proved to be a powerful tool to vividly illustrate the sizes and shapes of samples. Panels a and b of Figure 4 demonstrate the representative FESEM images of the precursor obtained by hydrothermal treatment at 100 °C for 12 h, using Sr(NO3)2, Al(NO3)3 · 9H2O, CO(NH2)2, and CTAB as the starting materials. It indicates that the sample is mainly composed of needle-like structures with diameters of hundreds of nanometers and lengths up to tens of micrometers, indicating their high aspect ratio. Next, by calcining the precursor in a reducing atmosphere of H2/Ar (15% + 85%) at 1300 °C for 5 h, we obtained the pure SrAl2O4 samples in terms of the XRPD pattern. The shape and size of these samples were given in Figure 4c-f. It reveals that the SrAl2O4 sample is basically made up of abundant wires, having diameters of hundreds of nanometers and lengths up to tens of micrometers. In particular, the TEM image in Figure 4f tells us that the
Novel Necklace-like MAl2O4:Eu2+, Dy3+ Phosphors
Figure 4. (a, b) FESEM images of the precursor obtained by hydrothermal treatment at 100 °C for 12 h and (c-f) FESEM and TEM images of SrAl2O4 samples obtained by calcining the precursor in a reducing atmosphere of H2/Ar (15% + 85%) at 1300 °C for 5 h, using Sr(NO3)2, Al(NO3)3 · 9H2O, CO(NH2)2, and CTAB as the starting materials.
SrAl2O4 samples are actually constructed with nanoparticles along the 1-D direction. Thus, we can conclude that the needlelike precursors in fact act as the self-sacrificing templates for praparing the final necklace-like SrAl2O4 samples. Panels a and b of Figure 5 show typical FESEM images of the precursor obtained by hydrothermal treatment at 100 °C for 12 h, using CaCl2 · 2H2O, Al(NO3)3 · 9H2O, CO(NH2)2, and CTAB as the starting materials. We can obviously see that large numbers of needle-like structures appear in the precursors. To prepare the CaAl2O4 samples, we calcined the corresponding precursor in a reducing atmosphere of H2/Ar (15% + 85%) at 1300 °C for 5 h. The FESEM and TEM images are demonstrated in Figure 5c-f. As expected, the resulting samples are composed of necklace-like structures formed by the self-assembly of nanoparticles. With hydrothermal treatment of the reaction system consisting of Ba(NO3)2, Al(NO3)3 · 9H2O, CO(NH2)2, and CTAB as the starting materials at 100 °C for 12 h, the resulting samples were depicted by the FESEM tool. On the basis of the FESEM images having various magnifications in Figures 6a-c, we can see that the as-prepared precursors are also comprised of needle-like structures. They are hundreds of nanometers in diameter and tens of micometers in length, showing their high aspect ratio. When further calcining the precursor in a reducing atmosphere of H2/Ar (15% + 85%) at 1300 °C for 5 h, we can obtain BaAl2O4 samples, as shown in Figure 6d-f. We can see that the BaAl2O4 samples mainly consist of necklace-like structures, which have diameters of hundreds of nanometers and lengths up to several micrometers. Apparently, the as-prepared needlelike precursors, acting as the self-sacrificing templates, play a crucial role in the formation of necklace-like BaAl2O4 structures.
J. Phys. Chem. C, Vol. 113, No. 7, 2009 2687
Figure 5. (a, b) FESEM images of the precursor obtained by hydrothermal treatment at 100 °C for 12 h and (c-f) FESEM and TEM images of CaAl2O4 samples obtained by calcining the precursor in a reducing atmosphere of H2/Ar (15% + 85%) at 1300 °C for 5 h, using CaCl2 · 2H2O, Al(NO3)3 · 9H2O, CO(NH2)2, and CTAB as the starting materials.
To investigate the role of CTAB in the formation of necklacelike alkaline earth aluminates, we conducted some parallel experiments in this study. When calcining the precursor in a reducing atmosphere of H2/Ar (15% + 85%) at 1300 °C for 5 h, using Sr(NO3)2, Al(NO3)3 · 9H2O, and CO(NH2)2 as the starting materials in the absence of CTAB, the resulting SrAl2O4 samples are actually composed of irregular submicrometer particles instead of necklace-like structures (not shown here). This result agrees well with the previous work on the preparation of SrAl2O4 samples.8 Consequently, we can conclude that adding CTAB into the present reaction system plays a crucial role in preparing the final aluminate structures. Meanwhile, as a kind of typical cationic surfactant, CTAB was widely chosen as the shape-directed agent to prepare 1-D nanostructures. For example, Xie18 and Li19 repectively prepared GaP, InP nanowires, and WS2 nanotubes by introducing CTAB to restrict their growth along the 1-D direction. In this study, a series of needle-like precursors (usually occurring as a mixture) were first hydrothermally synthesized with the help of CTAB in solution. Next, these precursors were further calcined at high reaction temperature to form the final aluminate phosphors. Besides, the valence state of Eu3+ in the compounds converts into Eu2+ in the reducing atmosphere of H2/Ar (15% + 85%). To determine the luminescent properties of the as-prepared aluminate phosphors, we carried out the PL measurement at room temperature. The excitation and emission spectra of the SrAl2O4:Eu2+, Dy3+ phosphors are shown in Figure 7. The corresponding excitation spectrum shows a broadband from 325 to 365 nm, and its green emission is a symmetrical band at 512 nm that originates from the energy transition between the ground state of 4f7 and the excited state of 4f65d1 of Eu2+ ions.20 The emission wavelengths are consistent with the result derived from
2688 J. Phys. Chem. C, Vol. 113, No. 7, 2009
Chen et al.
Figure 6. (a-c) FESEM images of the precursor obtained by hydrothermal treatment at 100 °C for 12 h and (d-f) FESEM and TEM images of BaAl2O4 samples obtained by calcining the precursor in a reducing atmosphere of H2/Ar (15% + 85%) at 1300 °C for 5 h, using Ba(NO3)2, Al(NO3)3 · 9H2O, CO(NH2)2, and CTAB as the starting materials.
Figure 7. The excitation and emission spectra of the SrAl2O4: Eu2+, Dy3+ phosphors.
Figure 8. The excitation and emission spectra of the CaAl2O4: Eu2+, Dy3+ phosphors.
the sol-gel route,5 but slightly shorter than the 521 nm by the high-temperature ceramic sintering.21 Regarding the CaAl2O4:Eu2+, Dy3+ phosphors, a blue emission at 440 nm appears at λex ) 321 nm, as shown in Figure 8, which is also derived from the transition of Eu2+ ions between 4f65d1 and 4f7 levels. Furthermore, the symmetry and nonwinding emission band illustrates the existence of only one kind of luminescent center in CaAl2O4:Eu2+, Dy3+, where the Eu2+ ions are substituted with the nine-coordinated Ca2+ sites.13
In case of the BaAl2O4:Eu2+, Dy3+ phosphors, Figure 9 indicates the symmetrical and broad emission band centered at 495 nm at λex ) 324 nm, which also comes from the transition of Eu2+ ions between 4f65d1 f 4f7 levels.11 Here, it should be pointed out that the emission wavelengths of the MAl2O4: Eu2+, Dy3+ (M ) Ca, Sr, Ba) phosphors do not shift from short to long wave with increasing the ordinal number of alkali earth metals (Ca, Sr, and Ba). This is due to the fact that Eu2+ and Dy3+ are subject to the action of different crystal fields after M2+ ions are substituted by them.22 The radius of Sr2+ and Eu2+
Novel Necklace-like MAl2O4:Eu2+, Dy3+ Phosphors
J. Phys. Chem. C, Vol. 113, No. 7, 2009 2689 afterglow intensity and decay time of phosphors is SrAl2O4 > BaAl2O4 > CaAl2O4. The reason may lie in the fact that SrAl2O4:Eu2+, Dy3+ and CaAl2O4:Eu2+, Dy3+ phosphors are of monoclinic structure, generally giving rise to the formation of an appropriate trap for producing afterglow, while BaAl2O4: Eu2+, Dy3+ having a hexagonal structure is not suitable for this. 4. Conclusion
Figure 9. The excitation and emission spectra of the BaAl2O4: Eu2+, Dy3+ phosphors.
In summary, the CTAB-assisted solution-phase synthesis and postannealing approach was developed to prepare necklace-like aluminate phosphors, including SrAl2O4:Eu2+, Dy3+, CaAl2O4: Eu2+, Dy3+, and BaAl2O4:Eu2+, Dy3+. This study reveals that CTAB plays a crucial influence on preparing the final products in a reducing atmosphere. The optical properties of the aluminate phosphors were also studied. Especially, in contrast to the conventional solid-state route, the present synthetic method has the advantage of avoiding crushing the hard phosphor blocks into small particles, which generally leads to a decrease in the luminescence intensity. Acknowledgment. This project is supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, Ministry of Education of China, and Research Program for Young Teachers in Anhui Provincial Higher Education Institutions (2008jq1014zd). The authors also gratefully thank Dr. Zheng Hua Wang and Dr. Zhi Zhao at Anhui Normal University and USTC for their respective assistances with FESEM and optical characterization. References and Notes
Figure 10. Afterglow decay curves of the aluminate phosphors after exciting for 5 min: (a) SrAl2O4:Eu2+, Dy3+; (b) CaAl2O4:Eu2+, Dy3+; and (c) BaAl2O4:Eu2+, Dy3+.
is close and their valence is the same, so there is no apparent effect on the crystal structure when replacing Sr2+ by Eu2+. Whereas the radius of Eu2+ is larger than that of Ca2+, and the crystal will thus expand and the inter-repulsive force will minimize, which shifts the emission to long wavelength. On the other hand, the radius of Eu2+ is smaller than that of Ba2+, and the crystal will therefore contract and the gravitation will also minimize, resulting in the emission to short wavelength.23 Figure 10 depicts the typical afterglow decay curves of the aluminate phosphors after exciting for 5 min and a 365 nm xenon lamp is used as the light source. We can clearly see that all the afterglow decay curves are composed of two regimes, i.e., the initial rapid-decaying process and the subsequent slowdecaying process. As we know, Eu2+ ions are the luminescent centers and Dy3+ ions are the traps in aluminate phosphors. The long afterglow property usually results from the trap energy level produced by doping Eu2+ and Dy3+ ions in the crystals. The rapid-decaying process is due to the short survival time of the electron in Eu2+ and the slow-decaying process is due to the deep trap energy center of Dy3+.24 Meanwhile, the order of
(1) Feldmann, C.; Justel, T.; Ronda, C. R.; Schmidt, P. J. AdV. Funct. Mater 2004, 13, 511. (2) Saines, P. J.; Elcombe, M. M.; Kennedy, B. J. J. Solid State Chem. 2006, 179, 613. (3) Sakai, R.; Katsumata, T.; Komuro, S.; Morikawa, T. J. Lumin. 1999, 85, 149. (4) Aitasalo, T.; Deren, P.; et al. J. Solid State Chem. 2003, 171, 114. (5) Lu, Y. Q.; Li, Y. X.; Xiong, Y. H.; et al. Microelectron. J. 2004, 35, 379. (6) Lin, Y. H.; Zhang, Z. T.; Zhang, F.; Tang, Z. L.; Chen, Q. M. Mater. Chem. Phys. 2000, 65, 103. (7) Zhu, C. F.; Yang, Y. X.; Chen, G. R.; et al. J. Phys. Chem. Solids 2007, 68, 1721. (8) Qiu, Z. F.; Zhou, Y. Y.; Lv, M. K.; et al. Acta Mater. 2007, 55, 2615. (9) Suriyamurthy, N.; Panigrahi, B. S. J. Lumin. 2007, 127, 483. (10) Xing, D. S.; Gong, M. L.; et al. Mater. Lett. 2006, 60, 3217. (11) Singh, V.; Natarajan, V.; Zhu, J. J. Opt. Mater. 2007, 29, 1447. (12) Chang, C. K.; Xu, J.; et al. Mater. Chem. Phys. 2006, 98, 509. (13) Park, Y. J.; Kim, Y. J. Mater. Sci. Eng. B 2008, 146, 84. (14) Zhao, C. L.; Chen, D. H. Mater. Lett. 2007, 61, 3673. (15) Singh, V.; Zhu, J. J.; et al. Opt. Mater. 2007, 30, 446. (16) Xia, Y. N.; Yang, P. D.; et al. AdV. Mater. 2003, 15, 353. (17) Zou, G. F.; Qian, Y. T.; et al. Nanotechnology 2006, 17, S313. (18) Xiong, Y. J.; Xie, Y.; et al. Chem. Eur. J. 2004, 10, 654. (19) Li, Y. D.; Li, X. L.; et al. J. Am. Chem. Soc. 2002, 124, 1411. (20) Lu, C. H.; Chen, S. Y.; Hsu, C. H. Mater. Sci. Eng. B 2007, 140, 218. (21) Palilla, F. C.; Levine, A. K.; Tomkus, M. R. J. Electrochem. Soc. 1968, 115, 642. (22) Qiu, G. M.; Chen, Y. J.; Geng, X. J.; et al. J. Rare Earths 2005, 23, 629. (23) Chen, Y. J.; Qiu, G. M.; Sun, Y. B.; et al. J. Rare Earths 2002, 20, 50. (24) Katasumata, T.; Nabae, T.; Sasajima, K.; et al. J. Cryst. Growth 1998, 183, 361.
JP806375P
