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Facile Preparation Method for Rare Earth Phosphate Hollow Spheres and Their Photoluminescence Properties Mingyun Guan,†,‡ Feifei Tao,† Jianhua Sun,† and Zheng Xu†,* State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, PR China, and School of Chemistry and Chemical Engineering, Jiangsu Teachers UniVersity of Technology, Changzhou, Jiangsu 213001, PR China ReceiVed March 12, 2008. In Final Form: May 5, 2008. ReVised Manuscript ReceiVed May 3, 2008 We have developed a template-free hydrothermal method of constructing rare earth phosphate hollow spheres using H6P4O13 as the PO43- source. The mechanism of hollow spheres formation was proposed on the basis of Ostwald ripening. The resulting hollow spheres, especially with the aid of doping of other lanthanide cations, exhibit emission spanning the whole UV-visible wavelength range.
1. Introduction In recent years, it has been of considerable interest to develop methods of fabrication of micrometer-sized superstructures composed of inorganic nanoparticles with size-dependent chemical and physical behavior in a controlled manner. Among superstructures, hollow spheres are appealing not only because of their importance in achieving a better understanding of the self-assembly process with artificial building blocks but also due for potential technical applications in a variety of fields, such as drug delivery, chemical storage, battery materials, and catalysis.1–7 Various chemical approaches to the construction of inorganic hollow spheres can be roughly classified into two categories: one is to use sacrificial cores as templates and the other relies on interfacial synthesis.8–11 These approaches are usually multistep processes involving at least shell formation and core removal. To simplify the preparation procedure of forming hollow spheres, the development of one-step template-free methods should be of great significance. Lanthanide phosphate (LnPO4) is of intense interest in materials science because of its unique chemical, physical, and optical properties. LnPO4 exhibits a sharp, narrow emission in the visible * To whom correspondence should be addressed. E-mail: zhengxu@ netra.nju.edu.cn. † Nanjing University. ‡ Jiangsu Teachers University of Technology. (1) (a) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711. (b) Zeng, S. Y.; Tang, K. B.; Li, T. W.; Liang, Z. H.; Wang, D.; Wang, Y. K.; Zhou, W. W. J. Phys. Chem. C 2007, 111, 10217. (c) Zhu, H. L.; Yao, K. H.; Zhang, H.; Yang, D. R. J. Phys. Chem. B 2005, 109, 20676. (d) Yu, D. B.; Sun, X. Q.; Zou, J. W.; Wang, Z. R.; Wang, F.; Tang, K. J. Phys. Chem. B 2006, 110, 21667. (e) Agrawal, M.; Pich, A.; Gupta, S.; Zafeiropoulos, N. E.; Simon, P.; Stamm, M. Langmuir 2008, 24, 1013. (f) Ma, Y. R.; Qin, L. M.; Ma, J. M.; Cheng, H. M. Langmuir 2003, 19, 4040. (2) Yang, H. G.; Zeng, H. C. Angew. Chem., Int. Ed. 2004, 43, 5206. (3) Wu, C. Z.; Xie, Y.; Lei, L. Y.; Hu, S. Q.; Yang, C. Z. AdV. Mater. 2006, 18, 1727. (4) Bao, J. C.; Liang, Y. Y.; Xu, Z.; Si, L. AdV. Mater. 2003, 15, 1832. (5) Zhou, P.; Li, Y. G.; Sun, P. P.; Zhou, J. H.; Bao, J. C. Chem. Commun. 2007, 1418. (6) Bigi, A.; Boanini, E.; Walsh, D.; Mann, S. Angew. Chem., Int. Ed. 2002, 114, 2267. (7) (a) Kulak, A.; Davis, S. A.; Dujardin, E.; Mann, S. Chem. Mater. 2003, 15, 528. (b) Titirici, M. M.; Antonietti, M.; Thomas, A. Chem. Mater. 2006, 18, 3808. (8) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111. (9) Zhong, Z. Y.; Yin, Y. D.; Gates, B.; Xia, Y. N. AdV. Mater. 2000, 12, 206. (10) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006. (11) Yang, M.; Ma, J.; Zhang, C. L.; Yang, Z. Z.; Lu, Y. F. Angew. Chem., Int. Ed. 2005, 117, 6885.
region arising from 4f electrons of lanthanide, which are well shield by a 5s2p6 shell. As a multifunctional material, LnPO4 has been extensively used as phosphors, heat-resistant materials, and up-conversion materials.12–20 Lanthanide phosphate can be used as a host lattice for other lanthanide ions (Eu3+, Tb3+, and Dy3+) dopants, leading to phosphors with varied emision colors.12 The various shapes of LnPO4, such as nanotubes, nanorods, and nanowires, have recently been synthesized.13–21 Herein we synthesized hollow spheres of LnPO4 via a template-free route and studied their optical properties. In the present preparation process, only tetraphosphoric acid (H6P4O13) and Ln cations such as Gd3+ and Eu3+ were used; no surfactant was involved. Doping led to a variety of colors in the visible range.
2. Experimental Section Materials. All chemicals were used without further purification. Tetraphosphoric acid (H6P4O13) and Ln(NO3)3 or Ln(CH3COO)3, where Ln was Gd, Eu, Sm, Ce, Nd, Dy, and La, were purchased from the Beijing Chemical Reagent Company. Synthesis of LnPO4 Hollow Spheres. Ten milliliters of an aqueous solution of Ln(NO3)3 or Ln(CH3COO)3 (0.12 M) was added to 15 mL of an aqueous solution of H6P4O13 (0.39 M) at 50 °C. After the mixture was stirred for 2 h, the resulting transparent solution was poured into a Teflon stainless steel autoclave. The autoclave was sealed and maintained at 100 °C for 12 h and then was cooled to room temperature. The resulting products were washed with distilled water and finally dried at 80 °C in air. (12) (a) Heer, S.; Lehmann, O.; Haase, M.; Gu¨del, H. U. Angew. Chem., Int. Ed. 2003, 42, 3179. (b) Yates, M. Z.; Ott, K. C.; Birnbaum, E. R.; McCleskey, T. M. Angew. Chem., Int. Ed. 2002, 41, 476. (c) Bu¨hler, G.; Feldmann, C. Angew. Chem., Int. Ed. 2006, 45, 4864. (13) Tang, C. C.; Bando, Y.; Golberg, D.; Ma, R. Z. Angew. Chem., Int. Ed. 2005, 44, 576. (14) Fang, Y. P.; Xu, A. W.; Song, R. Q.; Zhang, H. X.; You, L. P.; Yu, J. C.; Liu, H. Q. J. Am. Chem. Soc. 2003, 125, 16025. (15) Cao, M. H.; Hu, C. W.; Wu, Q. Y.; Guo, C. X.; Qi, Y. J.; Wang, E. B. Nanotechnology 2005, 16, 282. (16) Zhang, Y. W.; Yan, Z. G.; You, L. P.; Si, R.; Yan, C. H. Eur. J. Inorg. Chem. 2003, 22, 4099. (17) Bu, W. B.; Hua, Z. L.; Chen, H. R.; Shi, J. L. J. Phys. Chem. B 2005, 109, 14461. (18) Meyssamy, H.; Riwotzki, K.; Kornouski, A.; Naused, S.; Haase, M. AdV. Mater. 1999, 11, 840. (19) Yan, R. X.; Sun, X. M.; Wang, X.; Peng, Q.; Li, Y. D. Chem.sEur. J. 2005, 11, 2183. (20) Xing, Y.; Li, M.; Davis, S. A.; Mann, S. J. Phys. Chem. B 2006, 110, 1111. (21) Shchukin, D. G.; Sukhorukov, G. B.; Mo¨hwald, H. J. Phys. Chem. B 2004, 108, 19109.
10.1021/la800789x CCC: $40.75 2008 American Chemical Society Published on Web 06/27/2008
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Figure 1. (a) SEM and (b) TEM images of EuPO4 · H2O hollow spheres. (c) SEM image of GdPO4 · H2O hollow spheres. The inset shows a highermagnification image. (d) SEM image of Eu3+/GdPO4 · H2O hollow spheres. (e) XRD patterns of EuPO4 · H2O and GdPO4 · H2O hollow spheres.
Synthesis of Doped LnPO4 Hollow Spheres. Doped samples were prepared by a similar procedure, except for adding dopant ions to the Gd3+-containing solution in the initial step. For example, to synthesize 10% Eu/GdPO4 hollow spheres, 10 mL of an aqueous solution containing Gd(NO3)3 (1.2 mmol) and Eu(CH3COO)3 (0.13 mmol) was added to 15 mL of an H6P4O13 (0.39 M) aqueous solution. Characterization. The X-ray powder diffraction (XRD) patterns of the resulting samples were recorded on Shimadzu XD-3A X-ray diffractometer with Cu KR radiation (λ ) 0.15417 nm). Transmission electron microscopy (TEM) imaging was performed on a JEOL JEM-200CX electron microscope, and scanning electron microscopy (SEM) imaging was performed on a JEOL JSM 5610LV electron microscope. Photoluminescence spectra were obtained using an SLM48000DSCF photoluminescence spectrometer and an excitation wavelength of 325 nm using a He-Cd laser.
3. Results and Discussion As shown in Figure 1a, EuPO4 · H2O hollow spheres were 2.6-8.5 µm in size. Their TEM image shows the lighter center portion and the darker edge, which further confirms their hollow structure (Figure 1b). The SEM image of GdPO4 · H2O samples shows that their size is in the range of 5-10 µm (Figure 1c). The images of the broken spheres (inset of Figure 1c) shows a shell
thickness of ca. 1.2 µm. SEM images of Eu3+-doped GdPO4 · H2O (Figure 1d) and Tb3+- or Dy3+-doped GdPO4 · H2O (Supporting Information (SI)-1) showed that the doped products kept the structure of hollow spheres. Figure 1e shows the XRD patterns of EuPO4 · H2O and GdPO4 · H2O hollow spheres. All diffraction peaks can be indexed as pure hexagonal phase for EuPO4 · H2O (JCPDS no. 20-1044) and GdPO4 · H2O (JCPDS no. 39-232).19 To gain in-depth insight into the formation mechanism of LnPO4 hollow spheres, the structural evalution during the experiment was monitored. The samples obtained at various reaction times were inspected by SEM. Prior to SEM measurement, the samples were gently ground by pestle in mortar to break hollow spheres. The temporal evolution of the resulting structure is shown in Figure 2. Obviously, in the initial stage of reaction (1.5 h), spherical aggregates of nanocrystallites were formed. After 2 h of reaction, core-shell structures appeared (Figure 2b), though the interspace between the shell and the core was quite small. After 2.5 h, the size of the core noticeably decreased, leading to a larger interspace between the core and the shell (Figure 2c). After 12 h, complete hollow spheres were obtained (Figure 1a,b). In a controlled experiment, we used H3PO4
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Figure 2. SEM images of the products obtained at different reaction times: (a) 1.5, (b) 2, and (c) 2.5 h; enlarged SEM images of a half of a microsphere and a fragment in part a showed the nature of the solid spheres. (d, e) SEM images of CePO4 and SmPO4 · H2O hollow spheres, respectively. Scheme 1. Illustration of the Proposed Mechanism of Forming LnPO4 Hollow Spheres via Ostwald Ripening
instead of H6P4O13 as the PO43- source, and only EuPO4 · H2O nanorods were obtained (SI-2). The reactivity and association structure of H3PO4 and H6P4O13 in solution are well known to be quite different. Adding Eu(Ac)3 to the H6P4O13 solution with stirring for 2 h at 50 °C led to a transparent resulting solution, whereas adding Eu(Ac)3 to the H3PO4 aqueous solution led to opalescent colloidal solutions. This suggests that the reactivity of H6P4O13 is lower than that of H3PO4 because the generation of PO43- via the hydrolysis of H6P4O13 was quite slow. As such, Ln3+ ions should coordinate with the P-O group of H6P4O13 to form complexes in aqueous solution, and LnPO4 nanocrystals were formed in a H6P4O13 association structure. In the initial
Figure 3. Emission spectra of GdPO4 · H2O hollow spheres and those doped with a various amounts of Eu3+: 0, ∼10, ∼15, and ∼20%. The Eu doping faction was calculated on the basis of the molar ratio of Gd to Eu in the precursor mixtures.
stage of reaction that lasted 1.5 h, the hydrolysis of lanthanide polyphosphate complexes leads to the formation of solid
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Figure 4. (a) Emission spectra of GdPO4 · H2O hollow spheres and those doped with various amounts of Tb: 0, ∼10, ∼15, and ∼20%. The Tb doping fraction was calculated on the basis of the molar ratio of Gd to Eu in the precursor mixtures. (b) Emission spectra of DyPO4 and Dy-doped GdPO4 · H2O hollow spheres.
microspheres composed of LnPO4 nanocrystals. Because of the presence of association structure of H6P4O13 in the aqueous phase, the difference in the concentrations of PO43- inside and outside the association structure resulted in smaller nanoparticles inside and larger ones outside. Owing to the higher surface energy, the smaller particle preferred to dissolve during Ostwald repining. Mass transport first gave rise to a shell-core structure, and then the diameter of the core gradually decreased with reaction time. At the end, the cores disappeared, and the hollow spheres were formed. Scheme 1 shows our speculated mechanism of the formation of hollow spheres via Ostwald ripening. Similar phenomena were observed in the formation of the hollow structure.2,22–26 The method was successfully extended to other lanthanide phosphates; hollow spheres of SmPO4 · H2O, CePO4, NdPO4, DyPO4 · 1.5H2O, and LaPO4 · nH2O were also synthesized under similar conditions (Figure 2d,e and SI-3 and -4). The fluorescence of LnPO4 nanoparticles originates from the transition between f electron configurations of the rare earth elements, and the particle size has little influence on the luminescence properties; these properties are quite different from those of semiconductor quantum dots.27,28 The color of fluorescent LnPO4 may be adjusted by the rare earth dopant. Because trivalent Gd3+ has a stable 4d104f7 electron configuration, the f-f transition energy of Gd3+ is much higher than that of other Ln3+ cations because of half-filled 4f shells. Thus, GdPO4 · H2O can serve as a host material.19 Figure 3 shows the emission spectra of GdPO4 · H2O hollow spheres doped with various levels of Eu (10, 15, and 20%). GdPO4 · H2O hollow spheres exhibit a strong blue-violet luminescence centered at around 429 nm. It cannot be attributed to the transition from 6P7/2 to 8S7/2 that should emit light at 311 nm.29 Here we speculate that the emission at 429 nm comes from a deep level or trap-state emission.30,31 When Eu3+ was doped into the GdPO4 · H2O crystalline lattice, the (22) Zheng, Y. H.; Cheng, Y.; Wang, Y. S.; Zhou, L. H.; Bao, F.; Jia, C. J. Phys. Chem. B 2006, 110, 8284. (23) Liu, B.; Zeng, H. C. Small 2005, 1, 566. (24) Lou, X. W.; Wang, Y.; Yuan, C. L.; Lee, J. Y.; Archer, L. A. AdV. Mater. 2006, 18, 2325. (25) Yang, H. G.; Zeng, H. C. J. Phys. Chem. B 2004, 108, 3492. (26) Wang, W. S.; Zhen, L.; Xu, C. Y.; Zhang, B. Y.; Shao, W. Z. J. Phys. Chem. B 2006, 110, 23154. (27) Shchukin, D. G.; Sukhorukov, G. B.; Mo¨hwald, H. J. Phys. Chem. B 2004, 108, 19109. (28) Ewa, M, G.; Krystyna, D. T.; Sun, J. J.; Dosi, D.; Ian, M. K.; Sergiey, Y.; Marek, G. J. Am. Chem. Soc. 2006, 128, 14498. (29) Xia, J. R. Practise Fluminescence Analytic Method; Chinese People’s Public Security University Press: Beijing, 1992; p 240. (30) Hu, C. G.; Liu, H.; Dong, W. T.; Zhang, Y. Y.; Bao, G.; Lao, C. S.; Wang, Z. L. AdV. Mater. 2007, 19, 470. (31) Huang, M. H.; Wu, Y. Y.; Feick, H.; Tran, N.; Weber, E.; Yang, P. D. AdV. Mater. 2001, 13, 113.
energy transfer from the GdPO4 · H2O host to Eu3+ occurred, and the luminescence emission of Gd3+ was partially quenched. The characteristic emission bands of Eu3+ at 591, 614, 653, and 699 nm were observed, which arose from the 5D0-7F1, 5D0-7F2, 5D -7F , and 5D -7F transitions, respectively. The intensity of 0 3 0 4 the red emission increases with Eu3+ doping level, at the cost of Gd3+ fluorescence. Figure 4a shows emission spectra of Tb3+-doped GdPO4 · H2O hollow spheres with doping levels varying from 10 to 20% under 325 nm excitation. The characteristic emission bands of Tb3+ at 490, 547, 586, and 622 nm are attributed to the 5D4-7F6, 5D4-7F5, 5D -7F , and 5D -7F transitions, respectively. The intensity of 4 4 4 3 the green emission increases with the Tb3+ doping level. The emission spectra of DyPO4 and Dy3+-doped GdPO4 · H2O hollow spheres are shown in Figure 4b. Emission bands of DyPO4 are located at 403, 429, 481, and 577 nm. After Dy3+ doping, emission spectra of GdPO4 · H2O/Dy hollow spheres exhibit four emission bands centered at 409, 436, 481, and 577 nm. Compared with those of DyPO4, the intensities of the emission bands centered at 481 and 577 nm obviously increased, indicating the energy transfer from the GdPO4 · H2O host to Dy3+. The emission band between 350 and 450 nm mainly comes from host GdPO4 rather than low Dy doping. The intensity of that band was significantly reduced after doping, which again showed evidence of the energy transfer from Gd3+ to Dy3+. The GdPO4 · H2O/Dy hollow spheres exhibited a blue emission.
4. Conclusions Hollow spheres of pure and dopped rare earth phosphates have been prepared via a tempate-free hydrothermal method. The synthesis method is quite simple and effective and is easy to scale up to the gram level. The resulting LnPO4 hollow spheres, for instance, GdPO4 · H2O and those doped with other Ln cations such as Eu, Tb, and Dy, show strong emission over the whole UV-visible wavelength range, which should have promising applications highperformance luminescent devices and biologic diagnosis. Acknowledgment. We thank the National Natural Science Foundation of China (nos. 90606005, 20490210, and 20571040) for financial support. Supporting Information Available: SEM images of hollow spheres of GdPO4 · H2O/Tb3+, GdPO4 · H2O/Dy3+, LaPO4 · nH2O, DyPO4 · 1.5H2O, and NdPO4. XRD patterns of hollow spheres of SmPO4, NdPO4, DyPO4 · 1.5H2O, CePO4, and LaPO4 · nH2O. TEM image of EuPO4 · H2O nanorods. This material is available free of charge via the Internet at http://pubs.acs.org. LA800789X
