Hydrothermal Synthesis and Luminescence of Eu-Doped Ba0.92Y2

Aug 31, 2009 - crystals R1-yMyF3-y (R ) La-Er; M ) Ca, Sr, Ba, Cd).18 The synthesis of these ternary fluorides mainly involved solid- state reactions...
0 downloads 0 Views 3MB Size
Download PDF
16962

J. Phys. Chem. C 2009, 113, 16962–16968

Hydrothermal Synthesis and Luminescence of Eu-Doped Ba0.92Y2.15F8.29 Submicrospheres Yeju Huang, Hongpeng You,* Guang Jia, Yuhua Zheng, Yanhua Song, Mei Yang, Kai Liu, and Lihui Zhang State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P.R. China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, P.R. China ReceiVed: June 26, 2009; ReVised Manuscript ReceiVed: August 3, 2009

The nonstoichimetric Ba0.92Y2.15F8.29 submicrospheres that piled up by nanoparticles have been prepared via a solution-based method in a hydrothermal environment. The size distribution of the submicrospheres could be tuned by varying the amount of BaCl2. The fluoride source NaBF4 plays an important role in the formation of the submicrospheres. The chelator ethylenediaminetetraacetic acid regulates the growth of the primary nanoparticles as well as the aggregated submicrospheres. The photoluminescence properties of different concentrations of Eu3+-doped Ba0.92Y2.15F8.29 were investigated and the results revealed that the 8% concentration of Eu3+ ions is the optimum doping concentration and the Y3+ ions occupy the site of inversion symmetry. 1. Introduction The past few years have been witness to an unprecedented revolution in particle syntheses, which has led to a spectacular variety of building blocks of different shapes, compositions, pattern functionalities, and hierarchical structures self-assembled by these building blocks.1,2 As an important group of inorganic materials with unique optical and electronic properties, nanoand submicroscale fluoride materials have drawn increasing attention. For example, the syntheses of metal fluoride nanomaterials, such as CaF2 nanocubes,3 SrF2 nanospheres,4 and BaF2 nanocubes5 and nanorods6 have been reported for their application in UV lithography, UV-transparent optical lenses, and surface conditioning of glass. Besides, binary lanthanide fluorides (LnF3, Ln ) lanthanide elements)7-9 and ternary ALnF4 (A ) alkali metals, Ln ) lanthanide elements)10-13 fluorides were intensively researched with potential applications in display, laser, and biological labels in recent years. Compared with the fluorides mentioned above, alkaline-earth lanthanide ternary fluorides have obtained relatively little attention, although these barium yttrium fluoride crystals such as BaY2F8 and BaYF5 are excellent host matrixes that can be doped with divalent and trivalent lanthanide ions, exhibiting the strong broadband emission in the near UV spectra region (360-440 nm)14 and highly efficient infrared-to-visible up-conversion light.15 The related investigations are mainly dominated on the bulk BaLn2F8/BaYF5 crystals16,17 and nonstoichimetric single crystals R1-yMyF3-y (R ) La-Er; M ) Ca, Sr, Ba, Cd).18 The synthesis of these ternary fluorides mainly involved solidstate reactions. However, solid-state syntheses have several disadvantages such as complex experimental setups, higher operating temperature, repeated milling and grinding, uncompleted production, irregular morphology, and larger particle size distribution. Compared with solid-state reaction, solution-based syntheses of fluorides possess some advantages, such as inexpensive facilities, lower reaction temperature, economical energy, easily controllable reaction conditions, size-selective growth, morphological control, smaller particle size, and smaller * Corresponding author. Tel.: +86-431-85262798. Fax: +86-43185698041. E-mail: [email protected].

particle size distribution. Therefore, solution-based syntheses are often used in the preparation of nano- or microfluorides. However, solution-based routes are limited in the syntheses of barium lanthanide fluorides. There are only a few documented literatures on the syntheses of alkaline-earth lanthanide ternary fluorides via solution-based methods until now. Karbowiak et al. prepared Eu3+-doped BaY2F8 crystals without considering the size and morphology by heating the powders coprecipitated by BaCl2 and LnCl3 (Ln ) Y, Eu) with HF in solution.19 Yi et al. synthesized tetragonal BaYF5 nanocrystals by codecomposition of barium acetoacetonate and yttrium trifluoroacetate in organic solvents under high temperature and argon gas protection.20 To the best of our knowledge, there has been no literature available on the synthesis of cubic-phase Ba0.92Y2.15F8.29, although it may possess some properties comparable to those of BaY2F8 and BaYF5. Herein, we report a facile synthesis of cubic-phase Ba0.92Y2.15F8.29 submicrospheres via a solution-based method in a hydrothermal environment for the first time. Ethylenediaminetetraacetic acid (EDTA) and NaBF4 were used as the chelator and the fluoride source in the reaction, respectively. The influences of the amount of BaCl2, reaction time, the chelator EDTA, and the fluoride source NaBF4 were investigated in detail. To explore the luminescent property of the Eu3+ ions and the environment of the Y3+ ions in Ba0.92Y2.15F8.29 host lattices, the luminescence of the Eu3+ doped Ba0.92Y2.15F8.29 were also researched. 2. Experiment Section Rare-earth nitrates were obtained by dissolving Y2O3 and Eu2O3 (99.99%) in HNO3 solution under heating with agitation. All the other chemicals were analytical grade and used asreceived without further purification. In a typical procedure of preparing Ba0.92Y2.15F8.29 submicrospheres, 0.15 g of EDTA was first added into 20 mL of deionized water. Ammonia (25 wt %) was then added dropwise with vigorous stirring until EDTA was dissolved absolutely to form a transparent solution with a pH value of about 9. Subsequently, 2 mmol of BaCl2 and 1 mmol of Y(NO3)3 were introduced into the above solution. After the solution was stirred

10.1021/jp905982v CCC: $40.75  2009 American Chemical Society Published on Web 08/31/2009

Eu-Doped Ba0.92Y2.15F8.29 Submicrospheres

J. Phys. Chem. C, Vol. 113, No. 39, 2009 16963

Figure 1. (a) XRD pattern of the product obtained in the typical procedure and the standard data of cubic-phase Ba0.92Tm0.04Y1.00Yb1.11F8.29 (JCPDS 49-0287) as a reference and (b) the corresponding EDX spectrum.

for 10 min, 15 mL of aqueous solution containing 1.4 mmol of NaBF4 was poured into it. The resultant solution was transferred into a 50-mL Teflon autoclave, and additional deionized water was added into the autoclave until about 80% of its volume. Finally, the autoclave was tightly sealed and heated at 180 °C for 24 h followed by cooling to room temperature naturally. The resulting precipitates were washed with deionized water and ethanol each two times. The final product was dried at 60 °C in air. Powder X-ray diffraction measurements were performed on a Bruker D8 focus X-ray powder diffractometer with Cu KR radiation (λ ) 0.15405 nm). The size and morphology of the products were characterized using an Hitachi S-4800 field emission scanning electron microscope equipped with an energy dispersive X-ray (EDX) spectrometer, operated at an acceleration voltage of 10 kV. The transmission electron microscope (TEM) and selected area electron diffraction (SAED) patterns were obtained by a JEOL H-8100 transmission electron microscope at the accelerating voltage of 200 kV. The photoluminescence excitation and emission spectra were recorded with an Hitachi F-4500 fluorescence spectrophotometer equipped with a 150 W xenon lamp as the excitation source. All the measurements were performed at room temperature. 3. Results and Discussion The crystal structure and phase purity of the resulting product were examined by X-ray powder diffraction. The XRD pattern of the as-synthesized product is shown in Figure 1a. All the diffraction peaks are very coincident with the cubic-phase Ba0.92Tm0.04Y1.00Yb1.11F8.29 (JCPDS 49-0287, space group Fm3m, z ) 8, and cell parameter a ) 11.421 Å) except for a small shift toward the lower 2θ side, indicating a slight expansion of the crystal lattice (the calculated cell parameter a ) 11.462 Å) compared with the standard card. As a result of the similarity of lanthanide elements, it is thought that different lanthanide ions can occupy the same sites in crystal without changing the crystal structure. So it is feasible to consider that the Tm3+, Yb3+, and Y3+ ions are accommodated in the same crystal sites in the crystal Ba0.92Tm0.04Y1.00Yb1.11F8.29 and the product can be indexed into the chemical formula of Ba0.92Y2.15F8.29 without doping Tm3+ and Yb3+ ions. Since the radii of Tm3+ and Yb3+ ions are smaller than the radius of Y3+ ions, it is reasonable that the product without doping Tm3+ and Yb3+ ions has a larger crystal lattice than the standard card. To further confirm this inference, EDX spectrum analysis was carried out and the result

Figure 2. (a, b) SEM images, (c, d) TEM images, and (e) SAED pattern corresponding to d of the product obtained in the typical procedure.

is shown in Figure 1b. It confirms the presence of elements Ba, Y, and F in the product and the obtained atomic ratio of Ba/ Y/F is 1:2.34:9.05, which is consistent with the theoretical atomic Ba/Y/F ratio (1:2.33:9.01) in Ba0.92Y2.15F8.29. The average crystallite size of the product can be estimated from the Scherrer formula D222 ) Kλ/(β cos θ), where λ is the X-ray wavelength (0.15405 nm), β is the full-width at half-maximum, θ is the diffraction angle, K is a constant (0.89), and D222 means the size along the (222) direction. The estimated average crystallite size is 26.8 nm. The size and morphology of the as-prepared product were further examined by scanning electron microscopy and transmission electron microscopy. The overall view SEM image of the product shown in Figure 2a clearly displays uniform submicrospheres with an average size of approximately 330 nm. The magnified SEM image displayed in Figure 2b indicates that the submicrospheres with rough surfaces are piled up by small nanoparticles. The TEM image demonstrates that the submicrospheres are discretely freestanding with a uniform size distribution (Figure 2c). A careful observation of a single submicrosphere can also find the uneven fringes of the submicrosphere (Figure 2d). Combined with the crystal size obtained by the Scherrer formula based on the XRD pattern, we can make a conclusion that the submicrospheres are piled up by small nanoparticles. The corresponding SAED pattern shows dot rings, revealing that the submicrosphere may consist of nanocrystals (Figure 2e).

16964

J. Phys. Chem. C, Vol. 113, No. 39, 2009

Huang et al.

Figure 3. SEM images of the products produced with different amounts of BaCl2 as the barium source: (a) 0.4 mmol; (b) 0.7 mmol; (c) 1.5 mmol. (d) XRD patterns corresponding to a-c, respectively.

The real atom ratio of yttrium to barium (2.33) in Ba0.92Y2.15F8.29 is larger than the set ratio of yttrium to barium (0.5) in the initial solution, that is, there were many residual Ba2+ ions in the solution after reaction. To verify whether the superfluous Ba2+ ions were necessary, different amounts of BaCl2 (0.4, 0.7, and 1.5 mmol) were introduced into reactions with a constant amount of Y(NO3)3 and NaBF4. The SEM images in Figure 3 reveal that the three products have similar morphologies of submicrospheres, but the size distribution of the submicrospheres in parts (a) and (b) of Figure 3 is larger than that in Figure 3c. The corresponding XRD patterns demonstrate the same cubic phase of Ba0.92Y2.15F8.29 (Figure 3d). On the basis of the above results, we conclude that varying the amount of Ba2+ ions can not change the crystal structure of the products, but it can tune the size distribution of the products. So the superfluous Ba2+ ions were necessary to obtain submicrospheres with a narrow size distribution. This may be associated with the different precipitation ability of yttrium and barium ions in the solution because BaF2 has a larger solubility product (KSP) than YF3. We suppose that the higher concentration of Ba2+ ions would cause a faster speed of nucleation and aggregation, which could promote the size uniformity. To gain more insight into the formation process of the submicrospheres, time-dependent experiments were carried out at 180 °C with different reaction times. Figure 4 shows SEM images of the products obtained with different reaction times. Surprisingly, the submicrospheres were formed when the reaction continued for 0.5 h, though these submicrospheres possessed a large size discrepancy (Figure 4a). When the reaction time was increased to 3 h, it seemed that the small

submicrospheres grew faster and thus the size difference between the large and small ones was reduced (Figure 4b). When the reaction time was increased to 6 h, the products were still submicrospheres but the number of small particles decreased sharply (Figure 4c). When the reaction time was prolonged to 12 h, the obtained submicrospheres became uniform in size (Figure 4d). The XRD patterns of the products corresponding to Figure 4a-d are shown in Figure 4e. One can see that all the products can be readily indexed into the cubic-phase Ba0.92Y2.15F8.29 irrespective of the reaction time. As mentioned in the Experiment Section, the transparency of the the solution was maintained before the solution was transferred into the autoclave because no dissociated F- ions could be provided by NaBF4 at room temperature, indicating that no crystals can be formed at room temperature. These results suggest that the reaction may be triggered by the hydrolysis of NaBF4 at high temperature and pressure in the autoclave, which lead to a fast nucleation process. High temperature and pressure caused the hydrolyzation of NaBF4 to burst forth and give birth to many dissociated F- ions (BF4- + 3H2O f H3BO3 + 3HF + F-). Meanwhile, numerous Ba0.92Y2.15F8.29 nuclei formed and quickly aggregated into submicrospheres to minimize their surface energy.21,22 As a result, the small submicrospheres possessing higher surface energy had a faster growth rate than the large ones, which make the size uniformity possible after a long ripening process. The influence of the chelator EDTA on the product was also investigated. For comparison, experiments without introduction of the chelator EDTA into reaction were carried out and the results are illustrated in Figure 5. The panoramic SEM image

Eu-Doped Ba0.92Y2.15F8.29 Submicrospheres

J. Phys. Chem. C, Vol. 113, No. 39, 2009 16965

Figure 4. SEM images of the products produced with different reaction times: (a) 0.5 h; (b) 3 h; (c) 6 h; (d) 12 h. (e) XRD patterns corresponding to a-d, respectively.

indicates that the product consists of microspheres with a size of about 1.6 µm and a few irregular microplates (Figure 5a). The highly magnified SEM image shown in Figure 5b reveals that the microspheres are piled up with irregular nanoplates, which have no clear boundary as they fused together. Compared with the Ba0.92Y2.15F8.29 submicrospheres obtained by using EDTA as chelator, it reveals that EDTA may control the size of the primary nanoparticles and the aggregated submicrospheres. The XRD pattern demonstrates the coexistence of cubic Ba0.92Y2.15F8.29 and minor orthorhombic BaF2 (Figure 5c). It suggests that the chelator EDTA may play an important role in the synthesis of pure phase Ba0.92Y2.15F8.29 because pure phase

Ba0.92Y2.15F8.29 could not be obtained without EDTA despite the amount of BaCl2 being decreased. During the reaction, the EDTA molecules accommodated the coprecipitation rate of BaF2 and YF3 to form homogeneous Ba0.92Y2.15F8.29 nuclei by complexing with the Ba2+ ions and Y3+ ions, and then bound to the surface of the nuclei, which consequently regulate particle morphology and size.23,24 The influence of NaBF4 was also investigated by replacing NaBF4 with NH4F as the fluoride source in the reaction. The product obtained by using NH4F as the fluoride source consists of well-dispersed nanoparticles with a size of approximately 30 nm (Figure 6a). The X-ray diffraction peaks of the product

16966

J. Phys. Chem. C, Vol. 113, No. 39, 2009

Huang et al.

Figure 5. (a) Panoramic SEM image, (b) highly magnified SEM image, and (c) XRD pattern of the product produced without introduction of EDTA as chelator and the standard data of cubic-phase Ba0.92Tm0.04Y1.00Yb1.11F8.29 (JCPDS 49-0287) as a reference.

Figure 6. (a) SEM image, (b) XRD pattern of the product produced by using NH4F as the fluoride source and the standard data of tetragonal-phase BaYF5 (JCPDS 46-0039) and cubic-phase Ba0.92Tm0.04Y1.00Yb1.11F8.29 (JCPDS 49-0287) as references.

shown in Figure 6b are different from that of the cubic-phase Ba0.92Y2.15F8.29 (JCPDS 49-0287) as it shifts greatly toward the lower 2θ side and possess an additional diffraction peak at 2θ ) 78.83°. The product could be indexed to the tetragonal-phase BaYF5 (space group P4j21m, z ) 10, and cell parameters a ) 12.468 Å and c ) 6.78 Å) compared with standard data JCPDS 46-0039 and the literature.25 According to the results, we hypothesize that NaBF4 is the main drive force for the formation of the submicrospheres morphology instead of the chelator EDTA since fluoroborate salts (NaBF4, NH4BF4, KBF4, etc.) have been used frequently as fluoride sources in the syntheses of lanthanide fluorides26,27 without using any surfactant and the obtained product in this literature is hierarchal structures that self-assembled by nanoparticles. Indeed, it has been recognized that not only organic additives can regulate the crystal growth

but also inorganic ions can tune the crystal growth by adsorbing on certain surfaces. For example, Li et al. reported that the morphologies of microstructured β-NaYF4 produced by using NH4F and NaF as fluoride sources are different, as the cations NH4+ and Na+ in the reaction system have a different guiding effect on the formation of the β-NaYF4 crystals.28 On the basis of all the above experiment results, a general schematic illustration of the crystal growth process of the products is proposed in Figure 7. In short, the chelator EDTA is indispensable for the formation of pure cubic-phase Ba0.92Y2.15F8.29 and control of the particle size. The key factor for obtaining the sphere morphology of the product is NaBF4. Introducing an excessive amount of BaCl2 and prolonging the reaction time can narrow the size distribution of the submicrospheres.

Eu-Doped Ba0.92Y2.15F8.29 Submicrospheres

J. Phys. Chem. C, Vol. 113, No. 39, 2009 16967

Figure 7. A general schematic illustration of the crystal growth process.

Figure 8. Excitation spectra (a) and emission spectra (b) of different Eu3+ concentration-doped Ba0.92Y2.15F8.29.

To obtain more insight into the local environment in the crystal of Ba0.92Y2.15F8.29, Eu3+ ions were doped into Ba0.92Y2.15F8.29 submicrospheres. In fact, it is well-known that the Eu3+ is an important structure probe often used to investigate the local environment in a host matrix.29 The excitation and emission spectra of different Eu3+ concentrations (1, 2, 5, 8, and 10% Eu3+) of Ba0.92Y2.15F8.29 are shown in Figure 8. All the excitation spectra of the samples with different Eu3+ concentrations have similar excitation peaks with a maximum peak at 397 nm (Figure 8a). These excitation peaks are assigned to the f-f transitions within the Eu3+(4f6) configuration. It is worth noting that the emission intensity increases initially and then decreases with a maximum at 8% Eu3+ concentration. The emission spectra of samples with different Eu3+concentrations consist of four emission bands at about 554, 591, 615, and 696 nm (Figure 8b). These emission bands are ascribed to the 5D1 to 7F2 and 5 D0 to 7FJ (J ) 1, 2, 4) transitions of the Eu3+ ions, respectively. Similarly, the sample with 8% Eu3+ concentration exhibits the strongest emission intensity. The decrease of the emission intensity is due to the concentration quenching. It demonstrates that 8% is the optimum doping concentration of Eu3+ in Ba0.92Y2.15F8.29. All the emission spectra of the samples have the maximum band at 591 nm, which is attributed to the 5D0 f 7 F1 parity-allowed magnetic dipole transition. In general, the 5 D0 f 7F1 transition is insensitive while the 5D0 f 7F2 transition is hypersensitive to the crystal field environment.30,31 If Eu3+ is in an inversion center, the magnetic dipole transition is dominant, while in a site without inversion symmetry, the 5D0 f 7F2 electronic transition becomes the strongest one. Therefore, the intensity ratio of the transitions 5D0 f 7F2 to 5D0 f 7F1 is

referred to as a measure of the symmetry of the Eu3+ site. The intensity contrast between the bands of 591 and 615 nm indicates that the Eu3+ ions locate at a site of inversion symmetry in the Ba0.92Y2.15F8.29 matrix. This result reveals that the Y3+ ions occupy a site of inversion symmetry in the Ba0.92Y2.15F8.29 matrix because the Eu3+ ions substitute the Y3+ ions in the matrix. 4. Conclusion We have successfully synthesized the cubic Ba0.92Y2.15F8.29 submicrospheres via a hydrothermal method for the first time. Our investigation reveals that Ba0.92Y2.15F8.29 submicrospheres consist of nanoparticles. The amount of BaCl2 used in the reaction plays an important role in controlling the size distribution. The low amount of BaCl2 used in the reaction causes a broader size distribution of the product, while the excessive BaCl2 would produce submicrospheres with uniform size. Chelator EDTA is indispensable for obtaining pure cubic-phase Ba0.92Y2.15F8.29 and controlling the particle size. Furthermore, the usage of fluoride source NaBF4 is the crucial key in the formation of submicrosphere morphology. The luminescent investigation indicates that the Y3+ ions occupy a site of inversion symmetry in the Ba0.92Y2.15F8.29 matrix. Our simple and facile approach should promise a future large-scale synthesis of this nanostructured material for some important applications in nanotechnology. Acknowledgment. This work is financially supported by the National Natural Science Foundation of China (Grant No. 20771098.) and the National Basic Research Program of China (973 Program, Grant No. 2007CB935502).

16968

J. Phys. Chem. C, Vol. 113, No. 39, 2009

References and Notes (1) Glotzer, S. C.; Solomon, M. J. Nat. Mater. 2007, 6, 557. (2) Gao, F.; Lu, Q. Y.; Meng, X. K.; Komarneni, S. J. Mater. Sci. 2008, 43, 2377. (3) Sun, X. M.; Li, Y. D. Chem. Commun. 2003, 1768. (4) Jin, Y.; Qin, W. P.; Zhang, J. S. J. Fluorine Chem. 2008, 129, 515. (5) Gao, P.; Xie, Y.; Li, Z. Eur. J. Inorg. Chem. 2006, 3261. (6) De, G. H.; Qin, W. P.; Zhang, J. S.; Zhang, J. S.; Wang, Y.; Cao, C. Y.; Cui, Y. J. Solid State Chem. 2006, 179, 955. (7) Hu, H.; Chen, Z. G.; Cao, T. Y.; Zhang, Q.; Yu, M. X.; Li, F. Y.; Yi, T.; Huang, C. H. Nanotechnology 2008, 19, 375702 (9 pp). (8) Li, C. X.; Yang, J.; Yang, P. P.; Lian, H. Z.; Lin, J. Chem. Mater. 2008, 20, 4317. (9) Yang, X. L.; Xiao, S. G.; Ding, J. W.; Yan, X. H. J. Appl. Phys. 2008, 103, 093101. (10) Zhuang, J. L.; Wang, J.; Yang, X. F.; Williams, I. D.; Zhang, W.; Zhang, Q. Y.; Feng, Z. M.; Yang, Z. M.; Liang, C. L.; Wu, M. M.; Su, Q. Chem. Mater. 2009, 21, 160. (11) Ptacek, P.; Scha¨fer, H.; Ko¨mpe, K.; Haase, M. AdV. Funct. Mater. 2007, 17, 3843. (12) Liang, X.; Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. AdV. Funct. Mater. 2007, 17, 2757. (13) Wang, Z. J.; Tao, F.; Cai, W. L.; Yao, L. Z.; Li, X. G. Solid State Commun. 2007, 144, 255. (14) Liu, X. R.; Xu, G.; Richard, C. P. J. Solid State Chem. 1986, 62, 83. (15) Johnson, L. F.; Guggenheim, H. J.; Rich, T. C.; Ostermayer, F. W. J. Appl. Phys. 1972, 43, 1125.

Huang et al. (16) Bigotta, S.; Parisi, D.; Bonelli, L.; Toncelli, A.; Tonelli, M. J. Appl. Phys. 2006, 100, 013109. (17) Toccafondo, V.; Cerqueira, S. A.; Faralli, S. J. Appl. Phys. 2007, 101, 023104. (18) Sorokin, N. I.; Sobolev, B. P. Phys. Solid State 2008, 50, 416. (19) Karbowiak, M.; Mech, A.; Bednarkiewicz, A.; Strek, W. J. Lumin. 2005, 114, 1. (20) Yi, G. S.; Lee, W. B.; Chow, G. M. J. Nanosci. Nanotechnol. 2007, 7, 2790. (21) Li, J.; Zeng, H. C. J. Am. Chem. Soc. 2007, 129, 15839. (22) Liang, X.; Xu, B.; Kuang, S. M.; Wang, X. AdV. Mater. 2008, 20, 3739. (23) Wang, J.; Xua, Y. H.; Hojamberdiev, M.; Peng, J. H.; Zhu, G. Q. Mater. Sci. Eng., B 2009, 156, 42. (24) Wang, Z. J.; Tao, F.; Yao, L. Z.; Cai, W. L.; Li, X. G. J. Cryst. Growth 2006, 290, 296. (25) Liu, F.; Wang, Y. S.; Chen, D. Q.; Yu, Y. L.; Ma, E.; Zhou, L. H.; Huang, P. Mater. Lett. 2007, 61, 5022. (26) Wang, M.; Huang, Q. L.; Zhong, H. X.; Chen, X. T.; Xue, Z. L.; You, X. Z. Cryst. Growth Des. 2007, 7, 2106. (27) Zhong, H. X.; Wang, M.; Yang, H. L.; Hong, J. M.; Huang, Q. L.; Chen, X. T. Mater. Sci. Eng., B 2009, 156, 62. (28) Li, C. X.; Yang, J.; Quan, Z. W.; Yang, P. P.; Kong, D. Y.; Lin, J. Chem. Mater. 2007, 19, 4933. (29) Mahalingam, V.; Vetrone, F.; Naccache, R.; Speghini, A.; Capobianco, J. A. J. Mater. Chem. 2009, 19, 3149. (30) Yan, B.; Wang, C. Solid State Sci. 2008, 10, 82. (31) Zhang, M. F.; Fan, H.; Xi, B. J.; Wang, X. Y.; Dong, C.; Qian, Y. T. J. Phys. Chem. C 2007, 111, 6652.

JP905982V