Formation of YF3 Nanocrystals and Their Self-Assembly into Hollow

Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711−714. [Crossref], [PubMed], [CAS]. (...
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CRYSTAL GROWTH & DESIGN

Formation of YF3 Nanocrystals and Their Self-Assembly into Hollow Peanut-Like Structures Wang,†

Miao Qing-Li Xiao-Zeng You†

Huang,†

Hao-Xiang

Zhong,†

Xue-Tai

Chen,*,†,§

Zi-Ling

Xue,‡

and

2007 VOL. 7, NO. 10 2106-2111

Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, P. R. China, State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Fuzhou 350002, P. R. China, and Department of Chemistry, UniVersity of Tennessee, KnoxVille, Tennessee 37996-1600 ReceiVed April 25, 2007; ReVised Manuscript ReceiVed June 28, 2007

ABSTRACT: A simple hydrothermal route has been used to prepare hollow peanut-like structures of orthorhombic YF3 by using a suitable tetrafluroborate complex as the fluoride source. The as-synthesized products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Four tetrafluroborate complexessNaBF4, NH4BF4, KBF4, and HBF4swere examined in this synthetic procedure. YF3 nanocrystals formed from NaBF4, NH4BF4, or KBF4 were found to self-assemble into hollow peanut-like structures. Time-dependent studies of the reactions have been conducted to reveal the morphological evolution, and the possible mechanistic pathways in the formation of the hollow structures are proposed. Introduction The fabrication of inorganic materials with hollow interiors has recently attracted much attention, because they have exhibited potential applications in photonic devices, drug delivery, active-materials encapsulation, catalysis carriers, and size-controlled reactors.1-17 Many procedures have been developed to prepare hollow micro- and nanostructures.1-17 Most reported approaches are concerned with spherical hollow nanoor microstructures, and there have been relatively few investigations of nanoboxes,5-6 hollow octahedrons,7 spindle-like structures,8 hollow bullet-head-like structures,9 and peanut-like coreshell structures.10-11 In addition, most of the previous studies were focused on the preparation of hollow structures of metals, metal oxides, semiconductors, and polymers. To our knowledge, little work has been conducted of the hollow structures of metal fluorides.17 YF3 is a very important metal fluoride with potential applications as phosphors,18 ionic conductors,19 and scintillators.20 YF3 has been used as a host crystal for lanthanide-doped phosphors with interesting down/up conversion luminescent properties.18,21-24 Recently the fabrication of nanostructured YF3 has attracted increasing attention. Some relatively mild chemical procedures, such as hydrothermal methods,21-26 microemulsion,27 and precipitation,28,29 have been developed to prepare YF3 nanocrystals with different sizes and morphologies. Truncated octahedral nanocrystals of YF3 have been prepared by an EDTA-assisted hydrothermal route.24,25 YF3 nanocrystals with quadrilateral and hexagonal shapes have been synthesized by a reverse micelle method.27 Spherical, octahedral, and bundlelike YF3 nanocrystals were prepared through a room-temperature precipitation route by adjusting the fluoride sources and the molar ratio of the starting materials.29 In this paper, we report the preparation of hollow peanutlike structures of YF3 via a simple hydrothermal route without * To whom correspondence should be addressed. E-mail: xtchen@ netra.nju.edu.cn. Fax: +86-25-83314502. † Nanjing University. § Fujian Institute of Research on the Structure of Matter. ‡ University of Tennessee.

Figure 1. (a) XRD pattern and (b) XPS spectrum of YF3 prepared from NaBF4 and Y(NO3)3·6H2O after 12 h of reaction at 160 °C.

10.1021/cg070397j CCC: $37.00 © 2007 American Chemical Society Published on Web 09/08/2007

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Figure 2. TEM (a,b) and SEM (c,d) images of YF3 obtained from NaBF4 after 12 h of reaction time.

any surfactant or polymer in the reaction mixture. A series of tetrafluoroborate complexessNaBF4, NH4BF4, KBF4, and HBF4s were examined as fluoride sources, and their effects were systematically investigated. Although these four fluoride sources all gave YF3 nanocrystals with an orthorhombic phase, only the nanocrystals from NaBF4, NH4BF4, or KBF4 self-assembled into hollow peanut-like structures. To the best of our knowledge, there has been no report of the preparation of such hollow peanut-like nano- or microstructured metal fluoride. Experimental Section A typical procedure for the preparation of YF3 using NaBF4 as the fluoride source is given below. NaBF4 (4.0 mmol) and Y(NO3)3‚6H2O (1.0 mmol) were dissolved in 25 mL of distilled water in a plastic flask. After being stirred for 20 min at room temperature, the mixture was transferred into a 30-mL Teflon-lined stainless autoclave. The autoclave was sealed and heated at 160 °C for a set time and then slowly cooled to room temperature. A white solid product was collected by centrifugation and washed in an ultrasonic bath several times with distilled water and then ethanol. The solid was collected and dried at 70 °C for 3 h. Similar procedures were performed with HBF4, NH4BF4, and KBF4. X-ray diffraction (XRD) analyses were carried out on a Shimadzu XRD-6000 powder X-ray diffractometer with Cu KR radiation (λ ) 1.5418 Å). The sizes and morphologies of the resulting products were studied by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) on a TECNAI F20 S-TWIN or JEM-2010F microscope and by scanning electron microscopy (SEM) on a JSM-6700F microscope. The X-ray photoelectron spectra (XPS) were recorded on an ESCALAB MK II X-ray photoelectron spectrometer, using Mg K-X-ray as the excitation source.

Results and Discussion YF3 was prepared by the reaction between Y(NO3)3·6H2O and NaBF4 under hydrothermal conditions (160 °C) with a fixed 1:4 molar ratio of Y3+/NaBF4. Figure 1a shows the XRD pattern of the as-synthesized product obtained after 12 h of reaction time. All diffraction peaks of the product are perfectly indexed to the orthorhombic phase of YF3 (JCPDS Files, No.74-0911). No peak of impurities was observed, confirming the formation of pure orthorhombic YF3. The composition of the as-prepared product was further confirmed by XPS studies. Figure 1b shows the XPS spectrum of the product obtained after 12 h of reaction time. Other than the minor C and O peaks that are from the

XPS chamber, no peak of other elements except Y and F was observed. The peaks at ca. 685.1 and 159.7 eV correspond to the F 1s and Y 3d binding energy, respectively. These results are in agreement with the literature values.29,30 The morphology of as-synthesized YF3 obtained after 12 h of reaction time has been examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) (Figure 2). The product consisted of peanut-like particles with rough surfaces (Figure 2a), suggesting that they are constructed from many small primary nanoparticles. Furthermore there are many pores on their surfaces, which are clearly observed from the TEM image of a typical YF3 particle (Figure 2b). Some broken peanut particles in the SEM image revealed their hollow interior (Figure 2c,d). The wall thickness of the peanut structure is estimated as ca. 150-200 nm. This large thickness is probably why there is an absence of the image contrast in the TEM images that are often observed in the TEM measurements of hollow spheres. Even though there are a few known peanut-like structures,10,31-33 the as-prepared YF3 peanut-like structures are different. Our YF3 has an interior cavity, while the reported structures are solid peanuts or contain a core inside to form core-shell structure.10 This is to our knowledge the first report of the peanut morphology of rare earth fluorides. To obtain a better understanding of the formation and evolution of YF3 hollow peanut-like structure, a more detailed time-dependent investigation was conducted. The products obtained with a fixed 1:4 molar ratio of Y3+/NaBF4 at different reaction stages (from 1 to 9 h) were isolated and characterized by XRD, TEM, and SEM. XRD measurements showed that these products were YF3 with the orthorhombic structure (Figure S1, Supporting Information). The morphological evolution of YF3 during the reaction is shown in Figure 3. The TEM images showed that YF3 obtained after 1 h of reaction is irregular nanocrystals (Figure 3a,b). These nanoparticles are not monodispersed and exhibit a wide size distribution from 20 to 60 nm (Figure 3a). These YF3 nanocrystals have a cone-like shape at both heads. The HRTEM image reveals clear lattice fringes with a distance of 0.36 nm, indicating the nature of single crystallinity (Figure 3b). When the reaction time increased to 2 h, nanocrystals of YF3 appeared to start to self-assemble (Figure 3c). After 4 h, the nanocrystals had assembled into clusters (Figure 3d,e). Of particular interest is that these clusters are

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Figure 3. TEM and SEM images of the as-synthesized YF3 with different reaction times: (a,b) 1 h; (c) 2 h; (d,e) 4 h; (f) 5 h; (g,h) 6 h; (i,j) 9 h.

hollow inside (Figure 3e). When the reaction time increased further, two clustered aggregates of YF3 fused together and gradually changed to the peanut-like morphology (5 h, Figure 3f; 6 h, Figure 3g). In order to reveal the interior structure of these peanut-like particles, the solid sample of YF3 obtained after 6 h of reaction time was ground and then characterized by SEM. Figure 3h is the SEM image of the broken peanutlike particles after grinding, indicating that the half-peanut is hollow. After 9 h of reaction, the peanut structure is retained (Figure 3i). The SEM image of the product after grinding also reveals that the YF3 peanut obtained at this stage is hollow

(Figure 3j). The observations here revealed that the hollow peanut structure was formed via a three-step procedure: YF3 nanocrystals form at the early stages, then self-assembled into a loosely packed hollow half-peanut. Finally two half-peanut aggregates fuse into a hollow peanut-like structure. The possible mechanistic pathways in the formation of current hollow peanut-like structures are of interest to us. We conducted studies to find out how the initially formed nanocrystals of YF3 self-assemble into clustered aggregates in Figure 3d and how the two clustered aggregates fused into the peanut-like structure in Figure 3f. HETRM images were recorded on the cluster

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Figure 4. (a-c) EM images of clustered aggregates of YF3 formed at 4 h and (d-f) EM images of the peanut-like of YF3 formed after 5 h of reaction.

Figure 5. A schematic illustration of the formation of peanut-like YF3.

aggregate (Figure 4a). Due to the large thickness of YF3 hollow aggregate, the TEM image with clear lattice fringes could not always be obtained. Only images of some regions near the pores and the margin were obtained. Two typical HERTM images of the regions indicated in Figure 4a are shown in Figure 4b,c, in which the lattice fringes in the adjacent nanocrystals are extending nearly in the same direction. It is possible that the primary YF3 nanocrystals are aggregated via the specific direction by the self-assembly process via the oriented attachment mechanism.34-36 It is noted that the aggregation could not lead to the closely packed structure due to the cone-shaped edge of these YF3 nanocrystals, which leave many pores in these hollow clusters. We also recorded the HRTEM images for the waist part of the peanut-like structure constructed by two clustered aggregates after 5 h of reaction. Two typical HRTEM

images recorded in the regions in Figure 4d are shown in Figure 4e,f. Again, the lattice fringes of the adjacent nanocrystals of YF3 in the waist part are nearly in the same directions, which is probably due to the oriented aggregation of two fragments of clustered aggregates. A schematic illustration of the formation of peanut-like YF3 is shown in Figure 5. Such an oriented attachment mechanism has been revealed in the formation of several hollow structures.37,38 Controlled experiments have been conducted to probe what affects this route to give the hollow peanut-like structures of orthorhombic YF3. The reaction temperature did not show significant effect on the crystalline phases and morphologies, but the formation of the product with peanut structure is much slower at a lower reaction temperature. In addition, the molar ratio of the starting materials did not influence the crystalline

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Figure 6. TEM and SEM images of the YF3 obtained from (a) KBF4, 12 h; (b) NH4BF4, 12 h; and (c) HBF4, 12 h.

phases and morphologies of the products. For instance, a reaction using the molar ratio of Y3+/BF4- ) 1:0.75 gave YF3 with the same crystalline phase and morphology, but the yield was much lower. Several other tetrafluoroborate complexes (KBF4, NH4BF4, HBF4) have also been examined as fluoride sources with the aim to investigate their effects on the morphology of YF3. All the products obtained from these different sources were indexed to the orthorhombic phase of YF3 according to their XRD patterns (Figure S1, Supporting Information). Their compositions were further confirmed by XPS measurements (Figure S2). Figure 6a,b shows the SEM images of YF3 prepared using NH4BF4 and KBF4, respectively, after 12 h of reaction time. Similar to that prepared from NaBF4, the products from NH4BF4 or KBF4 after 12 h are hollow peanut particles. Furthermore, the time-dependent experiments revealed similar morphological evolution as that using NaBF4 (Figure S3). However, the particles prepared from the acid HBF4 are different from those obtained using NaBF4, NH4BF4, and KBF4. Even though XRD analysis showed that the product was orthorhombic (Figure S1), TEM and SEM images of the products obtained at different reaction stages showed that no ordered aggregate of YF3 was obtained from HBF4. Figure 6c is the TEM image of the products after 12 h of reaction time, indicating that the typical morphology was small irregular nanocrystals. BF4- has been known to release F- ion in the reaction mixture.39,40 In order to obtain additional information about the reaction between tetrafluoride complexes and Y(NO3)3, in particular the fate of BF4-, a solid sample was obtained by evaporating the clear solution after centrifugation and then analyzed by XRD. The XRD pattern in Figure S4 showed that it was a mixture of solid H3BO3 and Na2B2O4. We thus propose the reaction pathways to explain the formation of YF3 shown in Scheme 1.42 The formation of H3BO3 was also supported by the change in acidity of the reaction mixture before and after hydrothermal reaction. The pH values of the initial and final aqueous solution were 6.28 and 1.15, respectively, suggesting an increase of the acidity of the reaction mixture after hydrothermal reaction. Similar experiments have been carried out for the reactions with KBF4, NH4BF4, and HBF4. The XRD patterns of the solids obtained from the reaction mixture with KBF4, NH4BF4, and HBF4 are shown in Figure

Scheme 1

4S (b-d). In addition to H3BO3, KB5O8 and (NH4)3B15O20(OH)8(H2O)4 were also detected in the reaction mixtures from KBF4 and NH4BF4. However, only H3BO3 was found in the reaction mixture from HBF4. The pH values were found to be vary from 6.44 to 1.14 for KBF4, from 6.45 to 1.10 for NH4BF4, and from 1.08 to 0.97 for HBF4. We have previously used NaBF4 as the fluoride source in the preparation of EuF3 nanocrystals.41 A simple room-temperature precipitation route using NaBF4 yielded nanospindles of EuF3 with orthorhombic structure and nanodisks of EuF3 with a hexagonal structure by controlling the molar ratios of the starting materials.41 Cao et al. have prepared desk-like and dotlike CeF3 nanocrystals with hexagonal structures using KBF4 and NH4BF4 by an ultrasonic-assisted route.42 Our previous research has also revealed that the cations of the simple binary fluoride sources XF (X ) K+, H+, NH4+, Na+, Rb+, or Cs+) play important roles in the formation of EuF3 nanoparticles with different morphologies.43 In the current work for the controlled synthesis of YF3 involving HBF4, NaBF4, NH4BF4, and KBF4 as fluoride sources, the anions (BF4-) are identical. HBF4 gave only YF3 nanocrystals that did not self-assemble. However, YF3 nanocrystals from NaBF4, NH4BF4, or KBF4 gradually selfassemble into hollow peanuts of YF3. It is possible that the different byproducts formed in these reaction mixtures are responsible for the formation of YF3 with different morphologies. It is obvious that there are many inorganic species existing in the reaction mixture. The byproduct borate salts probably promote the formation of peanut-like structures, since there are no such species in the reaction mixture starting with HBF4. Conclusions In summary, hollow peanut-like structures of YF3 have been prepared by a hydrothermal route using suitable tetrafluroborate complexes as the fluoride source. Four tetrafluroborate

Formation of YF3 Nanocrystals

complexessNaBF4, NH4BF4, KBF4, and HBF4swere studied in the controlled syntheses. Except HBF4, YF3 nanocrystals from NaBF4, NH4BF4, or KBF4 gradually self-assemble into hollow peanuts. The time-dependent studies reveal a morphological evolution and suggest that Na+, NH4+, or K+ of the tetrafluoroborate complexes play an important role in the self-assembly of YF3 nanocrystals. Acknowledgment. This work was supported by the Major State Basic Research Development Program (Grant No. 2006CB806104), the Natural Science Grant of China (Grant No. 50572037), Grant of Instruments of Nanjing University, and the U.S. National Science Foundation. Supporting Information Available: The detailed XRD, XPS, TEM, and SEM images of the as-obtained YF3 derived from other fluoride sources and the XRD patterns of the solids by evaporating the clear solution after centrifugation. This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) Zeng, H. C. J. Mater. Chem. 2006, 16, 649-662. (2) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 11111114. (3) Lu, Y.; McLellan, J.; Xia, Y. N. Langmuir 2004, 20, 3464-3470. (4) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176-2179. (5) He, T.; Chen, D. R.; Jiao, Wang, X. L.; Y. L. AdV. Mater. 2006, 18, 1078-1082. (6) Wang, W. Z.; Poudel, B.; Wang, D. Z.; Ren, Z. F. AdV. Mater. 2005, 17, 2110-2114. (7) Yang, H. G.; Zeng, H. C. Angew. Chem., Int. Ed. 2004, 43, 59305933. (8) Geng, J.; Zhu, J.-J.; Lu, D.-J.; Chen, H.-Y. Inorg. Chem. 2006, 45, 8403-8407. (9) Yao, K. X.; Zeng, H. C. J. Phys. Chem. B 2006, 110, 14736-14743. (10) Xie, Y.; Huang, J. X.; Li, B.; Qian, Y. T. AdV. Mater. 2000, 12, 1523-1526. (11) Xu, F.; Zhang, X.; Zhang, S. Y.; Liu, X. M.; Tian, X. B. Inorg. Chem. 2004, 43, 822-829. (12) Nakashima, T.; Kimizuka, N. J. Am. Chem. Soc. 2003, 125, 63866387. (13) Peng, Q.; Dong, Y. J.; Li, Y. D. Angew. Chem., Int. Ed. 2003, 42, 3027-3030. (14) Yang, H. G.; Zeng, H. C. J. Phys. Chem. B 2004, 108, 3492-3495. (15) Liu, B.; Zeng, H. C. Small 2005, 1, 566-571. (16) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711-714.

Crystal Growth & Design, Vol. 7, No. 10, 2007 2111 (17) Wang, X.; Li, Y. D. Angew. Chem., Int. Ed. 2003, 42, 3497-3500. (18) Iparraguirre, I.; Azkargorta, J.; Balda, R.; Fernandez, J. Opt. Mater. 2005, 27, 1697-1703. (19) Trnovcova´, V.; Garashima, L. S.; Sˇ kubla, A.; Fedorov, P. P.; C ˇ icˇka, R.; Krivandina, E. A.; Sobolev, B. P. Solid State Ionics 2003, 157, 195-201. (20) Nikl, M.; Yoshikawa, A.; Vedda, A.; Fukuda, T. J. Cryst. Growth 2006, 292, 416-421. (21) Cui, Y.; Fan, X. P.; Hong, Z. L.; Wang, M. Q. J. Nanosci. Nanotechnol. 2006, 6, 830-836. (22) De, G. H.; Qin, W. P.; Zhang, J. S.; Zhang, J. S.; Wang, Y.; Cao, C. Y.; Cui, Y. J. Lumin. 2007, 122-123, 128-130. (23) Yan, R. X.; Li, Y. D. AdV. Funct. Mater. 2005, 15, 763-770. (24) Tao, F.; Wang, Z. J.; Yao, L. Z.; Cai, W. L.; Li, X. G. J. Phys. Chem. C 2007, 111, 3241-3245. (25) Tao, F.; Wang, Z. J.; Yao, L. Z.; Cai, W. L.; Li, X. G. Cryst. Growth Des. 2007, 7, 845-858. (26) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. Nature 2005, 437, 121124. (27) Lemyre, J. L.; Ritcey, A. M. Chem. Mater. 2005, 17, 3040-3043. (28) Shchukin, D. G.; Sukhorukov, G. B. Langmuir 2003, 19, 44274431. (29) Wang, M.; Huang, Q. L.; Hong, J. M.; Chen, X. T. Mater. Lett. 2007, 61, 1960-1963. (30) Condorelli, G. G.; Anastasi, G.; Fragala, I. L. Chem. Vap. Deposition 2005, 11, 324-329. (31) Zhang, L. Z.; Yu, J. C.; Xu, A.-W.; Li, Q.; Kwong, K. W.; Yu, S.H. J. Cryst. Growth 2004, 266, 545-551. (32) Wang, C. G.; Wang, T. T.; Ma, Z. F.; Su, Z. M. Nanotechnology 2005, 16, 2555-2560. (33) Xuan, S. H.; Hao, L. Y.; Jiang, W. Q.; Song, L.; Hu, Y.; Chen, Z. Y.; Fei, L. F.; Li, T. W. Cryst. Growth Des. 2007, 7, 430-434. (34) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969-971. (35) Banfield, J. F.; Welch, S. A.; Zhang, H. Z.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751-754. (36) Tsai, M. H.; Chen, S. Y.; Shen, P. Nano Lett. 2004, 4, 1197-1201. (37) Yang, H. G.; Zeng, H. C. Angew. Chem., Int. Ed. 2004, 43, 59305933. (38) Liu, B.; Zeng, H. C.; J. Am. Chem. Soc. 2004, 126, 8124-8125. (39) Crabtree, R. H.; Hlatky, G. G.; Holt, E. M. J. Am. Chem. Soc. 1983, 105, 7302-7306. (40) Hidai, M.; Kodama, T.; Sato, M.; Harakawa, M.; Uchida, Y. Inorg. Chem. 1976, 15, 2694-2697. (41) Wang, M.; Huang, Q. L.; Hong, J. M.; Chen, X. T.; Xue, Z. L. Cryst. Growth Des. 2006, 6, 1972-1974. (42) Zhu, L.; Li, Q.; Liu, X. D.; Li. J. Y.; Zhang, Y. F.; Meng, J.; Cao, X. Q. J. Phys. Chem. C 2007, 111, 5898-5903. (43) Wang, M.; Huang, Q. L.; Hong, J. M.; Chen, X. T.; Xue, Z. L. Cryst. Growth Des. 2006, 6, 2169-2173.

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