Selective Synthesis and Characterization of Nanocrystalline EuF3 with Orthorhombic and Hexagonal Structures Miao
Wang,†
Qing-Li
Huang,†
Jian-Ming
Hong,‡
Xue-Tai
Chen,*,†,§
and Zi-Ling
Xue|
Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, and Analytic Center, Nanjing UniVersity, Jiangsu, Nanjing 210093, People’s Republic of China, State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Fuzhou 350002, People’s Republic of China, and Department of Chemistry, The UniVersity of Tennessee, KnoxVille, Tennessee 37996-1600
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 8 1972-1974
ReceiVed March 2, 2006; ReVised Manuscript ReceiVed May 27, 2006
ABSTRACT: A simple solution-based procedure employing NaBF4 as the fluoride source has been developed to selectively prepare nanospindles of EuF3 with an orthorhombic structure and nanodisks of EuF3 with a hexagonal structure by controlling the molar ratios of the starting materials. These as-prepared nanocrystals of EuF3 were characterized by XRD, SEM, and TEM. The nanodisks of EuF3 self-assemble into single-crystal column-like nanostructures. The photoluminescence properties of these nanoscale EuF3 species have been investigated. Introduction
Experimental Section
Lanthanide compounds have been widely used in many research fields such as lamps and display devices,1 optical telecommunications,2 and lasers.3 As an important family of lanthanide compounds with unique physical and chemical properties, the binary fluorides LnF3 have attracted particular interest. Several research groups have reported the preparation of nanocrystals of binary lanthanide fluorides with different morphologies via chemical routes. Van Veggel et al.4 have reported the preparation and luminescence of rare-earth-iondoped LaF3 colloidal nanoparticles. Li et al.5,6 have successfully employed hydrothermal routes to yield fullerene-like nanoparticles and Ln3+-doped, bundle-like YF3 phosphors. Wang et al.7 have reported the synthesis and self-assembly of LaF3 nanoplates. Ritcey et al.8 have used reverse microemulsion to prepare hexagonal and triangular YF3 nanocrystals. Yan et al.9 have developed a single molecular precursor route to prepare monodisperse LaF3 triangular nanoplates. Our group has fabricated a novel ring-like nanostructure of EuF3 via a hydrothermal route at 160 °C using NaBF4 as the fluoride source.10 These reported routes usually only gave rise to the nanoscale binary lanthanide fluorides with one crystal structure.4-10 EuF3 has two phases, with orthorhombic and hexagonal structures exhibiting different properties. It is particularly important to develop a solution-based route to controllably prepare nanocrystalline EuF3 with two different crystal structures by varying the reaction parameters. We have recently observed that the use of different fluoride sources XF (X ) K+, H+, NH4+, Na+, Rb+, and Cs+) yields nanoscale EuF3 species with different morphologies and crystal structures.11 In addition, it was found that two different phases of nanocrystalline EuF3 could be prepared by a simple room-temperature solution route employing NaBF4 as the fluoride source with different molar ratios of the starting materials. Here we report the selective synthesis and characterization of nanocrystalline EuF3 with orthorhombic and hexagonal structures.
The synthesis here involved the solution-phase reaction between Eu(NO3)6‚6H2O and NaBF4 at room temperature. An aqueous solution containing NaBF4 and Eu(NO3)3‚6H2O in a molar ratio of 0.75:1 or 4:1 was stirred for a fixed reaction period. The resulting white solid precipitates were collected and washed several times with distilled water and ethanol in an ultrasonic bath. The final products were dried at 70 °C for 3 h. The samples were characterized by X-ray powder diffraction (XRD) with a Shimadzu XD-3A powder X-ray diffractometer with Cu KR radiation (λ ) 1.5418 Å), recorded with 2θ ranging from 20 to 60°. The sizes and morphologies of the products were studied by highresolution electron microscopy on a JEOL JEM-2010F microscope and scanning electron microscopy on a JEOL JSM-6700F microscope. The room-temperature luminescent spectra were performed on an AmincoBowman luminescence spectrometer.
* To whom correspondence should be addressed at the Coordination Chemistry Institute, Nanjing University. E-mail:
[email protected]. Fax: ++86-25-83314502. † Coordination Chemistry Institute, Nanjing University. ‡ Analytic Center, Nanjing University. § Fujian Institute of Research on the Structure of Matter. | The University of Tennessee.
Results and Discussion The reactions between Eu(NO3)3‚6H2O and NaBF4 in H2O were used to prepare nanoscale EuF3 at room temperature. The crystalline phase and purity of the samples were determined by X-ray powder diffraction (XRD), and typical diffraction patterns are shown in Figure 1. The XRD pattern (Figure 1a) of the product, which was obtained from the reaction mixture with a molar ratio of NaBF4 to Eu3+ of 0.75:1 after 3 h reaction time, can be readily indexed to the orthorhombic structure (JCPDS File No. 33-0542) with the calculated cell parameters a ) 6.6 Å, b ) 7.1 Å, and c ) 4.4 Å, indicating that pure orthorhombic EuF3 was prepared. The morphology, as revealed by SEM and TEM images (Figure 2a-c), showed that the products are spindle-like nanoparticles of EuF3 with a diameter of 50 nm and length up to 300 nm. The surface of these spindle-like particles is not smooth. It appears that they are constructed with some small nanowires (Figure 2d). However, the ED pattern recorded on one single spindle-like particle shows that they are single crystals (inset in Figure 2d). When the molar ratio of NaBF4 to Eu3+ in the reaction mixture was increased to 4:1 while other reaction conditions were kept identical, a new phase of EuF3 was obtained. The X-ray diffraction patterns of the products obtained after reaction for 3 and 12 h are shown in parts b and c of Figure 1, respectively. All the diffraction peaks in Figure 1b can be indexed to the hexagonal structure of EuF3 (JCPDS File No.
10.1021/cg060116s CCC: $33.50 © 2006 American Chemical Society Published on Web 06/29/2006
Nanocrystalline EuF3
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Figure 1. XRD patterns of the products: (a) nanospindle; (b) nanodisk; (c) nanodisk column; (d) mixture of nanodisk and nanodisk column.
Figure 2. SEM (a, b) and TEM (c, d) images of as-obtained nanospindles of EuF3.
Figure 3. EM images of as-obtained products: (a) TEM image of EuF3 nanodisks (the inset gives the ED pattern); (b-d) SEM images of EuF3 nanodisks; (e) SEM image of single nanodisk column; (f) TEM image of EuF3 nanodisk column (the inset gives the ED pattern); (g) TEM image of the product obtained after 6 h reaction time.
32-0373) with the calculated cell parameters a ) 6.9 Å and c ) 7.1 Å. No peaks of impurities were detected, which indicated the high purity of the product. This result indicated that two different crystalline phases of EuF3 could be selectively prepared by simply controlling the molar ratio of NaBF4 to Eu3+ in the current procedure. Similarly, the XRD pattern of the product obtained from the reaction mixture of the same ratio of NaBF4 to Eu3+ after 12 h reaction time also suggested the formation of EuF3 with a hexagonal structure (Figure 1c). However, the diffraction peaks are sharper than those of the product obtained after reaction for 3 h, suggesting the improved crystallinity of the product after prolonging the reaction time. Furthermore, the diffraction peak [002] is stronger than the peak [111], in contrast with those of the product obtained after 3 h of reaction. These observations are consistent with the preferential growth along [001] in the product after reaction for 12 h. The morphology and microstructures of the products obtained at a molar ratio of NaBF4 to Eu3+ of 4:1 were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 3a shows the TEM image of the product obtained after reaction for 3 h. The as-obtained disk-
like particles exhibited a round shape, and the diameter is about 1 µm. The ED patterns shown in the inset in Figure 3a shows that the nanodisks are single crystals. Some nanodisks appear ellipsoidal because they do not lie tightly on the TEM grid, and the projections under the electric beam departed somewhat from the real morphology. The disk-like morphology was further confirmed by the SEM image of the nanodisk (Figure 3b). The thickness of the nanodisks was estimated to be ca. 100 nm. The morphologies of the product after increasing the reaction time to 12 h were identified as nanodisk columns by SEM and TEM. The low-magnification SEM images in Figure 3c indicate that there are many column-like particles in the sample prepared after reaction for 12 h. Figure 3d-f reveals that the columns consist of nanodisks, which are tightly stacked face to face, and no obvious spacing was found between them in the column. The columns were several micrometers in length and 1 µm in diameter, which was consistent with the diameter of the nanodisk formed after reaction for 3 h. It is obvious that columns are formed from the self-assembly of nanodisks. The ED pattern of a single nanodisk column (Figure 3f inset) reveals the singlecrystal nature of the nanodisk column. This strongly suggested
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intense for the nanospindles. Furthermore, the peaks of nanodisk columns are stronger than those of nanodisks. The difference in luminescence properties can be ascribed to the various dimensions, morphologies, and crystal structures. Conclusions
Figure 4. Room-temperature emission spectra of as-obtained EuF3: (a) nanodisks; (b) nanodisk columns; (c) nanospindles.
that the formation of a nanodisk column was the result of selfassembly of the round nanodisks via the oriented aggregation mechanism.12 Similar formations of nanostructures made up of nanoscale building blocks have been demonstrated.7,13-18 It is interesting to note that nanodisk columns were only formed with a prolonged reaction time (e.g. 12 h). A similar reaction was also carried out to detect the immediate reaction product when the reaction time was 6 h. The XRD pattern showed that the product also has a hexagonal structure (Figure 1d). The TEM image revealed that the product is a mixture of nanodisks and nanodisk columns (Figure 3g). The morphological evolution found at different reaction stages further supports the conclusion that round nanodisks of EuF3 were formed first, followed by stacking of the disks to form a column-like structure until all the disks were consumed to give the nanodisk columns. The nanodisks were found to only form in the presence of excess NaBF4, which suggested that the excess NaBF4 must play an essential role in the fabrication of nanodisks and nanodisk columns of EuF3. In the current study, NaBF4 was used as the fluoride source. It has also been employed in our fabrication of ring-like nanostructures of EuF3 under hydrothermal conditions at 160 °C.10 The other reported procedures for the preparation of nanoscale binary lanthanide fluorides usually employed a simple fluoride compound such as NaF. If NaF was used as the fluoride source instead of NaBF4, no nanospindle, nanodisk, or columnlike structure of EuF3 was formed. These observations revealed the unique and essential role of NaBF4 in the preparation of nanocrystalline EuF3 reported here. The room-temperature luminescence spectra were recorded for solid samples of nanodisks and columns of hexagonal EuF3 and nanospindles of orthorhombic EuF3 (Figure 4). When these species were excited at 394 nm, emission peaks centered at 592, 616, 651, and 697 nm were observed, which could contribute to the transitions of 5D0 f 7F1, 5D0 f 7F2, 5D0 f 7F3, and 5D0 f 7F4, respectively.4a,19 Although the major peaks in the emission spectra are identical in these samples, the intensity patterns are different. The strongest emission peak occurs at 592 nm (5D0 f 7F1) in nanodisks and columns of hexagonal EuF3, while the intensity of the peak at 616 nm is the most
A simple room-temperature solution route has been developed to selectively prepare EuF3 nanocrystals with orthorhombic and hexagonal structures. This solution route has demonstrated the usefulness of NaBF4 as the fluoride source in the fabrication of nanoscale rare-earth fluorides in aqueous solution. With a NaBF4 to Eu3+ molar ratio of 0.75:1, nanospindles with orthorhombic structures were observed, while in a reaction mixture with a 4:1 molar ratio, hexagonal nanodisks were formed, which after 12 h reaction time assembled into column-like nanostructures. It is expected that the use of NaBF4 as the fluoride source could be extended to the preparation of other nanoscale metal fluorides. Acknowledgment. This work was supported by a Natural Science Grant of China (No. 50572037), a Natural Science Grant of Jiangsu Province (No. BK 2004087), the Program for Changjiang Scholars, the Innovative Research Team in University, a Grant of Instruments of Nanjing University, and the U.S. National Science Foundation. References (1) Ju¨stel, T.; Nikol, H.; Ronda, C. Angew. Chem., Int. Ed. 1998, 37, 3084-3103. (2) Polman, A. J. Appl. Phys. 1997, 82, 1-39. (3) Reisfeld, R.; Jorgensen, C. K. Lasers and Excited States of Rare Earths; Springer: Berlin, 1977. (4) (a) Stouwdam, J. W.; van Veggel, F. C. J. M. Nano Lett. 2002, 2, 733-737. (b) Stouwdam, J. W.; Hebbink, C. A.; Huskens, J.; van Veggel, F. C. J. M. Chem. Mater. 2003, 15, 4604-4616. (5) (a) Wang, X.; Li, Y. D. Angew. Chem., Int. Ed. 2003, 42, 34973500. (b) Wang, X.; Li, Y. D. Chem. Eur. J. 2003, 9, 5627-5635. (6) Yan, R. X.; Li, Y. D. AdV. Funct. Mater. 2005, 15, 763-770. (7) Cheng, Y.; Wang, Y. S.; Zheng, Y. H.; Qin, Y. J. Phys. Chem. B 2005, 109, 11548-11551. (8) Lemyre, J. L.; Ritcey, A. M. Chem. Mater. 2005, 17, 3040-3043. (9) Zhang, Y. W.; Sun, X.; Si, R.; You, L. P.; Yan, C. H. J. Am. Chem. Soc. 2005, 127, 3260-3261. (10) Wang, M.; Huang, Q. L.; Chen, X. T.; Hong, J. M. Xue, Z. L. Submitted for publication. (11) Wang, M.; Huang, Q. L.; Chen, X. T.; Hong, J. M. Xue, Z. L. Submitted for publication in Cryst. Growth Des. (12) (a) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969-971. (b) Banfield, J. F.; Welch, S. A.; Zhang, H. Z.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751-754. (13) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 1188-1191. (14) He, T.; Chen, D. R.; Jiao, X. L. Chem. Mater. 2004, 16, 737-743. (15) Zitoun, D.; Pinna, N.; Frolet, N.; Belin, C. J. Am. Chem. Soc. 2005, 127, 15034-15035. (16) Cho, K. S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 7140-7147. (17) Lee, E. J. H.; Ribeiro, C.; Longo, E.; Leite, E. R. J. Phys. Chem. B 2005, 109, 20842-20846. (18) Huang, F.; Zhang, H. Z.; Banfield, J. F. J. Phys. Chem. B 2003, 107, 10470-10475. (19) Flores-Acosta, M.; Pe´rez-Salas, R.; Aceves, R.; Sotelo-Lerma, M.; Ramı´rez-Bon, R. Solid State Commun. 2005, 136, 567-571.
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