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Morphological Control and Luminescent Properties of CeF3 Nanocrystals Ling Zhu,†,‡ Qin Li,†,‡ Xiangdong Liu,†,‡ Jiayan Li,†,‡ Yanfei Zhang,†,‡ Jian Meng,† and Xueqiang Cao*,† Key Laboratory of Rare Earth Chemistry & Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100049, China ReceiVed: December 28, 2006; In Final Form: February 14, 2007
The nanocrystals of CeF3 with the hexagonal structure and different morphologies such as the disk, the rod, and the dot have been successfully synthesized via a mild ultrasound assisted route from an aqueous solution of cerium nitrate and different fluorine sources (KBF4, NaF, NH4F). The use of different fluorine sources has a remarkable effect on the morphology of the final product. The luminescence and UV-vis absorption properties of CeF3 nanocrystals with different morphologies have been investigated. Compared with other shape nanocrystals, the luminescence intensity of the disklike nanocrystals is obviously enhanced. It is suggested that the function-improved materials could be obtained by tailoring the shape of the CeF3 nanocrystals.
Introduction Rare-earth nanocrystals with controllable shapes and sizes have received intense research attention during the past few years because of their potential applications in optics, optoelectronics, biological labeling, catalysis, and so forth.1-5 Furthermore, it has been demonstrated that the chemical and physical properties of nanometer regime materials are strongly related to their sizes and morphologies. For instance, the optical properties of luminescent nanomaterials are enormously affected by their shapes and sizes.6-10 With different shapes, the CdSe nanowires and nanoribbons showed different single strong band edge emissions at about 620 and 670 nm, respectively.7 By only changing the length of the CdSe quantum rods, the emission positions of these rods are much different from each other when excited at the same wavelength.8 The photoluminescence spectra of the pure hexagonal-phased YBO3:Eu nanocrystals with different particle sizes showed a size-dependent property because the ratio of the red emission transition (5D0f7F2) to the orange emission transition (5D0f7F1) (R/O) was much higher in the smaller particles.9 YVO4:Eu nanocrystals with nanobundle-like, ricelike, and rhombuslike showed different emission positions as well as the relative intensity of the predominant peak in their photoluminescence spectra.10 Therefore, the synthesis of nanoparticles with well-controlled shapes, sizes, and structures is both scientifically and technically important. Cerium fluoride (CeF3) has been attracting increasing attention in virtue of its technological importance as an inorganic scintillating crystal.11-13 Compared with the other conventional scintillators, CeF3 is considered as one of the most promising scintillators for the next generation experiments in high-energy physics because of their high density (6.16 g‚cm-3), fast response, and high-radiation resistance. Meanwhile, it is also an important fluorescent host material owing to its low * To whom correspondence should be addressed. Tel/Fax: +86-43185262285; e-mail:
[email protected]. † Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.
vibrational energies and the subsequent minimization of the quenching of the excited state of the rare-earth ions.14 Studies also indicate that CeF3 is a good solid lubricant as a result of its layered structure.15 CeF3 single crystals have usually been grown from the melt using Czochralski (CZ) technique13,16-18 and Bridgeman19-21 method. CeF3 films have been synthesized by MOCVD22 and molecular beam epitaxy.23 Several references reported the synthesis of CeF3 nano- and microcrystals by wet chemical methods including the reverse micelles or microemulsions15,24-27 and the polyol method.14,28 However, it is still a challenge to develop some novel structures of CeF3 with the controlled morphologies in mild reaction conditions. Herein, we introduce a facile and fast sonochemical-assisted route for preparing CeF3 crystals with controlled morphologies, and we further investigate into their microstructure, UV-vis absorption, and luminescence properties. Studies indicate that its nanostructure can be efficiently controlled by a facile and fast sonochemical-assisted route when using KBF4, NaF, and NH4F as the fluorine sources, respectively, and the optical properties of the products are different from each other owing to the difference in the morphologies, crystal sizes, and structures. In recent years, much interest has been devoted to the ultrasonic synthesis technique because it provides rapid and controllable reaction conditions and has the ability to form nanoparticles with uniform shape, narrow size distribution, and high purity.29 When liquids are irradiated with high-intensity ultrasound, acoustic cavitations (the formation, growth, and implosive collapse of the bubbles) provide the primary mechanism for sonochemical effects, during which very high temperature (>5000 K), pressure (>20 MPa), and cooling rate (>1010 K‚s-1) are achieved upon the collapse of the bubbles.30-31 Such remarkable environments provide a unique platform for the chemical reaction. Experimental Section 1. Synthesis. In a typical procedure, an aqueous solution of Ce(NO3)3 was mixed with KBF4 solution in a 150 mL plastic flask to give a final concentration of 20 mM Ce(NO3)3 and 80 mM KBF4. The total volume of the solution was 100 mL. The
10.1021/jp068974m CCC: $37.00 © 2007 American Chemical Society Published on Web 04/03/2007
Luminescent Properties of CeF3 Nanocrystals
Figure 1. XRD patterns of CeF3: standard (JCPDS Card 08-0045) (a), disklike (b), rodlike (c), and dotlike (d) nanocrystals.
transparent mixture solution was then exposed to ultrasound irradiation under ambient air for 3 h. The ultrasound irradiation was accomplished with a high-intensity ultrasonic probe (JCS206 Jining Co. China, Ti-horn, 23 kHz) immersed directly in the reaction solution. A white precipitate was centrifuged and was washed with distilled water and absolute ethanol in sequence. The final product (sample 1) was dried in vacuum at 60 °C for 12 h. A similar synthetic procedure was employed using NaF (sample 2) and NH4F (sample 3) as fluorine source, and other reaction conditions were unchanged. The products were characterized via X-ray powder diffraction (XRD), scanning electron micrographs (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), high-resolution transmission electron micrographs (HRTEM), ultraviolet and visible absorption spectra (UV-vis), and photoluminescence spectra (PL). 2. Characterization. The XRD patterns were performed on a Rigaku D/MAX-2500 diffractometer with Cu KR radiation (λ ) 1.5406 Å) and a scanning rate of 5°‚min-1. The operation voltage and current were maintained at 40 kV and 200 mA, respectively. SEM images were taken on an XL-30 fieldemission scanning electron microscope (Philips) equipped with energy-dispersive X-ray fluorescence analysis (EDXA). Samples for SEM observation were prepared by dropping a diluted suspension of the sample powders on the silicon substrate. TEM, HRTEM, and SAED were recorded on a JEOL-JEM-2010 operating at 200 kV (JEOL, Japan). Samples for TEM observation were prepared by dropping a diluted suspension of the sample powders onto a standard carbon-coated Formvar film (20-30 nm) on a copper grid (230 mesh). The UV-vis absorption spectra were measured on a TU-1901 spectrophotometer, using a quartz cell with width of 1 cm. The assynthesized powder was dispersed in ethanol and then was sonicated at room temperature for 10 min, and a colloidal solution was thus obtained; pure ethanol was used as a blank. PL spectra were recorded with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The operation parameters of PL test are the following: scan speed ) 240 nm/min, delay ) 0 s, EX slit ) 2.5 nm, EM slit ) 2.5 nm, and PMT voltage ) 700 V. All the measurements were performed at room temperature. Results and Discussion 1. Structure and Morphology Control of the Nanocrystal. Figure 1 shows the XRD patterns of the as-prepared products
J. Phys. Chem. C, Vol. 111, No. 16, 2007 5899 with different morphologies. The diffraction patterns of all the products can be indexed with a pure hexagonal structure, which is in good agreement with the literature (JCPDS No. 08-0045). The XRD patterns indicate that the well-crystallized CeF3 crystals can be easily obtained under the current synthetic conditions. For the dotlike product (sample 3), the average crystalline domain is 15.5 nm as calculated from the XRD line widths using Scherrer’s equation, which is consistent with the TEM observation (Figure 2j). However, for the low-dimensional particles (samples 1 and 2) that deviate from the sphere, it is hard to get size information on either diameter or length just from XRD patterns. An accurate crystalline domain should be obtained from the direct observation. The refined crystallographic unit cell parameters are shown in Table 1. The morphologies and microstructures of samples 1-3 were investigated by the SEM and TEM observation. Figure 2a-c shows the disklike morphology of sample 1 with uniform size and the well-defined round shape. The average diameter and thickness of the disk are 800 and 21 nm (Figure 2b), respectively. The clear lattice fringes in the HRTEM image (Figure 2e) confirm the high crystallinity of the as-prepared CeF3 nanodisks. The fringe space of 0.32 nm corresponds to the [111] planes of the structure. The SAED (Figure 2f) of a single disk indicates that it is single crystalline. The energy-dispersive X-ray spectroscopy analysis (EDXA) confirms that the nanodisks are composed of Ce and F in a molar ratio of 1:3 (Figure 2g). Figure 2h is the TEM image of sample 2, indicating the rodlike morphology with a diameter of 9 nm and length from 30 to 40 nm. The SAED of a single rodlike nanoparticle (inset of Figure 2i) shows that the nanoparticle is polycrystalline in nature. HRTEM image (Figure 2i) reveals the bend of fringe and particle edges, while fringe spacing of 0.31 nm shows the [111] plane of the nanoparticle. The TEM image of sample 3 (Figure 2j) reveals that it consists of the spherical-shaped nanoparticles with diameter from 10 to 15 nm. The polycrystallinity of sample 3 is confirmed by SAED patterns (inset of Figure 2k). Clear lattice fringes were observed in the primary nanoparticles as shown in HRTEM image (Figure 2k), indicating the good crystallinity of sample 3. To investigate the effect of the ultrasound on the formation of sample 1 with the disklike shape, a sample (sample 4) was synthesized with stirring but without the ultrasonic irradiation, while the other preparation conditions were identical to those of sample 1. As shown in Figure 3, sample 4 has similar morphology with sample 1 with the disklike shape, and the diameter of the disks is in the range of 200-600 nm, implying that the ultrasonic irradiation does not have a substantial influence on the morphology of the nanoparticles. With the assistance of the ultrasonic irradiation, the reaction solution became turbid after 30 min, while it turned cloudy after 2 h, and only a small quantity of product was obtained without the ultrasonic irradiation. Therefore, in the present case, the utilization of ultrasonic irradiation would induce the cavitation bubble collapse and strong shock wave in the reaction solution, which can accelerate the hydrolysis process of KBF4 and thus accelerate the reaction rate. Moreover, the size distribution of sample 1 is relatively narrower than that of sample 4. It is clear that the main advantages of the application of ultrasound in this experiment are the significant reduction in fabrication time and the improvement in size uniformity. Furthermore, no disklike structure of CeF3 was formed in the case of using NaF or NH4F as the fluorine source. Considering all of these results, it is safe to draw a conclusion that KBF4 is a key factor in the formation of the disk shape.
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Figure 2. Morphological characterization of the CeF3 nanocrystals: SEM images (a, b), TEM images (c, d), HRTEM image (e), SEAD (f), and EDX (g) of the disklike particles; TEM (h), HRTEM (i), and SAED (inset i) images of the rodlike particles; TEM (j), HRTEM (k), and SAED (inset k) images of the dotlike particles.
TABLE 1: The Refined Unit Cell Parameters of CeF3 Nanocrystals sample
a (Å)
c (Å)
V (Å3)
JCPDS 08-0045 sample 1 (disklike) sample 2 (rodlike) sample 3 (dotlike)
7.112 7.137 ( 0.002 7.101 ( 0.003 7.116 ( 0.002
7.279 7.297 ( 0.002 7.293 ( 0.003 7.276 ( 0.002
318.85 321.86 ( 0.01 318.47 ( 0.02 319.04 ( 0.01
The following hydrolysis process is proposed to explain the formation mechanism of the disklike structure:
BF4- + H2O f HF + [HOBF3]BF4- + 3H2O f H3BO3 + 3HF + F-
The equilibrium constant of this hydrolysis reaction is only 2.5 × 10-10, implying that the hydrolysis is a very slow process in which the F- ions will be kept in a low concentration, and it consequently leads to the slow crystallization of the final product. The slow process is probably helpful to the twodimensional growth of the CeF3 single crystal. However, in the case of samples 2 and 3, white precipitates appeared immediately after the reactant solution was exposed to ultrasound treatment, which indicated that the nucleation took place rapidly. In these cases, only polycrystalline products were obtained. Therefore, the reaction mechanism may respond to the preparation of the products with different crystal structures and morphologies. In the slow crystallization process, the single crystal was formed, whereas only polycrystalline was obtained. Moreover, consider-
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J. Phys. Chem. C, Vol. 111, No. 16, 2007 5901
Figure 3. SEM image of CeF3 particles prepared with KBF4 by stirring.
Figure 5. Excitation spectra of the disklike (a), rodlike (b), and dotlike (c) nanocrystals of CeF3.
Figure 4. UV-vis absorption spectra of the disklike (a), rodlike (b), and dotlike (c) nanocrystals of CeF3.
ing the different morphologies between samples 2 and 3, maybe the different cation ions (Na+, NH4+) in the reaction solution correspond to it.32 Different cations are adsorbed on the surfaces of particles in different ways, which lead to the growth of small crystals in different ways, and finally result in different shapes. Certainly, the actual growth mechanism of these nanostructures should be further investigated. 2. Optical Properties. It is well-known that cerium compounds such as CeF3, CePO4, and CeP2O7 display strong absorption for the ultraviolet.14,33-34 Figure 4 gives the UVvis absorption spectra of the CeF3 products. As a result of the f-d electron transitions, all the products show four well-resolved absorption peaks, which is in good agreement with the literature.14,35-37 The absorption spectrum of sample 1 shows four peaks with maxima appearing at 253, 236, 220, and 209 nm, respectively (Figure 4a), while the absorption spectrum of sample 3 has four peaks with maxima emerging at 249, 235, 219, and 210 nm, respectively (Figure 4c). The absorption peaks show a small shift to the higher energy as the size of the particles decreases from the disk to the dot. This different spectral
behavior observed for the CeF3 nanodisks and CeF3 nanodots might be attributed to the distorted lattices. It is generally considered that the degree of disorder in the nanomaterials is relatively high, and thereby a lower crystal field symmetry might be induced in such materials.35,38-39 The cell volume of CeF3 nanodisks calculated from the XRD pattern is 321.86 Å3, while the cell volume of CeF3 nanodots is 319.04 Å3. The different cell volumes indicate that they are in the different crystal field symmetries, thus giving rise to the shift in the absorption peaks of the CeF3 products with different morphologies.40-41 Because of the large effective radius of Ce3+, a 5d electron interacts strongly with the lattice, and in this case one would expect that the different crystal field interactions would result in the different UV-vis absorption properties. The excitation spectra (Figure 5) of the as-prepared CeF3 products (samples 1-3) have a broad band ranging from 230 to 300 nm, peaking at 260, 280, and 282 nm, respectively. To simplify the comparison and discussion, we used two bands to fit the excitation curves of the samples with a Gaussian distribution. As shown in Figure 5 (the green line), the excitation spectra of the three as-prepared CeF3 samples could be fitted with Gaussian function.23,37 It is easy to observe that the fitted broad bands of the three samples show similar positions with maxima appearing at 260 and 290 nm, which correspond to the transitions from the ground state 2F5/2 of Ce3+ to the different components of the excited Ce3+ 5d states split by the crystal field. However, the intensity radios of these two bands are different from each other in these three samples, which correspond to the shifts of the prime peaks. Furthermore, the excited electronic configuration of Ce3+ is 5d1 which is not shielded from the surroundings and is very sensitive to the change surrounding Ce3+ ions at or near the surface.42 Therefore, in the rodlike and dotlike CeF3 samples, the 5d1 level of Ce3+ may descend when the grain size becomes smaller and results in a red shift in the excitation spectra.24 The emission spectra (Figure 6) show that samples 1-3 exhibit the characteristic emission of Ce3+ 5d-4f which is
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Figure 6. Emission spectra of the disklike, rodlike, and dotlike nanocrystals of CeF3.
similar to the result of Wang et al.14 However, by comparing the curves of these three samples, it is observed that the location and relative intensity of the peaks are different. The corresponding strongest emission peaks for samples 1-3 appear at 323, 329, and 330 nm, respectively. The emission of Ce3+ could be fitted with two broad bands with maxima at 320 and 360 nm, respectively, which are assigned to the parity allowed transitions of the lowest component of the 2D state to the spinorbit components of the ground state, 2F5/2 and 2F7/2 of Ce3+, respectively.43 The different intensity radios of the two bands correspond to the slight shift in the emission spectra. Furthermore, the relative intensities of the PL peaks seem to be closely related to the morphology.10, 44-45 In the three samples, sample 3 shows a weak luminescence, whereas sample 1 displays the strongest luminescence under the same measurement conditions. The enhancement of PL intensity may be due to the different structures and surface defects. Sample 3 would possess a higher surface area and more defects because of its smaller particle size and the faster crystal growth, so it shows the lowest intense PL emission. These results indicate that the luminescent property of CeF3 is very sensitive to its structure and is strongly dependent on the structural defect. The luminescence properties of CeF3 doped with 5 mol % (molar ratio) Tb3+ ions with different morphologies were also investigated. The morphologies of the 5 mol % Tb3+ doped CeF3 samples have similar morphologies with the undoped CeF3, which further proves that the morphologies of the samples are only affected by the fluorine sources. The CeF3:Tb3+ nanoparticles show a strong green emission under the UV excitation. The excitation spectra of the disklike, rodlike, and dotlike CeF3: Tb3+ crystals (Figure 7) monitored with the 542 nm emission (5D4-7F5) of Tb3+ are similar to those of CeF3 (Figure 5), including a broad band ranging from 230 to 300 nm, peaking at 259, 267, and 275 nm, respectively. The shifts of the prime peaks of these three samples show similar ways as in the case of the undoped CeF3 sample (at 260, 280, and 282 nm, respectively). By excitation into the Ce3+ band at their optimal peaks (259, 267, and 275 nm, respectively), the CeF3:Tb3+ samples yield
Figure 7. Excitation spectra of the disklike (a), rodlike (b), and dotlike (c) nanocrystals of CeF3:Tb.
Figure 8. Emission spectra of the disklike, rodlike, and dotlike nanocrystals of CeF3:Tb.
both weak emission of Ce3+ (300-400 nm) and strong emission of Tb3+ (450-650 nm) (Figure 8). This indicates that an energy transfer from Ce3+ to Tb3+ occurs in the CeF3:Tb3+ nanoparticles, as observed in the result reported before.14,46-48 The Ce3+ luminescence failed to be completely quenched by Tb3+ owing to the high Ce3+ concentration in the samples. The emission of Tb3+ is due to the transition between the excited 5D4 state and the 7FJ (J ) 6-3) ground states of Tb3+ ions. It is interesting that the major peak positions in the emission spectra are identical to these three samples, while the emission intensity of the disklike CeF3:Tb3+ nanocrystal (black line of Figure 8) has been pronouncedly improved compared with the other shaped samples. These results indicate that the luminescence properties of the nanostructured particles are largely affected by factors such as the morphology, the particle size, and the crystal structure. Conclusion In summary, the hexagonal nanocrystals of CeF3 with different morphologies have been successfully synthesized in a
Luminescent Properties of CeF3 Nanocrystals mild aqueous solution system. The use of different fluorine sources has a significant effect on the morphology. The formation mechanism of the disklike structure has been investigated, and the slow hydrolysis process of KBF4 in aqueous solution clearly contributes to the creation of such a structure. Room-temperature photoluminescence of CeF3 and the Tb3+ ion-doped CeF3 products with different morphologies have also been investigated. The obtained samples showed different luminescence properties, which could be related to their different morphologies, particle sizes, and crystal structures. Acknowledgment. We gratefully acknowledge the financial support from NSFC-20331030. We also appreciate Ms. M. Y. Li and Mr. L. H. Ge for help in the FE-SEM and TEM measurements. References and Notes (1) Buissette, V.; Moreau, M.; Gacoin, T.; Boilot, J. P.; Chane-Ching, J.; Le Mercier, T. Chem. Mater. 2004, 16, 3767. (2) Diamente, P. R.; Burke, R. D.; van Veggel, F. C. J. M. Langmuir 2006, 22, 1782. (3) Boyer, J. C.; Vetrone, F.; Cuccia, L. A.; Capobianco, J. A. J. Am. Chem. Soc. 2006, 128, 7444. (4) Darbandi, M.; Hoheisel, W.; Nann, T. Nanotechnology 2006, 17, 4168. (5) Huang, P. X.; Wu, F.; Zhu, B. L.; Gao, X. P.; Zhu, H. Y.; Yan, T. Y.; Huang, W. P.; Wu, S. H.; Song, D. Y. J. Phys. Chem. B 2005, 109, 19169. (6) Hu, J.; Li, L.; Yang, W.; Manna, L.; Wang, L.; Alivisatos, A. P. Science 2001, 292, 2060. (7) Pan, A. L.; Yang, H.; Yu, R.; Zou1, B. S. Nanotechnology 2006, 17, 1083. (8) Li, L. S.; Hu, J. T.; Yang, W. D.; Alivisatos, A. P. Nano Lett. 2001, 1, 349. (9) Wei, Z. G.; Sun, L. D.; Liao, C. S.; Yin, J. L.; Jiang, X. C.; Yan, C. H.; Lu, S. Z. J. Phys. Chem. B 2002, 106, 10610. (10) Chen, L. M.; Liu, Y. N.; Huang, K. L. Mater. Res. Bull. 2006, 41, 158. (11) Moses, W. W.; Derenzo, S. E. Nucl. Instrum. Methods A 1990, 299, 51. (12) Auffray, E.; Baccaro, S.; Beckers, T.; Benhammou, Y.; Belsky, A. N.; Borgia, B.; Boutet, D.; Chipaux, R.; Dafinei, I.; de Notaristefani, F.; Depasse, P.; Dujardin, C.; El Mamouni, H.; Faure, J. L.; Fay, J.; Goyot, M.; Gupta, S. K.; Gurtu, A.; Hillemanns, H.; Ille, B.; Kirn, T.; Lebeau, M.; Lebrun, P.; Lecoq, P.; Mares, J. A.; Martin, J. P.; Mikhailin, V. V.; Moine, B.; Nelissen, J.; Nikl, M.; Pedrini, C.; Raghavan, R.; Sahuc, P.; Schmitz, D.; Schneegans, M.; Schwenke, J.; Tavernier, S.; Topa, V.; Vasil’ev, A. N.; Vivargent, M.; Walder, J. P. Nucl. Instrum. Methods A 1996, 383, 367. (13) Shimamura, K.; Vı´llora, E. G.; Nakakita, S.; Nikl, M.; Ichinose, N. J. Cryst. Growth 2004, 264, 208. (14) Wang, Z. L.; Quan, Z. W.; Jia, P. Y.; Lin, C. K.; Luo, Y.; Chen, Y.; Fang, J.; Zhou, W.; O’Connor, C. J.; Lin, J. Chem. Mater. 2006, 18, 2030.
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