Synthesis, Characterization, and Luminescence Properties of Uniform

Apr 18, 2007 - High-quality, large-scale, and uniform nanospindles with a mean length of ... have shown unique luminescence properties with correspond...
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J. Phys. Chem. C 2007, 111, 6652-6657

Synthesis, Characterization, and Luminescence Properties of Uniform Ln3+-Doped YF3 Nanospindles Maofeng Zhang, Hai Fan, Baojuan Xi, Xuyang Wang, Chao Dong, and Yitai Qian* Hefei National Laboratory for Physical Sciences at Microscale and Department of Chemistry, UniVersity of Science and Technology of China, Hefei 230026, People’s Republic of China ReceiVed: December 25, 2006; In Final Form: March 7, 2007

A new example of a spindlelike Ln3+-doped YF3 luminescent nanomaterial has been prepared through a facile method by low-temperature hydrothermal treatment of YCl3, NaF, and EDTA at 140 °C for 12 h. High-quality, large-scale, and uniform nanospindles with a mean length of 560 nm and a mean width of 240 nm can be easily obtained. The size of the products can be controlled by varying reaction conditions. The effects of the molar ratio of EDTA to Y3+, reaction temperatures, and reaction time on the nanospindle growth have been investigated in detail. The possible growth mechanism of nanospindles has also been discussed. The results of photoluminescence spectroscopic measurements reveal that the as-prepared Ln3+-doped YF3 nanospindles show strong red and green emission. It is found that the morphology and size of the products have great influence on their emission intensity. Since the products exhibit excellent luminescence, they can be expected to become good candidates for research in optical and optoelectronic devices.

Introduction The greatest advances in the design and fabrication of uniform functional nanostructured materials for a wide range of new applications have generated a tremendous amount of interest recently. It is well-known that the size and shape of inorganic submicrocrystals and nanocrystals have great influence on their physical properties.1-3 Thus, further explorations of novel nanomorphology, with controlled size and shape by convenient synthesis methods, is of great importance for the development of new functional devices. Among many synthesis techniques, the most popular techniques for novel nanostructure with controlled size and shape involve iron liquid, arc discharge, laser ablation, and biotemplate, as well as template-directed methods.4-8 Nevertheless, the above-mentioned methods need some special instruments and harsh conditions, and usually lead to impurities due to the incomplete removal of the templates. Hydrothermal synthesis, which may provide a more promising technique than conventional methods in terms of cost and potential for largescale production, is viewed as one of the useful ways to prepare novel nanomorphology. Studies on fluorides doped with rare-earth ions nanocrystals have shown unique luminescence properties with correspondingly useful applications in optical telecommunication, lasers, new optoelectronic devices, diagnostics, and biological labels.9-16 Such applications rely on the luminescence of lanthanide ions with sharp lines and high efficiency. In comparison with oxygenbased systems, fluorides possess very low vibrational energies and therefore the quenching of the excited states of the rare earth ions will be minimal.17 Based on this virtue, YF3 was chosen as the host matrix to perform luminescence. For example, Ritcey18 synthesized YF3 with hexagonal and quadrilateral shape nanoparticles and Li and co-workers prepared Ln3+-doped YF3 nanocrystals,19 fullerene-like, and ricelike YF3 nanoparticles20,21 and investigated their fluorescence and up-conversion luminescence properties. Recently, Tao et al.22 prepared uniform and * Corresponding author. E-mail: [email protected].

monodisperse YF3:Eu nanosized and submicrosized truncated octahedra and observed Eu2+ emission when excited at 393 nm. It is reported that the properties of inorganic nanomaterials depend strongly on the material shape, size, and crystallinity. However, the effects of Ln3+-doped YF3 nanomaterial’s shape, size, and crystallinity on the luminescent properties have received little attention. Herein, we report on the controlled synthesis of Ln3+-doped YF3 nanospindles. The as-prepared nanospindles consisted of many nanoparticles with a mean size of 30 nm, and these nanoparticles generally organized into ordered chains that were aligned approximately parallel to the spindle long axis. Li et al.23 reported that the oriented aggregation of nanophosphors could significantly enhance the luminescent emissions at the nanoscale. Similarly, it can be easily deduced that the ordered chains’ structure leads to luminescence improvement in Ln3+-doped YF3 nanospindles compared with other shapes of nanomaterials. It is reasonable to expect that this distinct nanostructure of fluoride might be a useful strategy in technological applications, including high-density optical storage devices, nanosensors, and color displays.24 In the present study, we demonstrate that Ln3+-doped YF3 nanospindles with controlled size, shape, and excellent luminescence can be successfully prepared by a facial hydrothermal process. The structure and morphologies of the products have been characterized by means of X-ray diffraction, selected area electron diffraction (SAED), high-resolution transmission electron microscopy, field-emission scanning electron microscopy (FESEM), and transmission electron microscopy. In addition, the optical properties of as-prepared products are characterized by photoluminescence emission spectroscopy. Experimental Section Synthesis. All of the chemical reagents used in this experiment were of analytical grade and used without further purification. Yttrium chloride (YCl3‚6H2O), europium chloride (EuCl3‚ 6H2O), terbium chloride (TbCl3‚6H2O), ethylenediaminetetraacetic acid (EDTA), and sodium fluoride (NaF) were purchased from

10.1021/jp068919d CCC: $37.00 © 2007 American Chemical Society Published on Web 04/18/2007

Uniform Ln3+-Doped YF3 Nanospindles

Figure 1. A typical XRD pattern of the product.

the Shanghai Chemical Reagent Company. In a typical experiment, 1 mmol of the hydrated LnCl3 (Ln ) Y3+, Eu3+, and Tb3+) and 0.3 mmol (0.088 g) of EDTA were dispensed into 40 mL of deionized water and magnetically stirred for 10 min. Next, sodium fluoride (2 mL; 3 mmol) was added dropwise and the solution was constantly stirred for 10 min. Then the mixture was transferred into a 60 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained in an electric oven at 140 °C for 12 h, and then cooled to room temperature naturally. The product was carefully collected and washed with distilled water and absolute ethanol several times, and finally dried in a vacuum at 60 °C for 4 h. Characterization. The phase and composition of the products were determined by a Rigaku D/Max-γA rotating-anode X-ray diffractometer equipped with monochromatic high-intensity Cu KR radiation (λ ) 1.541 78 Å). The morphologies of the samples were observed with a transmission electron microscope (TEM; Hitachi H-800) using an accelerating voltage of 200 kV with a tungsten filament. FESEM images were taken on a JEOL JSM-6300F SEM. The microstructures of the samples were analyzed with a high-resolution transmission electron microscope (HRTEM; JEOL-2010). The photoluminescence spectra of the samples were measured by a steady-state/lifetime spectrofluorometer (JOBIN YVON, FLUOROLOG-3-TAU) with a 450 W monochromatized xenon lamp. Results and Discussion Morphology and Microstructure of the Products. The crystal structures and the phase purity of the materials were determined by X-ray diffraction (XRD). A typical XRD pattern of the as-prepared sample is presented in Figure 1. All the reflection peaks of the different products can be easily indexed to a pure orthorhombic structure [space group pnma (62)] of YF3 with calculated lattice constants of a ) 6.352 Å, b ) 6.850 Å, and c ) 4.398 Å, which are in good agreement with the literature values of a ) 6.353 Å, b ) 6.85 Å, and c ) 4.393 Å (JCPDS Card No. 74-0911). No other impurity peaks were detected. It is worth noting that the crystal structure of YF3 has not been changed after doping with Ln3+ ions. The size and morphology of as-prepared samples were examined using FESEM and TEM. Typical SEM and TEM images of the Ln3+-doped YF3 products obtained at 140 °C for 12 h, as shown in parts a, b, and c, respectively, of Figure 2clearly reveal a highly monodisperse distribution spindlelike nanostructure with a mean length of 560 nm and a mean width of 240 nm (aspect ratio of ∼2.3:1). In addition, the SEM image shows that the nanospindles consist of many nanoparticles with

J. Phys. Chem. C, Vol. 111, No. 18, 2007 6653 a mean size of 30 nm. Moreover, the nanoparticles are often slightly oblate in shape and generally organized into ordered chains that are aligned approximately parallel to the spindle long axis. The SAED pattern reveals the polycrystalline nature of the sample with a preferential growth direction along the long axis direction. The structure information of the nanospindles was further afforded by HRTEM. The HRTEM image (Figure 2d) shows that the nanospindles are structurally uniform with interplanar spacings of about 0.341 and 0.289 nm, which corresponds to the (020) and (210) lattice spacings of YF3, respectively. It is noteworthy that SEM and TEM images were obtained of randomly selected areas of the sample and, as such, are representative of the overall shapes and sizes of our asprepared fluoride nanospindles. In the present preparation process, it was found that the morphology and size of the products are greatly affected by the molar ratio of EDTA to Y3+. Also, it is necessary to first mix Ln3+ with EDTA at room temperature for some time to stabilize Ln3+ ions. As we all know, EDTA, as an efficient chelator for rare-earth ions, can react with Ln3+ to form stable Ln-EDTA (1:1) complexes. To investigate the effect of EDTA on the formation of size-controlled and uniform YF3 nanospindles in our synthetic method, the amount of EDTA was varied with the other conditions unchanged to get molar ratios of EDTA/Y3+ of 0, 0.1, 0.2, 0.3, 0.5, 1, and 5. Typical TEM micrographs of the products are shown in Figure S1 and Figure 3, from which it becomes obvious that the morphology of YF3 is greatly influenced by the EDTA/Ln3+ ratio. Figure S1a shows a TEM image of the products prepared in the absence of EDTA, which is of a rodlike shape with particle sizes of 1.7 µm and 700 nm in dimension. However, only nanoparticles were produced when the molar ratio was 0.1 (Figure S1b). Increasing the molar ratio to 0.2, sphere and oblatelike particles with loose structures and a big particle size of ∼400 nm (Figure 3a) were obtained. Figure 3b shows that a high-quality, uniform spindlelike nanostructure was obtained at a molar ratio of 0.3. In particular, when the molar ratio was greater than 0.3 (for example, 0.5, 1, 5), the morphology and size of nanospindles remained almost unchanged except that the products contained an impurity of residual EDTA. It is thought that nucleation and aggregation stages are involved in the formation of nanospindles. The addition of an appropriate quantity of EDTA may have helped the separation between the two stages, since the two processes lie in different concentration zones. In a concentration where the molar ratio of EDTA/Ln3+ is equal to 0.1, the rareearth ions are presumably more favorable to particle nucleation than aggregation, resulting in formation of nanoparticles. By contrary, when the EDTA/Ln3+ ratio is greater than 0.1 (0.2, 0.3, etc.), particle aggregation plays a key role and results in spindlelike nanostructure. The result suggests that the appropriate molar ratio of EDTA/Ln3+ is crucial to the formation of pure and uniform spindlelike nanostructure. In addition, reaction temperature also plays an important role in the formation of nanospindles. The optimum reaction temperature is about 140 °C. When the temperature is too low (25 °C), only amorphous nanoparticles were obtained (Figure S2). At 50 °C, high-aspect-ratio elongated fibrous bundles with a mean length of 700 nm and width of several nanometers were produced (Figure 4a). At 80 °C, a coaligned spindlelike bundle structure was formed (Figure 4b). These individual spindles were approximately 700 nm by 260 nm in dimension, and the width was uniform although the length was somewhat variable. Increasing the reaction temperature to 120 °C, fibrous bundles began to aggregate, forming close-knit nanospindles (Figure 4c).

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Figure 2. (a,b) Typical FESEM and TEM imagines of the product; (c) TEM image of a single Ln3+-doped YF3 nanospindle; (d) HRTEM image of Ln3+-doped YF3 nanospindles with spacing of lattice fringes of 0.341 and 0.289 nm. The inset shows the corresponding SAED pattern.

Figure 3. Typical TEM images of the products prepared at 140 °C for 12 h and at molar ratios of EDTA/Y3+ of (a) 0.2 and (b) 0.3.

It is interesting that, when treated at 140 °C, good crystalline spindle-shaped aggregates were formed which consisted of nanoparticles (Figure 2a). Further increases in reaction temperature and time resulted in a larger spindle which was about 720 nm by 350 nm in dimension with an aspect ratio of ∼2.0:1 (Figure 4d); nevertheless the shape of the products was kept the same as those at low temperatures. The above results indicate that reaction temperature and time synergetically influence the size of the products, which can be controlled by varying reaction temperature and time. To reveal the intermediate steps in the formation of Ln3+doped YF3 nanospindles, a series of detailed time-dependent experiments was carried out for different numbers of hours. At the beginning, amorphous white precipitate immediately appeared after NaF was added into the mixed solution containing LnCl3 and EDTA (Figure S3a). When the reaction time was 4 h, the amorphous nanoparticles turned into crystalline oblate nanoparticles with a mean particle size of 30 nm (Figure S3b). It can be deduced that the final products with spindlelike nanostructure may be composed of these nanoparticles. As reaction time was prolonged for 7 h, some adjacent nanoparticles were joined together, driven by the collective behavior of nanoparticles and the effect of EDTA between them, and formed weak crystalline spindlelike connections (Figure 5a). The inset image (inset in Figure 5a) clearly shows the loose and somewhat

hollow structure of the products. At 10 h, it grew and formed relatively dense nanospindles (Figure 5b). However, there are still some incomplete nanospindles contained in the samples. To obtain high-quality and monodisperse products, the experiment was conducted for 12 h. The corresponding SEM shows that a large quantity of uniform Ln3+-doped YF3 nanospindles was produced (Figure 2a). Based on the above experimental results, a possible formation mechanism of the products is proposed. The whole process can be schematically illustrated in Figure 6. First, amorphous nanoparticles are formed in the aqueous solution under intensive stirring. Second, amorphous nanoparticles turned into crystalline monodisperse oblate nanoparticles under the effect of EDTA. Third, oblate nanoparticles began to aggregate in a certain way, most probably by electrostatic gravitation and intermolecular force, and finally spindlelike nanostructure were formed at prolonged reaction time. The presence of oblate nanoparticles prepared at 4 h indicates that the nanospindles formed by the aggregation of multiple nanoparticles in a linear aggregate that organized gradually into one spindle. The SAED together with the similarity of the shape and size of nanoparticles and the nanoparticles located at the surface of nanospindles as well as high monodispersity of the latter substantiates this mechanism. Optical Properties. As previously mentioned, fluoride crystals doped with rare-earth metal ions display unique

Uniform Ln3+-Doped YF3 Nanospindles

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Figure 4. TEM and SEM images of the products prepared at a molar ratio of EDTA/Y3+ of 0.3 and (a) 50 °C for 12 h, (b) 80 °C for 12 h, (c) 120 °C for 12 h, and (d) 180 °C for 24 h.

Figure 5. TEM images of the products prepared at different numbers of hours: (a) 7 h (inset shows a higher magnification image); (b) 10 h.

Figure 6. Schematic diagram showing the growth process of Ln3+doped YF3 nanospindles.

luminescence properties.25-27 As a demonstration of this principle, the as-prepared YF3 nanospindles doped with either Eu3+ or Tb3+ ions displayed either red or green luminescence, respectively. It is known that Eu3+ ions are a good probe for the chemical environment of the lanthanide ion; the relative intensities of 5D0 f 7F1 and 5D0 f 7F2 emission, which are typical magnetic and electronic dipole-dipole transitions, respectively, depend strongly on the local symmetry of the Eu3+ ions. In a site with inversion symmetry the 5D0 f 7F1 magnetic dipole transition is dominating, while in a site without inversion symmetry the 5D0 f 7F2 electric dipole transition is the strongest. Figure 7A shows the room-temperature emission spectra of YF3 nanospindles with a doping concentration of 5 mol % Eu3+ ions. As shown in the figure, the emission spectrum

consists of lines mainly located in the red spectral area from 570 to 630 nm. The dominating emission centered at 592 nm corresponds to the 5D0 f 7F1 magnetic dipole transition, which indicates that Eu3+ locates in a site with inversion symmetry in YF3 matrix. The peak at 614 nm can be ascribed to a 5D0 f 7F electric-dipole transition, which is similarly sensitive to small 2 changes in the chemical environment as well as to symmetry considerations surrounding the Eu3+ ions.28 The same procedure was used to dope YF3 nanpspindles with Tb3+. Figure 7B shows the emission spectra of Tb-doped YF3 at room temperature. However, the fluorescence intensity of YF3:Tb3+ is relatively weak compared with YF3:Eu3+. Successful doping with Tb3+ was evidenced from the splitting and the intensity pattern of the luminescence lines. The emission spectrum exhibits four well-resolved peaks, centered at 489, 544, 583, and 620 nm, which have been observed for Tb-doped fluoride, corresponding to Tb electronic transitions of 5D4 f 7F (where J ) 6, 5, 4, 3). Among them, the bright-green J emission at 544 nm which corresponds to 5D4 f 7F5 is the strongest. It is known that the properties of inorganic nanomaterials can be greatly changed by tailoring their morphology, size, and crystallinity. Some recent efforts have been devoted to the

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Figure 7. Emission spectra of as-prepared YF3 doped with (5 mol %): (A) Eu3+; (B) Tb3+.

Figure 8. (I) Emission spectra and (II) emission intensity centered at 592 nm of YF3:Eu3+ prepared at different temperatures for 12 h: (a) 50, (b) 80, (c) 120, (d) 140, (e) 160, and (f) 200 °C.

Figure 9. (I) Emission spectra and (II) emission intensity centered at 592 nm of YF3:Eu3+ prepared at 140 °C for different numbers of hours: (a) 4, (b) 7, (c) 10, (d) 12, (e) 16, and (f) 24 h.

synthesis of inorganic submicrocrystals and nanocrystals with well-defined nonspherical morphologies. To investigate the influence of morphology, size, and crystallinity on the optical property of the products, a series of detailed experiments were carried out at different reaction conditions, since the morphology, size, and crystallinity of the products are greatly affected by reaction temperature and time. Take YF3:Eu3+ for example, as shown in Figures 8 and 9. The fluorescence intensity of the LaF3:Eu3+ nanospindles is remarkably influenced by reaction temperature and time, but the peak positions of the emission spectra remain the same. Figure 8 shows the emission spectra and emission intensity of YF3:Eu3+ prepared at different temperatures for 12 h. It can be seen that emission intensity increases from 50 to 140 °C, and then decreases with increasing reaction temperatures to 200 °C. It is interesting to find that the emission intensity is changed with the morphology and size of the products. As mentioned

above, the morphology turned out to be fibrous bundles at 50 °C, coaligned spindlelike bundle structure at 80 °C, close-knit nanospindles at 120 °C, and spindle-shaped aggregates at 140 °C. Most important is that the nanospindles are composed of nanoparticles which are generally organized into ordered chain aggregates. It is reported that the oriented aggregation of nanophosphors could significantly enhance the luminescent emissions at the nanoscale. Above 140 °C, it results in larger nanospindles, and the size of the nanoparticles composing the nanospindles is larger. As we know, luminescence efficiency is increased with decreasing crystallite size, so the emission intensity of the products begins to decrease when reaction temperatures are higher than 140 °C. The size dependence of the fluorescence intensity can also be demonstrated from the quantum-confinement model as described by Bawendi et al.29,30 or in terms of the number of particles per unit area facing toward the incident light as described by Pramod et al.31

Uniform Ln3+-Doped YF3 Nanospindles Figure 9 shows the emission spectra and emission intensity of YF3:Eu3+ prepared at 140 °C for different numbers of hours. As shown in Figure 9, emission intensity was lowest when it reacted for 4 h, and the products were composed of oblate nanoparticles at this stage. After 7 h, it formed spindlelike connections and the nanoparticles began to form linear aggregates. The emission intensity of the products is much higher than that of nanoparticles prepared for 4 h. This coincides with the reported results that the total quantum yield of synthesized nanoparticles is improved up to ∼80% when five to seven of these nanophosphors are linearly aggregated to form an oriented structure. It has a slight increase when the reaction time is 10 h, because the products’ crystallinity is improved, which can be seen from XRD patterns of the products. Above 12 h, the products form large nanoparticles which result in the reducion of emission intensity, although the products have better crystallinity. It is thought that the effect of particle size on emission intensity plays an important role compared with crystallinity. Conclusion In summary, high-quality Ln3+-doped YF3 nanospindles with a mean length of 560 nm and a mean width of 240 nm have been successfully prepared via a facile hydrothermal method under mild conditions. Of interest to note is that fluoride nanospindles can be produced very readily using this method, without the use of a surfactant or template. Moreover, the morphology and size of the products could be controlled by varying reaction conditions. It is found that the morphology, size, and crystallinity of the products have great influence on their emission intensity. We believe that this approach might be applied to the synthesis of a new generation of important fluorides, even complex fluoride nanomaterials. The excellent luminescence of Ln3+-doped YF3 nanospindles provides the basis for a more thorough investigation of their optical and optoelectronic properties and might make applications in nanoscale devices possible. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 20431020) and the 973 Project of China (No. 2005CB623601). Supporting Information Available: Typical TEM images of the products prepared at different molar ratios of EDTA to Y3+; TEM image of the products prepared at 25 °C for 12 h; TEM images of the products prepared at different reaction times.

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