Facile Sonochemical Synthesis of Single-Crystalline Europium

A schematic illustration of the formation mechanism of the flower-shaped EuF3 crystals .... The related data for Eu3+ in EuF3 flower and disk are list...
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CRYSTAL GROWTH & DESIGN

Facile Sonochemical Synthesis of Single-Crystalline Europium Fluorine with Novel Nanostructure Ling Zhu,†,‡ Xiaoming Liu,†,‡ Jian Meng,† and Xueqiang Cao*,† CAS Key Laboratory of Rare Earths on AdVanced Materials and Valuable Utilization of Resources, 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

2007 VOL. 7, NO. 12 2505–2511

ReceiVed March 8, 2007; ReVised Manuscript ReceiVed July 30, 2007

ABSTRACT: A single-crystalline EuF3 nanoflower with a novel three-dimensional (3D) nanostructure has been successfully synthesized via a facile, fast, efficient, and mild ultrasonic irradiation solution route employing the reaction of Eu(NO3)3 and KBF4 under ambient conditions without any template or surfactant. The ultrasonic irradiation plays an important role and is necessary for the synthesis of EuF3 with the complex structure. The formation mechanism of this complex nanostructure is proposed in this paper. No template or surfactant is used in this method, which avoids the subsequent complicated workup for the removal of the template or surfactant. Furthermore, a substantial reduction in the reaction time as well as the reaction temperature is observed compared with the hydrothermal process.

1. Introduction The synthesis of inorganic nanocrystals is currently an active field of research in modern materials chemistry due to their widely application in catalysis, optoelectronics, microelectronics, magnetics, and biology.1–5 Furthermore, the special attention in nanoscience and nanotechnology has been pointed toward the fabrication of materials with uniform complex structures.6–10 Up to now, a number of synthetic approaches have been developed to prepare various types of functional materials such as metals, metal oxides, and semiconductors with complex structures using different surfactants, polymers, or templates for control of the sizes, dimensions, and morphologies of these materials. For instance, Sun et al. have reported the hydrothermal synthesis of branched R-MnO2 multipods in the presence of polyvinylpyrrolidone (PVP).11 By employing a double-hydrophilic block copolymer, poly(ethylene glycol)-block-poly(methacrylic acid) (PEG-b-PMAA), Qi et al. have synthesized unique penniform architectures based on BaWO4 nanowires.12 Cölfen et al. have reported the synthesis of flower-like and conelike BaSO4 particles with the help of the seed of two double hydrophilic block copolymers (DHBCs).13 Lin et al. have successfully synthesized nanorod bundles and caddice-spherelike particles of In(OH)3 by the microemulsion-mediated hydrothermal process of cetyltrimethylammonium bromide (CTAB)/ water/cyclohexane/n-pentanol.14 However, the introduction of surfactants, templates, or polymers into the reaction system involves a complicated process, introduces heterogeneous impurities in the products, and increases the production cost, which may restrict the wide development of research and applications. Therefore, developing a facile, mild, and templatefree method is still of great significance. Recently, rare earth fluorides have been attracting much attention due to the potential application in the fields of luminescent materials, magnets, catalysts, and other functional materials because they are expected to have unique properties such as luminescence and magnetic properties.15–20 Up to now, different morphologies of rare earth fluoride crystals based on * Corresponding author. E-mail: [email protected]. Tel/Fax: +86-43185262285. † Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.

different procedures and synthesis techniques have been prepared, such as fullerene-like lanthanide fluorides,21,22 bundlelike and rod-like YF3,23,24 hexagonal and triangular YF3 nanocrystals,25 self-assembly of LaF3 nanoplates,26 and monodispersed LaF3 triangular nanoplates.27 By using different fluorine sources, Chen et al. have reported the synthesis of EuF3 nanocrystals with different morphologies, such as nanodisks, nanorods, lunate shapes, nanospheres, branched, tree-like nanobundles, nanorods, and nanowires.28,29 However, to the best of our knowledge, the synthesis of EuF3 with flower-like complex nanoarchitectures has not yet been reported. In this paper, we report an ultrasonic approach to prepare well-defined 3D singlecrystal EuF3 with flower-like nanostructure at room temperature without the addition of any surfactant or template. The sonochemical method, which is simple and operated under ambient conditions, has been proven to be a useful route for the preparation of novel materials with unusual structures and properties. The chemical effect of ultrasonic irradiation arises from the acoustic cavitation, in other words, the formation, growth, and implosive collapse of bubbles in the liquid medium. The implosive collapse of the bubbles generates local hot spots through the adiabatic compression or shock wave formation within the gas phase of the collapsing bubble. These local hot spots have been shown to be about 5000 K with a pressure of 1800 atm and cooling rate higher than 108 K · s-1.30–32 These extremely transient high pressures and temperatures provide a unique environment for the growth of materials with novel structures. The advantages of this method include a rapid reaction rate, controllable reaction conditions, and the ability to form materials with uniform shapes, narrow size distributions, and high purities.

2. Experimental Procedures All the reagents used were of analytical grade, including Eu2O3 (Shanghai Chemical Reagent) and KBF4 (Beijin Chemical Reagent), and used as received without further purification. In a typical synthesis, an appropriate amount of Eu2O3 was first dissolved in 10% nitric acid and then mixed with a KBF4 solution in a 80 mL plastic flask to give a final concentration of 30 mM Eu(NO3)3 and 90 mM KBF4. The total volume of the solution was 50 mL. The resulting solution was sonicated at ambient temperature for 3 h by a high-intensity ultrasonic probe (JCS-206 Jining Co. China, Ti-horn, 23 kHz). A white precipitate was

10.1021/cg070224u CCC: $37.00  2007 American Chemical Society Published on Web 11/15/2007

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Figure 1. XRD pattern of the as-prepared nanocrystals of EuF3 flowers (a) and disks (b). centrifuged and washed with distilled water and absolute ethanol in sequence. The final product was dried under vacuum at 60 °C for 12 h. The product was characterized via X-ray powder diffraction (XRD), scanning electron micrographs (SEM), transmission electron microscopy (TEM), high-resolution transmission electron micrographs (HRTEM), selected area electron diffraction (SAED), and photoluminescence spectra (PL) and lifetime. The XRD pattern was performed on a Rigaku D/MAX-2500 diffractometer with Cu KR radiation (λ ) 0.15406 nm) and a scanning rate of 5 deg · min-1. SEM images were taken on a XL30 field-emission scanning electron microscope equipped with an energy-dispersive spectrometer. TEM, HRTEM, and SAED patterns were recorded on a JEOL-JEM-2010 operating at 200 kV (JEOL, Japan). The sample for TEM was prepared by dropping a diluted suspension of the sample powders onto a standard carbon-coated (20–30 nm) Formvar film on a copper grid (230 mesh). 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 the PL test are as follows: scanning speed ) 240 nm · min-1, delay ) 0 s, excitation (EX) slit ) 2.5 nm, emission (EM) slit ) 2.5 nm, and PMT voltage ) 700 V. The luminescence lifetime of Eu3+ was measured with a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) using a 399 nm laser wavelength (pulse width ) 4 ns) from YAG:Nd as the excitation source. All the measurements were performed at room temperature.

3. Results and Discussion 3.1. Structure and Morphology of the Product. XRD analysis was used to determine the structure and phase of the sample. As shown in Figure 1a, all the XRD peaks can be indexed to hexagonal EuF3 with lattice constants of a ) 6.923 Å, and c ) 7.086 Å, which are in good agreement with values of the standard card (JCPDS No. 32-0373, a ) 6.920 Å and c ) 7.086 Å). The strong and sharp diffraction peaks indicate that the product is well crystallized. The morphology and microstructure details of the asprepared EuF3 nanocrystals were investigated with SEM, TEM, HRTEM, and SAED. Figure 2 shows the typical SEM images with different magnifications. The SEM image clearly demonstrates that the majority of the crystals have a uniform flower-like shape (Figure 2a). A small portion (about 10%) of 2D nanodisks can also be observed. These nanodisks are the intermediate products between the initial nanoparticles and the final nanoflowers. The flowers are spherical with an average diameter between 0.9 and 1.0 µm, and the average thickness of the petals is about 0.14 µm. The highmagnification SEM image (Figure 2c) reveals that the surface of the flower is very smooth. Energy-dispersive X-ray

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spectroscopy analysis (EDXA) confirms that the nanoflowers are composed of Eu and F in a ratio of about 1:3 (Figure 2d). The EDXA result gives further support to the XRD analysis as discussed above. The TEM observations further reveal the flower-like shape of the as-prepared EuF3 product (Figure 3a). The SAED image (Figure 3c) shows regular diffraction spots and confirms that the EuF3 nanoflower is single-crystalline. The HRTEM image recorded on the edge of a flower (Figure 3b) shows the well-defined lattice fringes with interplanar spacing of 0.341 nm for the (110) faces of the hexagonal structure, indicating the high crystallinity of the product. 3.2. Effect of the Ultrasonic Irradiation and Possible Growth Mechanism. It has been well established that ultrasonic irradiation introduces unusual physical and chemical effects deriving from the acoustic cavitation. Such cavitation behavior leads to many unique properties in the irradiated solution and has been used extensively to generate novel materials with unusual properties.32–34 In this work, ultrasonic irradiation plays an important role and is found to be necessary for the synthesis of EuF3 with the complex 3D structure. A comparative experiment under vigorous electric stirring instead of ultrasonic treatment proves that only nanodisks could be obtained (Figure 4), as also observed by Chen et al.28 It has been well documented that ultrasonic irradiation offers a kinetic process different from other methods (such as hydrothermal or microemulsion) and can promise a very short reaction time and relatively stable reaction conditions.30–32 In the current case, the ultrasonic waves may accelerate the hydrolysis of KBF4, which is presumably helpful to the nucleation and growth of EuF3 nanocrystals. The experimental phenomena support the above hypothesis. With the assistance of ultrasonic irradiation, the reaction solution became turbid after 30 min, while it turned cloudy after 1.5 h and only a little bit product was obtained without ultrasonic irradiation. Furthermore, the cavitation and shock wave created by the ultrasound can accelerate solid particles to high velocities, leading to interparticle collisions and effective fusion at the point of collision.35 It is suggested that the ultrasound causes fusion of the adjacent particles and attachment of primary particles on the outer layers of the disks, which are the intermediate products between the initial nanoparticles and final nanoflowers, which will be well confirmed in the later discussion. Ultrasound appears to be particularly effective as a means of inducing nucleation and may affect the crystallization through the mechanism of cavitation and acoustic streaming, which is responsible for the formation of final complex 3D EuF3 single crystals. The advantage of the application of ultrasonic irradiation to the synthesis of nanomaterials is the significant reduction in fabrication time.36 In our case, the role of sonication is not only to accelerate the reaction between the raw materials but also to lead to the growth and crystallization of EuF3 with complex 3D nanostructure. To further investigate the details of the formation of EuF3 nanoflowers, the growth processes of the final product were carefully followed by time-dependent experiments. SEM images obtained after different reaction times show an obvious growth process from the small primary nanoparticles to the final flowers (Figure 5). With ultrasonic treatment for 30 min, the transparent reaction solution became turbid, showing the formation of small primary particles (Figure 5a), and after another 30 min these nanoparticles further grow in some specific orientations to form disk-like structures (Figure 5b). After the formation of these

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Figure 2. SEM images with different magnifications (a-c) and the EDS (d) of the EuF3 nanocrystals.

Figure 3. (a) TEM, (b) HRTEM, and (c) SAED images of EuF3 nanocrystals.

On the basis of the experimental results, a possible formation mechanism is proposed as shown in Figure 6. In the reaction aqueous solution, the hydrolysis of KBF4 occurs as follows: )))))))))))

Figure 4. SEM image of EuF3 obtained by stirring.

nanodisks, the intermediate product appears to be flowerlike (Figure 5c). After 3 h, the growth of the flower-like product is completed (Figure 5d).

BF4 + 3H2O 98 H3BO3 + 3HF + F

(1)

Eu3+ + 3F f EuF3 V

(2)

The equilibrium constant of this hydrolysis reaction is (eq 1) 6.41 × 10-12 at 25 °C,37 indicating that the hydrolysis process is very slow, which is helpful to keeping the low concentration of F- in the reaction solution, and consequently leads to the slow crystallization process. Moreover, the use of the raw material KBF4 is excessive (200% excess). It was observed that the pH value of the reaction solution decreased slowly and continuously from 7 to 2 when the synthesis was finished, implying that its hydrolysis proceeded in the whole ultrasonication process. The employing of the ultrasonic irradiation will accelerate the hydrolysis process, leading to the formation of the small primary nanoparticles. With the ongoing of the reaction, the primary nanoparticles further grow to the nanodisks, and the freshly formed nanoparticles will spontaneously “land”

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Figure 5. Time-dependent SEM images of flower-like crystal after ultrasound treatment of (a) 0.5, (b) 1, (c) 2, and (d) 3 h.

Figure 6. A schematic illustration of the formation mechanism of the flower-shaped EuF3 crystals.

on the as-formed disks and then undergo further growth to another disk, forming a complex structure. These processes could be related to a proposed mechanism of the so-called “orientated attachment” by Penn and Banfield and co-workers.38,39 In this mechanism, the larger particles are grown from small primary nanoparticles through an orientated attachment process, in which the adjacent nanoparticles are self-assembled by sharing a common crystallographic orientation and docking of these particles at a planar interface. Small particles may aggregate in an oriented fashion to produce a larger single crystal, or they may aggregate randomly and reorient, recrystallize, or undergo phase transformations to produce larger single crystals. This type of growth mode could lead to the formation of faceted particles or anisotropic growth if there is sufficient difference in the surface energies of different crystallographic faces. In our case, according to the experimental results, the latter seems to be more reasonable. Therefore, it is believed that the sonochemical formation of flower-like EuF3 undergoes three steps in sequence: (1) ultrasound-induced hydrolysis of KBF4, which leads to the formation of primary nanoparticles, (2) ultrasound-induced fusion of these primary nanoparticles accompanying the oriented growth to form the disk-like structure, and (3) a further growth and crystallization process, giving rise to the formation of the 3D flower-like product. Although the exact formation mechanism for this complex nanostructure is not yet exactly clear, it is believed that the growth of the flower-like nanostructures is not catalyst-assisted

or template-directed, because the only material sources used in our synthesis are pure rare earth oxide crystals and KBF4. 3.3. Photoluminescent Properties. The excitation and emission spectra for the EuF3 nanoflowers (solid line) and nanodisks (dotted line) are shown in Figure 7a,b, respectively. The excitation spectra of these two samples (Figure 7a) consist of the characteristic excitation lines of Eu3+ within its 4f6 configuration from 200 to 500 nm. The position of these sharp lines is practically similar to the characteristic absorption bands for f-f transition in trivalent europium,40 and the spectra reported for the Eu3+-doped rare earth fluoride.41,42 In general, most of the excitation lines can be clearly assigned (320 nm, 7 F0 f 5H6; 364 nm, 7F0 f 5D4; 378 nm, 7F0 f 5G2; 399 nm, 7 F0 f 5L6, strongest; 418 nm, 7F0 f 5D3; 467 nm, 7F0 f 5D2) except for those weak ones at 253, 270, 288, and 300 nm (which have little contribution to the excitation of Eu3+ and are of minor significance).43 It is well know that in the Eu-doped systems, charge transfer occurs by electron delocalization from the filled 2p shell of the ligand to the partially filled 4f shell of Eu3+. The transition energy depends strongly on the electronegativity of the ligand. For Eu3+ in oxide hosts, a charge-transfer band (CTB) of Eu3+-O2- is frequently observed between 200 and 300 nm in the excitation spectra. Owing to the high electronegativity of the pure fluoride systems, much higher energy is needed to remove an electron from F- than from O2-; consequently the CTB of Eu3+-F- is generally located below 200 nm (in the vacuum ultraviolet (VUV) region).43,44

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Figure 7. Room-temperature PL (a) excitation and (b) emission spectra for the as-prepared 3D EuF3 flowers via sonochemical method (solid line) and the 2D EuF3 disks prepared under vigorous electric stirring (dotted line) (Ex and Em slits, 2.5 nm).

Figure 8. Decay curves of Eu3+ luminescence (590 nm) of 3D EuF3 flowers (a) and the 2D EuF3 disks (b) (λex) 399 nm).

Excitation into the strongest 7F0 f 5L6 transition of Eu3+ at 399 nm yields the emission spectra of these two samples, as shown in Figure 7b. The spectra show that the samples have characteristic emission peaks of Eu3+ within the wavelength range from 550 to 720 nm, corresponding to the transitions from the excited levels of 5D0 to 7FJ (J ) 0–4).28,45 It is worth noting that the intensity of the magnetic dipole allowed 5D0f 7F1 transition (590 nm) greatly exceeds the intensity of the electron dipole allowed 5D0 f 7F2 transition (612, 617 nm), which demonstrates the formation of the centrosymmetrical structure of europium fluoride.45,46 Although the PL positions of these two samples are identical, the PL intensity is much different with that of the 3D flower structure much higher than that of 2D disk structures under the same measurement conditions. Normally, such differences in the PL spectra can be caused by factors like the extent of crystallinity, morphology, size distribution, homogeneity, and dimension of the luminescent material. XRD studies revealed that the EuF3 nanoflowers (Figure 1a) and nanodisks (Figure 1b) prepared with different methods are highly crystalline with the same crystal structure and similar crystal intensity. Therefore, the enhancement of PL intensity may be due to the different morphology and dimension of the luminescent material, which has similarly been observed by other researchers.47–50 The flower-like EuF3 crystal is expected to exhibit high light-collection efficiency and enhanced luminescence performance due to its complex 3D structure. The greatly enhanced luminescence performance observed in the

flower structure is exciting and may have significant technological applications in the inorganic scintillating field. The luminescent dynamics of Eu3+ at room temperature for the EuF3 nanoflowers and nanodisks were also measured and compared. Generally speaking, if there is one kind of luminescent center in the phosphor, the decay function can be fit by the first order. Two kinds of luminescent centers means second order and so on. Furthermore, the decay behavior can also be influenced by the energy transfer and impurities present in the host lattices. In our current case, because the as-prepared samples belong to centrosymmetrical structure, the decay curves corresponding to the 5D0 level of Eu3+ in these two samples are single exponential.51,52 Figure 8a,b displays the typical luminescent dynamics of Eu3+ (from 5 D0 to the ground states of 7F1) in EuF3 nanoflowers (a) and nanodisks (b). These curves can be well fitted by a singleexponential function as I(t) ) I0 exp(-t/ι) (I0 is the initial emission intensity at t ) 0, and ι is the 1/e lifetime of the emission center). The luminescence lifetimes of Eu3+ of the two samples are quite close, with 1.93 and 1.67 ms for the EuF3 nanoflowers and nanodisks, respectively. This result indicates that the factors that greatly effect the PL intensity have little effect on the lifetime of the samples. Since the intensity of the 5D0 f 7F1 transition does not depend on the chemical environments around the Eu3+ ion due to its magnetic dipole nature, it can be taken as a reference for the calculation of the luminescent quantum yield of Eu3+.53,54 On obtaining the intensity parameters of the Eu3+ emission

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Table 1. Integral Intensity Ratio of the 5D0-7F1 Transition to the Total 5D0-7Fj (j ) 1–4), [S(0–1)/∑S(0-j)] Transitions, Experimental 5 D0 Lifetime, ι, and Decay Rate, ktot, Calculated Radiative Rate, kι, and Nonradiative Rate, knι, and Quantum Yield, η, in EuF3 Flowers and Disks sample EuF3 flowers EuF3 disks

-1

-1

-1

S(0–1)/∑S(0-j) ι (ms) ktot (ms ) kι (ms ) knι (ms ) η (%) 0.656 0.648

1.93 1.67

0.518 0.599

0.076 0.077

0.442 0.522

structure may be a promising candidate for both fundamental research and functional applications. Acknowledgment. The authors are very grateful to Mr. L. H. Ge for invaluable assistance in the TEM observations. This work was financially supported by Grant NSFC-20331030.

14.7 12.9

References spectrum, the total radiative rate of 5D0 can be expressed by eq (3), where Kι(0–1) is the radiative rate of the 5D0-7F1 transition.55,56 Since in vacuum [Kι(0–1)]vac ) 14.65 s-1,60 when an average index of refraction n equal to 1.506 was considered, the value of Kι(0–1) ) n3[Kι(0–1)]vac≈ 50 s-1 and S(0-j) and S(0–1) are the integral intensities of the 5D0-7Fj and 5D0-7F1 transitions, respectively:54–60 4

∑S

(0-j)

Ki ) Kτ(0-1)

j)0

S(0-1)

(3)

The related data for Eu3+ in EuF3 flower and disk are listed in Table 1. From these data, the total decay rate of 5D0 (Ktot) can be calculated according to eq (4), and the quantum yield (η) values can be estimated by eq (5). 1 Ktot ) ) Kτ + Knτ τ η)

Kτ Kτ + Knτ

(4) (5)

Finally, the emission quantum yield (η) determined from eq (5) is around 14.7% and 12.9% for the EuF3 flowers and disks, respectively. The quantum yield values seem low considering that the material generally leads to high quantum yields for this rare earth ion. This may be due to the concentration quenching effect and a result of energy-transfer processes to the surface through adjacent dopant ions or because the luminescence of surface dopant ions is quenched.42,61,62 Because the samples were prepared in water, the surface of the nanocrystals can be covered with europium hydroxides, fluorine ions, borate, and the adsorbed water molecules. And these adsorbed species on the surface can quench the emission from Eu3+ ions, resulting in the low quantum yield of the samples in the present case. It should be noted that eq (5) describes the quantum efficiency of the Eu3+ 5D0 level and not the absolute emission quantum yield of the samples. The absolute emission quantum yield is a more general quantity that involves the ratio between the light absorption and the light emission. Therefore, the absolute emission quantum yield and the quantum efficiency of the 5D0 level are equal, if all the energy absorbed is transferred to the 5 D0 level.

4. Conclusions In summary, we demonstrate a facile, quick, and mild sonochemical route to the synthesis of 3D, novel, flower-like EuF3 with single crystalline structure in the absence of any additives. The experimental results indicate that the ultrasonic irradiation plays the key role in the formation of the flowerlike structure. The formation and evolution of EuF3 with 3D structure was investigated, and a possible mechanism was proposed to explain its formation. The synthesis route is easily controllable and well reproducible and may be feasible to develop into the scale-up production. This unique complex 3D

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