CRYSTAL GROWTH & DESIGN
Controlled Synthesis and Characterization of Nanostructured EuF3 with Different Crystalline Phases and Morphologies Miao
Wang,†
Qing-Li
Huang,†
Jian-Ming
Hong,‡
Xue-Tai
Chen,*,†,§
and Zi-Ling
2006 VOL. 6, NO. 9 2169-2173
Xue#
Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, P. R. China, Analytic Center of 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, The UniVersity of Tennessee, KnoxVille, Tennessee 37996-1600 ReceiVed March 27, 2006; ReVised Manuscript ReceiVed June 22, 2006
ABSTRACT: A variety of nanostructured EuF3 with different crystalline phases and morphologies have been prepared via a simple solution route employing the reactions between Eu(NO3)3‚6H2O and fluoride sources XF (X ) K+, H+, NH4+, Na+, Rb+, and Cs+) under ambient conditions. The cations of the fluoride sources were found to determine the crystal structures and morphologies of the as-prepared nanostructured EuF3. X-ray diffraction (XRD) showed that the reaction of Eu(NO3)3‚6H2O with KF or HF yielded nanostructured EuF3 with hexagonal structure, while orthorhombic EuF3 crystals were obtained in the reactions with NH4F, NaF, RbF, or CsF. Different nanostructures including incomplete hollow nanoplates, spherical nanoparticle aggregates, branched tree-like ramified nanostructures, nanorods, and nanowires have been prepared. The luminescent properties of the nanostructured EuF3 have also been studied. 1. Introduction Nanostructured materials have received extensive attention due to their unusual properties and potential applications. It remains a great challenge to precisely control the size, morphology, and crystal structure of nanomaterials and thus to tailor their properties.1-3 Many techniques and approaches have been developed to prepare various types of functional materials such as metals, metal oxides, and semiconductors. The use of organic molecules such as surfactants, polymers, and capping ligands has been a popular method to control the size, morphology, and crystal structure of nanomaterials.4-6 In contrast, the roles of inorganic species in the controlled synthesis of nanomaterials are not well-known. Murphy and co-workers7 have reported the synthesis of silver nanowires that was controlled by the amount of NaOH in the reaction without the use of a polymer or a surfactant. The work of Pileni and co-workers has demonstrated that the addition of inorganic salts controlled the morphology of copper nanostructures in a micelle system.8 Inorganic ions are also involved in the control of the morphologies of Ag nanoparticles, as Xia and co-workers have shown.9,10 In the preparation of nanoscale MoO3 under hydrothermal conditions, Cao et al.11 have found that its morphologies could be controlled by the use of different salts such as KNO3, Ca(NO3)2, and La(NO3)3. Rare-earth trifluorides (LnF3) have been actively studied for their potential applications in optoelectronic integrated circuits (OEICs), lasers emitting at short wavelength by up-conversion,12,13 and monolithic waveguide devices.14 Several solutionbased routes have been developed to prepare nanostructured binary rare earth fluorides.15-20 The hydrothermal route has been proven to be a very powerful method and used to fabricate nanostructured LnF3 with different morphologies such as fullerene-like nanoparticles,15 bundle-like particles,16 and nano* To whom correspondence should be addressed. Fax: ++86-2583314502. E-mail:
[email protected]. † Nanjing University. ‡ Analytic Center of Nanjing University. § Fujian Institute of Research on the Structure of Matter. # The University of Tennessee.
Figure 1. XRD patterns of the EuF3: (a) S1; (b) S2; (c) S3; (d) S4; (e) S5; (f) S6. Table 1. Summary of the Reaction Conditions and Morphologies of the Products crystalline phase
sample
fluoride source
S1
KF‚2H2O
3:1
hexagonal
S2 S3
HF NH4F
3:1 3:1
hexagonal orthorhombic
S4 S5 S6
NaF RbF CsF
3:1 3:1 3:1
orthorhombic orthorhombic orthorhombic
fluoride/Eu3+
morphology lunate nanoplates nanospheres branched tree-like nanobundles nanorods nanowires
plates17 under different reaction conditions. Other synthetic methods include reverse microemulsion for hexagonal and triangular YF3 nanocrystals18 and a single molecular precursor route for monodispersed LaF3 triangular nanoplates.19 Most of these reported routes, with the exception of the single molecular precursor approach, usually employ HF, NH4F, and alkaline fluorides as fluoride sources.15-18 We reasoned that the nature
10.1021/cg0601709 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/20/2006
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Wang et al.
Figure 2. SEM, TEM, and HRTEM images of hexagonal EuF3 (S1, S2): (a) SEM of S1; (b-d) TEM of S1; (e) SEM of S2; (f, g) TEM of S2; (h) HRTEM of a region of S2.
of fluoride source might play a key role in the control of the morphology of the nanosize products. To this end, we have introduced NaBF4 as fluoride source to selectively prepare nanospindles of EuF3 with orthorhombic structure and EuF3 nanodisks with hexagonal structure.20 We have found that a simple solution route under ambient conditions, employing the reactions between Eu(NO3)3‚6H2O and several fluoride sources XF (X ) K+, H+, NH4+, Na+, Rb+, and Cs+) yields nanoscale EuF3 with a variety of crystalline phases and morphologies. The preparation and characterization of these nanostructured materials and their luminescent properties are reported. 2. Experimental Section Synthesis. In a typical experiment, 3.0 mmol of KF‚2H2O and 1.0 mmol of Eu(NO3)3‚6H2O were dissolved in distilled water in a 50 mL
plastic flask with vigorous stirring at room temperature for 3 h. The white solid precipitates were collected and separated by centrifugation and washed in an ultrasonic bath several times with distilled water and ethanol. The final product (S1) was dried at 70 °C for 3 h. A similar synthetic procedure was employed using HF, NH4F, NaF, RbF, and CsF (instead of KF‚2H2O), while other reaction conditions are kept constant. The products were denoted as S2-S6, respectively (Table 1). Characterizations. XRD analyses were carried out on a SHIMADZU XRD-6000 powder X-ray diffractometer with Cu KR radiation, λ ) 1.5418 Å. The morphologies and microstructures of the as-synthesized samples were studied by scanning electron microscope (SEM, JEOL JSM-6700F) and transmission electron microscope (TEM, JEOL JEM-2010F). The room-temperature luminescent spectra were performed on a luminescence spectrometer (Aminco Bowman).
Nanostructured EuF3
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Figure 3. The EM images of orthorhombic EuF3 (S3-S6): (a, b) SEM and TEM images of S3; (c, d) SEM image of a single nanostructure of S3 (HREM and ED patterns of S3 insets); (e, f) SEM and TEM images of S4; (g) TEM and ED patterns of S5; (h) TEM, HREM, and ED patterns of S6.
3. Results and Discussion The crystalline phases of the as-prepared samples were identified by powder X-ray diffraction (Figure 1). The diffraction patterns of S1 and S2 prepared from KF and HF, respectively, are indexed to EuF3 with hexagonal structure (Figure 1a,b). The positions of the peaks are in good agreement with literature values (JCPDS file 32-0373). The XRD data of S3-S6 (Figure 1c-f) prepared from NH4F, NaF, RbF, and CsF, respectively, are indexed to orthorhombic EuF3 (JCPDS file 33-0542). It should be noted that some diffraction peaks of S5 and S6 are very weak or missing, which might be due to preferential growth of these one-dimensional nanostructures (see below). These results indicate that the hexagonal and orthorhombic phases of EuF3 have been selectively prepared by simply using different
fluoride sources; KF or HF yielded EuF3 with hexagonal structure (S1, S2), and NH4F, NaF, RbF, or CsF gave rise to EuF3 with orthorhombic structures (S3-S6). Most of previously reported routes only yield the nanostructured binary lanthanide fluorides with one crystalline phase.15-19 To our knowledge, there has been no report of selective synthesis of different crystalline phases controlled by the fluoride sources. The current work therefore provides a novel route to control the crystalline phases of EuF3. The morphologies and microstructures of S1-S6 were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 2 shows SEM and TEM images of EuF3 with hexagonal structures (S1 and S2). S1 consists of many C-shaped lunate plates with an outer
2172 Crystal Growth & Design, Vol. 6, No. 9, 2006
Figure 4. Room-temperature emission spectra of as-obtained EuF3.
diameter of ca. 100-200 nm and a thickness of 20-40 nm (Figure 2a,b). The nanoplate could also be viewed as an incomplete hollow plate. A single typical nanoplate is shown in Figure 2c, and clear lattice fringes are observed in the HRTEM image of a domain of single nanostructure (inset in Figure 2c). It is noteworthy that some C-shaped nanoplates stack face to face to form a chain (Figure 2d). The SEM image of S2 shows many nanospheres ca. 200 nm in diameter (Figure 2e). The surface of the sphere is not smooth. S2 is a spherical aggregate constructed by numerous primary nanoparticles (Figure 2f,g). There are many micropores in the nanospheres. Clear lattice fringes of 0.32 nm were found in the primary nanoparticles as shown in HRTEM images (Figure 2h), indicating the good crystallinity of S2. This is further revealed by the electronic diffraction patterns (inset in Figure 2h). SEM and TEM images of orthorhombic S3-S6 prepared from NH4F, NaF, RbF, and CsF as fluoride sources are shown in Figure 3. SEM and TEM images reveal that S3 contains branched tree-like ramified nanostructures. Figure 3c,d provides the top and side views of a single nanostructure. Such novel nanostructure is constructed by nanorods with diameters up to 10-20 nm and lengths ranging from 100 to 200 nm, which have grown around a central axis. The good crystallinity of S3 is confirmed by the clear lattice fringes in the HRTEM and ED patterns recorded on the tip region of a nanorod (Figure 3c insets). SEM and TEM images of S4 (Figure 3e,f) reveal that it consists of bundles 50 nm in diameter and 0.2-0.3 µm in length. These bundles appeared to be formed from small nanowires, and their surface is rough. The polycrystallinity of S4 is confirmed by ED patterns (Figure 3f insets). S5 is nanorods with diameters of 100 nm and lengths of 0.5-2 µm (Figure 3g). The nanorods are single-crystalline as revealed by spotlike ED patterns (Figure 3g insets). S6 is composed of uniform nanowires with an average diameter of 100 nm and lengths of several micrometers (Figure 3h). The HRTEM image and ED pattern of S6 show that the nanowire is single-crystalline (Figure 3h insets). Except the use of different fluoride sources XF (X ) K, H, NH4, Na, Rb, Cs), these nanostructured EuF3 with different crystal structures and morphologies were prepared under the identical reaction conditions, suggesting that the nature of fluoride sources plays a key role in the controlled synthesis of EuF3. It is well-known that the properties of microsized colloidal particles formed by precipitation from a homogeneous solution are highly sensitive to many factors such as pH, base, and anion sources.21-23 It was reported recently that inorganic species were also involved in controlling the shape of nanoparticles.7-11 In our current reactions, using Eu(NO3)3‚6H2O and XF (X ) K, H, NH4, Na, Rb, Cs) in molar ratio of 1:3, the only difference
Wang et al.
is the cations in the solutions. We believe that the cations X+ in the byproducts XNO3 are responsible for the morphologies of the as-prepared nanostructured EuF3. Pileni and co-workers suggested that the anions Cl- and Br- in the added salts selectively adsorbed on the different crystal faces of copper particles to control the morphology of the products.8 The roles of cations in our case, to some extent, are perhaps similar to those of anions. It is quite possible that cations are absorbed on the surface of the initially formed tiny EuF3 particles at the early stage of the reactions, due to the strong interactions between the cations and the fluoride anions on the particle surface. However the number of cations (K+, H+, NH4+, Na+, Rb+, and Cs+) of different sizes absorbed on the EuF3 crystal faces of the initial particles is probably different. As a result, the growth rate of the crystalline plane is different, leading to the distinct morphologies of the products S1-S6. Luminescent spectra were recorded on the solid samples of S1-S6 at room temperature (Figure 4). At the excited wavelength of 395 nm, the corresponding emission peaks of two different crystalline phases of EuF3 at 592, 615, 651, and 692 nm were observed, corresponding to the 5D0 f 7F1, 5D0 f 7F2, 5D f 7F , and 5D f 7F transitions in Eu3+, respectively.24 0 3 0 4 Although the positions of the major peaks in the emission spectra are identical in these samples, the emission and peak patterns are different. The strongest emission peak occurs at 592 nm (5D0 f 7F1) in S1-S4, while the peak at 616 nm (5D0 f 7F2) is the most intense for S5 and S6. The reasons for these differences are not clear yet. Yan and co-workers25-27 have investigated the luminescent properties of nanostructured YBO3/ Eu3+ and found that the ratio of 5D0 f 7F2 to 5D0 f 7F1 emissions is size-dependent. They suggested that as the particle size of nanostructured YBO3/Eu3+ became smaller, the lattice become more distorted, which was responsible for the difference of the intensity patterns. The degree of crystallinity and the level of disorder are the other two factors affecting the luminescent properties of nanostructured YBO3/Eu3+.25-27 It is generally believed that the morphology and the crystal structure should influence the luminescent properties of nanostructured particles. Therefore, the difference in luminescent properties of S1-S6 can be ascribed to the combined roles of their various dimensions, morphology, and the degree of crystallinity and crystalline phase. 4. Conclusions Nanostructured EuF3 with different crystalline phases and morphologies have been prepared by a simple precipitation reaction under ambient conditions using several fluoride sources. This is the first demonstration that the nature of cations in the fluoride sources played an important role in controlling the products of nanoscale metal fluorides. This novel approach may be extended to the synthesis of other nanostructured metal fluorides. Acknowledgment. This work was supported by the Natural Science Grant of China (No. 50572037), Natural Science Grant of Jiangsu Province (No. BK 2004087), Program for Changjiang Scholars, and Innovative Research Team in University, Grant of Instruments of Nanjing University, and the US National Science Foundation (Grant CHE-0516928). References (1) Alivisatos, A. P. Science 1996, 271, 933-937. (2) Hu, J. T.; Odom. T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435-445.
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CG0601709
