CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 5 854-858
Articles Shape-Controlled Synthesis and Characterization of YF3 Truncated Octahedral Nanocrystals Feng Tao, Zhijun Wang, Lianzeng Yao,* Weili Cai, and Xiaoguang Li Department of Materials Science and Engineering, UniVersity of Science and Technology of China, Hefei 230026, P. R. China ReceiVed June 28, 2006; ReVised Manuscript ReceiVed December 21, 2006
ABSTRACT: Highly uniform and monodisperse YF3 nanosized and sub-microsized truncated octahedra were successfully prepared in large quantities using a facile hydrothermal approach assisted by a capping reagent, ethylenediamine tetraacetic acid disodium salt (Na2H2EDTA). Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) observations showed that most of these nanosized and sub-microsized truncated octahedra are uniform in size with an average edge length of about 700 nm. High-resolution transmission electron microscopy (HRTEM) investigation and selected area electron diffraction (SAED) revealed that these samples are single crystals. Energy dispersive X-ray (EDX) analysis was used to check the chemical composition and purity of the products. The formation mechanism of the YF3 truncated octahedra is discussed. Introduction In recent years, great effort has been devoted to the synthesis of inorganic micro- and nanocrystals of controlled size and shape using various methods driven primarily by the fact that the shape and size of inorganic nanocrystals have tremendous effects on their properties.1-3 These nanomaterials form building blocks for new bottom-up approaches to materials assembly for a range of uses.4 There have been significant developments in this area in the past decade. A variety of nanomaterials with different shapes and sizes have been prepared such as nanowires,5 nanorods,6 nanocubes,7,8 nanopyramids,9 nanotriangles,10 and nanodisks.11 Recently, materials with an octahedral shape have been reported. Submicro- and nano-Cu2O octahedra were synthesized via a solution-phase route12 and in microemulsions using γ-irradiation.13 In2O3 octahedra were fabricated by controlling the appropriate external conditions using a chemical vapor deposition (CVD) method.14 Octahedral Cu2O nanocages have been synthesized by a one-pot catalytic solution route.15 Rutile-like SnO2 octahedra were fabricated via a one-pot solution route based on a two-dimensional aggregation of nanocrystallites.16 BaWO4 octahedral microparticles were obtained through surfactant-assistant methods.17 However, few papers have been reported on the synthesis of yttrium or lanthanide fluoride octahedron crystals so far. Since kollia showed that YF3 could be a laser material,18 YF3 crystalline materials have been commonly studied owing to their applications in the fields of solid-state lasers and scintillators. In addition to their low nonradiation transitions, good optical * Corresponding author. E-mail:
[email protected]. Tel: +86-5513600249.
properties make these materials good host matrices for visible or infrared light emissions and other optical applications.19 However, the YF3 crystals obtained are almost irregular nanoparticles. Since the properties of inorganic structures may be well tuned by tailoring the morphology and crystallinity, the crystal structure and crystallization have turned out to affect the optical properties of the luminescent centers. Some efforts have been devoted to the synthesis of inorganic structures with well-defined nonspherical morphologies. Recently, YF3 nanoparticles with quadrilateral and hexagonal shapes have been synthesized using a reverse microemulsion technique.20 LaF3 triangular nanoplates have been synthesized from a single-source precursor.21 Rare-earth fluoride nanocrystals were prepared in a water-ethanol mixed-solution system.22 Sodium rare-earth fluoride nanocrystals were also synthesized through co-thermolysis of Na(CF3COO) and RE(CF3COO)3 in oleic acid/ oleyamine/1-octadecene.23 In this paper, we report the preparation of truncated octahedral YF3 nanocrystals of high quality using a simple hydrothermal approach. Compared with the above reports, our synthesis procedure reduces the use of organic reagent and is relatively simple and easier to operate. Experimental Section The initial chemicals in this work, yttrium oxide (Y2O3, 99.99%), ethylenediamine tetraacetic acid disodium salt (Na2H2EDTA, AR), ammonium fluoride (NH4F, AR), and HNO3 (AR), were used without further purification. In a typical reaction, 0.511 g of Y2O3 powder was completely dissolved in dilute nitric acid to form a clear aqueous solution, the pH of which was adjusted to about 2-3. Then, 0.729 g of Na2H2EDTA was added to the solution according to a Y3+-to-Na2H2EDTA ratio of 2:1 to form a chelated yttrium complex. After the solution was magnetic
10.1021/cg060404o CCC: $37.00 © 2007 American Chemical Society Published on Web 04/03/2007
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Figure 1. XRD pattern of hydrothermal synthesized YF3 nanotruncated octahedral at 180 °C for 48 h.
stirred for 10 min, 0.51 g of NH4F was added into the solution. Subsequently, the solution was transferred to a 30-mL Teflon-lined autoclave. Distilled water was added into the Teflon vessel until about 80% of its volume. After the autoclave was tightly sealed, it was heated at 180 °C for 48 h and then cooled to room temperature naturally. The products were washed several times with distilled water and absolute ethanol in turn and finally dried in air at about 70 °C for 12 h. X-ray diffraction (XRD) of the samples was performed using an X-ray diffractometer (D/MAX-γA) with Cu KR radiation (λ ) 0.15418 nm) in the range of 20° e 2θ e 80°. The morphologies of the YF3 nanocrystals were investigated on a transmission electron microscope (TEM, H800) and a field emission scanning electron microscope (FESEM, JEOL-JSM-6700F) with an accelerating voltage of 200 kV. High-resolution transmission electron microscope (HRTEM, JEOL2010) and selected area electron diffraction (SAED) attached to the TEM were employed to characterize the crystal structure. The chemical composition and purity of the product were examined by electron energy dispersive X-ray (EDX) analysis.
Results and Discussion X-ray diffraction pattern of the as-obtained product is shown in Figure 1. It can be seen from Figure 1 that all of the diffraction peaks can be indexed according to the orthorhombic YF3 [space group: Pnma (62)] with lattice constants (a ) 0.6353, b ) 0.6853 and c ) 0.4393 nm) very close to the reported data in literature (JCPDS 74-0911). To further illustrate our synthesis strategy, the product was also characterized by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Typical FESEM images of the YF3 crystals prepared at 180 °C for 48 h are presented in Figure 2. Plenty of almost uniform, regular and truncated octahedra with an average length of about 700 nm and smooth surfaces can be seen in Figure 2a, and the inset is the magnification of some YF3 truncated octahedra. From the inset image, we can see the jointed part of the intersecting octahedra, and the imperfections are a result of mismatched closure. The similar results were also observed by Yang et al.16 Figure 2b is the magnified FESEM image of a single truncated octahedron with sharp edges. Well-defined truncated octahedral morphology is characteristic of single-crystalline orthorhombic YF3 crystals bound by eight {111} planes (see Figure 2b). The rhombic bases of the truncated octahedron are indexed to the {010} lattice plane. It is also apparent in Figure 2b that some small particles are attached to the surfaces of the octahetra, which suggests that these small particles may serve as building blocks for growing the nano-octahedra. Figure 2c is a schematic diagram of the truncated octahedron. Figure 3 displays a typical TEM image of YF3 crystals and SAED pattern of a single particle. Well-defined nanoparticles
Figure 2. FESEM of the obtained YF3 nanotruncated octahedral. (a) Almost uniform, regular and truncated octahedral; the inset is the magnification of some YF3 truncated octahedra. (b) Magnified FESEM image of a single truncated octahedral. (c) Schematic diagram of the truncated octahedron.
with different shapes are observed in Figure 3a; most are rhombuses, and the others are hexagons. Comparing Figure 3a with the FESEM image in Figure 2, it is known that the hexagons and rhombuses represent, in fact, the same particle morphology observed from different directions. Figure 3c is the SAED pattern corresponding to a single truncated octahedron (Figure 3b) showing a rhombic projection. The diffraction spots are indexed to the (002), (101), and (200) plane of orthorhombic YF3, respectively, which indicates that the octahedron is a welldeveloped single crystal. Figure 4 shows the HRTEM image of the rhombic base of a relatively small truncated octahedron. The measured lattice spacing is about 0.32 and 0.36 nm, corresponding to the distance of the {200} and {101} planes of the orthorhombic YF3, respectively. The calculated angle between [200] and [101] is about 55°, and that between [101] and [1h01] is about 70°, which are consistent with the measured values. This image also reveals the single-crystal nature of the product.
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Figure 5. EDX spectrum revealing the chemical purity of the YF3 nanotruncated octahedral.
Figure 3. TEM images and the SAED pattern of a single YF3 nanotruncated octahedral. (a) TEM image of well-defined nanoparticles with different shapes. (b) TEM image of a single YF3 truncated octahedron, and (c) SAED pattern corresponding to a single truncated octahedron in panel b.
Figure 4. The corresponding HRTEM image of a single YF3 nanotruncated octahedron. (a) HRTEM image of the rhombic base of a relatively small truncated octahedron. (b and c) Local magnified image of panel a.
Figure 5 gives the EDX spectrum of the product obtained at 180 °C. It can be seen from Figure 5 that only yttrium and fluorine are detected in the spectrum. The elemental analysis reveals that the atomic ratio of F to Y of the product is 73.9: 25.0 very close to the stoichiometric YF3 in agreement with the XRD, SAED, and HRTEM results. A series of experiments was carried out to understand the formation mechanism of the truncated YF3 octahedra. Figure 6 gives the FESEM images of the YF3 crystals prepared under different conditions. Our study indicates that Na2H2EDTA is indispensable to obtain the truncated YF3 octahedra. When Na2H2EDTA is absent, the products are irregular rice-like
nanoparticles, as shown in Figure 6a, which is consistent with the results in ref 22. The dimensions and uniformity of the product are found to depend on the Na2H2EDTA concentration. As the Na2H2EDTA concentration increases successively from 0.42 to 1.67 µmol/L, the size of the YF3 crystals decreases from ∼1.6 to 0.5 µm gradually. Moreover, relatively uniform truncated octahedra are obtained under a relatively low Na2H2EDTA concentration. A further increase in the Na2H2EDTA concentration, however, will result in aggregation of the truncated octahedral particles, and Na2H2EDTA, in this case, will actually serve as a cohesive agent. The crystal size and shape of the truncated YF3 octehedra seem to be independent of the reagent concentrations according to our experiments. When the Y3+-to-Na2H2EDTA ratio (2:1) and the Y3+-to-F- ratio (1:3) are kept unchanged, decreasing or increasing the reagent initiative concentrations does not cause the shape and size of crystals to change significantly as shown in Figure 6b. Prolonging the reaction time does not make the crystals grow larger as expected. If the reaction time is shortened from 48 to 24 h, however, the product is irregular particles and many small particles present simultaneously (see Figure 6c). This is because the reaction is not complete in such short time. When the temperature increases from 180 to 200 °C, the truncated octahedral crystals become larger and imperfect so that some defects or flaws can be seen on the surfaces of crystals as shown in Figure 6d. It is well-known that crystals in a solution growth environment favor a full exposure of the crystal faces. However, since the growth rate is anisotropic, planes appearing as the crystal faces should have the slowest growth rates over the others. Besides, it is discussed above that the particle size decreases with an increase in the Na2H2EDTA concentration, which means the growth rate of the crystals drops correspondingly. Therefore, it is reasonable to speculate that the interaction between Na2H2EDTA and various crystallographic planes of YF3 can greatly reduce the growth rate of the crystal, and the selective interactions of Na2H2EDTA with the {010} and {111} crystallographic planes should be stronger than those of the others. A significant drop in the growth rates along the normal directions of the {010} and {111} planes should contribute to the formation of YF3 truncated octahedra abundant in trapezoid {111} facets over the rhombic {010} bottom planes.24 We speculate that these YF3 truncated octahedra are formed as follows: Little YF3 particles start to form when a critical supersaturation of the particle-forming species is reached. Then, some of these particles form crystallite aggregates via oriented attachment25 to build the rhombic bases of the truncated octahedra, which are indexed to the {010} lattice planes. Then,
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Figure 6. FESEM images of nano-YF3 synthesized under different conditions. (a) At 180 °C for 48 h free of Na2H2EDTA, (b) at 180 °C for 48 h, but changing the reagent initiative concentration, (c) at 180 °C for 24 h, and (d) at 200 °C for 48 h.
Figure 7. FESEM images of LnF3 crystals hydrothermally prepared at 180 °C for 48 h, (a) EuF3 (O); (b) DyF3 (O); (c) SmF3 (H); (d) NdF3 (H).
the particle aggregates on the surfaces of the {010} planes grow along the [010] and [01h0] orientations to the two poles. These
aggregates can form small complex geometrical structures by means of a plane-by-plane process. When two or more such
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initial crystallites are very close to each other, it is possible that these crystallites may join with each other, that is, grow into large octahedra as shown in Figures 2a and 6c. This process is very similar to the results in ref 16 in which the introduced Na2H2EDTA plays an important role in the preferential growth of YF3 truncated octahedra. Some small truncated octahedral particles formed earlier may serve as seeds on which some other little YF3 particles grow and finally become large truncated octahedra through an Ostwald ripening process.26 This may explain why some small particles also exist in the product. Figure 7 presents the FESEM images of LnF3 (Ln ) Eu, Dy, Sm, and Nd) crystals hydrothermally synthesized at 180 °C for 48 h. Our study shows that the results for different rare-earth fluorides are similar to each other, but the images have some differences due to different crystal structures.22 The crystals structures of LnF3 belong to the hexagonal or orthorhombic system. When the crystal structure is the orthorhombic system, there is only a slight difference in the FESEM images as shown in Figure 7a,b. However, when the crystal structure is the hexagonal system, the bases of the crystals are hexagonal, and the crystals are dodecahedron-shaped as shown in Figure 7c,d. It is reasonable to imagine that, with the changing of the central atom and crystal structure, the complexation state on the different surfaces of the crystal varies slightly, which will lead to a different growth mode. Further investigation is undertaken to fully understand this growth process. Conclusion In summary, uniform, single-crystal sub-microsized and nanosized YF3 truncated octahedra are successfully synthesized via a simple hydrothermal route assisted by Na2H2EDTA. The Na2H2EDTA chelant plays a critical role in the formation of the YF3 truncated octahedra. A selective interaction between Na2H2EDTA and various crystallographic planes of YF3 is proposed, which may change the growth rates along different directions. The size of the truncated octahedra shrinks with an increase in the Na2H2EDTA concentration. These synthetic and mechanistic studies may be the basis for controlling the geometry of a wide range of nano/microparticles, and more
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