Highly Uniform and Monodisperse Ba2ClF3 Microrods: Solvothermal

Highly Uniform and Monodisperse Ba2ClF3 Microrods: Solvothermal Synthesis and Characterization Xiaoming Zhang, Chunxia Li, Cuimiao Zhang, Jun Yang, Zewei Quan, Piaoping Yang, and Jun Lin*

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 12 4564–4570

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ReceiVed June 13, 2008; ReVised Manuscript ReceiVed July 21, 2008

ABSTRACT: One-dimensional hexagonal Ba2ClF3 microrods with highly uniform morphology and size have been successfully synthesized via a facile solvothermal method at a low temperature (160 °C). X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to characterize the samples. The synthesis process, based on a phase-transfer and separation mechanism, allows the control of properties such as particle size and shape in low dispersity by bonding the surfactant oleic acid to the crystal surface. We propose a possible formation process and preliminary growth mechanism for the microrods based on the effect of oleic acid. In the synthetic process, oleic acid as a surfactant plays a crucial role in confining the growth of the Ba2ClF3 microcrystals. Under different reaction conditions, Ba2ClF3 microcrystals with different shapes and sizes can be obtained.

1. Introduction It is generally believed that the properties of nanomaterials strongly depend on the size, shape, and dimensionality. Controlled synthesis of nanocrystal with specific structures and research on their structural-based properties are important subjects in nanoscience.1 One-dimensional (1D) nanostructures, including nanorods, nanowires, nanotubes, and nanoprisms, have attracted extensive synthetic interest over the past years due to their numerous potential applications in the fabrication of electronic, optical, optoelectronic, and magnetic devices.2 More applications and new functional materials might emerge if shapecontrolled nanocrystals could be achieved with high complexity.3 Recently, many kinds of novel 1D structured materials have been successfully synthesized, such as III-V4 and II-VI5 semiconductors and elemental and oxide nanowires/nanorods.2f,6,7 Solid inorganic fluorides have a number of uncommon properties, for example, electron-acceptor behavior, a large opticaltransmission domain, high resistivity, and anionic conductivity. BaF2 is one of the dielectric fluorides (CaF2, SrF2, and BaF2) that have a wide range of potential applications in microelectronic and optoelectronic devices, such as wide-gap insulating overlayers, gate dielectrics, insulators and buffer layers in semiconductor-on-insulators structures, and more advanced three-dimensional structure devices.8 Barium fluorochloride (BaFCl) is an alkaline-earth dihalide system which belongs to the MFX (M: Ca, Sr, Ba; X: Cl, Br, I) family. These materials are interesting for both fundamental reasons and various applications.9-18 However, up to now, there are rare reports on the synthesis of Ba2ClF3.19 Crystalline pure Ba2ClF3 was usually obtained with hightemperature disposal technology.19 However, the high temperature simultaneously caused wide particle size distribution. Therefore, how to obtain crystalline pure Ba2ClF3 nano- or microctystals with controllable size and morphology is still a challenge. Since the 1990s, inorganic nanocrystals have attracted wide research interest because of their remarkable size-, shape-, or surface-dependent physical and chemical properties with * To whom correspondence should be addressed. E-mail: [email protected].

respect to their bulk counterparts.20 Many synthetic methods have been developed to prepare nanoscale materials with controllable size and shape successfully, such as the microemulsion method, high boiling point solvent process, precipitation method, thermolysis of organometallic precursor and hydroor solvothermal process.19,21-31 Up to now, metal-organic or various solution-based synthetic routes have been regarded as a powerful pathway to synthesize size- and shape-controlled nanocrystals.32,33 With the advantages of high purity and good homogeneity, the hydrothermal synthesis method is an important technology for the preparation of low dimension nanostructures of anisotropic nanomaterials. Recently, Li et al. developed a liquid-solid-solution synthetic route which can carefully design the chemical reactions that take place on the interfaces of the different phases.34 Through this method, the prepared nanocrystals (NCs) with a long alkyl chain outside can be well dispersed in nonpolar solvents such as chloroform or cyclohexane. This synthetic strategy has been applied to prepare a large variety of NCs with different sizes and properties, including sulfide and selenide semiconductors, rare-earth orthovanadate, rare-earth fluoride, iron oxide, metal, a series of indium hydroxides, oxyhydroxides and oxides NCs, etc.35 In this paper, we focus on the controllable synthesis of 1D Ba2ClF3 hexagonal microrods via a large-scale and facile solution based solvothermal method assisted with oleic acid molecules. The hexagonal Ba2ClF3 microrods we obtained have perfect uniformity, monodispersity, and well-defined crystallographic facets. To the best of our knowledge, there have been no reports on the synthesis of 1D Ba2ClF3 hexagonal microrods at such a low temperature hitherto. The synthesis process, based on a phase-transfer and separation mechanism, allows the control of properties such as particle size and shape in low dispersity by bonding the surfactant to the crystal surface. We propose a possible formation process and preliminary growth mechanism for the microrods based on the effect of oleic acid. We discuss the effects of the addition of oleic acid, the amount of NaOH, reactant and the reaction time. Through adjusting the synthetic parameters, microrods with different sizes and aspect ratios have been obtained.

10.1021/cg800619z CCC: $40.75  2008 American Chemical Society Published on Web 11/06/2008

Uniform and Monodisperse Ba2ClF3 Microrods

Crystal Growth & Design, Vol. 8, No. 12, 2008 4565

Table 1. Sample Denotations and Their Corresponding Detailed Experimental Conditions Together with Their Morphology and Size Propertiesa sample

BaCl2 · 2H2O

NaF

T (h)

OA (mL)

NaOH (g)

morphology and size

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12

2 mmol 2 mmol 2 mmol 2 mmol 2 mmol 2 mmol 2 mmol 2 mmol 1 mmol 3 mmol 2 mmol 2 mmol

4 mmol 4 mmol 4 mmol 4 mmol 4 mmol 4 mmol 4 mmol 4 mmol 2 mmol 6 mmol 2 mmol 6 mmol

24 24 6 12 48 24 24 24 24 24 24 24

0 7 7 7 7 7 7 7 7 7 7 7

0.3 0.3 0.3 0.3 0.3 0.2 0.4 0.5 0.3 0.3 0.3 0.3

irregular polyhedrons, d ≈ 1.5 - 6.9 µm microrods, d ≈ 400 nm, l ≈ 1.8 µm microrods and nanoparticles microrods, d ≈ 1.38 µm, l ≈ 2.26-4 µm microrods, d ≈ 1.39 µm, l ≈ 3.05-4.6 µm microrods, d ≈ 5.3-7.5 µm, l ≈ 6.2-8.5 µm microrods, d ≈ 2 µm, l ≈ 3.3-7.8 µm microrods, d ≈ 1.6 µm, l ≈ 4.8-7.5 µm microrods, d ≈ 1.2 µm, l ≈ 6.9-10.8 µm microrods, d ≈ 190 nm-1.8 µm, l ≈ 1.9-2.8 µm no clear shapes microrods, d ≈ 112-250 nm, l ≈ 1.3-4 µm

a

The amount of ethanol and deionized water is 20 and 18 mL, respectively. The reaction temperature is kept at 160 °C.

2. Experimental Section Synthesis. All chemicals were analytical reagents and supplied by the Beijing Chemical Reagent Company, and used as received without further purification. Deionized water was used throughout. In a typical synthesis, 7 mL of oleic acid, 20 mL of ethanol, and 0.3 g of sodium hydroxide were mixed together. And then the mixture was agitated to form a homogeneous milk white solution. Then 9 mL of an aqueous solution containing 2 mmol of BaCl2 · 2H2O and 9 mL of an aqueous solution containing the 4 mmol of NaF were added into the mixture under vigorous stirring in order. The mixture was agitated for about another 45 min and then transferred into a 50 mL autoclave, sealed and hydrothermal treated at the designed temperature of 160 °C for about 24 h. Then the autoclave was then cooled to room temperature naturally in the air and the products were deposited at the bottom of the vessel. The resulting solutions were centrifuged for 10 min at 4500 rpm to separate the solid powder products. The powders could not be redispersed in cyclohexane solvent, so they were purified several times using ethanol to remove the surplus oleic acid, sodium oleate, and other remnant substances. Additionally, different amount of oleic acid (0, 7 mL, 160 °C, 24 h), NaOH (0.2, 0.3, 0.4, 0.5, 160 °C, 24 h), reactants (1 mmol, 2 mmol, 3 mmol, 160 °C, 24 h) where the molar ratio of Ba2+ to F- is 1:2, different molar ratios (1:1, 1:3, 160 °C, 2 mmol Ba2+, 24 h) of Ba2+ to F- and hydrothermal treatment time (6 h, 12 h, 24 h, 48 h, 160 °C) were selected to investigate the effects of these factors on the morphological and structural properties of the samples. Table 1 lists the reaction conditions for the different samples together with their morphology and size properties. Characterization. The phase purity and crystllinity of the products were examined by powder X-ray diffraction (XRD) carried out on a Rigaku-Dmax 2500 diffractometer with Cu KR radiation (λ ) 0.15405 nm). The accelerating voltage and emission current were 40 kV and 200 mA, respectively. The specimens for the XRD test were prepared by centrifugated the solution containing the powders, drying the powders at 100 °C for about 10 h. The morphology and structure of the samples were inspected using a field emission scanning electron microscope (FE-SEM, XL30, Philips) and transmission electron microscope. Low-resolution transmission electron microscope (TEM) images and selected area electron diffraction (SAED) patterns were obtained using a JEOL 2010 transmission electron microscope operating at 200 kV. Samples for TEM were prepared by depositing a drop of the samples dispersed in cyclohexane onto a carbon coated copper grid. The excess liquid was wick away with filter paper. The solvent was evaporated at room temperature, and the grid was dried in air.

3. Results and Discussion 3.1. Phase Formation. The composition and phase purity of the as-prepared products were first investigated by X-ray diffraction (XRD). The XRD patterns of all the Ba2ClF3 samples are almost identical, and thus the XRD pattern of the as-prepared sample S2 is selected as a typical example to demonstrate the results, as shown in Figure 1a. The strong and sharp diffraction peaks indicate that the product is highly crystalline, and all the peaks can be readily indexed to a pure rhombohedral phase structure (space group is unknown so far) known from the bulk

Figure 1. XRD pattern of (a) the as-prepared Ba2ClF3 sample S2 and (b) the standard data of rhombohedral Ba2ClF3 as a reference (JPCDS No. 07-0029).

Ba2ClF3 crystals (JPCDS Cards No. 07-0029) as Figure 1b shows. No additional peaks of other phases have been found. The lattice of the rhombohedral (R) cell presents the hexagonal nature and thus the hkl indices are based on the corresponding hexagonal unit cell with a ) b ) 1.840 nm, c ) 1.245 nm. The calculated cell parameters of the as-prepared Ba2ClF3 crystals are equal to a ) b ) 1.837 nm, c ) 1.2567 nm, well compatible with those values of the corresponding bulk Ba2ClF3 crystals. Note that during the growth process of crystals, a series of factors especially synthetic conditions drastically influence the crystallization process and shape evolution of the samples. In the current case for Ba2ClF3, as a result of the preferential growth effect caused by different synthesis routes, the relative intensities of XRD lines (particularly the small angle ones) in our solvothermal process derived from the sample are different from those reported in the literature (JPCDS Cards No. 07-0029) to some extent. In addition, as can be seen from the XRD pattern in Figure 1a, high crystalline can be obtained at a relatively low solvothermal temperature (160 °C) and the sharper reflections for rhombohedral Ba2ClF3 imply its large size. 3.2. Morphologies and Growth Mechanism. The morphologies and crystal phase purity of the as-prepared microcrystals are mainly affected by the following experimental conditions, such as reaction time, the addition of oleic acid, reactant concentration, the quantity of NaOH and the ratio of NaF to Ba2+, which has been discussed in detail as follows. Effects of Oleic Acid. It has been reported that the selective ashesion of the capping ligand on certain crystal surfaces plays a crucial role in the epitaxial growth of nanocrystals and microcrystals.36-38 As is known, oleic acid as a surfactant has a long alkyl chain and can be effectively adsorbed on the crystal

4566 Crystal Growth & Design, Vol. 8, No. 12, 2008

Figure 2. SEM images of the as-prepared solvothermal products synthesized without oleic acid.

surfaces of the products, as the capping reagent to control the growth of the products. Therefore, we first investigate the important influence of oleic acid on the shape formation of hexagonal microrods in our current synthesis. The ethanol/water ratio, the reactants quantity, reaction temperature, and time were kept constant (20 mL/18 mL, 2 mmol, 160 °C and 24 h, respectively), 0 and 7 mL were selected to investigate the effect of oleic acid. Figures 2 and 3 show the corresponding sizes and morphology variation of Ba2ClF3 products S1 and S2, respectively. As can be seen in Figure 2, without the addition of oleic acid, Ba2ClF3 crystals tend to grow into an irregular polyhedral structure with a wide distribution range from 1.5 to 6.9 µm and some rodlike substances instead of uniform hexagonal microrod crystals. However, once oleic acid is introduced into the reaction system, a distinct change takes place in the morphology of the crystals. Figure 3 shows the typical SEM images for Ba2ClF3 sample S2. As can be seen from a low-magnification SEM image (Figure 3a), the as-prepared sample is almost entirely composed of such hexagonal microrods with perfect uniformity, monodispersity, and well-defined crystallographic facets. Analysis of a number of the microrods shows that these hexagonal microrods have an average diameter of 400 nm and a length of about 1.8 µm, respectively. These results indicate the high yield and good uniformity achieved with this approach. Furthermore, from the images of a higher magnification (Figure 3b) and typical individual microrods (parts c and d of Figure 3), it can be seen that these microrods have clear edges and smooth side planes. Figure 4 shows the typical TEM and HRTEM images and the SAED pattern of S2 sample, which provide further insight into the micrometer-scale details of the hexagonal rods. In Figure 4a, the regular rodlike images can be clearly observed, which correspond to the rods that are parallel to the copper grids. Typical widths and lengths of the rods are about 320-500 nm and 1.5 µm, respectively. These values are very consistent with those observed from SEM images (Figure 3). The image in the inset of Figure 4a shows the corresponding SAED pattern taken from an area of a single microrod. The rings in SAED pattern imply the polycrystalline nature of the microrods. The corresponding HRTEM image (Figure 4b) recorded from a hexagonal microrod shows high crystallinity of the products. It is well-known that the surfaces of a hexagonal microrod are typically {0001j} top/bottom planes and six energetically equivalent {101j0} family of prismatic planes, consisting of (101j0), (011j0), (1j010), (01j10), (11j00), and (1j100),39 as shown in Figure 3e. In our case, the interplanar

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distances between adjacent lattice planes are determined as 0.328 nm, corresponding to the d-spacing values of the (303) planes. From the contrast experiments, it is found that the addition of oleic acid plays a critical role in the morphology transition from irregular polyhedrons to hexagonal microrods. The exact mechanism for the change of morphology of Ba2ClF3 grown with and without oleic acid is not very clear. It might be explained in terms of the kinetics of the crystal growth process. In the past, many research projects have been done on the growth mechanism of hexagonal prismatic crystal.40-42 In this work, we speculate that in the growth process of Ba2ClF3 microcrystals, oleic acid as a shape modifier selectively binds to the surfaces that are parallel to the c-axis of the growing crystallines, which renders the epitaxial growth and results in microrods. The interaction between oleic acid molecular and particles would also facilitate crystals to grow along the determinate direction. It is likely that the anisotropic growth of rodlike Ba2ClF3 is governed by the selective adsorption effect of oleic acid on certain crystal planes of the growing Ba2ClF3 crystals. Such phenomenon can be compared with the fact that the growth of the rare earth fluorides, oxides, orthovanadates and so on can be controlled effectively through the absorption and/or complexation of oleic acid molecules on the outer surfaces of the growing nanocrystals.35 Effects of Reaction Time. Because we observe that there exist BaF2 and Ba2ClF3 two crystal phases in different reaction times, the phase transformation process was first investigated with an extended reaction time at the same solvothermal temperature (160 °C) without changing the other reaction conditions. The phase and composition of the products obtained at different reaction times were investigated first by XRD measurements. Figure 5 shows the XRD patterns of the products with two different phases obtained at different reaction times. All the diffraction peaks for BaF2 shown in Figure 5a are characteristic of a pure cubic phase [space group: Fm3jm(225); the unit cell structure of BaF2 is shown in Figure S1, Supporting Information]. The XRD peaks for each Ba2ClF3 sample in Figure 5c can be indexed to a pure rhombohedral crystal phase. When the reaction time is 1 h, the nanocrystals of the final product which can be dispersed into the nonpolar solvent such as cyclohexane were indexed to cubic BaF2 in Figure 5a. With the reaction time prolonged, the product can be separated into two parts: one part can be dispersed into cyclohexane, and the other cannot. The XRD results proved that the white precipitation in the final products that cannot be dispersed into the nonpolar solvent such as cyclohexane is the rhombohedral phase Ba2ClF3 as Figure 5c shown. As the time increased to 9 h, the final products were totally transformed to white precipitations, Ba2ClF3, with rhombohedral phase. The broadening of the reflections for BaF2 distinctly indicates the nanocrystalline nature, and the sharper reflections for Ba2ClF3 implied their larger sizes. The XRD results indicate that when the reaction time exceeded 1 h, a phase transition process appears from cubic BaF2 nanocrystals to rhombohedral Ba2ClF3 microcrystals. With the further reaction, the fraction of the Ba2ClF3 phase increased gradually. As the reaction time was prolonged to 9 h, the BaF2 phase disappeared completely and only the Ba2ClF3 phase existed. To date, there is no report on the synthesis of rhombohedral Ba2ClF3 crystal at such a low temperature. Thus, it is reasonable that the present synthetic conditions may be preferable for achieving rhombohedral Ba2ClF3 crystal, indicating that this method is facile and effective to obtain highly crystalline pure Ba2ClF3 rhombohedral crystal at a relatively low temperature (160 °C).



Figure 3. (a) Lower magnification, (b) higher magnification, (c, d) typical individual SEM images for the as-prepared sample S2, and (e) schematic diagram showing the anisotropy of the rhombohedral phase Ba2ClF3 crystal.

Figure 4. TEM (a) and HRTEM (b) images of the as-prepared product S2 and SAED pattern (inset a) of Ba2ClF3 microrods.

At the same time, the morphologies of the samples were carefully investigated by quenching the reaction at different time intervals. Figure 6 shows the SEM images of Ba2ClF3 samples obtained with 7 mL of oleic acid, 20 mL of ethanol, 0.3 g of NaOH and 2 mmol of Ba2+ at 160 °C for various reaction periods. As the time was extended to 6 h, a typical SEM image (Figure 6a) reveals that the sample is composed of a rodlike structure and some particles without clear morphology. We speculate that the rodlike structure is the rhombohedral Ba2ClF3 crystal, and the particles are the cubic BaF2 nanocrystals. With further reaction, BaF2 nanoparticles convert to Ba2ClF3 microrods gradually. These particles would serve as seeds for the growth of Ba2ClF3 hexagonal microrods. Then, the particle edge sharpening occurs concomitantly with particle growth. With the reaction proceeding for 12 h, the fairly uniform and well-defined Ba2ClF3 hexagonal microrods with an average diameter of 1.38 µm and a length of 3.2 µm (Figure 6b) are produced on a large scale with a relatively narrow diameter distribution. Furthermore, the six prismatic planes are very smooth. On the above analysis, it is obvious that the phase transition of cubic BaF2 phase to rhombohedral Ba2ClF3 phase results in the dramatic change in morphology. The reason for that should be related to the different characteristic unit cell structures for different crystallographic phases. The BaF2 nanocrystals have isotropic unit cell structures, which generally induce the isotropic growth of particles. In contrast, the rhombohedral (R) cell presents the hexagonal nature of its lattice; therefore, the Ba2ClF3 microc-

Figure 5. XRD patterns for the two different products at different reaction times. (a) The products that can be dispersed into cyclohexane, (c) the products that cannot be dispersed into cyclohexane. The standard data of cubic BaF2 (b, JPCDS No. 04-0452) and rhombohedral Ba2ClF3 (d, JPCDS No.07-0029) are given as references.

rystals have anisotropic unit cell structures, which can induce anisotropic growth along crystallographically reactive directions, resulting in the formation of hexangonal microrods. There are many reports on the anisotropic growth nature of materials that have hexagonal crystal structures, just like that of ZnO as wellknown.43 After 12 h and 24 h of reaction and growth, there are no further changes in the morphology and size as shown in Figure 6c,d. It indicates that longer times have no apparent influence on morphology and size. Usually, we choose 24 h as the optimal reaction time. Effects of the NaOH Quantity. The NaOH quantity was found to be an important synthetic parameter to influence the final morphology of the Ba2ClF3 microcrystals. The quantity of NaOH was changed from 0.2 to 0.5 g with the other synthetic parameters keeping constant. Figure 7 shows the SEM images of the as-prepared products with the quantity of NaOH from 0.2 to 0.5. Figure 7a indicates that when the amount of NaOH is 0.2 g, the microprisms have been obtained with an average diameter of 6.6 µm, an average length of about 7 µm, and the aspect ratios are 0.76 to 1.3. When the amount of NaOH increases to 0.3 g, the average diameter of the microprisms


Figure 6. SEM images for Ba2ClF3 samples at different reaction times: (a) 6 h, (b) 12 h, (c) 24 h, (d) 48 h.

Figure 7. SEM images of Ba2ClF3 samples with different amounts of NaOH keeping the other conditions constant. (a) 0.2 g, (b) 0.3 g, (c) 0.4 g. (d) 0.5 g.

reduces to about 1.2 µm and the length is about 2.3-4.3 µm, as shown in Figure 7b. Figure 7c,d shows the morphology of products derived from 0.4 and 0.5 g of NaOH, indicating that the average diameters of these microrods are about 2 and 1.6 µm, and the lengths are about 5.9 and 6.4 µm, respectively. Compared with the morphology prepared with 0.3 g of NaOH, the edges of the prismatic surfaces of the sample prepared with 0.2 g of NaOH are not clear and present a polyhedral prismlike structure with a low aspect ratio instead of a clear hexagonal rodlike structure. With the increase of the amount of NaOH up to 0.3 and 0.4 g, the corresponding products obtained under these conditions are still hexagonal rods. However, all the edges of surfaces of the microrods are much clearer, as shown in Figure 7b,c. When the amount of NaOH increases to 0.5 g, there is no further change in morphology and size. From the analysis above, we can conclude that with the amount of NaOH increasing, the morphologies of the microrods transfer to a smaller diameter and higher aspect ratio. We can ascribe this result to the increase of the amount of sodium oleate forming in the reaction. The results indicate that the capping ability of sodium oleate on the {101j0} prismatic surfaces increases significantly with the increase of its amount so that the crystal growth is completely restricted along [101j0] directions, leading to Ba2ClF3 hexagonal microrods with smaller diameters and higher aspect ratios, as observed. However, as shown in Figure 7c,d, there are some impurities on the surface of the as-prepared

Zhang et al.

hexagonal microrods which results in products that are not welldispersed. These impurities may be sodium oleate, which cannot wash clearly because of its large amount with 0.4 or 0.5 g of NaOH. On the basis of the above analysis, it can be concluded that the optimal addition of NaOH for the formation of Ba2ClF3 hexagonal microrods is about 0.3 g. Effect of Reactant and NaF Concentration. By fixing other reaction conditions, the effect of the reactant concentration on the morphology and size of the as-prepared microcrystals was investigated. Figure 8 clearly showed the final morphology and size of the as-prepared samples (S2, S9, and S10 in Table 1). By keeping the other reaction conditions constant, when the reactant content was increased from 1 to 3 mmol, the acquired microcrystals had completely different sizes. Figure 8a shows the typical SEM image of Ba2ClF3 microrods with reactant concentration as 1 mmol, clearly showing that the product is entirely composed of relatively uniform microrods with diameters of about 1.1-1.4 µm, lengths of 7-11 µm, and mean aspect ratios of about 9. When the reactant concentration was increased to 2 mmol, the final products also consist of uniform microrods; however, the diameters and lengths decreased to 270-460 nm and 1.2-2.5 µm with aspect ratios between 2.6 and 6.5. With a reactant concentration of 3 mmol, microrods with different sizes and aspect ratios were obtained as shown in Figure 8c. The larger size microrods have diameters of 1.2-1.8 µm and lengths of 1.5-2.5 µm; the smaller are 191-500 nm in diameter and 1.4-2.6 µm in length. As shown in Figure 8a-c, higher reactant concentration (3 mmol) preferably results in uneven microrods with different sizes in a wide size distribution and lower aspect ratio (Figure 8c), whereas a lower concentration will lead to microrods with a relatively uniform diameter and higher aspect ratio. Here, we attribute this result to the effect of the capping surfactant; it has been reported that the adsorption of the capping ligand on certain crystal surfaces plays a crucial role in the shape-control process. Because of the other reaction condition being constant, with the increase of the reactant concentration, the capping effect of oleic acid decreases gradually. The SEM images in Figure 8d-e shows the effects of the NaF content on the morphology and size of the as-prepared microcrystals. The molar ratios of F- to Ba2+ were changed from 1:1 to 1:3. The synthetic conditions were listed in Table 1. The molar ratios of Ba2+ to F- of samples S11 and S12 were 1:1 and 1:3, respectively. From Figure 8b,d,e, it can be seen that the morphology and size of the microcrystals have been affected by the NaF content. Under the settled conditions, when the ratio of Ba2+ to F- is 1:1 (which means the NaF is deficient), the as-prepared products do not have clear shapes. As the above description, when the ratio of Ba2+ to F- was 1:2, the stoichiometric proportion, the as-prepared products consist of uniform microrods; however, the diameters and lengths are 270-460 nm and 1.2-2.5 µm with the aspect ratios are about 2.6-6.5, respectively. When the ratio of Ba2+ to F- increased to 1:3, which means the NaF is in excess to Ba2+, the as-prepared products are composed of a large number of microrods, with an average diameter of about 170 nm, length of about 2.5 µm and a higher aspect ratio of about 14, as shown in Figure 8e. When the NaF content was insufficient, we obtained the structure without clear shapes. When the NaF content was increased from 4 to 6 mmol, the diameters and aspect ratios changed distinctly; however, when the NaF content was 6 mmol, the as-prepared products consisted of microrods without clear edges. From the above results, we can conclude that the optimal molar NaF content was 4 mmol, where the ratio



Figure 8. (a–e) SEM images of the microcrystals with different reaction contents when the ratio of Ba2+ to F- is 1:2, and different NaF contents when the amount of Ba2+ is kept at 2 mmol. For details, see the text.

Figure 9. Illustration for the formation process of the Ba2ClF3 microrods.

of Ba2+ to F- is the stoichiometric proportion, for the formation of Ba2ClF3 hexagonal microrods. Formation Mechanism for the Microrods. A possible schematic illustration of formation mechanism of this rodlike structure is shown in Figure 9. We employed a phase transfer and separation synthetic route by designing the chemical reaction taking place at the interfaces of different phases, which combines the aqueous-based and organic solvent-based synthetic processes.34 Oleic acid, which has a long alkyl chain and can be effectively adsorbed on the surfaces of the products, have been widely used as the capping reagent to control the growth of the nanocrystals or microcrystals in organic-solvent-based synthetic routes; however, they were never adopted in an aqueous synthetic method because of the limit of solubility in water. In our synthetic route, we employ a water/ethanol mixed solvent as the main continuous solution phase. Water is an ideal solvent for most inorganic salts, and ethanol is a good solvent for most of the surfactants such as oleic acid. This leads many soluble inorganic salts to be utilized as the precursor materials. In our experiments we agitated the solution that contained oleic acid, sodium hydroxide and ethanol to form sodium oleate first. In this procedure two phases formed, including the liquid phase consisting of the excess oleic acid and ethanol, and the solid phase containing sodium oleate.23,24 Then we added the aqueous

solution containing 2 mmol of BaCl2 under vigorous stirring, where the third phase formed (the solution phase). After the addition of aqueous solution, an ions exchange process occurred between sodium oleate and Ba2+ to form Ba oleate under agitation, and simultaneously a phase transfer process occurred in which the Ba2+ ions shift from the aqueous solution to the solid phase of (RCOO)2Ba. After the addition of F-, the oleic acid capped Ba2+ reacted with F- quickly to form oleic acid capped BaF2 amorphous precipitation under agitation. At the designed temperature the in situ generated oleic acid adsorbed and capped on the surface of the BaF2 nanocrystals were further confined to grow and crystallize. Since we choose the BaCl2 · 2H2O as the Ba2+ source, therefore in the aqueous solution, a large amount of Cl- exists with a concentration of 4 mmol. The preformed cubic BaF2 nanocrystals with isotropic unit cell structures that reacted with Cl- resulted in the formation of rhombohedral Ba2ClF3 microcrystals with anisotropic unit cell structures. At the same time, the growth of some crystal facets will be restricted for selective binding of oleic acid, which results in anisotropic growth of Ba2ClF3 microcrystals. When the reaction time is prolonged, the preformed BaF2 nanocrystals gradually transferred to 1D Ba2ClF3 microrods. Therefore, because of the adsorbed effect on certain crystal facet and anisotropic growth nature of hexagonal crystal structures, the morphology of the final as-prepared Ba2ClF3 microcrystals is a hexagonal microrods structure, which can be clearly seen in Figure 2c,d.

4. Conclusions We have reported a novel, efficient, environmentally benign, and solution-based solvothermal synthesis for 1D Ba2ClF3 hexagonal microrods with perfect uniformity, monodispersity, and well-defined crystallographic facets. The synthesis process, based on a phase-transfer and separation mechanism, allows the control of properties such as particle size and shape in low dispersity by bonding the surfactant to the crystal surface. The formation process and preliminary growth mechanism of the as-prepared microrods are based on the effect of oleic acid. It is observed that oleic acid acting as a capping reagent introduced into the reaction system can modulate the growth rate of different crystallographic facets to govern the formation of final


morphology. The quantity of oleic acid, NaOH, reactant and the reaction time will affect the morphology of the products. Through adjusting the synthetic parameters, microrods of different sizes and aspect ratios have been synthesized successfully. Acknowledgment. This project is financially supported by the foundation of “Bairen Jihua” of Chinese Academy of Science, the MOST of China (2003CB314707, 2007CB935502), and the National Natural Science Foundation of China (NSFC 50572103, 20431030, 50702057, 50872131). Supporting Information Available: The unit cell structure of BaF2 (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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CG800619Z

Highly Uniform and Monodisperse Ba2ClF3 Microrods: Solvothermal

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