General Strategy for Fine Manipulating Crystal Growth of Water

Mar 5, 2014 - used to manipulate the growth rate of different planes in aqueous ... assembling hierarchical micro/nanostructures and changing intrinsi...
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General Strategy for Fine Manipulating Crystal Growth of WaterSoluble Salts Jian Zhang,†,§ Zhongping Zhang,†,§ Qi Ji,‡ Yingchang Jiang,† Shudong Zhang,*,† and Zhenyang Wang*,† †

Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei, Anhui 230031, China School of Physics and Engineering, Zhengzhou University, Zhengzhou, Henan 450052, China

‡

S Supporting Information *

ABSTRACT: In this work, a general strategy was proposed for fine manipulating intrinsic growth of water-soluble salts belonging to different crystal systems. Various hollow microsphere hierarchical architectures assembled with hopperlike single crystal blocks of cubic KBr, orthorhombic (NH4)2SO4, or hexagonal Na2SO3 have been designed and prepared via microemulsions. Intrinsic growth manipulations of different water-soluble salts has been successfully achieved by preventing growth of the outside surface of each single crystal block via a diffusion-limit effect of the interfaces between the oil phase and aqueous droplets. As a result, hopperlike single crystals of water-soluble salts belonging to different crystal systems were formed. Furthermore, octahedral NaCl hopperlike single crystal, rather than their typical cubic shape, has also been fabricated by manipulating their growth rates along different lattice directions in combination with urea as additives. The large amount of fine manipulation of intrinsic growth of water-soluble salts will provide us with a deep understanding of crystallography of inorganic salts, as well as facilitate design and production of water-soluble salts architectures with corresponding shapes, according to the different requirements.

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INTRODUCTION Crystal structure has long been recognized as one of the most critical factors to determine properties of inorganic matter. Under equilibrium thermodynamic and kinetic conditions, the inorganic crystal is grown along the direction perpendicular to the crystallographic planes with the relative high surface energies to form their intrinsic shapes.1 Therefore, for the purpose of designing and reaching the desired properties, intensive efforts have been made to change the intrinsic growth nature of different inorganic crystals by either altering surface energy of certain planes or hindering growth along some directions.2 So far, organic ligands or additives have been widely used to manipulate the growth rate of different planes in aqueous solutions to form various nanostructures, nanosheets, and polyhedrons.3−7 Currently, in combination with the control of organic ligands/additives, complex heterogeneous and hierarchical architectures have been extensively assembled at air/liquid, liquid/liquid, or solid/liquid interfaces.8−10 Such methods have been successful to manipulate crystallographic morphology of different water-insoluble inorganic matters, including metals, oxides, sulfides, and insoluble salts.11−16 On © 2014 American Chemical Society

the other hand, water-soluble salts, as another large family of inorganic matter, have rarely been reported to be changed in their intrinsic crystal growth nature. What is more, even controllable growth of micro/nanostructures of these watersoluble salts remain challenging currently. Both controlled assembling hierarchical micro/nanostructures and changing intrinsic growth nature of these kind of water-soluble inorganic matters to obtain other shapes and structures are very important to comprehensively understand the growth mechanism of various crystals. In our previous work,17 taking the most representative matters of cube crystal water-soluble salts NaCl and KCl as samples, we have managed to alter their intrinsic growth nature to have formed their hopperlike single crystals via interfacial growth by hindering growth of one of their (100) planes. These results imply that fine manipulation in crystal structures of water-soluble salts might be achieved provided appropriate approaches are taken. Received: December 21, 2013 Revised: February 10, 2014 Published: March 5, 2014 1520

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Figure 1. Growth manipulation of water-soluble salts belonging to different crystal systems.

To validate this hypothesis, in this work, hopperlike single crystals and their self-assembled hierarchical micro/nanoarchitectures of three kinds of water-soluble salts belonging to different crystal systems [including cubic KBr, orthorhombic (NH4)2SO4, and hexagonal Na2SO3, respectively] have been control assembled by our proposed general interfacial growth strategy, which are all formed deviating from their intrinsic growth nature. As seen in Figure 1, our basic idea is to realize manipulation of various single crystals of water-soluble salts with different crystal systems via the diffusion-limited effect on the metastable liquid−liquid interface between oil phase and the aqueous droplet. Various hopperlike single crystals belonging to different crystal systems can be assembled, since growth of the water-soluble salt single crystal toward the organic phase is prevented. Even more interesting, taking cubic NaCl as a sample, fine manipulations of their crystal growth has also been achieved to change their shapes from cubes to octahedrons. The basic idea is to truncate all the eight corners of the cubes via introducing urea into the reaction system, thus to assemble an octahedral hopperlike single crystal.

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EXPERIMENTAL SECTION

Hierarchical architectures assembled by cubic KBr hopperlike single crystals were prepared in a similar way as our previous work. Typically, cyclohexane (8 mL) was first mixed with acetone (8 mL). An aqueous solution of 7.5 μL of KBr was subsequently injected into the above organic mixture through a syringe pinhole under vigorous agitation at room temperature for 10 min. After the reaction, the white precipitates were collected by the removal of supernatant. This same procedure was also adopted for preparation of (NH4)2SO4 and Na2SO3 hopperlike single crystals other than the fact that acetone was replaced by ethanol. For the growth of octahedral NaCl hopperlike single crystals, the same method was adopted to prepare cubic NaCl hopperlike single crystals, other than the fact that 0.8 M urea was injected into the reaction system together with the aqueous, as the structure-directing agent to truncate corners of the cubes. The phase and purity of the obtained products were determined by X-ray powder diffraction (XRD) using an X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). The structures of the samples were characterized using field-emission scanning microscopy (FE-SEM, Sirion200).

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Figure 2. SEM images of various hierarchical architectures assembled by hopperlike water-soluble salt single crystals belonging to different crystal systems. (a) SEM images of KBr hollow spherulites. (b) SEM images of (NH4)2SO4 hollow spherulites. (c) SEM image of Na2SO3 hollow spherulites. Their corresponding high-magnification SEM images show all of them are built up with hopperlike single crystals.

RESULTS AND DISCUSSION Figure 2 shows the scanned electron microscope images of three typical resulting samples composed of KBr, (NH4)2SO4, and Na2SO3, respectively. It can be seen that the precipitates of all three samples consist of microparticles, most of which are in the size range of 10−40 μm. All the individual salt microparticles have highly spherical and hollow interiors, thus 1521

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crystal block also has one deep hopperlike pit open outside. It is notable that they do not take on a clear hexagonal shape. This is because the selected solvent can also influence the final morphology of the water-soluble salt by changing growth rates along different crystal directions, although the XRD examinations have confirmed they belong to the hexagonal crystal structure. The evolution of intermediate products of orthorhombic (NH4)2SO4 and hexagonal Na2SO3 have also been observed by SEM to verify their growth process. As seen in Figure S3 (panels b and c) of the Supporting Information, at the first stage, hollow microspheres assembled with small orthorhombic (NH4)2SO4 or hexagonal Na2SO3 blocks came to form. With time increasing, pits appeared on outside surfaces of each single crystal block. Finally, hopperlike single crystal blocks were grown to form the microspheres almost the same as those shown in Figure 2 (panels b and c). On the basis of all above observations, it can be seen that our proposed method is suitable for manipulating intrinsic growth of many water-soluble salts with different crystal systems. Furthermore, much finer manipulation would be sought to shape crystals of water-soluble salts in this work, as indicated in Figure 1. It has been found for over a century that the nucleation, growth, and morphology of crystals can be significantly altered by the presence of low concentrations of additives, which can reduce the crystal growth rate and alter morphology by binding to crystal faces and interfering with propagation steps.1d,12 In this work, much finer manipulation of intrinsic growth of watersoluble salts was carried out by combining the former diffusionlimit effect of the W/O interface with roles of additives. Taking the typical cubic NaCl as the sample, urea was introduced by injecting 2 M NaCl aqueous solution contained 0.8 M urea into the mixture of cyclohexane and acetone, to truncate corners of the cubes to form the octahedral hopperlike crystal block to assemble the microsphere-shaped NaCl hierarchical architectures. As shown in Figure 3, the resulting products are mainly dominated with hollow microspheres, ranging from 10 to 40 μm in diameter. High magnification SEM images show that these microspheres are assembled with tens of octahedral NaCl single crystals around 2 μm. Every individual crystal has one deep hole opening outside. The corresponding XRD examinations confirm the resulting products are octahedral NaCl (JCPDS 01-0993), as shown in Figure S6 of the Supporting Information. To investigate their growth processes, three samples were taken to monitor evolution of intermediate products by SEM, when the NaCl aqueous has been injected into the mixture of cyclohexane and acetone for 1, 2.5, and 10 min, respectively. In accordance with Figure 3b, at the initial stage (1 min), hollow microsphere composed of small regular NaCl octahedral came to form. With time increasing up to 2.5 min, a pit was found to appear on the outside surface of every octahedral block. Finally, the pits grew larger and deeper to form the hopperlike octahedral crystal block (10 min). In accordance with these above observations, it can be seen that much finer manipulation has been achieved on NaCl intrinsic in the cubic crystal system. First, growth toward one of their preferential growth directions has been successfully restrained. Second, their shapes have been manipulated from their typical cube into octahedrons. This can be ascribed to the

suggesting the growth of crystals at the interface between water microdroplets and the organic phase. The structure and composition of the products can be confirmed by the corresponding X-ray diffraction (XRD) measurement as seen in Figure S1 of the Supporting Information. They are cubic KBr (JCPDS, 04-0531), orthorhombic (NH4)2SO4 (JCPDS, 720456), and hexagonal Na2SO3 (JCPDS, 05-0653), respectively. In Figure 2a, cubic hopperlike single crystals of KBr with sizes of 3−5 μm appeared when one of the single microspheres was further magnified (upper inset of Figure 2a). With further magnification, it can be clearly seen that every individual crystal has one deep pit opening outside of the sphere, and the highly regular cube maintains equal dimensions and sharp edges. Formation of this kind of hierarchical architecture results from diffusion-limited effect on the metastable liquid−liquid interface between the oil phase and the aqueous droplet in the emulsion system similar to our previous works.17 Typically, as shown in Figure S2 of the Supporting Information, in the emulsion, the slow diffusion of acetone into water droplets results in the supersaturation of KBr and thus leads to the initial nucleation and subsequent growth of the KBr cubic crystals at the surface of water droplets. Meanwhile, the cyclohexane molecules contact on their (001) plane facing organic phase to prevent its growth along this direction, and thus to form a hopperlike pit. The most direct evidence for the above growth process was obtained by monitoring the evolution of intermediate products by SEM (seen in Figure S3a of the Supporting Information). During a typical preparation experiment, three samples were taken when the KBr aqueous was injected into the organic mixture for 1, 2.5, and 10 min, respectively. At the initial stage (1 min), a hollow microsphere composed of small regular KBr cubes formed. With the increase of growth time up to 2.5 min, a single pit appeared on the outside faces of every cubic block. Finally, the hopperlike single crystals were formed (10 min). It is notable that this way to manipulate intrinsic growth is generally suitable for almost all of the water-soluble salts with the cubic crystal system. Growth of cubic KCl and NaCl (Figure S4 and S5 of the Supporting Information) have also been changed to assemble the same microspheres and hopperlike single crystals. To verify the versatility of our proposed way to manipulate growth of water-soluble salts, we carried out series of experiments to assemble hopperlike single crystal of salts with other crystal systems, including orthorhombic (NH4)2SO4 and the hexagonal of Na2SO3. As shown in Figure 2b, the diameter of these resultant orthorhombic (NH4)2SO4 microsphere particles distributes from 10 to 50 μm and the mean diameter is about 20 μm, which indicates a possible size of the water droplets. The microsphere also has a hollow interior structure assembled with the orthorhombic (NH4)2SO4 single crystal through the high-resolution SEM characterizations. It can also be clearly seen that every single crystal has one deep regular cubic hole outside. Similarly, dispersing the aqueous solution of Na2SO3 as droplets in ethanol leads to the formation of hollow microspheres taking the droplets as the templates. From the SEM image in Figure 2c, Na2SO3 microspheres assembled with visible hopperlike crystals can be clearly seen. Size of these hopperlike crystals is about 300 nm in width and 1 μm in length. The diameter of these hollow spheres ranged from 4 to 20 μm, which is dependent on the size of the water droplets, depending on experimental conditions. From further SEM observations, it can clearly be seen that every Na2SO3 single 1522

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have been carried out to investigate the urea concentration influence on microstructures of NaCl. Five samples were prepared by the same typical process other than the fact that 0, 0.35, 0.5, 0.65, and 0.8 M urea were injected into the organic mixtures together with NaCl aqueous, respectively. The SEM observations were given in Figure 4, where panels a−e correspond to urea concentrations of 0, 0.35, 0.5, 0.65, and 0.8 M in the starting reaction system, respectively. It can be clearly seen that, without urea in the starting reactants, the final shape of blocks in the NaCl microsphere hierarchical architectures is a cubic hopperlike single crystal, which is similar to our previous work on hopperlike NaCl crystals (seen in Figure 4a). When 0.35 M urea was introduced into the starting reactants, as shown in Figure 4b, all the eight corners of the basic cubes were cut and remain a deep pit on each of their outside surface. As shown in Figure 4 (panels c and d), it is clear that corners of the basic cubic were truncated deeper and deeper together with increasing urea concentrations from 0.5 to 0.65 M. Until the urea was increased up to 0.8 M, all the corners of the cubes were cut away and eight {111} planes are shown to form the final octahedrons (seen in Figure 4e). Equally, growth on the outside (111) plane was prevented to form the octahedral hopperlike NaCl single crystal due to the diffusion-limit effect on the W/O interface. The SEM images indicate that all these polyhedral are single crystalline and exhibit atomically defined facets with sharp edges and corners. In this case, the concentration of the urea is a key factor influencing the formation of the microstructure. It is commonly accepted that the geometrical shape of an fcc nanocrystal is mainly determined by the ratio (R) of the growth rate along the [100] versus that along the [111] direction.18 As R increases, the shape of a cubic crystal has been reported to evolve from a perfect cube (R = 0.58), a truncated octahedron (0.87 < R r{100} > r{111}. However, in this work, the surface energies have been modulated through the selective adsorption of urea on {111} crystal planes, which reduces the growth rate of the crystals along a [111] direction, while the growth along the [100] direction is accelerated, facilitating the formation of octahedral crystals.

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CONCLUSION In summary, series of water-soluble salt micro/nano-hierarchical architectures assembled with different hopperlike single crystal blocks have been prepared to illustrate the fine manipulation of intrinsic growth of water-soluble salts belonging to a different crystal system. Growth of the hollow microsphere hierarchical architectures could be due to super saturation driving crystallization along interfaces between the oil phase and aqueous droplets in the microemulsion. Formation of the hopperlike structures results from the diffusion-limit effect on the interface between aqueous and organic mixtures. Growth on different crystallographic planes has been finely manipulated by introducing proper additives absorbed onto certain planes to control their surface energies and the corresponding growth rate along different directions. In combination with the above-mentioned diffusion-limit effect and roles of additives, many fine manipulations of intrinsic growth of water-soluble salts belonging to different crystal systems have been achieved, which will favor us to deeply understand crystallography of water-soluble salts, as well as facilitating design and production of water-soluble salts architectures with corresponding shapes, according to the different requirements.

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ASSOCIATED CONTENT

S Supporting Information *

Schematic, X-ray diffraction patterns, and SEM images of the as-synthesized samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. J.Z. and Z.Z. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No.51202253, 51002159).

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REFERENCES

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