Supernet Structures of Calcium Carbonate Mesocrystals Formed in a

Oct 19, 2009 - Recently, mesocrystal has emerged as a special case of colloi- dal crystals that have a .... resolution transmission electron microscop...
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DOI: 10.1021/cg900584s

Supernet Structures of Calcium Carbonate Mesocrystals Formed in a Blend System of Anionic/Nonionic Surfactants

2009, Vol. 9 4720–4724

Chao You,† Qiang Zhang,*,‡ Qingze Jiao,† and Zhanda Fu† ‡

Department of Chemistry, College of Science, Beijing Institute of Technology, Beijing 100081, China, and †College of Chemistry and Environmental Science, Beijing Institute of Technology, Beijing 100081, China Received May 31, 2009; Revised Manuscript Received August 20, 2009

ABSTRACT: A new kind of vaterite calcium carbonate mesocrystal with a self-organized supernet structure was successfully achieved in the presence of polyoxyethylene (20) sorbitan monolaurate (Tween 20) and sodium dodecyl sulfate (SDS) using the liquid-diffusion method. A well-organized two-dimensional network structure is built up successfully with a mass of submicrometer fanlike single crystal units. Comparative experiments showed that the mixed system of Tween 20 and SDS played important roles in the morphological control of CaCO3. SDS and Tween 20 can form complicated reticular micelles in aqueous solution, and the adsorption of SDS molecules onto a certain crystal face leads to the generation of the least stable vaterite. X-ray powder diffraction (XRD), FT-IR spectrometer, transmission electron microscopy (TEM), selected area electron diffraction (SAED), and field-emission scanning electron microscope (FE-SEM) equipped with energy-dispersive X-ray (EDX) were used to characterize the crystals.

Introduction Biologically mineralized materials have gained lots of attention in recent years because of their unusual properties arising from their complex shape, hierarchical organization, and various polymorphs of the constituent minerals.1 Recently, mesocrystal has emerged as a special case of colloidal crystals that have a common crystallographic register, which makes a mesocrystal scatter like a single crystal.2 Mesocrystals are oriented superstructures formed from spherical or nonspherical nanocrystals, and the anisotropy of the nanobuilding units offers new possibilities forming superstructure mesocrystals.3 Calcium carbonate is one of the most abundant biological minerals and has many important applications in industry, such as pigments, papermaking, plastics, and so on. It can be produced by a wide variety of biological organisms with exquisite control over some even sized, tailored shaped, crystalline polymorphs and highly ordered composite structures of the crystals.4 As crystal nucleation and growth are highly sensitive processes, artificial synthesis of calcium carbonate crystalline material is usually controlled by various additives as templates or matrices,5 such as metallic ions,6 surfactants,7 hydrogel spheres,8 self-assembled monolayers (SAMs),9 cholesterols,10 proteins,11 and organic polymers.12 These additives play a critical role in forming calcium carbonate crystals with unique hierarchical morphologies. For example, stacks of pancakelike morphologies were obtained in the presence of a certain rigid hexacyclen block.5b Calcite superstructures with two different platonic shapes and minimal surfaces have been synthesized in the presence of poly(4-styrenesulfonate-comaleic acid) (PSS-co-MA).13 Hexagonal prism vaterite single crystals were obtained by a simple and unified method of lime-cured gelatin controlled crystallization.14 Aragonite *To whom correspondence should be addressed. E-mail: zhangqiang6299@ bit.edu.cn. pubs.acs.org/crystal

Published on Web 10/19/2009

needle-like agglomerates were precipitated from aqueous solution containing proteins. Even though many studies on the various novel calcium carbonate crystals in the presence of numerous crystal modifiers have been carried out, and much progress has been made toward determining the hierarchical structures of calcium carbonate minerals as well as establishing structure-function relationships for them, to the best of our knowledge, little work has been done with a focus on the metastable vaterite crystal with complex geometrical superstructures, especially through the mesocrystal formation processes. In this research, a new kind of vaterite calcium carbonate crystal with a self-organized supernet structure was synthesized, which has neither been found in natural biominerals nor been reported as synthetic crystals before. The material consists of sub-micrometer fanlike vaterite single crystals. Its fabrication has been carried out in a blended system of polyoxyethylene (20) sorbitan monolaurate (Tween 20) and sodium dodecyl sulfate (SDS) using the liquid-diffusion method. This is a new synthetic approach for the fabrication of calcium carbonate by controlling the crystal growth with mixed surfactants as oriented additives, which is favorable for forming amazing hierarchical morphologies because of the slow growth speed by diffusion of Ca2þ and CO32- similar to the gas diffusion method. Experimental Section Materials. Polyoxyethylene (20) sorbitan monolaurate and SDS, sodium carbonate, and calcium chloride were obtained from the Chemical Reagent Company of Beijing. All chemical reagents obtained were used without further purification. All glassware was cleaned and sonicated in ethanol for 10 min, then rinsed with doubly distilled water, and finally dried in air. CaCO3 Growth. The crystallization of CaCO3 was carried out in a closed system made of a big beaker and a small one at room temperature (25 °C). First, SDS and Tween 20 were put into a threenecked flask, and then doubly deionized water was added to make the total volume 50 mL. The aqueous solution was continuously r 2009 American Chemical Society

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Figure 1. Schematic representation of the liquid diffusion experimental setup. stirred at a constant rate of 200 rpm by a Teflon-coated magnetic stirring bar at room temperature. After 30 min, a 20 mL solution of calcium chloride (0.1 M) was added to the flask, and it was continuously stirred for at least 30 min with electromagnetic stirring (200 rpm). Second, 0.002 mol of sodium carbonate solid was placed into a big beaker with a volume of 250 mL, and after a 50 mL beaker filled with the above solution was put in the center of the big beaker, and then doubly deionized water was slowly added into the big beaker until it just submerged the border of the small beaker. Finally, the big beaker was covered with parafilm. When the sodium carbonate dissolved, CO32- diffused from the bottom to the top of the big beaker, whereas the Ca2þ moved from the top of the small beaker to the bottom of the big beaker. The crystallization of CaCO3 took place during the slow diffusion. After crystallization for 7 days, the crystals were obtained, briefly washed with doubly distilled water, and allowed to dry. Schematic representation of the experimental setup is shown in Figure 1. Characterization. The as-prepared crystals were characterized by X-ray powder diffraction (XRD) using a SHIMADZU-6000 X-ray diffractometer with a Cu KR radiation (λ = 1.54056 A˚). Infrared spectra were collected by using a Nicolet Impact 400 FT-IR spectrometer on KBr pellets. Scanning electron microscopy (SEM) analysis equipped with energy-dispersive X-ray (EDX) was performed on a field emission SEM microscope (HITACHI S-4800). Transmission electron microscope (TEM) imaging, selected area electron diffraction (SAED) patterns and high-resolution transmission electron microscopy (HRTEM) were performed on a JEOL-2010 highresolution transmission electron microscope at an accelerating voltage of 200 kV.

Results and Discussion Typical SEM images of the hierarchical self-organized supernet structure of calcium carbonate crystal obtained in the presence of [SDS] =2 g 3 L-1 and [Tween 20]=2 g 3 L-1, [Ca2þ]=10 mM, at 25 °C for 7 days are shown in Figure 2. From A to C, it can be observed that the morphologies of the crystal are quite unique and which have not been reported before. The two-dimensional superstructures consist of orientated fanlike crystal units with sizes ranging from 50 to 60 μm, and thicknesses ranging from 500 nm to 1 μm. An intact and typical fanlike crystal unit and a beautiful structure like an angel’s wing consisting of one intact and two half fanlike crystal units are shown in Figure 2, panels A and B, respectively. Another kind of structure like an old-style aeroplane consisting of three quite intact interlaced fans is displayed in Figure 2C. The SEM images under lower magnification (5.0 K and 1.0 K, respectively) are observed in Figure 2, panels D and E. The well self-organized two-dimensional network structure is clearly composed of a great deal of fanlike crystal units.

Figure 2. FESEM images of two-dimensional superstructures obtained in the presence of [SDS]=2 g 3 L-1, [Tween 20]=2 g 3 L-1, and [Ca2þ]=10 mM, at 25 °C for 7 days.

The X-ray diffraction pattern (Figure 3A) shows the presence of pure vaterite. The broadening features of the sample implied that the crystals could be composed of primary nanoparticles with a size of about ca. 40 nm with the Scherrer formula. The results suggested that the CaCO3 superstructure is so-called mesocrystals assembled from nanobuilding units. Generally, calcium carbonate exhibits one of three anhydrous crystalline polymorphs, that is, calcite, aragonite, and vaterite depending on the mineralization conditions and external environments. Thermodynamically, calcite is the most stable form and vaterite is the least stable one.15 It is wellknown that organic matrix plays a key role in the course of

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Figure 3. XRD patterns of the CaCO3 crystals obtained by the liquid diffusion method, [Ca2þ]=10 mM, at 25 °C for 7 days: (A) [SDS]=2 g 3 L-1; [Tween 20]=2 g 3 L-1; (B) [SDS]=2 g 3 L-1; (C) [Tween 20]=2 g 3 L-1.

Figure 4. TEM images and HRTEM images of the two-dimensional superstructures obtained in the presence of [SDS]=2 g 3 L-1, [Tween 20]=2 g 3 L-1, and [Ca2þ]=10 mM, at 25 °C for 7 days: (A) TEM image of the fan-like units; (B) HRTEM image; (C) HRTEM image (right corner inset is the electron diffraction pattern).

inorganic compounds deposition by lowering the activation energy of nucleation of specific crystal faces and controls the

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Figure 5. FESEM images of calcium carbonate crystals collected in the presence of [Ca2þ]=10 mM, at 25 °C for 7 days: (A) [Tween 20]= 2 g 3 L-1; (B) [SDS]=2 g 3 L-1.

polymorph through interfacial recognition. In addition, organic anions can also have a marked kinetic effect on crystallization, particularly on polymorph selectivity. These interactions can be highly specific.16,3a In light of this, we suggest that the interaction between the mixed surfactants and calcium ions would presumably induce the formation of less stable polymorphs,17 namely, the electrostatic interaction between SO32- of SDS and Ca2þ of a certain crystal plane can boost and stabilize the vaterite nucleation.1a To further analyze the more detailed morphology and structure of CaCO3 mesocrystals formed in SDS/Tween 20 mixed aqueous solution, TEM, HRTEM, and SAED were performed. Parts of two fanlike units are shown in Figure 4A and Figure 4B confirms that the fanlike crystals are built of slightly textured nanosized vaterite monocrystals with a size of about 30-40 nm, which is consistent with the size determined from the XRD results. Figure 4C shows the typical HRTEM image of the crystals. Lattice fringes with spacing of 0.376 nm is observed, corresponding to an interplanar distance of the (112) plane. The SAED image exhibits the regular diffraction pattern (inset in Figure 4C), which can be indexed as a pure orthorhombic vaterite single crystal, in good agreement with the XRD result presented above.2b It is significant to identify the interaction between mixed surfactants and calcium ions that control crystallization of calcium carbonate, in order to further apply this method to design and synthesize new materials rationally. In comparison, in the presence of only Tween 20, regular rhombohedra and partial irregular polyhedron CaCO3 crystals were obtained, which accumulated together in a cluster (Figure 5A). While only SDS was used as a surfactant, detached fanlike crystals were obtained, and no obvious network structures were observed (Figure 5B). XRD patterns of CaCO3 crystals obtained from different surfactants are shown in Figure 3. It can be seen that the crystals can be indexed as pure vaterite phase in the presence of SDS (Figure 3B), whereas the crystals obtained in the presence of Tween 20 are pure calcite phase (Figure 3C). These results indicate that the copresence of SDS and Tween 20 segments is necessary for the formation of network matrices. The cooperative effect between SDS and Tween

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Figure 6. FTIR spectrum of the two-dimensional superstructures obtained in the presence of [SDS]=2 g 3 L-1, [Tween 20]=2 g 3 L-1, and [Ca2þ]=10 mM, at 25 °C for 7 days.

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for calcium carbonate nucleation, due to their electrostatic interaction with calcium ions. The growth of calcium carbonate starts from multiple nucleation sites on the surface of the micelles.7a The surfactants have not been found in the ultimate products as demonstrated by FTIR analysis (Figure 6) and EDX (Figure 7). The characteristic peaks of calcium carbonate (at 1465, 1088, 875, and 746 cm-1) in the FTIR spectrum of the superstructures proved that the synthesized compounds were vaterite, without characteristic bands of the organic functional group in the mixed surfactants, such as ester CdO, SdO and ester C-O-C. At the same time, the EDX spectrum also reveals the presence of three elements: calcium, oxygen, and carbon, indicating that SDS and Tween 20 were not present in the synthesized superstructures. On the basis of the above results, we conclude that the surfactants just provide a complicated netty framework in the initial stage for the crystallization and play a role as a template in forming the calcium carbonate crystals. When carbonates approach the calcium ions, as the electrostatic interaction between them is stronger than the interaction between surfactants and calcium ions, the interaction between surfactants and calcium ions becomes weaker and weaker until they are thoroughly separated, so pure vaterite crystal is obtained without any surfactant which has been removed during the washing processes. Conclusion

Figure 7. Energy dispersive X-ray (EDX) spectrum of the twodimensional superstructures obtained in the presence of [SDS]=2 g 3 L-1, [Tween 20]=2 g 3 L-1, and [Ca2þ]=10 mM, at 25 °C for 7 days.

20 plays a vital role in the formation process of the twodimensional network structures. The SDS is responsible for producing fanlike crystal units to a great extent and the Tween 20 can link them together so as to build the supernet structures. On the basis of the above results, we can assume that a complicated netty micelle is formed by the anionic surfactant SDS and the nonionic surfactant Tween 20 in the aqueous solution. There are two opposing forces in this micelle system that control the association process: one is the hydrophobic effect of the hydrocarbon group, which pulls SDS and Tween 20 molecules out of the aqueous environment, and another is hydrophilic effect of sulfonic headgroup offered by SDS molecules. On one hand, the molecules of the Tween 20 insert into the molecules of SDS and partially shield the electrostatic repulsion between the head groups of the SDS. In other words, the electric charge density on the surface of the micelle is decreased. On the other hand, the hydrophobic effects are enhanced due to the interactions among hydrocarbon-hydrocarbon chains of the two surfactants. Together, these two interactions determine the formation of the network micelle. At the same time, SDS polar groups act as active sites

In summary, distinct two-dimensional reticular morphologies of vaterite mesocrystals have been synthesized in the presence of anionic/nonionic surfactants as crystal modifiers. Such self-organized hierarchical structure formation in the micrometre scale is unusual. A well-organized two-dimensional network structure is built successfully with a mass of sub-micrometer fanlike single crystal units. We propose that SDS and Tween 20 form complicated netty micelles in the aqueous solution, and the interaction between mixed surfactants and calcium ions plays a critical role in organizing calcium carbonate on the microscopic level. In addition, the adsorption of SDS molecules onto a certain crystal face leads to the generation of the least stable vaterite. Because of its larger surface area and special netty structure, the new synthetic calcium carbonate material may have a potential application in some areas. The use of various surfactants in biomimetic materials by the liquid diffusion method offers new insights into controlling the structure and morphology under easily attainable reaction conditions.18 Acknowledgment. This work was supported by Beijing Natural Science Foundation (No. 2082021) and Basic Research Foundation of BIT.

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