Complex Spherical BaCO3 Superstructures Self-Assembled by a Facile Mineralization Process under Control of Simple Polyelectrolytes Shu-Hong Yu,*,† Helmut Co¨lfen,‡ An-Wu Xu,§ and Wenfei Dong| Structure Research Laboratory of CAS and Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, People’s Republic of China, Department of Colloid Chemistry and Interface Department, Max Planck Institute of Colloids and Interfaces, MPI Research Campus Golm, D-14424, Potsdam, Germany, and School of Chemistry and Chemical Engineering, Zhongshan University, Guangzhou 510275, China Received June 6, 2003;
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Revised Manuscript Received July 27, 2003
ABSTRACT: Complex BaCO3 superstructures can be easily generated by using poly(sodium 4-styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) as structure directing agents in the mineralization process. More complex macroporous superstructures could be generated by cooperative templating effects of the molecular template and a foreign static template such as air bubbles. It has been an intensive focus to synthesize higher ordered inorganic crystals or hybrid inorganic/organic materials with a specific size, shape, orientation, organization, complex form, and hierarchy1-9 owing to the importance and potential to design new materials and devices in various fields such as catalysis, medicine, electronics, ceramics, pigments, and cosmetics. Recently, bioinspired morphosynthesis strategies have been emerging as an important tool for crystallization, taking advantage of using self-assembled organic superstructures to template inorganic materials with controlled morphologies and hierarchy.1 Metal carbonates, especially calcium carbonate (CaCO3), were chosen as one of the standard model systems due to their abundance in nature and also their important industrial application in paints, plastics, rubber, or paper.10 Biomimetic synthesis of biominerals such as CaCO3 crystals in the presence of organic templates and/or additives has been intensively investigated in recent years as reviewed recently.11 Langmuir monolayers,12 ultrathin organic films,13 self-assembled films,14 foam lamellae,15 cross-linked gelatin films,16,17 polymer substrates,10 crystalimprinted polymer surfaces,18 and polymeric matrixes19,20 have been used as effective templates or were employed to direct the controlled growth of CaCO3 crystals. It has been shown that special functional low molecular weight and polymeric additives can influence the crystallization of CaCO3 strongly,21 including complex liquidlike morphologies22 or stabilized amorphous CaCO3.23 Similarly, barium carbonate exists in nature as a thermodynamically most stable crystal modification among the heavy metal carbonates (ACO3, A ) Sr, Ba, Pb) even though the orthorhombic phase is metastable in the case of calcium carbonate.24 BaCO3 has also attracted a lot of recent research25-28 due to its close relationship with aragonite, a prevalent and important biomineral, with many important applications in the ceramic and glass industries as well as its use as a precursor for magnetic ferrites and/or ferroelectric materials.29 Usually, organic additives and/or templates with complex functionalization patterns are used to control the nucle* Corresponding author. E-mail:
[email protected]. Fax: 0086 551 3603040. † University of Science and Technology of China. ‡ Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces. | Interface Department, Max Planck Institute of Colloids and Interfaces. § Zhongshan University.
ation, growth, and alignment of inorganic crystals. Recently, it was shown that double hydrophilic block copolymers can exert significant effects on the crystal growth of inorganic and organic crystals.30 These functional polymers can adopt different functions ranging from the temporary stabilization of precursor phases to direct crystal growth via face selective adsorption, as demonstrated in our recent report about the morphosynthesis of various metal carbonates.31 In addition, highly ordered funnel-like BaCrO432 and BaSO4 superstructures with a remarkable self-similar growth pattern as well as fiber bundles with repetitive growth patterns30c,32 can be readily generated by using sodium polyacrylate as a structure directing agent. Although it was found that the double hydrophilic block copolymers were much more effective additives for crystal growth control, their functional low molecular weight homopolymer analogues without the stabilizing water soluble second block also proved effective in a limited window of experimental conditions30c,32b so that it is interesting to explore if the much easier available polyelectrolyte homopolymers can be used as structure directing additives. Recently, crystallization of spherical CaCO3 particles in the presence of poly(sodium 4-styrenesulfonate) (PSS) has been reported by Jada et al.32b Herein, we report that low molecular weight polyelectrolytes can be used for the self-assembly of complex spherical BaCO3 superstructures through a facile mineralization process under ambient conditions. The macroporous spherulites can also be generated by introducing air bubbles into the initial mineralization solution, thus acting as a foreign static template. All chemicals were obtained from Aldrich and used without further purification. Analytical-grade ammonia carbonate and BaCl2 were used as received. The polycation, poly(allylamine hydrochloride) (PAH), Mw 15 000 g mol-1, and the polyanion, poly(sodium 4-styrenesulfonate) (PSS), Mw 70 000 g mol-1, were obtained from Aldrich. All glassware (glass bottles and small pieces of glass substrates) was cleaned and sonicated in ethanol for 5 min, then rinsed with distilled water (18 ΩM cm-1), further soaked with a H2O-HNO3 (65%)-H2O2 (1:1:1, v/v/v) solution, then rinsed with doubly distilled H2O, and finally dried with acetone. The precipitation of BaCO3 was carried out in glass bottles with a volume of 5-15 mL, which were put into a closed desiccator at room temperature (22 ( 3 °C). Stock aqueous BaCl2 solution (10 mM) was freshly prepared in
10.1021/cg0340906 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/14/2003
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boiled doubly distilled water and bubbled with N2 overnight before use. The amount of 30 mg of polymer was added into 30 mL of 10 mM BaCl2 under vigorous stirring to ensure complete polymer dissolution to generate a 10 mM BaCl2 solution containing 1 g L-1 polymer for further crystallization experiments. After that, equal volume (10 mL) solutions were added into the glass bottles. The bottles were then covered with Parafilm, which was punched with three needle holes, and placed in a larger desiccator after glass coverslips were placed in the bottles to collect the crystals. Two small glass bottles (10 mL) of crushed ammonium carbonate were also covered with Parafilm punched with three needle holes and placed at the bottom of the desiccator. After different periods of reaction time from several hours to two weeks, the Parafilm was removed, and the precipitate was rinsed with distilled water and ethanol and allowed to dry at room temperature. The time dependent crystallization experiments were done by taking out the small pieces of glass substrates from the bottles to stop the reaction for examination. The initial pH of the solution was about 5. For producing the macroporous spheres, a similar preparation method was applied except that the starting solution containing the polymer was shaken vigorously by hand for 3 min. The precipitates were collected and washed with distilled water and dried in air for further characterization. The small pieces of coverslips were examined by optical microscopy and then gold coated for scanning electron microscopy (SEM). Element analysis was conducted by energydispersive spectra (EDS) on a DSM 940 A (Carl Zeiss, Jena) microscope. Powder X-ray diffraction (XRD) patterns were recorded on a PDS 120 diffractometer (Nonius GmbH, Solingen) with CuKR radiation. Dendritic structures BaCO3 (Witherite, JCPDS 05-0378, orthorhombic structure with cell constants a ) 5.315, b ) 8.904, c ) 6.433), which is isostructural with the mineral aragonite, were observed in the control experiments without adding polymer additives through either rapid mixing or slow gas diffusion experiments (Figure 1a). EDS analysis on this sample gives a stoichiometric molar ratio for BaCO3. However, mineralization of the solution of BaCl2 in the presence of PSS produced well-defined spherical BaCO3 microspheres with size of 2-3.5 µm (Figure 1b,c). The enlarged SEM image in Figure 1d shows that the spheres were built from smaller, elongated rodlike building blocks with a typical diameter of 50 nm and length of 200 nm, which apparently adopted the more equilibrated isostructural aragonite appearance. The formation of such complex superstructures may arise from a mesoscale self-assembly process in the present system. On the basis of our prior studies on BaSO430c or CaCO334 using double hydrophilic block copolymers, where an amorphous precursor was observed via TEM as well as on the recent understanding that amorphous precursor phases play an important role in mesoscale transformation and assembly processes,1a,33 we believe that polyelectrolyte stabilized amorphous nanoparticles act as the precursor for the follow-up mesoscale self-assembly. Because of the surface energy of the amorphous particles, the formation of a spherical aggregate is likely as it maintains minimum surface exposition. The addition of polyelectrolytes with strong inhibition ability will stabilize the amorphous nanobuilding blocks in the early stage and then stimulate a mesoscale transformation.1a,30c,34 This process should then lead to the observed mesostructured particles (Figure 1), which are self-assembled from tiny building blocks through a mesoscale transformation and crystallization process of the amorphous precursor particles as reviewed recently.1a Such complex morphology
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Figure 1. SEM images of BaCO3 particles obtained by a mineralization reaction for two weeks at room temperature. pH ) 5, [BaCl2] ) 10 mM. (a) Without additive; (b-d) in the presence of 1 g L-1 PSS. (d) The detailed surface structure of the spheres.
was observed in a wide range of initial pH values, suggesting that the mineralization reaction in this system was self-regulated for the precipitation of BaCO3 through the release of NH3 and CO2 from the decomposition of ammonium carbonate, which will dissolve in solution and further regulate the pH value throughout the reaction. It has to be noted that we cannot expect an at least temporary stabilization of primary particles, which can be obtained with double hydrophilic block copolymers, but have to expect a cross-linking of the primary particles due to bridging, as the PSS molar mass is in the order of magnitude of commercial scale inhibitors of the acrylic acidco-maleic acid type. The above structures shown in Figure 1b,c could be further modified into spheres containing holes on their surface by shaking the solution before mineralization. Many air bubbles were observed by vigorously shaking the initial solution after adding the PSS. After a mineralization
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Figure 2. (a,b) SEM images of BaCO3 spheres containing holes on the surfaces obtained by a mineralization reaction for two weeks at room temperature with shaking the solution before crystallization in the presence of 1 g L-1 PSS. pH ) 5, [BaCl2] ) 10 mM.
Scheme 1 a
a (a) Initial polymer solution. (b) Nucleation and aggregation of amorphous nanoparticles. (c) Particle crystallization within the aggregates. (d) Particle crystallization within the aggregates in the presence of foreign static templates such as air bubbles.
reaction for two weeks, spherical BaCO3 particles with textured surface structures were observed as shown in Figure 2, which are totally different from that without shaking (Figure 1b,c). The air bubbles generated by shaking acted as static templates accompanying the crystallization reaction, which is similar to that found in the microemulsion system reported by Mann et al.35 Rudloff et al. have demonstrated that complex CaCO3 morphologies can be obtained from block copolymers, which temporarily stabilized nanoparticle aggregates at the air/solution interface by CO2 evaporation.36 In the present case, the external template (air bubbles) was again combined with the molecular template effect to exert obvious cooperative effects on the architecture control of the inorganic crystals. It is remarkable that the BaCO3 structures obtained after shaking are less textured with respect to the nanocrystalline substructures than those obtained without shaking (Figure 1b,c). Apparently, the included air bubbles induce a fusion of the amorphous precursor particles around the bubbles. The structure formation of the particles are summarized in Scheme 1. In contrast to the previously reported negatively charged PSS additive, a different morphology was observed in the presence of PAH with a functional amino group, which is protonated at the applied acidic starting pH. Nearly spherical aggregates with a relatively smooth surface structure were observed as shown in Figure 3a. Figure 3b shows that the spherical particles consist of major subunits, which appear to have grown together. Interestingly, the intermediate particles with the dumbbell shape prior to sphere formation were observed (Figure 3c). Such dumbbell shaped particles were also observed as intermediates of spherical CaCO3 particles, which were obtained in the presence of double hydrophilic block copolymers.34 Figure 3d shows that the particle surface is composed of tiny
Figure 3. SEM images of BaCO3 particles obtained by a mineralization reaction at room temperature in the presence of 1 g L-1 PAH. pH ) 5, [BaCl2] ) 10 mM. (a and b) For two weeks and (c) 5 days. (d) The surface structure of a typical dumbbell (inset).
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Communications Scheme 2 a
a (a) Initial polymer solution. (b) Nucleation and aggregation of amorphous nanoparticles. (c) Crystallization of an elongated crystal. (d and e) Branching at the ends of the primary rod and formation of dumbbell intermediates based on a mechanism suggested by Busch and Kniep.37 (f) Final spherical superstructure.
nanoparticles with a size of about several nanometers instead of the larger rodlike subunits observed in the case of PSS, suggesting a strong inhibition of the amino group on the crystal growth of BaCO3. The presence of dumbbells as intermediates for sphere formation under the control of PAH is consistent with our previous observations of various metal carbonate systems using double hydrophilic block copolymers as crystal modifiers in solution31 and is also consistent with that reported by Kniep et al. for the growth of fluoroapatite37 or by Lo¨bmann et al. for calcium carbonate38 in gelatin gels instead of free solution, so that the rod-dumbbell-sphere morphogenesis mechanism appears to be rather universal. However, it has to be pointed out that the exact growth mechanism is still unknown, although some explanation was given in the literature based on the role of intrinsic electric fields, which direct the growth of dipole crystals.37,39 The particle formation process in the presence of PAH is summarized in Scheme 2. In contrast to the above PSS case, the amorphous particles do not seem to crystallize within the proposed aggregates (Scheme 2b) but instead, an elongated primary particle seems to form, possibly by a dissolution-recrystallization process (Scheme 2c), which branches at both tips to form the observed dumbbell structures (Scheme 2d,e) leading to the finally observed spheres (Scheme 2f). Thus, the role of the polyelectrolytes appears to be fundamentally different. Whereas the anionic PSS seems to promote crystallization of amorphous precursor particles within a spherical aggregate involving particle bridging by the polymer, the cationic PAH does not seem to bridge the primary particles so that crystallization of an elongated particle is favored. Whether or not the PAH selectively adsorbs on some crystal faces as shown in Scheme 2 cannot be elucidated, as already in the default experiment without additives, the particles are elongated and branched (Figure 1a). In summary, we demonstrated that simple polyelectrolytes, such as PSS and PAH, can be used as crystal growth modifiers to template unusual complex morphologies of BaCO3 crystals. Well-defined spheres with unusual surface structures or composed of densely packed nanoparticles were assembled through a mesoscale transformation and crystallization process. By introducing air bubbles as heterogeneous templates into the mineralization system, the structures can be further modified into hole-containing spheres, suggesting that more complex superstructures could be generated by cooperative templating effects of both molecular template guided mesoscale transformation
processes and a foreign static template. Using these simple polyelectrolytes or a combination with other static templates, research is being further extended for the morphogenesis of other minerals with complex superstructures. Acknowledgment. S.-H.Y. acknowledges the special funding support from the Century Program of the Chinese Academy of Sciences and the National Natural Science Foundation of China. H.C. thanks the Max Planck Society for financial support.
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