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Biomimetic synthesis of hollow microspheres of calcium carbonate crystals in the presence of polymer and surfactant
Colloids and Surfaces A: Physicochem. Eng. Aspects 393 (2012) 139–143
Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Biomimetic synthesis of hollow microspheres of calcium carbonate crystals in the presence of polymer and surfactant Lina Zhao a,b,∗ , Jiku Wang a,b a b
College of Chemistry, Jilin Normal University, Siping 136000, China Key Laboratory of Preparation and Applications of Environmental Friendly Materials, Jilin Normal University, Ministry of Education, Siping 136000, China
a r t i c l e
i n f o
Article history: Received 18 September 2011 Received in revised form 10 November 2011 Accepted 11 November 2011 Available online 22 November 2011 Keywords: Hollow microspheres Calcium carbonate Crystallization Calcite
a b s t r a c t Hollow CaCO3 microspheres were successfully synthesized using sodium carbonate and calcium chloride through a precipitation reaction method at room temperature. Polyvinylpyrrolidone (PVP), together with sodium dodecyl sulfonate (SDS), was employed as template for the controlled growth of hollow CaCO3 microspheres. The concentration of SDS was an important factor to control the synthesis of hollow CaCO3 microspheres. It suggested that the PVP–SDS complex micelles played a key role in controlling the growth of biominerals X-ray diffraction (XRD), FESEM and TEM confirmed that hollow CaCO3 microspheres consisting of calcite crystals were synthesized In the present work, we also proposed a hypothetical mechanism for the formation process of hollow CaCO3 microspheres. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Modern technologies require innovation approaches for complex materials with excellent properties. It is well-known that biomineralization is an elaborate process that produces biominerals with complex and fascinating morphologies as well as superior mechanical properties [1–3]. The controlled synthesis of inorganic materials of specific size and morphology is a key aspect in fields as diverse as modern materials, catalysis, medicine, electronics, ceramics, pigments and cosmetics [4,5]. Hollow CaCO3 particles with nanometer to micrometer dimensions represent an important class of materials because their unique structural, optical and surface properties may lead them to a wide range of application [6–8]. The strategy of using organic templates or modifiers with complex functional patterns to control the nucleation, growth, and alignment of inorganic crystals has been widely adopted for the biomimetic synthesis of inorganic materials with complex form [9,10]. A number of studies have been carried out to elucidate the effect of various organic molecules on the crystallization of inorganic crystals [11–13]. As widely and efficiently used processing additives in industrial end products, the various surfactants aroused the particular interest in the crystallization process of inorganic species [14,15]. The use of polymer–surfactant
∗ Corresponding author at: College of Chemistry, Jilin Normal University, Siping 136000, China. Tel.: +86 434 3292154. E-mail address: [email protected] (L. Zhao). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.11.012
supramolecular systems for the control of inorganic crystallization has become an area of intense research activity in recent years [16–19]. Although the influence of uncharged polymers [20–22], anionic surfactants, or the polymer–surfactant mixtures [23–25] on the crystallization process of inorganic crystals have been investigated, nevertheless, the aqueous systems containing polymer–surfactant mixtures have not been used for the research of crystallization and aggregation behavior of calcium carbonate [26]. In this work, hollow CaCO3 microspheres were successfully synthesized using sodium carbonate and calcium chloride through precipitation reaction at room temperature. Polyvinylpyrrolidone (PVP) and sodium dodecyl sulfonate (SDS) were employed together as template for the controlled growth of hollow CaCO3 microspheres. The effect of the concentration of SDS on the crystallization and aggregation of CaCO3 was investigated and discussed. The PVP–SDS complex micelles played a key role in controlling the growth of biominerals. 2. Experiment 2.1. Chemicals Polyvinylpyrrolidone (PVP) and sodium dodecyl sulfonate (SDS) were obtained from Beijing Chemical Factory (China). Na2 CO3 and CaCl2 ·2H2 O were brought from Shenyang Chemical Reagent Factory. All chemicals were of analytical grade and used without further purification. The water used in this work was distilled water made in our laboratory.
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Fig. 1. FESEM and TEM images of hollow CaCO3 microspheres.
2.2. Preparation 0.1 M of Na2 CO3 solution with 10 g/L of PVP and SDS, and 0.1 M of CaCl2 solution with certain concentration of SDS were prepared beforehand. Then, CaCO3 crystals were precipitated by quickly pouring 50 mL of the CaCl2 mixture into a three-necked flask with an equal volume of the Na2 CO3 mixture. The precipitation reaction was carried out at 20 ◦ C under continuous stirring of 300 rpm for 1 h. Subsequently, the precipitate was filtered and rinsed three times with distilled water. Finally, the product was dried in an oven at 80 ◦ C for at least 24 h and used for characterization. 2.3. Characterization Micrographs of the hollow CaCO3 microspheres were taken by a field emission scanning electron microscope (FESEM). It was carried out using a JEOL JSM-6700F at 15 kV. Transmission electron microscopy (TEM) images were characterized by JEOL-2100H electron microscope. The crystalline phase and structure of the synthesized CaCO3 were detected by X-ray diffraction (XRD) analysis with a Shimadju D/max-rA X-ray diffractometer with Cu K␣ radiation, under the accelerating voltage 40 kV, current 30 mA, and scanning rate 4◦ min−1 . Fourier transform infrared spectroscopy (FT-IR) was performed on FTIR-8400S using KBr pellets.
3. Results and discussion
impurity peaks were detected, indicating that the powders had high purity [27]. 3.3. FT-IR characterization of the synthesized CaCO3 crystals Fig. 3 shows the FT-IR spectrum of the CaCO3 particles. The characteristic absorption bands observed at 715 cm−1 , 873 cm−1 and 1462 cm−1 confirmed the formation of calcite, which were corresponding to the 4 , 2 and 3 CO3 2− absorption bands of calcite [28]. Weak bands were also observed at 2928 cm−1 and 2869 cm−1 that are associated with asymmetric and symmetric methyl and ethylene C–H stretches, respectively [29]. This suggests that surfactant molecules remained strongly associated with the calcite crystals even after the extensive washing process. 3.4. Influence of different temperature on the crystallization of CaCO3 particles To research the influence of different temperature on the crystallization of CaCO3 particles, experiments were carried out at 20 ◦ C, 40 ◦ C, 60 ◦ C and 80 ◦ C, respectively, keeping the other conditions invariable. Fig. 4 shows the FESEM images of hollow CaCO3 microspheres. It exhibited that the CaCO3 microspheres could be synthesized at 20 ◦ C. From Fig. 4(a), the well-defined hollow spheres with rough outer surface were observed. The size of the spheres ranged from 2 to 3 m. When the temperature was as high as 40 ◦ C, the CaCO3 microspheres could be obtained, but the
3.1. Morphology of the synthesized CaCO3 crystals Fig. 1 shows the typical FESEM images of CaCO3 hollow spherical particles obtained in the reaction. It exhibited that the mean diameter of the hollow spheres ranged from 2 to 3 m. It can be seen that CaCO3 microspheres with coarse surfaces were obtained (Fig. 1(a)). The magnified image shows that the hollow sphere with an opening hole is composed of nano-sized crystallites. The mean wall thickness was around 400 nm. The hollow structure of the product was further confirmed by TEM (see Fig. 1(b)). 3.2. XRD characterization of the synthesized CaCO3 crystals Fig. 2 shows the typical XRD pattern of the obtained hollow CaCO3 microspheres. The peaks at 2 = 2304◦ , 2940◦ , 3600◦ , 3940◦ , 4316◦ , 4748◦ and 4850◦ represent the (0 1 2), (1 0 4), (1 1 0), (1 1 3), (2 0 2), (0 1 8) and (1 1 6) planes of calcite crystals, respectively No
Fig. 2. XRD pattern of the hollow CaCO3 microspheres.
L. Zhao, J. Wang / Colloids and Surfaces A: Physicochem. Eng. Aspects 393 (2012) 139–143
Fig. 3. FT-IR spectra of the hollow CaCO3 microspheres.
opening structure could not be seen in Fig. 4(b), and the mean size of the spheres decreased dramatically to about 2 m. As the experiments were carried out at 60 ◦ C and 80 ◦ C, the obtained products were irregularly aggregated and no hollow spheres could be synthesized. Therefore, it should be pointed out that lower temperature is favorable to the crystallization of hollow CaCO3 microspheres. 3.5. Influence of SDS concentration on the crystallization of CaCO3 particles In order to investigate the influence of SDS concentration on the crystallization of CaCO3 , following experiments were carried out
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PVP concentration was fixed at 10 g/L, the SDS concentration varied from 5 mM to 50 mM, and the other experimental conditions were invariable. Different SDS concentrations resulted in different aggregation states of PVP–SDS complexes in the reaction system. The crystalline CaCO3 particles controlled by different concentrations of SDS were shown in Fig. 5. It was observed that cubic particles with smooth surfaces were prepared at low SDS concentration (5 mM) (Fig. 5(a)). This might be ascribed to the fact that surfactant monomers dispersed in the solution with a low concentration. Thus, PVP–SDS complex micelles could not form increasing the concentration of SDS to 10 mM, hollow microspheres composed of nano-sized crystallites were obtained (Fig. 5(b)). The PVP–SDS complex micelles served as a spherical template to generate hollow spheres of CaCO3 crystal in the precipitation system. As we can see in Fig. 5(c), flower-shaped CaCO3 spheres were formed when the SDS concentration reached up to 20 mM. When the concentration of SDS was 50 mM, however, a mixture of hollow microspheres disappeared and dispersed nano-sized particles were produced (Fig. 5(d)). From the above results, a conclusion can be drawn that the concentration of SDS played a key role in controlling the formation of hollow CaCO3 microspheres. 3.6. Growth mechanism of hollow CaCO3 microspheres The crystallization and aggregation mechanism, such as the enrichment of Ca2+ on polymer/surfactant superstructures, was still unclear. From the above discussion, a speculated growth mechanism of hollow CaCO3 microspheres was proposed. In the presence of polymers, an organic–inorganic interface region would perform the functions like the chemical microenvironment where nucleation of CaCO3 occurs [30]. PVP could interact strongly with anionic SDS to form PVP–SDS complex micelles with long chains intermingling in the headgroup region of the SDS micelles [31]. SDS could provide the nucleation sites for the crystallization of CaCO3 owing
Fig. 4. FESEM images of CaCO3 particles obtained in the presence of PVP and SDS at the different temperature: (a) 20 ◦ C; (b) 40 ◦ C; (c) 60 ◦ C; (d) 80 ◦ C.
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Fig. 5. FESEM images of CaCO3 particles obtained in the presence of PVP (1.0 g/L)and different concentrations of SDS: (a) 5 mM; (b) 10 mM; (c) 20 mM; (d) 50 mM.
Fig. 6. Schematic depiction for the formation of hollow CaCO3 microspheres modulated by PVP and SDS.
to the interaction of SDS and Ca2+ . Each polymer molecule could accommodate only a limited number of beads due to the bead–bead electrostatic repulsion. Not only PVP–SDS complex micelles could be formed, but also free surfactant micelles could exist in the solution, which may simultaneously exert additional controlling effect on the crystallization process [32]. At first, the addition of PVP into SDS solution led to the formation of the PVP–SDS complex micelles through the adsorption of head groups of surfactants on the polymer chains. And then, hollow spheres consisted of
calcite nanoparticles fabricate around the PVP–SDS complex micelles. Finally, the nanoparticles formed the shell of the hollow spheres on the surface by adsorption. Hypothetical formation mechanism of hollow CaCO3 microspheres was illustrated in Fig. 6. 4. Conclusions In summary, hollow CaCO3 microspheres have been synthesized successfully in the presence of PVP and SDS using a facile
L. Zhao, J. Wang / Colloids and Surfaces A: Physicochem. Eng. Aspects 393 (2012) 139–143
precipitation method. The PVP–SDS complex micelles were the nucleating centers and serves as spherical template to generate hollow spheres of CaCO3 crystals in the reaction system. The PVP–SDS complex micelles could control the nucleation, growth, and aggregation of CaCO3 crystalline particles. However, the concentration of SDS plays an important role in influencing the formation of hollow CaCO3 microspheres. The formation mechanism of hollow CaCO3 microspheres was proposed in the end. The preparation of welldefined hollow CaCO3 spheres represents a simple and useful route for the synthesis of functional materials applied in the biomimetic mineralization. Acknowledgements This research was financed by the National Natural Science Foundation of China (21077041) and supported by “the Twelfth Five” Science Technology Research of Jilin Education Hall. The authors also would like to thank Jilin Normal University for supplying the various instruments used in this study Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.colsurfa.2011.11.012. References [1] S. Mann, Biomineralization: Principles and Concepts in Bioinorganic Materials Chemistry, Oxford University Press, New York, 2001. [2] C.H. Lu, L.M. Qi, J.M. Ma, H.M. Cheng, M.F. Zhang, W.X. Cao, Controlled growth of micropatterned, oriented calcite films on a self-assembled multilayer film, Langmuir 20 (2004) 7378–7380. [3] H. Wei, Q. Shen, Y. Zhao, Y. Zhou, D. Wang, D. Xu, On the crystalization of calcium carbonate modulated by anionic surfactants, J. Cryst. Growth 279 (2005) 439–446. [4] Y.S. Han, G. Hadiko, M. Fuji, M. Takahashi, Effect of flow rate and CO2 content on the phase and morphology of CaCO3 prepared by bubbling method, J. Cryst. Growth 276 (2005) 541–548. [5] S. Mann, B.R. Heywood, S. Rajam, J.B.A. Walker, Structural and stereochemical relationships between langmuir monolayers and calcium carbonate nucleation, J. Phys. D Appl. Phys. 24 (1991) 154–164. [6] Q. Shen, H. Wei, Y. Zhao, D.J. Wang, L.Q. Zheng, D.F. Xu, Morphological control of calcium carbonate crystals by polyvinylpyrrolidone and sodium dodecyl benzene sulfonate, Colloids Surfaces A: Physicochem. Eng. Aspects 251 (2004) 87–91. [7] Y. Wen, L. Xiang, Y. Jin, Tribological investigation of PTFE composite filled with lead and rare earths-modified glass fiber, Mater. Lett. 57 (2003) 2557–2565. [8] L.M. Qi, J. Li, J.M. Ma, Biomimetic morphogenesis of calcium carbonate in mixed solutions of surfactants and double-hydrophilic block copolymers, Adv. Mater. 14 (2002) 300–303. [9] G. Hadiko, Y.S. Han, M. Fuji, M. Takahashi, Synthesis of hollow calcium carbonate particles by the bubble templating method, Mater. Lett. 59 (2005) 2519–2522. [10] P. Malkaj, J. Kanakis, E. Dalas, The effect of leucine on the crystal growth of calcium carbonate, J. Cryst. Growth 266 (2004) 533–538.
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[11] E. Dala, P. Klepetsanis, P.G. Koutsoukos, The overgrowth of calcium carbonate on poly(vinylchloride-co-vinyl acetate-co-maleic acid), Langmuir 15 (1999) 8322–8327. [12] K. Esumi, A. Suzuki, A. Yamahira, K. Torigoe, Role of poly(amidoamine) dendrimers for preparing nanoparticles of gold, platinum, and silver, Langmuir 16 (2000) 2604–2608. [13] S.H. Yu, H. Colfen, M. Antonietti, Polymer-controlled morphosynthesis and mineralization of metal carbonate superstructures, J. Phys. Chem. B 107 (2003) 7396–7405. [14] H. Wei, Q. Shen, Y. Zhao, Y. Zhou, D. Wang, D. Xu, On the crystallization of calcium carbonate modulated by anionic surfactants, J. Cryst. Growth 279 (2005) 439–446. [15] H. Tong, W.T. Ma, L.L. Wang, P. Wan, J.M. Hu, L.X. Cao, Control over the crystal phase, shape, size and aggregation of calcium carbonate via a l-aspartic acid inducing process, Biomaterials 25 (2004) 3923–3929. [16] L.E. Euliss, M.H. Bartl, G.D. Stucky, Control of calcium carbonate crystallization utilizing amphiphilic block copolypeptides, J. Cryst. Growth 286 (2006) 424–430. [17] M. Kitamura, Tribological investigation of PTFE composite filled with lead and rare earths-modified glass fiber, J. Colloid Interface Sci. 236 (2001) 318–327. [18] A. Sugawara, T. Ishii, T. Kato, Self-organized calcium carbonate with regular surface-relief structures, Angew. Chem. Int. Ed. 42 (2003) 5299–5303. [19] C.Y. Wang, Y. Sheng, H. Bala, X. Zhao, X.K. Ma, Z.C. Wang, A novel aqueous-phase route to synthesize hydrophobic CaCO3 particles in situ, Mater. Sci. Eng. C 27 (2007) 42–45. [20] C. Deslouis, D. Festy, O. Gil, V. Maillot, S. Touzain, B. Tribollet, Characterization of calcareous deposits in artificial sea water by impedances techniques: 2-deposit of Mg(OH)2 without CaCO3 , Electrochim. Acta 45 (2000) 1837–1845. [21] Q. Shen, H. Wei, L. Wang, Y. Zhou, Y. Zhao, Z. Zhang, D. Wang, G. Xu, D. Xu, Crystallization and aggregation behaviors of calcium carbonate in the presence of poly(vinylpyrrolidone) and sodium dodecyl sulfate, J. Phys. Chem. B 109 (2005) 18342–18347. ´ G. Falini, Influence of some polysaccharides on [22] J. Kontrec, D. Kralj, L. Breˇcevic, the production of calcium carbonate filler particles, J. Cryst. Growth 310 (2008) 4554–4560. [23] M. Kontopoulou, J.S. Parent, The dynamics of montmorillonite clay dispersion and morphology development in immiscible ethylene–propylene rubber/polypropylene blends, Polymer 48 (2007) 4520–4528. [24] X. Fan, Z. Zhang, G. Li, A.A. Rowson, Attachment of solid particles to air bubbles in surfactant-free aqueous solutions, Chem. Eng. Sci. 59 (2004) 2639–2645. [25] D.L. Tran, V.H. Tran, T.Q. Duong, J.S. Kim, Effect of nanosized and surface-modified precipitated calcium carbonate on properties of CaCO3 /polypropylene nanocomposites, Mater. Sci. Eng. A 501 (2009) 87–93. [26] B. Cheng, M. Lei, J.G. Yu, X.J. Zhao, Preparation of monodispersed cubic calcium carbonate particles via precipitation reaction, Mater. Lett. 58 (2004) 1565–1570. [27] C.Y. Wang, Y. Sheng, X. Zhao, Y. Pan, B. Hari, Z.C. Wang, Synthesis of hydrophobic CaCO3 nanoparticles, Mater. Lett. 60 (2006) 854–857. [28] S.I. Kuriyavar, R. Vetrivel, S.G. Hegde, A.V. Ramaswamy, D. Chakrabarty, S. Mahapatra, Insights into the formation of hydroxyl ions in calcium carbonate: temperature dependent FTIR and molecular modeling studies, J. Mater. Chem. 10 (2000) 1835–1840. [29] A.G. Xyla, P.G. Koutsoukos, Quantitative analysis of calcium carbonate polymorphs by infrared spectroscopy, J. Chem. Soc. Faraday Trans. 85 (1989) 165–3172. [30] X.X. Ji, G.Y. Li, X.T. Huang, The synthesis of hollow CaCO3 microspheres in mixed solutions of surfactant and polymer, Mater. Lett. 62 (2008) 751–754. [31] Z.P. Zhang, D.M. Gao, H. Zhao, C.G. Xie, G.J. Guan, D.P. Wang, S.H. Yu, Biomimetic assembly of polypeptide-stabilized CaCO3 nanoparticles, J. Phys. Chem. B 110 (2006) 8613–8618. [32] Y. Pan, X. Zhao, Y.P. Guo, X.T. Lv, S.X. Ren, M.R. Yuan, Z.C. Wang, Controlled synthesis of hollow calcite microspheres modulated by polyacrylic acid and sodium dodecyl sulfonate, Mater. Lett. 61 (2007) 2810–2813.
Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Biomimetic synthesis of hollow microspheres of calcium carbonate crystals in the presence of polymer and surfactant Lina Zhao a,b,∗ , Jiku Wang a,b a b
College of Chemistry, Jilin Normal University, Siping 136000, China Key Laboratory of Preparation and Applications of Environmental Friendly Materials, Jilin Normal University, Ministry of Education, Siping 136000, China
a r t i c l e
i n f o
Article history: Received 18 September 2011 Received in revised form 10 November 2011 Accepted 11 November 2011 Available online 22 November 2011 Keywords: Hollow microspheres Calcium carbonate Crystallization Calcite
a b s t r a c t Hollow CaCO3 microspheres were successfully synthesized using sodium carbonate and calcium chloride through a precipitation reaction method at room temperature. Polyvinylpyrrolidone (PVP), together with sodium dodecyl sulfonate (SDS), was employed as template for the controlled growth of hollow CaCO3 microspheres. The concentration of SDS was an important factor to control the synthesis of hollow CaCO3 microspheres. It suggested that the PVP–SDS complex micelles played a key role in controlling the growth of biominerals X-ray diffraction (XRD), FESEM and TEM confirmed that hollow CaCO3 microspheres consisting of calcite crystals were synthesized In the present work, we also proposed a hypothetical mechanism for the formation process of hollow CaCO3 microspheres. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Modern technologies require innovation approaches for complex materials with excellent properties. It is well-known that biomineralization is an elaborate process that produces biominerals with complex and fascinating morphologies as well as superior mechanical properties [1–3]. The controlled synthesis of inorganic materials of specific size and morphology is a key aspect in fields as diverse as modern materials, catalysis, medicine, electronics, ceramics, pigments and cosmetics [4,5]. Hollow CaCO3 particles with nanometer to micrometer dimensions represent an important class of materials because their unique structural, optical and surface properties may lead them to a wide range of application [6–8]. The strategy of using organic templates or modifiers with complex functional patterns to control the nucleation, growth, and alignment of inorganic crystals has been widely adopted for the biomimetic synthesis of inorganic materials with complex form [9,10]. A number of studies have been carried out to elucidate the effect of various organic molecules on the crystallization of inorganic crystals [11–13]. As widely and efficiently used processing additives in industrial end products, the various surfactants aroused the particular interest in the crystallization process of inorganic species [14,15]. The use of polymer–surfactant
∗ Corresponding author at: College of Chemistry, Jilin Normal University, Siping 136000, China. Tel.: +86 434 3292154. E-mail address: [email protected] (L. Zhao). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.11.012
supramolecular systems for the control of inorganic crystallization has become an area of intense research activity in recent years [16–19]. Although the influence of uncharged polymers [20–22], anionic surfactants, or the polymer–surfactant mixtures [23–25] on the crystallization process of inorganic crystals have been investigated, nevertheless, the aqueous systems containing polymer–surfactant mixtures have not been used for the research of crystallization and aggregation behavior of calcium carbonate [26]. In this work, hollow CaCO3 microspheres were successfully synthesized using sodium carbonate and calcium chloride through precipitation reaction at room temperature. Polyvinylpyrrolidone (PVP) and sodium dodecyl sulfonate (SDS) were employed together as template for the controlled growth of hollow CaCO3 microspheres. The effect of the concentration of SDS on the crystallization and aggregation of CaCO3 was investigated and discussed. The PVP–SDS complex micelles played a key role in controlling the growth of biominerals. 2. Experiment 2.1. Chemicals Polyvinylpyrrolidone (PVP) and sodium dodecyl sulfonate (SDS) were obtained from Beijing Chemical Factory (China). Na2 CO3 and CaCl2 ·2H2 O were brought from Shenyang Chemical Reagent Factory. All chemicals were of analytical grade and used without further purification. The water used in this work was distilled water made in our laboratory.
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Fig. 1. FESEM and TEM images of hollow CaCO3 microspheres.
2.2. Preparation 0.1 M of Na2 CO3 solution with 10 g/L of PVP and SDS, and 0.1 M of CaCl2 solution with certain concentration of SDS were prepared beforehand. Then, CaCO3 crystals were precipitated by quickly pouring 50 mL of the CaCl2 mixture into a three-necked flask with an equal volume of the Na2 CO3 mixture. The precipitation reaction was carried out at 20 ◦ C under continuous stirring of 300 rpm for 1 h. Subsequently, the precipitate was filtered and rinsed three times with distilled water. Finally, the product was dried in an oven at 80 ◦ C for at least 24 h and used for characterization. 2.3. Characterization Micrographs of the hollow CaCO3 microspheres were taken by a field emission scanning electron microscope (FESEM). It was carried out using a JEOL JSM-6700F at 15 kV. Transmission electron microscopy (TEM) images were characterized by JEOL-2100H electron microscope. The crystalline phase and structure of the synthesized CaCO3 were detected by X-ray diffraction (XRD) analysis with a Shimadju D/max-rA X-ray diffractometer with Cu K␣ radiation, under the accelerating voltage 40 kV, current 30 mA, and scanning rate 4◦ min−1 . Fourier transform infrared spectroscopy (FT-IR) was performed on FTIR-8400S using KBr pellets.
3. Results and discussion
impurity peaks were detected, indicating that the powders had high purity [27]. 3.3. FT-IR characterization of the synthesized CaCO3 crystals Fig. 3 shows the FT-IR spectrum of the CaCO3 particles. The characteristic absorption bands observed at 715 cm−1 , 873 cm−1 and 1462 cm−1 confirmed the formation of calcite, which were corresponding to the 4 , 2 and 3 CO3 2− absorption bands of calcite [28]. Weak bands were also observed at 2928 cm−1 and 2869 cm−1 that are associated with asymmetric and symmetric methyl and ethylene C–H stretches, respectively [29]. This suggests that surfactant molecules remained strongly associated with the calcite crystals even after the extensive washing process. 3.4. Influence of different temperature on the crystallization of CaCO3 particles To research the influence of different temperature on the crystallization of CaCO3 particles, experiments were carried out at 20 ◦ C, 40 ◦ C, 60 ◦ C and 80 ◦ C, respectively, keeping the other conditions invariable. Fig. 4 shows the FESEM images of hollow CaCO3 microspheres. It exhibited that the CaCO3 microspheres could be synthesized at 20 ◦ C. From Fig. 4(a), the well-defined hollow spheres with rough outer surface were observed. The size of the spheres ranged from 2 to 3 m. When the temperature was as high as 40 ◦ C, the CaCO3 microspheres could be obtained, but the
3.1. Morphology of the synthesized CaCO3 crystals Fig. 1 shows the typical FESEM images of CaCO3 hollow spherical particles obtained in the reaction. It exhibited that the mean diameter of the hollow spheres ranged from 2 to 3 m. It can be seen that CaCO3 microspheres with coarse surfaces were obtained (Fig. 1(a)). The magnified image shows that the hollow sphere with an opening hole is composed of nano-sized crystallites. The mean wall thickness was around 400 nm. The hollow structure of the product was further confirmed by TEM (see Fig. 1(b)). 3.2. XRD characterization of the synthesized CaCO3 crystals Fig. 2 shows the typical XRD pattern of the obtained hollow CaCO3 microspheres. The peaks at 2 = 2304◦ , 2940◦ , 3600◦ , 3940◦ , 4316◦ , 4748◦ and 4850◦ represent the (0 1 2), (1 0 4), (1 1 0), (1 1 3), (2 0 2), (0 1 8) and (1 1 6) planes of calcite crystals, respectively No
Fig. 2. XRD pattern of the hollow CaCO3 microspheres.
L. Zhao, J. Wang / Colloids and Surfaces A: Physicochem. Eng. Aspects 393 (2012) 139–143
Fig. 3. FT-IR spectra of the hollow CaCO3 microspheres.
opening structure could not be seen in Fig. 4(b), and the mean size of the spheres decreased dramatically to about 2 m. As the experiments were carried out at 60 ◦ C and 80 ◦ C, the obtained products were irregularly aggregated and no hollow spheres could be synthesized. Therefore, it should be pointed out that lower temperature is favorable to the crystallization of hollow CaCO3 microspheres. 3.5. Influence of SDS concentration on the crystallization of CaCO3 particles In order to investigate the influence of SDS concentration on the crystallization of CaCO3 , following experiments were carried out
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PVP concentration was fixed at 10 g/L, the SDS concentration varied from 5 mM to 50 mM, and the other experimental conditions were invariable. Different SDS concentrations resulted in different aggregation states of PVP–SDS complexes in the reaction system. The crystalline CaCO3 particles controlled by different concentrations of SDS were shown in Fig. 5. It was observed that cubic particles with smooth surfaces were prepared at low SDS concentration (5 mM) (Fig. 5(a)). This might be ascribed to the fact that surfactant monomers dispersed in the solution with a low concentration. Thus, PVP–SDS complex micelles could not form increasing the concentration of SDS to 10 mM, hollow microspheres composed of nano-sized crystallites were obtained (Fig. 5(b)). The PVP–SDS complex micelles served as a spherical template to generate hollow spheres of CaCO3 crystal in the precipitation system. As we can see in Fig. 5(c), flower-shaped CaCO3 spheres were formed when the SDS concentration reached up to 20 mM. When the concentration of SDS was 50 mM, however, a mixture of hollow microspheres disappeared and dispersed nano-sized particles were produced (Fig. 5(d)). From the above results, a conclusion can be drawn that the concentration of SDS played a key role in controlling the formation of hollow CaCO3 microspheres. 3.6. Growth mechanism of hollow CaCO3 microspheres The crystallization and aggregation mechanism, such as the enrichment of Ca2+ on polymer/surfactant superstructures, was still unclear. From the above discussion, a speculated growth mechanism of hollow CaCO3 microspheres was proposed. In the presence of polymers, an organic–inorganic interface region would perform the functions like the chemical microenvironment where nucleation of CaCO3 occurs [30]. PVP could interact strongly with anionic SDS to form PVP–SDS complex micelles with long chains intermingling in the headgroup region of the SDS micelles [31]. SDS could provide the nucleation sites for the crystallization of CaCO3 owing
Fig. 4. FESEM images of CaCO3 particles obtained in the presence of PVP and SDS at the different temperature: (a) 20 ◦ C; (b) 40 ◦ C; (c) 60 ◦ C; (d) 80 ◦ C.
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Fig. 5. FESEM images of CaCO3 particles obtained in the presence of PVP (1.0 g/L)and different concentrations of SDS: (a) 5 mM; (b) 10 mM; (c) 20 mM; (d) 50 mM.
Fig. 6. Schematic depiction for the formation of hollow CaCO3 microspheres modulated by PVP and SDS.
to the interaction of SDS and Ca2+ . Each polymer molecule could accommodate only a limited number of beads due to the bead–bead electrostatic repulsion. Not only PVP–SDS complex micelles could be formed, but also free surfactant micelles could exist in the solution, which may simultaneously exert additional controlling effect on the crystallization process [32]. At first, the addition of PVP into SDS solution led to the formation of the PVP–SDS complex micelles through the adsorption of head groups of surfactants on the polymer chains. And then, hollow spheres consisted of
calcite nanoparticles fabricate around the PVP–SDS complex micelles. Finally, the nanoparticles formed the shell of the hollow spheres on the surface by adsorption. Hypothetical formation mechanism of hollow CaCO3 microspheres was illustrated in Fig. 6. 4. Conclusions In summary, hollow CaCO3 microspheres have been synthesized successfully in the presence of PVP and SDS using a facile
L. Zhao, J. Wang / Colloids and Surfaces A: Physicochem. Eng. Aspects 393 (2012) 139–143
precipitation method. The PVP–SDS complex micelles were the nucleating centers and serves as spherical template to generate hollow spheres of CaCO3 crystals in the reaction system. The PVP–SDS complex micelles could control the nucleation, growth, and aggregation of CaCO3 crystalline particles. However, the concentration of SDS plays an important role in influencing the formation of hollow CaCO3 microspheres. The formation mechanism of hollow CaCO3 microspheres was proposed in the end. The preparation of welldefined hollow CaCO3 spheres represents a simple and useful route for the synthesis of functional materials applied in the biomimetic mineralization. Acknowledgements This research was financed by the National Natural Science Foundation of China (21077041) and supported by “the Twelfth Five” Science Technology Research of Jilin Education Hall. The authors also would like to thank Jilin Normal University for supplying the various instruments used in this study Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.colsurfa.2011.11.012. References [1] S. Mann, Biomineralization: Principles and Concepts in Bioinorganic Materials Chemistry, Oxford University Press, New York, 2001. [2] C.H. Lu, L.M. Qi, J.M. Ma, H.M. Cheng, M.F. Zhang, W.X. Cao, Controlled growth of micropatterned, oriented calcite films on a self-assembled multilayer film, Langmuir 20 (2004) 7378–7380. [3] H. Wei, Q. Shen, Y. Zhao, Y. Zhou, D. Wang, D. Xu, On the crystalization of calcium carbonate modulated by anionic surfactants, J. Cryst. Growth 279 (2005) 439–446. [4] Y.S. Han, G. Hadiko, M. Fuji, M. Takahashi, Effect of flow rate and CO2 content on the phase and morphology of CaCO3 prepared by bubbling method, J. Cryst. Growth 276 (2005) 541–548. [5] S. Mann, B.R. Heywood, S. Rajam, J.B.A. Walker, Structural and stereochemical relationships between langmuir monolayers and calcium carbonate nucleation, J. Phys. D Appl. Phys. 24 (1991) 154–164. [6] Q. Shen, H. Wei, Y. Zhao, D.J. Wang, L.Q. Zheng, D.F. Xu, Morphological control of calcium carbonate crystals by polyvinylpyrrolidone and sodium dodecyl benzene sulfonate, Colloids Surfaces A: Physicochem. Eng. Aspects 251 (2004) 87–91. [7] Y. Wen, L. Xiang, Y. Jin, Tribological investigation of PTFE composite filled with lead and rare earths-modified glass fiber, Mater. Lett. 57 (2003) 2557–2565. [8] L.M. Qi, J. Li, J.M. Ma, Biomimetic morphogenesis of calcium carbonate in mixed solutions of surfactants and double-hydrophilic block copolymers, Adv. Mater. 14 (2002) 300–303. [9] G. Hadiko, Y.S. Han, M. Fuji, M. Takahashi, Synthesis of hollow calcium carbonate particles by the bubble templating method, Mater. Lett. 59 (2005) 2519–2522. [10] P. Malkaj, J. Kanakis, E. Dalas, The effect of leucine on the crystal growth of calcium carbonate, J. Cryst. Growth 266 (2004) 533–538.
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