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Transformation of novel morphologies and polymorphs of CaCO3 crystals induced by the anionic surfactant SDS
Materials Chemistry and Physics 123 (2010) 534–539
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Transformation of novel morphologies and polymorphs of CaCO3 crystals induced by the anionic surfactant SDS Zhiying Chen, Caifen Li, Qianqian Yang, Zhaodong Nan ∗ College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, Jiangsu Province 225002, China
a r t i c l e
i n f o
Article history: Received 31 January 2010 Received in revised form 22 April 2010 Accepted 4 May 2010 Keywords: Calcium carbonate Sodium dodecyl sulfate (SDS) Morphology Polymorph
a b s t r a c t Anionic surfactant, sodium dodecyl sulfate (SDS), was used as a template to modify the formation of CaCO3 crystals. Aragonite was produced when the concentration of SDS was controlled at 0.5 mM. The rod-shaped aragonite aggregated by block-like particles was fabricated. Pure vaterite with a novel flowershaped morphology was obtained when the concentration of SDS was 1.0 mM. When the concentration of SDS was increased to 2.5 mM, a mixture composed of vaterite and calcite was produced. And the as-prepared sample showed various morphologies, including tube-shaped, rod-shaped and hexagonal structures. However, the most interesting result is that rhombohedral calcite may be transformed from hexagonal vaterite. This result may prove useful to the further investigation of the processes of biomineralization. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Biological materials have unique structures and morphologies, which obtain much better performance than that for their constitute natural minerals. Many organisms make use of calcium carbonate as a constituent of their skeleton or as a protective shell, such as in sea shells, snail shells, and bird’s eggs. CaCO3 can crystallize as calcite, aragonite, or vaterite. Calcite and aragonite are the most common biologically formed CaCO3 polymorphs. Vaterite, as a less stable polymorph, will transform into calcite through a solvent-mediated process [1]. Biomimetic synthesis and polymorph-control of CaCO3 crystals have been studied in great detail as reviewed by Cölfen [2]. Many investigations have demonstrated that the properties of CaCO3 crystals, such as the crystal polymorph, the particle size and morphology, are strongly dependent on the preparation methods and additives [3–5]. Watersoluble additives, such as anionic surfactants, have always been selected to modify the polymorph of CaCO3 [6–11]. In our previous report [12], the anionic surfactant, sodium dodecyl benzene sulfonate (SDBS), was used as a template to modify the forming processes of CaCO3 crystals, and various polymorphs of CaCO3 crystals were obtained. However, no effect of SDS on polymorphs of CaCO3 was found, the formation of calcite from SDS solutions may be attributed to the tridentate motif of oxygen atoms in HSO3 − terminated monolayers, favoring the nucleation of the (0 0 1) plane of calcite in which carbonate anions are parallel to the plane [8].
∗ Corresponding author. E-mail address: [email protected] (Z. Nan). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.05.010
In this paper, SDS was employed as a template to control the growth of CaCO3 crystals at 90 ◦ C. Structures transformations from aragonite to vaterite and calcite were found. Novel morphologies of CaCO3 crystals were also obtained. 2. Materials and methods 2.1. Materials All the chemicals, including urea, calcium acetate and anionic surfactant sodium dodecyl sulfate (SDS), purchased from Sinopharm Chemical Reagent Company, were of analytical grade and used as-received without further purification. De-ionized water was used as the solvent. 2.2. Methods For the experiments, 50 mM Ca(CH3 COO)2 and 0.25 M CO(NH2 )2 were put into a Teflon-lined stainless-steel autoclave, together with concentrations of SDS of 0.5, 1.0 or 2.5 mM, respectively. This volume of chemicals was up to 70% of the total volume of the Teflon cylinder (100 ml). The autoclave was maintained at 90 ◦ C for 24 h, and subsequently cooled down to room temperature. The resulting precipitate was washed with water and alcohol several times, and dried at room temperature for more than 24 h in vacuum. 2.3. Characterization X-ray powder diffraction (XRD) patterns were recorded using a Bruker D8 Advanced XRD diffractometer with Cu K␣ radiation at a scanning rate of 0.04◦ s−1 . Scanning electron microscope (SEM) images were taken with a Hitachi S4800, fitted with a field emission source, and working at 20 kV. All samples were mounted on copper stubs and sputter coated with gold prior to examination. Selected-area electron diffraction (SAED) was obtained on a 200 kV Hitachi, Model H-800 microscope. Infrared spectroscopic analysis was performed in transmission mode (FT-IR) using a Nicolet Aexus 470, with scanning from 4000 to 500 cm−1 by using KBr pellets. The curves of thermogravimetry (TG) were obtained by a NETZSCH STA 409 PC/PG.
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Fig. 1. XRD patterns of the as-obtained samples at 90 ◦ C with different concentrations of SDS solutions: (A) 0.5 mM; (B) 1.0 mM; (C) 2.5 mM.
3. Results and discussion The XRD patterns of the obtained samples with different concentrations of SDS at 90 ◦ C are given in Fig. 1. In comparison with their standard JCPDS files (aragonite: 5-0453; calcite: 25-0127; vaterite: 33-0268), the Bragg reflections were marked with C, A and V, which correspond to the calcite polymorph, aragonite polymorph and vaterite polymorph, respectively. When the concentration of SDS was 0.5 mM, pure aragonite was fabricated as shown in Fig. 1A. When the concentration of SDS was increased to 1.0 mM, pure vaterite was obtained as given in Fig. 1B. A mixture composed of calcite and vaterite was fabricated when the concentration of SDS was further increased to 2.5 mM, as seen in Fig. 1C. When the concentration of SDS was increased to 10.0 mM, a mixture composed of calcite and vaterite was still fabricated (the result was not shown here). The contents of aragonite, vaterite and calcite contained in the as-prepared samples were calculated according to the reference [13] and are shown in Fig. 2. In order to show the effect of SDS concentration on the polymorph selection of CaCO3 crystals, the result obtained in our previous report [14] is also given in Fig. 2, for which no SDS was added to the reacting system.
Fig. 2. Percentage of three different crystals of CaCO3 with different concentrations of SDS solutions. (䊉) Vaterite, () aragonite, and () calcite.
Fig. 3. (1) FT-IR spectra of CaCO3 crystals obtained with different concentrations of SDS solutions: (A) 0.5 mM; (B) 1.0 mM; (C) 2.5 mM; (D) SDS. (2) FT-IR spectra of pure calcite, vaterite and aragonite.
The results are further demonstrated by FT-IR as given in Fig. 3(1). According to references [15,16], vibrational bands at about 1083 and 854 cm−1 can be assigned to the characteristic symmetric carbonate stretching (1 mode) and a carbonate out-of-plane
Fig. 4. TG spectra of CaCO3 obtained with different concentrations of SDS solutions: (A) 0.5 mM; (B) 1.0 mM; (C) 2.5 mM.
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Fig. 5. SEM images of the samples obtained with different concentrations of SDS solutions: (A) 0.5 mM; (B) 1.0 mM; (C)–(H) 2.5 mM.
bending (2 mode) vibrations of aragonite, respectively, and a pair of bands at about 699 and 712 cm−1 can be attributed to the inplane bending modes (4 mode) of aragonite, as shown in Fig. 3A. The band at 745 cm−1 (4 mode) is a characteristic vibration band of vaterite, as given in Fig. 3B. The bands at 877 and 712 cm−1 can
be attributed to the 2 and 4 modes of calcite, as shown in Fig. 3C. However, the bands at 587, 622, 970, 1105, 1205 and 1236 cm−1 are also found in Fig. 3C. In order to investigate further these vibrational bands, the FT-IR spectrum of the SDS was determined and given in Fig. 3D. Comparing Fig. 3C with Fig. 3D, the bands at 587, 622, 970,
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Fig. 7. FT-IR spectra of CaCO3 obtained at 1.0 mM SDS and different reaction times: (A) 6 h; (B) 12 h; (C) 48 h; (D) 72 h. Fig. 6. XRD patterns of the as-obtained samples at 1.0 mM SDS and different aging times: (A) 6 h; (B) 12 h; (C) 48 h; (D) 72 h.
1105, 1205 and 1236 cm−1 can be attributed to SDS. In order to compare these results, FT-IR spectra of pure calcite, aragonite and vaterite were shown in Fig. 3(2). The result demonstrates that the molecule of SDS was adsorbed onto the facets of the as-prepared sample. In order to prove the adsorption of SDS onto the crystal facets, the temperatures of decomposition of the obtained samples were determined by TG technology. The obtained results are given in Fig. 4. No weight loss is found when the temperature is lower than 600 ◦ C, as shown in Fig. 4A. When the experimental temperature is lower than 400 ◦ C, a weight loss is calculated to be about 0.2 and 19.0 wt% as shown in Fig. 4B and C, respectively. These kinds of weight loss are corresponding to decomposition of the surfactant SDS. The weight loss corresponding to the decomposition of CaCO3 crystals occurs at about 650 ◦ C as seen in Fig. 4. When the concentration of SDS is increased from 0.5 to 2.5 mM, the adsorption of SDS onto the CaCO3 crystal facets is increased. These results are in agreement with those obtained by FT-IR as shown in Fig. 3. The present results are, however, different from those reported in reference [8], for which no effect of SDS on the polymorphs of CaCO3 was found at room temperature. In our previous report [12], SDBS induced the formation of vaterite at 90 ◦ C. The mechanism of structure correspondence was applied to explain the formation processes. In the present study, SDS can also induce the formation of vaterite when the concentration of SDS is equal to (or smaller than) 1.0 mM. When the concentration of SDS is higher than 1.0 mM, a mixture of polymorphs, composed of calcite and vaterite, was fabricated. The adsorption of SDS onto the facets of the CaCO3 crystals may affect the formed polymorph. When the concentration of SDS is smaller than 0.5 mM, pure aragonite was prepared. The adsorption of SDS onto the as-prepared samples increases with the increasing concentration of SDS. This kind of adsorption may decrease the energy of nucleation of vaterite and therefore vaterite was produced. Fig. 5 shows SEM images of the samples fabricated with different concentrations of SDS. Fig. 5A shows rod-shaped structures aggregated by smaller blocks. It is a novel morphology of aragonite which has never been reported before. A novel kind of flower-shaped morphology is found in Fig. 5B, for which pure vaterite was fabricated according to the results shown in Fig. 2. A mixture composed of calcite and vaterite was synthesized in 2.5 mM SDS as given in Fig. 2. Different morphologies are seen in Fig. 5C–H, for which Fig. 5D and F
are magnified images corresponding to Fig. 5C and E, respectively. Tube-shaped particles are found in Fig. 5C and D, for which the inner diameter is determined to be about 100 nm, the thickness of the tube to be about 15 nm, and the length of the tube to be about 300–800 nm. The tubes may be formed by the rolling up of a pseudolayered structure as indicated by the arrows shown in Fig. 5D. A similar mechanism was used to explain the formation of tubeshaped WS2 [17]. Rhombohedral structures are also seen in Fig. 5E, which is the typical morphology of calcite. At the same time, rodshaped structures are also found in Fig. 5E. A magnified image is given in Fig. 5F and this structure is assembled by block-shaped particles. Hexagonal particles are also shown with various magnitude in Fig. 5G and H and these hexagonal particles may be vaterite according to the references [18–20]. The present results demonstrate that the anionic surfactant SDS induces various effects with different applied aging times. The XRD patterns of the fabricated samples with the same concentration of SDS (1.0 mM) at different aging time are given in Fig. 6. No clear peak is seen in Fig. 6A. Fig. 6B and C can be indexed as pure vaterite, for which the aging time is 12 and 48 h, respectively. When the aging time is further increased to 72 h, pure calcite is fabricated as shown in Fig. 6D. These results are further emphasized by FT-IR as given in Fig. 7. Spectra A–C in Fig. 7 show that vibrational bands at about 877 (2 mode), and 742 cm−1 (4 mode) can be assigned to the vaterite. The vibrational bands at about 589 and 622 cm−1 can be clearly found in Fig. 7A, which are assigned to the SDS. These results prove that the molecules of SDS were adsorbed onto the facets of CaCO3 crystals. The bands corresponding to calcite are only found in Fig. 7D. The results obtained from FT-IR are in agreement with those obtained by XRD. Fig. 8 shows SEM images of the fabricated samples obtained after various aging times. When the aging time is 6 h, flowerand spherical-shaped crystals are observed as shown in Fig. 8A and B, respectively. Pure vaterite was fabricated according to the result obtained by Fig. 7A. When the aging time was increased to 12 h, broom-shaped crystals aggregates are found, as shown in Fig. 8C and pure vaterite was obtained. In order to see clearly the linear-shaped structure of these aggregates, an image at higher magnification is shown in Fig. 8D. When the aging time was increased to 24 h, the flower-shaped vaterite is produced as shown in Fig. 5B. When the aging time was further increased to 48 h, various morphologies of particles are found as shown in Fig. 8E and F. Pure vaterite was produced in this experimental condition. Comparing Fig. 8E with Fig. 8A, similar parts are found as indicated by
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Fig. 8. SEM images of the samples obtained at 1.0 mM SDS and different aging times: (A) and (B) 6 h; (C) and (D) 12 h; (E) and (F) 48 h; (G) and (H) 72 h. Inset in (F) shows electron diffraction pattern of vaterite.
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ever, hexagonal particles are found when a mixture composed of both vaterite and calcite is fabricated as shown in Fig. 5G and H. This phenomenon has not been reported previously as far as we know. 4. Conclusion
Fig. 9. Formation processes of CaCO3 crystals at various aging times: (A) 6 h; (B) 12 h; (C) 24 h; (D) 48 h; (E) 72 h.
the arrow. At the same time, hexagonal particles are also seen as shown in Fig. 8F. In order to further study these hexagonal particles, SAED image is shown in Fig. 8F (as inset). The obtained result demonstrates that these hexagonal crystals are vaterite. When the aging time is further increased to 72 h, belt-shaped crystals are obtained as shown in Fig. 8G, which pure calcite was produced. The magnified image in Fig. 8H shows that the belt-shaped particles are aggregated by smaller rhombohedral structures. Based on the experimental observations at different aging times, the proposed morphology sequence of CaCO3 crystals in 1.0 mM SDS is shown schematically in Fig. 9. Spherical vaterite was fabricated at first as Fig. 9A. The flower-shaped structure (given in Fig. 8A) may be aggregated with the spherical particles. When the aging time was increased from 6 to 12 h, the spherical vaterite transformed into rectilinear vaterite by a solution-mediated process as shown in Fig. 8D. The resulting rectilinear products were wrapped around a central block-shaped vaterite through a dissolution-recrystallization process and the flower-shaped structure (given in Fig. 5B) was obtained as shown in Fig. 9C. When the aging time was increased from 24 to 48 h, hexagonal vaterite (given in Fig. 8F) was produced as shown in Fig. 9D. At present, it is not fully understood how this form of hexagonal vaterite is produced. Vaterite often appears in a spherical morphology; however, hexagonal vaterite has been reported [18–20]. Usually vaterite is unable to expose (0 0 1) crystal faces, which consists of a hexagonal lattice of carbonate ions or calcium ions. Cölfen suggested that ammonium ions adsorbed on the (0 0 1) faces of vaterite and act as a stabilizing agent [21]. Besides ammonium ions, some organic additives can also adsorb onto the faces of vaterite and induce the formation of novel morphologies [20]. This phenomenon will be investigated in the future. We propose that the rectilinear vaterite may first transform into rod-shaped particles similar to those shown in Fig. 5F, then the rod-shaped particles may transform into hexagonal vaterite. The morphology of the sample shown in Fig. 8E demonstrates this kind of transformation, in which block-shaped particles were formed. The rod-shaped particles surrounded by block-shaped particles, and the spherical structure surrounded by hexagonal particles are all found as shown in Fig. 5F and H, respectively. The spherical structure may also be transformed from the rod-shaped particles. When the aging time is further increased from 48 to 72 h, the resulting hexagonal vaterite is completely transformed into rhombohedral calcite as shown in Fig. 9E. The process by which vaterite transforms into calcite has been studied in detail and in the present results, the morphology of vaterite transforms into a hexagonal morphology at first, then this hexagonal vaterite transforms into calcite. No hexagonal vaterite is found when pure vaterite is produced as shown in Fig. 5B. How-
SDS has been used to modify the polymorph and morphology of CaCO3 crystals. Aragonite was produced when the concentration of SDS was in the range from 0 to 0.5 mM. However, various morphologies were obtained with different concentrations of SDS. Rod-shaped aragonite with smooth faces was fabricated without addition of SDS as our previous report [12]. Rod-shaped aragonite aggregated by block-like particles was fabricated when the concentration of SDS was 0.5 mM. The nucleus of vaterite may become stable when the molecule of SDS was absorbed onto the vaterite. A novel flower-shaped vaterite was obtained when the concentration of SDS was 1.0 mM. When the concentration of SDS was further increased to 2.5 mM, a mixture composed of both vaterite and calcite was produced. The as-prepared sample shows various morphologies, including tube-shaped, rod-shaped and hexagonal structures. When the concentration of SDS was 1.0 mM, vaterite was fabricated at aging times shorted than 48 h. A variety of morphologies of vaterite were found at different aging times, including spherical, rectilinear and hexagonal structures. The resulting vaterite transformed into calcite during aging between 48 and 72 h. The most interesting result is that rhombohedral calcite may be transformed from hexagonal vaterite. The results obtained in the present work may be useful generally to the further study of the formation mechanisms of biominerilization and to the preparation of other functional materials. Conflict of interest No conflict of interest. Acknowledgments The financial support from the National Science Foundation of China (20753002) and the Natural & Scientific Grant of Jiangsu Province (BK2009181), China, is gratefully acknowledged. References [1] A. Lopezmacipe, J. Fomezmorales, R. Rodriguezclemente, J. Cryst. Growth 166 (1996) 1015. [2] H. Cölfen, Curr. Opin. Colloid. Interface Sci. 8 (2003) 23. [3] A.J. Xie, Y.H. Shen, X.Y. Li, Z.W. Yuan, L.G. Qiu, C.Y. Zhang, Y.F. Yang, Mater. Chem. Phys. 101 (2007) 87. [4] C. Li, L. Qi, Angew. Chem. Int. Ed. 47 (2008) 2388. [5] D. Liu, M.Z. Yates, Langmuir 22 (2006) 5566. [6] H. Wei, Q. Shen, Y. Zhao, D. Wang, D. Xu, J. Cryst. Growth 264 (2004) 424. [7] K. Hosoi, T. Hashida, H. Takahashi, N. Yamasaki, T. Korenaga, J. Mater. Sci. Lett. 16 (1997) 382. [8] H. Wei, Q. Shen, Y. Zhao, Y. Zhou, D. Wang, D. Xu, J. Cryst. Growth 279 (2005) 439. [9] Q. Shen, H. Wei, L. Wang, Y. Zhou, Y. Zhao, Z. Zhang, D. Wang, G. Xu, D. Xu, J. Phys. Chem. B 109 (2005) 18342. [10] N. Abdel-Aal, K. Sawada, J. Cryst. Growth 256 (2003) 188. [11] K.D. Demadis, S.D. Katarachia, Phosphorus Sulfur Silicon 179 (2004) 627. [12] Z. Nan, X. Chen, Q. Yang, X. Wang, Z. Shi, W. Hou, Colloid Interface Sci. 325 (2008) 331. [13] C.G. Kontoyannis, N.V. Vagenas, Analyst 125 (2000) 251. [14] Z. Nan, Z. Shi, B. Yan, R. Guo, W. Hou, J. Colloid Interface Sci. 317 (2008) 77. [15] L.F. Wang, I. Sondi, E. Matijevic, J. Colloid Interface Sci. 218 (1999) 545. [16] T.J. Mason (Ed.), Sonochemistry: The Uses of Ultrasound in Chemistry, The Royal Society of Chemistry, Cambridge, UK, 1990. [17] Y.D. Li, X.L. Li, R.R. He, J. Zhu, Z.X. Deng, J. Am. Chem. Soc. 124 (2002) 1411. [18] A.W. Xu, M. Antonietti, H. Cölfen, Y.P. Fang, Adv. Funct. Mater. 16 (2006) 903. [19] M. Sedlák, H. Cölfen, Macromol. Chem. Phys. 202 (2001) 587. [20] J.H. Huang, Z.F. Mao, M.F. Luo, Mater. Res. Bull. 42 (2007) 2184. [21] N. Gehrke, H. Cölfen, N. Pinna, M. Antonietti, N. Nassif, Cryst. Growth Des. 5 (2005) 1317.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Transformation of novel morphologies and polymorphs of CaCO3 crystals induced by the anionic surfactant SDS Zhiying Chen, Caifen Li, Qianqian Yang, Zhaodong Nan ∗ College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, Jiangsu Province 225002, China
a r t i c l e
i n f o
Article history: Received 31 January 2010 Received in revised form 22 April 2010 Accepted 4 May 2010 Keywords: Calcium carbonate Sodium dodecyl sulfate (SDS) Morphology Polymorph
a b s t r a c t Anionic surfactant, sodium dodecyl sulfate (SDS), was used as a template to modify the formation of CaCO3 crystals. Aragonite was produced when the concentration of SDS was controlled at 0.5 mM. The rod-shaped aragonite aggregated by block-like particles was fabricated. Pure vaterite with a novel flowershaped morphology was obtained when the concentration of SDS was 1.0 mM. When the concentration of SDS was increased to 2.5 mM, a mixture composed of vaterite and calcite was produced. And the as-prepared sample showed various morphologies, including tube-shaped, rod-shaped and hexagonal structures. However, the most interesting result is that rhombohedral calcite may be transformed from hexagonal vaterite. This result may prove useful to the further investigation of the processes of biomineralization. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Biological materials have unique structures and morphologies, which obtain much better performance than that for their constitute natural minerals. Many organisms make use of calcium carbonate as a constituent of their skeleton or as a protective shell, such as in sea shells, snail shells, and bird’s eggs. CaCO3 can crystallize as calcite, aragonite, or vaterite. Calcite and aragonite are the most common biologically formed CaCO3 polymorphs. Vaterite, as a less stable polymorph, will transform into calcite through a solvent-mediated process [1]. Biomimetic synthesis and polymorph-control of CaCO3 crystals have been studied in great detail as reviewed by Cölfen [2]. Many investigations have demonstrated that the properties of CaCO3 crystals, such as the crystal polymorph, the particle size and morphology, are strongly dependent on the preparation methods and additives [3–5]. Watersoluble additives, such as anionic surfactants, have always been selected to modify the polymorph of CaCO3 [6–11]. In our previous report [12], the anionic surfactant, sodium dodecyl benzene sulfonate (SDBS), was used as a template to modify the forming processes of CaCO3 crystals, and various polymorphs of CaCO3 crystals were obtained. However, no effect of SDS on polymorphs of CaCO3 was found, the formation of calcite from SDS solutions may be attributed to the tridentate motif of oxygen atoms in HSO3 − terminated monolayers, favoring the nucleation of the (0 0 1) plane of calcite in which carbonate anions are parallel to the plane [8].
∗ Corresponding author. E-mail address: [email protected] (Z. Nan). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.05.010
In this paper, SDS was employed as a template to control the growth of CaCO3 crystals at 90 ◦ C. Structures transformations from aragonite to vaterite and calcite were found. Novel morphologies of CaCO3 crystals were also obtained. 2. Materials and methods 2.1. Materials All the chemicals, including urea, calcium acetate and anionic surfactant sodium dodecyl sulfate (SDS), purchased from Sinopharm Chemical Reagent Company, were of analytical grade and used as-received without further purification. De-ionized water was used as the solvent. 2.2. Methods For the experiments, 50 mM Ca(CH3 COO)2 and 0.25 M CO(NH2 )2 were put into a Teflon-lined stainless-steel autoclave, together with concentrations of SDS of 0.5, 1.0 or 2.5 mM, respectively. This volume of chemicals was up to 70% of the total volume of the Teflon cylinder (100 ml). The autoclave was maintained at 90 ◦ C for 24 h, and subsequently cooled down to room temperature. The resulting precipitate was washed with water and alcohol several times, and dried at room temperature for more than 24 h in vacuum. 2.3. Characterization X-ray powder diffraction (XRD) patterns were recorded using a Bruker D8 Advanced XRD diffractometer with Cu K␣ radiation at a scanning rate of 0.04◦ s−1 . Scanning electron microscope (SEM) images were taken with a Hitachi S4800, fitted with a field emission source, and working at 20 kV. All samples were mounted on copper stubs and sputter coated with gold prior to examination. Selected-area electron diffraction (SAED) was obtained on a 200 kV Hitachi, Model H-800 microscope. Infrared spectroscopic analysis was performed in transmission mode (FT-IR) using a Nicolet Aexus 470, with scanning from 4000 to 500 cm−1 by using KBr pellets. The curves of thermogravimetry (TG) were obtained by a NETZSCH STA 409 PC/PG.
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Fig. 1. XRD patterns of the as-obtained samples at 90 ◦ C with different concentrations of SDS solutions: (A) 0.5 mM; (B) 1.0 mM; (C) 2.5 mM.
3. Results and discussion The XRD patterns of the obtained samples with different concentrations of SDS at 90 ◦ C are given in Fig. 1. In comparison with their standard JCPDS files (aragonite: 5-0453; calcite: 25-0127; vaterite: 33-0268), the Bragg reflections were marked with C, A and V, which correspond to the calcite polymorph, aragonite polymorph and vaterite polymorph, respectively. When the concentration of SDS was 0.5 mM, pure aragonite was fabricated as shown in Fig. 1A. When the concentration of SDS was increased to 1.0 mM, pure vaterite was obtained as given in Fig. 1B. A mixture composed of calcite and vaterite was fabricated when the concentration of SDS was further increased to 2.5 mM, as seen in Fig. 1C. When the concentration of SDS was increased to 10.0 mM, a mixture composed of calcite and vaterite was still fabricated (the result was not shown here). The contents of aragonite, vaterite and calcite contained in the as-prepared samples were calculated according to the reference [13] and are shown in Fig. 2. In order to show the effect of SDS concentration on the polymorph selection of CaCO3 crystals, the result obtained in our previous report [14] is also given in Fig. 2, for which no SDS was added to the reacting system.
Fig. 2. Percentage of three different crystals of CaCO3 with different concentrations of SDS solutions. (䊉) Vaterite, () aragonite, and () calcite.
Fig. 3. (1) FT-IR spectra of CaCO3 crystals obtained with different concentrations of SDS solutions: (A) 0.5 mM; (B) 1.0 mM; (C) 2.5 mM; (D) SDS. (2) FT-IR spectra of pure calcite, vaterite and aragonite.
The results are further demonstrated by FT-IR as given in Fig. 3(1). According to references [15,16], vibrational bands at about 1083 and 854 cm−1 can be assigned to the characteristic symmetric carbonate stretching (1 mode) and a carbonate out-of-plane
Fig. 4. TG spectra of CaCO3 obtained with different concentrations of SDS solutions: (A) 0.5 mM; (B) 1.0 mM; (C) 2.5 mM.
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Fig. 5. SEM images of the samples obtained with different concentrations of SDS solutions: (A) 0.5 mM; (B) 1.0 mM; (C)–(H) 2.5 mM.
bending (2 mode) vibrations of aragonite, respectively, and a pair of bands at about 699 and 712 cm−1 can be attributed to the inplane bending modes (4 mode) of aragonite, as shown in Fig. 3A. The band at 745 cm−1 (4 mode) is a characteristic vibration band of vaterite, as given in Fig. 3B. The bands at 877 and 712 cm−1 can
be attributed to the 2 and 4 modes of calcite, as shown in Fig. 3C. However, the bands at 587, 622, 970, 1105, 1205 and 1236 cm−1 are also found in Fig. 3C. In order to investigate further these vibrational bands, the FT-IR spectrum of the SDS was determined and given in Fig. 3D. Comparing Fig. 3C with Fig. 3D, the bands at 587, 622, 970,
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Fig. 7. FT-IR spectra of CaCO3 obtained at 1.0 mM SDS and different reaction times: (A) 6 h; (B) 12 h; (C) 48 h; (D) 72 h. Fig. 6. XRD patterns of the as-obtained samples at 1.0 mM SDS and different aging times: (A) 6 h; (B) 12 h; (C) 48 h; (D) 72 h.
1105, 1205 and 1236 cm−1 can be attributed to SDS. In order to compare these results, FT-IR spectra of pure calcite, aragonite and vaterite were shown in Fig. 3(2). The result demonstrates that the molecule of SDS was adsorbed onto the facets of the as-prepared sample. In order to prove the adsorption of SDS onto the crystal facets, the temperatures of decomposition of the obtained samples were determined by TG technology. The obtained results are given in Fig. 4. No weight loss is found when the temperature is lower than 600 ◦ C, as shown in Fig. 4A. When the experimental temperature is lower than 400 ◦ C, a weight loss is calculated to be about 0.2 and 19.0 wt% as shown in Fig. 4B and C, respectively. These kinds of weight loss are corresponding to decomposition of the surfactant SDS. The weight loss corresponding to the decomposition of CaCO3 crystals occurs at about 650 ◦ C as seen in Fig. 4. When the concentration of SDS is increased from 0.5 to 2.5 mM, the adsorption of SDS onto the CaCO3 crystal facets is increased. These results are in agreement with those obtained by FT-IR as shown in Fig. 3. The present results are, however, different from those reported in reference [8], for which no effect of SDS on the polymorphs of CaCO3 was found at room temperature. In our previous report [12], SDBS induced the formation of vaterite at 90 ◦ C. The mechanism of structure correspondence was applied to explain the formation processes. In the present study, SDS can also induce the formation of vaterite when the concentration of SDS is equal to (or smaller than) 1.0 mM. When the concentration of SDS is higher than 1.0 mM, a mixture of polymorphs, composed of calcite and vaterite, was fabricated. The adsorption of SDS onto the facets of the CaCO3 crystals may affect the formed polymorph. When the concentration of SDS is smaller than 0.5 mM, pure aragonite was prepared. The adsorption of SDS onto the as-prepared samples increases with the increasing concentration of SDS. This kind of adsorption may decrease the energy of nucleation of vaterite and therefore vaterite was produced. Fig. 5 shows SEM images of the samples fabricated with different concentrations of SDS. Fig. 5A shows rod-shaped structures aggregated by smaller blocks. It is a novel morphology of aragonite which has never been reported before. A novel kind of flower-shaped morphology is found in Fig. 5B, for which pure vaterite was fabricated according to the results shown in Fig. 2. A mixture composed of calcite and vaterite was synthesized in 2.5 mM SDS as given in Fig. 2. Different morphologies are seen in Fig. 5C–H, for which Fig. 5D and F
are magnified images corresponding to Fig. 5C and E, respectively. Tube-shaped particles are found in Fig. 5C and D, for which the inner diameter is determined to be about 100 nm, the thickness of the tube to be about 15 nm, and the length of the tube to be about 300–800 nm. The tubes may be formed by the rolling up of a pseudolayered structure as indicated by the arrows shown in Fig. 5D. A similar mechanism was used to explain the formation of tubeshaped WS2 [17]. Rhombohedral structures are also seen in Fig. 5E, which is the typical morphology of calcite. At the same time, rodshaped structures are also found in Fig. 5E. A magnified image is given in Fig. 5F and this structure is assembled by block-shaped particles. Hexagonal particles are also shown with various magnitude in Fig. 5G and H and these hexagonal particles may be vaterite according to the references [18–20]. The present results demonstrate that the anionic surfactant SDS induces various effects with different applied aging times. The XRD patterns of the fabricated samples with the same concentration of SDS (1.0 mM) at different aging time are given in Fig. 6. No clear peak is seen in Fig. 6A. Fig. 6B and C can be indexed as pure vaterite, for which the aging time is 12 and 48 h, respectively. When the aging time is further increased to 72 h, pure calcite is fabricated as shown in Fig. 6D. These results are further emphasized by FT-IR as given in Fig. 7. Spectra A–C in Fig. 7 show that vibrational bands at about 877 (2 mode), and 742 cm−1 (4 mode) can be assigned to the vaterite. The vibrational bands at about 589 and 622 cm−1 can be clearly found in Fig. 7A, which are assigned to the SDS. These results prove that the molecules of SDS were adsorbed onto the facets of CaCO3 crystals. The bands corresponding to calcite are only found in Fig. 7D. The results obtained from FT-IR are in agreement with those obtained by XRD. Fig. 8 shows SEM images of the fabricated samples obtained after various aging times. When the aging time is 6 h, flowerand spherical-shaped crystals are observed as shown in Fig. 8A and B, respectively. Pure vaterite was fabricated according to the result obtained by Fig. 7A. When the aging time was increased to 12 h, broom-shaped crystals aggregates are found, as shown in Fig. 8C and pure vaterite was obtained. In order to see clearly the linear-shaped structure of these aggregates, an image at higher magnification is shown in Fig. 8D. When the aging time was increased to 24 h, the flower-shaped vaterite is produced as shown in Fig. 5B. When the aging time was further increased to 48 h, various morphologies of particles are found as shown in Fig. 8E and F. Pure vaterite was produced in this experimental condition. Comparing Fig. 8E with Fig. 8A, similar parts are found as indicated by
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Fig. 8. SEM images of the samples obtained at 1.0 mM SDS and different aging times: (A) and (B) 6 h; (C) and (D) 12 h; (E) and (F) 48 h; (G) and (H) 72 h. Inset in (F) shows electron diffraction pattern of vaterite.
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ever, hexagonal particles are found when a mixture composed of both vaterite and calcite is fabricated as shown in Fig. 5G and H. This phenomenon has not been reported previously as far as we know. 4. Conclusion
Fig. 9. Formation processes of CaCO3 crystals at various aging times: (A) 6 h; (B) 12 h; (C) 24 h; (D) 48 h; (E) 72 h.
the arrow. At the same time, hexagonal particles are also seen as shown in Fig. 8F. In order to further study these hexagonal particles, SAED image is shown in Fig. 8F (as inset). The obtained result demonstrates that these hexagonal crystals are vaterite. When the aging time is further increased to 72 h, belt-shaped crystals are obtained as shown in Fig. 8G, which pure calcite was produced. The magnified image in Fig. 8H shows that the belt-shaped particles are aggregated by smaller rhombohedral structures. Based on the experimental observations at different aging times, the proposed morphology sequence of CaCO3 crystals in 1.0 mM SDS is shown schematically in Fig. 9. Spherical vaterite was fabricated at first as Fig. 9A. The flower-shaped structure (given in Fig. 8A) may be aggregated with the spherical particles. When the aging time was increased from 6 to 12 h, the spherical vaterite transformed into rectilinear vaterite by a solution-mediated process as shown in Fig. 8D. The resulting rectilinear products were wrapped around a central block-shaped vaterite through a dissolution-recrystallization process and the flower-shaped structure (given in Fig. 5B) was obtained as shown in Fig. 9C. When the aging time was increased from 24 to 48 h, hexagonal vaterite (given in Fig. 8F) was produced as shown in Fig. 9D. At present, it is not fully understood how this form of hexagonal vaterite is produced. Vaterite often appears in a spherical morphology; however, hexagonal vaterite has been reported [18–20]. Usually vaterite is unable to expose (0 0 1) crystal faces, which consists of a hexagonal lattice of carbonate ions or calcium ions. Cölfen suggested that ammonium ions adsorbed on the (0 0 1) faces of vaterite and act as a stabilizing agent [21]. Besides ammonium ions, some organic additives can also adsorb onto the faces of vaterite and induce the formation of novel morphologies [20]. This phenomenon will be investigated in the future. We propose that the rectilinear vaterite may first transform into rod-shaped particles similar to those shown in Fig. 5F, then the rod-shaped particles may transform into hexagonal vaterite. The morphology of the sample shown in Fig. 8E demonstrates this kind of transformation, in which block-shaped particles were formed. The rod-shaped particles surrounded by block-shaped particles, and the spherical structure surrounded by hexagonal particles are all found as shown in Fig. 5F and H, respectively. The spherical structure may also be transformed from the rod-shaped particles. When the aging time is further increased from 48 to 72 h, the resulting hexagonal vaterite is completely transformed into rhombohedral calcite as shown in Fig. 9E. The process by which vaterite transforms into calcite has been studied in detail and in the present results, the morphology of vaterite transforms into a hexagonal morphology at first, then this hexagonal vaterite transforms into calcite. No hexagonal vaterite is found when pure vaterite is produced as shown in Fig. 5B. How-
SDS has been used to modify the polymorph and morphology of CaCO3 crystals. Aragonite was produced when the concentration of SDS was in the range from 0 to 0.5 mM. However, various morphologies were obtained with different concentrations of SDS. Rod-shaped aragonite with smooth faces was fabricated without addition of SDS as our previous report [12]. Rod-shaped aragonite aggregated by block-like particles was fabricated when the concentration of SDS was 0.5 mM. The nucleus of vaterite may become stable when the molecule of SDS was absorbed onto the vaterite. A novel flower-shaped vaterite was obtained when the concentration of SDS was 1.0 mM. When the concentration of SDS was further increased to 2.5 mM, a mixture composed of both vaterite and calcite was produced. The as-prepared sample shows various morphologies, including tube-shaped, rod-shaped and hexagonal structures. When the concentration of SDS was 1.0 mM, vaterite was fabricated at aging times shorted than 48 h. A variety of morphologies of vaterite were found at different aging times, including spherical, rectilinear and hexagonal structures. The resulting vaterite transformed into calcite during aging between 48 and 72 h. The most interesting result is that rhombohedral calcite may be transformed from hexagonal vaterite. The results obtained in the present work may be useful generally to the further study of the formation mechanisms of biominerilization and to the preparation of other functional materials. Conflict of interest No conflict of interest. Acknowledgments The financial support from the National Science Foundation of China (20753002) and the Natural & Scientific Grant of Jiangsu Province (BK2009181), China, is gratefully acknowledged. References [1] A. Lopezmacipe, J. Fomezmorales, R. Rodriguezclemente, J. Cryst. Growth 166 (1996) 1015. [2] H. Cölfen, Curr. Opin. Colloid. Interface Sci. 8 (2003) 23. [3] A.J. Xie, Y.H. Shen, X.Y. Li, Z.W. Yuan, L.G. Qiu, C.Y. Zhang, Y.F. Yang, Mater. Chem. Phys. 101 (2007) 87. [4] C. Li, L. Qi, Angew. Chem. Int. Ed. 47 (2008) 2388. [5] D. Liu, M.Z. Yates, Langmuir 22 (2006) 5566. [6] H. Wei, Q. Shen, Y. Zhao, D. Wang, D. Xu, J. Cryst. Growth 264 (2004) 424. [7] K. Hosoi, T. Hashida, H. Takahashi, N. Yamasaki, T. Korenaga, J. Mater. Sci. Lett. 16 (1997) 382. [8] H. Wei, Q. Shen, Y. Zhao, Y. Zhou, D. Wang, D. Xu, J. Cryst. Growth 279 (2005) 439. [9] Q. Shen, H. Wei, L. Wang, Y. Zhou, Y. Zhao, Z. Zhang, D. Wang, G. Xu, D. Xu, J. Phys. Chem. B 109 (2005) 18342. [10] N. Abdel-Aal, K. Sawada, J. Cryst. Growth 256 (2003) 188. [11] K.D. Demadis, S.D. Katarachia, Phosphorus Sulfur Silicon 179 (2004) 627. [12] Z. Nan, X. Chen, Q. Yang, X. Wang, Z. Shi, W. Hou, Colloid Interface Sci. 325 (2008) 331. [13] C.G. Kontoyannis, N.V. Vagenas, Analyst 125 (2000) 251. [14] Z. Nan, Z. Shi, B. Yan, R. Guo, W. Hou, J. Colloid Interface Sci. 317 (2008) 77. [15] L.F. Wang, I. Sondi, E. Matijevic, J. Colloid Interface Sci. 218 (1999) 545. [16] T.J. Mason (Ed.), Sonochemistry: The Uses of Ultrasound in Chemistry, The Royal Society of Chemistry, Cambridge, UK, 1990. [17] Y.D. Li, X.L. Li, R.R. He, J. Zhu, Z.X. Deng, J. Am. Chem. Soc. 124 (2002) 1411. [18] A.W. Xu, M. Antonietti, H. Cölfen, Y.P. Fang, Adv. Funct. Mater. 16 (2006) 903. [19] M. Sedlák, H. Cölfen, Macromol. Chem. Phys. 202 (2001) 587. [20] J.H. Huang, Z.F. Mao, M.F. Luo, Mater. Res. Bull. 42 (2007) 2184. [21] N. Gehrke, H. Cölfen, N. Pinna, M. Antonietti, N. Nassif, Cryst. Growth Des. 5 (2005) 1317.