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Influence of surfactants on the morphologies of CaCO3 by carbonation route with compressed CO2
Colloids and Surfaces A: Physicochem. Eng. Aspects 324 (2008) 167–170
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Influence of surfactants on the morphologies of CaCO3 by carbonation route with compressed CO2 Chaoxing Zhang, Jianling Zhang, Xiaoying Feng, Wei Li, Yueju Zhao, Buxing Han ∗ National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
a r t i c l e
i n f o
Article history: Received 10 December 2007 Received in revised form 19 March 2008 Accepted 8 April 2008 Available online 15 April 2008 Keywords: CaCO3 CO2 Carbonation route SDS CTAB Tween 80
a b s t r a c t Three surfactants, sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB) and polyoxyethylene-80-sorbitan monooleate (Tween 80), were used to control the growth of CaCO3 crystals by carbonation route using Ca(OH)2 and compressed CO2 . The obtained CaCO3 particles were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The effects of surfactants on the morphology of the particles were studied. It was demonstrated that Tween 80 and SDS have obvious effect on the morphology of CaCO3 particles, while CTAB does not affect morphology considerably. The possible mechanism has been discussed on the basis of the binding of the surfactants to the certain face of the crystals. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Inorganic materials of different morphologies and sizes have showed excellent properties in catalysis [1,2], optical and electronic devices and sensors [3], magnetic storage and pigments [4,5]. Therefore, synthesis of inorganic materials has been an interesting topic for many years [6,7]. Calcium carbonate (CaCO3 ) is one of the most abundant biomineral in nature, which can form organic–inorganic hybrid composites with controlled structure and properties. Besides, CaCO3 is an important industrial material. It can be used in various fields, such as paper, paint, magnetic recording, textile, detergent, adhesive, plastic, cosmetic, food, etc. [8]. The morphology, polymorph, particle size and the chemical purity are the crucial parameters of the materials. Therefore, many methods have been used to control these parameters [9–13]. There are mainly two pathways to synthesize CaCO3 crystals. One is solution route, typically, CaCl2 (or Ca(OH)2 ) and Na2 CO3 (or (NH4 )2 CO3 ) are used as the sources to produce CaCO3 . Another way, which uses Ca(OH)2 and gas CO2 as the sources, is named as carbonation route. The latter one is preferred in terms of environment protection and effective use of mineral resources [14]. The morphology of industrial products is scalenoherdral calcite. However, with the usual carbonation route the crystal shape and modification of
CaCO3 are difficult to control [14]. Compressed or supercritical (sc) CO2 has showed excellent advantages in material preparations, due to their special properties that are dependent of its temperature and pressure. There have been some reports on the formation or modification of CaCO3 crystals by scCO2 [15–17]. For instance, Gu et al. proposed a method to prepare CaCO3 from Ca(OH)2 and scCO2 by direct contact of Ca(OH)2 powders with wet scCO2 at 50 ◦ C, and a conversion of 98% of Ca(OH)2 was reached [18]. Domingo et al. have synthesized rhombic calcite with carbonation route by highly compressible CO2 through the slurry of Ca(OH)2 at 25 ◦ C, and rhombic calcite with less agglomerates was obtained [8]. In this work, we studied the preparation of CaCO3 by carbonation route with compressed CO2 , where surfactants were used as the crystal growth modifiers. The effects of three commonly used surfactants, polyoxyethylene-80-sorbitan monooleate (Tween 80, nonionic surfactant), sodium dodecyl sulfate (SDS, anionic surfactant), and cetyltrimethylammonium bromide (CTAB, cationic surfactant), on the morphology of CaCO3 were investigated. It was demonstrated that the surfactants have different effects on the growth of CaCO3 crystals. 2. Experimental 2.1. Materials
∗ Corresponding author. Tel: +86 10 62562821; fax: +86 10 62559373. E-mail address: [email protected] (B. Han). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.04.010
Calcium hydroxide (Ca(OH)2 , analytical grade), SDS (reagent grade), CTAB (analytical grade) are all purchased from Beijing
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C. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 324 (2008) 167–170
into the autoclave, which was placed in the water bath of 20 ◦ C. Then CO2 was charged into the autoclave under vigorous stirring until a certain pressure was reached. After reaction for 1 h, the CO2 was released slowly. The products were collected and washed several times with water until the supernatant solution is clear. The white powder on the bottom of the autoclave was collected and dried under vacuum at 60 ◦ C for several hours. As the reference experiment, CaCO3 was also produced without using any surfactant. 2.3. Characterization The resulting products were characterized by SEM (Modle.S4300) and XRD (Model D/MAX2500, Rigaka) with Cu K␣ radiation. 3. Results and discussions Fig. 1. XRD patterns of the CaCO3 crystals produced at 7.43 MPa without surfactant (a), in the presence of Tween 80 (b), CTAB (c), and SDS (d), respectively.
Chemical Reagent Company. Tween 80 (reagent grade), was bought from Amresco Company. CO2 (>99.995% purity) was provided by Beijing Analytical Instrument Factory. Double-distilled water was used. All the above reagents were used without further purification. 2.2. Synthesis of CaCO3 The experimental apparatus was similar to that used previously [19]. It was mainly composed of an autoclave (23 ml), a highpressure pump, a constant temperature water bath, and a pressure gauge. In a typical experiment, a certain amount of Ca(OH)2 aqueous suspension (2.2 g/L) containing surfactant (2.0 g/L) was loaded
3.1. Without surfactant To study the influence of the surfactants on the morphology and size of the obtained CaCO3 particles, some reference experiments were carried out without use of surfactant at different CO2 pressures. Fig. 1a is the XRD pattern of the CaCO3 particles obtained at 7.43 MPa, from which the sharp diffraction patterns of calcite CaCO3 can be observed. The SEM images of the CaCO3 particles formed at different pressures are shown in Fig. 2. The typical rhombohedral calcite CaCO3 crystals with smooth surface can be clearly observed. And with increasing pressure from 4.90 to 7.43 MPa and 12.04 MPa (Fig. 2a–c, respectively), there is no remarkable difference in the morphologies and size of the obtained CaCO3 particles. This is similar to the
Fig. 2. SEM images of CaCO3 prepared at 4.90 MPa (a), 7.43 MPa (b) and 12.04 MPa (c) in the absence of surfactant.
Fig. 3. SEM images of CaCO3 prepared at 5.04 MPa (a), 7.43 MPa (b) and 12.02 MPa (c) in the presence of Tween 80.
C. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 324 (2008) 167–170
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Fig. 4. SEM images of CaCO3 particles prepared at 7.43 MPa in the presence of SDS (a) and CTAB (b).
result reported by Domingo et al. that change of CO2 pressure from 7 to 20 MPa did not appreciably influence the crystal morphology and the particle size distribution of the rhombic calcite CaCO3 particles synthesized with carbonation route by highly compressible CO2 and the slurry of Ca(OH)2 [8]. 3.2. In the presence of Tween 80 We investigated the effect of the surfactant on the growth of CaCO3 crystals at different CO2 pressures by carbonation route. The XRD pattern of the particles is shown in Fig. 1b. It can be seen that the calcite CaCO3 was formed in the presence of Tween 80. Fig. 3 shows the corresponding SEM images of CaCO3 obtained at different pressures. The cubic CaCO3 particles aggregated by many plate-like crystals along c-axis were produced. At the CO2 pressures of 5.04, 7.43 and 12.02 MPa, the average particle sizes were 2, 3 and 4 m, respectively, as can be known from Fig. 3a to c. Clearly, the morphology of the obtained CaCO3 was different from those of CaCO3 particles produced in the absence of the surfactant. This may be due to the specific binding of the surfactant to a particular crystal face. Tween 80 is a kind of nonionic surfactant, which makes it preferably adsorb on the neutral {1 0 4} face. Thus the reaction of hydrated CO3 2− and Ca2+ ions on the {1 0 4} face was inhibited to some extent and the growth speed of {1 0 4} face was slow, resulting in the final formation of the plate-like morphology. As to the face {0 0 1}, the reaction of hydrated CO3 2− ions and Ca2+ was not affected by the surfactant considerably, resulting in the appearance of the morphology [20].
In the presence of CTAB, typical rhombohedral CaCO3 particles were formed (Fig. 4b). By comparing with the CaCO3 particles obtained without surfactant at the same CO2 pressure (Fig. 2b), it can be found that the effect of CTAB on the morphologies of CaCO3 is not considerable. This is similar to that reported by Yu et al. that the addition of CTAB had no obvious effect on the morphology of the synthesized CaCO3 particles by solution route [21]. It may be due to the electrostatic repulsion interaction between the positively charged alkyl chain and the Ca2+ ion, which makes its adsorption onto the face of CaCO3 difficult. Therefore, addition of CTAB does not affect the morphology of the obtained CaCO3 particles. 4. Conclusion CaCO3 crystals with different morphologies were synthesized in the presence of surfactants by carbonation route with compressible CO2 . It was found that Tween 80, SDS, and CTAB have different effects on the formation of CaCO3 crystals. The main reason is that binding ability of the surfactants to crystal faces of CaCO3 is different. Combination of surfactants and compressible CO2 is the promising route to prepare CaCO3 materials of different morphologies. Acknowledgement The authors are grateful to the National Natural Science Foundation of China (20633080). References
3.3. In the presence of CTAB and SDS The effects of other two kinds of surfactants, cationic surfactant CTAB and anionic surfactant SDS, on the morphology of CaCO3 particles prepared were also investigated. Fig. 1c and d are the XRD patterns of the CaCO3 crystals produced at 7.43 MPa in the presence of CTAB and SDS, respectively, which are the same as those of the CaCO3 crystals obtained in the presence of Tween 80 shown in Fig. 1b. Fig. 4a is the SEM image of CaCO3 particles fabricated in the presence of SDS at 7.43 MPa. It can be seen clearly that the cubic CaCO3 particles with rough surface were obtained. In aqueous solution, the alkyl chain of SDS carried negative charge, which can be absorbed onto the positively charged face of CaCO3 , such as {0 0 1}, {1 0 1}, {1 1 0} faces, inhibiting their further growth of some faces and is favorable to the formation of cubic CaCO3 particles with rough surface.
[1] J. Kim, O. Bondarchuk, B.D. Kay, J.M. White, Z. Dohnalek, Catal. Today 120 (2007) 186–195. [2] M.C. McLeod, W.F. Gale, C.B. Roberts, Langmuir 20 (2004) 7078–7082. [3] F.M. Davidson, R.J. Wiacek, B.A. Korgel, Chem. Mater. 17 (2005) 230–233. [4] T. Adschiri, Y. Hakuta, K. Arai, Ind. Eng. Chem. Res. 39 (2000) 4901–4907. [5] A. Cabanas, M. Poliakoff, J. Mater. Chem. 11 (2001) 1408–1416. [6] H. Gu, M.D. Soucek, Chem. Mater. 19 (2007) 1103–1110. [7] A. Szegedi, M. Popova, V. Mavrodinova, M. Urban, I. Kiricsi, C. Minchev, Micropor. Mesopor. Mater. 99 (2007) 149–158. ´ [8] C. Domingo, E. Loste, J. Gomez-Morales, J. Garc´ıa-Carmona, J. Fraile, J. Supercrit. Fluids 36 (2006) 202–215. ¨ [9] S.H. Yu, H. Colfen, J. Hartmann, M. Antonietti, Adv. Funct. Mater. 12 (2002) 541–545. [10] N. Kensuke, T. Yasuyuki, C. Yoshiki, I. Yoshikatsu, Chem. Commun. 19 (1999) 1931–1932. ¨ [11] H. Colfen, L. Qi, Chem. Eur. J. 7 (2001) 106–116. [12] C. Damle, A. Kumar, S.R. Sainkar, M. Bhagawat, M. Sastry, Langmuir 18 (2002) 6075–6080. [13] Y.S. Han, G. Hadiko, M. Fuji, M. Takahashi, J. Cryst. Growth 289 (2006) 269–274. [14] C. Wang, Y. Sheng, X. Zhao, Y. Pan, H. Bala, Z. Wang, Mater. Lett. 60 (2006) 854–857.
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[15] C.A. Garcia-Gonzalez, N. el Grouh, A. Hidalgo, J. Fraile, A.M. Lopez-Periago, C. Andrade, C. Domingo, J. Supercrit. Fluids 43 (2008) 500–509. [16] G. Montes-Hernandez, F. Renard, N. Geoffroy, L. Charlet, J. Pironon, J. Cryst. Growth 308 (2007) 228–236. [17] L. Xin, S. Matsuya, M. Nakagawa, Y. Terada, K. Ishikawa, J. Mater. Sci. Mater.-M 19 (2008) 479–484.
[18] W. Gu, D.W. Bousfield, C.P. Tripp, J. Mater. Chem. 16 (2006) 3312–3317. [19] H.F. Zhang, Z.M. Liu, B.X. Han, J. Supercrit. Fluids 18 (2000) 185–192. [20] R. Sundara, M. Stephen, J. Chem. Soc., Chem. Commun. 24 (1990) 1789–1791. [21] J. Yu, X. Zhao, B. Cheng, Q. Zhang, J. Solid State Chem. 178 (2005) 861–867.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Influence of surfactants on the morphologies of CaCO3 by carbonation route with compressed CO2 Chaoxing Zhang, Jianling Zhang, Xiaoying Feng, Wei Li, Yueju Zhao, Buxing Han ∗ National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
a r t i c l e
i n f o
Article history: Received 10 December 2007 Received in revised form 19 March 2008 Accepted 8 April 2008 Available online 15 April 2008 Keywords: CaCO3 CO2 Carbonation route SDS CTAB Tween 80
a b s t r a c t Three surfactants, sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB) and polyoxyethylene-80-sorbitan monooleate (Tween 80), were used to control the growth of CaCO3 crystals by carbonation route using Ca(OH)2 and compressed CO2 . The obtained CaCO3 particles were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The effects of surfactants on the morphology of the particles were studied. It was demonstrated that Tween 80 and SDS have obvious effect on the morphology of CaCO3 particles, while CTAB does not affect morphology considerably. The possible mechanism has been discussed on the basis of the binding of the surfactants to the certain face of the crystals. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Inorganic materials of different morphologies and sizes have showed excellent properties in catalysis [1,2], optical and electronic devices and sensors [3], magnetic storage and pigments [4,5]. Therefore, synthesis of inorganic materials has been an interesting topic for many years [6,7]. Calcium carbonate (CaCO3 ) is one of the most abundant biomineral in nature, which can form organic–inorganic hybrid composites with controlled structure and properties. Besides, CaCO3 is an important industrial material. It can be used in various fields, such as paper, paint, magnetic recording, textile, detergent, adhesive, plastic, cosmetic, food, etc. [8]. The morphology, polymorph, particle size and the chemical purity are the crucial parameters of the materials. Therefore, many methods have been used to control these parameters [9–13]. There are mainly two pathways to synthesize CaCO3 crystals. One is solution route, typically, CaCl2 (or Ca(OH)2 ) and Na2 CO3 (or (NH4 )2 CO3 ) are used as the sources to produce CaCO3 . Another way, which uses Ca(OH)2 and gas CO2 as the sources, is named as carbonation route. The latter one is preferred in terms of environment protection and effective use of mineral resources [14]. The morphology of industrial products is scalenoherdral calcite. However, with the usual carbonation route the crystal shape and modification of
CaCO3 are difficult to control [14]. Compressed or supercritical (sc) CO2 has showed excellent advantages in material preparations, due to their special properties that are dependent of its temperature and pressure. There have been some reports on the formation or modification of CaCO3 crystals by scCO2 [15–17]. For instance, Gu et al. proposed a method to prepare CaCO3 from Ca(OH)2 and scCO2 by direct contact of Ca(OH)2 powders with wet scCO2 at 50 ◦ C, and a conversion of 98% of Ca(OH)2 was reached [18]. Domingo et al. have synthesized rhombic calcite with carbonation route by highly compressible CO2 through the slurry of Ca(OH)2 at 25 ◦ C, and rhombic calcite with less agglomerates was obtained [8]. In this work, we studied the preparation of CaCO3 by carbonation route with compressed CO2 , where surfactants were used as the crystal growth modifiers. The effects of three commonly used surfactants, polyoxyethylene-80-sorbitan monooleate (Tween 80, nonionic surfactant), sodium dodecyl sulfate (SDS, anionic surfactant), and cetyltrimethylammonium bromide (CTAB, cationic surfactant), on the morphology of CaCO3 were investigated. It was demonstrated that the surfactants have different effects on the growth of CaCO3 crystals. 2. Experimental 2.1. Materials
∗ Corresponding author. Tel: +86 10 62562821; fax: +86 10 62559373. E-mail address: [email protected] (B. Han). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.04.010
Calcium hydroxide (Ca(OH)2 , analytical grade), SDS (reagent grade), CTAB (analytical grade) are all purchased from Beijing
168
C. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 324 (2008) 167–170
into the autoclave, which was placed in the water bath of 20 ◦ C. Then CO2 was charged into the autoclave under vigorous stirring until a certain pressure was reached. After reaction for 1 h, the CO2 was released slowly. The products were collected and washed several times with water until the supernatant solution is clear. The white powder on the bottom of the autoclave was collected and dried under vacuum at 60 ◦ C for several hours. As the reference experiment, CaCO3 was also produced without using any surfactant. 2.3. Characterization The resulting products were characterized by SEM (Modle.S4300) and XRD (Model D/MAX2500, Rigaka) with Cu K␣ radiation. 3. Results and discussions Fig. 1. XRD patterns of the CaCO3 crystals produced at 7.43 MPa without surfactant (a), in the presence of Tween 80 (b), CTAB (c), and SDS (d), respectively.
Chemical Reagent Company. Tween 80 (reagent grade), was bought from Amresco Company. CO2 (>99.995% purity) was provided by Beijing Analytical Instrument Factory. Double-distilled water was used. All the above reagents were used without further purification. 2.2. Synthesis of CaCO3 The experimental apparatus was similar to that used previously [19]. It was mainly composed of an autoclave (23 ml), a highpressure pump, a constant temperature water bath, and a pressure gauge. In a typical experiment, a certain amount of Ca(OH)2 aqueous suspension (2.2 g/L) containing surfactant (2.0 g/L) was loaded
3.1. Without surfactant To study the influence of the surfactants on the morphology and size of the obtained CaCO3 particles, some reference experiments were carried out without use of surfactant at different CO2 pressures. Fig. 1a is the XRD pattern of the CaCO3 particles obtained at 7.43 MPa, from which the sharp diffraction patterns of calcite CaCO3 can be observed. The SEM images of the CaCO3 particles formed at different pressures are shown in Fig. 2. The typical rhombohedral calcite CaCO3 crystals with smooth surface can be clearly observed. And with increasing pressure from 4.90 to 7.43 MPa and 12.04 MPa (Fig. 2a–c, respectively), there is no remarkable difference in the morphologies and size of the obtained CaCO3 particles. This is similar to the
Fig. 2. SEM images of CaCO3 prepared at 4.90 MPa (a), 7.43 MPa (b) and 12.04 MPa (c) in the absence of surfactant.
Fig. 3. SEM images of CaCO3 prepared at 5.04 MPa (a), 7.43 MPa (b) and 12.02 MPa (c) in the presence of Tween 80.
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Fig. 4. SEM images of CaCO3 particles prepared at 7.43 MPa in the presence of SDS (a) and CTAB (b).
result reported by Domingo et al. that change of CO2 pressure from 7 to 20 MPa did not appreciably influence the crystal morphology and the particle size distribution of the rhombic calcite CaCO3 particles synthesized with carbonation route by highly compressible CO2 and the slurry of Ca(OH)2 [8]. 3.2. In the presence of Tween 80 We investigated the effect of the surfactant on the growth of CaCO3 crystals at different CO2 pressures by carbonation route. The XRD pattern of the particles is shown in Fig. 1b. It can be seen that the calcite CaCO3 was formed in the presence of Tween 80. Fig. 3 shows the corresponding SEM images of CaCO3 obtained at different pressures. The cubic CaCO3 particles aggregated by many plate-like crystals along c-axis were produced. At the CO2 pressures of 5.04, 7.43 and 12.02 MPa, the average particle sizes were 2, 3 and 4 m, respectively, as can be known from Fig. 3a to c. Clearly, the morphology of the obtained CaCO3 was different from those of CaCO3 particles produced in the absence of the surfactant. This may be due to the specific binding of the surfactant to a particular crystal face. Tween 80 is a kind of nonionic surfactant, which makes it preferably adsorb on the neutral {1 0 4} face. Thus the reaction of hydrated CO3 2− and Ca2+ ions on the {1 0 4} face was inhibited to some extent and the growth speed of {1 0 4} face was slow, resulting in the final formation of the plate-like morphology. As to the face {0 0 1}, the reaction of hydrated CO3 2− ions and Ca2+ was not affected by the surfactant considerably, resulting in the appearance of the morphology [20].
In the presence of CTAB, typical rhombohedral CaCO3 particles were formed (Fig. 4b). By comparing with the CaCO3 particles obtained without surfactant at the same CO2 pressure (Fig. 2b), it can be found that the effect of CTAB on the morphologies of CaCO3 is not considerable. This is similar to that reported by Yu et al. that the addition of CTAB had no obvious effect on the morphology of the synthesized CaCO3 particles by solution route [21]. It may be due to the electrostatic repulsion interaction between the positively charged alkyl chain and the Ca2+ ion, which makes its adsorption onto the face of CaCO3 difficult. Therefore, addition of CTAB does not affect the morphology of the obtained CaCO3 particles. 4. Conclusion CaCO3 crystals with different morphologies were synthesized in the presence of surfactants by carbonation route with compressible CO2 . It was found that Tween 80, SDS, and CTAB have different effects on the formation of CaCO3 crystals. The main reason is that binding ability of the surfactants to crystal faces of CaCO3 is different. Combination of surfactants and compressible CO2 is the promising route to prepare CaCO3 materials of different morphologies. Acknowledgement The authors are grateful to the National Natural Science Foundation of China (20633080). References
3.3. In the presence of CTAB and SDS The effects of other two kinds of surfactants, cationic surfactant CTAB and anionic surfactant SDS, on the morphology of CaCO3 particles prepared were also investigated. Fig. 1c and d are the XRD patterns of the CaCO3 crystals produced at 7.43 MPa in the presence of CTAB and SDS, respectively, which are the same as those of the CaCO3 crystals obtained in the presence of Tween 80 shown in Fig. 1b. Fig. 4a is the SEM image of CaCO3 particles fabricated in the presence of SDS at 7.43 MPa. It can be seen clearly that the cubic CaCO3 particles with rough surface were obtained. In aqueous solution, the alkyl chain of SDS carried negative charge, which can be absorbed onto the positively charged face of CaCO3 , such as {0 0 1}, {1 0 1}, {1 1 0} faces, inhibiting their further growth of some faces and is favorable to the formation of cubic CaCO3 particles with rough surface.
[1] J. Kim, O. Bondarchuk, B.D. Kay, J.M. White, Z. Dohnalek, Catal. Today 120 (2007) 186–195. [2] M.C. McLeod, W.F. Gale, C.B. Roberts, Langmuir 20 (2004) 7078–7082. [3] F.M. Davidson, R.J. Wiacek, B.A. Korgel, Chem. Mater. 17 (2005) 230–233. [4] T. Adschiri, Y. Hakuta, K. Arai, Ind. Eng. Chem. Res. 39 (2000) 4901–4907. [5] A. Cabanas, M. Poliakoff, J. Mater. Chem. 11 (2001) 1408–1416. [6] H. Gu, M.D. Soucek, Chem. Mater. 19 (2007) 1103–1110. [7] A. Szegedi, M. Popova, V. Mavrodinova, M. Urban, I. Kiricsi, C. Minchev, Micropor. Mesopor. Mater. 99 (2007) 149–158. ´ [8] C. Domingo, E. Loste, J. Gomez-Morales, J. Garc´ıa-Carmona, J. Fraile, J. Supercrit. Fluids 36 (2006) 202–215. ¨ [9] S.H. Yu, H. Colfen, J. Hartmann, M. Antonietti, Adv. Funct. Mater. 12 (2002) 541–545. [10] N. Kensuke, T. Yasuyuki, C. Yoshiki, I. Yoshikatsu, Chem. Commun. 19 (1999) 1931–1932. ¨ [11] H. Colfen, L. Qi, Chem. Eur. J. 7 (2001) 106–116. [12] C. Damle, A. Kumar, S.R. Sainkar, M. Bhagawat, M. Sastry, Langmuir 18 (2002) 6075–6080. [13] Y.S. Han, G. Hadiko, M. Fuji, M. Takahashi, J. Cryst. Growth 289 (2006) 269–274. [14] C. Wang, Y. Sheng, X. Zhao, Y. Pan, H. Bala, Z. Wang, Mater. Lett. 60 (2006) 854–857.
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[15] C.A. Garcia-Gonzalez, N. el Grouh, A. Hidalgo, J. Fraile, A.M. Lopez-Periago, C. Andrade, C. Domingo, J. Supercrit. Fluids 43 (2008) 500–509. [16] G. Montes-Hernandez, F. Renard, N. Geoffroy, L. Charlet, J. Pironon, J. Cryst. Growth 308 (2007) 228–236. [17] L. Xin, S. Matsuya, M. Nakagawa, Y. Terada, K. Ishikawa, J. Mater. Sci. Mater.-M 19 (2008) 479–484.
[18] W. Gu, D.W. Bousfield, C.P. Tripp, J. Mater. Chem. 16 (2006) 3312–3317. [19] H.F. Zhang, Z.M. Liu, B.X. Han, J. Supercrit. Fluids 18 (2000) 185–192. [20] R. Sundara, M. Stephen, J. Chem. Soc., Chem. Commun. 24 (1990) 1789–1791. [21] J. Yu, X. Zhao, B. Cheng, Q. Zhang, J. Solid State Chem. 178 (2005) 861–867.