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O microemulsion
Colloids and Surfaces A: Physicochem. Eng. Aspects 256 (2005) 57–60
Morphosynthesis of microskeletal silica spheres templated by W/O microemulsion Cheng Tao, Junbai Li∗ International Joint Lab, Key Lab of Colloid and Interface Science, The Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Zhong Guan Cun, Beijing 100080, China
Abstract AOT containing water-in-oil (W/O) microemulsion was used as an organized reaction environment for silica preparation originating from hydrolysis and crosslink of tetraethoxysilane (TEOS). The nucleation and growth of skeletal silica shperes with a diameter of 40 m were synthesized in association with microemulsion. Study on the structure and morphology, principally by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), indicates that the final product is subject to the composition of microemulsion and aging time. A possible mechanism involving collision and fusion among large quantities of droplets covered by surfactant molecules was proposed here, attempting to describe the growth of the network structured silica spheres. © 2004 Elsevier B.V. All rights reserved. Keywords: Microemulsion; SiO2 spheres; Framework
1. Introduction Fabrication of complex inorganic materials with controllable shape, morphology and structure is of great importance in many technical areas. For example, framework structured inorganic material has shown considerable promise in separation [1], catalysis [2] and materials chemistry [3]. Among the strategies employed to prepare the complex inorganic materials, it has been found that the organized reaction medium in which the formation of inorganic solids proceed is a key feature to offer a constrained environment for nucleation and growth [4–7]. Recently, it has been shown that multiphase systems, such as reverse micelle [8], oil-in-water droplet [9], and bicontinuous microemulsion [10], can be exploited as organized reactive environments for the controlled synthesis of complex inorganic materials. W/O microemulsion is thermodynamically stable with a continuous oil phase and a compartmentalized aqueous domain covered by surfactant molecules, thus able to constrain and pattern the deposi∗
Corresponding author. Tel.: +86 10 82614087; fax: +86 10 82612629. E-mail address: [email protected] (J. Li).
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.09.020
tion of spatially confined inorganic precursors. As a result, a wide range of materials including various morphological BaCO3 [8], BaSO4 [11], and CaCO3 [12] could be successfully produced. However, the synthesis of silica originating from tetraethoxysilane (TEOS), different than the products obtained from precipitation reaction, involves that the hydrolysis and crosslink processes occurred in water droplets and/or on W/O interface [13]. The structure and morphology of the products can be markedly influenced by the nature of microemulsion, such as temperature, composition, drop size, and interfacial stability. This implies the preparation targeting at the materials with special structure and morphology could be controllable by manipulating the processing steps of the synthesis, although sometimes the mechanism cannot be understood completely. With this approach we reported here that the framework structured silica spheres could be achieved through sol–gel process of TEOS in AOT containing W/O microemulsion. Our results, focusing on the morphology evolution, indicate that the skeletal spheres originate from the hydrolysis of silica source and the growth process is accompanied with the collision and fusion of W/O droplets covered by surfactant layers.
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C. Tao, J. Li / Colloids and Surfaces A: Physicochem. Eng. Aspects 256 (2005) 57–60
2. Experimental 2.1. Materials and synthesis Tetraethyl orthosilicate (TEOS), bis(2-ethylhexyl)sodium sulfosuccinate (AOT) and n-heptane were purchased from ACROS. The siliceous spheres synthesis was undertaken in W/O microemulsion prepared as follows: a solution containing 1.8 g AOT and 7.2 g C7 H16 were mixed with 1.0 mL 4 mol/L HCl solution (w = [H2 O]/[AOT] = 13.7, [H2 O]/[SiO2 ] = 11.6). The mixture was sonicated for 20 min to produce a transparent microemulsion. Based on the ternary phase diagram of the system a W/O microemulsion was obtained [14]. Then 1.0 g TEOS was carefully dropwised into the mixture during stirring, a milky emulsion with white precipitates was achieved. Afterwards the mixture was strongly stirred for 4 h at room temperature, filtrated and repeatedly washed with hot water and ethanol to collect white precipitate. The water used in all experimental steps is Millipore water (resistivity higher than 18.2 M cm). Finally, the synthesized sample was dried in air at room temperature, and the organic template was removed by calcination at 550 ◦ C for 3.5 h.
Fig. 1. SEM images of the microskeletal SiO2 spheres. (a) Aggregates of microskeletal SiO2 spheres. (b) Selected single SiO2 sphere with microskeletal structure. (c) Cross-section view of half a microskeletal SiO2 sphere. (d) Enlarged surface of the SiO2 sphere at a high magnitude.
2.2. Characterization Scanning electron microscopy (SEM) images were achieved by JEOL JSM-6301F system with an accelerating voltage of 20 kV. The transmission electron microscopy (TEM) measurements were performed on Phillips TECNAI 20 operating at 120 kV. X-ray powder diffraction patterns (XRD) were obtained on a Rigaku X-ray diffractometer D/max-2500 using Cu K␣1 radiation at 40.0 kV and 120.0 mA. N2 adsorption–desorption isotherms experiment was carried out at 77.35 K by using Quantachrome Autosorb Automated system. Samples were degassed at 150 ◦ C under high vacuum for at least 4 h before the measurement. The specific surface area was determined from the linear part of the BET equation (P/P0 = 0–1).
3. Results and discussion Fig. 1 shows SEM images of the calcined samples. A large quantity of aggregated microskeletal silica spheres were observed as presented in Fig. 1a. Estimated from most skeletal spheres, the average diameter is approximately 40 m. With a high magnitude SEM image of a single particle with the microskeletal structure and a well-defined spherical shape is visualized in Fig. 1b. A selected SEM image of one SiO2 halfsphere reveals that the inner sphere also posses regular pores (side view of Fig. 1c). Each building block of the sphere is well connected to each other, forming an ordered solid framework. From a larger magnification of partial spherical surface in Fig. 1d, the length and width of each block can be seen as more than 2 m and 0.5–2 m, respectively. The absence
of XRD peaks for the calcined samples implies that the SiO2 is amorphous, while the N2 adsorption–desorption isotherm also indicates that the sample with a surface area is 4.05 m2 /g does not have mesopores. To investigate the effect of the composition of the microemulsion on the morphology of the SiO2 products, two control experiments were performed. Firstly, the microemulsion defined as 1.8 g AOT, 16.2 g C7 H16 and 2.0 g HCl, the silica source of 1.5 g TEOS (w = [H2 O]/[AOT] = 27.4, [H2 O]/[SiO2 ] = 15.4) produced irregular morphological particles, as shown in Fig. 2a. However, if the W/O microemulsion is composed of 2.7 g AOT, 6.3 g C7 H16 and 1.0 g HCl (w = [H2 O]/[AOT] = 9.1, [H2 O]/[SiO2 ] = 23.1), the hydrolysis of 0.5 g TEOS resulted in “flake-like” SiO2 (Fig. 2b). The results suggest that the composition of microemulsion has a pronounced influence on the morphology of final products. The formation of skeletal as-synthesized SiO2 spheres in the W/O microemulsion may undergo various processes including small particle growth, fusion and aggregation [14]. The aggregation of the water droplet covered by surfactants in the microemulsion will directly influence upon the diameter of the particles, and the surfactant adsorption layers provide the pores structure. This explanation can be understood by taking the hydrolysis products of TEOS at different reaction time, which is corresponding to the different status of the microemulsion. TEM observation of the early growth stage of SiO2 showed that a predominance of separate spherical SiO2 particles after hydrolysis of 15 min under stirring. Most SiO2 particles are regular sphere shaped and possess a diameter of approximately 250–400 nm (Fig. 3a), suggesting that the surfactant covered water droplets facilitated the for-
C. Tao, J. Li / Colloids and Surfaces A: Physicochem. Eng. Aspects 256 (2005) 57–60
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Fig. 2. SEM images of (a) SiO2 particles synthesized in a ratio of 1.8 g AOT:16.2 g C7 H16 :2.0 g HCl:1.5 g TEOS in the microemulsion reaction and (b) SiO2 particles synthesized in a ratio of 2.7 g AOT:6.3 g C7 H16 :1.0 g HCl:0.5 g TEOS in the microemulsion reaction.
mation of SiO2 particles in hundreds of nanometer regime. Further hydrolysis (over 30 min) leads to the formation of lots of chainlike particles fused by small particles (Fig. 3b), confirming the particles have been integrated by droplets col-
Fig. 3. TEM images of TEOS hydrolysis products with different reaction time after (a) 15 min; (b) 30 min; (c) 1 h and (d) 2 h, respectively.
Fig. 4. Schematic illustration of the forming process for the microskeletal SiO2 spheres in W/O microemulsion.
lision and fusion. The fusion part of chainlike particles has significantly obscured the original spherical shape of particles. Then particles grew and linked to each other to form a network after undergoing one-hour reaction (Fig. 3c). Continuous growth of SiO2 network even leads to the formation of spherical curvature, which is consistent with the morphology of the final products (Fig. 3d). Therefore, it can be deduced that in the microemulsion the hydrolysis and crosslink of TEOS occurred accompanying with the growth of water droplets up to the micrometer size. However, the aggregation of droplets produced several new water interfaces where AOT adsorption layers were formed. Such layers may create some potential pore spaces. This process allows the formation of microskeletal SiO2 spheres after calcination. The possible mechanism can be described as shown in Fig. 4. The aqueous droplets containing the initial hydrolyzed products of TEOS in microemulsion are growing and moving rapidly during stirring to combine to each other (Fig. 4a and b). As the reactants TEOS further hydrolyze, the fused parti-
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C. Tao, J. Li / Colloids and Surfaces A: Physicochem. Eng. Aspects 256 (2005) 57–60
cles will be enlarged and further grown in the microemulsion (Fig. 4c). After collision and fusion of W/O microemulsion droplets covered with surfactant layers, a final larger species can be obtained [15,16]. Repeated growth of this process will create some highly branched network with interconnected channels [10]. The formation of skeletal spheres requires the driving force for reorganization and aggregation, which corresponds to a reduction in surface free energy. The surfactants always act as the boundaries of containers and templates. After the calcination they produce the interspace and allow the crosslink of SiO2 particles to form a network structure. Accordingly, it should be deduced that the phase structure of microemulsion generally results the morphological features of the final yields. This should be consistent with the previous observation and explanation [10], in which the inner connected framework of porous silica can well replicate the structure of bicontinuous microemulsion.
4. Conclusion In summary, microskeletal silica sphere in micrometer regime has been prepared employing reverse microemulsion as template. The morphology of the silica samples formed in the microemulsion is defined by the composition of microemulsion. Although the mechanism leading to the special morphology is not yet completely understood, the forming process should be subject to the collision and fusion of W/O droplets covered by surfactant layers, which produce the pore structures. Furthermore, this approach can be extended to many other sol–gel syntheses to produce inorganic materials with similar properties.
Acknowledgement This work was financially supported by the National Nature Science Foundation of China (NNSFC 29925307) and the collaborated project of German Max Planck Society. References [1] Y.N. Jun, D.M. Dabbs, I.A. Aksay, S. Erramilli, Langmuir 10 (1994) 3377. [2] P.T. Tanev, M. Chibwe, T.J. Pinnavania, Nature 368 (1994) 321. [3] P. Calvert, Biomimetic inorganic–organic composites, in: S. Mann (Ed.), Biomimetic Materials Chemistry, VCH, New York, 1996, pp. 315–336, Chapter 11. [4] F.J. Arriagada, K. Osseo-Asare, J. Colloids Interf. Sci. 170 (1995) 8. [5] L. Qi, J. Ma, H. Cheng, Z. Zhao, Colloids Surf. A 108 (1996) 117. [6] J.D. Hopwood, S. Mann, Chem. Mater. 9 (1997) 1819. [7] S. Mann, S.L. Burkett, S.A. Davis, C.E. Fowler, N.H. Mendelson, S.D. Sims, D. Walsh, N.T. Whilton, Chem. Mater. 9 (1997) 2300. [8] L. Qi, J. Ma, H. Cheng, Z. Zhao, J. Phys. Chem. B 101 (1997) 3460. [9] S. Schacht, Q. Huo, I.G. Voight-Martin, G.D. Stucky, F.F. Sch¨uth, Science 273 (1996) 768. [10] S.D. Sims, D. Walsh, S. Mann, Adv. Mater. 10 (1998) 151. [11] M. Li, S. Mann, Langmuir 16 (2000) 7088. [12] M. Li, S. Mann, Adv. Func. Mater. 12 (2002) 773. [13] C. Yu, B. Tian, J. Fan, G.D. Stucky, D. Zhao, Chem. Lett. (2002) 62. [14] F. Debuigne, L. Jeunieau, M. Wiame, J.B. Nagy, Langmuir 16 (2000) 7605. [15] D.W. Kim, S.G. Oh, J.D. Lee, Langmuir 15 (1999) 1599. [16] A.S. Bommarius, J.F. Holzwarth, D.I.C. Wang, T.A. Hatton, J. Phys. Chem. 94 (1990) 7232.
Morphosynthesis of microskeletal silica spheres templated by W/O microemulsion Cheng Tao, Junbai Li∗ International Joint Lab, Key Lab of Colloid and Interface Science, The Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Zhong Guan Cun, Beijing 100080, China
Abstract AOT containing water-in-oil (W/O) microemulsion was used as an organized reaction environment for silica preparation originating from hydrolysis and crosslink of tetraethoxysilane (TEOS). The nucleation and growth of skeletal silica shperes with a diameter of 40 m were synthesized in association with microemulsion. Study on the structure and morphology, principally by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), indicates that the final product is subject to the composition of microemulsion and aging time. A possible mechanism involving collision and fusion among large quantities of droplets covered by surfactant molecules was proposed here, attempting to describe the growth of the network structured silica spheres. © 2004 Elsevier B.V. All rights reserved. Keywords: Microemulsion; SiO2 spheres; Framework
1. Introduction Fabrication of complex inorganic materials with controllable shape, morphology and structure is of great importance in many technical areas. For example, framework structured inorganic material has shown considerable promise in separation [1], catalysis [2] and materials chemistry [3]. Among the strategies employed to prepare the complex inorganic materials, it has been found that the organized reaction medium in which the formation of inorganic solids proceed is a key feature to offer a constrained environment for nucleation and growth [4–7]. Recently, it has been shown that multiphase systems, such as reverse micelle [8], oil-in-water droplet [9], and bicontinuous microemulsion [10], can be exploited as organized reactive environments for the controlled synthesis of complex inorganic materials. W/O microemulsion is thermodynamically stable with a continuous oil phase and a compartmentalized aqueous domain covered by surfactant molecules, thus able to constrain and pattern the deposi∗
Corresponding author. Tel.: +86 10 82614087; fax: +86 10 82612629. E-mail address: [email protected] (J. Li).
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.09.020
tion of spatially confined inorganic precursors. As a result, a wide range of materials including various morphological BaCO3 [8], BaSO4 [11], and CaCO3 [12] could be successfully produced. However, the synthesis of silica originating from tetraethoxysilane (TEOS), different than the products obtained from precipitation reaction, involves that the hydrolysis and crosslink processes occurred in water droplets and/or on W/O interface [13]. The structure and morphology of the products can be markedly influenced by the nature of microemulsion, such as temperature, composition, drop size, and interfacial stability. This implies the preparation targeting at the materials with special structure and morphology could be controllable by manipulating the processing steps of the synthesis, although sometimes the mechanism cannot be understood completely. With this approach we reported here that the framework structured silica spheres could be achieved through sol–gel process of TEOS in AOT containing W/O microemulsion. Our results, focusing on the morphology evolution, indicate that the skeletal spheres originate from the hydrolysis of silica source and the growth process is accompanied with the collision and fusion of W/O droplets covered by surfactant layers.
58
C. Tao, J. Li / Colloids and Surfaces A: Physicochem. Eng. Aspects 256 (2005) 57–60
2. Experimental 2.1. Materials and synthesis Tetraethyl orthosilicate (TEOS), bis(2-ethylhexyl)sodium sulfosuccinate (AOT) and n-heptane were purchased from ACROS. The siliceous spheres synthesis was undertaken in W/O microemulsion prepared as follows: a solution containing 1.8 g AOT and 7.2 g C7 H16 were mixed with 1.0 mL 4 mol/L HCl solution (w = [H2 O]/[AOT] = 13.7, [H2 O]/[SiO2 ] = 11.6). The mixture was sonicated for 20 min to produce a transparent microemulsion. Based on the ternary phase diagram of the system a W/O microemulsion was obtained [14]. Then 1.0 g TEOS was carefully dropwised into the mixture during stirring, a milky emulsion with white precipitates was achieved. Afterwards the mixture was strongly stirred for 4 h at room temperature, filtrated and repeatedly washed with hot water and ethanol to collect white precipitate. The water used in all experimental steps is Millipore water (resistivity higher than 18.2 M cm). Finally, the synthesized sample was dried in air at room temperature, and the organic template was removed by calcination at 550 ◦ C for 3.5 h.
Fig. 1. SEM images of the microskeletal SiO2 spheres. (a) Aggregates of microskeletal SiO2 spheres. (b) Selected single SiO2 sphere with microskeletal structure. (c) Cross-section view of half a microskeletal SiO2 sphere. (d) Enlarged surface of the SiO2 sphere at a high magnitude.
2.2. Characterization Scanning electron microscopy (SEM) images were achieved by JEOL JSM-6301F system with an accelerating voltage of 20 kV. The transmission electron microscopy (TEM) measurements were performed on Phillips TECNAI 20 operating at 120 kV. X-ray powder diffraction patterns (XRD) were obtained on a Rigaku X-ray diffractometer D/max-2500 using Cu K␣1 radiation at 40.0 kV and 120.0 mA. N2 adsorption–desorption isotherms experiment was carried out at 77.35 K by using Quantachrome Autosorb Automated system. Samples were degassed at 150 ◦ C under high vacuum for at least 4 h before the measurement. The specific surface area was determined from the linear part of the BET equation (P/P0 = 0–1).
3. Results and discussion Fig. 1 shows SEM images of the calcined samples. A large quantity of aggregated microskeletal silica spheres were observed as presented in Fig. 1a. Estimated from most skeletal spheres, the average diameter is approximately 40 m. With a high magnitude SEM image of a single particle with the microskeletal structure and a well-defined spherical shape is visualized in Fig. 1b. A selected SEM image of one SiO2 halfsphere reveals that the inner sphere also posses regular pores (side view of Fig. 1c). Each building block of the sphere is well connected to each other, forming an ordered solid framework. From a larger magnification of partial spherical surface in Fig. 1d, the length and width of each block can be seen as more than 2 m and 0.5–2 m, respectively. The absence
of XRD peaks for the calcined samples implies that the SiO2 is amorphous, while the N2 adsorption–desorption isotherm also indicates that the sample with a surface area is 4.05 m2 /g does not have mesopores. To investigate the effect of the composition of the microemulsion on the morphology of the SiO2 products, two control experiments were performed. Firstly, the microemulsion defined as 1.8 g AOT, 16.2 g C7 H16 and 2.0 g HCl, the silica source of 1.5 g TEOS (w = [H2 O]/[AOT] = 27.4, [H2 O]/[SiO2 ] = 15.4) produced irregular morphological particles, as shown in Fig. 2a. However, if the W/O microemulsion is composed of 2.7 g AOT, 6.3 g C7 H16 and 1.0 g HCl (w = [H2 O]/[AOT] = 9.1, [H2 O]/[SiO2 ] = 23.1), the hydrolysis of 0.5 g TEOS resulted in “flake-like” SiO2 (Fig. 2b). The results suggest that the composition of microemulsion has a pronounced influence on the morphology of final products. The formation of skeletal as-synthesized SiO2 spheres in the W/O microemulsion may undergo various processes including small particle growth, fusion and aggregation [14]. The aggregation of the water droplet covered by surfactants in the microemulsion will directly influence upon the diameter of the particles, and the surfactant adsorption layers provide the pores structure. This explanation can be understood by taking the hydrolysis products of TEOS at different reaction time, which is corresponding to the different status of the microemulsion. TEM observation of the early growth stage of SiO2 showed that a predominance of separate spherical SiO2 particles after hydrolysis of 15 min under stirring. Most SiO2 particles are regular sphere shaped and possess a diameter of approximately 250–400 nm (Fig. 3a), suggesting that the surfactant covered water droplets facilitated the for-
C. Tao, J. Li / Colloids and Surfaces A: Physicochem. Eng. Aspects 256 (2005) 57–60
59
Fig. 2. SEM images of (a) SiO2 particles synthesized in a ratio of 1.8 g AOT:16.2 g C7 H16 :2.0 g HCl:1.5 g TEOS in the microemulsion reaction and (b) SiO2 particles synthesized in a ratio of 2.7 g AOT:6.3 g C7 H16 :1.0 g HCl:0.5 g TEOS in the microemulsion reaction.
mation of SiO2 particles in hundreds of nanometer regime. Further hydrolysis (over 30 min) leads to the formation of lots of chainlike particles fused by small particles (Fig. 3b), confirming the particles have been integrated by droplets col-
Fig. 3. TEM images of TEOS hydrolysis products with different reaction time after (a) 15 min; (b) 30 min; (c) 1 h and (d) 2 h, respectively.
Fig. 4. Schematic illustration of the forming process for the microskeletal SiO2 spheres in W/O microemulsion.
lision and fusion. The fusion part of chainlike particles has significantly obscured the original spherical shape of particles. Then particles grew and linked to each other to form a network after undergoing one-hour reaction (Fig. 3c). Continuous growth of SiO2 network even leads to the formation of spherical curvature, which is consistent with the morphology of the final products (Fig. 3d). Therefore, it can be deduced that in the microemulsion the hydrolysis and crosslink of TEOS occurred accompanying with the growth of water droplets up to the micrometer size. However, the aggregation of droplets produced several new water interfaces where AOT adsorption layers were formed. Such layers may create some potential pore spaces. This process allows the formation of microskeletal SiO2 spheres after calcination. The possible mechanism can be described as shown in Fig. 4. The aqueous droplets containing the initial hydrolyzed products of TEOS in microemulsion are growing and moving rapidly during stirring to combine to each other (Fig. 4a and b). As the reactants TEOS further hydrolyze, the fused parti-
60
C. Tao, J. Li / Colloids and Surfaces A: Physicochem. Eng. Aspects 256 (2005) 57–60
cles will be enlarged and further grown in the microemulsion (Fig. 4c). After collision and fusion of W/O microemulsion droplets covered with surfactant layers, a final larger species can be obtained [15,16]. Repeated growth of this process will create some highly branched network with interconnected channels [10]. The formation of skeletal spheres requires the driving force for reorganization and aggregation, which corresponds to a reduction in surface free energy. The surfactants always act as the boundaries of containers and templates. After the calcination they produce the interspace and allow the crosslink of SiO2 particles to form a network structure. Accordingly, it should be deduced that the phase structure of microemulsion generally results the morphological features of the final yields. This should be consistent with the previous observation and explanation [10], in which the inner connected framework of porous silica can well replicate the structure of bicontinuous microemulsion.
4. Conclusion In summary, microskeletal silica sphere in micrometer regime has been prepared employing reverse microemulsion as template. The morphology of the silica samples formed in the microemulsion is defined by the composition of microemulsion. Although the mechanism leading to the special morphology is not yet completely understood, the forming process should be subject to the collision and fusion of W/O droplets covered by surfactant layers, which produce the pore structures. Furthermore, this approach can be extended to many other sol–gel syntheses to produce inorganic materials with similar properties.
Acknowledgement This work was financially supported by the National Nature Science Foundation of China (NNSFC 29925307) and the collaborated project of German Max Planck Society. References [1] Y.N. Jun, D.M. Dabbs, I.A. Aksay, S. Erramilli, Langmuir 10 (1994) 3377. [2] P.T. Tanev, M. Chibwe, T.J. Pinnavania, Nature 368 (1994) 321. [3] P. Calvert, Biomimetic inorganic–organic composites, in: S. Mann (Ed.), Biomimetic Materials Chemistry, VCH, New York, 1996, pp. 315–336, Chapter 11. [4] F.J. Arriagada, K. Osseo-Asare, J. Colloids Interf. Sci. 170 (1995) 8. [5] L. Qi, J. Ma, H. Cheng, Z. Zhao, Colloids Surf. A 108 (1996) 117. [6] J.D. Hopwood, S. Mann, Chem. Mater. 9 (1997) 1819. [7] S. Mann, S.L. Burkett, S.A. Davis, C.E. Fowler, N.H. Mendelson, S.D. Sims, D. Walsh, N.T. Whilton, Chem. Mater. 9 (1997) 2300. [8] L. Qi, J. Ma, H. Cheng, Z. Zhao, J. Phys. Chem. B 101 (1997) 3460. [9] S. Schacht, Q. Huo, I.G. Voight-Martin, G.D. Stucky, F.F. Sch¨uth, Science 273 (1996) 768. [10] S.D. Sims, D. Walsh, S. Mann, Adv. Mater. 10 (1998) 151. [11] M. Li, S. Mann, Langmuir 16 (2000) 7088. [12] M. Li, S. Mann, Adv. Func. Mater. 12 (2002) 773. [13] C. Yu, B. Tian, J. Fan, G.D. Stucky, D. Zhao, Chem. Lett. (2002) 62. [14] F. Debuigne, L. Jeunieau, M. Wiame, J.B. Nagy, Langmuir 16 (2000) 7605. [15] D.W. Kim, S.G. Oh, J.D. Lee, Langmuir 15 (1999) 1599. [16] A.S. Bommarius, J.F. Holzwarth, D.I.C. Wang, T.A. Hatton, J. Phys. Chem. 94 (1990) 7232.