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Synthesis of hollow mesoporous silica spheres and carambola-like silica materials with a novel resin sphere as template
Materials Letters 135 (2014) 43–46
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of hollow mesoporous silica spheres and carambola-like silica materials with a novel resin sphere as template Aibing Chen n, Yifeng Yu, Haijun Lv, Yue Zhang, Tingting Xing, Yunhong Yu College of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 9 April 2014 Accepted 26 July 2014 Available online 4 August 2014
Hollow mesoporous silica spheres with wormlike mesoporous were synthesized by a dual-templating method using resorcinol/formaldehyde resin spheres as hard templates and cationic surfactant (CTAB) as soft template. The average particles diameters were adjusted by changing molar ratio of TEOS/CTAB. Carambola-like silica materials with hollow structure can be obtained using one-pot method. & 2014 Elsevier B.V. All rights reserved.
Keywords: Porous materials Nanoparticles Silica spheres Resin spheres CTAB
1. Introduction Hollow silica spheres with mesoporous shell have raised increasing interest for combining the characteristic of both macroporous and mesoporous structures. Hence, such porous structures can act as a micro reactor, and be used for controlled release of drug delivery, adsorption and catalyst [1–3]. Among many synthesis methods [4–7], a dual-templating method [8] has been widely applied and researched for the synthesis of hollow mesoporous silica spheres (HMSs). Surfactant (CTAB, block copolymer) and some organic/inorganic polymers as soft/hard-template can produce the mesoporous and hollow interior during the hydrolysis and condensation of the silica precursor. Le et al. [9] fabricated HMSs using nanosized calcium-carbonate particles as solid template and CTAB as soft template. But transmission electron microscopy (TEM) shows that there exists obvious aggregation among the particles. Blas et al. [10] prepared monodisperse, no aggregated hollow silica nanoparticles with a homogeneous ordered mesoporous shell by a dual-templating method, which employed polystyrene latexes with anionic or cationic surface charges and CTAB acted as the solid/soft templates. Qi et al. [11] reported a facile and scalable process to synthesize smooth, uniform and monodisperse HMSs using concentrated polystyrene latex as template. However, most of the preparation processes need to modify the hydrophobic, chemically inert surfaces of these templates, augmenting the preparation complexity. More polymerized agents and additives are introduced to increase the preparation cost and environment pollution, especially polystyrene latex. Therefore, it is still necessary to develop a facile, low-cost template for preparing HMSs. Recently, Liu
n
Corresponding author. Tel: þ 86 311 8632392. E-mail address: [email protected] (A. Chen).
http://dx.doi.org/10.1016/j.matlet.2014.07.155 0167-577X/& 2014 Elsevier B.V. All rights reserved.
et al. [12] fabricated resorcinol/formaldehyde (RF) resin spheres with uniform and controlled particles size with the extension of the Stöber method. We believe RF resin spheres can be a novel template for fabricating hollow inorganic materials. Herein, in the present work, we reported the synthesis of HMSs by the dual-templating method (RF resin spheres as sacrificial template, CTAB as soft template). The shell thickness can be tuned by changing the molar ratio of TEOS/CTAB. Simultaneously, we proposed a formation mechanism of HMSs, and corresponding characterizations showed that RF resin spheres become a novel template to replace other templates, such as polystyrene latexes. In addition, carambola-like silica materials were obtained by one-pot method.
2. Experimental Synthesis of HMSs. 0.2 g of RF resin spheres was added to a solution containing 10 ml of ethanol, 5 ml of deionized water, 0.1 ml of ammonia aqueous solution (25 wt%) and 0.1 g of CTAB under vigorous stirring at room temperature. The mixture stirred for 20 min before adding TEOS. The molar ratio of TEOS/CTAB were 1.7, 3.4, 5.0, 6.7 and 8.4. The mixture was kept at room temperature for 3 h, and subsequently heated for 24 h at 100 1C under a static condition. The solid products were collected by centrifugation and washed with deionized water. The templates were removed by calcining in air at 550 1C for 3 h. The resulting samples were labeled as HMSs-x, where x represented the TEOS/CTAB molar ratio. Synthesis of carambola-like silica materials. 0.1 ml ammonia aqueous solution was mixed with a solution containing 8 ml ethanol and 20 ml deionized water, and then stirred for more than 1 h. Subsequently, 0.1 g CTAB was added and continually stirred for 30 min. Then 0.2 g
44
A. Chen et al. / Materials Letters 135 (2014) 43–46
Fig. 1. SEM images of HMSs prepared at different molar ratios of TEOS/CTAB: (a) HMSs-5.0; (b) HMSs-6.7; (c) HMSs-8.4; TEM images of HMSs-3.4 (d), and HMSs-6.7 with different magnifications (e, f). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
resorcinol and formaldehyde solution (0.28 ml) was added under stirring for 30 min. The solution was stirred for 20 h at 30 1C. After addition of TEOS (TEOS/CTAB molar ratio of 3.4) for another 4 h, the solution was subsequently heated for 24 h at 100 1C under a static condition. The next step is consistent with the preparation process of HMSs. Characterization. The products were characterized by powder X-ray diffraction (XRD) pattern, using a Rigaku D/MAX-2500 XRD system with Cu Kα radiation. Scanning electron microscopy (SEM) was performed on a HITACHI S-4800-I scanning electron
microscope. TEM were obtained on a JEOL JEM-2010 electron microscope. N2 isotherm measurements were performed on a Micromeritics TriStar 3020 at 100 1C under a static condition.
3. Results and discussion In the initial experiment, we employed the direct coating method to introduce the silica source onto the surface of resin spheres without adding CTAB. Nevertheless, the crater-like silica materials were
A. Chen et al. / Materials Letters 135 (2014) 43–46
uniform particle size were composed of silica nanoparticles with an average size of ca.100 nm (as shown in red circles of Fig. 1b). Employing the same diameter of template, the average diameters of HMSs at different molar ratios of TEOS/CTAB were 700, 750 and 800 nm, indicating that the shell thickness of HMSs increased gradually. Obviously, some spheres were broken as seen in the SEM images, demonstrating that the samples have hollow structure (Fig. 1a–c). Fig. 1d–f shows the TEM images of HMSs fabricated at different TEOS/CTAB molar ratios. For HMSs-3.4, most HMSs collapsed forming irregular morphology (Fig. 1d). However, The HMSs-6.7 sample show perfectly spherical hollow structure under different magnifications (Fig. 1e and f). Moreover, the average diameter of the resultant sample
180
Intensity (a.u.)
Volume Assorbed (cm3 g -1)
160 140 120 100
Pore Volume (cm3 g-1)
fabricated, which is not desirable for present hollow structure. The reason for this case is most likely that RF resin spheres have negative charges [12]. Thereby it affected the morphology of the final HMSs materials. In order to prepare HMSs, we employed a dual template method using RF resin spheres as hard template and CTAB as soft template to fabricate HMSs. One of important factors to form HMSs is the TEOS/CTAB molar ratio. When the molar ratio was less than 5.0, only several broken pieces were found. The possible reason for the absence of hollow structures that low molar ratio of TEOS/CTAB cannot be coated onto the entire surface of RF resin spheres. When the molar ratio of TEOS/CTAB increased to 5.0, the intact spherical morphology was clearly observed in Fig. 1a. The surfaces of HMSs with
45
Pore Diameter (nm)
80 60 40 20
2
4
6
2 Theta (degrees)
8
10
0.0
0.2
0.4
0.6
0.8
Relative Pressure ( P/P0 )
Fig. 2. Small-angle XRD pattern (a), N2 adsorption/desorption isotherms (b) and its pore size distribution (inset) of HMSs-6.7.
Fig. 3. SEM images with different magnifications (a, b) and TEM image of carambola-like silica (c).
1.0
46
A. Chen et al. / Materials Letters 135 (2014) 43–46
Fig. 4. Schematic for the formation of HMSs with mesoporous shell.
was about 750 nm, and the shell thickness was about 65 nm in uniform size (Fig. 1e). Furthermore, the shell of HMSs consists of disordered mesoporous with the wormhole structure (Fig. 1f). The small-angle XRD pattern of HMSs-6.7 (Fig. 2a) has only one broad reflection at 2θ¼ca.2.31, which indicating a mesoporous silica structure lacks long-range ordering in the arrangement of mesopores. Similar results were reported by other research groups [13,14]. This is consistent with the wormlike pores structure observed in the TEM images. The presence of CTAB is the most important factor to form the mesoporous structure of silica shell. The N2 isotherms of the HMSs-6.7 is depicted in Fig. 2b. It exhibited typical-IV hysteresis with a sharp increase with nitrogen uptake, indicating the existence of mesoporous structure. The HMSs-6.7 displayed a narrow mesoporous size distribution, centered at 3.8 nm (the inset of Fig. 3b). The BET surfaces areas and pore volume of HMSs-6.7 are measured to be 359 cm2 g 1 and 0.26 cm3 g 1, respectively. In order to simplify the experiment procedure, we tried to fabricate HMSs by employing one-pot method. But HMSs were not obtained. Instead, a novel shape (carambola-like) is obtained, as shown in the SEM image of Fig. 3a and b. Carambola-like silica materials have smooth surfaces and uniform size (0.7 μm in diameter and 1.4 μm in length). The TEM image (Fig. 3c) offers direct evidence that carambolalike silica materials have hollow structure, and the shell thickness was approximately 23 nm. It is worth noting that the carambola-like silica materials can be fabricated in a wide range of condition, such as regulation of TEOS/CTAB molar ratio from 3.4 to 6.7. Based on the above discussion, we proposed a possible mechanism for synthesis of HMSs (Fig. 4). The negatively charged RF resin spheres can couple with the positively charged CTAB bilayer, and form CTAB coated RF aggregates. In that case, CTA þ exposed to the external part and form the positively aggregates. Silicon hydroxide species from the hydrolysate of TEOS were negatively charged. The positively aggregates interact with negatively charged silicon hydroxide species via electrostatic interaction, which lead to the formation of the mesostructured shell on the surface of RF resin spheres [15,16]. As the silica source gradually increases, the shell thickness can be tuned in a specific range.
were fabricated by the direct coating method without adding CTAB. Afterwards, HMSs with mesoporous structure and tunable shell thickness were successfully synthesized in the presence of RF resin spheres and CTAB co-templates. CTAB not only is used as soft template but also has electrostatic interaction with the surfaces of RF resin spheres. Therefore, the mesoporous structure of silica shell can be created by adding CTAB. The wormlike mesoporous centered at 3.8 nm were investigated in the shell of HMSs. And the average particles diameters of these HMSs were adjusted by changing the molar ratio of TEOS/CTAB. Simultaneously, the carambola-like silica materials were successfully synthesized by one-pot method, which possess large hollow cavity, regular shape and thin wall. These properties endow them with promising applications such as catalysis and controlled release.
Acknowledgment The work is supported by funds from the National Natural Science Foundation of China (20906019) and Science and Technology Research Projects in Hebei Universities (QN20131069 and ZD20131032).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
4. Conclusions RF resin spheres prepared by the extension of Stöber method can be used as a novel solid template. Crater-like silica materials
[13] [14] [15] [16]
Yang J, Lee J, Kang J, Lee K, Suh JS, Yoon HG, et al. Langmuir 2008;24:3417–21. Li ZZ, Chen JF, Li Y, Wen LX. J Nanosci Nanotechnol 2007;7:535–41. Li WJ, Sha XX, Dong WJ, Wang ZC. Chem Commun 2002;20:2434–5. Yamada Y, Mizutani M, Nakamura T, Yano K. Chem Mater 2010;22:1695–703. Qi GG, Wang YB, Estevez L, Switzer AK, Duan XN, Yang XF, et al. Chem Mater 2010;22:2693–5. Liu J, Hartono SB, Jin YG, Li Z, Lu GQ, Qiao SZ. J Mater Chem 2010;20:4595–601. Wu XJ, Xu DS. Adv Mater 2010;22:1516–20. Du X, He JH. Nanoscale 2011;3:3984–4002. Le Y, Chen JF, Wang JX, Shao L, Wang WC. Mater Lett 2004;58:2105–8. Blas H, Save M, Pasetto P, Boissière C, Sanchez C, Charleux B. Langmuir 2008;24:13132–7. Qi G, Wang YB, Estevez L, Switzer AK, Duan XN, Yang XF, et al. Chem Mater 2010;22:2693–5. Liu J, Qiao SZ, Liu H, Chen J, Orpe A, Zhao DY, et al. Angew Chem Int Ed 2011;50:5947–51. Tanev PT, Pinnavaia TJ. Science 1995;267:865–7. Bagshaw SA, Prouzet E, Pinnavaia TJ. Science 1995;269:1242–4. Zhang XH, Li YN, Cao CB. J Mater Chem 2012;22:13918–21. Li N, Zhang Q, Liu J, Joo JB, Lee AS, Gan Y, et al. Chem Commun 2013;49:5135–7.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of hollow mesoporous silica spheres and carambola-like silica materials with a novel resin sphere as template Aibing Chen n, Yifeng Yu, Haijun Lv, Yue Zhang, Tingting Xing, Yunhong Yu College of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 9 April 2014 Accepted 26 July 2014 Available online 4 August 2014
Hollow mesoporous silica spheres with wormlike mesoporous were synthesized by a dual-templating method using resorcinol/formaldehyde resin spheres as hard templates and cationic surfactant (CTAB) as soft template. The average particles diameters were adjusted by changing molar ratio of TEOS/CTAB. Carambola-like silica materials with hollow structure can be obtained using one-pot method. & 2014 Elsevier B.V. All rights reserved.
Keywords: Porous materials Nanoparticles Silica spheres Resin spheres CTAB
1. Introduction Hollow silica spheres with mesoporous shell have raised increasing interest for combining the characteristic of both macroporous and mesoporous structures. Hence, such porous structures can act as a micro reactor, and be used for controlled release of drug delivery, adsorption and catalyst [1–3]. Among many synthesis methods [4–7], a dual-templating method [8] has been widely applied and researched for the synthesis of hollow mesoporous silica spheres (HMSs). Surfactant (CTAB, block copolymer) and some organic/inorganic polymers as soft/hard-template can produce the mesoporous and hollow interior during the hydrolysis and condensation of the silica precursor. Le et al. [9] fabricated HMSs using nanosized calcium-carbonate particles as solid template and CTAB as soft template. But transmission electron microscopy (TEM) shows that there exists obvious aggregation among the particles. Blas et al. [10] prepared monodisperse, no aggregated hollow silica nanoparticles with a homogeneous ordered mesoporous shell by a dual-templating method, which employed polystyrene latexes with anionic or cationic surface charges and CTAB acted as the solid/soft templates. Qi et al. [11] reported a facile and scalable process to synthesize smooth, uniform and monodisperse HMSs using concentrated polystyrene latex as template. However, most of the preparation processes need to modify the hydrophobic, chemically inert surfaces of these templates, augmenting the preparation complexity. More polymerized agents and additives are introduced to increase the preparation cost and environment pollution, especially polystyrene latex. Therefore, it is still necessary to develop a facile, low-cost template for preparing HMSs. Recently, Liu
n
Corresponding author. Tel: þ 86 311 8632392. E-mail address: [email protected] (A. Chen).
http://dx.doi.org/10.1016/j.matlet.2014.07.155 0167-577X/& 2014 Elsevier B.V. All rights reserved.
et al. [12] fabricated resorcinol/formaldehyde (RF) resin spheres with uniform and controlled particles size with the extension of the Stöber method. We believe RF resin spheres can be a novel template for fabricating hollow inorganic materials. Herein, in the present work, we reported the synthesis of HMSs by the dual-templating method (RF resin spheres as sacrificial template, CTAB as soft template). The shell thickness can be tuned by changing the molar ratio of TEOS/CTAB. Simultaneously, we proposed a formation mechanism of HMSs, and corresponding characterizations showed that RF resin spheres become a novel template to replace other templates, such as polystyrene latexes. In addition, carambola-like silica materials were obtained by one-pot method.
2. Experimental Synthesis of HMSs. 0.2 g of RF resin spheres was added to a solution containing 10 ml of ethanol, 5 ml of deionized water, 0.1 ml of ammonia aqueous solution (25 wt%) and 0.1 g of CTAB under vigorous stirring at room temperature. The mixture stirred for 20 min before adding TEOS. The molar ratio of TEOS/CTAB were 1.7, 3.4, 5.0, 6.7 and 8.4. The mixture was kept at room temperature for 3 h, and subsequently heated for 24 h at 100 1C under a static condition. The solid products were collected by centrifugation and washed with deionized water. The templates were removed by calcining in air at 550 1C for 3 h. The resulting samples were labeled as HMSs-x, where x represented the TEOS/CTAB molar ratio. Synthesis of carambola-like silica materials. 0.1 ml ammonia aqueous solution was mixed with a solution containing 8 ml ethanol and 20 ml deionized water, and then stirred for more than 1 h. Subsequently, 0.1 g CTAB was added and continually stirred for 30 min. Then 0.2 g
44
A. Chen et al. / Materials Letters 135 (2014) 43–46
Fig. 1. SEM images of HMSs prepared at different molar ratios of TEOS/CTAB: (a) HMSs-5.0; (b) HMSs-6.7; (c) HMSs-8.4; TEM images of HMSs-3.4 (d), and HMSs-6.7 with different magnifications (e, f). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
resorcinol and formaldehyde solution (0.28 ml) was added under stirring for 30 min. The solution was stirred for 20 h at 30 1C. After addition of TEOS (TEOS/CTAB molar ratio of 3.4) for another 4 h, the solution was subsequently heated for 24 h at 100 1C under a static condition. The next step is consistent with the preparation process of HMSs. Characterization. The products were characterized by powder X-ray diffraction (XRD) pattern, using a Rigaku D/MAX-2500 XRD system with Cu Kα radiation. Scanning electron microscopy (SEM) was performed on a HITACHI S-4800-I scanning electron
microscope. TEM were obtained on a JEOL JEM-2010 electron microscope. N2 isotherm measurements were performed on a Micromeritics TriStar 3020 at 100 1C under a static condition.
3. Results and discussion In the initial experiment, we employed the direct coating method to introduce the silica source onto the surface of resin spheres without adding CTAB. Nevertheless, the crater-like silica materials were
A. Chen et al. / Materials Letters 135 (2014) 43–46
uniform particle size were composed of silica nanoparticles with an average size of ca.100 nm (as shown in red circles of Fig. 1b). Employing the same diameter of template, the average diameters of HMSs at different molar ratios of TEOS/CTAB were 700, 750 and 800 nm, indicating that the shell thickness of HMSs increased gradually. Obviously, some spheres were broken as seen in the SEM images, demonstrating that the samples have hollow structure (Fig. 1a–c). Fig. 1d–f shows the TEM images of HMSs fabricated at different TEOS/CTAB molar ratios. For HMSs-3.4, most HMSs collapsed forming irregular morphology (Fig. 1d). However, The HMSs-6.7 sample show perfectly spherical hollow structure under different magnifications (Fig. 1e and f). Moreover, the average diameter of the resultant sample
180
Intensity (a.u.)
Volume Assorbed (cm3 g -1)
160 140 120 100
Pore Volume (cm3 g-1)
fabricated, which is not desirable for present hollow structure. The reason for this case is most likely that RF resin spheres have negative charges [12]. Thereby it affected the morphology of the final HMSs materials. In order to prepare HMSs, we employed a dual template method using RF resin spheres as hard template and CTAB as soft template to fabricate HMSs. One of important factors to form HMSs is the TEOS/CTAB molar ratio. When the molar ratio was less than 5.0, only several broken pieces were found. The possible reason for the absence of hollow structures that low molar ratio of TEOS/CTAB cannot be coated onto the entire surface of RF resin spheres. When the molar ratio of TEOS/CTAB increased to 5.0, the intact spherical morphology was clearly observed in Fig. 1a. The surfaces of HMSs with
45
Pore Diameter (nm)
80 60 40 20
2
4
6
2 Theta (degrees)
8
10
0.0
0.2
0.4
0.6
0.8
Relative Pressure ( P/P0 )
Fig. 2. Small-angle XRD pattern (a), N2 adsorption/desorption isotherms (b) and its pore size distribution (inset) of HMSs-6.7.
Fig. 3. SEM images with different magnifications (a, b) and TEM image of carambola-like silica (c).
1.0
46
A. Chen et al. / Materials Letters 135 (2014) 43–46
Fig. 4. Schematic for the formation of HMSs with mesoporous shell.
was about 750 nm, and the shell thickness was about 65 nm in uniform size (Fig. 1e). Furthermore, the shell of HMSs consists of disordered mesoporous with the wormhole structure (Fig. 1f). The small-angle XRD pattern of HMSs-6.7 (Fig. 2a) has only one broad reflection at 2θ¼ca.2.31, which indicating a mesoporous silica structure lacks long-range ordering in the arrangement of mesopores. Similar results were reported by other research groups [13,14]. This is consistent with the wormlike pores structure observed in the TEM images. The presence of CTAB is the most important factor to form the mesoporous structure of silica shell. The N2 isotherms of the HMSs-6.7 is depicted in Fig. 2b. It exhibited typical-IV hysteresis with a sharp increase with nitrogen uptake, indicating the existence of mesoporous structure. The HMSs-6.7 displayed a narrow mesoporous size distribution, centered at 3.8 nm (the inset of Fig. 3b). The BET surfaces areas and pore volume of HMSs-6.7 are measured to be 359 cm2 g 1 and 0.26 cm3 g 1, respectively. In order to simplify the experiment procedure, we tried to fabricate HMSs by employing one-pot method. But HMSs were not obtained. Instead, a novel shape (carambola-like) is obtained, as shown in the SEM image of Fig. 3a and b. Carambola-like silica materials have smooth surfaces and uniform size (0.7 μm in diameter and 1.4 μm in length). The TEM image (Fig. 3c) offers direct evidence that carambolalike silica materials have hollow structure, and the shell thickness was approximately 23 nm. It is worth noting that the carambola-like silica materials can be fabricated in a wide range of condition, such as regulation of TEOS/CTAB molar ratio from 3.4 to 6.7. Based on the above discussion, we proposed a possible mechanism for synthesis of HMSs (Fig. 4). The negatively charged RF resin spheres can couple with the positively charged CTAB bilayer, and form CTAB coated RF aggregates. In that case, CTA þ exposed to the external part and form the positively aggregates. Silicon hydroxide species from the hydrolysate of TEOS were negatively charged. The positively aggregates interact with negatively charged silicon hydroxide species via electrostatic interaction, which lead to the formation of the mesostructured shell on the surface of RF resin spheres [15,16]. As the silica source gradually increases, the shell thickness can be tuned in a specific range.
were fabricated by the direct coating method without adding CTAB. Afterwards, HMSs with mesoporous structure and tunable shell thickness were successfully synthesized in the presence of RF resin spheres and CTAB co-templates. CTAB not only is used as soft template but also has electrostatic interaction with the surfaces of RF resin spheres. Therefore, the mesoporous structure of silica shell can be created by adding CTAB. The wormlike mesoporous centered at 3.8 nm were investigated in the shell of HMSs. And the average particles diameters of these HMSs were adjusted by changing the molar ratio of TEOS/CTAB. Simultaneously, the carambola-like silica materials were successfully synthesized by one-pot method, which possess large hollow cavity, regular shape and thin wall. These properties endow them with promising applications such as catalysis and controlled release.
Acknowledgment The work is supported by funds from the National Natural Science Foundation of China (20906019) and Science and Technology Research Projects in Hebei Universities (QN20131069 and ZD20131032).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
4. Conclusions RF resin spheres prepared by the extension of Stöber method can be used as a novel solid template. Crater-like silica materials
[13] [14] [15] [16]
Yang J, Lee J, Kang J, Lee K, Suh JS, Yoon HG, et al. Langmuir 2008;24:3417–21. Li ZZ, Chen JF, Li Y, Wen LX. J Nanosci Nanotechnol 2007;7:535–41. Li WJ, Sha XX, Dong WJ, Wang ZC. Chem Commun 2002;20:2434–5. Yamada Y, Mizutani M, Nakamura T, Yano K. Chem Mater 2010;22:1695–703. Qi GG, Wang YB, Estevez L, Switzer AK, Duan XN, Yang XF, et al. Chem Mater 2010;22:2693–5. Liu J, Hartono SB, Jin YG, Li Z, Lu GQ, Qiao SZ. J Mater Chem 2010;20:4595–601. Wu XJ, Xu DS. Adv Mater 2010;22:1516–20. Du X, He JH. Nanoscale 2011;3:3984–4002. Le Y, Chen JF, Wang JX, Shao L, Wang WC. Mater Lett 2004;58:2105–8. Blas H, Save M, Pasetto P, Boissière C, Sanchez C, Charleux B. Langmuir 2008;24:13132–7. Qi G, Wang YB, Estevez L, Switzer AK, Duan XN, Yang XF, et al. Chem Mater 2010;22:2693–5. Liu J, Qiao SZ, Liu H, Chen J, Orpe A, Zhao DY, et al. Angew Chem Int Ed 2011;50:5947–51. Tanev PT, Pinnavaia TJ. Science 1995;267:865–7. Bagshaw SA, Prouzet E, Pinnavaia TJ. Science 1995;269:1242–4. Zhang XH, Li YN, Cao CB. J Mater Chem 2012;22:13918–21. Li N, Zhang Q, Liu J, Joo JB, Lee AS, Gan Y, et al. Chem Commun 2013;49:5135–7.