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Synthesis and characterization of Ce-doped mesoporous anatase with long-range ordered mesostructure
Materials Letters 61 (2007) 4283 – 4286 www.elsevier.com/locate/matlet
Synthesis and characterization of Ce-doped mesoporous anatase with long-range ordered mesostructure Shuai Yuan a,⁎, Yi Chen a , Liyi Shi a,⁎, Jianhui Fang a , Jianping Zhang a , Jinlong Zhang b , Hiromi Yamashita c a
c
Research Center of Nanoscience and Nanotechnology, Shanghai University, 99 Shangda Road, Shanghai 200444, PR China b Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Received 22 December 2006; accepted 27 January 2007 Available online 6 February 2007
Abstract The effect of cerium cation on the formation of titania crystallites using H+ as catalyst under mild condition has been investigated. The results indicated that the presence of cerium cation can improve the selectivity of producing anatase crystallites and inhibit the growth of crystallites. The Ce-doped mesoporous titania with highly crystallized pore walls consisting of anatase nanoparticles was synthesized by anatase crystallites assembly. The long-range ordered mesostructure was characterized by low angle and wide angle X-ray diffraction (XRD), N2 adsorption– desorption, transmission electron microscopy (TEM) and selected area electron diffraction (SAED). © 2007 Elsevier B.V. All rights reserved. Keywords: Mesoporous; Nanomaterials; Anatase; Crystal structure; Cerium cation
1. Introduction According to the pore diameter (d) of porous solids, they can be divided into three categories, i.e., microporous (d b 2 nm), mesoporous (2 nm b d b 50 nm) and macroporous (d N 50 nm) materials [1]. Zeolites, the members of a great family of microporous materials, have been applied in many fields for a long time [2]. However, the blossom of mesoporous materials is considered to start from the successful synthesis of MCM-41 by Mobil in 1992 [3]. The difference between zeolites and mesoporous materials is not only in the pore size but also in the crystal structure of the pore walls. Zeolites have crystalline frameworks. On the contrary, mesoporous materials are usually constituted of amorphous pore walls and their applications are limited by poor thermostability and hydrothermostability. Recently, more and more efforts have been done on the preparation of mesoporous materials with crystallized walls to overcome the shortages of mesoporous materials with amorphous walls [4–7]. ⁎ Corresponding authors. Tel./fax: +86 21 66135215. E-mail addresses: [email protected] (S. Yuan), [email protected] (L. Shi). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.01.086
Mesoporous titania with highly crystallized walls is an important kind of mesoporous material and has more advantages than titania nanocrystals because of its high surface area, long-range ordered porous structure, facile recovery and low risk to human health [8]. For example, the rare earth-doped mesoporous titania combining the advantages of mesoporous materials and nanomaterials exhibited more applications in catalysis and optics than rare earth-doped titania nanocrystals [9,10]. Based on previous researches, the cerium cation-doped mesoporous titania with long-range order and highly crystallized pore walls was synthesized by using crystalline nanoparticles as assembly units [11,12]. 2. Experimental section 2.1. Chemicals Tetrabutyl titanate (Ti(OBu)4) was CP grade. Nitric acid (65– 68%) and cerium nitrate (Ce(NO3)3·6H2O) were all AR grade. EO20PO70EO20 (P-123) was a commercial product from Aldrich.
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2.2. Synthesis
Table 1 The XRD results of samples
The typical synthesis of crystalline nanoparticles was as follows: Ti(OC4H9)4 (5 ml, 14.7 mmol) was added to 15 ml aqueous solution of HNO3 (29.4 mmol HNO3) dropwise under violent stirring at 313 K in 20 min. After stirring for additional 60 min, the mixture was allowed to stand until two layers separated. The upper layer of butyl alcohol was removed to get a transparent sol. The transparent sol was kept at 313 K for 1 day to get a white precipitate. Then, the precipitate was filtrated and dried at 313 K. The product was denoted as H2Ce0, which meant the mole ratio of H+:Ti was equal to 2:1 and the mole ratio of Ce:Ti was equal to 0:1. The 5 at.% Ce-doped titania samples were prepared by adding 0.3200 g Ce (NO3)3·6H2O (0.74 mmol Ce(NO3)3) into the solution after the hydrolysis of Ti(OC4H9)4 for 30 min or before the addition of Ti(OC4H9)4, and the products were denoted as H2Ce5a and H2Ce5b, respectively. The synthesis of Ce-doped mesoporous titania was as follows: 14.7 mmol Ti(OBu)4 was added dropwise into 15 ml aqueous solution containing 14.7 mmol HNO3, 0.74 mmol Ce (NO3)3·6H2O and 0.34 mmol P-123, followed by stirring at 313 K for 60 min. The sol was transferred from a reactor to a large open Petri dish. The sol extended sufficiently and formed a uniform thin layer. The layer was maintained at 313 K for 48 h and at 413 K for 2 h. At last, the thin layer was calcined at 673 K for 1 h in airflow. The heating-up rate was 1 K min− 1, and the cooling rate was 5 K min− 1.
Samples
2.3. Characterization X-ray diffraction (XRD) patterns of all samples were collected in θ–2θ mode using a Rigaku D/MAX-2550 diffractometer (CuKα1 radiation, λ = 1.5406 Å). The crystallite size was estimated by applying the Scherrer equation. The weight fraction of rutile was calculated from the inte-
H1Ce0 H2Ce0 H2Ce5a H2Ce5b
Anatase
Rutile
Size (nm)
Fraction (%)
Size (nm)
Fraction (%)
2.7 9.0 8.8 7.7
74.3 26.3 50.6 60.4
1.6 25.5 21.9 20.1
25.7 73.7 49.4 39.6
grated intensities of anatase (101), rutile (110) with the equation Wrutile = 1 / (0.886AA /AR + 1) [13]. The porous texture of Ce-doped mesoporous titania was analyzed from nitrogen adsorption–desorption isotherms at 77 K by using a Micromeritics ASAP 2000 system. The sample morphology was observed under transmission electron microscopy (TEM) on a 2100 JEOL microscope (200 kV) using copper grids. 3. Results and discussion The wide angle XRD patterns of titania crystallites prepared in acid condition are shown in Fig. 1(A). The diffraction peaks in the range of 15–35° indicate that the samples are mixtures of anatase and rutile. For example, from the enlarged view of sample H1Ce0, it can be observed that the measured curve is fitted by one peak at 25.2° and another peak at 27.6° which are the (101) peak of anatase and the (110) peak of rutile, respectively. According to the formulas mentioned in characterization section, the diameters of anatase and rutile crystallites and their weight fractions are summarized in Table 1. From these data, it can be concluded that high H+ concentration can accelerate the growth of crystallites of both anatase and rutile, especially the formation of rutile [14,15]. However, the addition of cerium cations can inhibit the growth of crystallites and the formation of rutile. Compared with sample H2Ce5a, sample H2Ce5b contains more anatase phase, which indicates that the selectivity of producing anatase crystallites is higher when cerium cation was added before the hydrolysis of Ti(OBu)4. The XRD results reveal that it is possible to
Fig. 1. (A) Wide angle XRD patterns of samples (a) H1Ce0 (H+:Ti = 1:1, Ce:Ti = 0:1), (b) H2Ce0 (H+:Ti = 2:1, Ce:Ti = 0:1), (c) H2Ce5a (H+:Ti = 2:1, Ce:Ti = 0.05:1, Ce (NO3)3 was added after the addition of Ti(OBu)4) and (d) H2Ce5b (H+:Ti = 2:1, Ce:Ti = 0.05:1, Ce(NO3)3 was added before the addition of Ti(OBu)4). (B) The diffraction peak of H1Ce0 in the range from 15° to 35°.
S. Yuan et al. / Materials Letters 61 (2007) 4283–4286
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Fig. 2. (A) The XRD patterns and (B) N2 adsorption–desorption patterns of Ce-doped mesoporous titania after calcination at 673 K for 1 h.
produce nanosized anatase crystallites as the assembly units for the formation of long-range ordered mesoporous titania. By the similar process described in previous researches [11], the Cedoped mesoporous titania with crystallized pore walls was prepared. The XRD patterns of Ce-doped mesoporous titania after calcination are shown in Fig. 2(A). The single peak at 1.25° in the low angle XRD pattern indicates that the long-range order of mesoporous titania is still reserved, and the d100 value due to (100) reflection from hexagonal mesoporous structure is about 7.0 nm. The wide angle XRD shows that the mesoporous titania has a crystallized framework. The average size of anatase nanocrystals calculated from the Scherrer formula is about 6.7 nm. The mesostructure of calcined Ce-doped mesoporous titania was also characterized by N2 adsorption–desorption. Fig. 2(B) shows the type IV gas adsorption isotherm, which is due to the large mesoporous channel. The hysteresis loop is an intermediate between type H1 and H2, which indicates that the Ce-doped mesoporous titania has uniform cylindrical mesopores. The average pore diameter calculated by the BJH model is 3.8 nm, and the BET specific surface area is 194 m2 g− 1. The thickness of the pore wall calculated by subtracting the pore diameter from 2d100/√3 is about 4.3 nm.
Assuming anatase nanocrystals as spherical particles, the surface area can also be estimated by S = 6 / ρd [16]. Here, ρ is the density of anatase (3.84 g cm− 3), and d is the average diameter calculated from the FWHM of the (101) peak of anatase. The value obtained is 233 m2 g− 1 which is close to the BET specific surface area. The result indicates that the pore walls of Ce-doped mesoporous titania are made of highly crystallized anatase. The TEM image of Ce-doped mesoporous titania is shown in Fig. 3. It can be observed that the long-range ordered mesoporous structure with an area about 100 nm × 200 nm is constructed by nanoparticles. The pore diameter and thickness of the pore walls estimated from the TEM image are about 4.0 nm and 4.5 nm, respectively, which are close to the analysis results from XRD and N2 adsorption–desorption data. The selected area electron diffraction (SAED) pattern shows that the pore walls are made of only anatase nanocrystals. Why the average size of anatase nanocrystals obtained from the Scherrer formula can be larger than the thickness of the pore wall has been discussed by other researchers [17].
4. Conclusion In this paper, the effect of cerium cation on the formation of titania crystallites using H+ as catalyst under mild condition has been investigated. The presence of cerium cation can improve the selectivity of producing anatase and inhibit the growth of crystallites of both rutile and anatase, especially when cerium cation was added before the hydrolysis of titania precursor. Based on this result, Ce-doped mesoporous anatase with longrange ordered mesostructure was synthesized by titania crystallites assembly. The mesostructure of Ce-doped mesoporous anatase was confirmed by various characterizations. References
Fig. 3. TEM image and selected area electron diffraction pattern (inset) of Ce-doped mesoporous titania after calcination at 673 K for 1 h.
[1] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, J. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603. [2] G.J.de A.A. Soler-Illia, C. Sanchez, B. Lebeau, J. Patarin, Chem. Rev. 102 (2002) 4093. [3] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [4] D. Grosso, G.J.de A.A. Soler-Illia, E.L. Crepaldi, B. Charleux, C. Sanchez, Adv. Funct. Mater. 13 (2003) 37.
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[5] I. Kartini, P. Meredith, J.C. Diniz Da Costa, G.Q. Lu, J. Sol-Gel. Sci. Techn. 31 (2004) 185. [6] M.S. Wong, E.S. Jeng, J.Y. Ying, Nano Lett. 1 (2001) 637. [7] Y. Fang, H. Hu, J. Am. Chem. Soc. 128 (2006) 10636. [8] R.F. Service, Science 300 (2003) 243. [9] T. López, F. Rojas, R. Alexander-Katz, F. Galindo, A. Balankin, A. Buljan, J. Solid State Chem. 177 (2004) 1873. [10] K.L. Frindell, J. Tang, J.H. Harreld, G.D. Stucky, Chem. Mater. 16 (2004) 3524. [11] S. Yuan, Q. Sheng, J. Zhang, F. Chen, M. Anpo, Mater. Lett. 58 (2004) 2757.
[12] Q. Sheng, S. Yuan, J. Zhang, F. Chen, Microporous Mesoporous Mater. 87 (2006) 177. [13] H. Luo, C. Wang, Y. Yan, Chem. Mater. 15 (2003) 3841. [14] M. Gopal, W.J.M. Chan, L.C. Jonghe, J. Mater. Sci. 32 (1997) 6001. [15] Z. Tang, J. Zhang, Z. Cheng, Z. Zhang, Chem. Phys. 77 (2002) 314. [16] F. Bosc, A. Ayral, P.-A. Albouy, L. Datas, C. Guizard, Chem. Mater. 16 (2004) 2208. [17] S.Y. Choi, M. Mamak, S. Speakman, N. Chopra, G.A. Ozin, Small 1 (2005) 1.
Synthesis and characterization of Ce-doped mesoporous anatase with long-range ordered mesostructure Shuai Yuan a,⁎, Yi Chen a , Liyi Shi a,⁎, Jianhui Fang a , Jianping Zhang a , Jinlong Zhang b , Hiromi Yamashita c a
c
Research Center of Nanoscience and Nanotechnology, Shanghai University, 99 Shangda Road, Shanghai 200444, PR China b Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Received 22 December 2006; accepted 27 January 2007 Available online 6 February 2007
Abstract The effect of cerium cation on the formation of titania crystallites using H+ as catalyst under mild condition has been investigated. The results indicated that the presence of cerium cation can improve the selectivity of producing anatase crystallites and inhibit the growth of crystallites. The Ce-doped mesoporous titania with highly crystallized pore walls consisting of anatase nanoparticles was synthesized by anatase crystallites assembly. The long-range ordered mesostructure was characterized by low angle and wide angle X-ray diffraction (XRD), N2 adsorption– desorption, transmission electron microscopy (TEM) and selected area electron diffraction (SAED). © 2007 Elsevier B.V. All rights reserved. Keywords: Mesoporous; Nanomaterials; Anatase; Crystal structure; Cerium cation
1. Introduction According to the pore diameter (d) of porous solids, they can be divided into three categories, i.e., microporous (d b 2 nm), mesoporous (2 nm b d b 50 nm) and macroporous (d N 50 nm) materials [1]. Zeolites, the members of a great family of microporous materials, have been applied in many fields for a long time [2]. However, the blossom of mesoporous materials is considered to start from the successful synthesis of MCM-41 by Mobil in 1992 [3]. The difference between zeolites and mesoporous materials is not only in the pore size but also in the crystal structure of the pore walls. Zeolites have crystalline frameworks. On the contrary, mesoporous materials are usually constituted of amorphous pore walls and their applications are limited by poor thermostability and hydrothermostability. Recently, more and more efforts have been done on the preparation of mesoporous materials with crystallized walls to overcome the shortages of mesoporous materials with amorphous walls [4–7]. ⁎ Corresponding authors. Tel./fax: +86 21 66135215. E-mail addresses: [email protected] (S. Yuan), [email protected] (L. Shi). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.01.086
Mesoporous titania with highly crystallized walls is an important kind of mesoporous material and has more advantages than titania nanocrystals because of its high surface area, long-range ordered porous structure, facile recovery and low risk to human health [8]. For example, the rare earth-doped mesoporous titania combining the advantages of mesoporous materials and nanomaterials exhibited more applications in catalysis and optics than rare earth-doped titania nanocrystals [9,10]. Based on previous researches, the cerium cation-doped mesoporous titania with long-range order and highly crystallized pore walls was synthesized by using crystalline nanoparticles as assembly units [11,12]. 2. Experimental section 2.1. Chemicals Tetrabutyl titanate (Ti(OBu)4) was CP grade. Nitric acid (65– 68%) and cerium nitrate (Ce(NO3)3·6H2O) were all AR grade. EO20PO70EO20 (P-123) was a commercial product from Aldrich.
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S. Yuan et al. / Materials Letters 61 (2007) 4283–4286
2.2. Synthesis
Table 1 The XRD results of samples
The typical synthesis of crystalline nanoparticles was as follows: Ti(OC4H9)4 (5 ml, 14.7 mmol) was added to 15 ml aqueous solution of HNO3 (29.4 mmol HNO3) dropwise under violent stirring at 313 K in 20 min. After stirring for additional 60 min, the mixture was allowed to stand until two layers separated. The upper layer of butyl alcohol was removed to get a transparent sol. The transparent sol was kept at 313 K for 1 day to get a white precipitate. Then, the precipitate was filtrated and dried at 313 K. The product was denoted as H2Ce0, which meant the mole ratio of H+:Ti was equal to 2:1 and the mole ratio of Ce:Ti was equal to 0:1. The 5 at.% Ce-doped titania samples were prepared by adding 0.3200 g Ce (NO3)3·6H2O (0.74 mmol Ce(NO3)3) into the solution after the hydrolysis of Ti(OC4H9)4 for 30 min or before the addition of Ti(OC4H9)4, and the products were denoted as H2Ce5a and H2Ce5b, respectively. The synthesis of Ce-doped mesoporous titania was as follows: 14.7 mmol Ti(OBu)4 was added dropwise into 15 ml aqueous solution containing 14.7 mmol HNO3, 0.74 mmol Ce (NO3)3·6H2O and 0.34 mmol P-123, followed by stirring at 313 K for 60 min. The sol was transferred from a reactor to a large open Petri dish. The sol extended sufficiently and formed a uniform thin layer. The layer was maintained at 313 K for 48 h and at 413 K for 2 h. At last, the thin layer was calcined at 673 K for 1 h in airflow. The heating-up rate was 1 K min− 1, and the cooling rate was 5 K min− 1.
Samples
2.3. Characterization X-ray diffraction (XRD) patterns of all samples were collected in θ–2θ mode using a Rigaku D/MAX-2550 diffractometer (CuKα1 radiation, λ = 1.5406 Å). The crystallite size was estimated by applying the Scherrer equation. The weight fraction of rutile was calculated from the inte-
H1Ce0 H2Ce0 H2Ce5a H2Ce5b
Anatase
Rutile
Size (nm)
Fraction (%)
Size (nm)
Fraction (%)
2.7 9.0 8.8 7.7
74.3 26.3 50.6 60.4
1.6 25.5 21.9 20.1
25.7 73.7 49.4 39.6
grated intensities of anatase (101), rutile (110) with the equation Wrutile = 1 / (0.886AA /AR + 1) [13]. The porous texture of Ce-doped mesoporous titania was analyzed from nitrogen adsorption–desorption isotherms at 77 K by using a Micromeritics ASAP 2000 system. The sample morphology was observed under transmission electron microscopy (TEM) on a 2100 JEOL microscope (200 kV) using copper grids. 3. Results and discussion The wide angle XRD patterns of titania crystallites prepared in acid condition are shown in Fig. 1(A). The diffraction peaks in the range of 15–35° indicate that the samples are mixtures of anatase and rutile. For example, from the enlarged view of sample H1Ce0, it can be observed that the measured curve is fitted by one peak at 25.2° and another peak at 27.6° which are the (101) peak of anatase and the (110) peak of rutile, respectively. According to the formulas mentioned in characterization section, the diameters of anatase and rutile crystallites and their weight fractions are summarized in Table 1. From these data, it can be concluded that high H+ concentration can accelerate the growth of crystallites of both anatase and rutile, especially the formation of rutile [14,15]. However, the addition of cerium cations can inhibit the growth of crystallites and the formation of rutile. Compared with sample H2Ce5a, sample H2Ce5b contains more anatase phase, which indicates that the selectivity of producing anatase crystallites is higher when cerium cation was added before the hydrolysis of Ti(OBu)4. The XRD results reveal that it is possible to
Fig. 1. (A) Wide angle XRD patterns of samples (a) H1Ce0 (H+:Ti = 1:1, Ce:Ti = 0:1), (b) H2Ce0 (H+:Ti = 2:1, Ce:Ti = 0:1), (c) H2Ce5a (H+:Ti = 2:1, Ce:Ti = 0.05:1, Ce (NO3)3 was added after the addition of Ti(OBu)4) and (d) H2Ce5b (H+:Ti = 2:1, Ce:Ti = 0.05:1, Ce(NO3)3 was added before the addition of Ti(OBu)4). (B) The diffraction peak of H1Ce0 in the range from 15° to 35°.
S. Yuan et al. / Materials Letters 61 (2007) 4283–4286
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Fig. 2. (A) The XRD patterns and (B) N2 adsorption–desorption patterns of Ce-doped mesoporous titania after calcination at 673 K for 1 h.
produce nanosized anatase crystallites as the assembly units for the formation of long-range ordered mesoporous titania. By the similar process described in previous researches [11], the Cedoped mesoporous titania with crystallized pore walls was prepared. The XRD patterns of Ce-doped mesoporous titania after calcination are shown in Fig. 2(A). The single peak at 1.25° in the low angle XRD pattern indicates that the long-range order of mesoporous titania is still reserved, and the d100 value due to (100) reflection from hexagonal mesoporous structure is about 7.0 nm. The wide angle XRD shows that the mesoporous titania has a crystallized framework. The average size of anatase nanocrystals calculated from the Scherrer formula is about 6.7 nm. The mesostructure of calcined Ce-doped mesoporous titania was also characterized by N2 adsorption–desorption. Fig. 2(B) shows the type IV gas adsorption isotherm, which is due to the large mesoporous channel. The hysteresis loop is an intermediate between type H1 and H2, which indicates that the Ce-doped mesoporous titania has uniform cylindrical mesopores. The average pore diameter calculated by the BJH model is 3.8 nm, and the BET specific surface area is 194 m2 g− 1. The thickness of the pore wall calculated by subtracting the pore diameter from 2d100/√3 is about 4.3 nm.
Assuming anatase nanocrystals as spherical particles, the surface area can also be estimated by S = 6 / ρd [16]. Here, ρ is the density of anatase (3.84 g cm− 3), and d is the average diameter calculated from the FWHM of the (101) peak of anatase. The value obtained is 233 m2 g− 1 which is close to the BET specific surface area. The result indicates that the pore walls of Ce-doped mesoporous titania are made of highly crystallized anatase. The TEM image of Ce-doped mesoporous titania is shown in Fig. 3. It can be observed that the long-range ordered mesoporous structure with an area about 100 nm × 200 nm is constructed by nanoparticles. The pore diameter and thickness of the pore walls estimated from the TEM image are about 4.0 nm and 4.5 nm, respectively, which are close to the analysis results from XRD and N2 adsorption–desorption data. The selected area electron diffraction (SAED) pattern shows that the pore walls are made of only anatase nanocrystals. Why the average size of anatase nanocrystals obtained from the Scherrer formula can be larger than the thickness of the pore wall has been discussed by other researchers [17].
4. Conclusion In this paper, the effect of cerium cation on the formation of titania crystallites using H+ as catalyst under mild condition has been investigated. The presence of cerium cation can improve the selectivity of producing anatase and inhibit the growth of crystallites of both rutile and anatase, especially when cerium cation was added before the hydrolysis of titania precursor. Based on this result, Ce-doped mesoporous anatase with longrange ordered mesostructure was synthesized by titania crystallites assembly. The mesostructure of Ce-doped mesoporous anatase was confirmed by various characterizations. References
Fig. 3. TEM image and selected area electron diffraction pattern (inset) of Ce-doped mesoporous titania after calcination at 673 K for 1 h.
[1] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, J. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603. [2] G.J.de A.A. Soler-Illia, C. Sanchez, B. Lebeau, J. Patarin, Chem. Rev. 102 (2002) 4093. [3] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [4] D. Grosso, G.J.de A.A. Soler-Illia, E.L. Crepaldi, B. Charleux, C. Sanchez, Adv. Funct. Mater. 13 (2003) 37.
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S. Yuan et al. / Materials Letters 61 (2007) 4283–4286
[5] I. Kartini, P. Meredith, J.C. Diniz Da Costa, G.Q. Lu, J. Sol-Gel. Sci. Techn. 31 (2004) 185. [6] M.S. Wong, E.S. Jeng, J.Y. Ying, Nano Lett. 1 (2001) 637. [7] Y. Fang, H. Hu, J. Am. Chem. Soc. 128 (2006) 10636. [8] R.F. Service, Science 300 (2003) 243. [9] T. López, F. Rojas, R. Alexander-Katz, F. Galindo, A. Balankin, A. Buljan, J. Solid State Chem. 177 (2004) 1873. [10] K.L. Frindell, J. Tang, J.H. Harreld, G.D. Stucky, Chem. Mater. 16 (2004) 3524. [11] S. Yuan, Q. Sheng, J. Zhang, F. Chen, M. Anpo, Mater. Lett. 58 (2004) 2757.
[12] Q. Sheng, S. Yuan, J. Zhang, F. Chen, Microporous Mesoporous Mater. 87 (2006) 177. [13] H. Luo, C. Wang, Y. Yan, Chem. Mater. 15 (2003) 3841. [14] M. Gopal, W.J.M. Chan, L.C. Jonghe, J. Mater. Sci. 32 (1997) 6001. [15] Z. Tang, J. Zhang, Z. Cheng, Z. Zhang, Chem. Phys. 77 (2002) 314. [16] F. Bosc, A. Ayral, P.-A. Albouy, L. Datas, C. Guizard, Chem. Mater. 16 (2004) 2208. [17] S.Y. Choi, M. Mamak, S. Speakman, N. Chopra, G.A. Ozin, Small 1 (2005) 1.