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Preparation of oxide hollow spheres by colloidal carbon spheres
Materials Letters 60 (2006) 2991 – 2993 www.elsevier.com/locate/matlet
Preparation of oxide hollow spheres by colloidal carbon spheres Mingbo Zheng, Jieming Cao ⁎, Xin Chang, Jun Wang, Jinsong Liu, Xianjia Ma Nanomaterials Research Institute, College of Materials Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China Received 16 May 2005; accepted 14 February 2006 Available online 3 March 2006
Abstract Amorphous SiO2, anatase TiO2 and cassiterite SnO2 hollow spheres have been successfully prepared using hydrophilic colloidal carbon spheres as template in conjunction with the sol–gel method. The hollow spherical structures could be confirmed by TEM and SEM. The void sizes of these hollow spheres were about 60% smaller than the diameters of the template. The BET surface areas of hollow spheres were about 183.8, 32.5, and 74.3 m2/g, respectively. © 2006 Elsevier B.V. All rights reserved. Keywords: Colloidal carbon spheres; Template; Hollow spheres; Sol–gel preparation
1. Introduction Mesoscale hollow spheres of ceramic materials have recently attracted much interest because of their potential applications in catalysis, controlled delivery, artificial cells, light fillers, low dielectric constant materials, acoustic insulation, and photonic crystals [1]. Various methods, such as templating method [2], sonochemical method [3], hydrothermal method [4] and so on, have been reported as the procedures for the preparation of inorganic materials with hollow spherical structures. In recent years, templating against colloidal particles, such as silica particles and polystyrene latex particles [5–15], to fabricate such hollow spherical structure of inorganic materials has been proven as a successful method. Recently, colloidal carbon spheres as a novel green template have been reported [16,17]. In comparison with the synthesis of polymer or silica spheres, carbon spheres have two apparent features: 1) it is a green template. 2) The surface of colloidal carbon spheres is hydrophilic and has a distribution of –OH and –C_O groups, which makes surface modification unnecessary. However, to the best of our knowledge, only Ga2O3 [17], and WO3 [18] oxide hollow spheres have been synthesized by colloidal carbon spheres templating. TiO2, a very important metal oxide, has been widely used in photovoltaic cells, photo⁎ Corresponding author. Tel.: +86 25 84893633; fax: +86 25 84895289. E-mail address: [email protected] (J. Cao). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.02.030
catalysis, gas sensors, pigments, and so on [19]. SnO2 is a widebandgap semiconductor (with a room temperature bandgap of ∼3.6 eV) that has been widely used in gas sensors, optical devices, and lithium batteries [20]. Herein, we firstly synthesized hollow SiO2, TiO2 and SnO2 spheres using colloidal carbon spheres as template. We found that both alkoxide and metal salt were easily adsorbed on the surface of the template. 2. Experimental 2.1. Preparation of carbon spheres In a typical synthesis of colloidal carbon spheres, 0.02 mol of sucrose was dissolved in 20 ml of water to form a clear solution. The solution was then sealed in a 30 ml Teflon-lined autoclave and maintained at 180 °C for 8 h. The products were centrifuged, washed, and redispersed in water, and this cycle was repeated five times. Next, the products were centrifuged, washed, and redispersed in ethanol, and this cycle was repeated five times. The spheres were then oven-dried at 80 °C for 5 h. 2.2. Preparation of hollow spheres In a typical synthesis of SiO2 hollow spheres, the starting solution was prepared by mixing 2 ml of ethanol and 18 ml of water, which was adjusted to pH = 2 with HCl, and then adding 1 ml of TEOS into the solution with vigorous stirring. The
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M. Zheng et al. / Materials Letters 60 (2006) 2991–2993
Fig. 1. (a) SEM image of carbon spheres. (b) SEM image of SiO2 hollow spheres. (c) TEM image of SiO2 hollow spheres.
TEOS was hydrolyzed by stirring for 30 min. 0.2 g carbon spheres (2 μm) were added into the solution and stirred vigorously to coat silica onto the surface of spheres. The products were centrifugated after reaction time of 24 h. Next, the products were redispersed, washed, and centrifuged in ethanol, and this cycle was repeated three times. The samples were oven-dried at 40 °C for 12 h, and then were heated to 450 °C at 3 °C min− 1 for 4 h using a tube furnace in air. White powders were obtained as the samples were allowed to cool to room temperature. Synthesis of TiO2, SnO2 hollow spheres were similar to the synthesis of SiO2. 2.3. Characterization The size and morphology of hollow spheres were examined by scanning electron microscopy (SEM) (LEO1530), and transmission electron microscopy (TEM) (JEOL, JEM200CX, at 200 kV). The crystal structures of samples were characterized by X-ray diffraction (XRD) (Bruker D8 advance). The BET surface areas of samples were measured on a Micromeritics ASAP 2010 instrument. 3. Results and discussion Fig. 1a shows a typical SEM image of such colloidal carbon spheres with diameter of 2 μm and narrow size distributions. It should be noted that the diameter of colloidal carbon spheres can be controlled by the reaction time [16]. Fig. 1b, c shows the SEM and TEM micrograph of the SiO2 hollow structure, respectively. The TEOS-adsorbed carbon spheres were calcined in air to remove the carbon component, resulting in SiO2
Fig. 2. Typical (a) SEM and (b) TEM images of TiO2 hollow spheres. Inset is the SAED pattern.
hollow spheres. During the calcination, the template may transform into CO2, which escapes effectively without breaking the shell wall. The hollow spheres prepared from carbon spheres of 2 μm in diameter had average diameter of 800 nm. Similar with Sun and Li's results [17], the hollow spheres also had a rippled shell. The shell thickness was about 20 nm as measured by using a magnified TEM image. In comparison with traditional methods, the silica–carbon composite particles underwent serious shrinkage during the calcination process. The large shrinkage is probably caused by further dehydration of the loosely cross-linked structure of the carbon spheres [17]. The BET surface area of SiO2 hollow spheres was about 183.8 m2/g. We have also tried to extend this procedure to other alkoxide system. TiO2 hollow spheres were fabricated by templating a sol–gel precursor solution, i.e., tetrabutyl titanate (TBT) in ethanol with a volume ratio of TBT: C2H5OH = 1 : 5 (using 300 nm carbon spheres as template). Due to TBT hydrolyze easily in air, the stirring should be carried through in a glove box. When centrifugated samples were exposed to the moisture in air, the precursor hydrolyzed into metal oxide sols, which subsequently aggregated into a network of gel [12,21]. This gel formed a homogeneous, dense, thin coating around each carbon sphere. Fig. 2 shows the SEM and TEM images of TiO2 hollow spheres. Similar with SiO2 hollow spheres, they also had a large shrinkage. XRD pattern (Fig. 3a) and selected area electron diffraction (Fig. 2b inset) indicate anatase phase was formed during calcinations. The BET surface area of TiO2 hollow spheres was about 32.5 m2/g. We additionally choose metal salt as reactant, i.e., SnCl4 in ethanol with a molar ratio of SnCl4 : C2H5OH = 1 : 20 (using 2 μm carbon spheres as template). Metal cations are absorbed onto the surface layer
Fig. 3. XRD patterns of (a) TiO2 and (b) SnO2 hollow spheres prepared at 450 °C.
M. Zheng et al. / Materials Letters 60 (2006) 2991–2993
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Fig. 4. Typical (a) SEM and (b), (c) TEM images of SnO2 hollow spheres. Inset is the SAED pattern.
to form a composite shell. In the following calcination process, the metal atoms in the shell become denser, the spheres contract and crosslink to form metal-oxide hollow spheres, which are a smaller replica of the carbon sphere. Fig. 4 shows the SEM and TEM images of SnO2 hollow spheres. The fragments from broken hollow spheres were clearly observed. XRD pattern (Fig. 3b) indicates that the inorganic SnO2 framework consists of cassiterite nanocrystalline domains. Based on the line width of the diffraction peak corresponding to (110) reflection, the crystallite size was determined by using the Scherrer formula. The average size of crystallizes for the calcined product was about 13.5 nm. It was also demonstrated by the magnified TEM image (Fig. 4c). The BET surface area of SnO2 hollow spheres was about 74.3 m2/g. Energy dispersive X-ray analysis of calcined sample indicated the carbon content was about 3.1 wt.%.
4. Conclusions In summary, SiO2, TiO2 and SnO2 hollow spheres have been successfully prepared by using colloidal carbon spheres as template. The void sizes of these hollow spheres were about 60% smaller than the diameters of the carbon template. This approach provides a green, simple, and economical method to uniform hollow spheres. Our future work of hollow spheres of varied compositions prepared by using this method is under progress. It is anticipated that these novel materials will have potential applications in many fields. Acknowledgements This work was financially supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars.
References [1] F. Caruso, R.A. Caruso, H. Möhwald, Science 282 (1998) 1111; Z. Zhong, Y. Yin, B. Gates, Y. Xia, Adv. Mater. 12 (2000) 206; F. Caruso, Adv. Mater. 13 (2001) 11; Z. Yang, Z. Niu, Y. Lu, Z. Hu, C.C. Han, Angew. Chem., Int. Ed. Engl. 42 (2003) 1943. [2] F. Caruso, Chem. Eur. J. 6 (2000) 413. [3] J.J. Zhu, S. Xu, H. Wang, J.M. Zhu, H.Y. Chen, Adv. Mater. 15 (2003) 156. [4] C. Wang, K. Tang, Q. Yang, J. Hu, Y. Qian, J. Mater. Chem. 12 (2002) 2426. [5] R.A. Caruso, A. Susha, F. Caruso, Chem. Mater. 13 (2001) 400. [6] Z.J. Liang, A. Susha, F. Caruso, Chem. Mater. 15 (2003) 3176. [7] K. Kamata, Y. Lu, Y. Xia, J. Am. Chem. Soc. 125 (2003) 2384. [8] Y. Lu, J. McLellan, Y. Xia, Langmuir 20 (2004) 3464. [9] I. Tissot, C. Novat, F. Lefebvre, E. Bourgent-Lami, Macromolecules 34 (2001) 5737. [10] P. Jiang, J.F. Bertone, V.L. Colvin, Science 291 (2001) 453. [11] S. Eiden, G. Maret, J. Colloid Interface Sci. 250 (2002) 281. [12] S. Fujikawa, T. Kunitake, Chem. Lett. (2002) 1134. [13] Y. Hotta, P.C.A. Alberius, L. Bergström, J. Mater. Chem. 13 (2003) 496. [14] J.J.L.M. Cornelissen, E.F. Connor, H.-C. Kim, V.Y. Lee, T. Magibitang, P.M. Rice, W. Volksen, L.K. Sundberg, R.D. Miller, Chem. Commun. (2003) 1010. [15] G.C. Li, Z.K. Zhang, Mater. Lett. 58 (2004) 2768. [16] X. Sun, Y. Li, Angew. Chem., Int. Ed. Engl. 43 (2004) 597. [17] X. Sun, Y. Li, Angew. Chem., Int. Ed. Engl. 43 (2004) 3827. [18] X.-L. Li, T.-J. Lou, X.-M. Sun, Y.-D. Li, Inorg. Chem. 43 (2004) 5442. [19] T.F. Baumann, S.O. Kucheyev, A.F. Gash, J.H. Satcher, Adv. Mater. 17 (2005) 1546. [20] D.C. Pan, N.N. Zhao, Q. Wang, S.C. Jiang, X.L. Ji, L.J. An, Adv. Mater. 17 (2005) 1991. [21] L.L. Hench, J.K. West, Chem. Rev. 90 (1990) 33.
Preparation of oxide hollow spheres by colloidal carbon spheres Mingbo Zheng, Jieming Cao ⁎, Xin Chang, Jun Wang, Jinsong Liu, Xianjia Ma Nanomaterials Research Institute, College of Materials Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China Received 16 May 2005; accepted 14 February 2006 Available online 3 March 2006
Abstract Amorphous SiO2, anatase TiO2 and cassiterite SnO2 hollow spheres have been successfully prepared using hydrophilic colloidal carbon spheres as template in conjunction with the sol–gel method. The hollow spherical structures could be confirmed by TEM and SEM. The void sizes of these hollow spheres were about 60% smaller than the diameters of the template. The BET surface areas of hollow spheres were about 183.8, 32.5, and 74.3 m2/g, respectively. © 2006 Elsevier B.V. All rights reserved. Keywords: Colloidal carbon spheres; Template; Hollow spheres; Sol–gel preparation
1. Introduction Mesoscale hollow spheres of ceramic materials have recently attracted much interest because of their potential applications in catalysis, controlled delivery, artificial cells, light fillers, low dielectric constant materials, acoustic insulation, and photonic crystals [1]. Various methods, such as templating method [2], sonochemical method [3], hydrothermal method [4] and so on, have been reported as the procedures for the preparation of inorganic materials with hollow spherical structures. In recent years, templating against colloidal particles, such as silica particles and polystyrene latex particles [5–15], to fabricate such hollow spherical structure of inorganic materials has been proven as a successful method. Recently, colloidal carbon spheres as a novel green template have been reported [16,17]. In comparison with the synthesis of polymer or silica spheres, carbon spheres have two apparent features: 1) it is a green template. 2) The surface of colloidal carbon spheres is hydrophilic and has a distribution of –OH and –C_O groups, which makes surface modification unnecessary. However, to the best of our knowledge, only Ga2O3 [17], and WO3 [18] oxide hollow spheres have been synthesized by colloidal carbon spheres templating. TiO2, a very important metal oxide, has been widely used in photovoltaic cells, photo⁎ Corresponding author. Tel.: +86 25 84893633; fax: +86 25 84895289. E-mail address: [email protected] (J. Cao). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.02.030
catalysis, gas sensors, pigments, and so on [19]. SnO2 is a widebandgap semiconductor (with a room temperature bandgap of ∼3.6 eV) that has been widely used in gas sensors, optical devices, and lithium batteries [20]. Herein, we firstly synthesized hollow SiO2, TiO2 and SnO2 spheres using colloidal carbon spheres as template. We found that both alkoxide and metal salt were easily adsorbed on the surface of the template. 2. Experimental 2.1. Preparation of carbon spheres In a typical synthesis of colloidal carbon spheres, 0.02 mol of sucrose was dissolved in 20 ml of water to form a clear solution. The solution was then sealed in a 30 ml Teflon-lined autoclave and maintained at 180 °C for 8 h. The products were centrifuged, washed, and redispersed in water, and this cycle was repeated five times. Next, the products were centrifuged, washed, and redispersed in ethanol, and this cycle was repeated five times. The spheres were then oven-dried at 80 °C for 5 h. 2.2. Preparation of hollow spheres In a typical synthesis of SiO2 hollow spheres, the starting solution was prepared by mixing 2 ml of ethanol and 18 ml of water, which was adjusted to pH = 2 with HCl, and then adding 1 ml of TEOS into the solution with vigorous stirring. The
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Fig. 1. (a) SEM image of carbon spheres. (b) SEM image of SiO2 hollow spheres. (c) TEM image of SiO2 hollow spheres.
TEOS was hydrolyzed by stirring for 30 min. 0.2 g carbon spheres (2 μm) were added into the solution and stirred vigorously to coat silica onto the surface of spheres. The products were centrifugated after reaction time of 24 h. Next, the products were redispersed, washed, and centrifuged in ethanol, and this cycle was repeated three times. The samples were oven-dried at 40 °C for 12 h, and then were heated to 450 °C at 3 °C min− 1 for 4 h using a tube furnace in air. White powders were obtained as the samples were allowed to cool to room temperature. Synthesis of TiO2, SnO2 hollow spheres were similar to the synthesis of SiO2. 2.3. Characterization The size and morphology of hollow spheres were examined by scanning electron microscopy (SEM) (LEO1530), and transmission electron microscopy (TEM) (JEOL, JEM200CX, at 200 kV). The crystal structures of samples were characterized by X-ray diffraction (XRD) (Bruker D8 advance). The BET surface areas of samples were measured on a Micromeritics ASAP 2010 instrument. 3. Results and discussion Fig. 1a shows a typical SEM image of such colloidal carbon spheres with diameter of 2 μm and narrow size distributions. It should be noted that the diameter of colloidal carbon spheres can be controlled by the reaction time [16]. Fig. 1b, c shows the SEM and TEM micrograph of the SiO2 hollow structure, respectively. The TEOS-adsorbed carbon spheres were calcined in air to remove the carbon component, resulting in SiO2
Fig. 2. Typical (a) SEM and (b) TEM images of TiO2 hollow spheres. Inset is the SAED pattern.
hollow spheres. During the calcination, the template may transform into CO2, which escapes effectively without breaking the shell wall. The hollow spheres prepared from carbon spheres of 2 μm in diameter had average diameter of 800 nm. Similar with Sun and Li's results [17], the hollow spheres also had a rippled shell. The shell thickness was about 20 nm as measured by using a magnified TEM image. In comparison with traditional methods, the silica–carbon composite particles underwent serious shrinkage during the calcination process. The large shrinkage is probably caused by further dehydration of the loosely cross-linked structure of the carbon spheres [17]. The BET surface area of SiO2 hollow spheres was about 183.8 m2/g. We have also tried to extend this procedure to other alkoxide system. TiO2 hollow spheres were fabricated by templating a sol–gel precursor solution, i.e., tetrabutyl titanate (TBT) in ethanol with a volume ratio of TBT: C2H5OH = 1 : 5 (using 300 nm carbon spheres as template). Due to TBT hydrolyze easily in air, the stirring should be carried through in a glove box. When centrifugated samples were exposed to the moisture in air, the precursor hydrolyzed into metal oxide sols, which subsequently aggregated into a network of gel [12,21]. This gel formed a homogeneous, dense, thin coating around each carbon sphere. Fig. 2 shows the SEM and TEM images of TiO2 hollow spheres. Similar with SiO2 hollow spheres, they also had a large shrinkage. XRD pattern (Fig. 3a) and selected area electron diffraction (Fig. 2b inset) indicate anatase phase was formed during calcinations. The BET surface area of TiO2 hollow spheres was about 32.5 m2/g. We additionally choose metal salt as reactant, i.e., SnCl4 in ethanol with a molar ratio of SnCl4 : C2H5OH = 1 : 20 (using 2 μm carbon spheres as template). Metal cations are absorbed onto the surface layer
Fig. 3. XRD patterns of (a) TiO2 and (b) SnO2 hollow spheres prepared at 450 °C.
M. Zheng et al. / Materials Letters 60 (2006) 2991–2993
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Fig. 4. Typical (a) SEM and (b), (c) TEM images of SnO2 hollow spheres. Inset is the SAED pattern.
to form a composite shell. In the following calcination process, the metal atoms in the shell become denser, the spheres contract and crosslink to form metal-oxide hollow spheres, which are a smaller replica of the carbon sphere. Fig. 4 shows the SEM and TEM images of SnO2 hollow spheres. The fragments from broken hollow spheres were clearly observed. XRD pattern (Fig. 3b) indicates that the inorganic SnO2 framework consists of cassiterite nanocrystalline domains. Based on the line width of the diffraction peak corresponding to (110) reflection, the crystallite size was determined by using the Scherrer formula. The average size of crystallizes for the calcined product was about 13.5 nm. It was also demonstrated by the magnified TEM image (Fig. 4c). The BET surface area of SnO2 hollow spheres was about 74.3 m2/g. Energy dispersive X-ray analysis of calcined sample indicated the carbon content was about 3.1 wt.%.
4. Conclusions In summary, SiO2, TiO2 and SnO2 hollow spheres have been successfully prepared by using colloidal carbon spheres as template. The void sizes of these hollow spheres were about 60% smaller than the diameters of the carbon template. This approach provides a green, simple, and economical method to uniform hollow spheres. Our future work of hollow spheres of varied compositions prepared by using this method is under progress. It is anticipated that these novel materials will have potential applications in many fields. Acknowledgements This work was financially supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars.
References [1] F. Caruso, R.A. Caruso, H. Möhwald, Science 282 (1998) 1111; Z. Zhong, Y. Yin, B. Gates, Y. Xia, Adv. Mater. 12 (2000) 206; F. Caruso, Adv. Mater. 13 (2001) 11; Z. Yang, Z. Niu, Y. Lu, Z. Hu, C.C. Han, Angew. Chem., Int. Ed. Engl. 42 (2003) 1943. [2] F. Caruso, Chem. Eur. J. 6 (2000) 413. [3] J.J. Zhu, S. Xu, H. Wang, J.M. Zhu, H.Y. Chen, Adv. Mater. 15 (2003) 156. [4] C. Wang, K. Tang, Q. Yang, J. Hu, Y. Qian, J. Mater. Chem. 12 (2002) 2426. [5] R.A. Caruso, A. Susha, F. Caruso, Chem. Mater. 13 (2001) 400. [6] Z.J. Liang, A. Susha, F. Caruso, Chem. Mater. 15 (2003) 3176. [7] K. Kamata, Y. Lu, Y. Xia, J. Am. Chem. Soc. 125 (2003) 2384. [8] Y. Lu, J. McLellan, Y. Xia, Langmuir 20 (2004) 3464. [9] I. Tissot, C. Novat, F. Lefebvre, E. Bourgent-Lami, Macromolecules 34 (2001) 5737. [10] P. Jiang, J.F. Bertone, V.L. Colvin, Science 291 (2001) 453. [11] S. Eiden, G. Maret, J. Colloid Interface Sci. 250 (2002) 281. [12] S. Fujikawa, T. Kunitake, Chem. Lett. (2002) 1134. [13] Y. Hotta, P.C.A. Alberius, L. Bergström, J. Mater. Chem. 13 (2003) 496. [14] J.J.L.M. Cornelissen, E.F. Connor, H.-C. Kim, V.Y. Lee, T. Magibitang, P.M. Rice, W. Volksen, L.K. Sundberg, R.D. Miller, Chem. Commun. (2003) 1010. [15] G.C. Li, Z.K. Zhang, Mater. Lett. 58 (2004) 2768. [16] X. Sun, Y. Li, Angew. Chem., Int. Ed. Engl. 43 (2004) 597. [17] X. Sun, Y. Li, Angew. Chem., Int. Ed. Engl. 43 (2004) 3827. [18] X.-L. Li, T.-J. Lou, X.-M. Sun, Y.-D. Li, Inorg. Chem. 43 (2004) 5442. [19] T.F. Baumann, S.O. Kucheyev, A.F. Gash, J.H. Satcher, Adv. Mater. 17 (2005) 1546. [20] D.C. Pan, N.N. Zhao, Q. Wang, S.C. Jiang, X.L. Ji, L.J. An, Adv. Mater. 17 (2005) 1991. [21] L.L. Hench, J.K. West, Chem. Rev. 90 (1990) 33.