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TiO2 composite hollow spheres
Journal of Colloid and Interface Science 272 (2004) 340–344 www.elsevier.com/locate/jcis
Preparation and characterization of silver/TiO2 composite hollow spheres Caixia Song, Debao Wang, Guohua Gu, Yusheng Lin, Jingyi Yang, Lei Chen, Xun Fu, and Zhengshui Hu ∗ Department of Applied Chemistry, Qingdao University of Science and Technology, Qingdao, Shandong 266042, People’s Republic of China Received 16 June 2003; accepted 25 August 2003
Abstract Silver-coated poly(methyl acrylic acid) (PSA) core–shell colloid particles were prepared by an in situ chemical reduction method. Crystalline silver/titania composite hollow spheres were obtained by coating the as-prepared PSA/silver particles with an amorphous titania layer and subsequently calcining in Ar atmosphere. SEM and TEM investigation indicated that the size of the as-prepared PSA/silver and PSA/silver/TiO2 core–shell particles and silver/titania composite hollow particles was fairly uniform and the wall thickness of the hollow spheres was in the range of 40–80 nm. UV–vis absorption spectra were recorded to investigate their optical properties. 2003 Elsevier Inc. All rights reserved. Keywords: Silver/TiO2 ; Hollow spheres; Template; Core–shell particles
1. Introduction Nanometer- to micrometer-sized inorganic hollow spheres represent an important class of materials that are useful in variety of areas [1–3]. A number of methods have been employed to explore the preparation of hollow spheres of different materials [4–7] among which the sulfate-stabilized polystyrene (PS) latex templating method has been extensively studied in the literature. Shiho and Kawahashi [8,9] prepared several kinds of ceramic and magnetic hollow spheres by templating directly against the PS latex. The Caruso group [10,11] extensively studied the fabrication of inorganic hollow spheres using polyelectrolytemodified PS spheres as template via the layer-by-layer selfassembly technique. Ceramic hollow spheres have also been prepared by templating against the crystalline array of PS spheres [12]. Nanoparticles of noble metals have potential applications in optics, optoelectronics, catalysis, and so forth. However, unsupported nanoparticles are thermally unstable. Hence, the immobilization of these nanoparticles on supports, such as polymer latex particles [13,14] or high-surface-area ceramic hollow spheres, to form composite particles is of interest [12,15,16]. * Corresponding author.
E-mail address: [email protected] (Z. Hu). 0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2003.08.062
Previously, we successfully obtained uniform styrene– acrylic acid copolymer latex particles with a diameter of about 600 nm [17], which have surface properties similar to those of negatively charged PS particles. Here, we report a simple route for the synthesis of silver-coated PSA particles by in situ chemical reduction, the formation of PSA/silver/titania composite parcels by templating against the as-prepared PSA/silver particles via a simple wet-chemistry route, and the formation of silver/titania composite hollow spheres by removing the PSA core.
2. Materials and methods All the chemicals with the highest purity commercially available were used as received. Poly(styrene-methyl acrylic acid) (PSA) latex used as core particles were prepared by the radical copolymerization method using ammonium persulfate as the initiator [9,17]. To prepare PSA/silver core–shell particles, a colloid solution containing 1.8 g/l PSA latex, 0.008 mol/l AgNO3 , 0.03 mol/l urotropine, and 5.0 g/l poly(vinylpyrrolidone) (PVP) was aged for 4 h in a capped test tube at 85 ◦ C. After that, the suspension was cooled to room temperature, centrifuged, and washed three times with water. To prepare PSA/silver/TiO2 composite particles, the asprepared PSA/silver particles were dispersed in an aqueous solution containing 5.0 g/l PVP and 0.012 mol/l Ti(SO4 )2
C. Song et al. / Journal of Colloid and Interface Science 272 (2004) 340–344
under ultrasound. The colloid suspension was aged at 70 ◦ C for 3 h, then cooled, centrifuged, and washed several times with water and alcohol, finally dried under vacuum at 40 ◦ C. To prepare silver/TiO2 hollow particles, the PSA/silver/ TiO2 composite particles were heated to 550 ◦ C at a rate of 1 ◦ C min−1 in Ar atmosphere and kept at 550 ◦ C for 2 h. TEM images of the samples were taken on a Hitachi 800 transmission electron microscope. SEM images were obtained on a Hitachi (X-650) scanning electron microanalyzer and a JSM-6700F field emission scanning electron microscope. XRD patterns of the products were recorded using a Rigaku D/Max r–A X-ray diffractometer with CuKα radiation (λ = 0.154178 nm). Thermogravimetric analysis (TGA) was carried out on a TA-50 thermal analyzer (Shimadzu) in N2 atmosphere. UV–vis spectra were recorded on a Shimadzu UV-2401PC UV–vis recording spectrophotometer.
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SEM image in Fig. 2c further confirms that most of the PSA latex particles are well decorated with silver nanoparticles. XRD pattern in Fig. 3b confirms the formation of silverdecorated PSA particles. Three reflection peaks are indexed to the (111), (200), and (220) planes of fcc silver (JCPDS Card 4-783), respectively. The higher background in the lower 2θ region can be contributed to the reflection of the PSA latex particles.
3. Results and discussion Fig. 1 shows the schematic illustration of the formation procedure of PSA/silver core–shell particles, PSA/silver/ TiO2 particles, and silver/TiO2 hollow spheres. It has been demonstrated that negatively charged sulfatestabilized polystyrene latex can be covered with a smooth inorganic layer by the hydrolysis of metal ions [3,8,9]. It was of interest to apply this method to the case of silver using PSA latex as core particles. In the first stage, urotropine slightly decomposes to give formaldehyde and ammonia upon aging in colloid solution, and the adsorbed silver ions are reduced in situ by the freshly produced formaldehyde to form silver nuclei on the surface of PSA templates. The nucleation stage is then followed by diffusion of silver ions and reducing agent to the surface of PSA particles to feed the in situ reducing reaction, and silver nuclei grow to silver particles. Fig. 2a shows the TEM image of the PSA template particles with uniform size and diameter 600 nm on average. Fig. 2b shows typical TEM image of the as-prepared PSA/silver core–shell particles. It is noteworthy that although the PSA latex surface appears to be fairly evenly coated with silver nanoparticles, these metallic deposits do not form a homogeneous coating or shell but a coating of immobilized silver particles of 10–30 nm in diameters. The
Fig. 1. Schematic diagram of the formation procedure of PSA/silver core–shell particles and silver/TiO2 hollow spheres.
Fig. 2. (a) TEM image of PSA latex particles, (b) TEM images of PSA/silver particles, and (c) SEM image of PSA/silver particles.
Fig. 3. XRD patterns of (a) PSA latex particles, (b) PSA/silver particles, (c) PSA/silver/TiO2 particles, and (d) silver/TiO2 hollow spheres.
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Fig. 5. Thermogravimetric analysis curve of the PSA/silver/TiO2 composite particles. Fig. 4. SEM image and TEM image (inset) of PSA/silver/TiO2 particles. The bar in the inset is 150 nm.
The coating procedure of PSA/silver/TiO2 particles involves a simple wet-chemistry method, which consists of the immersion of the silver-coated PSA latex into an aqueous solution of Ti(SO4 )2 and subsequent aging. During the aging procedure, Ti(SO4 )2 hydrolyzed into TiO2 colloid, with which the PSA/silver particles were covered. It was also found the addition of PVP is necessary in all the coating process to prevent the aggregation of the core particles and the coated particles [9]. In contrast to organometallic precursor of Ti(IV) used in the literature [9,12], Ti(SO4 )2 aqueous solution is easy to handle, and the coating procedure is simple. Fig. 4 shows a SEM image of the as-prepared PSA/silver/ TiO2 composite particles. All the particles are dispersed relatively well and have a uniform size. The inset of Fig. 4 shows a TEM image of one of these composite particles at high magnification. Both the SEM and TEM images indicate that the surfaces of the PSA/silver particles become smooth after the coating process with TiO2 . No new reflection peaks appear in the XRD pattern (Fig. 3c), which that the TiO2 layer on silver coated PSA is amorphous (compared with Fig. 3b). Fig. 5 shows the TGA curve of the PSA/silver/TiO2 composite particles. The first weight loss in the range of 50– 260 ◦ C can be attributed to the release of water, whereas the second sharp weight loss corresponds to the decomposition of core PSA particles in the range of 260–475 ◦ C. It has been reported that hollow particles of metallic copper can be obtained after removing the core PS particles by calcinations of the composite particles in nitrogen atmosphere [18]. Thus the calcinations at 550 ◦ C in Ar were employed to remove the PSA core particles in present experiments. The formation of crystalline TiO2 was confirmed by XRD pattern in Fig. 3d. The XRD pattern can be indexed to (101), (200), (105), and (211) planes of tetragonal phase anatase TiO2 (JCPDS Card 83-2243) together with the diffraction of (111), (200), and (220) planes of fcc silver (JCPDS Card 4-783). No obvious XRD peaks arising from impurities are found, suggesting the main component of the hollow spheres being silver and titania. XRD results indicate that crystalline
titania were produced by calcinating the composite particles at 550 ◦ C in Ar atmosphere. SEM images shown in Figs. 6a and 6b reveal that the product consists of well-defined hollow spheres with uniform size distribution and smooth surfaces. Broken hollow spheres can also be observed. To further confirm the hollow nature of the spheres, TEM observations were carried out. As shown in Figs. 6c and 6d, the spheres’ shape was verified by tilting the TEM sample plane from 0◦ (Fig. 6c) to 22◦ (Fig. 6d), obviously revealing the hollow structure of the sample. The wall thickness of these hollow spheres is in the range 40–80 nm. Electron diffraction pattern obtained by focusing the electron beam on an individual hollow sphere is shown in Fig. 6e. The rings-like diffraction pattern can be indexed to tetragonal-phase TiO2 together with (111) diffraction of fcc silver, suggesting the formation of silver/TiO2 composite hollow spheres. UV–vis absorption spectra of the suspension of these composite particles were recorded to investigate their optical properties (Fig. 7). As can be seen from curve b in Fig. 7, PSA/silver core–shell particles have a broad absorption at 400–500 nm, which is the characteristic absorption of silver–latex composites [13]. For the suspension of PSA/silver/TiO2 particles, the absorption (curve c in Fig. 7) becomes weaker and broader, which may be the results of the scattering of the amorphous TiO2 layers. The suspension of the silver/TiO2 hollow spheres exhibits a UV–vis spectrum containing two peaks (curve d, Fig. 7): the weaker one located at about 300 nm can be assigned to Ag+ 4 clusters [19], and the stronger at about 500–600 nm shows obvious red shifts with respect to the usual plasmon peak position of the silver nanoparticles. It has been reported that the peaks of the silver colloid shift about 20 nm toward longer wave length when the TiO2 /silver core–shell structure is formed [20]. The obvious shift toward longer wavelengths in the present study might be related to the hollow structures of silver/TiO2 particles, although the exact mechanism is not clear. The UV–vis investigation indicated that the asprepared silver/TiO2 hollow spheres showed broad and relatively strong absorption in visible region. We suggest that they could potentially be useful for the investigation of var-
C. Song et al. / Journal of Colloid and Interface Science 272 (2004) 340–344
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Fig. 6. (a, b) SEM images of silver/TiO2 hollow spheres. (c, d) TEM images of silver/TiO2 hollow spheres tilting the sample plane from (c) 0◦ to (d) 22◦ . (e) ED pattern of silver/TiO2 hollow spheres.
have been successfully prepared by templating against PSA latex particles. The as-synthesized composite hollow spheres have a uniform size and wall-thickness of 40–80 nm. By replacing silver with other noble metals, and titania with other ceramic materials, we hope it would be serve as a general route to hollow spheres with different compositions and could offer the prospect of assembly novel nanoscale architectures in solution.
References
Fig. 7. UV–vis spectra of (a) PSA, (b) PSA/silver particles, (c) PSA/ silver/TiO2 , and (d) silver/TiO2 hollow spheres.
ious optical phenomena, as novel supported photocatalysts, or as candidates for photonic crystals [21].
[1] [2] [3] [4] [5] [6] [7] [8]
4. Conclusion In summary, silver-decorated PSA latex core–shell structures and crystalline silver/titania composite hollow spheres
[9] [10] [11] [12]
F. Caruso, Chem. Eur. J. 6 (2000) 413. Z. Zhong, Y. Yin, B. Gates, Y. Xia, Adv. Mater. 12 (2000) 206. N. Kawahashi, E. Matijevi´c, J. Colloid Interface Sci. 143 (1991) 103. C.E. Fowler, D. Khushalani, S. Mann, J. Mater. Chem. 11 (2001) 1968. S. Chah, J.H. Fendler, J. Yi, J. Colloid Interface Sci. 250 (2002) 142, doi:10.1006/jcis.2002.8328. E. Stefanie, M. Georg, J. Colloid Interface Sci. 250 (2002) 281, doi:10.1006/jcis.2002.8315. D. Zhang, L. Qi, J. Ma, H. Cheng, Adv. Mater. 14 (2002) 1499. H. Shiho, Y. Manabe, N. Kawahashi, J. Colloid Interface Sci. 226 (2000) 91, doi:10.1006/jcis.2000.6789. H. Shiho, N. Kawahashi, Colloid Polym. Sci. 278 (2000) 270. F. Caruso, R.A. Caruso, H. Möhwald, Science 282 (1998) 1111. D. Wang, F. Caruso, Chem. Mater. 14 (2002) 1909. Y. Yin, Y. Lu, B. Gates, Y. Xia, Chem. Mater. 13 (2001) 1146.
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[13] A.B.R. Mayer, W. Grebner, R. Wannemacher, J. Phys. Chem. B 104 (2000) 7278. [14] P.A. Schueler, J.T. Ives, F. DeLaCroix, W.B. Lacy, P.A. Becker, J. Li, K.D. Caldwell, B. Drake, J.M. Harris, Anal. Chem. 65 (1993) 3177. [15] H. Tamai, T. Ikeya, F. Nishiyama, H. Yasuda, K. Iida, S. Nojima, J. Mater. Sci. 35 (2000) 4945. [16] A. Sarkany, Zs. Revay, Appl. Catal. A 243 (2003) 347.
[17] Y. Guo, Thesis, Qingdao University of Science and Technology, Qingdao, 2002. [18] N. Kawahashi, H. Shiho, J. Mater. Chem. 10 (2000) 2294. [19] R. Tausch-Trekml, A. Henglein, J. Lilie, Phys. Chem. 82 (1978) 1335. [20] Y. Zhou, C.Y. Wang, H.J. Liu, Y.R. Zhu, Z.Y. Chen, Mater. Sci. Eng. B 67 (1999) 95. [21] D.J. Norris, Y.A. Vlasov, Adv. Mater. 13 (2001) 371.
Preparation and characterization of silver/TiO2 composite hollow spheres Caixia Song, Debao Wang, Guohua Gu, Yusheng Lin, Jingyi Yang, Lei Chen, Xun Fu, and Zhengshui Hu ∗ Department of Applied Chemistry, Qingdao University of Science and Technology, Qingdao, Shandong 266042, People’s Republic of China Received 16 June 2003; accepted 25 August 2003
Abstract Silver-coated poly(methyl acrylic acid) (PSA) core–shell colloid particles were prepared by an in situ chemical reduction method. Crystalline silver/titania composite hollow spheres were obtained by coating the as-prepared PSA/silver particles with an amorphous titania layer and subsequently calcining in Ar atmosphere. SEM and TEM investigation indicated that the size of the as-prepared PSA/silver and PSA/silver/TiO2 core–shell particles and silver/titania composite hollow particles was fairly uniform and the wall thickness of the hollow spheres was in the range of 40–80 nm. UV–vis absorption spectra were recorded to investigate their optical properties. 2003 Elsevier Inc. All rights reserved. Keywords: Silver/TiO2 ; Hollow spheres; Template; Core–shell particles
1. Introduction Nanometer- to micrometer-sized inorganic hollow spheres represent an important class of materials that are useful in variety of areas [1–3]. A number of methods have been employed to explore the preparation of hollow spheres of different materials [4–7] among which the sulfate-stabilized polystyrene (PS) latex templating method has been extensively studied in the literature. Shiho and Kawahashi [8,9] prepared several kinds of ceramic and magnetic hollow spheres by templating directly against the PS latex. The Caruso group [10,11] extensively studied the fabrication of inorganic hollow spheres using polyelectrolytemodified PS spheres as template via the layer-by-layer selfassembly technique. Ceramic hollow spheres have also been prepared by templating against the crystalline array of PS spheres [12]. Nanoparticles of noble metals have potential applications in optics, optoelectronics, catalysis, and so forth. However, unsupported nanoparticles are thermally unstable. Hence, the immobilization of these nanoparticles on supports, such as polymer latex particles [13,14] or high-surface-area ceramic hollow spheres, to form composite particles is of interest [12,15,16]. * Corresponding author.
E-mail address: [email protected] (Z. Hu). 0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2003.08.062
Previously, we successfully obtained uniform styrene– acrylic acid copolymer latex particles with a diameter of about 600 nm [17], which have surface properties similar to those of negatively charged PS particles. Here, we report a simple route for the synthesis of silver-coated PSA particles by in situ chemical reduction, the formation of PSA/silver/titania composite parcels by templating against the as-prepared PSA/silver particles via a simple wet-chemistry route, and the formation of silver/titania composite hollow spheres by removing the PSA core.
2. Materials and methods All the chemicals with the highest purity commercially available were used as received. Poly(styrene-methyl acrylic acid) (PSA) latex used as core particles were prepared by the radical copolymerization method using ammonium persulfate as the initiator [9,17]. To prepare PSA/silver core–shell particles, a colloid solution containing 1.8 g/l PSA latex, 0.008 mol/l AgNO3 , 0.03 mol/l urotropine, and 5.0 g/l poly(vinylpyrrolidone) (PVP) was aged for 4 h in a capped test tube at 85 ◦ C. After that, the suspension was cooled to room temperature, centrifuged, and washed three times with water. To prepare PSA/silver/TiO2 composite particles, the asprepared PSA/silver particles were dispersed in an aqueous solution containing 5.0 g/l PVP and 0.012 mol/l Ti(SO4 )2
C. Song et al. / Journal of Colloid and Interface Science 272 (2004) 340–344
under ultrasound. The colloid suspension was aged at 70 ◦ C for 3 h, then cooled, centrifuged, and washed several times with water and alcohol, finally dried under vacuum at 40 ◦ C. To prepare silver/TiO2 hollow particles, the PSA/silver/ TiO2 composite particles were heated to 550 ◦ C at a rate of 1 ◦ C min−1 in Ar atmosphere and kept at 550 ◦ C for 2 h. TEM images of the samples were taken on a Hitachi 800 transmission electron microscope. SEM images were obtained on a Hitachi (X-650) scanning electron microanalyzer and a JSM-6700F field emission scanning electron microscope. XRD patterns of the products were recorded using a Rigaku D/Max r–A X-ray diffractometer with CuKα radiation (λ = 0.154178 nm). Thermogravimetric analysis (TGA) was carried out on a TA-50 thermal analyzer (Shimadzu) in N2 atmosphere. UV–vis spectra were recorded on a Shimadzu UV-2401PC UV–vis recording spectrophotometer.
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SEM image in Fig. 2c further confirms that most of the PSA latex particles are well decorated with silver nanoparticles. XRD pattern in Fig. 3b confirms the formation of silverdecorated PSA particles. Three reflection peaks are indexed to the (111), (200), and (220) planes of fcc silver (JCPDS Card 4-783), respectively. The higher background in the lower 2θ region can be contributed to the reflection of the PSA latex particles.
3. Results and discussion Fig. 1 shows the schematic illustration of the formation procedure of PSA/silver core–shell particles, PSA/silver/ TiO2 particles, and silver/TiO2 hollow spheres. It has been demonstrated that negatively charged sulfatestabilized polystyrene latex can be covered with a smooth inorganic layer by the hydrolysis of metal ions [3,8,9]. It was of interest to apply this method to the case of silver using PSA latex as core particles. In the first stage, urotropine slightly decomposes to give formaldehyde and ammonia upon aging in colloid solution, and the adsorbed silver ions are reduced in situ by the freshly produced formaldehyde to form silver nuclei on the surface of PSA templates. The nucleation stage is then followed by diffusion of silver ions and reducing agent to the surface of PSA particles to feed the in situ reducing reaction, and silver nuclei grow to silver particles. Fig. 2a shows the TEM image of the PSA template particles with uniform size and diameter 600 nm on average. Fig. 2b shows typical TEM image of the as-prepared PSA/silver core–shell particles. It is noteworthy that although the PSA latex surface appears to be fairly evenly coated with silver nanoparticles, these metallic deposits do not form a homogeneous coating or shell but a coating of immobilized silver particles of 10–30 nm in diameters. The
Fig. 1. Schematic diagram of the formation procedure of PSA/silver core–shell particles and silver/TiO2 hollow spheres.
Fig. 2. (a) TEM image of PSA latex particles, (b) TEM images of PSA/silver particles, and (c) SEM image of PSA/silver particles.
Fig. 3. XRD patterns of (a) PSA latex particles, (b) PSA/silver particles, (c) PSA/silver/TiO2 particles, and (d) silver/TiO2 hollow spheres.
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Fig. 5. Thermogravimetric analysis curve of the PSA/silver/TiO2 composite particles. Fig. 4. SEM image and TEM image (inset) of PSA/silver/TiO2 particles. The bar in the inset is 150 nm.
The coating procedure of PSA/silver/TiO2 particles involves a simple wet-chemistry method, which consists of the immersion of the silver-coated PSA latex into an aqueous solution of Ti(SO4 )2 and subsequent aging. During the aging procedure, Ti(SO4 )2 hydrolyzed into TiO2 colloid, with which the PSA/silver particles were covered. It was also found the addition of PVP is necessary in all the coating process to prevent the aggregation of the core particles and the coated particles [9]. In contrast to organometallic precursor of Ti(IV) used in the literature [9,12], Ti(SO4 )2 aqueous solution is easy to handle, and the coating procedure is simple. Fig. 4 shows a SEM image of the as-prepared PSA/silver/ TiO2 composite particles. All the particles are dispersed relatively well and have a uniform size. The inset of Fig. 4 shows a TEM image of one of these composite particles at high magnification. Both the SEM and TEM images indicate that the surfaces of the PSA/silver particles become smooth after the coating process with TiO2 . No new reflection peaks appear in the XRD pattern (Fig. 3c), which that the TiO2 layer on silver coated PSA is amorphous (compared with Fig. 3b). Fig. 5 shows the TGA curve of the PSA/silver/TiO2 composite particles. The first weight loss in the range of 50– 260 ◦ C can be attributed to the release of water, whereas the second sharp weight loss corresponds to the decomposition of core PSA particles in the range of 260–475 ◦ C. It has been reported that hollow particles of metallic copper can be obtained after removing the core PS particles by calcinations of the composite particles in nitrogen atmosphere [18]. Thus the calcinations at 550 ◦ C in Ar were employed to remove the PSA core particles in present experiments. The formation of crystalline TiO2 was confirmed by XRD pattern in Fig. 3d. The XRD pattern can be indexed to (101), (200), (105), and (211) planes of tetragonal phase anatase TiO2 (JCPDS Card 83-2243) together with the diffraction of (111), (200), and (220) planes of fcc silver (JCPDS Card 4-783). No obvious XRD peaks arising from impurities are found, suggesting the main component of the hollow spheres being silver and titania. XRD results indicate that crystalline
titania were produced by calcinating the composite particles at 550 ◦ C in Ar atmosphere. SEM images shown in Figs. 6a and 6b reveal that the product consists of well-defined hollow spheres with uniform size distribution and smooth surfaces. Broken hollow spheres can also be observed. To further confirm the hollow nature of the spheres, TEM observations were carried out. As shown in Figs. 6c and 6d, the spheres’ shape was verified by tilting the TEM sample plane from 0◦ (Fig. 6c) to 22◦ (Fig. 6d), obviously revealing the hollow structure of the sample. The wall thickness of these hollow spheres is in the range 40–80 nm. Electron diffraction pattern obtained by focusing the electron beam on an individual hollow sphere is shown in Fig. 6e. The rings-like diffraction pattern can be indexed to tetragonal-phase TiO2 together with (111) diffraction of fcc silver, suggesting the formation of silver/TiO2 composite hollow spheres. UV–vis absorption spectra of the suspension of these composite particles were recorded to investigate their optical properties (Fig. 7). As can be seen from curve b in Fig. 7, PSA/silver core–shell particles have a broad absorption at 400–500 nm, which is the characteristic absorption of silver–latex composites [13]. For the suspension of PSA/silver/TiO2 particles, the absorption (curve c in Fig. 7) becomes weaker and broader, which may be the results of the scattering of the amorphous TiO2 layers. The suspension of the silver/TiO2 hollow spheres exhibits a UV–vis spectrum containing two peaks (curve d, Fig. 7): the weaker one located at about 300 nm can be assigned to Ag+ 4 clusters [19], and the stronger at about 500–600 nm shows obvious red shifts with respect to the usual plasmon peak position of the silver nanoparticles. It has been reported that the peaks of the silver colloid shift about 20 nm toward longer wave length when the TiO2 /silver core–shell structure is formed [20]. The obvious shift toward longer wavelengths in the present study might be related to the hollow structures of silver/TiO2 particles, although the exact mechanism is not clear. The UV–vis investigation indicated that the asprepared silver/TiO2 hollow spheres showed broad and relatively strong absorption in visible region. We suggest that they could potentially be useful for the investigation of var-
C. Song et al. / Journal of Colloid and Interface Science 272 (2004) 340–344
343
Fig. 6. (a, b) SEM images of silver/TiO2 hollow spheres. (c, d) TEM images of silver/TiO2 hollow spheres tilting the sample plane from (c) 0◦ to (d) 22◦ . (e) ED pattern of silver/TiO2 hollow spheres.
have been successfully prepared by templating against PSA latex particles. The as-synthesized composite hollow spheres have a uniform size and wall-thickness of 40–80 nm. By replacing silver with other noble metals, and titania with other ceramic materials, we hope it would be serve as a general route to hollow spheres with different compositions and could offer the prospect of assembly novel nanoscale architectures in solution.
References
Fig. 7. UV–vis spectra of (a) PSA, (b) PSA/silver particles, (c) PSA/ silver/TiO2 , and (d) silver/TiO2 hollow spheres.
ious optical phenomena, as novel supported photocatalysts, or as candidates for photonic crystals [21].
[1] [2] [3] [4] [5] [6] [7] [8]
4. Conclusion In summary, silver-decorated PSA latex core–shell structures and crystalline silver/titania composite hollow spheres
[9] [10] [11] [12]
F. Caruso, Chem. Eur. J. 6 (2000) 413. Z. Zhong, Y. Yin, B. Gates, Y. Xia, Adv. Mater. 12 (2000) 206. N. Kawahashi, E. Matijevi´c, J. Colloid Interface Sci. 143 (1991) 103. C.E. Fowler, D. Khushalani, S. Mann, J. Mater. Chem. 11 (2001) 1968. S. Chah, J.H. Fendler, J. Yi, J. Colloid Interface Sci. 250 (2002) 142, doi:10.1006/jcis.2002.8328. E. Stefanie, M. Georg, J. Colloid Interface Sci. 250 (2002) 281, doi:10.1006/jcis.2002.8315. D. Zhang, L. Qi, J. Ma, H. Cheng, Adv. Mater. 14 (2002) 1499. H. Shiho, Y. Manabe, N. Kawahashi, J. Colloid Interface Sci. 226 (2000) 91, doi:10.1006/jcis.2000.6789. H. Shiho, N. Kawahashi, Colloid Polym. Sci. 278 (2000) 270. F. Caruso, R.A. Caruso, H. Möhwald, Science 282 (1998) 1111. D. Wang, F. Caruso, Chem. Mater. 14 (2002) 1909. Y. Yin, Y. Lu, B. Gates, Y. Xia, Chem. Mater. 13 (2001) 1146.
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[13] A.B.R. Mayer, W. Grebner, R. Wannemacher, J. Phys. Chem. B 104 (2000) 7278. [14] P.A. Schueler, J.T. Ives, F. DeLaCroix, W.B. Lacy, P.A. Becker, J. Li, K.D. Caldwell, B. Drake, J.M. Harris, Anal. Chem. 65 (1993) 3177. [15] H. Tamai, T. Ikeya, F. Nishiyama, H. Yasuda, K. Iida, S. Nojima, J. Mater. Sci. 35 (2000) 4945. [16] A. Sarkany, Zs. Revay, Appl. Catal. A 243 (2003) 347.
[17] Y. Guo, Thesis, Qingdao University of Science and Technology, Qingdao, 2002. [18] N. Kawahashi, H. Shiho, J. Mater. Chem. 10 (2000) 2294. [19] R. Tausch-Trekml, A. Henglein, J. Lilie, Phys. Chem. 82 (1978) 1335. [20] Y. Zhou, C.Y. Wang, H.J. Liu, Y.R. Zhu, Z.Y. Chen, Mater. Sci. Eng. B 67 (1999) 95. [21] D.J. Norris, Y.A. Vlasov, Adv. Mater. 13 (2001) 371.