pubs.acs.org/Langmuir © 2009 American Chemical Society
Facile Fabrication Method and Characterization of Hollow Ag/SiO2 Double-Shelled Spheres Zhenxuan Wang, Xiaobing Chen, Min Chen, and Limin Wu* Department of Materials Science and Advanced Materials Laboratory, Fudan University, Shanghai 200433, China Received January 23, 2009. Revised Manuscript Received March 4, 2009 Hollow Ag/SiO2 double-shelled spheres with silver nanoparticle coating onto the interior wall of hollow silica have been successfully prepared via a novel and facile approach. In this method, negatively charged polystyrene (PS) beads were first modified by Sn2+ ions via electrostatic interaction, and then by Ag+ ions, which were reduced by Sn2+ ions and sodium-potassium tartrate to obtain PS/Ag composite spheres. When these spheres were coated by silica nanoparticles as shells from the hydrolysis and condensation reaction of tetraethoxysilane (TEOS) in isopropanol/ ammonia mixture at 15 °C, the PS beads were “dissolved” in the same medium subsequently even synchronously, directly forming hollow Ag/SiO2 double-shelled spheres. Neither additional dissolution nor calcination process was used to remove polymer templates. This structure of hollow spheres not only has high catalytic activity, but also will decrease the loss of Ag nanoparticles due to frictional and/or other mechanical forces and the possible aggregations, and have controllable and selective catalytic activity compared to these common nanocomposite spheres with noble metal nanoparticles coating on the surfaces of supporting beads.
Introduction Noble metal nanoparticles have received considerable attention due to their wide applications,1-8 e.g., catalysis,9,10 photonic crystals,11-13 plasmonics,14 surface-enhanced Raman scattering (SERS),15,16 biochemical tagging reagents,17 and so forth. Generally, those metal nanoparticles are synthesized on some supporting materials,18-20 such as polymer beads and inorganic oxide (silica, titanium oxide). For instance, Jiang et al.10 successfully synthesized SiO2/Ag composite spheres using SiO2 particles as seeds for continuous Ag+ ions adsorption *Corresponding author. E-mail:
[email protected]. (1) Aslan, K; Wu, M; Lakowicz, J. R.; Geddes, C. D. J. Am. Chem. Soc. 2007, 129, 1524–1525. (2) Yang, Z.; Yang, L.; Zhang, Z.; Wu, N.; Xie, J.; Cao, W. X. Colloids Surf., A 2008, 312, 113–117. (3) Kim, Y. H.; Lee, D. K.; Cha, H.; Kim, C. W.; Kang, Y. J. Phys. Chem. C 2007, 111, 3629–3635. (4) Jackson, J. B.; Halas, N. J. J. Phys. Chem. B 2001, 105, 2743–2746. (5) Graf, C.; Vossen, D. L. J.; Imhof, A.; Blaaderen, A. V. Langmuir 2003, 19, 6693–6700. (6) Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. Langmuir 1998, 14, 3740–3748. (7) Kobayashi, Y.; Maceira, V. S.; Liz-Marzan, L. M. Chem. Mater. 2001, 13, 1630–1633. (8) Li, T.; Moon, J.; Morrone, A. A.; Mecholsky, J. J.; Talham, D. R.; Adair, J. H. Langmuir 1999, 15, 4328–4334. (9) Deng, Z.; Chen, M.; Wu, L. J. Phys. Chem. C 2007, 111, 11692–11698. (10) Jiang, Z.; Liu, C.; Sun, L. J. Phys. Chem. B 2005, 109, 1730–1735. (11) Liang, Z.; Susha, A. S.; Caruso, F. Adv. Mater. 2002, 14, 1160–1164. (12) Zhang, W.; Lei, X.; Wang, Z.; Zheng, D.; Tam, W. Y.; Chan, C.; Sheng, P. Phys. Rev. Lett. 2000, 84, 2853–2856. (13) Wang, W.; Asher, S. A. J. Am. Chem. Soc. 2001, 123, 12528–12535. (14) Siiman, O.; Burshteyn, A. J. Phys. Chem. B 2000, 104, 9795–9810. (15) Nie, S.; Emory, S. R. Science 1997, 275, 1102–1105. (16) Dick, L. A.; McFarland, A. D.; Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2002, 106, 853–860. (17) Patolsky, K.; Lichtenstein, A.; Willner, I. J. Am. Chem. Soc. 2001, 123, 5194–5205. (18) Chen, Z.; Wang, Z.; Zhan, P.; Zhang, J.; Zhang, W.; Wang, H.; Ming, N. Langmuir 2004, 20, 3042–3046. (19) Kim, Y. H.; Lee, D. K.; Cha, H.; Kim, C. W.; Kang, Y.; Kang, Y. J. Phys. Chem. B 2006, 110, 24923–24928. (20) Tierno, P.; Goedel, W. A. J. Phys. Chem. B 2006, 110, 3043–3050.
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and reduction. The homogeneously formed Ag nanoparitlces on the surfaces of SiO2 showed excellent catalytic ability. Recently, we successfully prepared SiO2/Ag 9 and polystyrene (PS)/Ni 21 nanocomposite spheres using SiO2 particles and negatively charged PS beads as the templates, respectively, the Ag+ and Ni2+ ions were first adsorbed on the surfaces of templates via electrostatic interaction and then reduced by polyvinylpyrrolidone and hydrazine hydrate, respectively. In these works, the metal cations were first combined with a certain template particle, and then reduced by adding various reducing agents or catalyst to form the stable colloidal template/metal composite particles. Very recently, Moon et al.22 successfully decorated the interior surfaces of hollow silica colloids with Pt nanoparticles. In their method, thin platinum layer was first deposited on a-Se spherical colloids through chemical reduction in ethylene glycol or alcohols. The silica from ammonia-catalyzed hydrolysis and condensation of tetraethoxysilane (TEOS) was coated on the surfaces of composite spheres as shells. Selective removal of the Se cores generated Pt nanoparticles embedded in hollow silica spheres. The fabrication method of assembling noble metal nanoparticles on the interior wall of hollow spheres is very interesting since hollow spheres have low density, and these protected noble metal nanoparticles can be prevented from loss due to mechanical forces or from aggregation. Unfortunately, however, multistep processes are needed for the synthesis of coreshell composite particles, e.g., the surface-functionalization of templating particles/exchange of solvent/coating reaction for templating particles approach. In order to obtain hollow spheres from the core-shell composite particles, removing the core particles by selective dissolution in an appropriate solvent or by calcination at elevated temperature in air is indispensable. (21) Chen, M.; Zhou, J.; Xie, L.; Gu, G.; Wu, L. J. Phys. Chem. C 2007, 111, 11829–11835. (22) Moon, G. D.; Jeong, U. Chem. Mater. 2008, 20, 3003–3007.
Published on Web 04/06/2009
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In previous works,23,25 we reported that monodisperse hollow silica and titania spheres were fabricated via a one-step process, which means that the formation of the inorganic shells and dissolution of core polymer particles occur in the same medium. In that method, monodisperse positively charged PS beads were prepared by dispersion polymerization using cationic monomer 2-(methacryloyl) ethyltrimethylammonium chloride as the comonomer, which ensured the generating silica or titania nanoparticles from the hydrolysis and condensation of TEOS or tetra-n-butyl titanate could be rapidly captured by PS beads via electrostatic interaction in aqueous ammoniacal alcohol medium at 50 °C, in which PS beads were “dissolved” subsequently, even synchronously, to directly form hollow silica or titania spheres; neither additional dissolution nor calcination process was needed to remove the PS cores. In this work, we further assembled silver nanoparticles onto the interior wall of hollow silica and successfully prepared hollow Ag/SiO2 double-shelled spheres using our knowledge of “dissolution” of PS beads in aqueous ammoniacal alcohol medium. Preliminary investigation indicates that these hollow double-shelled spheres still have high catalytic activity, although silver nanoparticles are assembled on the interior wall of silica.
Experimental Section Materials. Styrene (St) was purchased from Shanghai Chemical Reagent Co. (China) and purified by treating with 5 wt % of aqueous NaOH solution to remove the inhibitor. Poly(vinylpyrrolidone) (PVP, Mw = 30 000), absolute ethanol, isopropanol, sulfuric acid, tin(II) chloride dehydrate (SnCl2 3 2H2O g 99.0%), hydrochloric acid (HCl 38-40%), potassium sodium tartrate (C4H4O6KNa 3 4H2O g 99.0%), silver nitrate (AgNO3 g 99.8%), aqueous ammonia solution (28 wt %), TEOS, potassium borohydride (KBH4 g 95%), rhodamine B (RB) were all purchased from Shanghai Chemical Reagent Co. (China) and used as received. Azobisisobutyronitrile (AIBN) was purchased from Aldrich (USA). Silica spheres with a diameter of 900 nm were purchased from Duke Scientific Corp. Ultrapure water (>17 MΩ cm-1) from a Milli-Q water system was used throughout the experiment. Synthesis of Monodisperse PS Beads. The monodisperse PS spheres were prepared via dispersion polymerization of St as follows: Typically, stabilizer (3.0 g of PVP), ethanol (45 g), and H2O (10 g) were charged into a 250 mL three-neck Flask, and then ultrasonically dissolved, followed by addition of a mixture of St (10 g) and AIBN (0.6 g). The above mixture was deoxygenated by bubbling nitrogen gas at room temperature for about 30 min and then heated to 70 °C under constant stirring for 2 h, followed by addition of a solution containing 10 g of St and 45 g of ethanol. The reaction was allowed to proceed for 24 h to make sure the conversion of St reached at least 95% and then cooled to room temperature. The obtained PS beads were separated from the reaction medium by centrifugation and redispersion cycles, and then dried in a vacuum oven at room temperature to yield dried PS powders for further use. Preparation of PS/Ag Core-Shell Spheres. The PS beads were further sulfonated by stirring a mixture of 1 g of PS beads and 100 mL of concentrated sulfuric acid at 40 °C for 4 h to obtain negatively charged PS beads. The suspension was centrifuged at 10 000 rpm for 10 min and washed with ethanol, and then redispersed into water to get 5 mg/mL of sulfonated PS dispersion. Afterward, 50 mL of the sulfonated PS dispersion was mixed with 50 mL of SnCl2 aqueous solution (5 mg/mL (23) Chen, M.; Wu, L.; Zhou, S.; You, B. Adv. Mater. 2006, 18, 801–806. (24) Anderson, M.; Martin, J. E.; Odinek, J. G.; Newcomer, P. P. Chem. Mater. 1998, 10(1), 311–321. (25) Cheng, X.; Chen, M.; Wu, L.; Gu, G. Langmuir 2006, 22, 3858–3863.
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and 0.1 M HCl) and stirred at room temperature for 30 min to obtain Sn2+-sensitized PS dispersion. After centrifugation and redispersion cycles, 50 mL of the Sn2+-sensitized PS dispersion (5 mg/mL in water) was poured into a mixture composed of 50 mL of [Ag(NH3)2]+ ion solution (2.5 mg/mL) and 50 mL of sodium-potassium tartrate (5 mg/mL) solution and stirred at room temperature for 30 min to obtain PS/Ag nanocomposite spheres.
Preparation of Hollow Ag/SiO2 Double-Shelled Spheres. Under vigorous stirring, the above PS/Ag nanocomposite spheres were mixed with 250 mL of isopropanol and 25 mL of deionized water, and then 4 mL of ammonium hydroxide was added, followed by adding 1.6 g of TEOS. This mixture was stirred at room temperature for 30 min, and then allowed to age at 15 °C overnight with gentle agitation. During the silica from ammonia-catalyzed hydrolysis and condensation of TEOS coated on the surfaces of PS/Ag composite spheres as shells, the PS beads were “dissolved” in the same medium subsequently, even synchronously, based on our previous report,23 directly forming the hollow Ag/SiO2 double-shelled spheres. The suspension was centrifuged and washed with a mixture of water and ethanol (5:4), and then redispersed in water for characterizations.
Catalytic Property of Hollow Ag/SiO2 Double-Shelled Spheres. A given amount of the hollow double-shelled spheres was first mixed with RB, and the volume was adjusted to 9 mL with deionized water. Then, 1 mL of KBH4 solution was rapidly injected into the above mixture under stirring, and the color of the mixture vanished gradually, indicating the dye had been reduced. The catalytic activity of hollow Ag/SiO2 double-shelled spheres was evaluated by monitoring the variation in optical density at the wavelength of the absorbance maximum (λmax) of the dye. Characterizations. The Size and Morphology. The size and morphology of the products were obtained from a transmission electron microscope (TEM: Hitachi H-600; High-resolution TEM: JEOL 2010) and a scanning electron microscope (SEM Philips XL30 apparatus). The particles dispersions were diluted and ultrasonized at 25 °C for 10 min and then dried onto carboncoated copper grids before examination. X-ray Diffraction (XRD). Crystal structure identification was carried out using Rigaku D/max-rB X-ray diffractometer with Cu KR irradiation (λ = 1.54056A˚) at a scanning rate of 0.02 deg/s in 2θ ranging from 5° to 80°. The samples for XRD were supported on glass substrate. Thermogravimetric Analysis (TGA). About 5 mg of the dried spheres was examined with a TGA instrument (PerkinElmer TGA-7 USA). All these powders were heated in N2 atmosphere from room temperature to 800 °C at a scanning rate of 20 °C/min. Brunauer-Emmett-Teller (BET) Analysis. Nitrogen adsorption-desorption measurements were performed at 77 K using an ASAP 2010 analyzer utilizing the BET model for calculation of surface areas. The pore size distribution was calculated from the desorption isotherm curves using the Barrett-Joyner-Halenda (BJH) method. UV-Visible Spectrum. UV-visible absorption spectra were recorded using a UV-visible spectrophotometer (Hitachi UV-3000, Japan). The samples were placed in a 1 cm 1 cm 3 cm quartz cuvette, and spectra were recorded at room temperature.
Results and Discussion Synthesis and Morphology of Spheres. Scheme 1 describes a brief procedure for fabrication of hollow Ag/SiO2 doubleshelled spheres. First, monodisperse PS templates synthesized from dispersion polymerization using AIBN as the initiator and PVP as the stabilizer were sulfonated to obtain negatively DOI: 10.1021/la9002934
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Wang et al. Scheme 1. Schematic Procedure to Fabricate Hollow Ag/SiO2 Double-Shelled Spheres
Figure 1. SEM and TEM images of (a,b) PS beads (900 nm) and (c,d) PS/Ag composite spheres. The inset of panel d is the SAED of silver nanoparticles.
Figure 2. (a) TEM image of the hollow Ag/SiO2 double-shelled spheres; (b) SEM image of hollow Ag/SiO2 double-shelled spheres; (c) highresolution TEM image of the shell, wherein silver nanoparticles of ca. 20 nm can be observed.
charged PS beads, which were then deposited by Sn2+ ions via electrostatic interaction. Second, the Sn2+-sensitized PS beads were mixed with [Ag(NH3)2]+ ion solution, wherein Ag+ ions were reduced by Sn2+ ions and deposited on the surfaces of PS beads as seeds, the remaining Ag+ ions were further reduced by 7648 DOI: 10.1021/la9002934
sodium-potassium tartrate to obtain PS/Ag composite spheres. Third, the silica nanoparticles from the hydrolysis and condensation reaction of TEOS in an isopropanol/ammonia mixture were coated on the surfaces of PS/Ag composite spheres as shells, while the PS beads were “dissolved” in the same medium Langmuir 2009, 25(13), 7646–7651
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subsequently, even synchronously, directly causing hollow Ag/SiO2 double-shelled spheres. Figure 1 presents the typical SEM and TEM images of the PS and PS/Ag hybrid spheres. It can be seen that all the PS beads are uniform with a diameter of ca. 900 nm, Ag nanoparticles are homogenously deposited on the surfaces of PS beads, forming an average shell thickness of 30 nm compared to PS size. The selected-area electron diffraction (SAED) pattern, as displayed in the inset of Figure 1d, has clear diffraction rings corresponding to the face-centered cubic (fcc) structure of silver nanoparticles.26 In this process, Sn2+ ions play an important role in the formation of the composite: the negatively charged PS beads were favorable for absorbing Sn2+ ions via electrostatic interaction. After the Sn2+ ion-sensitized PS beads were mixed with Ag+ ions, some of Ag+ ions were reduced to Ag nanoparticles by Sn2+ ions on the surface of PS beads, these Ag nanoparticles can serve as seeds for the formation of a silver shell structure, and the residual Ag+ ions were further reduced by sodiumpotassium tartrate, forming compact and continuous silver shells on the surfaces of PS beads.27 When these PS/Ag composite spheres were coated by the silica nanoparticles from the hydrolysis and condensation reaction of TEOS in an isopropanol/ammonia mixture at 15 °C, the PS beads were “dissolved” in the same medium subsequently, even synchronously. Figure 2 demonstrates the typical SEM and TEM images of SiO2-coated composite spheres. The strong contrast between the bright center, and the dark edge indicates a hollow structure (see Figure 2a), which can be further confirmed by the SEM image of broken shells under higher magnification in Figure 2b. The mean silica shell was measured to be about 40 nm thick compared to the mean size of PS/Ag composite spheres; the Ag nanoparticles of ca. 20 nm are decorated on the interior surfaces of the silica wall in Figure 2c. Analysis of Hollow Ag/SiO2 Double-Shelled Spheres. The typical hollow Ag/SiO2 double-shelled spheres were dried for further examination with XRD, as shown in Figure 3. All of the XRD diffraction peaks can be indexed to fcc silver, which is in good agreement with the value in the literature (JCPDS card No. 04-0783). No diffraction peak characteristic of Ag2O is observed in the prepared sample, indicating that the amount of Ag2O in the sample can be negligible, and the Ag nanoparticles in this composite structure are stable in air. The crystalline size of the Ag particles is estimated to be around 20 nm by Scherrer’s formula,24 which is consistent with the TEM result in Figure 2c. Figure 4 presents the typical TGA curves of the original PS beads, partly “dissolved” Ag/SiO2 double-shelled spheres, and the completely “dissolved” Ag/SiO2 double-shelled spheres. The weight-loss stage below 300 °C is a result of the evaporation of physically adsorbed water and residual solvent in the sample. The pure PS is decomposed completely from 350 °C through 450 °C, the partly “dissolved” Ag/SiO2 double-shelled spheres have less decomposition than the pure PS beads, while the completely “dissolved” hollow Ag/SiO2 double-shelled spheres do not show any degradation. This further confirms the hollow structure of the completely “dissolved” Ag/SiO2 double-shelled spheres. The typical nitrogen adsorption-desorption of the hollow double-shelled spheres is demonstrated in Figure 5; it displays a
Figure 5. Nitrogen adsorption and desorption isotherms obtained at 77 K (inset: pore size distribution from desorption branch; V = pore volume, D = pore size).
(26) Wang, Z.; Chen, M.; Wu, L. Chem. Mater. 2008, 20(10), 3251–3253. (27) Chen, Z.; Zhan, P.; Wang, Z.; Zhang, J.; Zhang, W.; Ming, N.; Chan, C.; Sheng, P. Adv. Mater. 2004, 16, 417–422.
typical type of IV isotherm of mesoporous materials. The BET surface area and the total pore volume are 13.3 m2/g and 0.105 cm3/g, respectively, while the pore size distribution (inset of
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Figure 3. XRD pattern of typical hollow Ag/SiO2 double-shelled spheres.
Figure 4. TGA curves of (a) original PS beads, (b) partly “dissolved” Ag/SiO2 double-shelled spheres, (c) completely “dissolved” Ag/SiO2 double-shelled spheres.
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Figure 6. TEM images of hollow Ag/SiO2 double-shelled spheres with various AgNO3 concentrations: (a) 2.5 mg/mL; (b) 3.75 mg/ mL; (c) 5.0 mg/mL. (d) Typical SEM image.
Figure 7. TEM images of hollow Ag/SiO2 double-shelled spheres prepared in (a) isopropanol and (b) ethanol.
Figure 5) ranges from 1 to 9 nm with a mean size of 3.7 nm mesopores in these hollow composite spheres. Effect of AgNO3 Amount. Figure 6 further illustrates the electron images of the hollow Ag/SiO2 double-shelled spheres prepared from different AgNO3 concentrations. As AgNO3 concentration increases, the hollow spheres are becoming darker, indicating that more Ag nanoparticles have deposited on the surfaces of PS beads. Effect of Alcohols. On the basis of our previous research,23 when the silica shells were coated on the PS beads via ammoniacatalyzed hydrolysis and condensation of TEOS in aqueous ammoniacal ethanol medium at 50 °C, the PS beads were “dissolved” subsequently, even synchronously, during the coating process to directly obtain silica hollow spheres. Neither additional dissolution nor calcination processes were needed. In this study, when the PS/Ag composite spheres were coated by silica shells from the ammonia-catalyzed hydrolysis and condensation of TEOS in aqueous ammoniacal isopropanol medium at 15 °C, the PS beads were also “dissolved” subsequently, even synchronously, during the coating process to directly obtain hollow Ag/SiO2 double-shelled spheres. This is probably because the “dissolving” PS macromolecule chains and their congregate can diffuse out gradually through the porous silica shells, thus causing hollow spheres. Moreover, aqueous ammoniacal isopropanol medium seems to “dissolve” PS beads more easily than aqueous ammoniacal ethanol medium, as demonstrated in Figure 7. This is probably because isopropanol has a closer solubility parameter to PS beads than ethanol. Effect of Ammonia Amount. In fact, the concentration of ammonia has a crucial influence on the silica coating process and PS dissolving process. When the amount of ammonia was 1 mL, the PS beads could not be “dissolved” in the medium, and silica was coated on PS/Ag composite spheres to form core-shell 7650 DOI: 10.1021/la9002934
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structures, as shown in Figure 8a. While the volume of ammonia was increased to 2 mL, some of the PS cores were “dissolved” (see Figure 8b). As the amount of ammonia was further increased to 4 mL, the high contrast of the shells with the cores denotes that the PS beads inside have been completely “dissolved”, and homogeneous, dense silica layers formed, causing monodisperse hollow Ag/SiO2 double-shelled spheres, as demonstrated in Figure 8c. Catalytic Property of Hollow Ag/SiO2 Double-Shelled Spheres. It has been experimentally demonstrated that noble metal nanoparticles have high catalytic activities for hydrogenation, hydroformylation, carbonylation, and so forth.28 However, in most cases, they coalesce during the catalytic processes, because nanosized metal particles in the solution are active and tend to coalesce as a result of Van der Walls forces and high surface energy unless they are protected. To work out this problem, some studies have focused on rivetting these metal nanoparticles on the surfaces of some supports such as PS or SiO2 beads.9,21,29,30 Here, we also explored the catalytic property of hollow Ag/ SiO2 double-shelled spheres preliminarily by studying the change of the adsorption intensity at λmax of the dye. For the sake of comparison, the SiO2/Ag composite spheres with Ag nanoparticles coating on the outer surfaces of silica beads were also prepared and investigated for the catalytic property. Figure 9 displays the evolution of the UV-vis spectra of the dye during the reaction of RB reduced by KBH4. While the system contained hollow Ag/SiO2 double-shelled spheres, the absorbance at λmax of the dye quickly decreased with the reaction time, and the reduction of dyes by KBH4 finished within a few minutes. Additionally, with increasing the concentration of hollow Ag/SiO2 double-shelled spheres or Ag nanoparticles, the reduction rate of the dyes increased, as shown in Figure 9a,b,c. Moreover, the system containing hollow Ag/SiO2 doubleshelled spheres has a comparable catalytic property to that with equal concentration of Ag nanoparticles coating on equal size of SiO2 beads, as demonstrated in Figure 9a,d. This indicates that these Ag nanoparticles moored on the interior wall of hollow sphere still have a high catalytic activity. This is because, although these Ag nanoparticles are protected by a silica shell, the hollow spheres have a surface area as high as 13.3 m2/g and a porous silica shell with a mean pore size of 3.7 nm, which allow the diffusion of guest molecules into and out of the interior. Compared to these Ag nanoparticles decorated on the surfaces of some supporting polymer or inorganic oxide beads, e.g., PS/Ag or SiO2/Ag nanocomposite spheres, the hollow Ag/SiO2 double-shelled spheres seem to have some additional advantages: e.g., the loss of Ag nanoparticles due to frictional or other mechanical forces can be avoided, and the possible aggregation occurring in PS/Ag or SiO2/Ag nanocomposite spheres can remarkably decrease. More importantly, since the pore size and porosity of silica shell can be tuned by varying the hydrolysis and condensation rate and concentration of TEOS, the catalytic activity of hollow Ag/SiO2 double-shelled spheres composites can be controllable and so selective for some guest compounds. In fact, during our catalytic tests, we also found that the reduction rate of RB decreased if as-prepared hollow spheres with thicker silica-shells were used, which agrees well with other reports about tuning the catalytic activity of silver particles by the thickness of the silica shell.28 (28) Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. J. Phys. Chem. B 1999, 103, 6770–6773. (29) Kim, J.; Bryan, W.; Lee, T. R. Langmuir 2008, 24, 11147–11152. (30) Olavi, S.; Alexander, B. J. Phys. Chem. B 2000, 104, 9785–9810.
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Figure 8. TEM images of silica coating PS/Ag composite spheres with various amounts of ammonia: (a) 1 mL; (b) 2 mL; (c) 3 mL.
Figure 9. UV-visible spectra of RB during reduction catalyzed by (a-c) hollow Ag/SiO2 double-shelled spheres and (d) SiO2/Ag core-shell composite spheres. [KBH4] = 1 10-2 mol/L, [RB] = 2 10-5 mol/L. (a) [Ag]0 = 5.0 10-5 mol/L; (b) [Ag]0 = 7.5 10-5 mol/L; (c) [Ag]0 = 10.0 10-5 mol/L; (d) [Ag]0 = 5.0 10-5 mol/L. The arrows mark the increase of reaction time.
Conclusion In this study, a facile and novel method for fabrication of hollow Ag/SiO2 double-shelled spheres with silver nanoparticles assembling on the interior wall of silica was proposed, in which the negatively charged PS beads were first deposited by Sn2+ ions via electrostatic interaction, and then by Ag+ ions which were reduced by Sn2+ ions and sodium-potassium tartrate to obtain PS/Ag composite spheres. When these spheres were coated by silica shells from the ammonia-catalyzed hydrolysis and condensation of TEOS in aqueous ammoniacal isopropanol medium, the PS beads were “dissolved” in the same medium subsequently, even synchronously. The concentration of ammonia has a crucial influence on the formation of a hollow structure, and the thicknesses of Ag and silica shells can be controlled by the concentrations of Ag+ ions and TEOS. Catalytic investigation indicates these silver nano-
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particles assembled on the interior wall of silica still have high catalytic activity, even though they are protected by a silica shell. It can be expected that this structure of hollow Ag/SiO2 doubleshelled spheres can decrease the loss of Ag nanoparticles due to frictional or other mechanical forces and the possible aggregations, and has controllable and selective catalytic activity compared to these nanocomposite spheres with noble metal nanoparticles decorating the surfaces of some supporting polymer or inorganic oxide beads. Acknowledgment. Financial support from the Foundation of Science and Technology of Shanghai (07DJ14004), the Shanghai Leading Academic Discipline Project (B113), the Shanghai Excellent Leader of Academic Discipline Project, and the Shuguang Scholar-Tracking Foundation of Shanghai is appreciated.
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