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Preparation of Metallodielectric Composite Particles with Multishell Structure Z. Chen, Z. L. Wang,* P. Zhan, J. H. Zhang, W. Y. Zhang, H. T. Wang, and N. B. Ming National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China Received July 22, 2003. In Final Form: February 2, 2004 In this article, we demonstrated the synthesis of metallodielectric composite particles comprising a metal shell on a dielectric core and an outer coating of an insulating dielectric layer by depositing silver on silica supporting cores followed by coating of titania. A combination of surface reaction and surface seeding techniques is exploited for the formation of a complete silver shell on silica spheres. The additional outer coating of titania on silver shell particles is then performed by hydrolyzing tetra-n-butyl titanate in ethanol at room temperature. The morphologies of silver shells and titania coating are studied with electron microscopy, and their existences are confirmed with X-ray diffraction and energy-dispersive X-ray measurement.
Introduction Colloidal metal particles (2.5 compared with 1.45) and negligible absorption from visible to the wavelengths of about 12 µm. We exploit a combination of surface seeding/electroless plating methods to prepare silver shell nanoparticles by use of silica colloids as supporting cores. Specifically, the silica colloidal surfaces were modified with Sn2+ ions (via electrostatic attraction); silver nanoseeds were formed by a redox reaction in which Sn2+ ions were oxidized to Sn4+ and at the same time Ag+ ions were reduced into metallic Ag that anchored on the surface of silica beads. This step is analogous to the pretreatment step in electroless plating and thus can be applied to the formation of other kinds of metal shells.16 To obtain multishell composite particles, we adopted a modification of the method described by Imhof21 to further coat a titania layer on the silver/PS composite particles. Experimental Section (i) Materials. In our experiments, the following materials purchased from Shanghai Chemicals have been used: silver nitrate (AgNO3 g 99.8%), formaldehyde solution (methanal 37.0%), ammonia water (NH3 g 29.3 wt %), stannous(II) chloride dihydrate (SnCl2‚2H2O g 99.0%), hydrochloric acid (HCl 39.0%), sodium chloride (g 99.5%), poly(vinylpyrrolidone) (PVP, molecular weight 36 000), and tetra-n-butyl titanate (g98.0%). The negatively charged silica spheres with a diameter of 1050 nm used were purchased from Duke Scientific Corp. Absolute ethanol (g99.7%) and distilled water were used in all the preparations. (ii) Synthesis. Silica core-silver shell particles and silicasilver-titania multishell particles were synthesized in a multistep reaction process. A detailed description of the procedures will be given as follows. (20) Holland, B. T.; Blanford, C. F.; Stein, A. Science 1998, 281, 538. (21) Imhof, A. Langmuir 2001, 17, 3579.
Figure 2. (a,b) TEM and (c) SEM micrographs of the silicasilver core-shell particles. TEM images are under magnifications of 20 000 (a) and 50 000 (b). The thickness of the silver shell was estimated to be 75 ( 25 nm. Silver Shells with a Silica Core. For the preparation of silver nanoshells on colloids, silica spheres (13.4 mg) were used as supporting cores. These spheres were sensitized under ultrasonic agitation in a sensitizing solution (2.0 mL) that was prepared by mixing SnCl2 (2.5 g), HCl (10 mL), and tin particles (3.0 g) with distilled water (140 mL). The addition of tin particles in the solution was made to prevent transition of stannous ions from the bivalent state to the quaternary state. After 6 min of agitation, the beads were separated from the solution by centrifugation and then flushed with distilled water two times. Subsequently, these beads, which were covered with bivalent stannous ions, were immersed in an activation solution that consists of silver nitrate (0.16 g), distilled water (80 mL), and a controlled amount of ammonia water under ultrasonic agitation. Consequently, silver ions were reduced by stannous ions and a layer of silver nanoparticles was formed at the surface of the PS beads. These silver nanoparticles act as seeds for the growth of a continuous silver nanoshell in the following step. Here the addition of ammonia water in the activation mixture was controlled in the following way. First, ammonia water was dribbled slowly into the solution under agitation. Immediately, the reaction of silver ions with ammonia water led to the formation of Ag2O precipitates. Upon continuous dribbling of ammonia,
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Figure 3. EDX analysis (a) and XRD pattern (b) of the sample shown in Figure 2c, indicating the presence of silver. the preformed Ag2O reacted with ammonia water, which formed [Ag(NH3)2]OH complex. As a consequence, the amount of the precipitates was observed to decrease. Because superfluous ammonia water can slow the activation process, further dribbling of ammonia water was stopped when the precipitates were observed to disappear. After activating the surface of the silica beads for 10 min, the beads were separated from the solution through repeated centrifugation. The unattached silver nanoparticles in the solution were carefully removed after repeated centrifugation and dispersion. In the next step, using silver nanoparticles anchored on the surface of the silica beads as seeds, a complete silver shell was grown via electroless plating. The plating solution was prepared by mixing solutions A (2.39 mL) and B (2.39 mL). Solution A was a mixture of silver nitrate (3.5 g), deionized water (100 mL), and ammonia water (3.1 mL); solution B was prepared from formaldehyde solution (1.1 mL), deionized water (3.9 mL), and ethanol (95.0 mL). The time for plating was about 30 min. After this procedure, complete silver shells on silica spheres were formed. Titania Shell Covering the Silver Shell. A coating of titania on colloidal particles is difficult to control because titania precursors are highly reactive, which easily causes titania to form secondary particles or supporting cores to aggregate.22 In our experiment, the coating reaction is done in ethanol at room temperature by hydrolyzing tetra-n-butyl titanate, a procedure similar to that reported by Imhof.21 More specifically, the prepared silica-silver core-shell particles were first diluted with 21.5 mL of absolute ethanol containing 200 mg of PVP in an ultrasonic bath. The addition of PVP has been used as a stabilizer for titania species23 and for silica coating on gold particles.12b Then the mixture was put in a reactor under magnetic stirring (>600 rpm) at 21 °C. Another 15 mL of absolute ethanol solution containing 0.225 mL of tetra-n-butyl titanate was added rapidly into the mixture under vigorous stirring. Finally, the particles were centrifuged and redispersed in aqueous ethanol solution several times. The above procedure was repeated in order to increase the coating thickness of titania. (22) (a) Srinivasan, S.; Datye, A. K.; Hampden-Smith, M.; Wachs, I. E.; Deo, G.; Jehng, J. M.; Turek, A. M.; Peden, C. H. F. J. Catal. 1991, 131, 260. (b) Hanprasopwattana, A.; Srinivasan, S.; Sault, A. G.; Datye, A. K. Langmuir 1996, 12, 3173. (c) Guo, X.-C.; Dong, P. Langmuir 1999, 15, 5535. (23) Sato, T.; Kohnosu, S. Colloids Surf., A 1994, 88, 197.
Figure 4. TEM (a,b) and SEM (c) micrographs of the metallodielectric multishell particles with a thin outer layer of titania. The magnification of the TEM image is 20 000 (a) and 50 000 (b). The spherical shape of the supporting silica core was preserved for the multishell particles. The surface roughness of the metallodielectric particles decreases after an outer coating of a thin titania layer. (iii) Characterization. Several methods were used to analyze the surface morphology and composition of the prepared multishell composite particles. Transmission Electron Microscopy (TEM). Samples for TEM analysis were prepared by dropping a drop of suspension on a Formvar-coated copper grid that was placed on a piece of filter paper in order to remove the solvent. The deposition of silver nanoparticles on the surfaces of silica spheres and the morphologies of titania-covered silver shells were observed with a JEM 200CX transmission electron microscope. Scanning Electron Microscopy (SEM). SEM was also used for obtaining the surface morphology of the core-shell and the multishell metallodielectric particles. The samples were prepared by dropping several drops of suspension onto a glass substrate. A thin layer of gold was sputtered onto the surface of samples for SEM measurements that were carried out with a JSM 5610LV scanning electron microscope.
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Figure 6. (a) EDX analysis of the sample shown in Figure 4c. (b) EDX analysis of the sample shown in Figure 5c. Both confirm the presence of titania in the multishell particles. In the spectrum, gold element is from gold sputtering and silicon element is from silica core beads and the silica substrate.
Figure 5. TEM (a,b) and SEM (c) micrographs of the coreshell particles covered with a thicker layer of titania (thickness, 70 ( 20 nm). (a) TEM image under 20 000 magnification. (b) TEM image of the same particles under a higher magnification of 45 000. The diameter of the particles has increased apparently after a heavy loading of titania. Energy-Dispersive X-ray (EDX) Analysis. SEM measurements were followed by EDX analysis to identify the composition of the two kinds of core-shell composite particles. The EDX analyses were performed with a Vantage X-ray Microanalysis System that is linked to a JSM 5610LV scanning electron microscope. X-ray Diffraction (XRD). As an alternative way to identify the composition of prepared silver-coated silica particles, XRD analysis was carried out on a Japan Rigaku D/max-RA X-ray diffraction meter using Cu KR radiation (λ ) 1.5418 Å).
Results and Discussion (i) Silver Shells with a Silica Core. TEM and SEM micrographs of the silica spheres (used as supporting cores) are shown in Figure 1a,b, respectively. The particles have a diameter of 1050 nm and a polydispersity of less than 5%. Figure 2a,b shows TEM micrographs of silver-coated silica beads under a magnification of 20 000 and 50 000, respectively. As seen from these TEM images, a complete silver coverage was achieved after the seeding growth
process. The silver shell thickness was estimated to be about 75 ( 25 nm. This was compared to the size of nanoparticle seeds formed after the activating process, which was about 20 nm on average, in agreement with ref 16a. A lot of roughness was created due to clumping of silver nanoparticles after the thick coating (Figure 2b). Shown in Figure 2c is the SEM image of the same coreshell particles, which also confirms high roughness in the silver wall. Figure 3a presents the EDX analysis of the aggregates, which clearly indicates the presence of silver from the core-shell spheres. In the pattern, gold element is from gold sputtering and silicon element is from silica core beads and the silica substrate. Figure 3b shows the X-ray diffraction pattern of the same sample. Sharp diffraction peaks corresponding to the cubic structure of metallic silver were observed, showing evidence of the presence of silver nanoparticles with high crystallinity. No diffraction peak characteristic of Ag2O was observed in the prepared sample, which indicates that the amount of Ag2O in the sample is essentially negligible. (ii). Titania-Covered Silver Shells. Figure 4a shows a TEM micrograph of the multishell particles with a thin outer layer of titania. A higher magnification (50 000) TEM image of the metallodielectric particles is shown in Figure 4b. Shown in Figure 5c is the SEM micrograph of the same particles. From both TEM and SEM micrographs, it is seen that coating a thin dielectric layer of titania leads to certain decrease in the surface roughness as compared with that of the silver shell particles. After a sequential coating of silver and titania, the spherical shape of the supporting cores was preserved. However, the polydispersity of the final composite particles was increased to about 8%.
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Multishell particles with a thicker layer of titania were also synthesized. Figure 5a shows the TEM micrograph of the resultant multishell particles with a thicker layer of titania that were prepared by repeating the previous seeding growth process four times (0.225 mL of tetra-nbutyl titanate each time). A TEM image of the multishell particles under a higher magnification (45 000) is shown in Figure 5b. By comparison with the TEM image shown in Figure 4b, the size of the particles was much increased after a thick deposition of titania. Here the thickness of the titania shell was estimated to be 70 ( 20 nm. Shown in Figure 5c is the SEM micrograph of the metallodielectric particles. It is observed that although a multistep coating can lead to an increase in weight loading of titania, secondary titania particles were also formed. To minimize the formation of these separate titania particles in the seeding growth process, a certain amount of NaCl was added, a method as described by Imhof.21 In our experiments, we removed these secondary nanoparticles through centrifugation and redispersion. EDX analyses were performed for the silver shell particles with both thin and thick outer layers of titania, and the results are shown in Figure 6a,b, respectively. The peak characteristic of titanium was observed for both multishell particles as expected, indicating the presence of titania on the surfaces of these metallodielectric particles. The weak intensity of the peak in Figure 6a was
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mainly due to the low coverage of titania on the silver shell particles (see Figure 4), whereas a thicker coating of titania (see Figure 5) results in a stronger peak as seen in Figure 6b. Conclusion In summary, metallodielectric beads comprising a silica-silver-titania multishell structure have been prepared. The silver shells are first formed through an enlargement of silver nanoparticle seeds via the deoxidation of silver ions by methanal. Covering of these silver shell-silica core particles with an outer titania shell is achieved by a hydrolyzing tetra-n-butyl titanate. The thickness of the outer titania shell can be changed by controlling the amount of tetra-n-butyl titanate added. The procedure reported here could enable potential application of these multishell composite spheres as building blocks for various ordered metallodielectric microstructures. Acknowledgment. This work is supported by a grant for the State Key Program for Basic Research of China and by NSFC under Grant Nos. 10174031 and 90101030. W.Z. also acknowledges partial support from NSFC under the “Excellent Youth Foundation”. LA035326A
