Simple Strategy for Preparation of Core Colloids Modified with Metal

Feb 2, 2007 - A simple method was developed for preparation of core colloids modified with Au (Ag) nanoparticles (NPs). The core ... The advantages of...
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J. Phys. Chem. C 2007, 111, 3651-3657

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Simple Strategy for Preparation of Core Colloids Modified with Metal Nanoparticles Chungui Tian, Baodong Mao, Enbo Wang,* Zhenhui Kang, Yanli Song, Chunlei Wang, and Siheng Li Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal UniVersity, Ren Min Street No. 5268, Changchun, Jilin, 130024, People’s Republic of China ReceiVed: October 27, 2006; In Final Form: January 4, 2007

A simple method was developed for preparation of core colloids modified with Au (Ag) nanoparticles (NPs). The core colloids vary from polymer colloids (polystyrene spheres) to dielectric inorganic spheres (SiO2) and semiconductors (ZnO) particles. The composite colloids were obtained by heating the solution of the composites composed of core colloids, poly(ethylenimine) (PEI), and metal ions. Poly(ethylenimine) used here played two important roles. It links the metal ions and the core colloids, and meanwhile acted as a reductant for in situ reduction of metal ions to metal nanoparticles. There is little effect of the colloid concentration on the formation of core colloids modified with metal nanoparticles and the reaction may be easily scaled up. X-ray photoelectron spectroscopy and IR studies demonstrated that the amine groups in PEI were oxidized to carboxyl groups after the reaction. UV-visible spectra studies indicated that all the formed metal NPs were immobilized on the core surfaces and no free metal NPs formed in the solution. Based on the results, an improved “surface seeding and shell growth technique” was adopted to prepare core colloids with controllable metal coverage. Accordingly, optical plasmon resonance of the composite colloids could be tuned in a broader spectrum range by changing the metal coverage.

1. Introduction Au and Ag nanoparticles (NPs) are of great interest due to their excellent optical, electric, and catalytic properties and potential applications in many areas.1 Many recent efforts have been devoted to the deposition of Au and Ag NPs onto the surface of the supports (colloid inorganic and organic spheres).2,3 The driving force is from the great advantages of these hybrid materials in conductivity, optical activity, and catalytic activity.4 For example, assembling metal NPs onto the surface of the supports allows them to retain high activity on cycling, enhances their pH and temperature stability, and enables easy separation from the reaction medium by centrifugation for reuse.5 Many methods have been developed for depositing metal NPs on core colloids. Generally, immobilization of metal NPs on core was achieved by adsorption of preprepared metal NPs on core surface with organic ligands or polyelectrolyte as “glue”,6-8 reduction of metal salts in a dispersion solution of polymer colloids,9 in situ formation by dispersion copolymerization of styrene and a polyamine in the presence of metal salts,10 electroless plating approach,11 solvent-assisted route,12 tollenssoaked process,13 polyol process,14 illumination,15a adsorption of metal ions on core surface followed by reduction with sodium borohydride,15b or deposition-precipitation method.15c A common feature of all these approaches is that the reductant (and/ or metal ions) and the core colloids are separated before the formation of the metal NP-core colloid composites. This may lead to complex operation processes, presence of residual metal NPs and reductant in the system, or lower yield of composite colloids. On the other hand, the core component has an important effect on the properties of resulted composites. For example, the * Corresponding author: e-mail [email protected] or wangenbo@ public.cc.jl.cn.

polystyrene (PS) and SiO2 spheres play the role of a carrier and the corresponding metal NP-core colloid composites mainly show the properties of the metal NPs. In the metal NPsemiconductor oxide composites, interfacial charge-transfer process is promoted by capture of the photoinduced charge carriers with metal NPs. Thus, depositing metal NPs on the surface of semiconductor oxides can improve the properties of both metal NPs and semiconductor particles, and the combination of metal NPs and semiconductor oxides is particularly interesting for catalytic applications.16 Thus, developing an effective strategy to deposit metal NPs on the surface of various core colloids is an ongoing challenge. Poly(ethylenimine), PEI, is a cationic polymer with branched structure in which plentiful amine groups can bond with both transition metal ions and negatively charged colloids.7,17 In addition, PEI could act as a reductant in the preparation of metal NPs.18 Recently, by using PEI as both a linker and an “in situ” reductant, we developed a simple method to prepare Ag-PS composites.19 The method is based on the formation of a composite composed of core colloids (PS spheres), reductant (PEI), and metal ions. Heat treatment can transform the composites into Ag-PS colloids. In the synthesis, PEI acted both as a linker between metal ions and core colloids and as the reductant for in situ reduction of metal ions to metal NPs. In this paper, the method was further improved and extended as a general strategy to deposit Au (Ag) NPs onto various core colloids, including inorganic spheres (SiO2), organic polymer colloids (PS), and semiconductor oxide NPs (ZnO). The advantages of the method include simple operation, easy largerscale production, and generality for various core colloids. Furthermore, based on the results obtained from the method, an improved “surface seeding and shell growth” technique was used to prepare core colloids with different metal coverage. The metal coverage was easily tuned by changing the initial mass

10.1021/jp067077f CCC: $37.00 © 2007 American Chemical Society Published on Web 02/02/2007

3652 J. Phys. Chem. C, Vol. 111, No. 9, 2007 ratio of metal ions to core colloids. Accordingly, optical plasmon resonance of the composites could be tuned in a broader spectrum range by changing the metal coverage.

Tian et al. SCHEME 1: Procedure for Preparation of Core Colloids Modified with Metal NPs

2. Experimental Section 2.1. Materials. Silver nitrate (AgNO3), hydrogen tetrachloroaurate (HAuCl4), hydrogen hexachloroplatinate (H2PtCl6), poly(ethylenimine) (PEI), tetraethyl orthosilicate (TEOS), styrene, potassium pyrosulfate, dihydrate zinc acetate, absolute ethanol, sodium citrate (C6H5O7Na3‚2H2O), formaldehyde (HCHO, 37%), ammonia solution (25%), and ascorbic acid were purchased from Beijing Chemical Co. Ltd. Water was distilled twice. 2.2. Synthesis. 2.2.1. Synthesis of Core Colloids. SiO2 spheres with diameter about 300 nm (Figure S1a, Supporting Information) were prepared by the well-known Sto¨ber method.20 PS colloids with diameter about 400 nm (Figure S1b, Supporting Information) were synthesized by emulsion polymerization in a water-ethanol system.21 ZnO NPs were prepared by an ethanol-thermal technique.22 Detailed parameters of experiments are given in the Supporting Information. 2.2.2. Modification of Core Colloids with PEI. For modification of PS spheres with PEI, quantitative PS colloids were dispersed into phospho-buffer solutions (solution A), and then PEI aqueous solution was added into solution A, generating solution B. The mass ratio of PEI to PS was about 1:10. Solution B was stirred at room temperature for 60 min. After three centrifugation/wash cycles, PS colloids modified with PEI were obtained. SiO2 (ZnO) particles were coated with PEI in 0.1 M KCl aqueous solution, and the pH of the solution was adjusted to 8 with 0.1 M HCl. The mass ratio of PEI to SiO2 (ZnO) was about 1:10. PEI-SiO2 (ZnO) composites were obtained after coating for 2 h and three centrifugation/wash cycles. 2.2.3. Preparation of Ag-SiO2, Au-SiO2, Au-PS, Ag-PS, and Au-ZnO. The procedures are similar for the preparation of Ag-SiO2, Au-SiO2, Au-PS, Ag-PS, and Au-ZnO composites. As an example, the synthesis of Ag-SiO2 is presented. Specifically, PEI-SiO2 colloids (2 g) were dispersed in 50 mL of water (solution A). Then 50 mL of aqueous solution containing 0.4 g of AgNO3 was mixed with solution A. After that, the solution was heated at 100 °C for 1 h and Ag-SiO2 composites were obtained. The products were collected through centrifugation/wash cycles. 2.3. Characterization. Transmission electron microscopy (TEM) (Hitachi, H-7500 at 120 kV) was used to characterize the particle size and morphology. The samples for TEM were prepared by dropping the particles dispersed in ethanol onto a copper grid that was then dried under ambient conditions prior to being introduced into the TEM chamber. X-ray powder diffraction (XRD) analysis was conducted on a Rigaku D/maxIIB X-ray diffractometer at a scanning rate of 4°/min with 2θ ranging from 20° to 100° by use of Cu KR radiation (λ ) 1.5418 Å). X-ray photoelectron spectroscopy (XPS) analysis was performed on a VG Escalab MK II with a Mg KR (1253.6 eV) achromatic X-ray source. UV-visible spectra were recorded with a 756 CRT UV-vis spectrophotometer. IR spectra were recorded in the range 400-4000 cm-1 on an Alpha Centauri FT/IR spectrophotometer by use of KBr pellets.

colloids were coated with PEI (stage 1). Then the aqueous solution of metal ions was mixed with a dispersion solution of core colloids modified with PEI. Formation of the metal ionPEI-core colloid composites can be seen by the color change of the precipitations (Table S1, Supporting Information). In the method, excessive metal ions in the solution do not affect the formation of metal NP-core colloid composites because the redox reaction only takes place within the metal ion-PEIcore colloid composites.19 Hence, with no need to remove residual metal ions, the dispersion solution was heated at 100 °C for 1 h, and then core colloids modified with metal NPs were obtained (stage 2). The formation of core colloids modified with metal NPs was directly seen by color change of the dispersion solution (Table S1, Supporting Information). The composites were characterized by TEM, XRD, XPS, and UV-vis spectroscopy. TEM and XRD characterizations of the Ag-PS composites were presented in detail in our previous work.19 Figure 1a,b shows TEM images of the as-synthesized Au-PS composites. It can be seen that many small particles are immobilized on the surface of PS spheres. In the XRD pattern shown in Figure 1c, five diffraction peaks are observed at 2θ ) 38.2°, 44.3°, 64.5°, 77.6°, and 81.6°, corresponding to the (111), (200), (220), (311), and (222) reflections of facecentered cubic (fcc) structured metal Au, respectively (PDF 040784). TEM images of the Au-SiO2 and Ag-SiO2 composites are shown in Figure 2, indicating the deposition of many small particles on the surface of SiO2 colloids. The XRD patterns shown in Figure S2 (Supporting Information) indicate the formation of Au and Ag NPs in the products. In addition, the UV-vis spectra show an absorption peak at 420 nm for AgSiO2 (Figure 3, curve a) and at 530 nm for Au-SiO2 (Figure 3, curve b), further indicating the formation of Ag and Au NPs on the SiO2 surface.

3. Results and Discussion The present method was used to deposit Au and Ag NPs on SiO2, PS, and ZnO colloids. The process is a modification of our previous method19 and is shown in Scheme 1. First the core

Figure 1. TEM images of Au-PS composites: (a) higher and (b) lower magnification. (c) XRD pattern of the Au-PS composites.

Core Colloids Modified with Metal Nanoparticles

Figure 2. TEM images of Au-SiO2 (a, c) and Ag-SiO2 (b, d).

The PS and SiO2 spheres mainly play the role of carrier in the composites. Depositing metal NPs on the semiconductor oxide surface can improve the properties of both the metal NPs and the semiconductor oxides. For metal NPs, their deposition onto oxide supports could enhance their pH and temperature stability. Also, the metal NPs (especially for Au) on the oxide supports were highly active catalysts for many reactions, for example, CO oxidation, hydrogenation of CO2, and hydrochlorination of ethylene.1a,23 On the other hand, the photocatalytic activity of semiconductor oxide could be improved by deposition of metal NPs on their surface. For example, the Au/TiO2 and Au/ZnO composites present improved photocatalytic activity for degradation of acid orange and methyl orange compared with the corresponding single-component semiconductors.16c,24 The present method can be used to deposit the metal NPs on the surface of the semiconductor oxides. Figure 4 shows the TEM images of ZnO particles and Au-ZnO composites. For unmodified ZnO, the particle surface is smooth as shown in Figure 4a. Figure 4b shows the TEM image of Au-ZnO. It can be seen that small particles are immobilized on the surface of the ZnO. XRD patterns of Au-ZnO are shown in Figure 5. Both the diffraction peaks of ZnO and Au can be observed. The peaks at 2θ ) 31.7°, 34.4°, 36.2°, 47.5°, 56.6°, 62.8°, and 67.8° were indexed to the (100), (002), (101), (102), (110), (103), and (112) diffractions of wurtzite-type ZnO, respectively (PDF 36-1451). At the same time, the peaks at 2θ ) 38.2°, 44.6°, 64.5°, and 77.6° correspond to the (111), (200), (220), and (311) reflections of fcc-structured metal Au, respectively. In addition, the UV-vis spectra of the Au-ZnO composites show an adsorption peak at about 540 nm, indicating the formation of Au NPs on the ZnO surface (Figure 3, curve c). The peak at about 370 nm is from ZnO particles as shown in Figure S3 (Supporting Information). XPS is an effective tool for analyzing elements and their corresponding valence state. For Ag-PS composites, the Ag 3d5/2 peak is located at about 368.1 eV, indicating the formation of Ag0 in the samples (Figure 6c).10 The Au 4f7/2 binding energy for Au-PS composites is located at 83.6 eV, which is close to

J. Phys. Chem. C, Vol. 111, No. 9, 2007 3653 the literature value for Au0 (Figure 6d).25 XPS of Ag in the Ag-SiO2 composites is shown in Figure 6a, in which the Ag 3d5/2 peak is located at 368 eV, indicating the formation of Ag0.26 The binding energy of Au 4f7/2 of Au-SiO2 is located at about 83 eV, which is characteristic for Au0 (Figure 6b).25 The XPS studies further demonstrated the formation of metal NPs on the surface of the core colloids. In the metal NP-core colloid composites, there exist certain interactions between the metal NPs and core colloids. The interaction is reflected by a shift of the binding energy value of Au 4f and Ag 3d. For Au/SiO2, the Au 4f7/2 binding energy locating at 83 eV (Figure 6b) experiences a significant shift (1 eV) with respect to that of bulk metallic gold (84 eV). This is consistent with the result reported by Radnik et al.27 As described in the previous reports, the shift of Au 4f binding energy might be ascribed to two factors. One is the small size of the Au NPs (when particle size is below 1 nm). The other is electron transfer from the support to the metal NPs.28 In the present study, the metal NPs on the core surface are well above 1 nm. Thus the first possibility was discarded, and electron transfer from the support to the metal NPs should be the major contribution to the shift of Au 4f binding energy. In addition, the shift (interaction) was significantly affected by the nature of supports. For example, the Au 4f7/2 binding energy presents different values for Au/ZrO2 (83.9 eV) and Au/TiO2 (83.4 eV).28 The effect of supports is also observed in the present work for Au-PS, for which Au 4f7/2 binding energy (Figure 6d, 83.6 eV) showed a shift of 0.4 eV compared with that for bulk gold. In contrary to Au, the Ag 3d XPS shows only a slight shift for both Ag-PS (Figure 6c, 368.1 eV) and Ag-SiO2 (Figure 6a, 368 eV) with respect to that for bulk metallic silver (367.9 eV). The results indicate that little electron transfer occurred between Ag NPs and core colloids. From the XPS results, it was concluded that the shift of binding energy of metal NPs was affected not only by the kind of supports but also by the nature of metal NPs. In the synthesis, PEI acted as a reductant for in situ reduction of metal ions to metal NPs. Although much previous work reported the function of PEI as a reductant, the oxidized products of PEI have not been fully clarified.29 The difficulties may be as follows. First, excessive PEI was used in the synthesis, leading to the presence of a lot of unreacted PEI in the products.29a,b Second, the presence of other organic molecules is also an obstacle for characterization of the oxidized products.29c In the present study, excessive metal ions were added in dispersion solution of core colloids modified with PEI, ensuring the full oxidation of PEI. In addition, other components (SiO2, PS, metal NPs) in the composites have no interference on the characterization of the oxidized product of PEI. Thus, the present method also provides a convenient way for characterizing oxidized products of PEI. For pure PEI, the C1s XPS peak is located at about 284.6 eV, which is assigned to the C1s in C-C and C-H,30 and no other peaks are observed as shown in Figure S4 (Supporting Information). In contrast, the C1s signals of AgSiO2 show two peaks at about 284.6 and 288.3 eV, respectively (Figure 7a). For Au-SiO2, the two peaks are located at 284.5 and 288.4 eV, respectively (Figure 7b). The peak at about 284.6 eV is assigned to the C1s in C-C and C-H, which is close to that of pure PEI. The peak at about 288.4 eV is assigned to the C1s of carboxyl, indicating the presence of carboxyl group in the product.30 The C1s XPS of Ag-PS and Au-PS also show two peaks at about 284.6 and 288.6 eV (Figure 7c,d). Also, as shown in Figure S5 (Supporting Information), the O1s signals for Au-PS and Ag-PS were observed at about 533 eV, which

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Figure 3. UV-visible spectra of Ag-SiO2 (a), Au-SiO2 (b), and Au-ZnO (c).

Figure 6. XPS of Ag 3d of Ag-SiO2 (a), Au 4f of Au-SiO2 (b), Ag 3d of Ag-PS (c), and Au 4f of Au-PS (d).

Figure 4. TEM images of ZnO (a), (Au-ZnO)8 (b), (Au-ZnO)6 (c), and (Au-ZnO)10 (d).

Figure 7. C1s XPS of (a) Ag-SiO2, (b) Au-SiO2, (c) Ag-PS, and (d) Au-PS.

Figure 5. XRD patterns of Au-ZnO composites.

may be ascribed to O in the oxidized product of PEI. In addition, the C1s and N1s XPS of pure PEI, Ag-SiO2, and Au-SiO2 are given in Figure 8. N1s and C1s XPS of pure PEI are shown

in curve a and a′, respectively. Curves b and b′ are C1s and N1s XPS of the oxidized products of PEI in Ag-SiO2, and curves c and c′ are C1s and N1s XPS of the oxidized products of PEI in Au-SiO2. From XPS it can be seen that the relative ratio of peak area of N1s to C1s decreased in Au-SiO2 and Ag-SiO2 compared with that in pure PEI, indicating that the

Core Colloids Modified with Metal Nanoparticles

Figure 8. C1s XPS of unreacted PEI (curves a′), Ag-SiO2 composites (curves b′), and Au-SiO2 composites (curves c′) and N1s XPS of unreacted PEI (curves a), Ag-SiO2 composites (curves b), and AuSiO2 composites (curves c).

amine groups in PEI were consumed after the reaction. XPS studies indicated that the amine groups in PEI were oxidized to carboxyl groups after formation of core colloids modified with metal NPs. IR spectra of the samples support the results of XPS. Figure S6 (Supporting Information) shows the IR spectra of PEI-SiO2 (curve a) and Ag-SiO2 (curve b). For both samples, the peak at about 1095 cm-1 is attributed to Si-O-Si symmetrical stretching vibration.31 The peak at 2924 cm-1 is ascribed to the stretching vibration of C-H, indicating the presence of PEI on the SiO2 surface. In addition, the intensity of stretching vibration of C-H is much lower than that of Si-O-Si vibration, indicating that the amount of PEI is very low compared with the SiO2 colloids. For Ag-SiO2, two new peaks emerge at 1455 and 1396 cm-1, which are ascribed to the antisymmetric and symmetric stretching of O-C-O, respectively.32a-c The stretching vibration of CdO of carboxyl (about 1700 cm-1) could not be observed, which should be ascribed to the coordination of carboxyl with metal NPs by the “bidentate model”.32a Figure S7 (Supporting Information) shows the IR spectra of Au-ZnO (curve a) and PEI-ZnO (curve b). The two new peaks at 1492 and 1452 cm-1 in curve a are also ascribed to the antisymmetric and symmetric stretching of O-C-O. In addition, the peak of N-H stretching vibration of PEI overlaps with that of O-H vibration in SiO231 and ZnO.32d Thus, the intensity change of N-H vibration is not obvious before and after the formation of metal NPs on the SiO2 and ZnO surface. To demonstrate the change of N-H vibration, the IR spectra of PEI-PS and AgPS are given in Figure S8 (Supporting Information). For PEIPS, the peak at 3446 cm-1 is ascribed to the N-H stretching vibration. For Ag-PS, the intensity of the N-H peak greatly decreased, implying that the amine groups in PEI were consumed. The vibration mode of O-C-O is not apparent after the formation of Ag-PS due to the intensive peaks of PS in this range. The IR studies further demonstrated that the amine groups in PEI were oxidized to carboxyl groups after formation of metal NPs on the core surface. The results are consistent with those of XPS. PEI acts as a linker between metal ions and core colloids and also as a reductant for in situ formation of metal NPs. Thus the adsorption content of PEI on core surfaces has an important effect on the resulting composite colloids. The effect was studied with Au-ZnO composites as a model. With unmodified ZnO as core colloids, Au-ZnO composites could not be obtained. On the other hand, the surface charge of both the core colloids and PEI can be tuned by pH. Thus, pH value affects the

J. Phys. Chem. C, Vol. 111, No. 9, 2007 3655 adsorption amount of PEI on the ZnO surface and, consequently, the formation of the Au-ZnO composites. In the experiments, PEI-ZnO composites prepared at pH 6, 8, and 10 were used to prepare Au-ZnO composites that were denoted as (AuZnO)6, (Au-ZnO)8, and (Au-ZnO)10, respectively. TEM images of the samples are shown in Figure 4b-d. As shown in Figure 4c,d, the Au coverage on ZnO is low and many ZnO particles are not modified with Au NPs for (Au-ZnO)6 and (Au-ZnO)10. In contrast, the Au coverage in (Au-ZnO)8 (Figure 4b) is higher than that in (Au-ZnO)6 and (Au-ZnO)10, and many Au NPs are successfully immobilized onto the ZnO surface. The TEM studies indicated that pH had a certain impact on the core colloids modified with metal NPs. The above results demonstrate that the Au (Ag) NPs could be deposited on the core surface. In the reaction system, metal ions in the metal ion-PEI-core colloid composites coordinate with amine groups of PEI, which prevent the interaction between PEI and excessive metal ions. Accordingly, redox reaction takes place only between PEI and metal ions coordinated with them under heat treatment.19 Because each metal ion-PEI-core colloid composite is an individual redox reactor, the effect of the concentration of colloids on the formation of the resulting metal NP-core colloid composites is expected to be small, and large quantities of composites can be readily obtained. For example, more than 4 g of composites could be prepared in 100 mL of water. Hence, the present method provided a convenient avenue for large-scale preparation of metal NPcore colloid composites with various core components. The core colloids modified with Au (Ag) NPs have important applications in catalytic, biological, and optical areas. Also, metal NPs on core colloids can act as “seeds” for the growth of thick metal coating on the core surface. Previously, seeding colloids were obtained by reduction of Ag+-PS with NaH2PO2, reduction of Ag+-SiO2 with KBH4,12 and adsorption of preprepared metal NPs on the surface of the modified core colloids.33 Excessive metal NPs and reductant usually coexist with the seeding colloids. These impurities must be removed to ensure that the shell growth mainly occurs on the core surface. It would be perfect if there were no free metal NPs and residual reductant coexisting with seeding colloids after the seeding reaction. If so, the shell growth process can be performed directly following the surface seeding step with no need of the centrifugation/ wash. In our experiment, all the seed metal NPs should form in situ and be immobilized on the core surface. As a result, no free metal particles coexist with seeding core colloids. The result is demonstrated by UV-vis absorption spectra shown in Figure S9 (Supporting Information). The UV-vis absorption spectrum of the Ag-SiO2 colloids exhibits an absorption peak at about 420 nm, which comes from the plasmon resonance of single Ag NPs. In contrast, the supernatant obtained from the centrifugal solution (2000r, 5 min) is colorless and shows no absorption peak in the range of 400-1000 nm (Figure S9b). The peak at about 300 nm is attributed to the absorption of excess Ag+ ions (Figure. S9a).34 In addition, TEM studies showed that no free metal NPs coexisted with core colloids modified with metal NPs. The results indicated that all formed metal NPs were immobilized on the core surface and only Ag+ ions coexist with Ag seeding core colloids. Thus the dispersion solution obtained after the seeding reaction could be used for shell growth process without centrifugation/wash treatment.35 Based on the above-mentioned experimental results, an improved surface seeding and shell growth technique was adopted for the preparation of the core colloids with controllable metal coverage, with Ag-SiO2 as a model. Aqueous solution

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Figure 10. UV-visible spectra of Ag-SiO2 composites with different Ag coverage: (a) seeding SiO2 spheres, (b) SiO2-Ag10:1, (c) SiO2Ag5:1, (d) SiO2-Ag1:1, and (e) SiO2-Ag2:3.

Figure 9. Typical TEM images of Ag-SiO2 composites with different Ag coverage: (a) SiO2-Ag10:1, (b) SiO2-Ag10:5, (c) SiO2-Ag1:1, and (d) SiO2-Ag2:3.

containing PEI-SiO2 and AgNO3 with mass ratio of 10:1, 10: 5, 1:1, and 2:3 was heated at 100 °C for 1 h at first (the products were denoted as SiO2-Ag10:1, SiO2-Ag10:5, SiO2-Ag1:1, and SiO2-Ag2:3, respectively). After the solution was cooled to room temperature, sodium citrate and ammonia were added into the solution, and then formaldehyde was added dropwise into the dispersion solution to reduce Ag+ to Ag0. In the process, sodium citrate played the role of coordination agent with Ag+, and the ammonia was used to adjust the pH of the solution in basic range (about 10). The color of the dispersion solution changed gradually to red or to black on the basis of the mass ratio of AgNO3 to PEI-SiO2, indicating the increase in Ag coverage on the surface of SiO2 colloids. TEM images of the products are shown in Figure 9. From these images it can be seen that the metal coverage on the core surface increases with increasing initial mass ratio of metal ions to core colloids. The result indicates that the Ag-SiO2 composites with thicker metal coverage could be obtained by an improved surface seeding and shell growth technique. At the same time, the metal coverage could be easily controlled by adjusting the initial mass ratio of AgNO3 to SiO2. The optical plasmon resonance of single metal NPs is confined to relatively narrow wavelength ranges. By depositing the particles on the core colloids, the optical plasmon resonance of the metal NPs may be extended to a broader spectrum range than that of single metal particles, and the absorption peak position depends on the metal coverage. UV-vis spectra of AgSiO2 composites with different metal coverage are shown in Figure 10. The absorption peak for seeding SiO2 spheres is located at about 420 nm, which is due to Mie plasmon resonance from the single Ag NPs (curve a).36 When the mass ratio of AgNO3 to SiO2 increased to 1:10 (SiO2-Ag10:1), the peak shifts from 420 to 435 nm (curve b). For SiO2-Ag10:5, a new absorption peak at longer wavelength (about 600 nm) emerged, which may be ascribed to the collective absorption behavior of Ag particles on the core surface (curve c).33d,35,36 For SiO2-

Ag1:1, the peaks at about 600 nm shift to about 650 nm (curve d). As the metal coverage increases further, the peak at longer wavelength shifts to about 700 nm (SiO2-Ag2:3, curve e). At the same time, the peak intensity at longer wavelength gradually increases as the metal coverage increases. The results may be explained as follows. When the metal coverage is low, for example, after the seeding reaction, the composites mainly show the optical plasmon resonance of individual Ag particles. In contrast, the composites mainly display collective absorption behavior of Ag particles when thicker Ag coating was formed. The UV-vis studies indicate that the optical plasmon resonance of Ag-SiO2 composites could be easily tuned in a broader spectrum range by changing the metal coverage. 4. Conclusions In summary, a simple method was developed to deposit metal NPs on the surface of various core colloids. The metal NPs formed in situ on the surface of the core colloids modified with PEI. There is little effect of colloid concentration on the formation of core colloids modified with metal NPs and the reaction may be easily scaled up. In addition, no “free” metal clusters and residual reductant coexist with the core colloids modified with metal NPs in the system. Based on the results, an improved surface seeding and shell growth technique was used to prepare the core colloids with controllable metal coverage. The core colloids modified with Au (Ag) NPs have important applications in catalytic, optical, and biological areas. Also, they could act as seeding colloids for deposition of other metals on core surfaces to obtain bimetallic coating (for example, PtAu-PS; Figure S10, Supporting Information). The present method is a promising synthetic strategy due to its simple operation, easy larger-scale production, and generality for various core colloids. Acknowledgment. We are grateful for financial support from the National Natural Science Foundation of China (20371011). Supporting Information Available: Synthesis of PS, SiO2, and ZnO; procedure for preparation of Ag-SiO2 composites with different metal coverage and for preparation of AuPt-PS composites; TEM images of PS, SiO2, and PtAu-PS; XRD of Au-SiO2 and Ag-SiO2; additional XPS data; and UV-vis and IR spectra of the samples. This material is available free of charge Via the Internet at http://pubs.acs.org.

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