Carbon Spheres with Controllable Silver ... - ACS Publications

(32) In the present work, we report a facile method to dope monodisperse preprepared porous carbon spheres (CSs) with Ag NPs to fabricate Ag-NP/C ...
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J. Phys. Chem. C 2010, 114, 977–982

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Carbon Spheres with Controllable Silver Nanoparticle Doping Shaochun Tang, Sascha Vongehr, and Xiangkang Meng* National Laboratory of Solid State Microstructures, Department of Materials Science and Engineering, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed: October 27, 2009; ReVised Manuscript ReceiVed: NoVember 22, 2009

We report a facile method to dope submicrometer carbon spheres with Ag nanoparticles (NPs) to fabricate Ag-NP/C composites via microwaving suspensions of nanoporous carbon spheres in aqueous Ag(NH3)2+ solutions with poly(N-vinylpyrrolidone) as reducer. The composite particles are synthesized in high yield within a short reaction time, and the size, number density, and to some extent even the locations of NPs in/on the carbon spheres can be controlled by adjusting reaction parameters. The controllability is discussed based on the experimental results from transmission electron microscopy, X-ray photoelectron spectroscopy depth profiling, and X-ray diffraction. By controlling the Ag doping, the composite spheres exhibit not only a tunable plasmon resonance shift but also an excellent catalytic activity toward the reduction of 4-nitrophenol by sodium borohydride. Introduction The synthesis of nanosized metal particles has been of continuous research interest because of their many important potential applications.1,2 Most properties, for example, surface plasmon resonances (SPR), surface-enhanced Raman scattering, and catalytic activity, depend on nanoparticle (NP) size,3 shape,4 and interparticle coupling.5,6 Numerous processes have been developed to synthesize metal nanoparticles (NPs), and the NPs of some metals can be produced in high yield using solutionbased methods, but the subsequent collection and assembly of the individual NPs from solutions are major challenges.7 Partially in order to address these problems, increasing effort has been aimed toward the introduction of metal NPs on/into spherical dielectrics such as silica, polystyrene and carbon to create composite structures. Dispersion of active metal NPs on/ in the submicrosubstrates prevents their aggregation even at high particle densities. Such composites exhibit novel optical, catalytic, magnetic, and optoelectronic properties5,8–15 and find applications in many fields. The composites are more conveniently separated and reused in catalytic applications and may serve as building blocks for functional devices such as photonic crystals.5 Control over the composites’ structure is necessary to tune their properties and optimize them for such applications as the detection of macromolecules such as DNA and antibodies via SPR shifts16 or catalysis of selected chemical reactions.17 Depositing metal NPs on the outside of dielectric spheres8–11,18 is often characterized by nonuniform distributions, low densities, and poor long-term stability of NPs.6 Embedding metal NPs inside enhances stability, lowers toxicity, and can alter electrooptical properties because of the NPs’ differing collective interactions6 and the matrix embedding. So far, there are few reports on the creation of highly dispersed metal NPs contained inside dielectric spheres5,19,20 Methods aimed at silica dielectrics, for instance, all form the NPs in situ in an ongoing modified Sto¨ber reaction, which makes it difficult to control the size and density of the NPs. It remains challenging to introduce NPs * To whom correspondence should be addressed. E-mail: [email protected]. Tel: +86 25 8368 5585. Fax: +86 25 8359 5535.

into preprepared spheres and have the NPs well dispersed with widely controllable sizes and densities. Poly(N-vinylpyrrolidone) (PVP) is often employed as a stabilizer in the preparation of composite spheres.21–24 Recently, PVP was also used to reduce noble metal ions (Au, Ag, Pd, and Pt) to prepare metallic nanostructures.25–27 Microwave (MW) heating allows a rapid synthesis of nanostructures28–31 and matrix deposition of metal NPs.32 In the present work, we report a facile method to dope monodisperse preprepared porous carbon spheres (CSs) with Ag NPs to fabricate Ag-NP/C composites by microwaving suspensions of CSs in aqueous Ag(NH3)2+ solutions with the assistance of PVP as reducer. The [Ag(NH3)2]+ ions first enter the porous structures of the CSs during an extended soaking. The synthesis offers good controllability of size and density of NPs, high-yield, and short reaction time. Depending on the reaction conditions, Ag NPs can be formed in different locations within and on the spheres. The locations are determined by transmission electron microscopy (TEM) analyses. This method has better controllability than current approaches for metal-NP-doped spheres, and to the best of our knowledge it has not yet been reported. The growth of Ag NPs inside the CSs and their increase of size with depth in the case of high microwave power are discussed. The Ag-NP-doped composite structures show tunable SPR properties. Their catalytic activity toward the reduction of 4-nitrophenol (4-NP) by sodium borohydride (NaBH4) is higher than that of halloysite nanotube-supported Ag NPs and Ag-NP-doped hollow poly(Nisopropylacrylamide) spheres because of the small size, high dispersion, and high number density of the Ag NPs. Experimental Section Materials. Silver nitrate (AgNO3, AR), sucrose (AR), PVP (K30, AR), and ammonia aqueous solution (25 wt %, GR) were purchased from Sinopharm Chemical Reagent Co. Ltd. and used as received without further purification. Deionized water with a resistivity of >18.0 MΩ · cm from a JL-RO 100 Millipore-Q Plus water purifier was used throughout the experiments. Synthesis of Carbon Spheres. Monodisperse CSs were prepared by a modified hydrothermal synthesis as described in detail elsewhere.33 Typically, an aqueous sucrose solution of

10.1021/jp9102492  2010 American Chemical Society Published on Web 12/15/2009

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0.3 mol/L was added into an 80 mL autoclave to fill a half volume. The autoclave was put into an oven and held at 190 °C for 3 h. After being cooled in air, the product was purified by repeated centrifugation/redispersion cycles with deionized water and anhydrous ethanol and finally dried at 40 °C in an oven. Synthesis of Ag-NP-Doped Carbon Spheres. The porous CSs were soaked in various concentrations (c) of aqueous [Ag(NH3)2]+ solutions. For a typical c ) 2 × 10-4 mol/L Ag(NH3)2+ ion solution, 3.5 mg of silver nitrate was dissolved in 100 mL of deionized water, and aqueous ammonia solution was dripped in until the intermediately formed precipitates disappeared again. A 0.25 g amount of preprepared CSs was dispersed in 50 mL of the solution and vigorously stirred for different immersion times (tI) of 0-12 h at room temperature (R.T.). After addition of 0.05 g of PVP and vigorous stirring for 1 min, the suspension was put into a commercial LG WD700 (MG-5041T) microwave oven with 2450 MHz frequency for different reaction times (tR). The “warm”, “low”, and “medium” levels generate microwave pulses of 6, 18, and 24 s every 30 s, respectively. The instantaneous output power during a pulse is P ) 700 W, i.e., the power level is the average power 〈P〉 over the 30 s cycle (hence warm, low, and medium correspond to 〈P〉 ) 140, 420, and 560 W, respectively). The resulting suspensions are opaque and dark black with a yellowish taint that is removed through subsequent washings during a purification procedure like that described above in the synthesis of CSs. Characterizations. TEM ObserWation. The shape, size, and structure of pure CSs and the analyses of the size, distribution, morphology, and crystal structure of the silver NP doping were both studied using transmission electron microscopy (TEM), selected area electron diffraction (SAED), and high resolution TEM (HRTEM) on an FEI TECNAI F20 microscope operating at 200 kV accelerating voltage. X-ray Diffraction (XRD). The composition and crystallographic properties of the products were investigated by XRD on a Rigaku Ultima III diffractometer (using Cu KR ) 1.5418 Å radiation) at a scanning rate of 2°/min in the range 20° < 2θ < 90°. The XRD samples were supported on glass substrates. X-ray Photoelectron Spectroscopy (XPS). The XPS measurements of the samples were performed in a Thermo VG Scientific MultiLab ESCA2000 system with a CLAM4 hemispherical analyzer and at a base pressure below 3 × 10-10 mbar. The XPS Mg KR source operating at 50 W provides X-rays at 1253.6 eV. Photoelectrons are collected through the hemispherical energy analyzer with pass energy of 20 eV. Depth profiling was performed using a 3 keV Ar+ ion beam. XPS data were collected between the sputtering cycles of 30 s. The porous carbon etching rate is at most several times that of nonporous amorphous carbon films, which is only about 2 nm/min.34 Therefore, this procedure provides a depth profile analyses of any carbon objects that are larger than several tens of nanometers in diameter (d > 10 nm), i.e., the depth profile is not that of the mixture of those objects below the Ar+ ion beam. InductiWely Coupled Plasma (ICP). ICP spectroscopy was performed on a J-A1100 ICP spectrometer (Jarrell-Ash Company, America). The Ag-NP/C composites were first dissolved by concentrated nitric acid (65 wt %). Then a clear solution, after removing carbon, was diluted to ppm level for subsequent ICP measurements. Brunauer-Emmett-Teller (BET). The BET measurements were conducted at 77 K on a sorptometer (Micromeritics

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Figure 1. TEM (a) and HRTEM (b) images of the CSs synthesized from a typical hydrothermal reaction of a 0.3 mol/L sucrose solution for 3 h at 190 °C.

Instrument Corporation ASAP 2020). The samples were prepared by evacuation for 6 h at 523 K and 10-3 Torr. Optical Properties of the Ag-NP/C Composites. UV-vis absorption spectroscopy was carried out on a Shimadzu UV3600 UV-vis-NIR spectrophotometer with wavelength in the range of 300-800 nm. Before measurements, the samples were ultrasonically dispersed in water. Catalytic Properties. To study the catalytic activity, 1.0 mg of the Ag-NP/C composite was added into 2.8 mL of 4-NP aqueous solution (5.0 × 10-5 M) under constant stirring at R.T. A freshly prepared aqueous solution of NaBH4 (0.20 mL, 1.0 × 10-1 M) was then added. The mixture was immediately transferred into a quartz cuvette with an optical path length of 1 cm, and UV-visible absorption spectra were recorded to monitor changes in the reaction mixture on the same spectrophotometer as used above but with the wavelength in the range of 200-600 nm. Results and Discussion Figure 1a shows the product from a typical hydrothermal reaction (190 °C, 3 h). The CSs are narrowly size distributed with an average diameter of 800 nm. The HRTEM image in Figure 1b shows the existing nanopores33 which allow precursor ions to infiltrate internal structure of the CSs. Figure 2a shows a low resolution TEM image of the product obtained with c ) 2 × 10-3 mol/L and tI ) 2 h after microwaving the suspension on the lowest power level (140 W) for a reaction time tR of 10 min. The spheres show a large number of dark spots that must be inside the spheres, as none are visible on the surface of the spheres’ image. TEM image showing a single composite sphere (Figure 2b) reveals that the NPs have a high number density yet are still well dispersed as mostly isolated, single particles. Some NPs overlap because TEM makes two-dimensional (2D) projections of 3D objects. The apparent NP number density decreases toward the outside of the sphere projections, further corroborating their dispersion inside the sphere. The SAED pattern (inset Figure 2b) of the entire Ag-NP/C composite sphere displays several diffraction rings due to numerous crystals within the sphere. The interplanar spacings of 0.23, 0.21, 0.14, and 0.12 nm correspond to the (111), (200), (220), and (311) planes of face-centered-cubic (fcc) silver (JCPDS 4-783), respectively. Higher magnification (Figure 2c) reveals the NPs to be almost spherical. A sample of 100 NPs resulted in a diameter of d ) (10 ( 2) nm. Still higher magnification (Figure 2d) clearly shows that the spherical NPs embed in the matrix and are separated from each other although some of them seem to overlap in the TEM projection. The XRD pattern of the typical sample (shown in Figure 2) is presented in Figure 3. The main diffraction peaks at 38.1°, 44.3°, 64.4°, 77.3°, and 81.7° are indexed as (111), (200), (220),

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Figure 4. XPS depth profile of Ag3d signal from the composite spheres of the sample shown in Figure 2 after sputtering for 0, 0.5, 1, 2.5, and 3 min, corresponding to curves from a to e, respectively.

Figure 2. TEM images at low (a) and high magnifications (b, c) of a typical product prepared with c ) 2 × 10-3 mol/L, tI ) 2 h, 〈P〉 ) 140 W, and tR ) 10 min. The inset of b shows the corresponding SAED pattern of a single composite sphere.

Figure 3. XRD pattern of Ag-NP/C composite spheres from the same sample as that shown in Figure 2. The inset is an enlarged region (smaller range) of the XRD pattern.

(311), and (222) reflections, confirming the presence of silver in the composite. On the basis of the Scherrer formula D ) Kλ/(B cos θ) (where K ) 0.9 is a dimensionless shape factor, λCuKR) 0.15418 nm, θ is the Bragg angle, and B is the diffraction peak full width at half-maximum in radians), the average grain size of the silver NPs is calculated to be about D ) 11 nm based on the observable four crystal directions and is consistent with the TEM observations. The amorphous nature of the spheres’ carbon is demonstrated by the wide diffraction peak around 2θ ) 23° (inset in Figure 3). The composites in this study are about 800 nm in diameter, and XPS depth profiling can be employed with confidence. Figure 4 shows the XPS depth profile of the Ag3d signal taken from the sample shown in Figure 2 after Ar+ sputtering for various durations up to 5 min. That the XPS spectra do not change much for etching times above 2.5 min (the spectra taken after longer than 3 min of sputtering are not shown) indicates that the spheres are homogeneously doped with Ag NPs. The

intensity increase with depth (Figure 4a to 4e) confirms that the Ag content increases, as expected from Figure 2c. No Ag3d can be discerned before sputtering (black curve a), indicating that no silver resides on the surface. After sputtering for 0.5 min, the Ag3d peaks start to appear (purple curve b). The XPS signals for Ag3d5/2 and 3d3/2 are resolvable after 1 min of sputtering (blue curve c), and their observed positions at binding energies of 368.3 and 374.4 eV, respectively, confirm the silver to be in its unoxidized Ag0 state. The two peaks shifted by 0.4 eV to higher binding energies compared with bulk silver (Ag3d5/2 is usually at 368.0 eV and 3d3/2 6.0 eV higher). This may be partly from the interaction of the metal with the carbon support.35 After sputtering for 2.5 min, the two peaks become even better resolved and smoother (green curve d). The peak shift increased to 0.8 eV, which indicates that it is actually more influenced by the interparticle coupling: the shift reflects the increase in the Ag content, i.e., the increase of the number density of NPs. The weight ratio of these Ag-NP/C composites from ICP data is 55.2% of Ag versus 44.8% carbon. Besides the possibility of achieving a homogeneous distribution, the size, density, and location of NPs in the composites can be controlled by adjusting reaction conditions. Figure 5a shows a typical product obtained with tI ) 4 h, c ) 5 × 10-4, 〈P〉 ) 140 W and tR ) 10 min. A large number of Ag NPs with a narrow size distribution (4-5 nm in diameter) are formed inside, and still no NPs are present on the surface of the spheres. The small NPs are well dispersed, and their sites are far from the outermost edge of the sphere images. This further demonstrates the formation of NPs inside of the CSs although TEM provides 2D projections. A HRTEM image (right lower inset of Figure 5a) reveals that these NPs are single-crystalline. When tI was prolonged to 12 h while other reaction conditions remained the same (c ) 5 × 10-4, 〈P〉 ) 140 W, and tR ) 10 min), NPs with a higher number density and larger sizes of d ) (15 ( 2) nm were obtained, as shown in Figure 5b. When instead c was increased to 2 × 10-3 mol/L and the MW power was increased by one level to 420 W while other conditions remained the same (tI ) 4 h, tR ) 10 min), the diameter of the NPs increases to about 15-30 nm, with the larger ones toward the center (Figure 5c). No NPs are present on the sphere surfaces. What seems like aggregates in the sphere centers results from the overlap of isolated particles in the 2D TEM projection. The diffraction intensity of the corresponding SAED patterns from single composite particles (insets of the figures) increases gradually from Figure 5a to 5c (or Figure 5d to 5e),

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Figure 5. TEM images of products from the reactions with c, tI, MW power 〈P〉, and tR being 5 × 10-4 mol/L, 4 h, 140 W, 10 min (a); 5 × 10-4 mol/L, 12 h, 140 W, 10 min (b); 2 × 10-3 mol/L, 4 h, 420 W, 10 min (c); 2 × 10-4 mol/L, 4 h, 140 W, 10 min (d); 2 × 10-4 mol/L, 4 h, 420 W, 3 min (e); and 2 × 10-4 mol/L, 4 h, 560 W, 3 min (f). The insets are the corresponding HRTEM image (right lower corner of a) and SAED patterns (right upper corners of a-d) of the single composite particle shown.

Figure 6. TEM images with different magnifications (a, b) of the product from a control experiment with c ) 2 × 10-3 mol/L, tI ) 0 h, 〈P〉 ) 140 W, and tR ) 10 min.

confirming the increase in size and density of NPs (i.e., total amount of Ag) in the composites. Figure 5d shows a typical product obtained with tI ) 4 h, c ) 2 × 10-4, 〈P〉 ) 140 W, and tR ) 10 min. Small silver NP (3-5 nm) with only few large-size outliers are formed in this case, and no NPs are present on the surface of the spheres. When the MW power was increased by one level to 420 W and tR was only 3 min while other reaction conditions remained the same (c ) 2 × 10-4, tI ) 4 h), larger NPs of 7-9 nm were obtained with a high number density, as shown in Figure 5e. The higher power induced a high nucleation density and fast growth so that many NPs form within a short tR. The NPs are homogeneously distributed over the volume but get larger toward the center of the spheres. When the power level was further increased to 560 W, the NPs have a slightly higher density and many of them are present also on the sphere surface, which can be confirmed by the TEM image (see Figure 5f). In a control experiment without immersion, i.e., tI ) 0 h (c ) 2 × 10-3 mol/L, 〈P〉 140 W, tR ) 10 min), few NPs form inside the sphere but some large ones are found on the sphere surface (Figure 6a,b). TEM images (not shown) taken from different angles (tilting of stage by ( 15°) further demonstrate locations of the NPs. The increase in total amount of Ag NPs inside spheres with increasing tI was already seen above at a

Figure 7. UV-vis adsorption spectra of bare carbon spheres (A) and Ag-NP/C composite particles such as in Figures 5a, 5b, 2b, and 5c (curves B to E, respectively).

lower concentration (c ) 5 × 10-4 mol/L), e.g., comparing tI ) 4 h (Figure 5a) with 12 h (Figure 5b). This is because most ions had not entered the spheres, but those that did seem to have distributed homogeneously inside. When tI was increased to 2 h instead, NPs grew only inside the spheres (Figure 2). Thus, a proper tI is important for the desired doping and can even influence the location of NPs in/on the spheres (see Table 1). The absorption spectrum of pure Ag NPs exhibits a SPR band at around 420 nm due to the plasmon resonance.9 Figure 7 shows UV-vis spectra of pure and doped CSs. The bare CSs have no distinct absorption peak (black spectrum A). The C-Ag composite spheres with small and few NPs (the product shown in Figure 5a) have a spectrum (pink curve B) with a SPR resonance at 425 nm. The spectra C (green), D (blue), and E (purple) correspond to the composites shown in Figures 5b, 2b, and 5c, respectively. When increasing the size and number of the silver NPs, the overall absorption increases, the SPR absorption peaks become broader and stronger, and their maxima red-shift from 429 (C) to 460 nm (D). The extinction band

Carbon Spheres with Silver Nanoparticle Doping

Figure 8. Successive UV-vis absorption spectra of the reduction of 4-NP by NaBH4 in the presence of Ag-NP/C composite spheres (shown in Figure 2). The inset shows the logarithm of the absorbance at 400 nm vs reduction time.

broadens greatly and the maximum shifts to 500 nm (E) when sizes and density of the NPs further increase (from Figure 2b to 5c). These changes are attributed to the dipole-dipole coupling between neighboring metal NPs.5,6,9 Especially the broadening also results from the inhomogeneity in size and shapes.36 Since the size and number density of the silver NPs are controlled by the reaction conditions, the SPR optical properties of the composite spheres are tunable. The small size, high dispersion, and high number density of silver NPs obtained promise significant catalytic activity.3 Here, the reduction of 4-NP using NaBH4 was used as a model reaction for determining the catalytic activity of the Ag-NPdoped composite. The UV-vis absorption spectrum of the aqueous mixture of 4-NP and NaBH4 has an absorption maximum at 400 nm due to the 4-NP ion in alkaline conditions. In agreement with previous results, the absorption peak of 4-NP changed from 317 to 400 nm immediately after the addition of NaBH4 solution, which corresponds to a color change of light yellow to yellow-green due to the formation of 4-nitrophenolate ion.37,38 Without addition of catalyst, a reduction will not proceed, the maximum absorption peak remains unaltered, and the mixture remians yellow. However, when the Ag-NP/C composite (shown in Figure 2) was added, reduction commences and the time-dependent absorption spectra (Figure 8) show a decrease in intensity of the absorption peak at 400 nm and concomitant development of a new peak at 300 nm corresponding to 4-aminophenol, the reduction product of 4-NP. The yellow color has completely disappeared after 25 min of reduction. The small amounts of products involved suggest that the Ag-NP/C composites are effective catalysts for similar reactions. No reaction was observed in a control experiment using CSs and PVP as catalyzer, indicating that only Ag-NP/C composites play a catalytic role in the reduction of 4-NP. The rate constant k is

J. Phys. Chem. C, Vol. 114, No. 2, 2010 981 determined from the plot of ln A (A is the absorbance at 400 nm) vs reduction time (inset of Figure 7). Reported ratios of rate constants k over the weight of catalyst employed are 1.30, 0.41, and 0.09 s-1 g-1 for coral-like dendrite, banana leavelike dendrite, and spherical Ag nanostructures,39 respectively. These are all lower than the 1.69 s-1 g-1 of the Ag-NP/C composites synthesized in this work, although a substantial part of the weight is actually the carbon in this case. Compared with other substrate-supported Ag NPs, the rate constant k of AgNP/C composites is much higher than previously reported ratios for silver NPs supported on halloysite nanotubes (0.087 s-1g-1)40 and Ag-NP-doped hollow poly(N-isopropylacrylamide) spheres (0.014 s-1g-1).41 One may suspect that only the overall energy radiated should be of influence, i.e. that using 140 W for tR ) 10 min equals maintaining 560 W for tR ) 2.5 min. However, this is not the case. At MW power levels above 140 W, the NPs get larger toward the sphere center. This may indicate a higher pressure p inside the locally heated dielectric spheres, especially at short warm-up times from R.T. to the boiling point. Solution and heat escaping toward the outside result in pressure and temperature gradients. The higher values toward the middle lead to larger NPs there. The outermost layers may even be free of NPs due to a fast outflow of solution, consistent with the high BET surface after reaction. The BET surface area of the CSs changes from 13 m2/g before doping to 21 m2/g afterward. The pores have opened up further because of the strong local MW heating28,32 of the spheres that triggers pressurized expulsion of solution from the inside through the outer layers. The high power of 560 W allows some ions to be reduced on the sphere surfaces (Figure 5c) although the ions inside the spheres are more easily reduced because of higher pressure p inside the locally heated dielectric spheres. The use of MW may be vital here in order to have hot spots28,32 occurring inside the CSs, thereby leading to the short reaction times compared with experiments at or below the solvent boiling temperatures,9 where reduction with help of PVP usually takes on the order of hours. Ultrasound-assisted reactions can have local hot spots due to bubble cavitations, too, but such mechanisms cannot work inside the dielectric sphere pore structure. Indeed, our control experiments using ultrasound instead of MW did not dope inside the CSs. Such attempts have only been successful when trying to grow NPs on the surface of CSs,33 consistent with possible bubble cavitations at the surface. Our control experiments also show that no silver NPs are formed when just microwaving in the absence of PVP reducer. The MW-assisted doping with NPs can be divided into stages, the main ones being the ions infiltrating the CSs, nucleation, and growth. When CSs are immersed in the silver solution, [Ag(NH3)2]+ ions diffuse into the spheres along interconnected pores. After addition of PVP and stirring, the CSs are covered in PVP. Under MW irradiation, [Ag(NH3)2]+ ions are reduced in the presence of PVP and form Ag nuclei.28,32 Because carbon

TABLE 1: A Summary of the Features of the Ag NPs Dependent on the Reaction Conditions immersion time (tI, h) c[Ag(NH3)2]+ (L/mmol) power (〈P〉, W) reaction time (tR, min) NP size (d, nm) 2 4 12 4 4 4 4 0

2 0.5 0.5 2 0.2 0.2 0.2 2

140 140 140 420 140 420 560 140

10 10 10 10 10 3 3 10

10 ( 2 4(1 15 ( 2 15-30 4(1 8(1 9(1 10-25

location of NPs homogeneously inside homogeneously inside inside inside, larger toward center inside with few large outliers inside in and outside outside

Figure 2 5a 5b 5c 5d 5e 5f 6

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is relatively electrically conductive,42 it is sufficient that the PVP is a surfactant and sticks to the sphere surfaces, in order to be able to provide the electrons taken up by the reduction. Our procedure can embed multimetal NPs within carbon while retaining the sphere surface for yet further loading with still other metals to form quite complex layered structures. Optical, electronic, and catalytic properties of metal NPs are size-dependent, and such layered structures integrate differently sized NPs of various metals into a single system.43 Conclusions In summary, CSs doped with highly dispersed Ag NPs have been synthesized in high yield by microwaving a suspension of preprepared nanoporous CSs in Ag(NH3)2+ solutions with the aid of PVP reducer. Different nanoparticle-related features such as size, number density, and preferred location can be controlled by adjusting reaction conditions. A discussion of the pressure and temperature gradients expected from different MW power levels during the reaction, for instance, gave a coherent picture of how the reaction conditions influence the different distributions of NPs inside and on the substrates. The Ag-NP/C composite particles show tunable SPR properties and significant catalytic activity toward the reduction of 4-nitrophenol by sodium borohydride. Their catalytic activity is higher than that of halloysite nanotube-supported Ag NPs, Ag-NP-doped hollow poly(N-isopropylacrylamide) spheres, and even than that of dendritic Ag nanostructures, which may be attributed to the small size, high dispersion, and high number density of the Ag NPs. MW-assisted incorporation of NPs into preprepared CSs represents a new concept in nanofabrication because this method, as opposed to for example ultrasound-assisted methods, can dope preformed dielectric spheres also in their interior. This method is a promising route to dope porous matrixes for the synthesis of other metal-dielectric hybrid materials. Exploring the synthesis and properties of the many complex layered structures thereby possible should inspire alot of future research. Acknowledgment. This work was supported by the State Key Program for Basic Research of China (2004CB6193052010CB631004), the National Natural Science Foundation of China (50831004), and the Innovation Fund of Jiangsu Province (BY2009148). References and Notes (1) Cao, Y. W.; Jin, R. C.; Mirkin, C. A. Science 2002, 297, 1536. (2) Rosi, N. L.; Mirkin, C. A. Chem. ReV. 2005, 105, 1547. (3) Mori, K.; Kumami, A.; Tomonari, M.; Yamashita, H. J. Phys. Chem. C 2009, 113, 16850. (4) Tian, N.; Zhou, Z.; Sun, S.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732. (5) Wang, W.; Asher, S. A. J. Am. Chem. Soc. 2001, 123, 12528. (6) Peceros, K. E.; Xu, X. D.; Bulcock, S. R.; Cortie, M. B. J. Phys. Chem. B 2005, 109, 21516.

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