Polyoxometalate-Assisted Galvanic Replacement Synthesis of Silver

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Polyoxometalate-Assisted Galvanic Replacement Synthesis of Silver Hierarchical Dendritic Structures Rongji Liu,†,|| Shiwen Li,† Xuelian Yu,† Guangjin Zhang,*,† Ying Ma,‡ Jiannian Yao,*,‡ Bineta Keita,*,§ and Louis Nadjo§ †

Institute of Process Engineering, Chinese Academy of Sciences, 100190, Beijing, China Beijing National Laboratory for Molecular Science, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, 100190 Beijing, China § Laboratoire de Chimie Physique, Equipe d’Electrochimie et de Photoelectrochimie, UMR 8000 CNRS, Universite Paris-Sud 11, 91405 Orsay Cedex, France Graduate University of Chinese Academy of Sciences, 100190 Beijing, China

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bS Supporting Information ABSTRACT: A simple organic-free replacement process was realized between an aqueous silver nitrate solution and an aluminum wafer, in the presence of a polyoxometalate (POM), [PW12O40]3 (PW12). The same protocol was used with a carbon-coated copper grid and a silicon wafer. The POM was photochemically reduced and used as both a reductant and a stabilizer of silver nanoparticles, Ag@POM, in the homogeneous phase. The presence of the POM and Ag@POM colloids was found to modulate the reaction kinetics, and the POM serves as a kind of soft template, maintaining the anisotropic conditions and directing the Ag nanoparticles in the solution to the seeds. Multiple morphologies were obtained, including nanoplates decorated at their periphery with fractal structures, dendrites, and nanospheres. The objects were studied by FESEM, TEM, XRD, XPS, and EDX. Through the variation of the silver salt to POM ratio and the initial concentrations of reactants, in addition to time-dependent observations, it has been possible to distinguish a kinetically dominated route resulting in nanoplates and the nonequilibrium and anisotropic growth yielding fractal and dendritic structures. Altogether, these observations have suggested a global growth mechanism. SERS study of Rhodhamine B on the dendrites gives particularly sharp results.

’ INTRODUCTION Silver nanostructures are widely used in the area of photography,1 enhanced fluorescence,2,3 catalysis,4,5 antimicrobial activities,6 superhydrophobic surfaces,7 and sensing.8 It has been well-established that all these potential applications are attributed to their physicochemical and optoelectronic properties, which are size- and shape-dependent. Therefore, much attention has been focused on the size- and shape-controlled synthesis of silver nanostructures. Silver nanostructures show a large variety of shapes, such as spherical nanoparticles (NPs),9 11 chainlike nanostructures,12 polyhedrons,9,10,13 wires,11,14 17 rods,14 dendrites,4 nanoinukshuk,18 and ribbons or saws.19 Among them, silver dendrites have received the focus of strong interest because of their intriguing structures. In addition, silver dendrites are found to be the most effective materials for surface enhanced Raman spectroscopy (SERS),20 which has been reported by many previous works.8,21 25 Many protocols are now available to obtain welldefined silver dendrites, including the electrochemical13,26,27 or photochemical technologies,28 traditional wet-chemical reactions using polyvinyl pyrrolidone as the surfactant,29 and galvanic r 2011 American Chemical Society

displacement reaction.24,30,31 Various silver dendrites are synthesized, including the flower-like dendrites,26,32 fernlike dendrites,26,33 coral-like dendrites,26 fractal-like dendrites,34 and cactus-like dendrites.4 Recently, a green wet chemical synthetic method of metal NPs has been developed using polyoxometalates (POMs) as both reductive and capping agents,11,35 37 and thus, the core shell nanostructures of Ag@POM,11 Au@POM,36 and Pt@POM37 had been formed. These POMs have built-in reduction capabilitie;,35 alternatively, they can be electrochemically or photochemically reduced.38 POMs are early transition metal oxygen anionic clusters, which exhibit remarkably rich redox and photochemical properties. Various shapes of silver nanostructures, including NPs, nanowires,11 and ribbons or saws,19 have been synthesized by the so-called POM method. Recently, a green wet chemical route for the synthesis of silver and palladium dendrites Received: January 27, 2011 Revised: June 22, 2011 Published: June 24, 2011 3424

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Figure 1. FESEM images of complex silver dendrites formed on the aluminum wafer. (a) Typical complex silver dendrites. (b) Magnified image corresponding to the green rectangular area of Figure 1a. (c) Magnified image corresponding to the rectangular area of Figure 1b. (d) Magnified image corresponding to the area labeled A of Figure 1a (total [POM] = 0.3 mM, total [Ag+] = 0.3 mM).

was described, starting simply from VOSO4.39 However, to the best of our knowledge, there is no report dealing with POMassisted synthesis of silver dendrites. Herein, we report our initial findings in the size- and shapecontrolled synthesis of silver dendrites by a galvanic displacement reaction in the presence of a selected POM. The incentive for this investigation is 2-fold: first, the selection of two metals with very different reduction potentials ( 1.66 V for Al and +0.337 V for Cu compared to +0.799 V for Ag) might induce different morphologies of the dendrites; second, even though surfactants or templates are not needed for this synthesis, the presence of a POM, in an organic-free replacement system, might prove interesting to modulate the reaction kinetics and favor the anisotropic growth of silver nanostructures. In this issue, [PW12O40]3 (PW12 for short) was selected as the POM. The obtained dendrites showed a good enhancement of Raman scattering.

’ EXPERIMENTAL SECTION Materials. Phosphotungstic acid [PW12O40]3 (PW12) (Reagent grade) and Rhodamine B (RhB) (Analytic grade) were purchased from Sigma-Aldrich, and silver nitrate and isopropanol were purchased from Beijing Chemical Reagents Corporation (Analytic grade). All the chemicals were used without further purification. Ultrapure water with a resistivity of 18.2 MΩ cm was produced using a Milli-Q Simplicity 185 filtration system (Millipore, USA). Synthesis of Silver Dendrites. Several protocols for dendrite synthesis were followed and compared: direct POM-free galvanic displacement, galvanic displacement in the presence of the oxidized form of the POM, galvanic displacement in the presence of the oneelectron photoreduced POM, and finally, galvanic displacement in the presence of preformed Ag@POM colloids. For these last two methods, which will be described in more detail, PW12 was first reduced photochemically. A 500 W Hg lamp was used as a UV light source. In a typical synthesis, 3 mL of PW12 with a concentration ranging from 0.03 to 1.0 mM was added in a spectrophotometer cell (1 cm path length) and mixed with 11 μL of isopropanol. Then the mixed solution was irradiated by the UV light for 30 min. The concentration of the oneelectron reduced form can be measured spectrophotometrically using an

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extinction coefficient of ε752 = 2000 M 1 cm 1. This quantification indicated that roughly 90% PW12 was actually photoreduced, a result of paramount importance in the following. Second, silver nitrate solution with a certain silver ion concentration (SIC) was injected into the solutions of the “reduced PW12”. The solutions were mixed by shaking by hand, agitated for ca. 3 s and allowed to stand. The whole reaction was performed at room temperature and ambient pressure in the dark. Finally, the solution was aged for 24 h, and thus, a mother solution of AgNP@POM was prepared. Whatever the synthesis protocol, 2 μL of the mother solution (light yellow, yellow, or brown, based on the concentration of silver nitrate) was dropped onto the carbon-coated copper grid, aluminum wafer, or silicon wafer for analysis. The droplet was left to dry completely under the illumination of incandescent lamp for ca. 12 min before being analyzed. All the wafers were treated in sequence with acetone, ethanol, and pure water before the experiment. For determination of the time-dependent growth of silver dendrites, the mother solution (initial concentration was 0.3 mM) was dropped onto three different aluminum wafers. Then these droplets were left to dry partly under the illumination of an incandescent lamp for 2, 4, and 8 min, and the residual liquid was immediately sucked out with filter paper.

Adsorption of RhB on Silver Dendrites for SERS Measurement. For SERS measurements, the silver dendrites formed on the aluminum wafer were incubated in 50 nM RhB aqueous solution (10 mL) for 10 h. Subsequently, the substrate was rinsed thoroughly with distilled water and finally dried in a dark atmosphere at room temperature for the test.40 Characterization. The products were characterized with a D/max 2500, Rigaku, X-ray diffraction (XRD) instrument. The X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220iXL electron spectrometer from VG Scientific using 300 W Al KR radiation. The scanning electron microscopy (SEM) analysis was performed on a Hitachi model S-4300 field-emission scanning electron microscope (FESEM). The transmission electron microscopy (TEM) analysis was performed on a JEOL JEM-2100 transmission electron microscope with an accelerating voltage of 200 kV. Raman spectra were obtained with a Renishaw Raman system model 1000 spectrometer. The 632.8-nm radiation from a 20-mW air-cooled argon ion laser was used as the exciting source. The laser diameter was 1 μm, and the laser power at the sample position was 4.0 mW. The data acquisition time was 10 s.

’ RESULTS AND DISCUSSION Complex Ag Dendrites Formed on the Aluminum Wafer. Al wafer is first emphasized as a substrate. Analogies and differences observed in comparing our results with literature reports upon plain galvanic replacement on Al or galvanic displacement carried out in the presence of various amounts of the oxidized form of PW12 are relegated to the Supporting Information section (Figures 1SI to 3SI and corresponding comments). In the present experiments, the fractal structure obtained with 0.03 mM AgNO3, roughly in 10 min, will be shown and compared with other synthesis modes in due course in the following. Lessons learned from the preceding series of experiments prompted us to use the protocols of the work described hereafter. Actually, well-behaved dendrites representative of the present work were obtained by dropping, on the Al wafer, a mother solution nominally 0.3 mM in POM and 0.3 mM in Ag+, with the following real composition: Ag+ (0.035 mM), Ag@POM (0.265 mM), POM (0.035 mM), where Ag@POM designates Ag NPs encapsulated by the POM. Figure 1 shows the FESEM images of these silver dendrites (Due to the limitation of the 3425

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Figure 2. FESEM and TEM images of Ag dendrites formed on the carbon-coated copper grid. (a) FESEM image of large scaled fractal-like shape structures. (b) Magnified FESEM image of one Ag dendrite. (c) TEM image of Ag dendrite. (d) Magnified TEM image of Ag dendrite (total [POM] = 0.3 mM, total [Ag+] = 0.3 mM).

Figure 3. FESEM images of silver nanostructures formed on the aluminum wafer. (a) Platelike silver dendrites ([Ag+] = 0.03 mM). (b) Irregular ramous silver nanostructures ([Ag+] = 0.03 mM, [Ag NP] = 0.265 mM). (c) Fractal-like shape structures ([Ag+] = 0.03 mM, [Ag NP] = 0.265 mM, [POM] = 0.265 mM). (d) Ag@POM composite particles ([Ag NP] = 0.265 mM, [POM] = 0.265 mM).

visual field, only one dendrite was shown in this picture. In fact, the output of this structure is about 95%). Such a unique structure exhibits some large platelike structures in the bottom (round area labeled A) and fractal-like shaped structures in tip of the object (area labeled C). The intermediate structures which contain both the small platelike structure and fractal-like structure of unsymmetrical dendrites were observed in the center of the stems (annular area labeled B). The branches (∼95%) of these dendrites grow from one side of the stems, with most of the sub-branches (∼80%) being symmetrical, which can be clearly observed in Figure 1b. The stems of the silver dendrites are tortuous; there are lots of randomly ramified branches, with part of them (5%) being bare. The diameters of the stems and branches are ∼100 200 nm. The lengths of the stems and branches are ∼40 50 μm and ∼2 4 μm, respectively. Figure 1c is a magnified image corresponding to the rectangular area of Figure 1b. The silver NPs were aggregated in the tips of the dendrite. The diameter of the silver NPs is ∼50 100 nm. Figure 1d is a magnified image of the platelike structures in the bottom of the dendrite, with a rather smooth surface, which might be compared with the nanoplate silver mesocrystals described previously.31 The XRD pattern recorded from these complex silver dendrites clearly shows the (111), (200), (220), and (311) Bragg reflections of face-centered cubic (fcc) silver (see Figure 4SIA). XPS analysis in Figure 5SI corroborates the preceding results: the 3d3/2 and 3d5/2 contributions of Ag are cleanly observed. With the charge effect corrected by fixing the photoelectric peak 1s of carbon at 284.8 eV, the 3d5/2 level is located at 368.2 ( 0.3 eV and the 3d3/2 level at 374.2 ( 0.3 eV. These values prove unambiguously that silver is present only in the metallic form. EDX data for Figure 1 confirm the presence of both Ag and W (Figure 6SI). Substrate Dependence of Ag Dendrites. When the same mother solution (Ag+ (0.035 mM), Ag@POM (0.265 mM), POM (0.035 mM)) was dropped onto a copper grid, different morphologies were obtained. Figure 2 shows the FESEM and

TEM images of silver dendrites that formed on a carbon-coated copper grid. Numerous fractal-like structures of silver dendrites were found in a large scale area (Figure 2a). From the magnified FESEM image in Figure 2b, the diameter of the stems and branches is ∼25 30 nm, and the length of the stems is ∼500 nm to 1.5 μm, while the length of the branches is ∼100 250 nm, which is much smaller than those observed on the aluminum wafer. The angles between the stems and the branches vary in a wide range, between 20 and 90°. The corresponding TEM images of the dendrite are shown in Figure 2c and d. The dendrite is clearly observed to be constituted by aggregation of silver NPs of several tens of nanometer in size. Also observed in the solution are NPs not yet aggregated. When the same mother solution was dropped onto the silicon wafer, only NPs were observed (see Figure 7SI). Altogether, these results indicate that the substrate is very important for the growth of the observed unique Ag hierarchical structure. It also reminds us that if we observe a species which may react with copper element during the TEM analysis, other means of characterization are necessary for crosschecking.41 In this issue, the role of the POM in our experiments should be questioned. However, when Ag+ and POM were removed from the mother solution after centrifugation and washing, only Ag NPs that formed during the reaction could be observed, no matter what substrate was used (see Figure 8SI). The presence of a small amount of Ag+ in the mother solution is further clarified as follows. Due to incomplete photoreduction of the POM as shown by quantitative analysis (roughly 90% reduction), superfluous silver ions exist in the final mother solution, containing normally 0.3 mM POM and 0.3 mM AgNO3. When these Ag+ ions were contacted with substrates with lower reduction potentials, such as copper and aluminum, the galvanic displacement will occur to reduce Ag+ to Ag0. This reaction constitutes the key progress in the formation of the big Ag dendrite, while the presence of the POM still needs further consideration. To explore the growth mechanism, we further investigate the influence of components in the mother solution. 3426



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Figure 4. FESEM images of different silver nanostructures formed with different γ. [POM] = 0.3 mM. (a) Ag@POM NPs dispersed on the substrate, γ = 0.25. (b) Complex and dense silver dendrites, γ = 2. (c) Dense bulk of Ag, γ = 3.

Influence of the Components in the Mother Solution. First, a pure solution of AgNO3 with the concentration of 0.03 mM was dropped onto the aluminum wafer and dried as previously indicated. The typical FESEM image of the formed dendritic structures is illustrated in Figure 3a. In general, platelike dendrites with the dimension of about 4 8 μm were found. The XRD pattern recorded from these silver dendrites also shows the (111), (200), (220), and (311) Bragg reflections of face-centered cubic (fcc) silver (see Figure 4SIB), and XPS analysis corroborates the preceding results (see Figure 9SI). EDX data for Figure 3a confirm both the presence of Ag and the absence of W (Figure 10SI). Second, another pure solution of AgNO3 was mixed with the colloidal solution of Ag NPs. The concentration of AgNO3 was kept at 0.03 mM, and the total concentration of Ag was kept at 0.3 mM. Then, the mixed solution was dropped onto the aluminum wafer and dried under the same conditions. The typical FESEM image of the formed silver nanostructures is illustrated in Figure 3b. It is observed that irregular ramus silver microstructures with dimension of ∼2 3 μm were formed, with a tendency to generate fractal-like structures. Besides, there are also many Ag NPs with the diameter of ∼50 100 nm observed around the structure. Both XPS analysis and EDX data for Figure 3b indicate the presence of Ag and the absence of W (Figure 11SI). For further comparison, the POM was added to the above mixture of Ag+ and Ag NPs. The concentration of the POM was kept at 0.3 mM. Then, the mixed solution was dropped onto the aluminum wafer and dried as previously indicated. It is observed that well-behaved fractal-like shape structures were formed, as shown in Figure 3c. The dimension of the dendrite is from ∼6 to 10 μm, which is larger than that of the former dendrites that formed without POM. EDX data for Figure 3c confirm the presence of both Ag and W (Figure 12SI). The shape of the formed dendrite is very close to that observed in Figure 1. The last comparison was made by mixing the POM with pure Ag NPs at the same concentrations. Only large particles with the diameter of ∼1 2 μm were observed, and no dendrite was found on the same substrate (Figure 3d). EDX analysis of these particles shows the presence of both Ag and W (Figure 13SI). Altogether, these results unambiguously prove that both free Ag+ and the POM in the mother solution play key roles in the formation of large dendrites on the substrate. Influence of the Excess Parameter [Ag+]/[POM]. The excess parameter is defined as γ = [Ag+]/[POM]. Figure 4 shows the different silver nanostructures formed for different γ values. For γ = 0.25, numerous NPs were obtained on the substrate (Figure 4a). In this case, since the reduced POM is in large excess, there is no free Ag+ in the solution, and Ag@POM clusters are quantitatively formed. When γ is increased to 1, typical dendrites were obtained as described above. When γ was further

increased to 2, complex and dense silver dendrites with many branches overlapping each other were obtained (Figure 4b). The top left inset of Figure 4b shows the dense platelike structures in the center, and the bottom left inset of Figure 4b shows dense fractal-like shape structures in the tip. When γ reaches 3, the dense bulk of Ag is observed (Figure 4c). Close observation of the bulk indicates it to be composed of a large number of Ag NPs that aggregated together (shown in the inset of Figure 4c). This is believed to be mainly related to the drastic decrease of the template effect of the POM in conjunction with an acceleration of reaction kinetics with the increase in γ. Thus, the excess Ag+ can react with the substrate and the formed Ag NPs can further aggregate with those Ag NPs that are in the mother solution to form the bulk of Ag. Influence of the Initial Concentrations. A series of experiments were performed by changing the initial concentration at a given molar ratio. Keeping γ = 1, the initial concentration of the POM was varied from 0.2 to 1 mM. The obtained dendrites at C0 = 0.3 mM were already shown above, which provides a dendritic structure with multilevel growth. For C0 = 0.2 mM, a similar dendritic structure (Figure 5a) as shown in Figure 1a was obtained, but the size of the dendrite is smaller. When C0 was increased to 0.4 mM, much thicker and more dense silver dendrites (Figure 5b) were obtained. These dendrites were very different from those obtained at lower C0. No NPs that aggregated on the dendrites were observed, indicating the good crystallinity of the dendrite. The length of the stems is ∼5 10 μm, while the branches and subbranches are ∼1 3 μm and ∼200 500 nm, respectively. Many sub-branches flanked the branches asymmetrically by an angle of ∼60°, and they become shorter and shorter from the bottom to the tip of the branch. We believe that the above observation implies a monomer-concentration driven anisotropic crystal growth. Further increasing C0 to 1.0 mM, the densely packed platelike silver nanostructures reminiscent of nanoplate silver mesocrystals described previously31 were obtained (Figure 5c). It is observed that the structures show more and more platelike morphologies with the increase in initial concentrations. This may be explained by the kinetic control of the mechanism. It indicates that directional aggregation and oriented attachment are both important in the formation of these types of structures. A diffusion process may also take place through the aggregation of Ag NPs on the substrates accompanying the evaporation of solvent, which was proposed by many previous works.8,41,42 With the increase of initial concentrations, there are more Ag NPs and free Ag+ in the mother solution. The reaction velocity of the galvanic displacement may increase, and the aggregation of Ag NPs may be accelerated. Thus, more platelike structures will be formed due to the faster fusion of Ag NPs under the same conditions. 3427



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Figure 5. FESEM images of different silver nanostructures formed on the aluminum wafer at different initial concentrations. γ was defined as 1. (a) Silver dendrites with multilevel growth (C0 was 0.2 mM). (b) Platelike silver dendrites (C0 was 0.4 mM). (c) Dense packed platelike silver nanostructures (C0 was 1.0 mM).

Figure 6. Schematic picture of the growth process of the silver dendrites.

Growth Mechanism. Altogether, the aforementioned results can be used to outline the dendrite growth mechanism sketched in Figure 6. First, the galvanic displacement reaction between Ag+ and the aluminum wafer (or the carbon-coated copper grid) constitutes the key prerequisite to initiate and induce the dendrite deposition process. That the dendritic shape finally involves the arrangement of primary Ag nanoparticles is demonstrated by Figure 1c. Schematically, Ag NPs come into being ramus, followed by more ramus from self-assembled particles; finally, small Ag dendrites are formed. The earlier formed small dendrites serve as seeds for further aggregation of Ag NPs. First, it is necessary that nonequilibrium conditions prevail for dendrite deposition. Second, due to the large electrical potential difference between the oxidation and reduction pairs in our experiments, (ΔEAl/Ag = 2.46 V and ΔECu/Ag = 0.46 V), the galvanic replacement process is anticipated to proceed very fast. However, it is known that fast reaction often causes uneven deposition and particles aggregation, which is unfavorable for anisotropic growth of Ag nanostructures. Thus, the anionic POM should modulate the reaction kinetics and also serve as a kind of soft template, maintaining the anisotropic conditions and directing the Ag NPs in the solution to the seeds. It is noteworthy that POMs are reported to selectively adsorb on the surface of Ag NPs,11 thus favoring the formation of dendrites. The uneven distribution of

POMs on the Ag NPs thus created distinctive local environments for the regioselective growth, which was also proposed for the growth of 3D dendritic gold nanostructures by Chen et al.43 Alternative explanations for the later dendritic growth stage may involve the linear aggregation of nanoparticles, which could be caused by either dipole-induced linear aggregation or electrostatic charge repulsion governed aggregation.43 45 It may be more likely in this paper that the epitaxial growth of the stems and branches followed the electrostatic charge repulsion governed aggregation because of the negatively charged Ag NPs surrounded by an anionic POM.11 We supposed that the electrostatic repulsion experienced by a NP during end-on attachment to a nanochain is weaker than that of a NP attaching to the sides, which was also proposed for the linear assembly of gold nanospheres.44,45 Time-dependent experiments were carried out, and the structures that formed at different reaction times further clarify our assumption on the growth mechanism. As shown in Figure 7a, at about 2 min from the growth beginning, small dendritic structures with the size of 30 40 μm were observed. The dendritic structure is incompact, and the center of the dendrite is composed of many Ag NPs (shown in the inset of Figure 7a). When the growth time is increased (to 4 min), it is observed that the density of branches and sub-branches increases (shown in Figure 7b). The size of the dendrite reaches 40 50 μm, and there are some leaflike structures growing in the center of the dendrite (shown in the inset of Figure 7b). When the growth time increased to 8 min, the size of the dendrite reached 50 60 μm (Figure 7c). Dendrites with much more dense stems and branches are observed. And in the center of the dendrite, the leaflike structures also grow more dense (shown in the inset of Figure 7c). Finally, when the growth is finished at 12 min, we observe much bigger and more complex silver dendrites with much more substems and branches, as shown in Figure 1a. Finally, relying on the experimental observations of the present work, we speculate that the qualitative combination of the diffusion-limited aggregation (DLA)46 mechanism to different extents might be reasonably accurate for describing the growth mechanism of the dendritic structures. Seeds of initial structures are first formed by the galvanic displacement of Ag+ with the substrate. Cluster formation occurs by sticking of particles together with a random path to a selected seed in contact, allowing the particles to form a growing structure, followed by the formation of 3D platelike structures in the center. The POM provides significant control over the nucleation and 3428



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Figure 7. FESEM images of the silver dendrites with different growth times (t): (a) t = 2 min; (b) t = 4 min; (c) t = 8 min. The insets show the central part of the structures, respectively (total [POM] = 0.3 mM; total [Ag+] = 0.3 mM).

Figure 8. (a) Optical micrograph of the silver dendrites. ([POM] = 0.3 mM, [Ag+] = 0.3 mM). (b) SERS spectra of RhB adsorbed on the part of the silver dendrites labeled A (line A), the part of the silver dendrites labeled B (line B) as shown in Figure 8a, and the blank aluminum wafer (line C).

directional aggregation, which gives rise to a nonequilibrium system, thereby favoring the formation of silver dendrites. Dendritic growth occurs at the tips and stems of branches. As the stem grows in length, new shorter branches are formed continuously at the tips. The POM selectively adheres to different crystallographic planes on the surfaces of NPs to enable anisotropic crystal growth. During the growth process, both the aggregation-based mechanism and the Ostwald ripening coexist to assist the formation of the complex dendritic structures with multilevel crystallization.31 It should be noted that the fractal structure was confined to the substrate surface in 2D and did not grow upward to form a half-spherical dendritic structure as reported in other literature.43,47 One possible reason is that the simultaneous galvanic exchange may have played a role. The Ag0 formed by galvanic exchange during the process served as a binder sticking/fusing the Ag NPs together. Another possible reason may be the electrostatic charge repulsion of the POMs. SERS Analysis. Silver dendrites are known to provide a remarkable substrate for SERS because of their unique structures. In the present case, our concern is to check the effectiveness of this process with Ag dendrites synthesized in the presence of PW12. SERS spectras of RhB adsorbed on different parts of the silver dendrites are shown in Figure 8. In the spectra shown in Figure 8b, lines A and B represent the spectra of RhB (50 nM) that was adsorbed on the tip and center of the silver dendrites, respectively (domains labeled A and B in Figure 8a). The spectrum C is that of pure RhB obtained by drying a solution of RhB (0.2 mM) on a flat aluminum wafer.

A remarkable enhancement in the SERS spectrum of RhB adsorbed on the fractal-like shape structures is observed, compared with those adsorbed on the platelike structures. In addition, there is only a weak SERS signal for pure RhB adsorbed on the flat aluminum wafer. The strong peaks at about 1610, 1569, 1483, 1443, and 1371 cm 1 are assigned to the aromatic C C stretching modes, and the band centered at about 1261 cm 1 is assigned to the C C bridge-bands stretching, while the bands centered at about 1187 and 1129 cm 1 are assigned to the C H in-plane bending of the aromatic ring moiety of the molecule.5 The most important reason for the difference may be related to the corresponding morphological change of the underlying Ag dendrites.5 Many pieces of research48 50 demonstrated that dendritic nanostructures consist of numerous nanostructures whose distance is located in the range of effective plasma resonance, resulting in the amplification of the electromagnetic (EM) field and enhancement of the effective rate of polarization for adsorbed molecules. As was indicated above, there are many nanoparticles with diameters in the range ∼50 100 nm, aggregating in the fractal-like shape structures, and that is the suitable size for the EM enhancement. So the enhancement is mainly attributed to the EM mechanism. In addition, the fractal-like shape structures have a very rough surface and a large surface area compared to those of the platelike structures. All these properties are beneficial to SERS observation. It should be noted that the Raman enhancement effect of the current Ag dendrite looks much stronger than those of the reported cases.5 3429



’ CONCLUSIONS In summary, we have reported the size- and shape-controlled synthesis of complex silver dendrites based on a galvanic displacement reaction assisted by the presence of a POM. These structures can be tuned by using different SICs and different molar ratios of metallic salt and POM. The POMs were first used as a reductant but also serve as a stabilizer, soft template, and shape controller. Both the galvanic displacement and the presence of the POM (in large excess) are very important for the synthesis of the complex silver dendrites described in this work. At last, we have investigated the SERS of the complex silver dendrites, using RhB as the analyte. The results show that such complex silver dendrites supported on an aluminum surface are suitable for the Raman lines enhancement and can be widely exploited for Raman analysis and imaging. The study of different morphologies of silver nanostructures on the different substrates emphasizes the need for caution while interpreting TEM data in the absence of confirmatory SEM or other related assistant imaging data. It is expected that the present experimental method can be extended to other noble metals to investigate the growth behavior of nanocrystals. Moreover, dendrites could enhance the efficiency and stability of POM-based modified electrodes. Further study of the electrochemistry and electrocatalytic properties of silver dendrites is in progress. ’ ASSOCIATED CONTENT

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Supporting Information. FESEM characterization of structures observed on an Al wafer during a galvanic displacement reaction with AgNO3 in the presence of the polyoxometalate [PW12O40]3 (PW12), XRD, XPS, and EDX analysis of the dendrites formed under different conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone/fax: 86-10-62528935. E-mail address: zhanggj@ home.ipe.ac.cn. *Phone/fax: 86-10-82616517. E-mail address: [email protected]. *E-mail address: [email protected].

’ ACKNOWLEDGMENT This work was supported by the One Hundred Talent Program of the institute of process engineering, Chinese Academy of Sciences, National Natural Science Foundation of China (No. 21071146, 51002155, 20733006), the CNRS (UMR 8000), and the Universite Paris-Sud 11, France. ’ REFERENCES (1) Gould, I. R.; Lenhard, J. R.; Muenter, A. A.; Godleski, S. A.; Farid, S. J. Am. Chem. Soc. 2000, 122, 11934–11943. (2) Drozdowicz-Tomsia, K.; Xie, F.; Goldys, E. M. J. Phys. Chem. C 2010, 114, 1562–1569. (3) Shanmugam, S.; Viswanathan, B.; Varadarajan, T. K. Nanoscale Res. Lett. 2007, 2, 175–183. (4) Rashid, M. H.; Mandal, T. K. J. Phys. Chem. C 2007, 111, 16750–16760. (5) Huang, J. F.; Vongehr, S.; Tang, S. C.; Lu, H. M.; Shen, J. C.; Meng, X. K. Langmuir 2009, 25 (19), 11890–11896.

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