Fabrication of Flower-Like Silver Structures through Anisotropic Growth

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Fabrication of Flower-Like Silver Structures through Anisotropic Growth Tao Liu,† Dongsheng Li,†,* Deren Yang,† and Minhua Jiang‡ †

State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China ‡ State Key Laboratory of Crystal Growth, Shandong University, Jinan, People’s Republic of China ABSTRACT: Using a simple chemical reaction, a new nanostructure of silver, which we call a “flower-like silver structure”, is produced. The flower-like silver structure consists of a silver core and many rod-like tips protruding out in three dimensions. Besides common face-centeredcubic (FCC) phase of silver, there exists hexagonal-close-packed (HCP) phase in these tips. The appearance of HCP silver is the result of rapid growth of silver nuclei when using CH2O or C2H4O as the reducing agent. The formation of the rod-like tips is caused by the anisotropic growth determined by the HCP phase and the directing role of formic acid, which is the oxidation product of CH2O. It is also found that the concentration of reactants, the kind of reducing agents and the sequence of adding reactants can influence the morphology and phase constitution of the final products.

1. INTRODUCTION Noble metal nanostructures have attracted extensive attention due to their fascinating physical, chemical properties and their relevant applications in optics,1 catalysis,2 conducting paste,3 surface-enhanced Raman scattering (SERS),4 and sensing.5 All these applications are influenced by the size, shape and morphology of metal nanostructures.6,7 Therefore, the control of the morphology is a primary focus. Until now, Au spheres,8 Ag plates,9 Pd cubes,10 and Ag rods,11 have been generated by a variety of methods. Except the structures with simple shapes mentioned above, a class of sophisticated flower-like or star-shaped structures have been synthesized recently, such as gold star,12 nanoflowers (Pd, Pt, and Au),13 starfish-like rhodium nanocrystals,14 and so on. These structures generally consist of a central metal sphere and several sharp tips branching out in three dimensions, which are capable of focusing electromagnetic field of excitation light at their ends to realize extremely large field enhancement for SERS.15,16 Moreover, the surface roughness and possible highindex planes of star-shaped structures may have special applications in catalysis due to much higher activity.17 Despite these fascinating properties, synthesis of noble metal nanoflowers with a high degree of structural anisotropy is still a great challenge. During the last decades, several synthetic methods have been reported, including seed-mediated growth in N,Ndimethylformamide (DMF) for gold star,12 Ostwald ripening for starfish-like rhodium nanocrystals,14 pH-dependent self-assembly of sodium N-(4-n-dodecyloxybenzoyl)-l-isoleucinate (SDBIL) for nanoflowers of Pd and Pt star,13 and so on. Comparing with flower-like structures of Au, Pd, Pt, and Rh, there were fewer reports about Ag nanoflowers. Even though there are some reports about branched silver meso/nano structures. For example, r 2011 American Chemical Society

L. Hong et al. prepared silver microparticles constructed of mutually intersecting 2D nanoflakes using electrochemical approach.18 Tang et al. also prepared silver microstructure composed of nanopetals using AA and citrate acid, and they considered that the particles formed through a film-fold process.19 Jena et al. prepared branched Ag nanoflowers with the size of 40
60 nm using rutin as the reducing agent. They investigated a slow growth process of spherical particles to branched silver particles.20 Here, a simple method is proposed to prepare flower-like silver structures. This new strategy comprises an anisotropic growth process caused by the metastable HCP phase and the absorption of formic acid, which results in many rod-like tips protruding out in three dimensions from a silver core. Different with the former strategies, the reaction proceeds without the limitation of the designated capping agents. Besides, the method proposed here is very simple and fast (less than 10 min).

2. EXPERIMENTAL DETAILS 2.1. Materials. 37% CH2O, 28% NH3 3 H2O and 40% C2H4O aqueous solution was supplied by Sino-pharm Chemical Reagent Co. Ltd. Polyvinylpyrrolidone (PVP, k30), AgNO3, glucose, and NaBH4 with analytical pure grade were supplied by Sino-pharm Chemical Reagent Co. Ltd. Rhodamine 6G (98%, R6G) was obtained from Sigma company. Home-made deionized water was used. 2.2. Experiment Details. In a typical synthetic procedure, 0.2 mL aqueous solution of 37% CH2O was added into 200 mL 0.25 mM AgNO3 aqueous solution, then 0.4 mL 28% NH3 3 H2O was injected Received: December 17, 2010 Revised: April 6, 2011 Published: April 18, 2011 6211

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Figure 1. SEM images of the flower-like silver structures with low magnification (a, b) and higher magnification (c, d). quickly to the mixture under stirring. The reaction was carried at 45 °C. The mixture turned gray in a few seconds upon the addition of ammonia, indicating the formation of flower-like silver structures due to the reduction of AgNO3 by CH2O. After 30 s, 20 mL 5% PVP aqueous solution was last added to stabilize the silver particles. The product was collected by centrifugation at 4000 r/min. 2.3. Characterization. The morphology and crystalline structure of the samples was examined by a transmission electron microscope (TEM, Philips CM200), a high resolution transmission electron microscope (HRTEM, Philips-FEI Tecnai G2 F30 S-Twin) and a scanning electron microscope (SEM, Hitachi S-4800). Elemental constitution was analyzed by the energy dispersive spectrometer (EDS) equipped in the HRTEM system. The phase constitution of the samples was characterized by X-ray diffraction (XRD) using an X’Pert PRO X-ray diffractmeter equipped with the graphite monochromatized Cu KR radiation. The extinction spectra of the samples were measured on an UV
vis spectrophotometer (Hitachi U-4100).

3. RESULTS AND DISCUSSION Reaction Process. The flower-like silver structures were produced by a one-step reaction. The reduction ability of CH2O depends on the pH value of the solution. If only formaldehyde was added, the reduction rate would be too slow due to low pH.21 When the ammonia was added, the pH value of the solution was raised, so Agþ can be reduced by CH2O. The other reaction parameters including the reaction temperatures and the concentration of reactants are adjusted to make the redox reaction completed in a few seconds. A similar reaction has been reported to synthesize spherical silver particles.22 However, capping agents like PVP are generally dissolved before the start of the reaction to avoid aggregations. Here, in order to guarantee the free growth of silver particles, PVP was last added, but it still can stabilize the silver particles from our observation. If PVP was not added, the silver particles would aggregate and precipitate onto the bottom of the beaker. It must be pointed out that the reaction discussed here is similar to the Tollens reaction. But there are several differences.

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Figure 2. EDS (a) and XRD (b) spectrum of the flower-like silver structures.

First, the reaction here is much faster than Tollens reaction. Excessive amount of NH3 3 H2O and CH2O is used here and the redox reaction is completed within several seconds, while the Tollens reaction often takes several minutes or longer time. Second, the ammonia was added last. Third, the amount of ammonia is larger than the Tollens reaction. It makes the pH value of the solution raised quickly, which will raise the reaction rate. Fourth, there is stirring, which make the transportation of the reactants faster to raise the growth rate of the silver structures. Stirring also make silver particles grow freely in three dimensions, while in the silver mirror reaction, silver particles grow on the walls of the container. Morphology and Structure Characterizations. Compared with spherical particles, silver structures with a drastically different morphology were obtained in our experiments. Figure 1 shows the SEM images of the samples. The low-magnification SEM images in Figure 1a and 1b reveal that the sample is composed of flower-like silver structures in high yield, though the whole size distribution is a little broad. The SEM image with higher magnification in Figure 1c shows a large flower-like particle, whose rod-like tips are about 200 nm in length and 80
100 nm in diameter. Figure 1d shows a SEM image of several smaller flower-like particles with shorter tips, whose length is about 100 nm or less. All the SEM images show that in each flower-like particle, many (10
20) rod-like tips protrude in three dimensions from one core. The elemental constitution of the sample is analyzed by an EDS spectrum shown in Figure 2a. First, the elemental constitution of the sample is analyzed by an EDS spectrum shown in Figure 2a, from which only peaks of silver element are observed. The peaks of Cu and C elements come from the Cu grid used for TEM characterization. Thus, the sample is composed of silver crystals only and no other compounds exist. Figure 2b displays the XRD pattern of the sample. Four FCC peaks at 38°, 44°, 64°, and 77° correspond to (111), (200), (220), and (311) planes of 6212

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Figure 3. TEM images with (a) low and (b) high magnification. (c and d) HRTEM images of the rod-like tips.

silver with face-centered-cubic (FCC) phase, respectively. Besides, there are two obvious peaks at 36° and 40°, which are indexed to the (0004) and (1102) planes of HCP silver. That means that both the HCP phase and FCC phase exist in the flower-like structures. Other XRD peaks of HCP phase are not obvious due to the low concentration of HCP phase. The similar phenomenon has been reported in rice-shaped23 and plate-belt heteronanostructures.24 The crystal structure of the sample was further analyzed by the TEM images of the flower-like particles in Figure 3. Figure 3a shows several particles, and Figure 3b show a single particle. From Figure 3b, we can observe that the diameters decrease at the end of the rod-like tips. The HR-TEM images of two different rod-like tips are shown in Figure 3, parts c and d. The lattice spacing of each group of the parallel fringes in Figure 3c is 0.243 nm, corresponding to the (1010) plane of HCP silver, while the crystal spacing shown in Figure 3d is 0.237 nm, corresponding to the (111) plane of FCC silver. Therefore, not all the tips have the HCP structure. SAED (selected area electron diffraction) patterns of two tips (a and b) shown in Figure 4a are shown in Figure 4b and 4c, respectively. In Figure 4b, the two spots near the transimission spot correspond to crystal planes with spacing of 0.243 and 0.166 nm. In addtion, the angle between the reciprocal vectors of the two spots is 103°. According to the crystal spacings and the angle, the two spots can be assigned to (0006), (1011) of HCP Ag. CBED (convergent beam electron diffraction) patterns of regions 1, 2, and 3 of tip a are shown in Figure 4d
f. It can be seen that the three regions have the same diffraction pattern. We infer that tip a has the HCP structure. In Figure 4c, the two spots near the transimission spot correspond to crystal planes with spacing of 0.236 and 0.144 nm. And the angle between the reciprocal vectors of the two spots is 90°. The two spots can be assigned to (111), (202) of FCC Ag. CBED of regions 4 and 5 of tip b are shown in Figure 4, parts g and h. The two regions have the same diffraction pattern. So tip b has the FCC structure. From the above discussion, we can infer

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that only some rod-like tips have the HCP structure, and other tips have the FCC structure. This can explain the reason why the intensity of the HCP peaks in the XRD spectrum shown in Figure 2b is not very high. Though HCP phase is unstable for the bulk silver, but it can exist in nanocrystals. It has been reported that HCP phase silver has lower surface energy than FCC phase silver in small crystals. So at the step of forming small silver nuclei, HCP phase forms to decrease the large surface energy. Particles with HCP phase easily grow into rods due to the highly anisotropic bonding in crystal structures, such as ZnO, Se, and CdSe and so on.25Thus, the silver crystals of HCP phase will probably develop into silver rods. The mechanism is much different from the templatedirected synthesis. As gold and silver have a highly symmetric FCC structure and usually tend to form spherical shape to reduce their surface energy. In most of the synthetic strategies of Au or Ag rods in solution, surfactants are used to direct the anisotropic growth of seed crystals by selective absorption on different crystal planes. One typical example is the preparation of gold nanorods using CTAB as the soft template.26 Growth Mechanism and Influence of Reaction Conditions. It was further found that the reaction conditions such as the reactant concentration, the kind of the reducing agent and the sequence of adding reactants would influence the morphology and phase constitution of the final products. When the concentration of AgNO3 was raised to 1 mM, and the amount of CH2O and NH3 3 H2O was also raised by 4 folds. Flower-like silver structures were also obtained, as shown in Figure 5. Parts a and b of Figure 5 give the low-magnification SEM, and Figure 5c shows a single flower-like silver particle with high magnification. As shown in Figure 5c, comparing with the sample obtained with 0.25 mM AgNO3, there are more silver rods (several tens) with smaller diameter (about 40 nm) on one core. From the XRD pattern shown in Figure 5d, stronger HCP peaks are observed, which suggests the existing of larger amount of HCP silver. According to the basic kinetic theory of chemical reactions, the increase of the reactant concentration will raise the formation rate of Ago monomers and the corresponding rates for nucleation and growth. High monomer concentration is favorable for the formation of highly branched structures.27 From this reaction, we can obtain more flower-like silver structures by simply raising the concentrations of the reactants. The sequence of adding reactants plays an important role in the morphology and phase structure of the silver particles. When ammonia was added before CH2O, many spherical particles appeared, as shown in Figure 6a. Though there are a few sharp tips, flower-like structures are hard to be found. Less number of tips may be caused by the slower reaction rate due to the complexation of NH3 with Agþ.28 However, HCP phase still exists. As shown in Figure 6c, except for four FCC peaks, two HCP peaks appear in the XRD spectra. It suggests that fast reaction is not very important for the appearance of HCP phase, but very essential for the growth of rod-like tips. When PVP was added before the start of the reaction, only spherical particles shown in Figure 6b were obtained. Its XRD pattern in Figure 6c shows that the product has a pure FCC phase. The adsorption of PVP molecules onto the surface of silver particles will reduce the surface energy. This suggests that the appearance of HCP phase is a result of decreasing surface energy. Thus, the free growth of silver particles without the limitation of capping agents is a key factor of obtaining HCP phase. 6213

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Figure 4. (a) TEM image of a flower-like silver particle. (b) SAED pattern of tip a shown in part a, (c) SAED pattern of tip b shown in part a, and (d
h) CBED patterns of the spots 1
5 shown in part a, respectively.

Figure 5. SEM images of the samples prepared with 1 mM AgNO3 taken at low (a) and high magnifications (b, c). (d) XRD of the sample shown in part a.

The kind of the reducing agent is crucial for the formation of flower-like silver structures. Compared with other reducing agents, CH2O has moderate reductive activity. And the oxidation product of CH2O is formic acid (CH2O2), which can be easily removed from the product through centrifugation without introducing any impurities. More importantly, from our observation, CH2O is a special reduction agent here as the use of CH2O results in the appearance of HCP phase. In control experiments, glucose—a weaker reducing agent and NaBH4—a stronger reducing agent were also used as reducing agents. When glucose was used, the reaction temperature was raised to 80 °C as the reaction proceeds very slow. Then quasispherical particles of 40
60 nm in diameter were obtained, as shown in Figure 7a. When NaBH4 was used, aggregations of small silver particles shown in Figure 7b were obtained.

Obviously, the silver particles produced by NaBH4 were unstable. They aggregated quickly before the addition of PVP solution. On the other hand, particles obtained by the reduction of NaBH4 and glucose have a pure FCC phase and no HCP peaks appear in the XRD spectra shown in Figure 7c. Thus, the reduction ability of the reducing agent is not essential for the appearance of HCP phase. Another reducing agent, acetaldehyde (C2H4O), which also has an aldehyde group in its structure, was also used, while keeping other reaction parameters same (the concentration of AgNO3 was 1 mM). As shown in Figure 8a, spherical silver particles of about 500 nm with sharp tips were obtained. In the XRD pattern shown in Figure 8b, besides four FCC peaks, there are also two HCP peaks. We infer that the formation of HCP phase may correlate with the aldehyde group (
CHO) in the reducing agents. Considering glucose also has aldehyde group, the reducing ability should not be too weak to form HCP silver. The special role of aldehyde group played may correlates with carboxyl group (
COOH), the oxidation product of aldehyde group. First, carboxyl groups may be beneficial for the formation of HCP phase, as there exist small amount of silver nanoparticles with HCP phase when AgNO3 is reduced by trisodium citrate, which also has carboxyl groups.29 Second, the carboxyl group also plays as a weak stabilizer and a shape dictator. It has been reported that formic could adsorb onto the surface of platinum nanoparticles by its carboxyl group.27From our observation, the flower-like silver structures can keep disperse for a short time, and this may be caused by the adsorption of formic acid. Similarly, particles prepared with glucose in the control experiment were found to be stable before the addition of PVP due to the carboxyl group produced in the redox reaction. The adsorption of formic acid on the silver particles will also contribute much to the anisotropic growth of silver particles, similar to the role that formic acid plays in the preparation of platinum nanodendrites. As formic acid is unstable and easily decomposes, so PVP is added last to stabilize the silver structures. The morphology of the flower-like silver particles we prepared is different from the former structures,18
20 as the straight rodlike tips with high aspect ratio are different from the flexural branches. In addition, there exist HCP silver, which was not obverted in the former structures. The appearance of HCP silver 6214

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Figure 6. SEM image of the samples produced (a) by adding NH3 3 H2O before the injection of CH2O, and (b) by adding PVP first. Other reaction parameters are same with that of the sample shown in Figure 5a. (c) XRD spectra of the samples shown in parts a and b.

Figure 7. TEM images of the samples prepared with (a) glucose and (b) NaBH4. (c) XRD spectra of the samples shown in parts a and b.

Figure 9. The extinction spectra of the flower-like silver structures prepared with 0.25 mM AgNO3 (a) and 1 mM AgNO3 (b). All the spectra are normalized with respect the peak intensity of high-energy absorption band.

Figure 8. (a) SEM image of the samples prepared with C2H4O. The inset is taken at higher magnification. (b) XRD pattern of samples in part a.

is the result of free and rapid growth of silver nuclei when using CH2O or C2H4O as the reducing agent. The growth of the flower-like silver particles here can be considered as an overgrowth process.30 The rod-like tips grow from the central core very fast. The anisotropic growth process is associated with two

reasons: one is the anisotropic growth of HCP Ag determined by the intrinsic crystal dissymmetry, and the other is the directing role of HCOOH, which has been discussed in the last paragraph. Optical Properties. The optical properties of flower-like noble metal structures have attracted a lot of attention.31
33 Comparing with the extinction spectrum of Au nanospheres, there is another strong absorption band at about 700 nm in the extinction spectrum of Au nanoflowers, which is caused by the longitudinal plasmon resonance of the tips.12 Similarly, the plasmon mode of flower-like silver structures is formed by hybridization of plasmons associated with the core and the tips.31 The extinction spectra of the flower-like silver structures prepared with 0.25 mM AgNO3 (a) and 1 mM AgNO3 (b) are shown in Figure 9. Obviously, flower-like structures have an absorption peak locating at about 400 nm, which is caused by the dipolar oscillation of the core. Comparing with typical extinction 6215

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Telephone: þ86-0571-8795 1667. Fax: þ86-0571-8795 2322.

’ ACKNOWLEDGMENT The authors express thanks for the financial support from the Central Universities (Program No. 2010QNA4001), the Natural Science Foundation of Zhejiang Province (Y1080068), and Program 973 (No. 2007CB613403). Figure 10. SERS spectrum of 10
6 M R6G dropped on flower-like silver structures and normal Raman spectrum of 10
2 M and 10
4 M R6G.

’ REFERENCES spectra of small spherical silver particles, the absorption intensity increases in the 900
1300 nm region. The strong extinction in the long wavelength region may be caused by longitudinal plasmon resonance of the rod-like tips. As the flower-like silver structures prepared with 1 mM AgNO3 have more rod-like tips, the intensity of low energy absorption band become stronger with respect to the high energy band, which agrees with the discussion in the literature.12 SERS Applications. Because of the highly branched structure and many sharp ends, flower-like silver structures prepared here are supposed to have high SERS activity. Preliminary test of its SERS effect has been done. First, the flower-like particles were deposited onto a silicon substrate, and then 25 μL 10
4 M ethanol solution of R6G was dropped onto it. After the evaporation of ethanol, the Raman spectra were performed under object lens of an Olympus Photo Microscopy with the 514.5 nm laser and a PIXIS: 100BR charge-coupled device (CCD) detector. The laser power was about 1 mW. Normal Raman spectrum of 10
4 M R6G and 10
2 M R6G under the same condition was also collected. The Raman spectrum is shown in Figure 10. The Raman peaks for 10
4 M R6G without silver structures are not detectable due to the low concentration. Because of the enhancement of flower-like silver structures, the typical Raman peaks of R6G between 600 and 1600 nm are obvious,34 and the positions of these peaks coincide with the normal Raman spectrum of 10
2 M R6G. Thus, the flower-like silver structures could serve as active SERS substrates. More detailed work needs to be done to raise the enhancement effect.

4. CONCLUSION In summary, flower-like silver structures were synthesized through a fast anisotropic growth process, which was caused by the HCP phase and the adsorption of the oxidation product of CH2O. The appearance of HCP silver was realized by free growth without the limitation of the capping agents. Adsorption of PVP molecules on the surface of silver particles during the growth process will prohibit the absorption of formic acid and the formation of HCP silver. Higher reactant concentrations result in more rod-like tips with smaller size on one core. The flower-like silver structure can serve as an effective SERS substrate, and it will find more applications such as in photoluminescence enhancement and catalysis. The strategy of obtaining flower-like particles here can be extended to synthesize other metallic nanostructures.

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