Large-Scale Synthesis of Micrometer-Sized Silver Nanosheets

Feb 19, 2010 - Hongjun Chen,† Frank Simon,‡ and Alexander Eychmüller*,†. Physikalische Chemie, TU Dresden, Bergstr. 66 b, 01062 Dresden, German...
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J. Phys. Chem. C 2010, 114, 4495–4501

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Large-Scale Synthesis of Micrometer-Sized Silver Nanosheets Hongjun Chen,† Frank Simon,‡ and Alexander Eychmu¨ller*,† Physikalische Chemie, TU Dresden, Bergstr. 66 b, 01062 Dresden, Germany, and Polymer Interfaces, Leibniz-Institut fu¨r Polymerforschung Dresden e.V, Hohe Str. 6, 01069 Dresden, Germany

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ReceiVed: September 24, 2009; ReVised Manuscript ReceiVed: December 21, 2009

In this paper, we report on a facile and environmentally friendly large-scale synthesis of micrometer-sized silver nanosheets without the introduction of any seeds, polymers, surfactants, or sacrificial substrates. Through the addition of a small quantity of H2PdCl4, AgNO3 can easily be reduced by H2O2 in basic aqueous solution at room temperature and under normal pressure. The edge length of the silver nanosheets obtained may reach lateral dimensions of up to 10-15 µm in size and about 28 nm in thickness. Due to their large surface area, these micrometer-sized silver nanosheets may find potential applications as substrates for surface-enhanced Raman scattering, electrochemical surface plasmon resonance, metal-enhanced fluorescence, and scanning tunneling microscopy. Introduction The synthesis of metal nanoparticles has been intensively studied in the last couple of decades due to their unique properties being different from their bulk materials.1 The intrinsic properties of metal nanoparticles are mainly determined by their size, shape, composition, crystallinity, and structure.2 Shape-controlled synthesis has recently received considerable attention because varying the shape of metal nanoparticles allows one to fine-tune their properties over a wide range.3 Due to the sensitivity of their optical response, metal nanoparticles are of tremendous importance for applications as nanobiosensors,4 in surface-enhanced Raman scattering (SERS),5 and for the generation of nanophotonic devices.6 As silver has the highest electrical conductivity of all metals, silver nanoparticles can be used as electronical contacts.7 Moreover, silver nanoparticles are also widely used as selective oxidation catalysts for the conversions of ethylene to ethylene oxide or of methanol to formaldehyde.8 A number of methods have been developed for the synthesis of silver nanoparticles with various shapes, such as nanocubes,9 nanoplates,10 nanorods,11 nanowires,12 nanobelts,13 nanofilaments,14 and branched nanocrystals.15 In this vein, the synthesis of silver nanoplates has abruptly become interesting in the past few years.10,16,17 An overview of the current research on the synthesis and the properties of silver nanoplates has recently been published.18 Up to now, various chemical and physical procedures have been devised to fabricate silver nanoplates with smooth surfaces. However, the chemical and physical methods developed so far were mostly limited to the preparation of silver nanoplates with edge lengths less than 500 nm, mostly being around 100 nm.10,16 It is more difficult to control the crystal growth of silver in two dimensions, especially for sizes much larger than 1 µm. Compared to the wealth of procedures for the fabrication of silver nanoplates smaller than 500 nm, there are only a few works published on the synthesis of silver nanosheets much larger than 1 µm in size (here, we call nanoplates larger than 1 µm nanosheets).17 Some silver nanosheets are made through the * Corresponding author. E-mail: [email protected]. † TU Dresden. ‡ Leibniz-Institut fu¨r Polymerforschung Dresden e.V.

galvanic displacement reaction using sacrificial substrates (including n-type GaAs, Sn plates, Cu foils, and Cu grids).17a-e Some silver nanosheets are synthesized through the hydrothermal method (160 °C, 24 h; 250 °C, 10 h)17f,g or in hot organic solvents (hexane mixed with oleylamine, 80 °C, 24 h).17h In addition, Dong’s group reported a step-by-step seed-mediated growth approach (with a total of nine steps) to synthesize silver nanosheets larger than 1 µm in size (about 1073 nm).17i There are some references which report on the synthesis of silver nanosheets at room temperature through an electrochemical method17j or using very high concentrations of anionic dextran (100 g L-1).17k However, some of these methods are conducted with the assistance of various surfactants or polymers, like oleylamine,17h poly(vinyl pyrrolodione),17j poly(9-vinylcarbazole),17e or polysaccharide.17k The use of surfactants or polymers may introduce heterogeneous impurities. Furthermore, the surfactant and polymer coating layers on the silver nanosheets may interfere with the labeling of specific molecules. Although using sacrificial substrates provides silver nanosheets at room temperature without using any surfactants or polymers, these silver nanosheets are growing perpendicular to the substrates, which may limit their further applications (like electrochemical surface plasmon resonance (SPR) and scanning tunneling microscopy (STM)). The step-by-step seed-mediated growth approach may also be conducted at room temperature, but it requires the silver seeds to have first been synthesized with nine subsequent gradual growth steps to finally obtain micrometersized silver nanosheets, which, obviously, is quite a tedious and time-consuming process. Therefore, the synthesis of silver nanosheets without using any seeds, surfactants, polymers, and sacrificial substrates through a simple wet-chemical method is still a challenging task. Here, we demonstrate a mild, room temperature wet-chemical route to the large-scale synthesis of micrometer-sized silver nanosheets using H2O2 as a reducing agent. Compared to the reports referred to above, our reaction has some obvious advantages: (i) It is a facile and environmentally friendly method conducted at room temperature and under normal pressure without using any seeds or sacrificial substrates. (ii) The absence in the reaction solution of organic long-alkyl-chain surfactants or polymers provides a relatively “clean” environment to grow

10.1021/jp909206x  2010 American Chemical Society Published on Web 02/19/2010

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silver nanosheets. As a consequence, these relatively “clean” surfaces can be easily modified with different molecules for further applications. (iii) The edge lengths of the silver nanosheets obtained may reach 10-15 µm in size with a potential use as SERS, electrochemical SPR, or STM substrates. Therefore, it is still meaningful to investigate new wet-chemical methods for the preparation of micrometer-sized silver nanosheets. Experimental Section Chemicals. Silver nitrate, sodium acetate, palladous chloride, rhodamine 6G (R6G), and amoxicillin were purchased from Sigma-Aldrich. Ammonia solution (25%) was purchased from VWR. Hydrogen peroxide solution (30.62% W/V) was purchased from Fisher scientific UK limited. All reagents were used as received without further purification. The water used was purified through a Millipore system. Synthesis of Micrometer-Sized Silver Nanosheets. In a typical experiment, 10 µL of 10 mM H2PdCl4 solution was mixed with 5 mL of 0.5 M NaAc solution first and then 5 mL of 20 mM AgNO3 solution was added. After complete mixing, 50 µL of H2O2 was added into the solution. The color of the mixed solution quickly changed into gray accompanied by metallic shining. After 30 min, the products were collected at the bottom of the flask. After washing three times, the products were used for further characterization. The products for EDS and XRD characterization were further treated in 1% ammonia solution for 5 min to eliminate the AgCl produced. The products after a reaction time of 1 min (shown in Figure 8) were obtained by centrifugation, which was used to eliminate the influence of the AgCl produced, the NaAc, and unreacted AgNO3, and to quickly stop the reaction. In another experiment, we used tissue to adsorb the reaction solution and by this quickly stop the reaction, a SEM image of which is shown in Figure S1 of the Supporting Information. Instrumentation. The X-ray diffraction (XRD) pattern was collected on a Siemens D5000 X-ray diffractometer using Cu (40 kV, 200 mA) radiation. The samples for XRD were prepared by placing a drop of colloidal solution on a clean glass plate. Energy-dispersive spectroscopy (EDS) and scanning electron micrographs (SEM) were performed on a Zeiss DSM 982 Gemini instrument. UV-vis absorption spectra were recorded with a Cary 5000 spectrophotometer (Varian). X-ray photoelectron spectra (XPS) were performed on an AXIS ULTRA (Kratos Analytical, England) using Al as the exciting source. The charging calibration was performed by referring the C1s to the binding energy at 285 eV. The samples for SEM, EDX, and XPS characterization were prepared by transferring the asprepared products onto a silicon slide. SERS spectra were recorded with a Renishaw 2000 (made in UK) equipped with a HeNe laser, which gave an excitation line of 633 nm. CCD was used as the detector; the accumulation time was 10 s, and the incident power was 10 mW. The active SERS substrate was fabricated by transferring the as-prepared Ag nanosheets onto silicon slides; then, 5 µL of 10-6 R6G or 2 × 10-5 M amoxicillin solution was dipped on the silicon slides and we waited until the solvent had evaporated. Results and Discussion Synthesis and Characterization of the Micrometer-Sized Silver Nanosheets. To investigate the morphology of the asprepared samples on a large scale, the obtained samples were characterized by SEM. Parts A, B, and C of Figure 1 display typical low-, medium-, and high-magnification SEM images of the samples, respectively. As shown in Figure 1A and B, the

Figure 1. Typical low- (A), medium- (B), and high-magnification (C) SEM images of the Ag nanosheets.

sample mainly consists of a large quantity of nanosheets with irregular shape. The high-magnification image (Figure 1C) shows that most edges of these nanosheets are rough, while the edge length of these nanosheets may reach 10-15 µm and about 28 nm in thickness (inset in Figure 1C). Figure 2 shows the UV-vis absorption spectrum of the Ag nanosheets, which was obtained by dispersing the Ag nanosheets in water under ultrasonication. The spectrum exhibits a broad absorption band range from ∼320 to ∼1200 nm. The out-ofplane quadrupolar Plasmon-resonance peak for the silver nanosheets is located around 330 nm, which is basically constant and independent of the nanosheet size. While the in-plane dipole resonance peak is very sensitive to the size and the aspect ratio of the silver nanosheets, it is reported that the in-plane dipole resonance peak of the silver sheets about 1073 nm in size and about 22 nm in thickness is beyond 2400 nm.17j Therefore, the in-plane dipole resonance peak of the present silver nanosheets

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Figure 2. UV-vis absorption spectrum of the silver nanosheets dispersed in water.

Figure 4. High-resolution XPS spectra of the Ag (3d) (A), Pd (3d) (B), and Cl (2p) (C) electron regions.

Figure 3. XRD patterns (A) and EDS (B) of the Ag nanosheets.

should be in the far-IR region, which is beyond the detection limit of our spectrometers. The out-of-plane dipole and the inplane quadrupole resonance peaks, as usual,16d,e are not clearly distinguished from the UV-vis absorption spectrum most probably due to the polydispersity and the truncations of the silver nanosheets. Similarly, broad absorption features were also observed by Deng et al. using hot organic solvents for the synthesis of micrometer-sized Ag nanosheets, ascribing the broad absorption to an extended delocalization of the in-plane electrons and a significant red-shift in the SPR band.17h A typical XRD pattern of the as-prepared samples is shown in Figure 3A. One sharp and strong diffraction peak together with four weak diffraction peaks are assigned to the (111), (200), (220), (311), and (222) reflections of the face-centered cubic (fcc) structure of metallic silver, respectively. The lattice constant calculated from this XRD pattern is 4.07 Å which is consistent with the reported data (R ) 4.0862 Å, JCPDS file: 04-0783). These results confirm that the Ag nanosheets are

composed of silver with fcc crystallinity. Also mentioned is that the ratio between the intensities of the (200) and (111) diffraction peaks is much lower than the conventional value (0.02 vs 0.45). This indicates that the silver nanosheets are primarily dominated by {111} facets, and thus their {111} planes are highly oriented parallel to the surface of the supporting substrate. The composition of the as-prepared samples was analyzed by EDS, as shown in Figure 3B. The strong peaks of silver demonstrate that the samples are composed of silver, while the other peaks of Si and C may stem from the substrate and the adsorbed Ac- ions, respectively. To further characterize the Ag nanosheets, XPS was used. Figure 4 shows the high-resolution spectra of the Ag (3d), Pd (3d), and Cl (2p) electron region, respectively. In Figure 4A, the Ag 3d5/2 peak as well as the Ag 3d3/2 peak were deconvoluted into two component peaks labeled with A (BE ) 367.921 and 373.947 eV) and B (BE ) 368.262 and 374.287 eV). The binding energy of the component peak B is in agreement with that of metallic silver,19 and the binding energy of the component peak A is typical for AgCl.20 As shown in Figure 4B, the Pd 3d spectrum was also deconvoluted into two component peaks A (BE ) 336.404 and 341.677 eV) and B (BE ) 337.559 and 342.833 eV). The component peak A corresponds to Pd (0), while the slightly higher binding energy (∼0.9 eV) than that of bulk polycrystalline Pd is the result of a Pd/Ag alloyed structure.19 The binding energy of the second component peak B corresponds to PdCl2.21 The Cl (2p) peak (Figure 4C) further demonstrates the presence of Cl- ions adsorbed on the Ag nanosheets. Through normalization of the peak areas, the

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Figure 5. Typical SEM images of the Ag nanosheets using 10 µL (A) and 100 µL (B) of H2O2.

elemental ratios of [Pd]:[Ag] and [Cl]:[Ag] are calculated as 0.017 and 0.208, respectively. It should be noted that within the detection limit no Pd signals in EDS or XRD have been discerned, demonstrating that the quantity of Pd (either Pd/Ag alloy or PdCl2) is very small, which is also in agreement with the above XPS result. The influence of H2O2 on the morphology of the Ag nanosheets was investigated. As seen in Figure 5, the morphology of the Ag nanosheets formed was not changed significantly when the quantity of the H2O2 was reduced from 50 to 10 µL or increased to 100 µL. The only larger difference between these samples is that the Ag nanosheets obtained at 100 µL of H2O2 have more smooth edges than those obtained at 10 µL of H2O2. We also examined the function of the additive H2PdCl4. From the control experiment without H2PdCl4 or just using NaCl as a replacement of H2PdCl4 and under otherwise identical experimental conditions, it is seen that distinctly irregular smaller platelets are formed (Figure 6A and B). Small, irregularly shaped Ag nanosheets can still be obtained even without H2PdCl4 which may rule out that AgCl crystals formed in the solution play a role as seeds for the growth of Ag nanosheets. Compared with Figure 6A and B, it can be clearly seen that the size of the Ag nanosheets synthesized in the presence of NaCl is much larger than that of those prepared without NaCl, demonstrating the preferential adsorption of Clions to the sites of the {111} facet, a similar role as the one that the Br- ions play in CTAB,10d which obviously suppresses the growth on the {111} facet and promotes a highly anisotropic crystal growth along the [110] or [100] direction,17 and, thus, leads to the growth of much larger Ag nanosheets. If further compared with the Ag nanosheets synthesized using H2PdCl4

Figure 6. Typical SEM images of the Ag nanosheets without H2PdCl4 (A) or using NaCl instead of H2PdCl4 (B) or using NH3 · H2O instead of NaAc (C).

(cf. Figure 1) and combined with the above XPS analysis, we may infer that the Pd2+ ions have a similar function as the Clions, namely, to facilitate the Ag nanosheet growth to 10-15 µm in the edge length. In other words, the Ag nanosheets as large as 10-15 µm in size are the result of the synergistic effect of Pd2+ and Cl- ions, not that of either one alone. It is wellknown that H2PdCl4 may react with the Ag nanosheets to form a Pd/Ag alloyed structure through a galvanic replacement reaction.14,22 Since we were interested in as pure as possible Ag nanosheets, the quantity of H2PdCl4 used was taken as small as possible and the influence of the Pd salt on the Ag-plate formation was not investigated in more detail. It is noted that a basic environment is also a prerequisite for this reaction. If NH3 · H2O was used replacing NaAc under otherwise identical experimental conditions, the irregular Ag nanosheets can still be obtained accompanied by some Ag nanobelts (Figure 6C),

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Figure 7. Typical SEM image of the Ag nanosheets using H2PtCl6.

which demonstrates that the main function of NaAc is to tune the pH value, while the other functions of protection and further directing the growth of the Ag nanosheets has a minimal effect in this reaction. Using H2PtCl6 instead of H2PdCl4 also yields the micrometer-sized Ag nanosheets (as shown in Figure 7). To study the growth of the silver nanosheets and receiving hints as to the underlying growth mechanism, the status of a sample at an earlier stage, namely, after a reaction time of only 1 min, is presented in Figure 8. From Figure 8A, it can be seen that some Ag nanosheets are already almost fully formed, but the edges of these Ag nanosheets are not yet as large as those obtained after 30 min (as shown in Figure 1). Also visible are many half-baked Ag nanosheets exhibiting numerous holes, especially close to the edges of the plates. Figure 8B is a highmagnification SEM image of one of those half-baked Ag nanosheets on which many fusion boundaries and holes can clearly be seen. In addition, it is observed that, although these holes have different sizes and shapes, most of them appeared inside the fusion boundaries. This phenomenon can also be found on the surface of some Ag nanosheets obtained after 30 min of reaction, as shown in Figure 8C. Although the fusion boundaries can still be seen, the holes almost disappear and leave some dark spots only on the surface of the Ag nanosheets (as indicated in the red circles). One small half-filled hole can still be identified in this image (as indicated by the red arrow). On the basis of the above analysis, we can tentatively propose a growth mechanism of the silver nanosheets as follows: (i) When H2O2 is added, AgNO3 is reduced, producing large numbers of Ag atoms which further form into numerous small irregularly shaped Ag nanosheets (shown in Figure S2 of the Supporting Information). (ii) Because the {110} lateral facets have a relatively high surface energy and in order to reduce the overall surface energy of the system, these small irregular Ag nanosheets are interconnected along the {110} lateral facets, which leads to the formation of large hexagonal nanosheets. It seems that the oriented attachment is a prerequisite for the fusion of the small irregular Ag nanosheets. We suggest that this step of the growth process resembles similarities to the assembling of jigsaw puzzles in which the irregular small Ag nanosheets “intelligently” recognize fusion options resulting in larger Ag nanosheets. The fusion boundaries appearing on some of the larger Ag nanosheets and half-baked Ag nanosheets may provide sufficient evidence to support this viewpoint (see Figure 8B and C). A similar mechanism has also been reported in the literature.16b,17h (iii) Because these small irregularly shaped Ag nanosheets are not perfectly matched, some holes remain in the fusion boundaries. Subsequently, these holes are filled by Ag

Figure 8. Typical low- (A) and high-magnification (B) SEM images of an earlier stage of the Ag nanosheet formation after a reaction time of 1 min and typical high-magnification SEM image (C) of the Ag nanosheets after a reaction time of 30 min.

atoms and gradually level up to form more perfect Ag nanosheets. Obviously, such processes should be thermodynamically favored and hence will occur in solution during the reaction. SERS of R6G and Amoxicillin on the Micrometer-Sized Silver Nanosheets. The relatively “clean” surface of the micrometer-sized Ag nanosheets enables them to be easily modified, which is highly beneficial for SERS. As the SERS enhancement is very sensitive to the separation distances between the surfaces of the SERS substrates and the to be detected or analyzed molecules, short distances may lead to higher SERS enhancement.17a,23 The absence of polymers or long-alkyl-chain surfactants is beneficial for precisely controlling the separation distance and also facilitating the modification of the molecules of interest. A typical sample of the micrometer-

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Figure 9. SERS spectra of R6G (a) and amoxicillin (b) on the Ag nanosheets.

sized Ag nanosheets (as shown in Figure 1) was used to evaluate its SERS activity with the use of R6G and amoxicillin as probing molecules. R6G was chosen owing to its relatively large Raman cross section and its clear vibrational features reported by many authors.4c,24,25 Figure 9a shows the SERS spectrum of 10-6 M R6G molecules modified Ag nanosheets and the SERS signals with all peaks being consistent with the fingerprint features of the SERS spectrum of R6G reported.4c,24,25 Amoxicillin, an antibiotic drug, was used as another kind of probing molecule to further investigate the SERS activity of the nanosheets which may further find practical applications in human health and safety.25 From Figure 9b, it is evident that an intense SERS spectrum characteristic of amoxicillin molecules was obtained with well-resolved Raman peaks. The results confirm that the micrometer-sized silver nanosheets exhibit a capability to enhance Raman signals of molecules adsorbed on their surfaces which is directly connected with the surface-plasmon-induced enhancement of local electrical fields over the area of the micrometer-sized silver nanosheets.17a Conclusions In this paper, we provide a simple and rapid method for the large-scale synthesis of micrometer-sized silver nanosheets in aqueous solution at room temperature and under normal pressure without using any seeds, surfactants, polymers, or sacrificiial substrates. Well-defined single-crystal Ag nanosheets with 10-15 µm edge length and about 28 nm thickness have been fabricated by this method. It is found that H2PdCl4 plays a vital role in forming the micrometer-sized silver nanosheets. The nanosheets can be used as SERS substrates. In view of their large {111} plane and relatively “clean” surface, the as-prepared Ag nanosheets may hold promise as electrochemical SPR, metalenhanced fluorescence, or STM substrates for further potential applications. Acknowledgment. H.C. appreciates the support from the Alexander von Humboldt (AvH) Foundation. The authors are grateful to Ellen Kern for the SEM images and to Christoph Ziegler for the acquisition of the SERS data. Supporting Information Available: Figures showing additional SEM images. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Feldheim, D. L.; Foss, C. Metal Nanoparticles: Synthesis, Characterization, and Applications Dekker: New York, 2002.

Chen et al. (2) (a) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (b) Hu, M.; Chen, J.; Li, Z. Y.; Au, L.; Hartland, G. V.; Li, X.; Marquez, M.; Xia, Y. Chem. Soc. ReV. 2006, 35, 1084. (3) (a) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (b) Wu, H.; Chu, H.; Kuo, T.; Kuo, C.; Huang, M. H. Chem. Mater. 2005, 17, 6447. (4) (a) Malinsky, M. D.; Kelly, K. L.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2001, 123, 1471. (b) Haes, A. J.; Van Duyne, R. P. J. Am. Chem. Soc. 2002, 124, 10596. (c) Lu, L.; Kobayashi, A.; Tawa, K.; Ozaki, Y. Chem. Mater. 2006, 18, 4894. (5) (a) Wang, Y.; Zou, X.; Ren, W.; Wang, W.; Wang, E. J. Phys. Chem. C 2007, 111, 3259. (b) Zou, X.; Dong, S. J. Phys. Chem. B 2006, 110, 21545. (c) Zhang, J.; Li, X.; Sun, X.; Li, Y. J. Phys. Chem. B 2005, 109, 12544. (6) (a) Fang, N.; Lee, H.; Sun, C.; Zhang, X. Science 2005, 308, 534. (b) Ozbay, E. Science 2006, 311, 189. (c) Hunter, E.; Fendler, J. H. AdV. Mater. 2004, 16, 1685. (7) (a) Wiley, B.; Sun, Y.; Xia, Y. Acc. Chem. Res. 2007, 40, 1067. (b) Wang, Y.; Camargo, P. H. C.; Skrabalak, S. E.; Gu, H.; Xia, Y. Langmuir 2008, 24, 12042. (c) Wiley, B.; Wang, Z.; Wei, J.; Yin, Y.; Cobden, D.; Xia, Y. Nano Lett. 2006, 6, 2273. (8) Nagy, A.; Mestl, G. Appl. Catal., A 1999, 188, 337. (9) (a) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (b) Yu, D.; Yam, V. W.-W. J. Am. Chem. Soc. 2004, 126, 13200. (c) Chen, H.; Wang, Y.; Dong, S. Inorg. Chem. 2007, 46, 10587. (10) (a) Chen, S.; Carroll, D. L. Nano Lett. 2002, 2, 1003. (b) Sun, Y.; Xia, Y. AdV. Mater. 2003, 15, 695. (c) Washio, I.; Xiong, Y.; Yin, Y.; Xia, Y. AdV. Mater. 2006, 18, 1745. (d) Chen, S.; Carroll, D. L. J. Phys. Chem. B 2004, 108, 5500–5506. (11) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 617. (12) (a) Caswell, K. K.; Bender, C. M.; Murphy, C. J. Nano Lett. 2003, 3, 667. (b) Sun, X.; Li, Y. AdV. Mater. 2005, 17, 2626. (c) Wiley, B.; Sun, Y.; Mayers, B.; Xia, Y. Chem.sEur. J. 2005, 11, 454. (13) (a) Sun, Y.; Mayers, B.; Xia, Y. Nano Lett. 2003, 3, 675. (b) Bai, J.; Qin, Y.; Jiang, C.; Qi, L. Chem. Mater. 2007, 19, 3367. (14) Chen, H.; Jia, J.; Dong, S. Nanotechnology 2007, 18, 245601. (15) (a) Hao, E.; Bailey, R. C.; Schatz, G. C.; Hupp, J. T.; Li, S. Nano Lett. 2002, 4, 327. (b) Lu, L.; Kobayashi, A.; Kikkawa, Y.; Tawa, K.; Ozaki, Y. J. Phys. Chem. B 2006, 110, 23234. (16) (a) Jin, R.; Cao, Y. C.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (b) Jin, R.; Cao, Y. C.; Hao, E.; Me´traux, G. S.; Schatz, G. C.; Mirkin, C. A. Nature 2003, 425, 487. (c) Pastoriza-Santos, I.; Liz-Marza´n, L. M. Nano Lett. 2002, 2, 903. (d) Zhang, J.; Liu, H.; Zhan, P.; Wang, Z.; Ming, N. AdV. Funct. Mater. 2007, 17, 1558. (e) Bastys, V.; Pastoriza-Santos, I.; Rodrı´guez-Gonza´lez, B.; Vaisnoras, R.; Liz-Marza´n, L. M. AdV. Funct. Mater. 2006, 16, 766. (f) Ledwith, D. M.; Whelan, A. M.; Kelly, J. M. J. Mater. Chem. 2007, 17, 2459. (g) Cao, Z.; Fu, H.; Kang, L.; Huang, L.; Zhai, T.; Ma, Y.; Yao, J. J. Mater. Chem. 2008, 18, 2673. (h) Song, J.; Chu, Y.; Liu, Y.; Li, L.; Sun, W. Chem. Commun. 2008, 1223. (i) Xiong, Y.; Washio, I.; Chen, J.; Cai, H.; Li, Z.; Xia, Y. Langmuir 2006, 22, 8563. (j) Xiong, Y.; Siekkinen, A. R.; Wang, J.; Yin, Y.; Kimb, M. J.; Xia, Y. J. Mater. Chem. 2007, 17, 2600. (k) Me´traux, G. S.; Mirkin, C. A. AdV. Mater. 2005, 17, 412. (l) Maillard, M.; Giorgio, S.; Pileni, M. P. AdV. Mater. 2002, 14, 1084. (m) Iyer, K. S.; Bond, C. S.; Saunders, M.; Raston, C. L. Cryst. Growth Des. 2008, 8, 1451. (n) Hao, E.; Kelly, K. L.; Hupp, J. T.; Schatz, G. C. J. Am. Chem. Soc. 2002, 124, 15182. (o) Maillard, M.; Giorgio, S.; Pileni, M.-P. AdV. Mater. 2002, 14, 1084. (p) Jiang, L.-P.; Xu, S.; Zhu, J.-M.; Zhang, J.-R.; Zhu, J.-J.; Chen, H.-Y. Inorg. Chem. 2004, 43, 5877. (q) Yener, D. O.; Sindel, J.; Randall, C. A.; Adair, J. H. Langmuir 2002, 18, 8692. (r) Chen, S.; Fan, Z.; Carroll, D. L. J. Phys. Chem. B 2002, 106, 10777. (s) Xue, C.; Mirkin, C. A. Angew. Chem., Int. Ed. 2007, 46, 2036. (t) Xie, J.; Lee, J. Y.; Wang, D. I. C.; Ting, Y. P. ACS Nano 2007, 1, 429. (u) Tang, B.; Xu, S.; An, J.; Zhao, B.; Xu, W. J. Phys. Chem. C 2009, 113, 7025. (v) Yang, J.; Wang, H.; Zhang, H. J. Phys. Chem. C 2008, 112, 13065. (w) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 5599. (x) Hulteen, J. C.; Van Duyne, R. P. J. Vac. Sci. Technol., A 1995, 13, 1553. (17) (a) Sun, Y.; Wiederrecht, G. P. Small 2007, 3, 1964. (b) Sun, Y. Chem. Mater. 2007, 19, 5845. (c) Fang, J.; Ding, B.; Song, X. Cryst. Growth Des. 2008, 8, 3616. (d) Qu, L.; Dai, L. J. Phys. Chem. B 2005, 109, 13985. (e) Wang, C.-W.; Ding, H.-P.; Xin, G.-Q.; Chen, X.; Lee, Y.-I.; Hao, J.; Liu, H.-G. Colloids Surf., A 2009, 340, 93. (f) Yang, J.; Lu, L.; Wang, H.; Shi, W.; Zhang, H. Cryst. Growth Des. 2006, 6, 2155. (g) Du, J.; Han, B.; Liu, Z.; Liu, Y. Cryst. Growth Des. 2007, 7, 900. (h) Deng, Z.; Mansuipur, M.; Muscat, A. J. J. Phys. Chem. C 2009, 113, 867. (i) Liu, G.; Cai, W.; Liang, C. Cryst. Growth Des. 2008, 8, 2748. (j) Zou, X.; Ying, E.; Chen, H.; Dong, S. Colloids Surf., A 2007, 303, 226. (k) Yang, J.; Qi, L.; Zhang, D.; Ma, J.; Cheng, H. Cryst. Growth Des. 2004, 4, 1371.

Synthesis of Micrometer-Sized Silver Nanosheets (18) Pastoriza-Santos, I.; Liz-Marza´n, L. M. J. Mater. Chem. 2008, 18, 1724. (19) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D.; Chastain, J. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1992. (20) Gaarenstroom, S. W.; Winograd, N. J. Chem. Phys. 1977, 67, 3500. (21) Kumar, G.; Blackburn, J. R.; Albridge, R. G.; Moddeman, W. E.; Jones, M. M. Inorg. Chem. 1972, 11, 269. (22) Sun, Y.; Tao, Z.; Chen, Jun.; Herricks, T.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 5940–5941.

J. Phys. Chem. C, Vol. 114, No. 10, 2010 4501 (23) Kennedy, B. J.; Spaeth, S.; Dickey, M.; Carron, K. T. J. Phys. Chem. B 1999, 103, 3640. (24) (a) Campbell, M.; Lecomte, S.; Smith, W. E. J. Raman Spectrosc. 1999, 30, 37. (b) Chen, H.; Wang, Y.; Qu, J.; Dong, S. J. Raman Spectrosc. 2007, 38, 1444. (c) Wang, Y.; Chen, H.; Dong, S.; Wang, E. J. Raman Spectrosc. 2007, 38, 515. (25) Zhang, B.; Wang, H.; Lu, L.; Ai, K.; Zhang, G.; Cheng, X. AdV. Funct. Mater. 2008, 18, 2348.

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