Facile Synthesis of Monodisperse Silver Nanospheres in Aqueous

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Facile Synthesis of Monodisperse Silver Nanospheres in Aqueous Solution via Seed-Mediated Growth coupled with Oxidative Etching Xiang Lin, Shuang Lin, Yuanlan Liu, Mengmeng Gao, Haiyan Zhao, Benkang Liu, Wuliji Hasi, and Li Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04343 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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Facile Synthesis of Monodisperse Silver Nanospheres in Aqueous Solution via SeedMediated Growth coupled with Oxidative Etching Xiang Lin,a Shuang Lin,b Yuanlan Liu,a Mengmeng Gao,c Haiyan Zhao,a Benkang Liu,a Wuliji Hasi,b and Li Wang a* a.

School of Physics and Materials Engineering, Dalian Nationalities University, Dalian,

116600 P. R. China. b.

National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute

of Technology, Harbin, 150080, P. R. China. c.

School of Physics and Optoelectronic Technology, Dalian University of Technology,

Dalian, 116024, PR China Corresponding Author * E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Due to their perfect geometrical symmetry, spherical metal nanoparticles have attracted much attention for various applications, including fundamental studies and construction of plasmonic devices. In this work, monodisperse silver nanospheres (Ag NSs) in aqueous solution were directly prepared by a continuous process of seed mediated growth followed by oxidative etching. Silver nanocubes (Ag NCs) were synthesized by a seed-mediated growth method and subsequently were transformed to Ag NSs by simple injection of Cu2+ to the fresh prepared Ag NCs solution. Without any centrifugation steps at both growth and etching stages makes this procedure more convenient and efficient. The etching process and morphology evolution of silver nanostructure have been monitored by UV-Vis spectromater, SEM and XRD. Monodisperse Ag NSs with broadly tunable diameters (from 37 to 68 nm) have been successfully prepared. The optical property of Ag NSs has been studied and the experimental results show fairly good consistency with simulation results. Furthermore, these Ag NSs prepared by our approach could be constructed into ordered superlattice by self-assembly technique based on their high monodispersity and sphericity.

KEYWORDS: Monodisperse Ag NSs; Single-crystal; Seed-Mediated Growth; Oxidative etching; Self-assembly.

INTRODUCTION Owing to their unique optical properties (localized surface plasmon resonance, LSPR), noble metal nanocrystals have been investigated widely in a range of applications, such as sensing,


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catalysis, metal-enhanced fluorescence, drug delivery and surface-enhanced Raman scattering (SERS).

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Because the LSPR properties of metal nanocrystals are highly dependent on their

sizes and shapes, it is critical to manipulate the size and shape of metal nanocrystals in order to optimize their performance. Over the past decades, many research groups have developed different methods for the synthesis of Au and Ag nanocrystals with a wide range of diverse shapes, including sphere, rod, cube, octahedron, triangular thin plate and bipyramid. 7-20 Among these examples, gold and silver spherical nanoparticles are model objects and ideal building components in many theoretical and experimental studies owing to their perfect geometrical symmetry. It is much easier to work with a perfectly spherical particle in fundamental studies compared with the polyhedron nanocrystals, because the electrodynamic response of spherical nanoparticles can be analytically solved with Mie theory in a classical electromagnetic framework.21-22 In addition, the spherical particles can serve as building blocks for bottom-up assembly of 2D and 3D metal superlattices and construction of plasmonic structures.23-26 Over the past five years, various protocols have been explored by a number of groups for the synthesis of single crystal Au spherical nanoparticles. Lee et al. reported a cyclic process of slow growth followed by slow chemical etching to prepare ultra-smooth, highly spherical monocrystalline Au particles.27 Zheng et al. developed a successive, seed-mediated growth route using single-crystal Au spheres as the seeds under a slow injection rate of the precursor.28 Ruan et al. prepared gold nanospheres (Au NSs) with sizes ranging from 20 nm to 220 nm using a simple seed-mediated growth method aided with mild oxidation.29 Liu et al. developed a twostep strategy for synthesis of Au NSs through a laser irradiation-induced shape conversion of Au nanooctahedra under ambient atmosphere.30 In this paper, Au nanoparticles were fabricated into


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periodic monolayer arrays by self-assembly technique utilizing their high monodispersity and perfect spherical shape. Hanske et al. proposed a simple and up-scalable protocol for the synthesis of smooth Au NSs via seed-mediated growth coupled with oxidative etching.25 And the synthesized Au NSs were employed as building blocks to produce uniform arrays of micronsized 3D pyramidal supercrystals using a template-assisted approach. Overall, most of the above methods for synthesizing Au NSs used oxidative etching process which has been demonstrated as a post-synthesis tool to control the size, structure and shape of metal nanocrystals.31-32 Despite these successful methods for synthesis Au NSs, there are very few reports on the synthesis of single-crystal Ag NSs. After extensive literature search, we were only able to find two methods for the synthesis of silver spherical nanoparticles. Xia and coworkers reported a method based on wet etching for the production of high quality, single-crystal Ag NSs.33 By rapidly mixing a suspension of PVP capped Ag NCs with ferric nitrate or ferricyanide-based etching solution, nanocubes could be transformed to nanospheres with roughly the same diameter as the original cubes. Xiong demonstrated the morphological evolution of Ag NCs into Ag NSs when capping agent of PVP is replaced by citric acid.34 Citrate photoreduction can trigger the transformation from cubic to spherical shapes, and the shape evolution is governed by oxidative etching. We found that these methods require time-consuming centrifugation steps before the oxidative etching process. In addition, the Ag NCs used as starting materials for shape evolution were synthesized by using a polyol reduction at 150–180 ℃. These methods have many drawbacks, such as difficult to replicate due to its sensitivity to trace impurities, less cost effective and environment friendly (use of organic solvents), complicate and time-consuming.35


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In this work, we reported a facile strategy for the synthesis of monodisperse Ag NSs in aqueous system using a simple seed mediated growth method assisted with oxidative etching. Ag NCs with high uniformity were synthesized by seed mediated growth method in cetyltrimethylammonium chloride (CTAC) aqueous solution. The shape evolution could be started by simple injection of Cu2+ to the fresh prepared Ag NCs solution. Without any centrifugation steps at growth and etching stages, this method is quite simple and convenient. The morphology evolution process of silver nanocrystal has been investigated. Monodisperse Ag NSs with broadly tunable diameters (from 37 to 68 nm) are successfully prepared. The experimental results and calculated results regarding to the optical property of Ag NSs reveals rather consistently. What’s more, these Ag NSs were constructed into ordered superlattice by self-assembly technique. We believe these Ag NSs can be used as an excellent and promising candidate for fabrication of plasmonic structures and devices. EXPERIMENTAL SECTION Chemicals and Materials. Cetyltrimethylammonium bromide (CTAB) was purchased from Sigma-Aldrich. Sodium borohydride (NaBH4) and Ascorbic acid were obtained from Acros Organics. Cetyltrimethylammonium chloride (CTAC, 97%) and copper nitrate trihydrate (Cu(NO3)2·3H2O) were obtained from Aladdin Chemical. Silver nitrate (AgNO3) and chloroauric acid (HAuCl4) were purchased from Sinopharm Chemical reagent Co. Ltd. Ascorbic Acid (AA) was purchased from Acreas Reagent Company. All the reagents were used as received. Deionized water (>18.0 MΩ) was used in all the experiments. Synthesis of Ag NCs.


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Ag NCs were prepared via a seed-mediated growth procedure that was modified from previous work.36 Briefly, CTAC solution (10 mL 0.5 mM) was prepared in a 30 mL scintillation vial. Next, AgNO3 solution (0.1 M 5 mL) and ice-cold NaBH4 solution (0.02 M, 10 mL) was prepared successively. Then, 25 µL AgNO3 solution was added to the vial under stirred. Almost immediately, 0.45 mL of NaBH4 solution was quickly added to the solution, turning the solution to yellow. The solution was then left undisturbed at 30 °C for 40 min and was used as the seed solution in the next step. To synthesis Ag NCs of ~38 nm in edge length (generation 0, g0), 0.64 g of CTAC was dissolved in 88 mL of deionized water in a conical flask. Next, 1 mL of the seed solution and AgNO3 solution (10 mL 0.1 M) were introduced to the conical flask. The mixture was kept in a water bath at 60 °C for 20 min. Then 1 AA solution (10 mL 0.1M) was added and the solution was stirred in the capped conical flask for 3 h at 60 °C. After the reaction finished, 30 mL of the Ag NCs solution were extracted and used as seed directly for the synthesis of Ag NSs. Then the 70 mL Ag NCs solution was used as seed for the growth of bigger Ag NCs. In the same vessel, 27 mL AgCl solution (0.192 g CTAC was dissolved in 24 mL of deionized water, then 3 mL of 0.1 M AgNO3 solution were introduced to the solution and the mixture was kept in a water bath at 60 ℃ for 20 min) was injected to the AgNCs solution. Then, AA solution (3 mL 0.1 M) was added and the solution was stirred in the capped conical flask for 3 h at 60 °C to obtain Ag NCs with edge length of ~42 nm (g1). By repeating this process, different generations (g2 to g7) of Ag NCs of progressively larger sizes were obtained. Preparation of Ag NSs with diameters in the range of 37~68 nm.


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The grown Ag NCs extracted from each generation (30mL in volume) were used as seed for the synthesis of Ag NSs without centrifugation and other treatment. In detail, 5 mL Ag NCs solution was diluted 3 times with 10 mL deionized water, and then Cu(NO3)2 (225 µL 5 mM) was injected to the diluted Ag NCs solution under stirring at 60 °C. The mixture was stirred at 60 °C until oxidation was completed. Characterization. UV−visible spectra were acquired with a Lambda 750 UV/Vis/NIR spectrophotometer. Spectral analysis was performed in the range of 300−800 nm at room temperature using quartz cuvettes with optical path length of 1 cm. The scanning electron microscope (SEM) images were taken using a Hitachi S-4800 field emission scanning electron microscopy (Hitachi, Japan). The transmission electron microscope (TEM) images of the particles were obtained using a JEM2100 TEM instrument (JEOL, Japan). FDTD simulation. FDTD simulations were carried out by FDTD Solutions 8.0 software developed by Lumerical Solutions Inc. Since the aqueous dispersions were dilute, the Ag NSs can be considered isolated. Therefore, the calculation of the extinction spectra of Ag NS colloid is equal to simulate a single Ag NS. A total-field scattered-field (TFSF) light source with a wavelength range of 300 to 750 nm was selected. The perfectly matched layer (PML) boundary condition was applied for x, y and z axis directions and the background refractive index was set to 1.33 (for water at room temperature). The mesh size was set as 0.1 nm for 37 nm Ag NS and 0.5 nm for Ag NSs with other sizes.


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RESULTS AND DISCUSSION This novel procedure for preparing the ultra-smooth Ag NSs combines the concepts of waterbased seed-mediated growth with directly oxidative etching of the resulting monodisperse Ag NCs. To transform Ag NCs into Ag NSs, many methods have been reported. However, most of these methods need several centrifugation steps which are time-consuming and complicate. According to a previous work, Cu2+ has been employed as etching agent for the morphological evolution from Au@Ag nanocuboids to Au@Ag nanorices with AA and CTAC coexist in an aqueous environment.37 It inspires us that Cu2+ could also be used to transform Ag NCs to Ag NSs. Since AA and CTAC have already existed in the final reaction solution of Ag NCs in this work, the etching process could be initiated by introduction of Cu2+ to the unwashed Ag NCs solution. Without any centrifugation steps in the whole process makes this procedure extremely convenient and efficient.


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Figure 1. (a) Schematic illustration of the synthesis process of Ag NSs using a seed-mediated growth coupled with chemical etching method; (b) TEM image of the Ag NCs which can be transformed into smooth Ag NSs (c) by chemical etching reaction; (d, e) SAED pattern of single Ag NC and Ag NS. The scale bars are 50 nm. The synthesis process is illustrated in Fig. 1a. At first, Ag NCswere synthesized via a seedmediated growth procedure that was modified from previous work,36 as shown in Fig. S1. In a typical experiment, silver seed of about 1~5 nm were prepared by using AgNO3, NaBH4 and CTAC as precursor, reducing agent and surfactant, respectively. The seed solution was added to an aqueous CTAC solution, followed by the introduction of AgNO3 as the silver source and AA as the reducing agent. The mixture was stirred for 3 hours at 60 ℃ to obtain Ag NCs. Finally, Cu2+ was injected to the solution to transform Ag NCs to Ag NSs. The oxidative etching is a continuous process and could be shut off by centrifugation and redispersion in water. All the three agents, AA, CTAC and Cu2+, are required for the etching process. As shown in the TEM image (Fig. 1b), the obtained Ag NCs exhibit high uniformity in both size and shape. A small fraction of right bipyramid byproduct was formed which is consistent with the literature. The TEM image of Ag NSs is shown in Fig. 1c, indicating these Ag nanoparticles possess perfectly circular profile. The selected area electron diffraction (SAED) patterns in Fig. 1d and 1e suggest that both the Ag NCs and Ag NSs are single crystalline. Because the color and LSPR band of silver nanoparticles sensitively depend on its shape,38 the morphological evolution from cube to sphere could be monitored by optical extinction spectroscopic measurements. Fig. 2a shows the color change at different etching times. The color


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changes from orangered to yellow green and becomes more and more transparent. More information could be obtained from the UV–vis spectra, as plotted in Fig. 2b.

Figure 2. (a) Photographs of the reaction solution during the etching process at different stages. (b) UV–vis spectra of the Ag NCs solution at different stages of the etching process. The UV–vis spectra of Ag NCs and Ag NSs display distinct optical features. The spectrum of Ag NCs solution shows five LSPR peaks at about 347, 396, 416, 474 and 560 nm, respectively, as shown in Fig. S2. According to the previous peak assignments for Ag NCs, peaks at 347, 396 and 474 nm are assigned to dipole resonance modes, while the peak around 416 nm can be attributed to the quadrupole resonance mode. In addition, the peak around 560 nm is contributed from the right bipyramids, which have a more elongated structure than cubes. It has


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demonstrated that the right bipyramids are growth from single twinned seeds.39-40 On the other hand, the spectrum of Ag NSs solution shows only one peak at 411 nm, which confirms the highly symmetric structure of nanospheres. During the etching process, a significant blue shift of the strongest peak (peak IV) from 474 to 411 nm can be observed in the absorption spectra, corresponding to color change from orangered to yellow green. This blue-shift is mainly due to less charge separation caused by high symmetry of spherical shape. After 120 min, the shoulder peak on the right of the main peak (peak V) disappears, implying the passivation of vertices of Ag right bipyramids. After about 135 min, peak I-III disappear, which proves the shape transformation from nanocubes to nanospheres. To further look into the shape evolution of the etching process, SEM was used to characterize the structure change of nanostructures. Since monodispersed nanoparticles could assembly into ordered superstructures by droplet evaporation, a droplet of each nanostructure during the reaction was dropped onto a silicon slide and allowed to evaporate at a sealed culture dish at room temperature. In the initial stage of the etching reaction (0-120 min), the edge lengths of nanocubes decrease from 58.7 nm to 53.5 nm and the nanocubes become more rounded, which is because silver atoms at vertices and edges were etched preferentially (Fig. 3a-c). And for the three cubic nanocrystals, two-dimension (monolayer) square ordered arrays formed on the substrates. With the continuous etching of Ag, Ag nanoparticle with spherical shape and relatively smooth surface were obtained (Fig. 3d-f). Because the etching reaction of Ag nanostrctures is a continuous process, the nanospheres became smaller as the reaction proceeded. For the spherical nanoparticles, three dimension (multilayer) hexagonal close-packed superlattices were formed on the substrates (Fig. 3d-f and S3). The main reason for the selfassembly of silver nanostructures with different shapes into superstructures with different


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structures and morphologies is the inter-nanoparticle van der Waals interactions.41 It is worth noting that the silver right bipyramids, which accounted for a small proportion of total nanocrystals, also underwent a shape evolution during the etching process and eventually transformed into nanospheres with a little bigger size. The shape evolution of silver right bipyramids is described clearly in Fig. S4 (see Supporting Information for detail).

Figure 3. SEM images of Ag NCs and Ag NSs obtained after etching for a) 0 min, b) 105 min, c) 120 min, b) 135 min, b) 150 min, and f) 165 min. The scale bars are 100 nm. In order to evaluate the role of Cu2+ in the shape evolution of AgNCs, several control experiments have been performed. When Cu2+ ions were replaced by Fe3+ or Fe2+ ions, the shape of Ag nanocubes remained unchanged, as shown in Fig. S5. These results suggest that the Cu2+ ion is irreplaceable in the Cu2+/AA-induced etching reaction. The contribution of dissolved O2 was further confirmed by a control experiment. An argon protective device has been used to isolate O2 from the system by bubbling argon (Fig. S6). The experiment result is shown in Fig. S5 (b) and (f). There is no significant shape change could be observed, which indicates that O2 is crucial for the shape evolution. On the other hand, after extensive literature search,42 the mechanism of the etching reaction can be written as:


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Cu2++ascorbate → Cu2+-ascorbate complex → Cu++ascorbyl (1) Cu++O2 → Cu2++O2-

(2)

Cu++O2-+2H+ → H2O2+Cu2+

(3)

2O2-+2H+ → H2O2+O2

(4)

Ag+H2O2+2H++Cl- →AgCl+2H2O

(5)

In presence of dissolved O2, Cu2+/AA system can produce H2O2 which has been demonstrated to have the ability to etch Ag nanocrystals such as Au@Ag nanorods and Ag triangular nanoplates.43-44 Finally, the Ag+ cations generated in situ combined with the Cl− anions dissociated from CTAC to form AgCl microparticles.37 X-ray diffraction (XRD) characterization was also employed to investigate the crystallinity of Ag nanostructures during the etching process. As shown in Fig. 4, the {200} facet is dominant for Ag NCs and truncated Ag NCs due to their preferential orientation of deposition on substrates with their {100} faces. The XRD peak intensity ratios I{200}/I{111} of the truncated Ag NCs is lower compared with that of nanocubes, which is due to more exposure of {111} crystal planes. As the etching reaction progress, the {111} facet is dominant for the final Ag NSs and other crystal planes including {220}, {311}, and {222} are also exposed at surfaces. The intensity ratio between each reflection peaks is consistent with the standard spectrum of face center cubic (fcc) silver (JCPDS no. 04-0783), which can be attributed to the random orientation of Ag NSs. This result further confirmed that the symmetry of silver nanoparticles increased from Oh symmetry (for Ag NCs and truncated Ag NCs) to Cs symmetry (for Ag NSs).45-46


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Figure 4. XRD patterns of silver structures from nanocubes to truncated nanocubes and nanospheres. The standard XRD patterns for fcc silver (JCPDS no. 04-0783) is shown as a bar diagram at bottom. After investigation of the morphology evolution process, we further synthesized Ag NSs with a range of controlled sizes from 37 nm to 68 nm. Firstly, a series of Ag NCs with uniform edge length and high purity were prepared. The initial Ag NCs with an average size about 38 nm served as the first generation product, Ag NCs with large sizes were successively grown using AgNCs with different sizes as the seeds. By this strategy, we can get high quality Ag NCs with narrow size distribution and fewer impurities. The average edge lengths of Ag NCs are 38.03 nm, 42.27 nm, 46.94 nm, 52.09 nm, 58.69 nm, 67.02 nm, 73.19 nm and 82.86 nm, respectively. (Fig. S1). Among them, Ag NCs with edge lengths of 38.03 nm, 46.94 nm, 58.69 nm and 73.19 nm were used as starting nanostructure for preparation of Ag NSs. After the chemical etching process, these Ag NCs were transformed into Ag NSs with sizes of 36.7±4.2 nm, 41.6±4.3 nm, 51.1±4.8 nm, 61.0±4.5 nm, and 67.9±6.3 nm, respectively (Fig. 5a–e). Fig. 5f shows the size distributions of Ag NSs. It can be seen from the histogram that some Ag NSs are bigger in size. According to the morphology evolution process of silver nanocrystal shown in SEM and TEM


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images, the bigger Ag NSs are produced from the silver right bipyramids (as demonstrated in Fig. S3).

Figure 5. (a–e) SEM images of Ag NSs with tunable diameters. The average diameter of each sample is 36.7±4.2 nm, 41.6±4.3 nm, 51.1±4.8 nm, 61.0±4.5 nm, and 67.9±6.3 nm. The scale bars are 100 nm. (f) The histograms of diameters of Ag NSs corresponding to Fig. 5a–e. The highly monodisperse Ag NSs with finely tunable sizes obtained in this work allowed us to systematically investigate their LSPR properties as a function of their size. The UV-vis spectra of the Ag NSs are shown in Fig. 6(a-e), demonstrating the size-dependent optical properties of the Ag NSs. The LSPR peaks display a red-shift from 407 to 413, 421, 433, and 442 nm as the size of Ag NS increases from 36.7±4.2 nm to 41.6±4.3 nm, 51.1±4.8 nm, 61.0±4.5 nm and 67.9±6.3 nm, respectively. The LSPR peaks show a linear correlation with the size of the Ag NS, and the fitting curve can be expressed as λmax= 1.099d+ 366.4 (R2 = 0.99), where λmax and d are the LSPR peak position and the particle size, respectively. Peak broadening is clearly observed owing to increasing radiative losses for the Ag NSs with larger sizes.


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Figure 6. (a~e) Experimental extinction spectra of Ag NSs with different diameter. (f) The experimental LSPR peak position vs Ag NSs size. To determine the contributions of the light scattering and absorption to the total extinction of the Ag NSs, FDTD simulations were performed, as shown in Fig. 7. The simulated extinction spectra were in good agreement with the experimental results. With the increase of the diameter, both scattering and absorption increases accompanied by a red shift, and the total extinction intensity increases as the particle size increases (Fig. 7f). For small Ag NSs (37 nm), the extinction efficiency is dominated by absorption. As the particle becomes larger, the portion contributed by scattering increases, for 50 nm Ag NSs, absorption and scattering contribute to extinction almost equally, for large Ag NSs (68 nm), the contribution of scattering to extinction is stronger than that of absorption. It is worth noting that as the particle size increases, a shoulder


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peak on the left of the main peak appeares. According to the previous peak assignments for Ag NSs, the shoulder peak should be attributed to the quadrupole resonance mode.47

Figure 7. Experimentally measured (dash) and simulated (solid) UV-vis extinction spectra for Ag NSs with different diameters: (a) 37 nm, (b) 42 nm, (c) 51 nm, (d) 61 nm, (e) 68 nm. The simulated absorbance efficiency (black) is contributed by absorption (blue) and scattering (red). (f) The theoretically calculated extinction cross sections of the Ag NSs by Mie theory. CONCLUSIONS In summary, a facile and effective strategy has been developed for the synthesis of Ag NSs in aqueous solution by a two-step continuous process, including synthesis of Ag NCs with high uniformity via a seed-mediated growth method and subsequently oxidative etching of monodispersed Ag NCs into Ag NSs. The shape transformation can be started by simply introducing Cu2+ into the fresh prepared Ag NCs solution. This new synthesis strategy avoids the multiple centrifugation steps required in conventional oxidative etching processes, which makes it more convenient and efficient. Monodisperse Ag NSs with broadly tunable diameters (from 37


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to 68 nm) have been prepared. The optical property of silver NSs has been studied by experimental measurement and simulation calculation and this two results are in reasonable agreement. Based on their high monodispersity and perfect spherical shape, these Ag NSs were constructed into ordered superlattices. We believe these Ag NSs will find application in the fundamental studies of plasmonic phenomena, fabrication of plasmonic devices and chemical/biochemical sensing. ASSOCIATED CONTENT Supporting Information. SEM images and extinction spectra of Ag NCs; SEM and TEM images of silver right bipyramids; SEM images of large scale self-assembly structures of Ag NSs with low magnifications; Control experiments: Cu2+, Cu2+ with Ar, Fe3+, Fe2+; Schematic illustration and photograph of the argon protective device. ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (21501021), International S&T Cooperation Program of China (2011DFA31770) and the Open Fund of State Key Laboratory of Molecular Reaction Dynamics, DICP, CAS (SKLMRD-K201812). REFERENCES 1. Rycenga, M.; Cobley, C. M.; Zeng, J.; Li, W.; Moran, C. H.; Zhang, Q.; Qin, D.; Xia, Y., Controlling the synthesis and assembly of silver nanostructures for plasmonic applications. Chem. Rev. 2011, 111 (6), 3669-712. 2. Dreaden, E. C.; Alkilany, A. M.; Huang, X.; Murphy, C. J.; El-Sayed, M. A., The golden age: gold nanoparticles for biomedicine. Chem. Soc. Rev. 2012, 41 (7), 2740-2779. 3. Shafiei, F.; Monticone, F.; KhaiQ.Le; Liu, X.-X.; Hartsfield, T.; Alu, A.; Li, X., Metalenhanced fluorescence of colloidal nanocrystals with nanoscale control. Nat. Nanotechnol. 2006, 1 (2), 126-130.


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Table of Contents Graphic (TOC)


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Facile Synthesis of Monodisperse Silver Nanospheres in Aqueous

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