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DOI: 10.1021/cg901497p

Size-Controlled and Size-Designed Synthesis of Nano/Submicrometer Ag Particles

2010, Vol. 10 3378–3386

Bo Chen, Xiuling Jiao, and Dairong Chen* Key Laboratory for Special Functional Aggregate Materials of Education Ministry, School of Chemistry & Chemical Engineering, Shandong University, Jinan 250100 P. R. China Received November 30, 2009; Revised Manuscript Received June 20, 2010

ABSTRACT: A series of nano/submicrometer Ag particles with uniform size-distribution are prepared using ascorbic acid to reduce AgCl in aqueous solution and at ambient conditions. NaOH is used to adjust the redox potential of ascorbic acid, and polyvinylpyrrolidone (PVP) is used to balance the relative rates at which various crystallographic planes of Ag grow. Under the reaction conditions, the nucleation and growth steps are separated, thus controlling the anisotropic growth of the Ag crystals. The products are studied using transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), field emissionscanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and UV-vis spectrophotometry. On this basis, an equation for the relationship between the synthesis conditions and the size of the product was established, which can be applied to produce quantitatively size-controlled and size-designed Ag particles. Scale-up of the synthetic procedure is also conducted to test the prospect of large-scale synthesis of Ag particles for practical industrial applications.

1. Introduction Metal nanomaterials, especially noble metal nanomaterials, have fascinating optical, electronic, magnetic, thermal properties and widespread applications in catalysis, electronics, photography and information storage, medicine, and sensing.1 Ag nanostructures exhibit a strong ultraviolet-visible (UV-vis) absorption band arising from a localized surface plasmon resonance (LSPR) and have potential applications in surface enhanced Raman scattering (SERS),2 surface enhanced fluorescence (SEF)3 and as chemical and biological sensors.4 Though the electromagnetic or chemical enhancement mechanism for LSPR is still under debate,5 both computational and experimental studies have demonstrated that the numbers and positions of the LSPR are closely related to the Ag nanoparticle’s physical parameters such as shape, size, and composition.6 One can tailor and fine-tune the properties of Ag particles by controlling these parameters. Therefore, the synthesis of shape- and size-controlled Ag nanoparticles is significant for practical applications. Many reports have focused on the synthesis of shapecontrolled Ag nanostructures, including quasi-spheres, decahedrons, cubes, prisms, rods, wires, tubes, branches, sheets or plates, and belts.7 Generally, size-controlled Ag particles can be realized by adjusting the reaction parameters.8 For example, by varying the reaction time, Evanoff and Chumanov synthesized Ag particles with diameters between 15 and 200 nm.8a By varying the concentration of sodium borohydride (NaBH4) employed in the reaction, Metraux and Mirkin have provided a straightforward and rapid route to Ag nanoprisms with control over prism thickness.8b By adjusting intensity and spectral properties of the irradiating light, Pietrobon and Kitaev synthesized decahedral Ag nanoparticles with controllable regrowth to larger sizes.8e Also, Yin’s laboratory has recently demonstrated that the aspect ratio and optical properties of Ag nanoplates can be precisely tuned *Corresponding author. E-mail: [email protected]. Tel: 86-053188364280. Fax: 86-0531-88364281. pubs.acs.org/crystal

Published on Web 07/06/2010

over a wide range through a UV-light-induced reconstruction process.8f However, in these examples of qualitative sizecontrol, the results can only be roughly speculated before the experiment is done (e.g., size-decreased or increased, but not a precise estimating). Quantitative size-control, where the product is size-designed by adjusting the reaction conditions to produce the desired particle sizes predictably and accurately, has not yet been established. Actually, it is well-known that the chemical synthesis of metal nanocrystals is influenced by several thermodynamic and kinetic factors, and much difficulty remains in capturing the distinct stages of nucleation and growth of nanocrystals.9 Also it is very hard to establish a quantitative function to describe the relationship between the synthesis conditions and the size of the product. Therefore, realizing qualitative and especially quantitative synthesis of size-controlled Ag particles is still a great challenge. Here, we present an approach using ascorbic acid to reduce AgCl in aqueous solution at ambient conditions, where we accomplish the qualitative and quantitative synthesis of sizecontrolled and size-designed nano/submicrometer Ag particles. Ag halides (except AgF), as nearly insoluble compounds, keep the concentration of free Agþ to a very low level in water, which provides an opportunity for kinetic control of the reaction rate and the separation of the nucleation and crystal growth. Gedanken et al. have described a reduction method to synthesize long, straight continuous Ag nanowires using AgBr as precursor.10 Also, Yu and Yam have reported an approach to generate monodisperse Ag nanocubes using AgBr.11 Similarly, Qian et al. have presented a simple hydrothermal route for the synthesis of uniform Ag nanowires using freshly prepared AgCl.12 In addition, ascorbic acid, as a mild reducing agent, can reduce Ag halides under alkaline conditions.13 Compared with AgBr or AgI, the solubility of AgCl is higher, and we find it is facile to adjust the rate of ascorbic acid to reduce AgCl by introducing a small amount of NaOH. Therefore, the rates of nucleation and growth of Ag in the reaction system are adjusted, and qualitative size-control can be achieved using ascorbic acid to reduce AgCl to prepare a r 2010 American Chemical Society

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series of uniform Ag nanoparticles. Moreover, by adjusting the redox reaction rates, the nucleation and growth stages of the Ag crystal can be defined. Using excess polyvinylpyrrolidone (PVP) adsorbed to the surface of Ag crystals, the addition of the Ag atoms to the crystallographic planes becomes isotropic, and hence the anisotropic growth of the Ag crystals is inhibited.7b On this basis, we derived a quantitative function to describe the relationship between the synthesis conditions and the size of the product, which can be applied to prepare quantitatively size-controlled and size-designed Ag particles. Remarkably, this quantitative size-control can accurately calculate the size of the product before the experiment is done, and a series of submicrometer Ag particles with sizes in accordance with the experimental design are prepared. Considering the extensive applications of submicrometer Ag particles, such as catalysts, conductive adhesives, display devices, passive components, inkjet printing, photon emission, and higher order multiples resonances substrates,14 this designed synthesis idea and procedure not only have experimental significance, but also have a practical application value because of the simple procedure, mild reaction conditions, and ready application to large-scale industrial synthesis. 2. Experimental Section 2.1. Chemicals. AgNO3 (Shanghai Chemical Co.), ascorbic acid (C6H8O6, Shanghai Chemical Co.), NaCl (Tianjin Reagent Co.), PVP (K30, Beijing Reagent Co.), NaOH (Tianjin Reagent Co.). All reagents are analytical grade and used without further purification. 2.2. Synthesis. Preparation of AgCl Colloid. In a typical synthesis, 85.0 mg of PVP is added into 20.0 mL of water under magnetic stirring in a 50 mL beaker; after it is completely dissolved, 85.0 mg of AgNO3 is added. When the reagent is completely dissolved, 200 μL of 5.0 mol 3 dm-3 NaCl is added under rapid stirring, and then stirred for 15 min in the dark. This freshly prepared AgCl colloid acted as a precursor for the next process. Qualitative Size-Controlled Synthesis of Ag Nanoparticles. For the size-controlled synthesis of Ag nanoparticles, 20.0 mL of 50.0 mmol 3 dm-3 ascorbic acid is added to a set volume of 0.5 mol 3 dm-3 NaOH under magnetic stirring, and 2.5 mL of freshly prepared AgCl colloid is also added. The mixture is then stirred for 2 h in the dark. The products are collected by centrifugation and washed with water. On the basis of the quantity of NaOH added, a series of Ag nanoparticles of different sizes with uniform size-distribution are prepared. The product can also be preserved in the refrigerator at 4.0 °C without centrifugation and washing in order to be used as a seed for the next process. Quantitative Size-Controlled and Size-Designed Synthesis of Submicrometer Ag Particles. For the size-controlled and size-designed synthesis of submicrometer Ag particles, 100.0 mg of PVP and 20.0 mL of 50.0 mmol 3 dm-3 ascorbic acid are added to a 50 mL beaker under magnetic stirring. After they are completely dissolved, a set volume of Ag nanoparticles prepared as above is added as seed (external seed). Then 0.5 mL of 0.5 mol 3 dm-3 NaOH and 2.5 mL of freshly prepared AgCl colloid are added in turn, and stirred for a set number of hours in the dark. The products are collected by centrifugation and washed with water. On the basis of the size and quantity of the external seed, a series of submicrometer Ag particles of designed size are prepared. All the synthetic procedures are conducted at room temperature and pressure. The redox reaction requires dark conditions because of the sensitivity of AgCl to light.15 In order to ensure the reproducibility and controllability, the temperature should be maintained at 25 ( 2 °C and the ascorbic acid solution should be freshly prepared. 2.3. Characterization. The morphology and microstructure of the products were characterized by a transmission electron microscope (TEM, JEM 100-CXII) with an accelerating voltage of 80 kV, high resolution transmission electron microscope (HR-TEM, GEOL-2010) with an accelerating voltage of 200 kV, and field emission-scanning electron microscope (FE-SEM JSM-6700F). X-ray diffraction

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(XRD) patterns were collected on a Rigaku D/Max 2200PC diffractometer with a graphite monochromator and Cu KR radiation (λ = 0.15418 nm), UV-vis absorption spectra of the products dispersed in water were collected on a UV-vis spectrophotometer (Lambda-35, Perkin-Elmer). X-ray photoelectron spectra (XPS) were recorded on a PHI-5300 ESCA spectrometer (Perkin-Elmer) to characterize the product surfaces with its energy analyzer working in the pass energy mode at 35.75 eV, and the AlKR line was used as the excitation source. The binding energies were referenced to the C1s line at 284.6 from adventitious carbon.

3. Results and Discussion 3.1. Qualitative Size-Controlled Synthesis of Ag Nanoparticles: Spontaneous Seed. Spontaneous seeds are controllably generated in the reaction system, so this approach is adopted to synthesize qualitatively size-controlled Ag nanoparticles. Considering the reaction of ascorbic acid to reduce AgCl, the reaction rate and properties of corresponding products are affected by a set of reaction parameters that may include the dosage of ascorbic acid, the amount of Cl-, the reaction temperature, as well as the introduction of capping agent such as PVP. Notably, the rate of oxidation of ascorbic acid is strongly dependent on the pH of the medium,16 so the dosage of NaOH is reasonably selected as a simple and effective approach to controlled synthesis of Ag nanoparticles. With other reaction conditions unchanged, the size of the Ag particles changes over a certain range depending on the amount of NaOH added to the reaction system. As shown in Figure 1, under suitable reaction conditions, the dosage of NaOH uniformly affects the size of the Ag nanoparticles. When the dosage of NaOH is reduced, the size of the asprepared Ag particles becomes larger. We can also scale up the reaction by 16 times using the same relative ratios of reagents and achieve the same results. Over 100 mg of Ag nanoparticles with a uniform size-distribution were produced in this way (see Figure S1, Supporting Information). According to the XRD of the as-prepared Ag nanoparticles (Figure 2a), the products are metallic Ag with a facecentered cubic (fcc) structure (JCPDS No. 04-0783), which indicates that ascorbic acid can reduce all the AgCl into metallic Ag under these experimental conditions. The spectra also show that the size of the corresponding Ag particles increases from 20 to 100 nm as the amount of added NaOH changes from 2.8 to 2.2 mL. However, a regular trend is not observed in the full width at half-maximum (fwhm) and the intensity of the XRD patterns, which implies that the particles observed from the FE-SEM images are likely not single-crystals, which will be further discussed below. The products are dispersed in water for UV-vis absorption spectra tests. According to the different particle sizes, the localized surface plasmon resonances (LSPR) of Ag nanoparticles show a regular trend. As shown in Figure 2c, as the particle sizes vary from 20 to 100 nm, the LSPR gradually shift to longer wavelengths while the maximum in the absorption spectra changes from 411 to 485 nm. This observation accords well with previously reported computational and experimental results.6b-e As the particle size increases, the quadrupole LSPR of the large Ag particles becomes more significant, revealing a shoulder peak at 380-400 nm. These higher order multipole resonances are excited due to the phase retardation of the field inside the even larger Ag particles, which result in a broad appearance of the absorption spectra, especially those of Ag particles having a larger size.14c

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Figure 1. FE-SEM images of as-prepared Ag nanoparticles, insets are the corresponding size-distribution histograms. The dosage of NaOH and the size of the as-prepared Ag particles are (a) 2.8 mL, 20 nm, (b) 2.6 mL, 35 nm, (c) 2.5 mL, 50 nm, (d) 2.4 mL, 65 nm, (e) 2.3 mL, 80 nm, and (f) 2.2 mL, 100 nm. TEM images are available in Figure S2, Supporting Information.

Figure 2. XRD patterns (a, b) and UV-vis absorption spectra (c) of the products, the dosage of NaOH is (a) 2.8-2.2 mL, (b) 2.2-2.0 mL, and (c) 2.8-2.2 mL, respectively. The reaction times are 2.0 h.

By analyzing the above experimental results, we believe that NaOH changes the redox potential of ascorbic acid.13 When the dosage of NaOH is relatively high, the power of ascorbic acid to reduce AgCl is relatively strong. When AgCl colloid is introduced into the reaction system, the number of spontaneously nucleated seeds is large. Therefore, according to the theory of crystal nucleation and growth,17 if the same quantity of AgCl colloid is provided, the size of the final product will be small; otherwise, the size of the product will be large. This mechanism can be confirmed from the experimental phenomena: the color change of the reaction system in the whole process is hard to observe because the reduction reaction is conducted under dark conditions, although different changes can be observed at the instant AgCl is

introduced into the system. When the amount of NaOH is 2.2 mL, the color of the solution changes slightly from colorless to light yellow, which indicates that a small amount of Ag seeds is formed in the reaction system. When more NaOH is added, the color change becomes more obvious, and the color becomes deeper. When the amount of NaOH is 2.8 mL, the color of the solution instantly changes to a deep greenish brown, which indicates that the Ag seeds generated in the reaction system are plentiful. This mechanism can be further demonstrated by the following two experiments. One experiment aimed to acquire Ag nanoparticles with a smaller size. We tried adding more NaOH to the reaction system; however, this proved to be unfeasible. As shown in Figure 3a, when the dosage of NaOH is 3.0 mL, unlike our

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Figure 3. TEM images of the as-prepared products; the amount of NaOH added into reaction system and the reaction time are (a) 3.0 mL, 2 h, (b) 2.1 mL, 6 h, and (c) 2.0 mL, 12 h.

prediction, the prepared Ag particles are irregular in shape and are somewhat larger than 20 nm. As shown by the arrow in Figure 3a, some relatively large particles are formed, likely arising from particle aggregation. As more Ag seeds are generated in the reaction system and a large amount of small silver nanoparticles is instantly formed in a short period of time, these small particles with extremely high surface energies are likely to coalesce because they have more opportunities to collide and aggregate, thus undergoing further growth.18 These aggregates form the large particles observed in the final products. Another experiment aimed to acquire Ag nanoparticles with larger sizes. We added less than 2.2 mL of NaOH at the same reaction time of 2.0 h. As shown in Figure 2b, when the amount of NaOH is 2.0 mL, a majority of the product is AgCl and a minority of metallic Ag is generated. When the amount of NaOH is 2.1 mL, most of the AgCl is reduced to metallic Ag, but AgCl still exists in the product. Only if the amount of NaOH is 2.2 mL or larger can the product be exclusively metallic Ag after 2 h. AgCl cannot be completely reduced to metallic Ag in 2 h reaction time if a relatively low amount of NaOH is used because the power of ascorbic acid to reduce AgCl is so weak that the amount of Ag seeds produced is small, and then the growth rate of the crystals is also slow. By prolonging the reaction time, the reaction can be completely conducted, but the sizes of the final Ag particles are not uniform. As shown in Figure 3b, if the reaction time of the experiment with 2.1 mL of NaOH is prolonged to 6 h, a multiple size-distribution is shown in the product. Large particles around 100 nm and a mass of small particles with diameters in the tens of nanometers also exist. If the reaction time of the experiment with 2.0 mL of NaOH is prolonged to 12 h (Figure 3c), similar results are acquired. The large particles are bigger than 100 nm, but the small particles are still present. We presume that this multiple size-distribution may result from the slow reaction rate, which allows repetitive nucleation to occur, and hence crystal nucleation and growth occur together for a relatively long time.17 At long reaction times, Ostwald ripening might also influence the size-distribution.17b,19 It is worth noting that rod-shaped particles can also be observed in Figure 3b,c. Under these reaction conditions, since the reaction rate is quite slow, the amount of Ag seeds is small, the growth rate of crystals is also slow, and therefore the newly generated Ag atoms in the system have abundant time for selecting various crystallographic planes to form rod-shaped particles.7b,8g,20 In short, if ascorbic acid is used to reduce AgCl and appropriate reaction conditions are chosen, qualitative size-control to prepare Ag nanoparticles is achieved, and the size of the product can be roughly speculated according

to the dosage of NaOH before the experiment is done. In this process, the key is to choose an appropriate dosage of NaOH, and then appropriate reaction rates can be obtained to ensure a proper amount of Ag seeds are instantly formed when AgCl is introduced. Meanwhile, in order to prevent particles from aggregating the nucleation rate cannot be too fast, so the seed which is formed first in the reaction system can undergo a relatively uniform growth stage, and finally, the product with uniform size-distribution can be acquired. 3.2. Quantitative Size-Controlled and Size-Designed Synthesis of Submicrometer Ag Particles: External Seed. The previous part discussed the qualitative size-controlled synthesis of Ag nanoparticles, and this control is realized by adding different amounts of NaOH to adjust the redox potential of ascorbic acid to reduce AgCl. Generally, ascorbic acid itself cannot reduce AgCl to generate metallic Ag, but the reaction begins by adding a set quantity of NaOH. The power of ascorbic acid to reduce AgCl is closely related to the pH, and therefore the reaction rate is closely related to the dosage of NaOH.13 By analyzing the process, a state must exist, at which the reaction can just progress and the ascorbic acid can just reduce AgCl into metallic Ag. However, the reaction rate is too slow. Consequently, the rate of Ag atoms generated is so slow that the concentration of Ag atoms is too low to meet the requirement for spontaneous nucleation. At this moment, if some external seeds are present in the reaction system, providing an opportunity for these Ag atoms to grow, then the separation of nucleation and growth is achieved.17 If such a state is defined, these external seeds can be added to the system at an appropriate moment, and the amount of external seeds and AgCl involved in the reaction can also be controlled, so the size of the product may be calculated before the experiment is done. Therefore, the objective of quantitative size-controlled and size-designed synthesis of Ag particles can be achieved. In order to achieve the quantitative size-controlled and size-designed synthesis of Ag particles, the key is to define the reaction conditions that separate the states of crystal nucleation and growth, which also means defining a proper reaction rate, and then we can determine such a rate by testing the amount of NaOH added into the reaction system. In the experiments on the qualitative size-controlled synthesis of Ag nanoparticles, we have already described that when the dosage of NaOH is 2.0 mL the reaction rate is so slow that 12 h are needed to reduce all the AgCl into metallic Ag. In this case, the final Ag particles are still products of crystal growth of spontaneous seeds. Therefore, we conducted a test of quantitative size-controlled and size-designed synthesis by choosing a dosage of NaOH less than 2.0 mL and introducing external seeds of Ag nanoparticles prepared in the qualitative size-controlled synthesis.

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Figure 4. TEM images of as-prepared products, (a, b) two sizes of Ag particles coexisting at NaOH dosages of 1.5 and 1.0 mL, respectively. (c, d) Rod- and wire-shaped Ag particles produced when the dosage of NaOH is 0.5 mL.

First, we conduct experiments using 50 nm Ag particles as external seeds and adding 1.5, 1.0, and 0.5 mL of NaOH. As shown in Figure 4a, two sizes of Ag particles obviously exist when the dosage of NaOH is 1.5 mL. The larger particles are around 100-200 nm, and should be the products of crystal growth around the external seed, but the other smaller particles are around tens of nanometers in diameter and these should be the product of crystal growth from spontaneous seeds. This phenomenon indicates that spontaneous nucleation still cannot be avoided under such experimental conditions. Therefore, the condition of separation of crystal nucleation and growth has not been met. Similarly, as shown in Figure 4b, particles with two sizes sill exist when the dosage of NaOH is 1.0 mL, but the proportion of small particles is lower, and the size of the large particles is larger. This phenomenon indicates that growth on external seeds is more complete, and although the spontaneous nucleation still occurs, the amount of spontaneous seeds is reduced. When the amount of NaOH is reduced to 0.5 mL, the small Ag nanoparticles almost disappear, which indicates that spontaneous nucleation of Ag crystals is basically stopped and the reaction conditions are close to the state where crystal nucleation and growth are completely separated. Though the reaction conditions with 0.5 mL of NaOH separate crystal nucleation and growth, another problem, which is to inhibit the anisotropic growth of the Ag crystals, has to be solved in order to realize the objective of quantitative size-controlled and size-designed synthesis of Ag particles. We have mentioned in the above discussion that when the amount of NaOH added into the reaction system is small, the reaction rate is too slow, the amount of Ag seeds is small, and the growth rates of crystals are also slow, and therefore the newly generated Ag atoms in the system have abundant time to select various crystallographic planes to form rod-shaped particles.7b,8g,20 Under the reaction conditions where crystal nucleation and growth are separated, the

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reaction rate is even slower, so the trend to form rod-shaped particles is more obvious, and micrometer-sized Ag nanowires can even be generated (Figure 4c,d). Once these rodand wire-shaped particles are formed, it is hard to realize the quantitative size-controlled and size-designed synthesis of Ag particles. Therefore, the anisotropic growth of the Ag crystals must be inhibited in the reaction system. In order to inhibit the formation of rod- and wire-shaped Ag particles, the essential mechanism of such particle formation should be first comprehended. Generally, the thermodynamic factors producing anisotropic crystal growth are the different surface energies of the various crystallographic planes. Under appropriate reaction conditions, the atoms preferentially add to the planes with higher energy in order to reduce the total surface energy.21 For crystalline Ag with fcc structure,1h the surface energies of the low-index crystallographic planes can be estimated as γ{111} < γ{100} < γ{110}. In order to maximize the expression of {111} facets and minimize the total surface energy, a twinned decahedral crystal with 5-fold symmetry can be generated. In order to compensate for the lattice strain caused by the twinning defects, this 5-fold twinned crystal tends to grow along the axial direction, and then a pentagonal rod-shaped Ag particle enclosed by five newly formed {100} side facets is formed (Figure 5a). These {100} facets could be stabilized through chemical interactions with the oxygen (and/or nitrogen) atoms of the PVP added to the reaction system beforehand. In comparison, the interaction between PVP and the {111} facets of the Ag crystals should be much weaker. Therefore, once the rod-shaped particle has been formed, it can readily grow into a longer nanowire because its side surfaces are tightly passivated by PVP and its ends are largely uncovered and remain attractive toward new Ag atoms.22 During characterization of the products with FE-SEM, we happened to observe the adsorption of PVP on the Ag particles (as shown by the arrow in Figure 5b). In order to further illustrate such regulation of Ag crystal growth, the fine structures of the seeds need to be clearly comprehended to find a way to inhibit the anisotropic growth of the Ag crystals. We used HR-TEM to observe the external seeds, the Ag nanoparticles which were prepared in the qualitative sizecontrolled synthesis experiments. We chose as-prepared 50 nm Ag particles as the substrate. Under low resolution, as shown in Figure 5c, the particles are well dispersed and of uniform size. HR-TEM shows that single Ag particles have various morphologies, among these the most common are decahedron (Figure 5d), icosahedron (Figure 5e), and quasi-sphere (Figure 5f), and a few are cube (Figure 5g) and hexagon (Figure 5h). We believe that these various morphologies are caused by various seeds.1g,h,7b,d Because of various morphologies, the Ag particles prepared in the experiment of qualitative size-controlled synthesis, though they have relatively uniform size-distribution, are far from monodisperse. When these particles are adopted as external seeds, their diversity will be further enlarged by the crystals’ anisotropic growth. In particular, as discussed in the previous part, rod- and wire-shaped particles are likely to form when decahedrons are used as the seeds. Considering the various shapes of the seed particles, it seems only the sphere can restrain such anisotropic growth to the largest extent because of the sphere’s absolute symmetry. However, we found it is very hard to acquire exclusively sphere-shaped Ag nanoparticles. Traced to its source, the problem can be solved by a thermodynamic approach. According to the

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Figure 5. Inhibition of the anisotropic growth of Ag crystals: (a) sketch of the twinned decahedron, rod- and wire-shaped crystals, (b) FE-SEM image, indicating the adsorption of PVP on the surface of a Ag particle, (c) TEM image of as-prepared 50 nm Ag particles, (d, e, f, g, h) HR-TEM images of as-prepared 50 nm Ag particles for the particles with the shape of decahedron, icosahedron, quasi-sphere, cube, and hexagon, respectively; insets are sketches of the corresponding structures.

literature PVP is selectively adsorbed on various Ag crystallographic planes.1h We have also observed this adsorption in our experiment (Figure 5b), where we adopt PVP to protect the Ag particles from agglomeration,1a,7b,23 at a dosage of 1.5 times the Ag concentration (calculated by the monomer). Hence, we added a great excess of PVP so it will adsorb to all the facets of the Ag particles. This method should balance the difference of the surface energies of each facet of the Ag particles and restrain the anisotropic growth.7b The experiment shows that this means is effective. When the dosage of PVP is 15 times the amount of AgCl, no rod- and wireshaped particles are generated in the reaction system, which indicates the anisotropic growth of Ag crystal is effectively inhibited. As mentioned above, the reaction conditions for separation of Ag crystal nucleation and growth are determined, the anisotropic growth is also inhibited, and hence the conditions for quantitative size-controlled and size-designed synthesis of Ag particles are satisfied. Presuming the final product is a quasi-sphere, the volume should be related to the cube of the diameter. So the following equation can be employed to predict the size of the product Ag particles (see Supporting Information). rffiffiffiffiffiffiffiffiffiffiffiffi 3 N þn ð1Þ D ¼ d n In this formula, D is the mean size of the final products, d is the mean size of external seeds, and N and n are the molar amount of AgCl and external seeds in the system, respectively. Therefore, in order to obtain a previously designed size of Ag particles, two variables should be determined: one is the particle size of the external seeds, namely, the d value in eq 1, the other is the ratio of the total number of Ag atoms in the reaction system and the number of the Ag atoms contained in the external seeds, namely, (N þ n)/n in eq 1 (named r value below). Since we use the Ag nanoparticles prepared in the qualitative size-controlled synthesis, the particle size, d, of the seeds is known, and the molar amounts of seeds and AgCl can be controlled by the reagent dosage; therefore, quantitative size-controlled and size-designed synthesis of Ag particles can be realized. We design the following tests to prove the feasibility of the idea.

In test one, with as-prepared 50 nm Ag particles adopted as the external seeds, we design to prepare Ag particles with sizes 2, 3, 4, and 5 times the size of the seeds, namely, 100, 150, 200, and 250 nm; according to eq 1 the r values should be 8, 27, 64, and 125, respectively. Then, if the dosage of AgCl is fixed, the corresponding dosage of seeds can be calculated (see Supporting Information). As shown in Figure 6, the experiment result accords well with our design: the particles are quasi-sphere (or polyhedron shaped); no rod- and wireshaped particles exist, which indicates the anisotropic crystal growth is inhibited. The corresponding size-distribution histograms also show that the mean sizes of the products coincide with the designed sizes (Figure 6b-e). In addition, the XRD patterns in Figure 6f show that the external Ag seeds evidently have experienced a process of crystal growth. The larger the particle size is, the better the crystallinity is, and a regular trend appears in the XRD pattern; namely, the fwhm becomes smaller, and the intensity gradually grows stronger. In test two, 35 nm Ag particles are adopted as external seeds, the dosage of AgCl is fixed and the same as the amount in test one, and the dosage of seeds is determined by calculation to meet the r value of 250. Then according to eq 1, the size of the product Ag particles should be 220 nm, i.e., 2501/3 (namely, 6.3) multiplied by 35 nm (see Supporting Information). As shown in the FE-SEM image and the sizedistribution histogram of the as-prepared product (Figure 7a), these Ag particles just have an average size of 220 nm and also have a uniform size-distribution, which again coincides well with the prior experimental design. To have a better understanding about the surface chemical state of the products, XPS analysis was performed on asprepared 150 nm Ag particles. As shown in Figure 8, two XPS levels assigned to Ag3d3/2 and Ag3d5/2 were observed at binding energies of 373.83 and 367.91 eV, which shift down to lower binding energies in comparison with Ag0 metal (374.2 eV for Ag3d3/2 and 368.3 eV for Ag3d5/2),24a indicating that the chemical environment around Ag atoms changed.24 In addition, for the O1s XPS spectrum, the binding energy was 532.03 eV, showing an upper shift compared with that of PVP.24c It is proposed that the adsorption of the O atom in the carboxyl group on the Ag particles’ surface will induce an

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Figure 6. FE-SEM images of 50 nm Ag seeds (a) and as-prepared products of quantitative size-controlled and size-designed synthesis with sizes of (b) 100 nm, (c) 150 nm, (d) 200 nm, and (e) 250 nm. Reaction times are 16 h, insets are the corresponding size-distribution histograms, (f) shows the XRD patterns corresponding to (a-e).

Figure 7. FE-SEM images of as-prepared products of quantitative size-controlled and size-designed synthesis with sizes of (a) 220 nm and (b) 200 nm. Among these, (a) adopts 35 nm Ag particles as seeds, the r value is 250, the reaction time is 24 h, and (b) adopts 50 nm-Ag particles as seeds, the r value is 64, and the reaction time is 16 h.

upper shift in the case of the O1s binding energy, and a lower shift in the case of the Ag3d binding energies.25 Therefore, the XPS analysis derives a clear indication of the strong interaction between the surface of as-prepared Ag particles and the O atoms of the carboxyl groups in the PVP. HR-TEM was performed on as-prepared 200 nm Ag particles (see Supporting Information, Figure S3). The results indicate that prepared larger Ag particles are single- or twin-crystal structure of the nanoparticles. Combining with the results of XPS analysis, therefore, it is reasonable to deduce that the larger particles are grown by adding newly generated Ag atoms, with an isotropic aspect basically, in the presence of PVP coating on the facets of the particles. This

further reveals that the preconditions of separation of nucleation and crystal gowth as well as the isotropic uniform growth are reasonable. We conducted a scale-up test using 50 nm Ag particles as external seeds. The dosage of seeds is fixed to be the same as that in Figure 6b, but we increased the dosage of AgCl to match the r value of 64, which is the same as that in Figure 6d (The other reagent dosages in this system are increased accordingly). By carefully analyzing the above test conditions this test represents a 9-fold scale-up of the conditions in Figure 6d. Therefore, their products should be the same size; that is to say, this scale-up test should also produce Ag particles with diameters of 200 nm (see Supporting Information).

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Figure 8. XPS (a) Ag3d, (b) O1s spectra of as-prepared 150 nm Ag particles.

As shown in the FE-SEM images and the size-distribution histogram of the as-prepared product (Figure 7b), the average size of the Ag particles is the predicted 200 nm; furthermore, they also have uniform size-distribution, though the reaction system is scaled-up 9 times. All the above experimental results prove that, by adopting this reaction system for separating Ag crystal’s nucleation and growth and inhibiting the anisotropic growth, Ag particles with a designed size can be synthesized by simple calculations to determine the experimental parameters according to eq 1. Specifically, the size of the external seed in the reaction system is first determined, and then the d value in eq 1 is determined. Second, according to the size of the Ag particles designed, namely, the D value, we calculate the r value (N þ n)/n, namely, the ratio of the total number of the Ag atoms in the reaction system and the number of the Ag atoms contained in the external seeds. Finally, the certain reagent dosage is determined according to the scale of the synthesis. Consequently, the Ag particles with the designed size are prepared and the objective of quantitative size-controlled and sizedesigned synthesis of Ag particles can be achieved. Incidentally, we also tried to synthesize micrometer-sized Ag particles according to this procedure. However, it is difficult to acquire products with uniform size-distribution. Moreover, it requires an extraordinarily long reaction time to ensure the AgCl is completely reduced by this procedure. The significance of larger Ag particles in practical application is relatively obscure, and so we will not conduct more investigations on this subject. 4. Conclusions In conclusion, we present a method of using ascorbic acid to reduce AgCl in aqueous solution and at ambient conditions to prepare a series of nano/submicrometer Ag particles with uniform size-distribution. In this procedure, by using NaOH to adjust the redox potential of ascorbic acid to AgCl and controlling spontaneous nucleation in reaction system, the objective of qualitative size-controlled synthesis of Ag nanoparticles is achieved. Furthermore, we establish an equation for the relationship between the synthesis conditions and the size of the product, and apply it to produce quantitatively sizecontrolled and size-designed Ag particles. The desired particle sizes can be predicted by simple calculations before the experiment is done, thus realizing the objective of quantitative sizecontrolled and size-designed synthesis of Ag nanoparticles. The current synthetic procedure is readily applicable to large-scale synthesis because of its simplicity and the mild reaction conditions, namely, in the use of aqueous and ambient conditions and the nontoxic and cheap reagents. For example, in the qualitative size-controlled synthesis of Ag particles, we increase the dosage of all reagents by a factor of 16.

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In this case, the synthesis follows the same pattern and more than 100 mg of Ag nanoparticles are produced with a uniform size-distribution. Similarly, the experiment of quantitative size-controlled and size-designed synthesis of 200 nm Ag particles is scaled-up by 9 times and the same reproducible experiment result is obtained. Undoubtedly, this characteristic of being facilely scalable is important for practical industrial applications. The reaction system which we present here not only has application in the qualitative and quantitative size-controlled and size-designed synthesis of Ag particles but could also be used to tailor and fine-tune the properties of Ag particles by designing and synthesizing them with different sizes, shapes, and compositions. We have succeeded in separating Ag crystal nucleation and growth, and we believe that this process can be used in the design and synthesis of fascinating Ag nanostructures if other styles of seeds are adopted. On the basis of determining the reaction conditions for the separation of crystal nucleation and growth, as well as inhibiting the anisotropic growth of crystals, this size-controlled and sizedesigned synthesis idea would be extended under different conditions and reaction systems. Such investigations are in progress in our laboratory. Acknowledgment. This work is supported by the Major State Basic Research Development Program of China (973 Program) (No. 2010CB933504), the Science Funds for Distinguished Young Scientists of Shandong Province (JQ200903), and the Doctoral Foundation of Shandong Province (2007BS04042). The authors thank Dr. Pamela Holt for editing the manuscript for English. Supporting Information Available: Detailed data for the experiments of size-controlled and size-designed synthesis Ag particles, and the scale-up experiments, the deducing of equation 1. TEM and HR-TEM images of prepared products. This material is available free of charge via the Internet at http://pubs.acs.org.

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