Roles of Pt seeds and chloride anions in the preparation of silver nanorods and nanowires by microwave-polyol method

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Colloids and Surfaces A: Physicochem. Eng. Aspects 316 (2008) 266–277

Roles of Pt seeds and chloride anions in the preparation of silver nanorods and nanowires by microwave-polyol method Masaharu Tsuji a,b,c,∗ , Kisei Matsumoto b,c , Peng Jiang a,d , Ryoichi Matsuo b,c , Xin-Lin Tang b,d , Khairul Sozana Nor Kamarudin a,e b

a Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka 816-8580, Japan Department of Applied Science for Electronics and Materials, Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan c CREST, Japanese Science and Technology, Kawaguchi, Saitama 332-0012, Japan d National Center for Nanoscience and Technology, China, Beijing 100080, People’s Republic of China e Department of Gas Engineering, Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia

Received 15 July 2007; received in revised form 17 August 2007; accepted 3 September 2007 Available online 8 September 2007

Abstract Silver (Ag) nanomaterials with well-defined crystal structures have been rapidly synthesized in three minutes by a microwave (MW)-polyol method. Various precursors such as H2 PtCl6 and Pt(acac)2 have been used to detect real roles of Pt seeds and anions for the formation of the Ag nanomaterials. Furthermore, effect of Cl− anions to final morphologies of the Ag nanostructures has also been explored by pre-adding Cl− precursor such as NaCl or KCl in the ethylene glycol (EG) solutions only including AgNO3 and poly(vinylpyrrolidone) (PVP). The experimental results show that pre-formed metallic Pt seeds are probably not responsible for nucleation and subsequent evolution of 1D Ag products, but Cl− ions indeed influence the formation of the 1D Ag nanostructures as well as perfect crystallization of other Ag nanoparticles with well-defined crystal structures including single- and twinned-FCC crystals. It has been further evidenced that the presence of Cl− ions can accelerate re-dissolution of formed spherical Ag particles and is favorable to the growth of the 1D Ag and other Ag nanostructures with well-defined crystal structures such as single-crystal cubes and twinned bi-pyramids in the MW-assisted polyol reduction process. At the same time, it also indicates that H2 PtCl6 probably does not act as a nucleation agent for the 1D Ag products but as a precursor of Cl− ions to affect final morphologies of the Ag products. Possible factors affecting shape-selected process of the Ag nanostructures have been discussed in detail. © 2007 Elsevier B.V. All rights reserved. Keywords: Pt seeds; Chloride anion; Silver nanostructures; Microwave heating; Polyol method

1. Introduction One-dimensional (1D) nanostructures (rods, wires, and tubes) of silver (Ag) have received considerable attention from a broad range of researchers because of their wide applications in catalysts, scanning probes, and various kinds of electronic and photonic nanodevices [1,2]. Here, we define nanorods and nanowires as the nanomaterials with aspect ratios of 2–20 and >20, respectively. Since catalytic, optical, and electric properties of the 1D nanostructures depend strongly on their shapes and

∗

Corresponding author. Tel.: +81 92 583 7815; fax: +81 92 583 7815. E-mail address: [email protected] (M. Tsuji).

0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.09.014

sizes, extensive studies on shape- and size-controlled syntheses of the 1D nanostructures have been carried out. Microwave (MW)-polyol method is a promising route for rapid preparation of metallic nanomaterials [3]. Recently, we have succeeded in rapid preparation of such anisotropic Ag nanostructures as nanorods, nanowires, nanosheets, nanoplates, and nanocubes by using this approach [3,4]. When MW was irradiated into the mixture of AgNO3 /H2 PtCl6 ·6H2 O/PVP in EG solution, anisotropic 1D Ag nanorods and nanowires were produced preferentially. Previous studies on anisotropic growth of Ag nanomaterials have demonstrated that Pt seeds play a key role and PVP serves as a protecting reagent [2–5]. Sun et al. [5] have performed a pioneer study on the preparation of the 1D Ag nanostructures by reducing AgNO3 in the presence of PVP and PtCl2 (a precursor

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of metal Pt) in EG at 180 ◦ C using oil bath. They have suggested that the formation of the 1D Ag nanostructures proceeds via two steps at least. At first step, Pt nanoparticles with an average diameter of 5 nm are formed by reducing PtCl2 with EG: 2HOCH2 –CH2 OH → 2CH3 CHO + 2H2 O

(1)

2CH3 CHO + PtCl2 → CH3 CO–COCH3 + Pt + 2HCl

(2)

At second step, AgNO3 and PVP are added into the reaction system drop-by-drop, leading to the nucleation and growth of Ag particles. They thought that the pre-synthesized Pt nanoparticles serve as nuclei for the epitaxial growth of new Ag nanostructures because crystal structures and lattice parameters for Pt and Ag are similar. As a result, Ag nanoparticles with diameters of 20–30 nm are formed in the solution. Besides the growth process above, most of Ag atoms nucleate through a homogeneous nucleation process. This yields the Ag nanoparticles with diameters in the range of 1–5 nm. When the dispersion of the bimodal Ag nanoparticles is continuously heated at high temperature, the small Ag nanoparticles progressively disappear to supplement Ag for those larger ones via a process called Ostwald ripening [6]. With the assistance of PVP protection agent, some of the larger decahedral particles can grow into the 1D Ag nanostructures with cross-section dimensions of 30–40 nm. The growth process would not continue until all the Ag nanoparticles with diameters <5 nm are consumed up. Therefore, only the Ag nanowires and the larger nanoparticles can survive stably. In the present study, we have conducted a further detailed study on the real role of Pt seeds as a nucleation regent for the preparation of 1D Ag nanostructures by using the MW-polyol method. When Pt catalysts were prepared from two different agents, H2 PtCl6 ·6H2 O and Pt(acac)2 , the 1D Ag nanostructures could be easily prepared only by using H2 PtCl6 ·6H2 O, although Pt seeds could be produced when using either of them. This observation might imply that Pt catalysts probably do not act as seeds for evolution of the 1D Ag nanostructures. In contrast, it is possible for Cl− ions resulting from H2 PtCl6 ·6H2 O to play an important role in assisting the formation of the 1D Ag nanostructures. More experimental evidences have been provided to elucidate the role of Cl− ions by using NaCl or KCl as a source of Cl− ions. In addition, besides the 1D Ag products, the formation of some well-crystallized cubic and bi-pyramid Ag nanocrystals with sharp edges has been found to be related to existence of Cl− ions. Possible formation processes of these Ag nanostructures under MW-polyol process are discussed. 2. Experimental The MW-polyol apparatus used in this study is similar to that reported previously [3,4]. A MW oven was modified by installing a condenser, a thermocouple through holes of the ceiling and a magnetic stirrer coated with Teflon at the bottom. A three-necked flask (100 ml) was placed in the MW oven and connected to the condenser. When Ag nanostructures were prepared using PVP (average molecular weights 10, 40, and 360 k in term of monomeric units), 1D products were produced with the highest yield using 360 k. Therefore, we used

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PVP (360 k) in this study. To examine effects of Pt seeds and Cl− anions, five kinds of experiments were carried out. In the first experiment, the solution containing AgNO3 , H2 PtCl6 ·6H2 O or platinum(II) bis-acetylacetonate denoted as Pt(acac)2 , and PVP in EG was irradiated by MW (Shikoku Instrumentation: ␮ Reactor) in a continuous wave (CW) mode at 650 W for 3 min. In the second experiment, AgNO3 /PVP/EG solution was used without addition of H2 PtCl6 ·6H2 O or Pt(acac)2 as a nucleation agent. In the third experiments, H2 PtCl6 ·6H2 O/PVP/EG or Pt(acac)2 /PVP/EG solution was used without addition of AgNO3 to examine what kinds of shapes and sizes of Pt seeds could be really produced. In the fourth experiment, AgNO3 /MCl (M = Na or K)/PVP/EG solution was used to detect effect of Cl− anions. The above four experiments were carried out in onestep process. In the fifth experiment, two-step seeded growth experiment was carried out. In this case, pre-synthesized Pt nanoparticles and Cl− from H2 PtCl6 ·6H2 O were used for the preparation of Ag nanostructures by the reduction of AgNO3 . Detailed reagent concentrations in each experiment will be given in figure captions. In all the five experiments described above, the solution was rapidly heated to the boiling point of EG (198 ◦ C) in about 1 min and kept at the temperature for 2 min. The total heating time was controlled for only 3 min. After MW irradiation, product solutions of Ag or Pt nanostructures were centrifuged at 13,000 rpm for 60 min. The relative centrifugal force was 1700 G in the centrifugal separation. The precipitate was collected, and re-dispersed in deionized water for JEOL JEM-2010 transmission electron microscope (TEM) observation. Specimens containing Ag or Pt nanostructures were prepared by dropping the colloidal solutions on carbon-covered Cu grids. Absorption spectra of the product solutions were measured in the UV–vis–near infrared (NIR) region by using a Shimadzu UV-3600 spectrometer. Original product solutions were diluted in EG by a factor of 100 before spectral measurements. 3. Results and discussion 3.1. Effects of Pt particles Fig. 1 shows TEM images and crystal structure schemes of five kinds of typical Ag nanostructures obtained in this study. Selected area electron diffraction (SAED) patterns were measured by focusing electron beam perpendicularly to well-defined facets of individual triangular bi-pyramid and cubic crystals. Obtained SAED patterns demonstrate typical {1 0 0} facet feature for face-centered cubic (FCC) crystal, as shown in Fig. 1. Spherical Ag particles (Fig. 1a) exhibit polycrystalline structure because of ring-type SAED patterns (not be shown in Fig. 1), whereas cubic (Fig. 1b) and triangular bi-pyramids (Fig. 1c) Ag particles have single crystal and single-twinned crystal structures surrounded by solely {1 0 0} facets, respectively. Average sizes of spherical Ag particles were determined by measuring their diameters. For those Ag particles having special shapes such as triangular bi-pyramid and cube, we generally considered their average edge lengths as average sizes of them, as shown by arrows in Fig. 1. Two types of 1D and quasi-1D Ag prod-

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Fig. 1. TEM images, TEM-SAED patterns, and crystal structures of spherical, cubic, triangular bi-pyramid, and 1D Ag nanocrystals prepared under MW heating prepared from AgNO3 (46.5 mM)/H2 PtCl6 ·6H2 O(115 ␮M)/PVP(264 mM)/EG and AgNO3 (46.5 mM)/NaCl(690 ␮M)/PVP(264 mM)/EG solutions under MW heating. Arrows indicate definition of sizes of each particle.

ucts were obtained. They possess five-fold twinned (Fig. 1d) [1–5,7] and plate-like twinned structures (Fig. 1e), respectively [8,9]. For the 1D and quasi-1D products, their sizes were just provided by average diameters and lengths. Fig. 2 depicts crystal structures and possible growth mechanisms for different Ag or Au nanostructures evolved from various (FCC) Ag or Au nanoparticle precursors [2,4c,4d,5c,7c,8–12]. It is expected that spherical particle (Fig. 2a), cubic crystal (Fig. 2b), triangular bi-pyramid and plate-like 1D crystals (Fig. 2c), and five-fold twinned-rods and twinned-wires (Fig. 2d) grow from small quasi-multipletwinned spherical particles, cubooctahedron, triangular and hexagonal plates, and decahedral particles, respectively. In our MW-polyol synthesis experiments, although a mixture of all these nanostructures was obtained in most cases, we mainly focus our attention upon the influence of Pt catalysts and Cl− anions to the formation of the 1D Ag products in the present study. Fig. 3a–b shows TEM images of the Ag products obtained by MW heating AgNO3 /PVP/EG solutions without and by the addition of H2 PtCl6 ·6H2 O, respectively. Magnified TEM images are shown in Fig. 3a-2 and b-2. When lacking H2 PtCl6 ·6H2 O, majority products are isotropic spherical Ag nanoparticles (yield 96%) with an average diameter of 98 ± 35 nm. A small amount of triangle Ag particles (yield 4%) with an average size of 207 ± 31 nm co-exists in the products (see Fig. 3a and the case

of [NaCl] = 0 in Tables 1 and 2). Once adding H2 PtCl6 ·6H2 O into the reactant solution, besides spherical and triangular Ag nanoparticles, cubic nanoparticles and 1D Ag nanorods and nanowires were also obtained. In general, triangular, cubic, and 1D Ag nanoparticles obtained by the addition of H2 PtCl6 ·6H2 O have not sharp edges and corners. It was found that the 1D product only with five-fold twinned structure was produced in the AgNO3 /PVP/H2 PtCl6 ·6H2 O/EG system (Fig. 3b). The yields and average sizes of spherical, cubic, triangular particles and 1D products are shown in Tables 1 and 2. Highly crystalline 1D Ag nanostructures are prepared only in the presence of H2 PtCl6 ·6H2 O. It seems therefore reasonable to assume that Pt catalysts are necessary for the preparation of five-fold twinned 1D Ag products. When H2 PtCl6 ·6H2 O was reduced in EG in the presence of PVP by MW heating, small spherical Pt nanoparticles with average diameters of 5 ± 2 nm could be formed, as shown in Fig. S1a (supplementary data). Although these particles were believed to act as nucleation catalyst of anisotropic Ag nanostructures, we find that this prediction is incorrect as shown below. To further elucidate the exact role of Pt catalysts, Pt(acac)2 was also employed as an alternative nucleation agent. Fig. 3c1 and c-2 shows typical TEM images of the products obtained from the AgNO3 /Pt(acac)2 /PVP/EG system. Note that dominant Ag products (96%) are isotropic spherical particles with an average diameter of 160 ± 50 nm (Tables 1 and 2). Only

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Fig. 2. Crystal structures and growth mechanisms of various shapes of Ag nanostructures.

Fig. 3. TEM images of Ag nanostructures prepared from (a) AgNO3 (46.5 mM)/PVP(264 mM)/EG, (b) AgNO3 (46.5 mM)/H2 PtCl6 ·6H2 O(0.115 mM)/PVP(264 mM)/EG, and (c) AgNO3 (46.5 mM)/Pt(acac)2 (0.115 mM)/PVP(264 mM)/EG solutions under MW heating. Magnified images are shown in a-2 and b-2, whereas c-1 and c-2 are two typical images measured in the same scale.

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Table 1 Yields (%) of Ag nanostructures obtained at various H2 PtCl6 ·6H2 O, Pt(acac)2 , NaCl, or KCl concentrations H2 PtCl6 ·6H2 O, Pt(acac)2 , NaCl, or KCl

mM

Rod and wire

Spherical

H2 PtCl6 ·6H2 O

0.115

23

65

Pt(acac)2

0.115

4

96

–

–

NaCl

0 0.069 0.173 0.345 0.69 1.38

0.4 7 32 4 25 21

96 40 14 14 43 32

0 47 21 78 16 23

4 6 33 4 16 24

KCl

0.69

26

20

26

28

a small amount of short 1D Ag nanorods (4%) can be seen. This result is significantly different from the case of the AgNO3 /H2 PtCl6 ·6H2 O/PVP/EG system (Fig. 3a), in which more Ag 1D products occur by MW heating. When increasing or decreasing the concentration of Pt(acac)2 by a factor of 10, we found that main product was still spherical Ag particles, as shown in Fig. S2 (supplementary data). In general, when Pt(acac)2 was reduced in EG without addition of AgNO3 by MW heating, small Pt nanoparticles with diameters of 2–3 nm could also be obtained, as shown in Fig. S1b (supplementary data). Assuming that the formation of the 1D Ag products depends on the small Pt nanoparticles, the 1D Ag products should rise or fall with the increase or decrease of the Pt nuclei. In fact, there is nearly no change in the yield of the 1D Ag products when Pt(acac)2 concentration varies. It indicates that morphologies of the Ag products are essentially independent of the concentration of Pt(acac)2 . These results remind us to think that the small Pt nanoparticles probably do not act as the seeds for evolution of the 1D Ag products. 3.2. Effects of Cl− anions The most significant difference between H2 PtCl6 ·6H2 O and Pt(acac)2 as nucleation agents is that Cl atoms are involved in the former reagent and become a source of Cl− anions during the process of reduction reaction. The concentration of Cl− anions produced after MW irradiation of H2 PtCl6 ·6H2 O/PVP/EG solution for 3 min was measured by using titration apparatus (Kyoto Electronic AT610). A standard AgNO3 solution was used as a

Cubic

Triangular

3

9

titration reagent. We found that ∼0.67 mM of Cl− anions were generated after 0.115 mM of H2 PtCl6 ·6H2 O in EG solvent was irradiated for 3 min by MW. If 0.115 mM of PtCl6 2− anions are decomposed via process (3), the Cl− concentration is estimated to be 0.230 mM in terms of the chemical equation, PtCl6 2− → Pt + 2Cl2 (Cl2 + 2Cl, or4Cl) + 2Cl−

(3)

For comparison, assuming that all Cl atoms in 0.115 mM of PtCl6 2− are completely converted to Cl− via reaction (4), the Cl− concentration is expected to be 0.69 mM: PtCl6 2− + 4e− → Pt + 6Cl−

(4)

Our measured Cl− concentration in final product solution is much closed to the value calculated by reaction (4), indicating that Cl atoms in PtCl6 2− are nearly completely converted to Cl− under our experimental conditions. Although the solubility product of AgCl in H2 O is very small at room temperature (pKsp = 9.75), it will be much larger than that in EG at 198 ◦ C. Therefore, Cl− anions can take part in the oxidative etching of Ag nanostructures at high temperature where the reduction of Ag+ to Ag0 in EG occurs. To clarify the effect of Cl− anions, various amounts of NaCl were premixed in the AgNO3 /PVP/EG solution. Figs. 4 and 5 show typical TEM images of the products obtained from the AgNO3 /NaCl/PVP/EG solutions with low (0–0.345 mM) and high (0.69–6.9 mM) Cl− concentrations, respectively. Bottom TEM images of Fig. 4 and middle and bottom TEM images of Fig. 5 are magnified or different TEM images of top ones.

Table 2 Average sizes (nm) of Ag nanostructures obtained at various H2 PtCl6 ·6H2 O, Pt(acac)2 , NaCl, or KCl concentrations H2 PtCl6 ·6H2 O, Pt(acac)2 , NaCl, or KCl

mM

Rod and wire; diameter, length

Spherical size

Cubic size

Triangular size

190 ± 50

170 ± 20

H2 PtCl6 ·6H2 O

0.115

110 ± 20, 2450 ± 2300

150 ± 40

Pt(acac)2

0.115

145 ± 40, 620 ± 190

160 ± 50

NaCl

0 0.069 0.173 0.345 0.69 1.38

– 69 ± 19, 2130 ± 2030 114 ± 55, 2320 ± 1200 116 ± 51, 6370 ± 3080 87 ± 34, 1640 ± 1070 79 ± 26, 2370 ± 1890

98 120 125 248 144 111

KCl

0.69

118 ± 86, 2160 ± 1800

244 ± 109

± ± ± ± ± ±

35 37 62 99 49 36

–

–

– 83 ± 21 142 ± 31 158 ± 23 141 ± 20 117 ± 16

207 ± 31 110 ± 50 221 ± 49 267 ± 37 207 ± 31 192 ± 31

192 ± 49

298 ± 82

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Fig. 4. TEM images of Ag nanostructures prepared from AgNO3 (46.5 mM)/NaCl(0–0.345 mM)/PVP(264 mM)/EG under MW heating. Magnified images are shown in x-2 (x = a–d).

The yield and average size of each type of particle in the range of 0–1.38 mM of Cl− concentration are summarized in Tables 1 and 2. In the case of no NaCl, main product is spherical Ag particles (96%) besides small amount of short 1D products (0.4%) and triangular particles (4%). At higher NaCl concentrations such as 2.76 and 6.9 mM, it is difficult to determine yield and average size of each kind of particle because a large amount of AgCl is deposited on Ag particles. As listed in Table 1, the yields for the 1D Ag products can achieve 4–32% when changing the NaCl concentration in the region of 0.069–1.38 mM, even without existence of Pt catalysts. In addition, highly crystalline Ag cubes and triangular bi-pyramids are preferentially produced simultaneously. Edges and corners of the regular Ag particles become sharp in the presence of Cl− anions due to high crystallization. In general, in the presence of Cl− anions, a mixture of 1D products, spherical particles, cubes, and triangular bi-pyramids is obtained by our MW-polyol synthesis method. There is no obvious dependent relationship of the shape and size of the product to NaCl concentration, but monodispersed cubic Ag crystals can be obtained with high yield (78%) at 0.345 mM of NaCl concentration, indicating that the MW-polyol method is a promising one to rapidly synthesize monodispersed cubic Ag nanoparticles in the presence of NaCl. Although 1D products, cubes, and triangular bi-pyramids can also be produced in the higher concentration range of 0.69–6.9 mM, amorphous particles also appear at the same time, as shown by arrows in Fig. 5b-3, c-3, and d-3. These particles can be easily dissolved when an electron beam was irradiated to them in TEM measurements. We have measured selected area electron diffraction (SAED) patterns and TEM-EDS (energy dis-

persive X-ray spectroscopy) data for the elemental analysis of amorphous particles. Their SAED patterns agreed with Debye rings of AgCl, and both Ag and Cl components were observed in EDS data of such amorphous particles. Thus, it was confirmed that these amorphous particles were AgCl particles. It is very interesting to evaluate effect of Cl− anion on Ag particle crystallization by using different Cl− anion sources. Taking the AgNO3 /H2 PtCl6 ·6H2 O(0.115 mM)/PVP/EG system as an example, Cl− concentration was measured to be 0.67 mM after MW irradiation to the solution for 3 min. In comparison with the case of the AgNO3 /NaCl/PVP/EG system with a NaCl concentration of 0.69 mM, the yield for the 1D products is similar, but the yield for spherical particles is much higher while that for cubes and triangular bi-pyramids is lower for the former than for the latter (see Table 1). The fact suggests that the initial presence of Cl− ions is more effective for the formation of highly crystalline nanoparticles than the supplement of Cl− ions from PtCl6 2− during the process of reduction reaction by MW heating. All of the above experiments were carried out in one-pot style. Besides it, two-step synthesis was also performed. That is, Pt seed solution was prepared from the H2 PtCl6 ·6H2 O(0.029 mM)/PVP(132 mM)/EG solution at the first step, and then AgNO3 (23 mM) was added at the second step. In the two-step synthesis, obtained products are composed of spherical, cubic, triangular particles, and 1D nanostructures too, as shown in Fig. 6a. The difference is that edges of cubic and triangular particles are much sharper than those observed in the one-step process from the AgNO3 /H2 PtCl6 ·6H2 O/PVP/EG solution (Fig. 3b). The experiment also implies that initial pres-

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Fig. 5. TEM images of Ag nanostructures prepared from AgNO3 (46.5 mM)/NaCl(0.69–6.9 mM)/PVP(264 mM)/EG under MW heating. Middle and bottom images shown in x-2 and x-3 (x = a–d) are magnified or different TEM images of top ones.

ence of a small amount of Cl− anions in the solution is effective for the preparation of higher crystalline Ag nanoparticles. To examine the effects of cations, KCl was used as another source of Cl− anions. The TEM images obtained from the AgNO3 /KCl/PVP/EG solution at a KCl concentration of 0.69 mM is shown in Fig. 6b. Products with various shapes including 1D nanostructure, spherical particles, cubes, and triangular bi-pyramids are prepared as in the case of the AgNO3 /NaCl/PVP/EG solution. This also positively evidences that Cl− anions are indeed responsible for the formation of anisotropic 1D Ag nanostructures, regular cubic, and triangular bi-pyramid-like Ag nanoparticles. Although shapes of the product particles are independent of cation species, there are significant differences in the yields and sizes of the Ag products when using different MCl reagents such as NaCl and KCl (Tables 1 and 2). The relative yields of cubic and triangular particles to that of spherical ones are larger for KCl than those for NaCl. The sizes of product particles for KCl are larger than those for NaCl. Based on the observation, it seems that cation

ions probably influence Ag product distributions under present experimental conditions. For example, Ag nanoparticles with single crystal and multiple-twinned crystal structures can reach much higher yields when using KCl than NaCl as a source of Cl− ions. 3.3. UV–vis–NIR spectra It has been well known that UV–vis–NIR absorption spectra of Ag nanostructures depend strongly on the shapes and sizes of Ag nanostructures, as reported by Mock et al. and Wiley et al. [13,14]. Surface plasmon resonance (SPR) band for spherical Ag nanoparticles with diameters of 20–40 nm has a peak at ∼410 nm. The peak shifts to ∼480 nm with the size increase from 40 to 90 nm [14]. The main SPR peaks for Ag nanorods and nanowires appear at ∼350 and ∼380 nm [4,5,7]. Sun et al. [5] attributed these two peaks to the SPR bands of the longitudinal mode of Ag nanowires similar to that of bulk Ag and the transverse mode of Ag nanowires, respectively. However,

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Fig. 6. TEM images of Ag nanostructures prepared from (a) AgNO3 (23 mM)/H2 PtCl6 ·6H2 O(0.029 mM)/PVP(132 mM)/EG solution through the two-step process and (b) AgNO3 (46.5 mM)/KCl(0.69 mM)/PVP(264 mM)/EG solution. Magnified images are shown in a-2 and b-2.

according to a recent report by Gao et al. [7c,7d], the same peaks were attributed to the transversal modes of the 1D products with pentagonal cross sections, corresponding to the out-of-plane quadrupole resonance and out-of plane dipole resonance modes, respectively. When the ∼410 nm peak due to spherical particles is strong, it is overlapped with the ∼380 nm peak. In such a case, the ∼380 nm peak is not clearly observed. When long Ag nanowires are produced, a long tail band is observed above 450 nm. Gao et al. [7d] attributed this tail band to the overlapping of the in-plane quadrupole and dipole resonance modes of the Ag nanowires with peaks at 445 and 514 nm, respectively. SPR bands of cubic (edge length: ∼80 nm) and triangular bipyramidal (edge length: 75 nm) Ag crystals have been observed at 320–800 nm with a peak at ∼470 and 320–900 nm with a peak at ∼520 nm, respectively [2,14,15]. When cubic and triangular Ag particles are mixed with spherical and 1D Ag products, the above peaks are also overlapped with the SPR bands of spherical and 1D particles. UV–vis–NIR absorption spectra were measured here under various experimental conditions. For example, Fig. 7a shows absorption spectra obtained from the AgNO3 /PVP/EG, AgNO3 /H2 PtCl6 ·6H2 O/PVP/EG and AgNO3 /MCl (M = Na or K)/PVP/EG systems. In all cases, an interband absorption of Ag [16] can be observed below 320 nm region. The spectrum of the AgNO3 /PVP/EG solution has a single peak at ∼430 nm. This peak can be attributed to the SPR band of spherical nanoparticles

273

with an average diameter of ∼100 nm. The UV–vis–NIR spectra become broad in the AgNO3 /H2 PtCl6 ·6H2 O/PVP/EG and AgNO3 /MCl (M = Na or K)/PVP/EG systems. The peaks are located at 400–430 nm and weak shoulders, as shown by arrows, are observed at ∼350 nm. The long tail band above 450 nm can be attributed to the overlapping of the in-plane quadrupole and dipole resonance modes of Ag nanowires, triangular bipyramids, and cubes, and the ∼350 nm band can be ascribed to the transversal modes of the 1D Ag products. Fig. 7b shows UV–vis–NIR absorption spectra obtained from AgNO3 /NaCl/PVP/EG system at various NaCl concentrations. The interband absorption of silver can be observed below 320 nm region in all cases. At the lowest NaCl concentration of 0.069 mM, only weak symmetrical SPR band with a peak at ∼410 nm can be observed. This spectrum is consistent with the high yields of spherical particles (120 nm, 40%) and cubes (83 nm, 47%) and a low yield of 1D Ag products (7%). The spectrum at 0.35 mM has a main peak at 580 nm and weak shoulder peaks at 360 and 430 nm. Under this condition, Ag cubic crystals with an average size of ∼160 nm are preferentially produced. Thus, the main peak can be attributed SPR band of cubic Ag crystals. The UV–vis–NIR spectra at the other concentrations are similar. They consist of a main SPR peak at 410 nm with long tail bands extending to longer wavelength region and a weak shoulder peaks at ∼350 nm. These spectra can be ascribed to SPR bands of a mixture of spherical, triangular, and cubic Ag particles as well as 1D Ag products, shown in Table 1. 3.4. Growth mechanism of Ag nanostructures Oxidative etching of Ag nanostructures by Cl− /O2 in the polyol method has recently been studied by Xia and co-workers [17]. They found that addition of chloride resulted in enhanced oxidation and preferential etching of twinned particles, leaving only the single-crystal particles (or seeds) to grow. These single crystal nanoparticles were monodispersed in size, and particles with diameters ranging from 20 to 80 nm were obtained by simply controlling the reaction time. The shapes of these nanoparticles were a mixture of cubes and tetrahedrons characterized by truncated corners and edges. In multiple-twinned decahedral particles, there exist defects among each single crystal tetrahedron subunits because each tetrahedron can only contribute an angle of 70.5◦ , leaving a 7.5◦ gap. Such defects in decahedral particles provide high energy sites for atomic addition, leading to 1D Ag products via anisotropic growth along 1 1 0 direction. In contrast, in an oxidative environment, preferential etching and dissolution of the multiply-twinned particles result from the higher reactivity of twinned particles versus single-crystal particles due to instability of multiple-twinned particles. According to the nucleation and growth model of 1D Ag products by Sun et al. [5b], bimodal Ag nanoparticles were obtained at first step. When the dispersion of bimodal Ag nanoparticles was continuously heated at high temperature at second step, the small Ag nanoparticles progressively disappeared to feed larger ones via a process called Ostwald ripening [6]. In the present investigation, on the basis of the experimental results,

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Fig. 7. UV–vis–NIR spectra observed from the (a) AgNO3 (46.5 mM)/PVP(264 mM)/EG, AgNO3 (46.5 mM)/H2 PtCl6 ·6H2 O(0.115 mM)/PVP(264 mM)/EG, and AgNO3 (46.5 mM)/MCl(M = Na and K: 0.69 mM)/PVP(264 mM)/EG solutions and (b) AgNO3 (46.5 mM)/NaCl(0.069-6.9 mM)/PVP(264 mM)/EG solutions under MW heating.

we think that shape-selected oxidative etching by Cl− ions probably plays a significant role for the dissolution of smaller spherical Ag nanoparticles. In the case of Sun et al. [5b], Cl− anions can be produced from decomposition of PtCl2 in polyol process. In our experiments, we found that initial existence of Cl− anions in the presence of O2 (dissolved in solvent) are more favorable to form 1D, cubic, and bi-pyramid Ag particles especially with sharp edges. It might be due to the fact that small seed particles prepared in the initial stage can be etched by Cl− anions. When Cl− ions are absent or its concentration is low, smaller Ag crystals have more probability to aggregate and further evolve into larger spherical particles due to not enough adsorption or oxidation by Cl− ions. For example, the products obtained from AgNO3 /NaCl/PVP/EG solution at a lower NaCl concentration of 0.069 mM usually have little sharp edges except for cubes, while sharp edges are general features for the products at the NaCl concentration above 0.173 mM. It should be point out that we did not find a general tendency between the shape and size of the Ag products and Cl− ion concentration. Precise reasons for this are not clear. One possibility is that the formation of AgCl may keep Cl− ion concentration constant at high temperatures. We also found here that an intermediate NaCl concentration of 0.345 mM is an optimum concentration for the synthesis of monodispersed Ag cubes (∼80%, ∼160 ± 20 nm) at high yield within only 3 minutes under MW irradiation. Fig. 8 shows the mechanism of shape selective oxidative etching and growth of Ag nanostructures under MW-polyol method. In our conditions, spherical particles are dissolved due to oxidative etching, while seeds of single crystals and twin-crystals can survive and grow to larger crystals having clear facets. Combined our analysis above, we think that the oxidative etching and growth mechanism proposed by Xia and co-workers [17] might be partly applied to shape-selected oxidative etching for the Ag nanoparticles. The decrease in yields of the spherical Ag particles and increase in the single crystal Ag cubes in the presence of Cl− anions in the present study are consistent with the previous finding by Xia and co-workers [17] but,

some twinned Ag triangular bi-pyramids and multiple-twinned Ag nanorods and nanowires are still preferentially produced under our experimental conditions. As confirmed by our recent MW-polyol investigation on core-shell Au–Ag nanorods and nanowires using decahedral Au nanoparticles as seeds [11], the five-twinned Ag rods and wires were formed from five-twinned decahedral nanoparticles. This shows that the preferential etching for the initial formed five-twinned Ag particles by Cl− anions does not occur in our MW-polyol synthesis experiments, but only takes place for spherical Ag nanoparticles, because spherical Ag nanoparticles are more active than single and twinned crystals for Cl− oxidative etching. In general, spherical nanoparticles have more defects on surface than single crystal (cubic) and twinned crystals (bi-pyramids and 1D products) with welldefined facets. Therefore, Cl− ions have more opportunities to attack and further dissolve unstable spherical Ag nanoparticles. Recently, we have explored a study on shape-selected oxidative etching of Au nanoparticles by AuCl4 − and Cl− anions by using MW-polyol method [12], in which we found that spherical particles and even octahedral particles with single-crystal structure are much easier to be etched than twinned plate-like and multiple-twinned Au particles. This means that there also exists an exceptional case that twinned particles are more stable than single crystals against oxidative etching. It is obvious that the etching mechanism on noble metallic nanoparticles is still not understood completely. Perhaps, material nature is related to the stability of the noble metallic particles. More investigations need to be carried out before shedding light on essential root. In general, shapes of final products depend on not only stability of crystal structures against etching but also the density of Ag0 atoms around the growing nuclei. Crystal growth of silver halides with FCC crystal structure has been extensively studied due to their application in photographic technologies. According to the modeling of twinning processes of AgBr precipitation, the probability of subsequent twinning of an adjoining face must be high for the formation of multi-twinned crystals [18]. That is, high-related ion concentration around the nuclei can be possible to create the environment with high twinning probability for the

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Fig. 8. Mechanism of shape selective oxidative etching and crystal growth of Ag nanostructures under microwave heating.

formation of twinned particles. Under the swift MW irradiation in the presence of Cl− etchant, Cl− anions can be rapidly heated due to field-induced motion by the MW electric field and followed by energy transfer via the interaction between salt ions and solvent [19]. The etching rate of spherical Ag particles by Cl− anions will also be accelerated under MW irradiation. Under such a condition, instability of high solution supersaturation due to high rate of addition of growth species (Ag0 ) will be locally established. Therefore, not only single crystal nanocubes, but also twinned particles leading to triangular bi-pyramids as well as five-twinned particles leading to 1D products can be produced under present experimental conditions by the MW-polyol method. In this study, we used MW heating as a rapid preparation method of Ag nanostructures. In the last section, we discuss about effects of MW irradiation. There are two kinds of effects of MW dielectric heating: thermal and non-thermal [20]. Thermal effects arise from different temperature regimes under MW heating, whereas non-thermal effects result from effects inherent to the MWs. Possible effects are as follows: (a) Thermal effects (effects of rapid and uniform heating): MW provides rapid and uniform heating of reagents, solvents, intermediates, products, and etchant like Cl− anions. Fast

heating accelerates the reduction of metal precursors, the nucleation of the metal cluster, and etching rate, resulting in monodispersed small nanostructures. Due to rapid and homogeneous MW heating and fast etching rate of unstable particles, a better crystallinity can be obtained. Therefore, such single-crystalline nanostructures as polygonal plates, rods, and wires could be synthesized efficiently in the present system. (b) Effects of hot spots and hot surfaces: When some solids heated by MW are involved in the reaction system, hot spots are created on the solid–liquid surfaces stabilized by a surfactant (PVP). An aprotic polar molecule N-methylpyrrolidone (NMP) has a large dielectric loss constant [21], MW irradiation accelerates the coherent heating of PVP as a polymer of NMP and PVP-stabilized metal surfaces. Thus, hot surfaces on solid metals are also created by adsorption of surfactant with a large dielectric loss constant. The uniform formation of hot spots and hot surfaces also accelerate the reduction of metal precursors and the nucleation of the metal cluster, leading to uniform nanostructures with small sizes. This effect may operate in the present Ag–PVP system. (c) Superheating: Superheating of solvents over boiling points of solvent often occurs as a consequence of the MW dis-

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sipation over the whole liquid volume [21]. This effect is especially significant in the presence of a large amount of ions. In our present case, this effect was not observed when the temperature of reagent solution was monitored. Therefore, it will be insignificant in the present system. (d) Non-thermal effects: non-thermal effects as those that occur under the same temperature profiles of solvent between MW and oil-bath heating during the reaction. Formation of hot spots and hot surfaces are typical non-thermal effects for the preparation of metallic nanostructures. This effect may participate in the present system. Although MW heating induces various thermal and nonthermal effects described above in the formation of Ag nanostructures in the presence of Cl− anions, further detailed study using both MW and conventional oil-bath heating will be necessary to determine their relative importance. 4. Conclusion In conclusion, rapid synthesis of silver nanomaterials with well-defined crystal structures has been realized by a microwave-polyol method. Various precursors such as H2 PtCl6 and Pt(acac)2 have been employed to elucidate roles of Pt seed and Cl− anion for the formation of 1D Ag nanostructures. It has been found that pre-existing metallic Pt seeds are not responsible for nucleation and subsequent evolution of 1D Ag products but Cl− anions indeed influence the formation of 1D Ag nanostructures as well as perfect crystallization of other Ag nanoparticles with well-defined crystal structures, i.e. single- and twinned-FCC crystals. Cl− anion effect to 1D and other Ag particle products has also been evidenced by our other experiments, in which Cl− precursor such as NaCl or KCl has been added in the reaction systems without Pt element. The results clearly show that the presence of Cl− ions can accelerate re-dissolution of formed spherical Ag particles and is favorable to the growth of 1D Ag and other Ag nanostructures with well-defined crystal structures such as single-crystal cubes and twinned bi-pyramids in the MW-assisted polyol reduction process. From another side, it implies that H2 PtCl6 probably does not act as a nucleation agent for 1D Ag products but as a precursor to release Cl− ions to affect the final morphologies of the Ag products. The study also demonstrates that MW-polyol method is a promising route for the rapid preparation of a large number of noble metal nanostructures with well-defined crystal structures, which is very important for future application of the nanostructures in biosensor field. Acknowledgements This work was supported by JST-CREST, Joint Project of Chemical Synthesis Core Research Institutions, and Grant-inAid for Scientific Research on Priority Areas “unequilibrium electromagnetic heating” and Grant-in-Aid for Scientific Research (B) from the Ministry of Education, Culture, Sports,

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