Rapid Preparation of Silver Nanorods and Nanowires by a Microwave

CRYSTAL GROWTH & DESIGN

Rapid Preparation of Silver Nanorods and Nanowires by a Microwave-Polyol Method in the Presence of Pt Catalyst and Polyvinylpyrrolidone

2007 VOL. 7, NO. 2 311-320

Masaharu Tsuji,*,§,# Kisei Matsumoto,# Nobuhiro Miyamae,# Takeshi Tsuji,§ and Xu Zhang§,† Institute for Materials Chemistry and Engineering, Kyushu UniVersity, Kasuga 816-8580, Japan, Graduate School of Engineering Sciences, Kyushu UniVersity, Kasuga 816-8580, Japan, and Department of Chemistry, Harbin Normal UniVersity, Harbin 150080, Heilongjiang, People’s Republic of China ReceiVed July 2, 2006; ReVised Manuscript ReceiVed October 31, 2006

ABSTRACT: Silver nanorods and nanowires have been synthesized by using a microwave (MW)-polyol method. When AgNO3 was reduced by ethylene glycol (EG) in the presence of Pt seeds and a long chain length of poly(vinylpyrrolidone) (PVP: Mw ) 360 k), mixtures of one-dimensional (1-D) silver nanorods and nanowires and three-dimensional (3-D) spherical, triangular-bipyramidal, and cubic nanoparticles were synthesized within a few minutes. The average diameters, lengths, and yields of these products were measured as a function of concentration of Pt, PVP, or AgNO3, heating time, or MW power to determine optimum conditions for the synthesis of 1-D products. 1-D products could be easily separated from other 3-D nanoparticles by repeating centrifugal separation in water. Longer and thicker 1-D products could be prepared by using 1-D products as seeds and repeating the reduction of AgNO3 in EG under MW irradiation. In this two-step preparation, some bent 1-D wires due to the combination of {111} facets of two rods and wires were produced. Possible growth mechanisms of 1-D products involving bent structures, and 3-D spherical, triangularbipyramidal, and cubic nanoparticles were discussed by reference to growth mechanisms of Au core-Ag shell nanocrystals prepared using the two-step MW polyol method. Introduction One-dimensional (1-D) nanostructures (rods, wires, and tubes) of silver have received considerable attention from a broad range of researchers because of their wide application to catalysts, scanning probes, and various kinds of electronic and photonic nanodevices.1,2 Here, we define nanorods and nanowires as materials with aspect ratios of 2-20 and >20, respectively. Since catalytic, optical, and electric properties of 1-D nanostructures depend strongly on their shapes and sizes, extensive studies on shape- and size-controlled syntheses of 1-D nanostructures have been carried out. The microwave (MW)-polyol method is a promising method for rapid preparation of metallic nanomaterials.3 We have recently succeeded in fast preparation of such anisotropic Ag nanostructures as nanorods, nanowires, nanosheets, nanoplates, and nanocubes by using this method.3-6 When MW was irradiated into mixtures of AgNO3/H2PtCl6‚ 6H2O/poly(vinylpyrrolidone) (PVP) in ethylene glycol (EG) solution, anisotropic 1-D Ag nanorods and nanowires were prepared preferentially. We found that the key to the formation of anisotropic nanomaterials is the use of Pt as a seeding material and PVP as a protecting reagent. In our preliminary study using three different chain-lengths of PVPs (Mw ) 10, 40, and 360 k),5 we found that the shapes of Ag nanostructures depended strongly on the chain length of PVP, and long chain lengths of PVP were favorable for the preparation of long nanorods and nanowires with high yields. However, more detailed studies were required to determine optimum experimental conditions for the synthesis of 1-D Ag nanostructures. In the present study, we have made further detailed studies on the preparation of 1-D Ag products by the MW-polyol * To whom correspondence should be addressed: E-mail: tsuji@ cm.kyushu-u.ac.jp. § Institute for Materials Chemistry and Engineering, Kyushu University. # Graduate School of Engineering Sciences, Kyushu University. † Harbin Normal University.

method from AgNO3/H2PtCl6‚6H2O/PVP/EG solutions using a long chain length of PVP (Mw ) 360 k). We examined the dependence of shapes, sizes, and optical properties of Ag nanostructures on such experimental parameters as concentration of Pt seeds, surfactant PVP, or reagent AgNO3, heating time, and MW power by using transmission electron microscope (TEM) and UV-visible spectroscopy. Effects of centrifugal separation of 1-D Ag products were studied in water. In addition, the preparation of thicker and higher aspect ratios of 1-D Ag products were attempted by using rods and wires as seeds and repeating reduction of AgNO3 in EG under MW heating. We discuss possible growth mechanisms of 1-D nanorods and nanowires, 3-D spherical, triangular-bipyramidal, and cubic nanostructures under MW irradiation in the presence of nucleation agent Pt and surfactant PVP. We have recently succeeded in synthesizing new Au core-Ag shell nanocrystals, denoted as Au@Ag, using the two step MW-polyol method.7,8 Their crystal structures and growth mechanisms were clarified. In this study, the crystal structures and growth mechanisms of 1-D products and triangular-bipyramidal, and cubic nanocrystals of silver are discussed with reference to Au@Ag nanoparticles with similar crystal structures. Experimental Section The MW-polyol apparatus used in this study was similar to that reported previously.3-6 An MW oven was modified by installing a condenser and 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. The solution containing AgNO3, H2PtCl6‚6H2O, and PVP (average molecular weight 360 k in term of monomeric units) in EG was irradiated by MW (Shikoku Keisoku) in a continuous wave (CW) mode at 400650 W (typically 650 W). The solution was rapidly heated to the boiling point of EG (198 °C) after about 1.5 min and held in this temperature for 1-7 min. A typical total heating time was 3 min. When two-step growth of 1-D products was attempted, Ag nanostructures prepared by the first step were centrifuged at 13 000 rpm

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312 Crystal Growth & Design, Vol. 7, No. 2, 2007

Figure 1. (a-f) TEM images of Ag nanostructures prepared from AgNO3 (23.25 mM)/H2PtCl6‚6H2O/PVP (132 mM)/EG at various H2PtCl6‚6H2O concentrations. MW power and heating time was 650 W and 3 min, respectively. for 60 min. The relative centrifugal force was 1700 G in the centrifugal separation. The precipitate was collected and used as seeds in the second step. AgNO3 was reduced again in the presence of 1-D Ag seeds under the same conditions as the first step. Au@Ag core-shell particles have been prepared by two steps.7,8 First, 2.4 mM HAuCl4‚4H2O was resolved in 20 mL of EG solution. Then, 1 M of PVP was added to the above solution. It was heated by MW irradiation in a CW mode for 2 min. After the heated solution was cooled to room temperature, AgNO3 was added at various [Ag]/ [Au] molar ratios of 7-10. The solution was heated again by MW irradiation for 2 min to overgrow the Ag shell. After MW irradiation, product solutions of Ag and Au@Ag nanostructures were usually centrifuged at 13 000 rpm for 60 min. The precipitate was collected and redispersed in deionized water for JEOL JEM-2010 and 3000F transmission electron microscope (TEM) observation. Specimens containing Ag and Au@Ag nanostructures were prepared by dropping the colloidal solutions on Cu grids covered with carbon. Both average diameters and lengths were measured for nanorods and nanowires. On the other hand, average sizes of prepared nanostructures were determined by measuring diameters for spherical, pentagonal, and hexagonal particles and edge lengths for triangular and cubic particles. Absorption spectra of product solutions were measured in the UV-visible region using a Shimadzu UV-3600 spectrometer. Original product solutions were diluted in EG by a factor of 200 before spectral measurements.

Results and Discussion Effects of Pt Concentration. To examine the effects of Pt seeds, MW was irradiated into the AgNO3/PVP/H2PtCl6‚6H2O/ EG solution for 3 min at six different H2PtCl6‚6H2O concentrations, as shown in Figure 1a-f. In Table 1 are summarized average sizes and yields of each product. In the absence of H2PtCl6‚6H2O, major products are isotropic spherical Ag nanoparticles with an average diameter of 32 ( 4 nm and little anisotropic nanostructures can be obtained (Figure 1a). On the other hand, when small amounts of H2PtCl6‚6H2O were added, besides spherical nanoparticles, 1-D nanorods and nanowires

Tsuji et al.

were prepared in all H2PtCl6‚6H2O concentrations studied in this study (Figure 1b-f). In addition, other anisotropic nanostructures such as triangular cubic, pentagonal, and hexagonal nanoparticles were obtained in the H2PtCl6‚6H2O concentration range of 11.5-115 µM. Expanded TEM images of triangular and cubic particles are shown in Figure 2a,b. Changes in TEM images of triangular and cubic particles observed from various view angles within (25o were the same as those reported for triangular and cubic Au@Ag particles (Supplementary Figure S1 and S3 in ref 8). When an electron beam was irradiated perpendicularly to one of the triangular planes of triangular crystals, only typical square diffraction patterns could be obtained (see Supplementary Figure S1, Supporting Information). This indicated that the incident electron beam was perpendicular to the {100} facets and crystals were fully covered by the six equivalent {100} facets. When the electron beam was irradiated perpendicularly to one of the square plane of cubic crystals, similar cubic diffraction patterns could be observed (see Supplementary Figure S2, Supporting Information). On the basis of these findings, it was concluded that crystals structures of triangular and cubic Ag crystals (Figure 2c,d) were the same as those in the cases of triangular and cubic Au@Ag core-shell particles, respectively (Figure 2e,f). It should be noted that these two crystals are surrounded solely by the {100} facets. These triangular-bipyramidal and cubic crystals of Ag have also been prepared highly selectively by Xia et al. under the conventional oil-bath heating from mixtures of AgNO3/PVP/EG solution.1,2,9,10 NaBr was added for the preparation of triangular-bipyramidal crystals.10 The average diameter of 1-D products increases from 34 ( 7 to 48 ( 11 nm by increasing the H2PtCl6‚6H2O concentration from 11.5 to 28.8 µM and becomes nearly constant (50 ( 5 nm) above that. More significant changes are observed in the length of 1-D products, although the standard deviations of length are larger than those of the diameter. Both average length and yield of 1-D products have maximum values of 1000 ( 880 nm and 56%, respectively, at a relatively low H2PtCl6‚6H2O concentration of 28.8 µM, and decrease to 230 ( 150 nm and 4%, respectively, with increasing the H2PtCl6‚6H2O concentration to 288 µM. These data imply that the H2PtCl6‚6H2O concentration must be kept relatively low for the preferential formation of 1-D products. The average sizes of spherical particles in the presence of Pt catalysts are 75-99 nm, which are 2.3-3.1 times larger than that observed without addition of H2PtCl6‚6H2O. The yield of spherical particles increases from 27 to 96% by increasing the H2PtCl6‚6H2O concentration from 11.5 to 288 µM. Triangular and cubic crystals occupy 10-22% of total products, and their sizes also have maximum values of 142 ( 31 and 91 ( 7 nm, respectively, at 28.8 µM. Very small amounts of pentagonal (yield: 1.5%) and hexagonal (2.0%) particles are produced only at a low H2PtCl6‚6H2O concentration of 11.5 µM. When MW was irradiated into an H2PtCl6‚6H2O/PVP/EG solution without the addition of AgNO3, only spherical Pt nanoparticles with a mean diameter of ∼2 nm were prepared (see Supplementary Figure S3, Supporting Information). Pt4+ ions are expected to be more easily reduced to Pt metals than Ag+ ions. Therefore, similar small nanoparticles, which act as seeds of Ag nanostructures, will also be formed at the initial stage of the reduction process in AgNO3/H2PtCl6‚6H2O/PVP/ EG solutions. On the basis of the present facts, small amounts of Pt catalyst must be added to prepare anisotropic 1-D products and polygonal particles. Pt catalyst will assist the preparation of multiple twin particles (MTPs) of Ag, which becomes seeds

Rapid Preparation of Silver Nanorods and Nanowires

Crystal Growth & Design, Vol. 7, No. 2, 2007 313

Table 1. Average Sizes and Yields of Products Obtained at Various H2PtCl6‚6H2O Concentrations [H2PtCl6‚H2O]

rod and wire

spherical

triangular

cubic

pentagonal

hexagonal

(µM)

diameter, length (nm) yield (%)

size (nm) yield (%)

size (nm) yield (%)

size (nm) yield (%)

size (nm) yield (%)

size (nm) yield (%)

400 > 500 W. This suggests that the contribution of large spherical particles is largest at 650 W. At higher MW power of 500 and 650 W, strong absorption bands of PtCl62- are observed in the 220-320 nm region with a peak at ∼260 nm. When small Pt nanoparticles are present in solution without taking part in the nucleation of Ag nanoparticles, they are heated at high temperatures. The critical particle radius below which a particle is not stable and should spontaneously dissolve becomes large with increasing temperature. At a high temperature of 197 °C, the small isolated Pt particles were no longer stable in solution,

and they started to dissolve and form again PtCl62- ions. Such a process was found to be significant at high MW power. UV-visible absorption spectra obtained at various PVP concentrations are shown in Supplementary Figure S6a (Supporting Information). The absorbance of the main SPR band with a peak at ∼430 nm is observed in all cases. It has the maximum at a medium PVP concentration of 53 mM due to the largest contribution of the spherical particles. The absorption spectrum at 71 mM is broadest with a long tail band in the longer wavelength region. This spectral observation is consistent with the formation of the longest 1-D products with the highest yield. In addition to the main SPR band, weak transverse SPR peaks are observed at ∼350 nm. A strong absorption peak due to PtCl62- ions is observed at ∼260 nm only at the highest PVP concentration of 427 mM. At high PVP concentration, Pt catalysts are heavily covered by PVPs. Therefore, large numbers of isolated small Pt particles, which are dissolved again as Pt4+ ions at a high temperature, will be produced. The absorption spectra at various AgNO3 concentrations are shown in Supplementary Figure S6b (Supporting Information). At the low AgNO3 concentration range of 5.8-23 mM, symmetrical SPR bands with a peak at ∼430 nm are observed due to large contribution from spherical particles. At high AgNO3 concentrations of 46 and 92 mM, the major SPR peak slightly shifts to blue (∼410 nm) probably due to the increase in the contribution of the ∼380 nm band of nanowires, and long tail bands due to 1-D products are observed. A minor transverse SPR band of nanowires at ∼350 nm becomes also prominent at high AgNO3 concentrations. These spectral changes are consistent with the TEM observations. Centrifugal Separation of 1-D Products. To separate 1-D products from other spherical nanoparticles, centrifugal separation was attempted. We have examined effects of solvent,


Figure 6. UV-visible absorption spectra of Ag nanostructures obtained (a) at various H2PtCl6 concentrations and (b) after various heating times. Other conditions were the same as those in Figures 1 and 3.

centrifugal time, and repeated times. When EG and H2O were used as solvent, better separation was attainted by using H2O. When centrifugal time was examined at 1, 2, 3, and 10 min, the shortest centrifugal time of 1 min was found to be best for the separation. Then, we repeated centrifugal separation in H2O at 1 min. For example, TEM images obtained before centrifugal separation and after 1, 6, and 8 times centrifugal separation are shown in Figure 7. The average yield of 1-D products is 37% before centrifugal separation and increases to 50, 71, and 79% after 1, 6, and 8 times separation, respectively. These results show 1-D rods and wires can be condensed by repeating centrifugal separation. Two-Step Growth of 1-D Products. To obtain 1-D products with long lengths, two-step growth of 1-D products was examined. Figure 8 shows TEM images of products before aging and after aging. Aging was carried out by using products obtained after one time centrifugal separation at 13 000 rpm (1700 G) for 60 min in EG as seeds of the second MW-polyol


synthesis under the same AgNO3 and PVP concentrations as those of the first one. The average diameter and length before aging are 45 ( 9 and 630 ( 600 nm, while those after aging are 69 ( 14 and 1660 ( 1590 nm, respectively. The yields of 1-D products before and after aging are 21 and 36%, respectively. These results indicate that 1-D products with larger diameters and longer lengths can be prepared more efficiently through the two-step growth. It should be noted that bent rods and wires have often been observed in the two-step growth using aging process, as shown in Figure 8c,d. The mechanism of these bent 1-D products will be discussed in the next paragraph. The UV-visible-near-infrared spectra of Ag products before and after aging are shown in Figure 9. Because of the increase in the fraction of longer 1-D products in comparison with the spherical nanoparticles, spectrum becomes broad having a long tail band in the visible and near-infrared region after aging. Growth Mechanisms of Silver Nanostructures. In the present study, mixtures of 1-D nanorods and nanowires, and 3-D spherical, triangular-bipyramidal, and cubic nanoparticles were prepared. On the basis of present findings for the preparation of Ag nanostructures, our recent study on Au@Ag core-shell nanoparticles,8 and reported growth mechanism of Ag nanostructures,2,16c we propose here two major routes for the formation of Ag nanostructures in MW-polyol process in the presence of H2PtCl6‚6H2O and PVP (360 k), as shown in Scheme 1. One is the homogeneous nucleation of Ag and the formation of spherical polygonal nanoparticles via quasi MTPs, where PVP acts as the protecting agent to prevent severe aggregation of spherical particles (Process 1 in Scheme 1). When Ag nanostructures are prepared from AgNO3/PVP/EG mixtures without addition of H2PtCl6‚6H2O, only spherical nanoparticles are produced. Thus, the formation of spherical particles is independent of the presence of Pt catalysts. The other is heterogeneous nucleation of Ag leading to various kinds of flat 2-D polygonal plates and 3-D polyhedral particles involving single crystals, single twin platelike particles, and MTPs, as demonstrated by us for Au@Ag particles8 and by Wang for general FCC crystals.21 These precursor particles are finally grown to 1-D nanorods and nanowires, triangular-bipyramidal, and cubic crystals via Processes 2a, 2b, and 2c in Scheme 1. For the preparation of these single-crystal structures, the addition of small amounts of Pt catalyst is required. According to theoretical calculations and experimental observation, the potential energy surface of a silver particle has several minima, instead of one global minimum.11,22 The potential energy barriers between these minima are low enough at small particle sizes so that thermal fluctuations could provide sufficient energy to provoke shifts between the twinned and single-crystal morphologies. This model explains why silver nanoparticles obtained in this study consist of a mixture of single crystalline, single twinned, and multiply twinned structures. Silver nanorods and nanowires are grown via Process 2a. The MTPs consists of 10 {111} facets. It has been believed that Ag nanorods and nanowires are grown from decahedral MTPs as seeds with the assistance of selective adsorption of PVP on five side {100} facets.1,2,5,15,16,23 To examine the formation mechanism of Ag rods and wires, Au@Ag core-shell nanoparticles were prepared. Since Au and Ag have the same FCC crystal structures and similar lattice constants (Au: 0.4079 nm and Ag: 0.4086 nm), similar shapes and sizes of nanostructures may be synthesized. Fortunately, Au and Ag can easily be distinguished by dark and bright contrasts of TEM images. Figure 10a shows the growth mechanism of a typical rod type of Au@Ag core-shell particle prepared by using an Au MTP as


Tsuji et al.

Figure 7. (a-d) TEM images of Ag nanostructures obtained before and after 1, 6, and 8 times centrifugal separation in H2O for 1 min at 13 000 rms.

Figure 9. UV-visible-near-infrared spectra of Ag nanostructures prepared by one step and two steps MW-polyol reduction.

Scheme 1.

Growth Mechanism of Each Ag Nanostructure under the MW-Polyol Method

Figure 8. (a-d) TEM images of Ag nanostructures prepared from AgNO3 (46 mM)/H2PtCl6‚6H2O (57.5 µM)/PVP (132 mM)/EG at one step and two steps MW-polyol reduction. In the second step, H2PtCl6‚ 6H2O was not added.

seed.8 It is clearly seen that pentagonal Ag shell having five {100} facets are overgrown on the center Au decahedral MTP having ten {111} facets. The growth rates to the two opposite directions are nearly the same, and the diameter of the nanorod is close to the diameter of MTP. By analogy with this Au@Ag core-shell nanorod, it is reasonable assume that a similar growth mechanism is dominant in the one-step synthesis

of 1-D Ag products. In this case, once the five-twinned Ag seeds have been formed, the morphologies of Ag nanoparticles change very quickly from MTPs to Ag nanorods and then to nanowires. The diameters of Ag nanorods and nanowires are determined by the diameters of decahedral Ag MTPs. In our preliminary study on Au@Ag core-shell particles,7 longer and thicker Au@Ag nanorods are prepared, when an Au rod becomes a



Figure 10. Growth mechanisms of Au@Ag and Ag nanorods under MW-polyol method.

seed of Au@Ag nanorod, as shown in TEM images in Figure 10b. However, yield of these 1-D products was much lower than Au@Ag nanorods having MTPs as core particles, because the number density of rods was much lower than that of MTPs in Au core crystals. In the present study on the preparation of 1-D Ag products through the two-step growth, we found that besides fast crystal growth to two opposite directions, as found in Au@Ag nanorods and nanowires, slow crystal growth occurs in a short axis direction (Figure 10b). This implies that Ag+ can be reduced not only on the two top {111} sides but also on the five side {100} facets in the second step, even when they are covered by PVP. In the two-step synthesis of 1-D particles, small amounts of multiple bent 1-D nanorods and nanowires are produced. Such a defect could never been observed for 1-D products prepared in the one-step growth. These structures are probably formed via a combination of two top {111} facets of rods and wires during the second step, as shown in Scheme 2. Since top {111} facets are inclined along the long axis of 1-D products, such bent structures with evident turn sites must be produced. An important finding of this study is that 1-D products can be synthesized in wider experimental conditions by using PVP (360 k) than those by using PVP (40 k).6 It is known that Ag+ ions are initially coordinated with the NCO group of PVP.16b Thus, one reason may be larger numbers of Ag+ ions are initially coordinated to PVP in the case of a long-chain PVP, and reduction occurs along the longer PVP chain. Then, longer 1-D products are formed preferentially. The triangular-bipyramidal crystal is grown via Process 2b. We found that Au@Ag triangular-bipyramidal crystal is overgrown on triangular twin plates, as shown in Figure 2c.8 By analogy with this mechanism, the formation of a triangularbipyramidal crystal proceeds through triangular and hexagonal twin plates. The 2-D FCC structures are grown via 2-D growth of ABCABC layers. When such a defect as ABC(B)AC is

Scheme 2. Growth Mechanism of Bent 1-D Products via Two-Step Growth under MW Heating

inserted in the crystal layer, triangular and hexagonal single twin 2-D plates are grown. These triangular and hexagonal plates have often been observed in the preparation of Ag+ nanostructures in polyol process.1,2 In this study, little triangular plates can be obtained probably because crystal growth from triangular and hexagonal plates to triangular-bipyramidal crystal is very fast. After the formation of small triangular-bipyramidal crystals, they are expected to further grow to larger ones, as found for Au@Ag core-shell crystals obtained at high [AgNO3]/[HAuCl4‚ 4H2O] molar ratios above ∼11.8 Cubic crystal is grown via Process 2c. Since the formation of octahedral particles having {111} facets is unfavorable, the formation mechanism of cubic crystal of Ag will be different


from that of cubic Au@Ag particles. After reduction of Ag+ ions in EG, polyhedral particles such as cubooctahedral particles consisting of {111} and {100} facets21 will be formed with the assistance of Pt catalyst. Selective adsorption of PVP on {100} facets suppresses crystal growth of this facets, and finally cubic crystals, fully covered by the {100} facets, will be produced. After the formation of small cubic crystals, they are expected to further grow to larger ones, as found for Au@Ag core-shell cubic crystals obtained at high [AgNO3]/[HAuCl4‚4H2O] molar ratios above ∼11.8 It should be noted that all three nanostructures (1-D, triangular-bipyramidal, and cubic crystals) are surrounded by {100} facets, except for rods and wires, where two top active surfaces consist of {111} facets. In our recent study on Au@Ag core-shell particles, we have found that {100} facets are most favorable in the crystal growth of Ag shells from AgNO3/PVP/ EG system in the presence of such Au core seeds as triangular twin plate, octahedral crystal, and MTP having only {111} facets.8 Thus, triangular-bipyramidal, cubic, and rod and wires Ag shells, having {100} facets as main surfaces, are selectively produced. Although the detailed role of the nucleation reagent H2PtCl6‚6H2O has not been clarified in this work, Pt4+ and Clin H2PtCl6, and O2 (dissolved in EG) probably assist in the formation of Ag nanocrystals surrounded by low energy {111} and {100} facets. To clarify the exact role of the Pt catalyst, Cl- ion, and O2 (resolved in EG), further detailed experimental studies will be required. Conclusion The MW-polyol method has been applied to the fast synthesis of silver nanostructures. Various mixtures of 1-D nanorods and nanowires and 3-D spherical and triangular-bipyramidal, and cubic nanoparticles were prepared within a few minutes. It was found that the shapes and sizes of silver nanostructures depended strongly on such experimental parameters as concentrations of Pt, PVP, and AgNO3, and heating time. For the preparation of long 1-D nanorods and nanowires at high yields, a low Pt concentrations (10-100 µM), and a high AgNO3 concentration (>10 mM) were required. It was found that 1-D products could be produced in wider experimental conditions by using PVP (360 k) than those by using PVP(40 k).6 The favorable formation of nanorods, nanowires, triangular-bipyramidal, and cubic naonocrystals indicated the preferential adsorption of PVPs on the {100} facets of silver. Repeating centrifugal separation in water and aging step were effective for the preparation of 1-D products in high yields. Acknowledgment. This work was partially supported by JST-CREST and Joint Project of Chemical Synthesis Core Research Institutions.

Tsuji et al. Supporting Information Available: Figures giving additional TEM and SAED patterns, and UV-visible absorption spectra obtained in this study. This material is available free of charge via the Internet at http://pubs.acs.org.

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Rapid Preparation of Silver Nanorods and Nanowires by a Microwave

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