Solvothermal Synthesis of Multiple Shapes of Silver Nanoparticles and ...

J. Phys. Chem. C 2007, 111, 9095-9104

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Solvothermal Synthesis of Multiple Shapes of Silver Nanoparticles and Their SERS Properties Yong Yang,* Shigemasa Matsubara, Liangming Xiong, Tomokatsu Hayakawa, and Masayuki Nogami* Department of Materials Science and Engineering, Nagoya Institute of Technology, Showa, Nagoya, 466-8555, Japan ReceiVed: December 22, 2006; In Final Form: April 13, 2007

The controllable synthesis of nanocrystals with different shapes is very important and challenging. In the present work, silver nanoparticles with different structural architectures, from nanorods, triangular plates, hexagonal plates, and nanocubes to polyhedrons have been synthesized successfully in high yield by a solvothermal process. Especially, the unique silver enneahedral nanoplates are also observed. These nanoparticles exhibit tunable surface plasmon resonance properties from the visible to near-infrared regions. Those nanoparticles were also self-assembled on glass substrates and evaluated as potential surface-enhanced Raman scattering (SERS) substrates using trans-1,2-bis(4-pyridyl)ethylene molecules. Thanks to the enhanced local field effect around their sharp corners and edges, those Ag triangular plates exhibit enhanced SERS properties and can serve as high-sensitivity substrates for SERS-based measurements.

I. Introduction Surface plasmons (SPs) are collective electronic excitations at the interface between metals and dielectrics, and are currently being explored for their potential applications in subwavelength optics,1 data storage,2 nonlinear optics,3 surface-enhanced Raman scattering (SERS),4 catalysis, and bio-photonics.5 The precise wavelength of plasmon resonance depends on several parameters, such as metal shape and size, crystallization, structure, and the nature of the environment.6 One can, in principle, control any one of these parameters to tailor plasmon properties for various applications. Especially, the controllable preparation of metal nanoparticles with different shapes is of great interest because it allows one to fine-tune the properties with a greater versatility in many cases. For example, the theoretical studies and experimental works7 on Ag and Au nanostructures have suggested that the number and position of surface plasmon resonance, as well as the effective spectral range for SERS, can be tuned by controlling the shape of metal nanoparticles. Therefore, the synthesis of noble metal nanoparticles with various shapes, such as prisms,8 rods,9 core-shells,10 cups,11 rings,12 disks,13 and cubes,14 have been continually reported by various synthesis methods, such as a modified polyol process,15 the photoinduced method,7 microwave rapid heating,16 and the seed-mediated growth method.17 Recently, silver nanostructures have attracted much attention because of their unique surface plasmon features, which have exhibited application potential as optical labels, nonlinear optical devices, near-field optical probes, and active substrates for surface-enhanced Raman scattering (SERS). The nanometersized Ag nanoparticles absorbed by a single Rhodamine 6G molecule have been reported to exhibit the SERS signal enhancement on the order of 1014 and the possibility of studying Raman scattering even at the single-molecule level.4a Silver nanoprisms have been synthesized using a photoinduced method for converting silver nanospheres into triangular nanoprisms.7,18 * Corresponding authors. E-mail:[email protected]; nogami@ mse.nitech.ac.jp. Tel/Fax: +81 52 7355285.

Mirkin and Murphy’s groups have reported a seeding methodology to prepare Ag and Au nanoprisms and control their edge lengths.17,19,20 Xia’s groups have developed a modified polyol process, in which ethylene glycol serves as both the solvent and the reducing agent, to synthesize Ag nanocubes with controllable corner truncation, right bipyramids, pentagonal nanowires, and so on.21 Tsuji and He have applied a microwave heating method16 to synthesize Ag triangular nanoplates. However, the solvothermal reduction route has seldom been reported as being used in the synthesis of anisotropic silver nanocrystallites until now. Solvothermal synthesis is a technique in which the reaction occurs in a pressure vessel that allows normal solvents such as water or alcohols to be heated to temperatures far in excess of their normal boiling points.22 The solvothermal reduction route is used widely to prepare highquality crystallized and monodisperse nanocrystals including metal oxides, nitrides, and novel semiconductor materials because of its advantages such as high pressure. In general, nanocrystallites with narrower size distributions, and a higher degree of crystallization can be obtained by solvothermal heat treatment than that by conventional oil-bath heating. Here we report a one-step solvothermal reduction route to synthesize high-yield silver nanocrystallites with different shapes including triangular, hexagonal, and enneahedral plates, nanocubes, nanorods, and polyhedrons. We have also investigated their tunable SPR properties and prepared Ag nanoparticle monolayers with well-defined shapes on glass for sensitive SERS substrates. Their SERS properties have been studied by using trans-1,2-bis(4pyridyl)ethylene (BPE) as molecule probes to explore the influence of their shape on SERS enhancement. II. Experimental Section Chemicals and Materials. Silver nitrate (AgNO3, 99+%), poly(vinyl pyrrolidone) (PVP, MW ) 40000) and N,N-dimethylformamide (DMF) were purchased from Tokyo Chemical. Co., Ltd. and used as received without further purification. All other reagents were from Aldrich and were used as received. Ultrapure

10.1021/jp068859b CCC: $37.00 © 2007 American Chemical Society Published on Web 06/05/2007

9096 J. Phys. Chem. C, Vol. 111, No. 26, 2007

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TABLE 1: Shapes of Silver Nanoparticles and Corresponding Experimental Conditions AgNO3 /mM 19.7 48.5 203 41.7 17.3 50.0 100 25.0 41.6

PVP (monomer unit)/ mM 0 42 42 210 210 830 2000 4170 420/20 mL ethanol

MR of PVP/ AgNO3(%)

temp/ °C

time/ h

shape/profile

0 0.9 0.2 5.0 12.1 16.6 20 167 10.1

140 160 160 160 140 180 140 160 160

1 2 2 4 6 8 16 6 4

Ag sphere rod + sphere rod + triangle small triangle plate hexagonal plate big triangle plate cube quasi-sphere hexagonal plate

deionized water (resistance ) 18.1 MΩ·cm) was prepared by a Narnstead Nanopure H2O purification system and used throughout the experiment. Synthesis of Ag Nanoparticles with Different Shapes. Silver nanocrystals were prepared by a one-step seedless solvothermal reduction route, in DMF as the solvent with the presence of surface-regulating polymer poly(vinyl pyrrolidone). PVP serves not only as the mild reductant and the stabilizer to prevent aggregation of particles but also as the capping agent to control the growth rate of different crystalline faces. Typically, the reaction solutions were prepared by dissolving different amounts of AgNO3 and PVP in 20 mL DMF or ethanol; and after stirring for 10 min, the solution turned light-yellow. The solutions were transferred into Teflon-lined stainless autoclaves and kept at different temperatures for an appropriate time. The detailed experimental conditions are shown in Table 1. For the synthesis of cubic particles, AgNO3 and PVP were added to a solution of HCl (1 mM) in DMF. After natural cooling to room temperature, the samples were concentrated and partially separated from DMF and PVP by centrifugation. Typically, 1.5 mL of as-prepared solution was centrifuged at 6000 rpm for 30 min. The supernatant was removed, and the solid was dispersed in 1.5 mL of deionized water and centrifuged again. After repeating the process 3 times, the solid residue was dispersed in a suitable volume of ultrapure water. Preparation of Ag Nanoparticle Monolayers. The silver NPs were self-assembled on 3-aminopropyltrimethoxysilane (APTMS)-modified glass slides according to the following procedure.23 The specially cleaned glass slides were placed in a dilute solution of APTMS solution (4 g APTMS in 36 g CH3OH) for 12 h and rinsed with copious amounts of CH3OH upon removal. The APTMS-modified glass slides were subsequently immersed in different Ag colloidal solution for 6 days and withdrawn at a speed of 10 mm/min, followed by extensive rinsing with water. Finally, these films were dried at room temperature. Characterization. The microstructure and morphology of Ag nanocrystals were measured with a JEOL JEM-2000EXII transmission electron microscope (TEM) operating at 200 kV. The surface morphologies of these monolayers were examined with dynamic force mode (DFM) using an atomic force microscope (Seiko II, SPA-300HV). The UV-vis-NIR spectra of the Ag colloids were measured directly with a Jasco Ubest 570 UV-vis-NIR spectrophotometer at room temperature. SERS measurements were performed by a Jasco NRS-2000 microscopy Raman spectrometer with a LN2-cooled chargecoupled-device (CCD) detector and a holographic notch filter. The molecule probe used in this study was trans-1,2-bis(4pyridyl)ethylene (BPE), which exhibited a high Raman scattering cross section and could absorb strongly to a Ag substrate. All samples for the SERS measurement were prepared by

dimension/ nm 10∼30 50 50 150 50-80 60-80 40

% yield >90 55 (rod) 75 (rod) 95 75 50 65 80 75

casting 30 µL of 1 mM BPE in methanol onto the Ag monolayer and allowing the solvent to evaporate. The SERS excitation wavelength was provided by the 514.0 nm line of a NEC Ion laser. The laser power at the sample position was about 15 mW for BPE on the SERS substrates. All of the spectra reported here were the result of a single 10-s accumulation at room temperature. III. Results and Discussion The shape and dimension of the Ag nanoparticles depend on the molar ratio of PVP (monomer unit)/AgNO3 (MR), the concentration of AgNO3, reaction temperature, and time. The prepared silver nanoparticle colloids were analyzed using transmission electron microscopy (TEM) and atomic force microscopy (AFM). Figures 1-9 show TEM photographs of all kinds of silver nanostructures obtained at different experimental conditions. Without the addition of PVP, only spherical silver nanoparticles of a diameter of 10-30 nm were produced, as shown in Figure 1. Liz-Marzan has reported that Ag+ ions can be reduced into small Ag nanoparticles in DMF even at room temperature and in the absence of any external agent.24 Our present results confirm that DMF can play the role of a weak reducing agent to reduce Ag+ ions to small Ag nanoparticles, which can serve as seeds for the further growth of Ag nanostructures. After the addition of a small quantity of PVP (MR ) 0.9), some Ag nanorods and spherical nanoparticles were observed, as shown in Figure 2a. The diameter of silver nanorods is around 20 nm, and their length is around 40 nm. While increasing the AgNO3 concentration and decreasing the molar ratio of PVP/

Figure 1. TEM images of Ag nanoparticles prepared without the addition of PVP.

Multiple Shapes of Silver Nanoparticles

Figure 2. TEM images of Ag nanoparticles synthesized when the molar ratio between PVP and AgNO3 was (a) 0.9 and (b) 0.2.

AgNO3 (MR ) 0.2), the spherical nanoparticles yield decreased and particles with a triangular outline appeared; the length and aspect ratio of nanorods increased at the same time (Figure 2b). These findings imply that a low PVP/AgNO3 ratio and a high AgNO3 concentration are required for the synthesis of long Ag nanorods with large aspect ratios. By increasing the molar ratio of PVP/AgNO3 (MR ) 5.0) continuously, the nanorods disappeared and only triangular particles were observed. Monodisperse triangular particles in a very high yield can be obtained in the fine adjustment of reactant and centrifugation conditions. Figure 3a shows that these particles are of uniform triangular shape and size with an edge length of around 50 nm. The corners of these triangular silver nanoparticles are rounded with various curvatures. The AFM image (Figure 3b) of these nanoparticles self-assembled on glass substrates shows that these nanoparticles are thin and flat nanoplates, and the thickness of these triangular nanoparticles is evaluated to be around 16 nm from AFM cross-section scanning results. Figure 3c and d shows high-resolution images of an individual triangular nanoplate, as well as the corresponding selected-area electron diffraction pattern obtained by the electron beam perpendicular to the top face, suggesting that each plate is a single-crystal structure. Three sets of spots can be identified from the 6-fold rotational symmetric diffraction spots based on their d spacing: the strongest intensity spot with a spacing of 1.4 Å can be indexed to the {220} lattice plane of FCC Ag, indicating that the triangular top faces are composed of {111} planes; and the inner set with a lattice spacing of 2.5 Å is believed to originate from the forbidden 1/3{422} reflection;25 the outer set with a lattice spacing of 0.8 Å can be indexed to the {422} reflection. The HRTEM image in Figure 3e (111 zone) shows a hexagonal lattice image with a spacing of 1.43 Å, indexed as {220} planes, whereas the HRTEM image of an individual triangular plate lateral facet in Figure 3f (100 zone) shows lattice spacing of 1.24 Å, indexed as {311} planes of FCC Ag. These data indicate that the triangular nanoplate is bound by two {111} planes as the top and bottom faces and three {100} planes as the side faces (the inset of Figure 3c).25-27 Recently, Sigmund28 and Tsuji16b,c have suggested that triangular nanoprisms are twinned {111} crystals, and this twinning produces convex and concave side facets in order to explain the presence of these 1/3{422} forbidden spots.29 But we have not observed any ridges and reentrant grooves in our TEM measurement, as suggested by these authors. Similar silver nanoprisms were reported recently by Xia’s group30 through the polyol synthesis process by using ethylene glycol as a solvent and reductant at high temperature, and the pre-prepared small

J. Phys. Chem. C, Vol. 111, No. 26, 2007 9097 Ag or Pt nanoparticles as seeds. The yield of silver nanoprisms by their method was very high, and the citrate was thought to be the key component in the morphological transformation from spheres into thin plates bounded by {111} facets. Liz-Marzan et al. 24c have also observed the silver nanoprisms in DMF solution using PVP as a stabilizer at 156 °C. In our present work without using any seeds, the yield is as high as that obtained by using the Xia’s Ag-seeds wet chemical reaction, and higher than that obtained by using Liz-Marzan’s method, which indicates that the higher pressure in the solvothermal process is helpful for the formation and growth of silver nanoprisms. Compared to the data without addition of PVP, these results also suggest that PVP plays a critical role in producing silver triangular plates with good stability and size/ shape uniformity. It has been suggested26 that the hydroxyl end group of PVP has a mild reducing power. The formation of triangular Ag nanoplates can be attributed to the kinetic control of the nucleation and growth of Ag nanoplates determined by the reduction power of PVP. The slow reduction rate originating from the PVP reducing power would lead to the small Ag nanoparticles with stacking faults formed at the initial nucleation stage and the growth procedure were controlled kinetically. These stacking faults and kinetically controlled growth procedure are critical characteristics of the growth of Ag nanoplates.26 At the same time, the DMF plays an important role in the formation of silver nanoprisms. No silver triangular prisms, but most of the hexagonal plate nanoparticles, were found when the DMF solvent was replaced by ethanol (see Figure S1). The mechanism might originate from the fact that the reduction rate in ethanol might be slower than that in DMF. He et al. have observed isolated spherical Ag nanoparticles, rather than hexagonal plates by microwave-assisted heat-treatment of AgNO3 and PVP in ethanol.16e This indicates that the solvothermal process is more helpful for the formation of anisotropic nanoparticles. Under high-pressure provided by vaporized DMF (boiling point of 153 °C),31 atomic diffusion is constrained, which makes atomic mobility more difficult. The lower atomic mobility slows the growth of the crystal, which makes the Ag crystal growth process kinetically controlled. Xia has pointed out that the kinetically controlled nucleation and growth processes are helpful to form the Ag anisotropic structure.26 The hexagonal nanoplates appeared when the molar ratio of PVP/AgNO3 was increased to 12.1. Figure 4 shows TEM images of the as-synthesized hexagonal nanoplates. Hexagonal plates with side lengths above 50 nm (45%), smaller hexagonal plates with side lengths of 30 nm (30%), and triangular plates with side lengths of 50 nm (25%) were found. One representative TEM image of an individual hexagonal nanoplate is shown in the inset of Figure 4a, and the flat plate structure can be observed. The edge lengths are 30, 56, 40 56, 30, and 65 nm, respectively. The thickness of this plate is evaluated to be around 16 nm from AFM cross-section scanning results. Figure 4b shows a typical electron diffraction pattern recorded by directing the electron beam perpendicular to the hexagonal flat plane of an individual hexagonal plate. The diffraction patterns are composed of three sets of diffraction spots with 6-fold rotational symmetry, which can be indexed to the {422}, {220}, and forbidden 1/3{422} reflections, respectively. Furthermore, the ED pattern from the side face recorded in the [100] orientation is also shown in Figure 4c and illustrates that the side faces of each nanoplate are bound by the {100} or {111} planes. These assignments imply that the flat top and bottom faces of the hexagonal plates are bound by {111} planes, three side faces by {111} planes, and the other three side faces by {100} planes,


Yang et al.

Figure 3. (a) TEM images of Ag nanoparticles synthesized when MR ) 5.0. (b) AFM image of one individual triangular nanoplate self-assembled on a glass substrate. (c) A representative TEM image of one individual silver triangular plate; the inset is the schematic crystal structure of the triangular nanoplate. (d) The corresponding selected area electron diffraction (SAED) pattern. (e) High-resolution TEM images of one individual triangular plate with a spacing of 0.143 nm, indexed as {220} of FCC Ag. (f) High-resolution lattice image with a spacing of 0.124 nm, indexed as {311} of FCC Ag.

as shown in the Figure 4d. The crystal structure of our hexagonal plates is consistent with the result reported by Xia26,27 but is

different from that reported by Sigmund.28 Sigmund et al. have suggested one model, originally proposed by Hamilton and


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Figure 4. (a) TEM images of Ag nanoparticles synthesized with an MR of 12.1; the inset shows a representative hexagonal nanoplate. (b) SAED pattern recorded by directing the electron beam perpendicular to the hexagonal flat faces. (c) SAED pattern from the side face recorded in the [100] orientation. (d) The schematic crystal structure of hexagonal plate; the gray and white colors represent the {100} and {111} facets, respectively. (e) TEM image of an enneahedral nanoplate marked by the yellow dot oval. (f) AFM image of an enneahedral plate. (g) SAED pattern recorded by directing the electron beam perpendicular to the enneahedral flat faces.

Seidensticker for germanium dendrites,32 where all side faces of silver hexagonal plates are composed of {111} planes by forming ridges and reentrant grooves at the twin planes.

Some hexagonal plates (marked by yellow in Figure 4a) seem to be truncated triangular plates in low-resolution TEM. Very interestingly, those truncated triangular plates are in fact


Yang et al.

Figure 5. (a) TEM image of Ag nanoparticles synthesized with an MR of 16.6. (b) AFM image of silver triangular plates self-assembled on a glass substrate.

Figure 6. (a) TEM image of Ag nanoparticles synthesized with an MR of 20; the inset is the SEM image of this sample. (b) A representative TEM image of one individual silver nanocube. (c) The corresponding selected area electron diffraction (SAED) pattern.

enneahedral nanoplates with very sharp corners in highresolution TEM images, as shown in Figure 4e. The inner angles of this enneahedral plate are 120° and 150°, respectively. The three longer side lengths of an enneahedral plate are 60 nm, and the six shorter side lengths are 25 nm. The thickness of this enneahedral plate is evaluated to be around 18 nm by AFM cross-section scanning results (Figure 4f). Moreover, the electron diffraction pattern (Figure 4g) obtained by an electron beam perpendicular to the flat faces of a single enneahedral plate also displays interesting {422}, {220}, and forbidden 1/3{422} reflection spots. The ED pattern also indicates that the flat top and bottom faces of silver enneahedral plates are bound by the {111} planes. The detailed structure of the nine side facets is not clear yet, and further investigations are necessary to clarify this point. Here, this kind of enneahedral plate is thought to be in the intermediate growth stage from hexagonal plate to bigger triangular prism because the enneahedral plates have such low yield. Sigmund has reported almost similar gold plates obtained

Figure 7. TEM image of Ag nanoparticles synthesized with an MR of 167.

from gold dissolved in aqua regia followed by the addition of a base to break the gold chloride complex. Their six short edges are not straight, and the inner angles are 100° and 160°, respectively. Those gold plates seem to have the corners of the triangular plate dissolved28 and are different from our silver enneahedral plates. The bigger triangular plates were obtained with the longer heat-treating time and MR ) 16.6. As shown in Figure 5, the edge lengths of the bigger triangular plates are around 150 nm and their thickness is evaluated to be around 18 nm from AFM cross-section scanning result. It is worth noting that the inscribed circle (40-45 nm) and the thickness (16-18 nm) of the big triangular prisms are about the same as those of typical hexagonal and enneahedral nanoplates. It indicates that those bigger triangular plates originate from the transformation of hexagonal nanoplates in our experiments. Jin and Mirkin et al. have reported one photoinduced method8 for converting silver


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Figure 8. UV-vis-NIR absorbance optical spectra of silver nanoparticles with different shapes.

Figure 9. SERS spectra of BPE absorbed on Ag nanoparticle monolayers with different shapes.

nanospheres into silver triangular nanoprisms and suggested one light-induced fusion growth of Ag nanoprisms through an edgeselective particle-fusion mechanism. Xia et al. have reported that the shape evolution of Ag nanaoplates has a process from circular nanoplate to hexagonal and then triangular plates together with a large increase in their lateral dimensions.26,27 In our present work, it seems that the process of shape evolution from a hexagonal plate toward a big triangular plate (Scheme 1) is present. As shown in the inset of Figure 3c, the flat top

and bottom faces of the hexagonal plates are bound by {111} planes, three side faces by {111} planes, and the other three side faces by {100} planes, respectively. PVP has been reported to interact more strongly with silver atoms on the {100} facets than those on the {111} facets.30 Selection adsorption of PVPs on the three {100} facets will lead to preferential addition of silver atoms to the {111} facets. Because the growth rate in the [111] direction is greater than that in the [100] direction, the three {100} side facets of hexagonal plates will become


Yang et al.

SCHEME 1: Shape Evolution from Hexagonal Plate to Triangular Platea

a

The gray and white colors represent the {100} and {111} facets, respectively.

SCHEME 2: Probable Growing Mechanism of Big Triangular Platesa

a

The hexagonal, enneahedral, and triangular plates have the same radii of inscribed circle and thickness.

enlarged, and three {111} side facets will disappear and grow up into three corners of bigger triangular plates by the Ostwaldripening process,33 which will result in the formation of the bigger triangular plates bound by two {111} planes as the top and bottom faces, and three {100} planes as side faces. In this process, the radii of a hexagonal inscribed circle and the thickness of hexagonal plates would not change until the triangular plates form, which has been proven by our experimental results. Considering that the enneahedral plate mentioned above has the same radii of the inscribed circle and the same thickness as the hexagonal and big triangular plates, the enneahedral plate is thought to be in an intermediate growth stage from hexagonal plates to bigger triangle plates. As shown in Scheme 2, the Ag+ ions were first reduced into small Ag seeds by DMF and PVP. As reaction time was prolonged, these Ag seeds grew into circular plates, then hexagonal plates, and triangular plates with increased edge lengths of the side faces by the template direction of PVP, where Ostwald ripening was most likely the driving force for the shape evolution. Figure 6 shows TEM and SEM images taken from Ag nanoparticles synthesized at MR ) 20. As shown in Figure 6a, 65% of the particles have a truncated cubic shape with side lengths of 50-80 nm. The inset of Figure 6a is SEM images of this sample, indicating that some corners and edges of these nanocubes are slightly truncated. Figure 6b and c shows one cubic Ag nanoparticle and the typical electron diffraction pattern recorded by directing the electron beam perpendicular to the top face of an individual cube, which indicates that each silver nanocube was a single-crystal bound primarily by {100} faces. The presence of HCl in the solution was believed to play an important role in selectively etching and dissolving twinned silver seeds.34 Furthermore, the presence of protons might slow down the reduction reaction and thereby facilitate the formation of single-crystal seeds. Once a large proportion of single seeds form, selection of PVP on the {100} facets will lead to a growth rate in the [111] direction greater than that in the [100] direction, and ultimately cubic particles result.15,34 When the molar ratio of PVP/AgNO3 was relatively high (MR ) 167), most of the nanoparticles were observed to be present with quasi-spherical polyhedrons (Figure 7), although a few slightly anisotropic particles such as short rods, and rhombic and triangular plates

were present. Their size is around 60-80 nm. In this case of enough PVP, every surface of the initially formed Ag seeds was completely covered with a thick coating of PVP; the further growth of seeds could lead to form isotropic nanoparticles. Our experiments suggest that the solvothermal process is helpful to form anisotropic Ag nanoparticles and PVP plays a critical role in synthesizing Ag nanoparticles with different shapes. Generally, the surface free energy of the FCC crystal is on the order of γ{110} > γ{100} > γ{111}. Under normal pressure without PVP, Ag atoms are generated and diffused at a sufficiently high rate, and the final product will have no choice but to take the thermodynamically favored shapes (multiply twinned decahedron) and give the most stable {111} face on the top surface of Ag nanoparticles. Under high pressure provided by vaporized DMF in the solvothermal process, atomic diffusion is constrained, which makes atomic mobility more difficult. The lower atomic mobility slows the growth of the crystal and makes the Ag crystal growth process kinetically controlled, which is helpful to form Ag triangular nanoplates and anisotropic structures. Several groups21 have reported that PVP can serve as the reducing and capping agent. PVP, which contains an N-CdO group, is easily bound to the surface of these Ag microcrystals and suppresses the growth speed of these crystal facets. The interaction strengths between PVP and different crystal facets of Ag microcrystals are substantially different and can therefore lead to anisotropic growth of Ag nanoparticles. Furthermore, our experiments also indicate that the molar ratio of PVP/AgNO3 also plays an important role in determining the anisotropic shape and morphologies of the final Ag nanopaticles, which results from a different reducing rate induced by PVP with different concentrations. When high concentrations of PVP were induced and the molar ratio of PVP/ AgNO3 was higher than 167, quasi-spherical polyhedrons were obtained where PVP served mainly as the reducing and capping agent. When the molar ratio of PVP/AgNO3 was decreased, the shape of Ag nanoparticles became more anisomerous and anisotropic, from cube, to hexagonal plate, then triangular plate, and finally nanorod in our experiments. It results from the different reducing rates induced by different molar ratios of PVP/ AgNO3, and the lower PVP concentration leads to slower

Multiple Shapes of Silver Nanoparticles reducing rates and a kinetically controlled growth process that is helpful to produce anisotropic nanostructures. Optical Properties. Figure 8 shows the UV-vis-NIR optical absorbance spectra of different colloids containing silver quasispheres, cubes, triangular nanoprisms, hexagonal nanoplates, and nanorods. The silver quasi-spheres exhibit an absorption band at 430 nm, which is attributed to the SPR of spherical Ag nanoparticles.6 The silver nanorod colloids display two SPR bands: one conventional 440 nm band attributed to the transverse SPR mode and another absorbance band at long wavelength due to the longitudinal SPR mode of the silver nanorod. The longitudinal SPR bands of two kinds of Ag nanorod colloids are located at 675 and 1000 nm, respectively, suggesting that the longitudinal SPR bands can be tuned by the aspect ratio of nanorods. Silver nanocubes show three SPR bands located at 345, 432, and 497 nm, which are associated to the resonances inherent to the cubic geometry.35 The peak at 432 nm corresponds to the dipolar resonance, whereas the peaks at 345 and 497 nm are due to high-multipolar excitations.36 Small triangular nanoplates display three SPR bands at 350, 445, and 535 nm, respectively. These bands can be attributed to the out-of-plane quadrupole resonance, the in-plane quadrupole resonance, and the in-plane dipole plasmon resonance, respectively, based on the theoretical calculation results by Jin8 and Schatz.7,37 The big triangular nanoplates exhibit four SPR bands at 340, 400, 525, and 860 nm, respectively. These bands can be attributed to the out-of-plane quadrupole resonance, the out-of-plane dipole, the in-plane quadrupole resonance, and the in-plane dipole plasmon resonance, respectively. Compared with the in-plane resonance bands of small triangular plates, the inplane quadrupole resonance (525 nm) and the in-plane dipole plasmon resonance (860 nm) of big triangular plates red-shift due to the larger edge lengths. The hexagonal nanoplates also show four SPR bands located at 350 (out-of-plane quadrupole resonance), 408 (the out-of-plane dipole), 464 (the in-plane quadrupole resonance), and 700 nm (the in-plane dipole plasmon resonance), respectively. Here, the long-wavelength resonance at 860 nm for big triangular plates blue-shifts to 700 nm for hexagonal plates, indicating that the in-plane dipole plasmon resonance is very sensitive to the sharpness of the corners on the triangular plates. Recently, metal-nanostructures including nanorods, nanowires, nanotriangles, and so on have been applied as SERS substrates to improve the sensitivity and reliability of conventional chemical and biological sensors. However, the influence of metal nanoparticles’ shape on their SERS enhancement is also a challenge. In the present work, these Ag nanoparticles with different shapes were self-assembled on glass substrates and served as ideal substrates for studying the SERS enhancement dependence of metal shapes. Figure 9 shows the surfaceenhanced Raman spectra of trans-1,2-bis(4-pyridyl)ethylene absorbed on different Ag nanostructured monolayers at an excitation wavelength of 514.0 nm. These peaks around 1200 and 1600 cm-1 are attributed to BPE signals.4b,38 In comparison with the SERS spectrum of spherical Ag nanoparticles, the magnitude of signals from cube, hexagonal, small and big triangular plates, and nanorods (aspect ratio >4) were 1.9, 2.4, 3.0, 3.6, and 4.5 times stronger. These results indicate that the triangular plates are very active substrates for the SERS detection of molecule species, which most likely originates from their sharp corners and edges. Recently, the theoretical studies on special metal structures37 have suggested that the local field effects can be enhanced by several orders of magnitude in the nanoparticles with sharp corners or edges (e.g., triangular plates)

J. Phys. Chem. C, Vol. 111, No. 26, 2007 9103 and can be responsible for the enhanced surface Raman spectrum. Furthermore, the largest SERS enhancement observed in nanorods should be attributed to the formation of SERS hot spots or a large number of BPE molecules residing at junctions between easily aggregated nanorods.39 Yadong Li et al. have also studied the SERS of Rhodamine B dye adsorbed on silver nanoparticles, nanowires, and nanoplates. They observed that the Raman enhancement of the Ag particles was better than that of the Ag nanowires and the Ag plates.40 They attributed it to the fact that the nanowire and nanoparticle substrates exposed more {100}, {110}, and {311} crystal faces, which had higher free energies and were more favorable for the interaction with organic molecules, and then enhanced the Raman signals by chemical effects. M. A. El-sayed et al. have also studied the Raman spectra of several molecules adsorbed on gold nanospheres (NSs) (12 nm in diameter) and nanorods using an offplasmon resonance excitation condition and observed that the Raman enhancement of the Au nanorods is stronger than that of the nanoparticles, which is attributed to the fact that the nanorods have more {110} active facets than nanospheres.41 S. B. Chaney et al. 4c and our group4e have reported that the Raman enhancement of the Ag nanorods is stronger than that of the Ag nanoparticles, which is attributed to the local field effects enhanced by the metal ellipsoidal structure. Xia et al. 26a have studied the SERS of BPE absorbed on Pd cubooctahedra, hexagonal, and triangular plate substrates and observed that the sequence of Raman enhancement quality is triangular plate > hexagonal plate > cubooctahedra, which is also consistent with our results. In principle, the SERS enhancement of some chemical molecules absorbed on metal substrates depends on many parameters such as metal size, shape and aggregation extent, surface particle packing density, surface plasmon position, excited laser wavelength, and so on. It is difficult to decide which factor will play a main role in the enhancement of SERS by far-field Raman spectra. If one wants to find out the enhancement mechanism for SERS enhancement influenced by metal shape and size, even crystal facets, then it is necessary to apply near-field Raman spectroscopy combined with in situ AFM for detecting the Raman signals of a single metal particle with different shapes and crystal facet orientations. Further investigations are necessary to clarify this point, and our related research is in progress. IV. Conclusions In summary, we have successfully applied one solvothermal route to synthesize silver nanoparticles with different shapes such as triangular, hexagonal, and enneahedral nanoplates, nanocubes, nanorods, and polyhedrons. Our experiments suggest that the solvothermal process is more helpful to form anisotropic Ag nanoparticles than a conventional oil-bath heating method. PVP plays a critical role in synthesizing Ag nanoparticles with different shapes. These Ag nanoparticles with different shapes were self-assembled on glass substrates and served as ideal substrates for studying the SERS enhancement dependence of metal shapes. The sequence of Raman enhancement quality is nanorods > triangular plates > hexagonal plates > cubes > spheres. In addition to the potential applications in SERS substrates to improve the sensitivity and reliability of conventional chemical and biological sensors, our materials are also of interest for use in second-harmonic generation (SHG)42 and third-order nonlinear optical materials3 because of the enhanced absorption of light from visible to the near-infrared wavelengths. Acknowledgment. Financial support and granting of the postdoctoral fellowship (P04416) of this work from the Japan

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