A Versatile Route to the Controlled Synthesis of Gold Nanostructures

CRYSTAL GROWTH & DESIGN

A Versatile Route to the Controlled Synthesis of Gold Nanostructures Hao Ming Chen,† Ru-Shi Liu,*,† and Din Ping Tsai‡,§ Department of Chemistry, Department of Physics, and Center of Nanostorage Research, National Taiwan UniVersity, Taipei 106, Taiwan

2009 VOL. 9, NO. 5 2079–2087

ReceiVed April 16, 2008; ReVised Manuscript ReceiVed February 23, 2009

ABSTRACT: This investigation demonstrates a versatile route for the synthesis of multishaped gold nanoparticles (such as spherical nanoparticles, bipyramids, nanorods, nanowires, T- and star-shaped nanoparticles, and triangular nanoplates) that can be controlled by varying the conditions. Morphological, structural, and spectral changes that are associated with the seed-mediated growth of the nanoparticles in the presence of cetyltrimethylammonium bromide (CTAB) were systematically examined. A mechanism of the fabrication of these multishaped gold nanostructures is also proposed. This approach for generating variously shaped gold nanostructures may be useful in the design of novel materials with improved optical and structural properties. Introduction Noble metal nanoparticles, especially of gold and silver, have attracted substantial interest recently because of their unique size-dependent properties.1-7 The strong absorption and the scattering of light by gold and silver nanoparticles, as well as their stability, have made them popular inorganic nanocrystals. The various applications of these noble metal nanoparticles follow from their unique structural properties at nanometer dimensions. These properties can be tailored for potential applications by properly controlling the size and shape of the nanomaterials. Accordingly, shape control has received considerable attention, because it allows their properties to be finetuned for great versatility.8 One-dimensional (1D) gold nanostructures have attracted substantial attention because of their size-dependent optical properties, which can be tuned by varying the aspect ratio of the rods.7,9 Various synthetic methods for preparing gold nanorods are available.10-14 The most popular chemical method for synthesizing gold nanorods is the seed-mediated growth of nanoparticles in the presence of a surfactant. The most extensively used surfactant is cetyltrimethylammonium bromide (CTAB). Preferential adsorption of CTAB to the different crystal faces of gold leads to the inhibition of growth perpendicular to the long axis of the rods, thereby promoting growth at the ends of the rods.9 The growth mechanism of gold nanorods in the presence of CTAB has been widely studied to determine the influence of various experimental parameters. We reported on a modified seed-mediated technique for fabricating gold nanorods/wires wherein the shape of the gold nanomaterials evolved from fusiform into 1D rods.10e The presence of silver ions strongly dominated the formation of fusiform nanoparticles. The evolution of the gold ions from Au-Cl complexes to Au rods was elucidated using X-ray absorption.15 The theoretical simulation of X-ray absorption spectra further revealed the evolution of gold and revealed that Ultrafine small clusters of gold (Au13) formed after a reducing agent was added to the growth solution. A redesigned seedassisted growth method involving the serial addition of growth * To whom correspondence should be addressed. E-mail: [email protected]. † Department of Chemistry, National Taiwan University. ‡ Department of Physics, National Taiwan University. § Center of Nanostorage Research, National Taiwan University.

solution was adopted to fabricate 1D gold nanorods/wires with a tunable size from 50 nm to 1.7 µm.15 This work demonstrates a versatile route for the controlled synthesis of gold nanoparticles that could alter the resulting products into desired shapes by utilizing a method similar to that described elsewhere.10e,15 To achieve the controlled fabrication of nanomaterials, we need to investigate fundamental aspects of synthetic conditions; these include the amount of growth solution, the introduction of foreign ions, and the reaction temperature. A mechanism of the fabrication of these multishaped gold nanostructures is also proposed. The current synthetic scheme and the mechanism of generation of multishaped gold nanostructures may offer great opportunities for the design of novel materials with improved optical and structural properties. Experimental Section Chemicals and Materials. Hydrogen tetrachloroaurate(III) hydrate, trisodium citrate dehydrate (99%), silver nitrate (99%), ascorbic acid (AA) (99%), and cetyltrimethylammonium bromide (CTAB) (99%) were obtained from Acros Organics and used without further purification. The water used throughout this investigation was reagent-grade, produced using a Milli-Q SP ultrapure-water purification system from Nihon Millipore Ltd., Tokyo. Preparation of Gold Seeds. Aqueous 1% trisodium citrate (0.35 mL) was added to 10 mL of 0.25 mM aqueous HAuCl4. After the solution had been stirred for 3 min, 0.3 mL of ice-cold, freshly prepared 0.01 M aqueous NaBH4 was added, and then the solution was stirred for 5 min. The seed solution was maintained at room temperature for ∼2 h before use. Preparation of Multishaped Gold Nanoparticles. (a) Growth Solution without Foreign Ions. An aqueous solution of 0.1 M CTAB and 0.25 mM HAuCl4 was used as the growth solution. This solution was stored at 27 °C throughout the experiment. Gold seeds (0.1 mL) were placed in a beaker. Three volumes (1, 10, and 100 mL) of growth solution were mixed with 0.06 mL (first), 0.6 mL (second), and 6 mL (third) of freshly prepared ascorbic acid solution (10 mM), respectively. The growth solution became colorless when the ascorbic acid solution was added. These three colorless solutions were added to the quiescent gold seed solution stepwise in intervals of 30s. (b) Growth Solution with Mild (Large) Foreign Ions (Silver). An aqueous solution of 0.1 M CTAB, 0.004 mM (0.04 mM) AgNO3, and 0.25 mM HAuCl4 was used as the growth solution. This solution was stored at 27 °C throughout the experiment. Gold seeds (0.1 mL) were placed in a beaker. Three volumes (1, 10, and 100 mL) of growth solution were mixed with 0.06 mL (first), 0.6 mL (second), and 6 mL (third) of freshly prepared ascorbic acid solution (10 mM), respectively.

10.1021/cg800396t CCC: $40.75  2009 American Chemical Society Published on Web 03/30/2009

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Figure 1. TEM images of gold products synthesized under three conditions: (a-c) in the absence of silver ions; (d-f) in the presence of 0.004 mM silver ions; (g-i) in the presence of 0.04 mM silver ions.

Figure 2. HRTEM image and corresponding electron diffraction pattern of gold (a) short rod and (b) long rod. These three colorless solutions were added to the quiescent gold seed solution stepwise at intervals of 30 s. (c) Growth Solution with Thermal Treatment. An aqueous solution of 0.1 M CTAB and 0.25 mM HAuCl4 was used as the growth solution. This solution was stored at the desired temperature throughout the experiment. Gold seeds (0.1 mL) were placed in a beaker. Three volumes (1, 10, and 100 mL) of growth solution were mixed with 0.06, 0.6, and 6 mL of freshly prepared ascorbic acid solution (10 mM), respectively. These three colorless solutions were added to the quiescent gold seed solution stepwise at 30 s intervals. Notably, all experiments

were operated under thermal conditions and the products were aged for 48 h. (Details are shown in the Supporting Information.) Characterization. The UV/vis spectra of the colloidal nanoparticle solution were obtained using a SHIMADZU UV-1700 spectrophotometer with a 1 cm quartz cell at room temperature. Transmission electron microscopy (TEM) was utilized to characterize the overall morphology of the samples. The TEM images were captured using a JEOL JEM2010 electron microscope. The high-resolution transmission electron microscope (HRTEM) images and electron diffraction patterns were collected on a JEOL JEM-2100F electron microscope. The specimens

Versatile Route to Controlled Synthesis of Au Nanostructures

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were obtained by placing several drops of the colloidal solution onto a carbon-covered copper grid and evaporating it in air at room temperature. Prior to the specimen was prepared, the colloidal solution was sonicated for 1 min to promote the dispersion of particles on the copper grid. Atomic force microscopic (AFM) images were taken using the SPI3800N Probe Station and the SPA 400 operated in the AFM mode with standard silicon nitride tips. Typically, the surface was scanned at 2 Hz at a resolution of 256 lines per image and a set point of 1.4 - 4.0 V.

Results Effect of Silver Ions and Volumes of Growth Solution on Morphology. Figures 1a-c present the products prepared in the absence of silver ions. Figure 1a indicates that the major products were spherical particles if the first addition of the growth solution was initially added to the seed solution, but a few short nanorods were obtained because a few gold ions were available. Following, the second and third growth solutions were used to grow the nanorods and the mean lengths were approximately 59 and 570 nm, respectively. Figure 1c depicts the high-yield gold nanorods that have relatively uniform diameter and length and are accompanied by a small percentage of formed rough nanoplates. Figure 2a shows high-resolution TEM images of the diffraction contrast along the growth axis of the nanorods, revealing that the structure is not single crystalline. The structure was twinned along the long axis. Electron diffraction analysis verified the superposition of specific crystallographic zones that correspond to the ξ and κ zones of the face-centered cubic structure. In the third addition, The HRTEM image and selective area electron diffraction of figure 2b showed that nanorods (third) were isometric penta-fold twinned around their growth axis, which was along the [110] direction. All sections were separated by the (111) planes, and every rod had five {100} side faces and ten {111} end faces, consistent with an earlier study of nanorods prepared in CTAB medium.16 When silver ions (0.004 mM) were introduced to the reaction system, the product changed from 1D nanorods to bipyramids (as shown in images d and e in Figure 1). A comparison with Figure 1a indicates that numerous bipyramids formed upon the first addition of growth solution, indicating that silver ions are crucial to the growth of bipyramids. Selected area electron diffraction and HRTEM imaging were performed to investigate the crystalline structure of gold bipyramids (Figure 3). The electron diffraction pattern recorded under tilt conditions can be indexed only as a superposition of the two crystallographic zones, ξ and κ, of the face-centered cubic structure, indicating the presence of twinning. HRTEM images provide further evidence of the twinned structure. The gold bipyramids in this work are penta-fold twinned around the growth axis, with the side {110} facets tilted toward the {111} facets. The TEM image (Figure 1e) demonstrates that the growth product following the second addition contained bipyramids and irregularly faceted particles, and that the yield of bipyramids was approximately one-third and that of irregularly faceted particles was approximately two-thirds. The formation of the irregularly faceted particles may have been caused by the blocking of silver on particular facets. Upon the third addition of growth solution, nanorods and some irregularly faceted particles were observed, and the mean length of the nanorods was ∼550 nm. The aspect ratio of nanorods prepared without silver ions (∼22.5) exceeded that of nanorods prepared with silver (∼17). The blocking effect of silver on certain facets may suppress the growth of rods, further facilitating the change in the growth produce from anisotropic to isotropic.17 Hence, the aspect ratio of gold nanorods is slightly decreased. Notably, the presence of silver

Figure 3. HRTEM images and corresponding electron diffraction pattern of gold bipyramids.

Figure 4. Extiction spectra of samples prepared under various conditions: (a) in the absence of silver ions; (b) in the presence of 0.004 mM silver ions; and (c) in the presence of 0.04 mM silver ions.

not only causes the formation of irregularly nanoparticles but also inhibits the anisotropic growth of gold. Accordingly, the attempt to increase the aspect ratio of nanorods by increasing the silver ion content (0.04 mM) was unsuccessful. However, the resulting product was dramatically altered as more silver ions were introduced into the reaction system. Following the first addition of growth solution to grow the seeds, the resulting products contained irregularly faceted particles and bipyramids (Figure 1g). Specifically, the morphology changed with the inclusion of irregularly faceted particles and bipyramids. The surfaces of the irregularly faceted particles and bipyramids became very rough. The already nucleating small gold cluster developed on an otherwise smooth facet, and some small tips and islands were obtained upon the surfaces of irregular particles and bipyramids. When more growth solution was introduced into the system, these small tips acted as nucleation sites for the subsequent growth of gold. Consequently, multipod-shaped (T-shaped and branched-shaped) and

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Figure 5. (a-f) TEM images of gold nanocrystals synthesized at various temperatures as labeled in each image. Table 1. Mean Number of Differently Shaped Particles Synthesized at Various Temperatures preparation temperature (°C)

rod (%)

trianglar plate (%)

irregular plate (%)

sphere (%)

40 50 60 70 80 90

81 ( 3 18 ( 3 0 0 0 0

14 ( 2 58 ( 4 91 ( 3 23 ( 3 7(2 0

3(1 21 ( 2 6(2 20 ( 4 8(3 1(1

2(1 3(1 3(1 57 ( 5 85 ( 3 99 ( 1

star-shaped nanoparticles were found (images h and i in Figure 1). The quasi-one-dimensional structure has a larger curvature than spheres at the tips for the same volume. Because the local field enhancement is proportional to the curvature of the surface, such nanomaterials are expected to exhibit significant field enhancement.5,18 Briefly, the results of the addition of silver ions at higher concentrations are probably due to the interaction of the bromide counterions of the surfactant monomers (see below). UV/Vis Spectral Analysis of Each Product. Figure 4 displays the spectra each product in this investigation. Figure 4a presents the absorption spectra of nanorods that were prepared under silver-free conditions. As the additional volume of the growth solution increases, the longitudinal plasmon band is redshifted with an increase in intensity. When the growth solution was first added to the seeds, only a small shoulder was observed, revealing that most of the resulting nanoparticles were spherical. As more growth solution was added, the extinction spectrum exhibited a two-band feature, confirming the formation of 1D nanostructures. Figure 4b displays the absorption spectra of the growth products (bipyramids) in the presence of 0.004 mM silver ions. The longitudinal plasmon band was red-shifted as the amount of growth solution increased. The extinction value at the

longitudinal peak is comparable to that at the peak at shorter wavelength. As the amount of growth solution increases, the intensity of the longitudinal plasmon band appears to increase slightly, whereas the transverse plasmon band wavelength remains at ∼520 nm. In contrast, when the amount of growth solution was increased, the longitudinal plasmon band was much broader, with a full width at half-maximum (fwhm) of ∼150 nm. The fwhm of the longitudinal plasmon band increases with the aspect ratio. Because the change in electron density that corresponds to the longitudinal mode is slower than that of the transverse mode, it becomes slower and the fwhm of the longitudinal absorption band increases as the aspect ratio increases.19 Notably, the transverse plasmon peak becomes broader because of the presence of another peak near the transverse peak, associated with the formation of some irregularly faceted particles (as presented in Figure 1e), causing the red-shift in the extinction spectrum because of the larger diameter of these irregular particles. Figure 4c displays the absorption spectra of the products that were prepared from larger amount of silver ions (0.04 mM). These spectra notably differ from those of the products prepared in the presence of 0.004 mM silver ions, in a manner consistent with the TEM results. When the first addition of growth solution was employed, the spectrum thus obtained was that of a 1D gold nanostructure and the transverse band had a similar position but a far wider peak than the nanorods. This observation revealed the presence of irregular particles. Upon the second addition, the longitudinal plasmon bands shifted toward a lowenergy position at 1070 nm through the increase in aspect ratio associated with the presence of gold bipyramids. Specifically, a minor plasmon peak, present at ∼880 nm, is attributable to the formation of branched gold nanocrystals. Branched gold nanocrystals have been synthesized through the addition of a suitable amount of NaOH were the branched nanocrystals have



Figure 6. (a) Magnified TEM image of a single triangular gold nanoplate. (b) High-resolution TEM image of the edge of a triangular nanoplate. The inset presents the electron diffraction pattern of a triangular nanoplate. (c) High-resolution TEM image of (icosahedral) products sampled at 90 °C.

Figure 7. (a) AFM image of a triangular gold nanoplate. (b) Topographic height analysis of the plate along the line indicated in the AFM micrograph.

monopod, bipod, tripod, and tetrapod structures.20 The absorption spectrum of these branched particles includes two broad absorption peaks centered at 580 and 835 nm. The positions of the absorption peaks were similar to those observed herein, revealing the formation of a branched nanostructure in this work. When more growth solution was applied (third addition), starshaped gold nanoparticles were formed. Although the gold nanostar samples have marked structural heterogeneity, their extinction spectra include broad yet well-defined peaks in the UV/vis region. Elongated gold nanoparticles have been wellestablished theoretically and experimentally to yield a longitudinal plasmon peak that is red-shifted to an extent that is proportional to their aspect ratio.21 The peak centered at 805 nm in Figure 4c is likely to be a similar longitudinal plasmon resonance associated with the elongated tip structure of the nanostars. The peak at 605 nm represents the transverse plasmon band of the tips and is seen for gold nanorods and large or slightly asymmetric spherical colloids in the solution. Effect of Temperature on Morphology. We observed that the morphology of the products was highly sensitive to the temperature of the growth solution employed in the experiments.

Therefore, a series of experiments was performed to investigate the effect of the temperature on the formation of gold nanocrystals. Figure 5 shows the corresponding TEM images of the products, and the mean percentage distribution of products with different shapes is presented Table 1. The table indicates that the amounts of triangular plates and spheres are functions of temperature. As the reaction temperature is increased, the yield of nanorods generally declined. Figure clearly presents triangular plates, hexagonal and spherical particles. A significant number of triangular plates with other shapes were observed at reaction temperatures of 40 ∼ 60 °C. Most of the products were triangular, with an average size of 134 ( 11 nm below the reaction temperature of 60 °C. Interestingly, the sides of the particles were slanted (Figure 6a), suggesting that rather than being flat prisms, they were, more accurately, tetrahedral with a truncated corner. Figure 6b presents an HRTEM of the triangular nanoplate recorded along the [1j1j1] zone axis. The fringes are separated by 1.43 Å, which can be ascribed to the {220} reflection for an fcc lattice of gold. The inset in Figure 6b shows a typical electron diffraction pattern recorded by directing the electron beam perpendicular to the triangular flat

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Scheme 1. Schematic Illustration Summarizing All Reaction and Structural Changes at Three Different Conditions: (a) Absence of Silver Ions; (b) Mild Amount of Silver Ions; and (c) Large Amount of Silver Ions

faces of an individual nanoplate. The 6-fold rotational symmetry revealed by the diffraction spots implies that the triangular faces are presented by {111} planes. Two sets of spots are identified on the basis of the d-spacing, and the outer set of spots with a spacing of 1.4 Å results from the {220} reflection of fcc Au (square). The inner set with a spacing of 2.4 Å is believed to originate from the forbidden (1/3){422} reflection (circle). This forbidden reflection has also been identified from silver or gold nanostructures in the form of nanoplates.22 This finding is consistent with the geometrical model, in which each triangular nanoplate is bound by two {111} planes as the top and bottom faces and three {100} planes as the side faces.23 Further increasing the reaction temperature yielded a large number of spherical particles, with the disappearance of triangular plates. TEM analysis revealed that over 90% of the

particles had a projected hexagonal shape with a mean size of 42 ( 9 nm (Figure 5f) as the reaction temperature was increased to 90 °C. A high-resolution image of a single particle revealed a twinning, indicating that the particle was composed of multiple crystal domains (Figure 6c). Twinning is one of the most common planar defects in nanocrystals and is commonly observed in face-centered cubic metallic nanocrystals, which generally have {111} twins. The two most typical examples of multiply twinned particles are decahedrons and icosahedrons.23 In the present investigation, most of the products were icosahedral. Twinning is the mechanism of formation of these particles, possibly because they have smaller surface and volume energy. This work presents for the first time uniform metal particles in solution under the present reaction conditions.



Scheme 2. Schematic Illustration of Shaped Evolution for Synthesis of Gold Nanostructures under Desired Temperature

Atomic force microscopy was applied to further elucidate the morphology of newly synthesized nanoplates. Figure 7a shows a representative AFM micrograph of a gold nanoplate. Interestingly, AFM measurements verify the TEM observation that triangular particles appeared flat, with lateral dimensions much larger than their height. The bottom part of Figure 7b displays a topographic height analysis along the direction indicated by the black line in the image. The nanoplate has a height of ∼37.5 nm and is quite smooth over the surface. Nowadays, the synthesis of triangular nanoplates with sharp corners and edges is important because of their ability to maximally enhance the electromagnetic field, which makes them attractive substrates for detecting surface-enhanced Raman scattering (SERS).7,24 In brief, triangular and icosahedral nanocrystals can be produced by increasing the reaction temperature. Discussion Effect of Foreign Ions. (a) Absence of Foreign Ions. In the case of silver-free growth, the CTA+ headgroup binds to the side surface with some preference. The preferential binding is based on steric effects; the Au atoms spaced on the side faces are more comparable in size to the CTA+ headgroup than are those on the close-packed {111} face of gold, which is at the ends of the nanorods.14 Such binding stabilizes the side faces, which have higher surface energy and stress than other faces. Thus, gold atoms can be added along the [110] common axis on {111} faces, which more weakly bind and block CTA+ headgroups. Additionally, it may probably regard the higher adsorption energy of nitrogen atoms on gold surfaces;25a the CTA+ headgroup is preferentially bound to the lowest-adsorption energy (100) surfaces rather than the (111) surface of gold. As a result, a gold nanorod with a cyclic penta-twinned crystal with five {111} twin boundaries arranged in the [110] direction of elongation was formed (as depicted in Scheme 1a). The length of the nanorods increased with the amount of growth solution. (b) Presence of Silver Ions. To grow nanoparticles in the presence of silver ions, the fact that silver ions rather than nitrate ions play an important role in controlling the morphology of the products must be considered. To verify this point, we conducted a control experiment in which NaNO3 is substituted for AgNO3 as an NO3- source. The resulting products were spherical nanoparticles, even at high concentrations of NaNO3 (see S1 in the Supporting Information), indicating that silver ions importantly influenced the shapes. The effect of some species of anions on the CTAB templates is elucidated.25b EDX studies demonstrate that 2-5% of silver was associated with the particles. Most interestingly, the silver was preferentially

located at the center (∼5%) to the tip (below 2%). Bromine was also identified in these spectra, so the effect of silver ions on the morphology of the products can be explained by considering the following possibilities: (i) Ag0 atoms may be formed under the experimental conditions applied herein. Recently, Guyot-Sionnest described how the underpotential deposition of Ag0 on the growing gold nanorods contributes to the role of silver in the growth of gold nanorods.26 Ag+ ions were reduced on surface of the growing rods at a potential of less than the standard reduction potential to generate monolayers of Ag0, even if ascorbic acid is too weak to reduce Ag+ at low pH. (ii) Silver bromide is formed in this experiment. Because the silver bromide is formed in two sets of experiments, the concentrations of silver ions (Ag+) were 4.0 × 10-6 M and 4.0 × 10-5 M and that of bromide was 0.1 M (ksp AgBr ) 5.35 × 10-13 at 25 °C).27 However, some reports have demonstrated the chemisorptions of silver bromide on the gold nanocrystal faces.28 Accordingly, Ag0 and/or AgBr may be the initial species that may deposited on the gold seeds in this experiment. In the case of a mild amount of silver ions is used (as in Scheme 1b), compact silver and/or silver bromide atomic layers are formed preferentially on the {110} facets of the gold nanocrystals. This atomic layer over Au {110} acts as a strong binding agent that protects the facet from further growth; therefore, the total growth rate of gold on a {110} facet may be significantly reduced. However, the ends of the nanorods grow faster, allowing 1D growth in the longitudinal direction. Following the second addition, more gold atoms supported the growth of nanorods at the tips because of the highly localized curvature.29 The charge density was lower at the tips than at the sides, such that the rate of collision of atoms was faster at the tip. This effect allows the bipyramids to lengthen along the longitudinal axis. Upon the third addition to grow the gold, the product was 1D nanorods, because more gold atoms were simultaneously provided to grow the gold rods; the CTAB templates, rather than the binding effect of Ag0 and AgBr, simultaneously dominated this growth. Notably, a significant amount of irregular gold nanocrystals were observed (Figure 1f), perhaps because of the blocking effect of Ag0 and AgBr in our experiment. When a large amount of silver ions (as in Scheme 1c) were used, the blocking effect of Ag0 and AgBr fails to dominate the growth of gold. The amounts of Ag0 and AgBr increase far above the small amount of silver ions because the amount of silver ions increases by a factor of 10. Silver is known to have a catalytic effect on the growth of gold in the electroless metal plating of surfaces.30 An examination of S3 (see the Supporting Information) reveals that twin defects appear along the nanostar

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tips. These defects are similar to those found in gold nanorods, where poor CTAB binding to twin defects is thought to be a source of anisotropic growth. Recently, star-shaped gold nanoparticles have been prepared using a strong reductant in the reaction system.31 The nucleation for anisotropic growth at multiple sites on the nanostars is attributable to highly defective surfaces at the tips of the nanostars. A significant amount of Ag0 and/or AgBr that is present on the surface of the seeds because of underpotential deposition or chemisorptions produces the generation of defects and/or islands on the surface of the seeds that provide active sites for subsequent growth. Subsequent gold atoms begin to grow from these active sites on the surface of seed particles, yielding star-shaped products. As presented in Scheme 1c, irregularly faceted particles and bipyramids were formed upon the first addition. Adding more growth solution (second) leads to the formation of irregularly faceted particles with tips. The gold nanostars were fabricated by the subsequent growth of gold atoms on the tips because of the presence of defects. Notably, the resulting products arise from kinetic effects rather than thermodynamic effects, and these shapes appear at least preliminarily not to be intermediates in the formation of nanorods. Effect of Temperature. The factors that influence the formation of gold nanoplates were studied by varying the reaction temperature. Scheme 2 depicts the evolution of the shape of the gold nanostructures at various temperatures. At high temperature, AuCl4- ions are reduced sufficiently rapidly to supply gold nuclei or gold seed particles. The interactions of CTAB with the gold seed particles at high temperature may favor the growth of gold nanoplates. Numerous studies have elucidated this kinetically controlled growth of nanoplates.32 The solution became colorless after it was heated to a very high temperature, implying that the Au-Br-CTA complex would not have been formed, suggesting the instability of the CTAB templates for stabilizing the gold nanocrystals. We suspect that gold nanoplates and icosahedral nanocrystals are the products of the kinetic control of gold growth procedures. As the rate of reduction increases, however, the nucleation and growth become kinetically controlled and the product can take a range of shapes (nanoplates). A higher rate of gold reduction results in a supersaturation of gold atoms on the growing surfaces, and so promotes the formation of kinetically controlled shapes (nanoplates). In contrast, if the reduction rate is too high, these seeds with defects could also evolve into other structures instead of nanoplates. A higher reaction temperature also corresponds to higher surface diffusion, causing Oswald ripening and producing thermodynamically favored shapes (icosahedral nanocrystals). Notably, triangular gold nanoplates were not derived from other shapes (rods, small particles, and others). Some small plates are observed in the early stage, further elucidating the final products of kinetic control. Recently, Millstone et al. demonstrated that Au nanoprisms can be used as seeds, and the particle growth can be reinitiated simply by exposing the particles to Au ions in the presence of a reducing agent in a step-by-step manner.32d These results verify that gold nanoplates developed from a small one rather than from other shapes. Conclusions The morphological, structural, and spectral changes involved in the seed-mediated growth of gold nanostructures in the presence of CTAB were systematically investigated. A versatile route for the synthesis of gold nanostructures that allows the morphology of the products to be changed markedly (to, for example, spherical nanoparticles, bipyramids, nanorods, nanow-

Chen et al.

ires, T- and star-shaped nanoparticles, and triangular nanoplates) by varying the experimental conditions was demonstrated. The effect of silver ions on the growth of gold nanoparticles was studied in detail. A mechanism of the fabrication of these multishaped gold nanostructures was also proposed. Gold nanoplates and icosahedrons with uniform size were prepared using a thermal method with aqueous CTAB surfactant. The developed synthetic approach and mechanism for the generation of multishaped gold nanostructures may provide great opportunities for the design of novel materials with improved optical and structural properties. Acknowledgment. We thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contracts NSC 97-2113-M-012-MY3 and NSC 97-2120-M-002-013. Supporting Information Available: Absorption spectra of single addition preparation with presence of desired NaNO3 concentration (S1); EDX spectrum of gold bipyramids collected at the (a) tip and (b) center (S2); HRTEM images of star-shaped (S3) (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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CG800396T

A Versatile Route to the Controlled Synthesis of Gold Nanostructures

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