J. Phys. Chem. C 2008, 112, 5463-5470
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Aspartic Acid Synthesis of Crystalline Gold Nanoplates, Nanoribbons, and Nanowires in Aqueous Solutions Yen Nee Tan,† Jim Yang Lee,*,†,‡ and Daniel I. C. Wang†,§ Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore-MIT Alliance, National UniVersity of Singapore, 4 Engineering DriVe 3, Singapore 117576, and Department of Chemical Engineering, Massachusetts Institute of Technology, Room 16-429, 77 Massachusetts AVenue, Cambridge Massachusetts 02139 ReceiVed: January 18, 2008; In Final Form: January 29, 2008
A facile, environmentally benign shape-controlled synthesis of anisotropic crystalline gold nanostructures at room temperature has been developed using aspartic acid (Asp) as the reducing, particle-stabilizing, and shape-directing agents. Great latitude in the control of particle size and shape, including formation of hitherto unreported nanoribbons and nanowires, could be accomplished through simple manipulation of the experimental conditions. The gold precursor concentration [HAuCl4] and the concentration ratio of aspartic acid to gold precursor, R ([Asp]/[HAuCl4]), were found to be critical factors in morphosynthesis. Their effects on crystal growth into different final morphologies were examined and correlated with the growth kinetics. The experimental results also suggested a growth mechanism where the difference in rates between reduction of the gold precursor on {111} and {100} family of planes, competition between the reduction and capping functions of Asp, and Asp surface coverage determined the morphology of the nanostructure formed.
Introduction The optical, electronic, and magnetic properties of nanoscale materials are known to depend strongly on their size and shape.1-4 Yet many reported methods of preparation have emphasized only particle size control and growth kinetics. Shape-controlled synthesis was a relatively recent development by comparison. Two approaches are commonly used to generate anisotropic structures at the nanoscale. In template synthesis a mesoporous inorganic material or micellar aggregates of surfactant molecules are used to constrain crystal growth to follow the contour of the templates.5-8 Although a template-directed synthesis can be simple in execution, it is high cost and limited by the availability and variety of the templates. The second approach uses polymers, organic molecules, or small ions in the growth solution to accelerate or inhibit addition of atoms to specific crystal planes so as to induce anisotropicity.9-11 While these methods have led to the successful synthesis of a number of single-crystalline one-dimensional (1D)12-14 metal nanowires and nanorods and two-dimensional (2D)15-17 metal nanoplates, the growth mechanism and, in particular, effects of reaction kinetics on the morphology of the nanostructures are not well understood. Concerns for the environmental impact on the use of organic solvents in some of these syntheses and use of the synthesized nanomaterials in biological applications have motivated the search for more environmentally benign alternatives17 to chemical synthesis. The biomineralization18-24 and biomimetic25-27 approaches to nanomaterial synthesis satisfy the “green chemistry” requirement and, additionally, are able to produce highly * To whom correspondence should be addressed. Phone: 65 6516 2899. Fax: 65 6779 1936. E-mail:
[email protected]. † Singapore-MIT Alliance, National University of Singapore. ‡ Department of Chemical and Biomolecular Engineering, National University of Singapore. § Massachusetts Institute of Technology.
complex inorganic structures beyond the capability of conventional chemical synthesis. Several microorganisms, including bacteria, yeast, and fungi, have been used for the synthesis of metals,18-21 sulfides,22,23 and magnetite.24 Extracts of selected higher organisms such as plant leaves could also perform such syntheses. A well-cited example is the work of Sastry and coworkers,28 who used a boiled extract of lemongrass leaves as reducing cum shape-directing agent to prepare gold nanoplates. However, the complexity of the biological system renders the analysis and identification of active species in the nucleation and growth of metal nanoparticles a daunting task. Recent works from our group have implicated proteins as the principal biomolecules involved in the algal synthesis of gold nanoparticles.29 Since proteins are composed of amino acids, it is reasonable to postulate that the intrinsic activity of proteins is derived from the constituent amino acids. Indeed, several studies have shown that natural amino acids such as tyrosine,30 tryptophan,31 lysine,32 cysteine,33,34 and aspartic acid35 could be used as the reductant and/or capping agent in metal nanoparticle synthesis. However, differences in molecular structures among the amino acids, such as the type of functional group in the side chain and the number of electron-donating groups per molecule, could result in differences in reaction kinetics and hence different product morphologies. While using most amino acids in the syntheses produces spherical nanoparticles as the main product,30-33 aspartic acid (Asp)35 is unparalleled in its ability to form gold nanoplates in aqueous solutions without an external capping agent. However, there was no follow-up study to investigate the mechanism of formation with this particular amino acid. We were interested in understanding the mechanism underlying the use of aspartic-acid-assisted synthesis and the possibility of extending the anisotropy to higher order structures. We found that, under appropriate conditions, Asp also promoted formation of network-like ribbon and wire structures. It is the objective
10.1021/jp800501k CCC: $40.75 © 2008 American Chemical Society Published on Web 03/18/2008
5464 J. Phys. Chem. C, Vol. 112, No. 14, 2008 of this study to identify, as much as possible, the role of Asp in the nucleation and growth of gold clusters into different end shapes and sizes. The growth process of gold nanostructures in the Asp solution was reconstructed by following the changes in the UV-vis spectra of the reaction mixture with time. In addition, detailed microstructural characterizations of the gold nanoparticles formed under different experimental conditions were used to obtain a better understanding of the shape evolution process. Presented below are the details of this investigation. Experimental Section Materials. Hydrogen tetrachloroaurate(III) trihydrate, HAuCl4‚ 3H2O (g99.5%) and L-aspartic acid, C4H7NO4 (g99.5%), were purchased from Aldrich Chemicals and Fluka Chemicals, respectively. All chemicals were used as received without further purification. All glassware and magnetic stir bars were cleaned in aqua regia (three parts of concentrated HCl and one part of concentrated HNO3 by volume) and rinsed with deionized water thoroughly. Ultrapure water (18 MΩ from an Elga LabWater purification system) was used as the solvent throughout. Synthesis of Gold Nanostructures. The synthesis of gold nanostructures commenced by mixing calculated quantities of HAuCl4 and Asp aqueous solutions at room temperature. After the mixture was homogenized in a vortex mixer, it was left to age at room temperature to reaction completion. The products prepared with different chloroaurate concentrations (0.05-1.0 mM) and Asp to HAuCl4 concentration ratios were sampled at the end of the reaction and characterized. Characterizations. UV-vis spectra of the as-synthesized gold nanoparticle solution were recorded on a Shimadzu UV2450 spectrophotometer at 1 nm resolution. The nanoparticles were imaged by a JEOL JSM- 6700F field-emission scanning electron microscope (FESEM) operating at 5 kV and by a JEOL JEM-2010 transmission electron microscope operating at 200 kV. All samples for microscopy were prepared by dispensing 10 µL of the nanoparticle solution onto a 3-mm carboncoated copper grid followed by drying in vacuum at room temperature. Results and Discussion The shape-controlled synthesis of gold nanostructures by Asp was investigated by varying the reaction conditions. Parameters such as the gold precursor concentration and the Asp to gold precursor concentration ratio (R) were found to be of paramount importance to the morphosynthesis. The following is an account of their effects on the growth kinetics, which affected crystal growth into different end morphologies. Formation of Discrete Nanoplates and Nanospheres. Reaction kinetics was followed by monitoring the changes in the absorbance of the peak corresponding to formation of elemental gold. The absorption of nanogold in the UV-vis region is caused by particle surface plasmon resonance (SPR). For a given preparation, the SPR wavelength remained fairly constant throughout the reaction while the absorbance of the SPR peak varied with time. However, the SPR wavelength would vary from sample to sample because of the different sizes and shapes of nanoparticles formed under different reaction conditions. The conditions for formation of discrete nanostructures are discussed first. Effect of [Asp]. The effect of [Asp] was examined first by keeping the gold precursor concentration constant. Figure 1A shows the changes in the absorbance of the SPR peak of nanogold for solutions containing a fixed [HAuCl4] of 0.50 mM and different Asp concentrations (2.50, 0.50, and 0.25 mM),
Tan et al. monitored at wavelengths of 550, 580, and 580 nm, respectively. The time course of absorbance evolution followed a characteristic sigmoidal shape consisting of three distinguishable reaction regimes: (I) the induction period, where the absorbance remained low and visibly unchanged since the start of the reaction, (II) the growth period, where the absorbance underwent the most rapid changes; and (III) termination, where the SPR absorbance reached a plateau value, signaling the end of the reaction. Analysis of the time-resolved UV-vis spectra of reaction solutions showed that a decrease in the Asp concentration had a strong retardation effect on the reaction kinetics. A generally long induction period was observed under our experimental conditions, suggesting that nucleation was a difficult process, caused probably by the low supersaturation of the Au nuclei formed. Lowering the reduction facility (low [Asp] concentration) would render nucleation even more difficult, thereby causing the observed increase in the induction period. Similar trends also prevailed at other chloroaurate concentrations (0.25-1.0 mM, data not shown). Typical TEM images of gold nanoparticles obtained with 0.50 mM gold precursor solutions containing different Asp concentrations (Figure S1, Supporting Information) show that only discrete particles of spheres and plate-like nanostructures were found under these conditions. The size and shape distributions of the nanostructures from counting over 400 nanoparticles in randomly selected regions in the TEM images are plotted in Figure 1B. Two trends are immediately apparent. (1) The average size of the nanoparticles, regardless of shape, decreased with the increase in Asp concentration. This may be understood in terms of the beneficial effect of a high Asp concentration on the rate processes, especially the nucleation rate. A large number of nuclei were formed, quickly depleting the gold precursor concentration in the reaction environment and hence the amount of gold atoms available for growth by addition. Smaller gold nanoparticles were formed. The capping action of Asp also stabilized the small nanoparticles from growth by coalescence. (2) A strong influence of the Asp concentration on particle shapes. The fraction of nanoplates in the product decreased noticeably with the increase in Asp concentration. It is hypothesized that at low Asp concentrations, Asp adsorbed specifically on energetically favorable sites on the growing nuclei. The specificity was lost at high Asp concentrations when both specific and nonspecific adsorptions of Asp occurred. The more uniform coverage of Asp on the nuclei masked out the intrinsic difference between sites, resulting in isotropic growth of the nuclei into spherical particles. Effect of [HAuCl4]. The effect of the gold precursor concentration on the shape and size evolution of the nanostructures was analyzed at [Asp] ) 1.00 mM using gold precursor concentrations in the range from 0.10 to 0.50 mM (Figure 2A). For the sample corresponding to [HAuCl4] ) 0.50 mM, the decrease in the SPR intensity after reaching the absorption plateau (indicating completion of reaction) was caused by sedimentation of relatively large gold nanoparticles. The Au SPR peaks in Figure 2A all emerged at about the same time. The observed independence of the induction period on gold precursor concentration indicates that the induction period was controlled by the Asp concentration, which was not varied under this set of experiments. In general, the higher the gold precursor concentration, the shorter the time for complete reaction. This is taken to indicate that while the Asp concentration affected more the kinetics in the early stages of the reaction dominated by nucleation (the induction period), the gold precursor concentration had a greater influence in the later part of the reaction
Synthesis of Crystalline Gold Nanoplates
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Figure 1. (A) Reaction kinetics study carried out at [HAuCl4] ) 0.50 mM and [Asp] ) 2.50 mM (squares), 0.50 mM (circles), and 0.25 mM (triangles). The marked induction, growth, and termination periods pertain to the sample prepared at [Asp] )2.50 mM. (B) Distributions of shapes (solid) and average size (hollow) of gold nanoparticles synthesized in solutions with [HAuCl4] ) 0.50 mM and different aspartic acid concentrations.
dominated by crystal growth. Analysis of the effect of gold concentration on shapes and sizes (Figure 2B) revealed an entirely different trend from that in Figure 1B. When the gold precursor concentration was varied at a constant [Asp] ) 1.00 mM, the fraction of nanoplates and nanospheres in the product remained relatively constant while the average size of the nanoparticles increased with the gold precursor concentration. It is hypothesized that the kinetics of particle growth was promoted by an increase in the gold precursor concentration. Consequently, the nuclei that were formed in the nucleation step would then have more gold atoms for accretion and growth. A detailed TEM characterization of the nanoplates in the product was carried out. Figure 3A shows the electron diffraction pattern obtained by directing the electron beam perpendicularly to the flat face of a nanoplate. The inner spots (triangles) with the weakest intensity correspond well with the formally forbidden 1/3{422} reflections, and the outer stronger spots (circles) could be indexed to the {220} reflections. The 6-fold rotational symmetry of the diffraction spots indicates that the flat faces were bound by the {111} planes of fcc Au. Figure 3B shows the diffraction pattern of one of the sides of a nanoplate recorded
in the [011] orientation when the nanoplate was titled by 35°. The spots framed with circle, triangles, and square could be indexed to the {220}, {200}, and {111} reflections, respectively. This pattern confirms that the sides of the nanoplates were the {100} planes. Hence, a gold nanoplate was bound by two {111} planes at the top and bottom and by three {100} planes at the sides. This is shown schematically in the inset of Figure 3B. These assignments are similar to previous results on nanoplates of Ag, Au, and Pd36-38 bounded by atomically flat surfaces. The Wulff construction predicts that the thermodynamically favorable shape for a fcc metal is a cuboctahedron under vacuum condition.39,40 In a solution-phase synthesis, however, capping molecules could alter the surface free energies via adsorption and thus induce new shapes.41 The anisotropy in gold nanoplates was caused by a faster growth rate in the 〈100〉 direction over the 〈111〉 direction. It is believed that adsorption of Asp on the {111} planes inhibited the growth of the {111} facets. Formation of Network-like Nanoribbons and Nanowires. Previous studies have revealed strong correlations between the size and shape of metal nanoparticles and the capping agent to metal-ion concentration ratio.42-45 For example, Carotenuto et
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Figure 2. (A) Reaction kinetics study carried out at [Asp] ) 1.00 mM and [HAuCl4] ) 0.50 mM (squares), 0.35 mM (circles), and 0.10 mM (triangles). The downward-pointing arrows indicate the end of the reaction. (B) Distributions of shapes (solid) and average particle size (hollow) of gold nanoparticles synthesized in solutions with an aspartic acid concentration of 1.00 mM and different gold precursor concentrations.
Figure 3. (A) ED pattern taken from an individual gold triangular nanoplate (The TEM image of which is shown as the inset to Figure 3A) by directing the electron beam perpendicular to its triangular faces. (B) ED pattern taken from the side of this nanoplate by tilting the nanoplate by 35°. The inset to Figure 3B shows a schematic drawing of the triangular nanoplate where the {100} and {111} facets are colored in white and gray, respectively.
al.42 observed an increase in the size of Au nanoparticles prepared at low [PVP]/[HAuCl4] ratios, while Zhou et al.32 reported that the distribution of icosahedral shapes and platelike structures was dependent on the [PVP]/[HAuCl4] ratio. In this study, where Asp functioned as both the reducing and the capping agent, it was expected that the [Asp]/[HAuCl4] ratio,
or R, should likewise have a strong influence on the kinetics and the shape and size distributions of the nanoparticles. To verify this we carried out experiments at fixed R ratios while the gold precursor concentrations varied from 0.05 to 1.00 mM. The results in Figure 4 show the interesting finding that while discrete nanoplates and nanospheres were the most prominent
Synthesis of Crystalline Gold Nanoplates
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Figure 4. Two-dimensional map of gold nanostructures synthesized at different combinations of gold precursor concentrations and R ([Asp]/ [HAuCl4]) ratios with the TEM images showing the reaction products synthesized at the selected reaction conditions: (A) R ) 0.5 and [HAuCl4] ) 0.50 mM, (B) R ) 5.0 and [HAuCl4] ) 0.50 mM, (C) R ) 0.5 and [HAuCl4] ) 0.10 mM, and (D) R ) 5.0 and [HAuCl4] ) 0.10 mM.
products in the synthesis, network-like gold nanostructures were the exclusive products under the condition of R < 1 and gold precursor concentrations lower than 0.2 mM. The different UV-vis light absorption patterns are indications of the different morphologies of the nanostructures formed.46-47 Generally, the SPR of anisotropic particles have more than one absorption maximum because of the presence of multiple principal axes. For example, the SPR of Au nanoplates has an absorption peak at 570 nm and an absorption band in the 7001200 nm region due to the out-of-plane quadrupole resonance and in-plane dipole resonance, respectively.43,48-50 Figure S2A in the Supporting Information shows the UV-vis spectrum of gold nanoplates obtained at [HAuCl4] ) 0.50 mM and R) 0.5 of which the absorption peak at 580 nm corresponds well with the out-of-plane quadrupole resonance. Absorption above 900 nm was not measured because of an equipment limitation in our spectrometer. On the contrary, absorption was broad and almost flat in the 500-800 nm region for the product prepared at [HAuCl4] ) 0.10 mM and R ) 0.5 (Figure S2B). The flat absorption profile is characteristic of the networked gold nanostructures found in the studies carried out independently by Pei et al.45 and Wang et al.51 The broadness was caused by superposition of the longitudinal resonance of gold nanostructures with different aspect ratios. The good agreement between TEM and UV-vis measurements confirmed formation of network-like structures of gold under the condition of low [HAuCl4] and R ratio. Careful TEM examinations (Figure 5A and B) revealed two distinct types of nanostructures present in the network. (1) Nanowires with circular cross-sections (Figure 5C). The electron diffraction pattern of an individual gold nanowire (inset of Figure 5C) shows a series of concentric rings attributable to different gold crystallographic planes, suggesting that it was a polycrystalline object. (2) Nanoribbons with rectangular crosssections (Figure 5D). The electron diffraction pattern of an individual nanoribbon (inset of Figure 5D) shows that it was single crystalline. The lattice fringes in the HRTEM images of
Figure 5. TEM images of network-like gold nanostructures. (B) Higher magnification TEM image showing that the networked structures consist of both nanowires and thin, flat nanoribbons. TEM image and ED pattern (inset) of isolated (C) nanowire and (D) nanoribbon.
gold nanoribbons in Figure 6 indicate that the nanostructure was formed upon fusion of nanocrystals with individually high crystallinity. The two arrows in Figures 6A show the bonding region between two adjoining nanoparticles. The indentation highlights a feature of crystal growth by oriented attachment.52,53 A lattice spacing of 0.204 nm, corresponding well with the lattice spacing of Au {100} planes,54 was measured (Figure 6B), which could be used to suggest the growth direction. Hence, surface energy was reduced by the oriented attachment of the {100} planes, while the adsorbed Asp molecules on the {111} planes prevented the close contact of planes in the 〈111〉 direction.
5468 J. Phys. Chem. C, Vol. 112, No. 14, 2008
Figure 6. HRTEM images of a gold nanoribbon formed by oriented attachment growth. Arrows in A indicate the indentation. (B) The measured lattice spacing of 0.204 nm is used to indicate crystal growth in the 〈100〉 direction.
Proposed Mechanism. Asp can act both as a reducing agent and as a capping agent in the nanogold synthesis. Asp was administered in large excess (0.01% HAuCl4 with 0.003 mM Asp, R ≈ 38)35 in a previous synthesis to prevent the product nanoparticles from coming into close contact. This study demonstrated a greater latitude in shape tuning (from discrete anisotropic particles to networks of nanowires and nanoribbons) by manipulating the [Asp]/[HAuCl4] ratio in regions of low Asp concentrations. The stoichiometry of the Asp reduction of [HAuCl4] has not been reported in the literature but may be estimated based on the number of possible electron-donating groups in an Asp molecule as follows: The reduction capability of carboxyl groups has been known for a long time, since the use of citric acid for gold nanoparticle synthesis.55 Recently, Blanchard et al.56 reported that amine could also be used as a reducing agent in formation of gold nanoparticles. Simple inspection of the functional groups in an Asp molecule (two carboxyl groups and one amine group) suggests an “idealized” reaction stoichiometry of [Asp]/[HAuCl4] ) 1. The experimental usage of a R < 1 ratio thus clearly indicates an understoichiometric condition for reduction. In the current synthesis, Asp was a morphogenic agent. As shown in Figure 7A, once the nuclei had grown to a sufficiently
Tan et al. large size where different crystal planes could be differentiated on the surface, Asp began to interact specifically with the surface, resulting in changes in the distribution of Asp molecules on the surface. It has been found that at the reaction condition of R > 1 and chloroaurate concentrations in the range of values tested (0.10-1.00 mM) only discrete particles of nanoplates and nanospheres were formed. This is because there was a sufficient amount of Asp present to stabilize the nanoparticles by surface passivation. The size and shape distributions of nanoplates and nanospheres in the product were dependent on the amount of Asp used. A relatively high R . 1 value would ensure complete coverage of Asp on the nuclei. The adsorption was rendered nonselective. The overwhelming accumulation of Asp on different crystallographic planes would homogenize the surface structure and disabled the differential growth in different crystallographic directions (Figure 7B). A high percentage of nanospheres were formed as a result. In addition, smaller nanoplates and nanospheres were formed because the small particles were adequately stabilized by Asp present in excess to inhibit growth. On the other hand, formation of the network-like nanoribbons and nanowires required more stringent experimental conditions. The first factor observed in our studies was a [Asp]/[HAuCl4] ratio below the stoichiometric requirement. At R < 1, the gold nanoparticles initially formed upon chemical reduction of gold precursor were thermodynamically unstable because of limited ligand (Asp) protection. There were two ways to reduce the high surface energy of these unstable small particles: (A) rapid growth of the particles into larger entities through the continual addition of gold atoms from the reduction of AuCl4- or (B) assembly and fusion of the unstable particles into larger aggregates. Particles under such an unstable state have shown more tendency to undergo fusion into stable assemblies.57,58 The second factor was a low gold precursor concentration which, as shown in the earlier section, would lead to slower particle growth. The low rate of reduction at low [HAuCl4] implies that the Au atoms were not formed fast enough and in sufficient
Figure 7. Schematic illustration of the proposed mechanism for formation of different gold nanostructures under different reaction conditions. (A) Anisotropic growth of nanoplates facilitated by the specific adsorption of Asp at [111] planes. (B) Isotropic growth of nanospheres due to overwhelming adsorption of Asp on all planes. (C) Oriented attachment growth of nanoribbon in 〈100〉 directions under the condition of a limiting supply of Asp. (D) Fusion growth of polycrystalline nanowires from the random collision of bare gold nanoparticles with limited or no Asp protection.
Synthesis of Crystalline Gold Nanoplates quantity to grow the nuclei into large particles to reduce the surface energy; thus, the unstable small particles were more likely to stabilize through the assembly route (B). The last and most important factor was the intrinsic property of Asp which determines whether a single-crystalline nanoribbon or polycrystalline nanowire will form in the reaction mixture. A schematic representation of the proposed mechanism for formation of gold nanoribbons is shown in Figure 7C. As discussed previously, the preferential adsorption of Asp molecules on the {111} planes inhibited growth in the 〈111〉 direction relative to other directions, most notably the 〈100〉 direction, and increased the anisotropy in the product. In the case of nanoribbon formation, this specific interaction of Asp facilitated the oriented attachment of unstable small particles in directions where no or limited adsorption of the capping molecules would occur to interfere with growth. However, the bonding process probably occurred in a random fashion on the exposed {100} crystal, forming irregular elongated ribbon structures. On the other hand, since Asp was the limiting reagent in the synthesis of the network-like structures, it is reasonable to assume that some of the particles formed were not protected by the capping agent. These bare nanoparticles would stabilize through a random collision and fusion mechanism similar to that prevailing in systems with insufficient nonspecific capping agents (e.g., sodium citrate45 and 2-mercaptosuccinic acid59). Polycrystalline nanowires were formed through the random attachment and fusion of crystallites with different crystallographic orientations (Figure 7D). It is believed that network formation began with either single ribbons or single wires and expanded through ordered or random attachment of additional construction units as the reaction progressed. However, it should be mentioned that there is not any synthetic control over getting nanowires or nanoribbons in high yield exclusively of the other under our experimental conditions. Conclusions A room-temperature one-step synthesis based on the Asp reduction of HAuCl4 was used to prepare a variety of Au nanostructures from discrete nanospheres and nanoplates to network-like single-crystalline nanoribbons and polycrystalline nanowires. In the synthesis, Asp was both the reducing agent and the capping agent. The significant findings include the following. (1) At constant [HAuCl4] the induction period in the reaction was dependent on the Asp concentration. The Asp concentration also determined the size and distribution of shapes in the product nanostructures. At high Asp concentrations, a high nucleation rate resulted in formation of smaller nanoparticles with uniform coverage of Asp and isotropic growth prevailing. (2) At fixed [Asp], [HAuCl4] controlled the growth rate of the particles and hence the size of the nanocrystals finally formed. A high gold precursor concentration resulted in more accretion of gold atoms on the nuclei and a larger product size. (3) The [Asp] to [HAuCl4] ratio, or R, determined the complexity of the nanostructures formed. In particular, networks of gold nanoribbons and nanowires were formed at R < 1 and low gold precursor concentrations. The different rates of reduction of chloroaurate on {111} and {100} planes and the competition between the reduction and capping functions of the Asp molecules, which were made more significant at low [Asp], were responsible for generating the anisotropy and network-like structures. Acknowledgment. This work was supported by the Singapore-MIT Alliance. Y.N. would like to acknowledge the Singapore-MIT Alliance for her research scholarship.
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