Shape Adjustment between Multiply Twinned and Single-Crystalline

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Shape Adjustment between Multiply Twinned and Single-Crystalline Polyhedral Gold Nanocrystals: Decahedra, Icosahedra, and Truncated Tetrahedra Daeha Seo,† Choong Il Yoo,† Im Sik Chung,‡ Seung Min Park,§ Seol Ryu,*,§ and Hyunjoon Song*,† Department of Chemistry and School of Molecular Science (BK21), Korea AdVanced Institute of Science and Technology, Daejeon 305-701, Korea, BioNanotechnology Research Center, Korea Research Institute of Bioscience and Biotechnology, Daejeon 305-333, Korea, and Department of Chemistry, Kyung Hee UniVersity, Seoul 130-701, Korea ReceiVed: NoVember 16, 2007

Polyhedral gold nanocrystals with decahedral, icosahedral, and truncated tetrahedral shapes are synthesized by a simple one-pot polyol process in the prescence of poly(vinyl pyrrolidone) (PVP). High PVP concentration up to 360 equiv of the gold precursor, HAuCl4, effectively stabilizes decahedral seeds to yield uniform decahedra with various edge sizes. Decreased PVP concentration subsequently leads to selective formation of icosahedra and truncated tetrahedra. This results from a combination between the relative energy difference of the polyhedral structures and the oxidative etching rate of the seeds by Cl-/O2 during the reaction. The distinct morphologies of gold nanocrystals exhibit characteristic extinction patterns in the UV-vis-NIR ranges, and these properties are successfully analyzed by the discrete dipole approximation (DDA) calculation. Most extinctions stem from the polar and azimuthal dipolar excitations, and azimuthal quardrupole resonance appears between two dipolar bands in the 88-nm decahedra. Given these shape- and size-dependent optical properties, gold nanocrystals hold considerable promise for biomedical and photonic applications.

1. Introduction Nanostructured materials have distinct properties from bulk forms of the same composition. As the particle size decreases, the ratio of surface atoms versus total volume rapidly increases, and surface energy has a stronger influence on the physical and chemical properties of the materials. For instance, the melting point of gold is originally 1064 °C as a bulk material, but decreases to ∼380 °C for 1.5-nm particles due to its high surface energy.1 Particle shapes are also dominated by surface energy difference. A sphere is the most thermodynamically stable structure in an isotropic system, because it has the minimum number of surface atoms per volume. In common lattice structures, however, low index planes such as the {100}, {111}, and {110} planes are more stable than other faces due to their high density of surface atoms, and these facets tend to form particle surfaces to make polyhedral structures.2 Spherical particles, meanwhile, have high-index crystallographic planes on their surface. Numerous gold and silver nanostructures have been synthesized to date including spheres,3 cubes,4 plates,5 rods,6 and wires.7 Shaping of these nanostructures can be analyzed by the deposition of metal atoms on the seed surface under reductive environments. For this phenomenon, many distinct factors actually influence the final morphology of the nanoparticles. Fundamental conditions include precursor and ligand concentrations, solvents, reaction temperature and time, and experimental procedure. The Xia group has reported the effects of various additives (HCl, FeCl3, NaNO3, KBr, etc.) on silver nanocrystal * To whom correspondence should be addressed. E-mail: [email protected] (S. Ryu); [email protected] (H. Song). † Korea Advanced Institute of Science and Technology. ‡ Korea Research Institute of Bioscience and Biotechnology. § Kyung Hee University.

shapes by adjusting reduction rates of the metal precursor.8 The Liz-Marza´n group has demonstrated gold decahedra and octahedra synthesis from the different seed structures, respectively.9 We have also shown that silver ions can be employed to tune gold nanocrystals from octahedral to cuboctahedral and cubic by differentiating {100} and {111} surface growth rates.10 Several mechanisms have been proposed to explain the selective formation of certain morphologies; however, these mechanisms are limited to a narrow range of experimental conditions due to the complexity of each reaction system. A simple one-pot reaction scheme, on the other hand, could be expected to provide important information on the general mechanisms underlying nanoparticle shape formation. The most stable morphology of metal clusters with a small number of atoms is known to be a truncated decahedron or icosahedron according to calculations,11 but it is very difficult to understand the relationships between relative stability of small clusters (or seeds) and resulting nanoparticle structures. The original seed structure may or may not be maintained during the particle growth by the help of surface stabilizing ligands, and structural change mainly determines the final nanocrystal morphology. Shape control of the particles alters their physical and chemical properties. Shape- (and size-) dependent optical properties have already been demonstrated theoretically by several groups. The extinction spectra of spherical and spheroidal shapes were interpreted by the Mie theory, an exact analysis of the Maxwell equation.12 More complex structures could be approximately analyzed by a number of numerical methods, such as discrete dipole approximation (DDA),13 the multiple multipole method,14 the boundary element method (BEM),15 and the finite difference time domain (FDTD) method.16 In particular, DDA is a powerful approach to analyze the optical properties of isolated particles under a complex

10.1021/jp7109498 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/30/2008

2470 J. Phys. Chem. C, Vol. 112, No. 7, 2008 surrounding medium, and it gives good calculation results for the optical spectra of polyhedra and plates of noble metal nanocrystals.13,17 In the present work, we developed a simple one-pot polyol process with HAuCl4 and poly(vinyl pyrrolidone) (PVP) in diethylene glycol (DEG). Multiply twinned decahedra and icosahedra were synthesized in very high yields at high PVP concentrations, whereas single-crystalline truncated tetrahedra were obtained at low PVP concentration under the identical reaction conditions. This exclusive shape adjustment between multiply twinned and single-crystalline nanocrystals stems from a combination of the relative energy difference of the polyhedral structures and the oxidative etching rate of the seeds by Cland O2. We also measured UV-vis-NIR extinctions of polyhedra and analyzed them on the basis of DDA calculations. 2. Experimental Section 2.1. Chemicals. Tetrachloroaurate trihydrate (HAuCl4‚3H2O, 99.9+%), poly(vinyl pyrrolidone) (PVP, Mw ) 55,000), diethylene glycol (DEG, 99%), and tetraethylene glycol (TEG, 99%) were purchased from Aldrich and used without further purification. 2.2. Synthesis of Gold Decahedra. In 25 mL of DEG was dissolved 2.0 g of PVP, and the polymer solution was refluxed for 5 min. To this boiling solution was added 2.0 mL of DEG solution containing 20 mg of HAuCl4, and the reaction mixture was allowed to reflux for 10 min. The mixture was cooled and diluted with 20 mL of ethanol. The precipitates were collected after centrifugation at 6000 rpm for 30 min and washed with ethanol thoroughly to afford gold decahedra with an average edge length of 88 nm. For 67- and 48-nm edge decahedra, 5.0 and 7.0 g of PVP were used under the same reaction conditions, respectively. The products were collected by centrifugation at 12,000 rpm for 40 min and were then washed with ethanol several times. 2.3. Synthesis of Gold Icosahedra and Truncated Tetrahedra. All synthetic procedures were the same as the gold decahedra synthesis except the usage of PVP amounts: 0.10 g for gold icosahedra and 0.050 g for gold truncated tetrahedra, respectively. In the case of gold icosahedra, filtration of the precipitates using an aluminum oxide filter with 200-nm pores (Whatman, anodisc 25, 0.2 µm) could remove aggregates and yielded highly uniform icosahedral particles. 2.4. Optimal Synthesis of Gold Truncated Tetrahedra. One gram of PVP was dissolved in 12.5 mL of TEG, and the solution was heated at 200 °C. One milliliter of TEG solution containing 10 mg of HAuCl4 was added to the polymer solution and stirred at 200 °C for 25 min. After cooling the reaction mixture, the product was collected by centrifugation at 6000 rpm for 30 min and washed with ethanol thoroughly. 2.5. Characterization of Gold Nanocrystals. Scanning tunneling microscopy (SEM) images were obtained using a Philips XL30S FEG operated at 3 kV. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images, and electron diffraction (ED) patterns were obtained on a Tecnai F20 FE-TEM operated at 200 kV. X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-IIIC (3 kW) diffractometer using Cu KR radiation. The samples were prepared by placing a few drops of the colloidal solutions either on small pieces (5 mm × 5 mm) of silicon wafer (P-100) for SEM and XRD or on copper grids coated with lacey carbon film (Ted Pella, Inc.) for TEM and ED. The UV-vis-NIR absorption spectra were collected on a Jasco V530 UV/vis spectrophotometer using colloidal suspensions in water. For

Seo et al. removing the physisorbed PVP on the particle surface, the nanoparticles were carefully washed with ethanol containing 60% HNO3 aqueous solution, followed by centrifugation/ dispersion cycles with ethanol several times. 2.6. Theoretical Simulation of Optical Spectra. The optical properties of gold polyhedra in the UV-vis-NIR wavelength range were simulated by the DDA method.13 DDA calculation approximates a metal particle as a collection of induced dipoles in a small unit interacting with each other and an incident light. Using the DDSCAT code developed by Draine and Flatau,18 the extinction cross section Cext for each structure was calculated for a given wavelength of an incident light. The extinction spectrum was averaged rotationally over 320 orientations of the nanoparticle relative to the direction of incident light. Water was chosen to be the dielectric medium (solvent), and the wavelength of incident light was varied with an interval of 10 nm. The dielectric function of gold has been taken from the literature.19 3. Results and Discussion 3.1. Synthesis of Polyhedral Gold Nanocrystals. Gold nanocrystals were synthesized by the method based on polyol process. The reaction solvent was diethylene glycol (DEG), which is polar enough to dissolve both PVP and HAuCl4 very well, and has a high boiling point at 245 °C compared to that of ethylene glycol (197 °C). The synthesis is basically through a one-pot reaction. The gold precursor solution was added to the boiling DEG dissolving a large amount of PVP (∼360 equiv of gold concentration), and then the reaction mixture was refluxed for 10 min to afford regular decahedral particles. Figure 1 shows that the particles are uniform both in shape and in size. The average edge length is estimated as 88 ( 9 nm by counting more than 200 particles. The SEM image of a particle in Figure 1b clearly exhibits an ideal decahedral shape (Figure 1c) involving equilateral triangles on the surface, and the TEM image (Figure 1d) demonstrates its pentagonal projection with slightly rounded apexes. Similar decahedral shapes were reported in gold9 and silver,20 meaning that decahedron is one of the most stable structures in nanoscopic size ranges. The electron diffraction (ED) pattern of a decahedron in Figure 1e was obtained by tilting and aligning the particle along [110] zone axis, because the decahedron prefers to stand against its triangular face on a flat surface. A set of spots with a five-fold symmetry is observed as well as the spots from various facecentered cubic reflections, in good agreement with the simulated pattern of a decahedral cross section along the five-fold axis in copper nanorods.21 A high-resolution TEM (HRTEM) image of an edge area in a decahedron (Figure 1f) displays symmetric lattice fringe images against the twin boundary, where a few parallel layers are observed. Each triangular face represents a single crystalline nature by continuous lattice fringes. An HRTEM image of the center area (Figure 1g) obviously shows splitting of pentagonal axis in two. Parallel twin lines fill the gap between two separate axes. This splitting is attributed to the structural stress release in large decahedral particles. An ideal decahedral shape has an internal angular gap (7° 20′) to make a complete three-dimensional structure using equilateral triangular faces in a face-centered cubic lattice.22 As the particle grows, the structural stress becomes larger, and the stress release mechanism dominates particle shapes in order to stabilize entire polyhedral structures.11 In our synthesis, regular decahedral shapes were preserved by axis splitting during the growth, instead of forming truncated or round decahedral particles.

Shape Adjustment of Polyhedral Gold Nanocrystals

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Figure 2. (a,b) SEM images of 67-nm edge decahedra. (c,d) SEM images of 48-nm edge decahedra. The bars represent 500 nm (a,c), and 100 nm (b,d).

Figure 1. (a) SEM image of 88-nm gold decahedra. (b) SEM, (c) ideal model, and (d) TEM images of a decahedron. (e) ED pattern of a decahedron along [110] zone axis. HRTEM images of (f) edge and (g) center areas in a decahedron. The bars represent 500 nm (a), 100 nm (b, d), and 2 nm (f, g).

Figure 3. (a) SEM image of gold icosahedra. (b) SEM and (c) TEM images of an icosahedron. (Inset) ED pattern of a decahedron along [101] zone axis. (d) HRTEM image of an edge area in an icosahedron. The bars represent 200 nm (a), 100 nm (b, c), and 2 nm (d).

Increase of the surface capping reagent without changing the reaction conditions commonly suppresses particle growth and yields smaller particles in gold cube and octahedron synthesis.10 Similarly, larger amounts of PVP with the same gold precursor concentration generated smaller decahedral particles in high yields without altering their shapes. Figure 2a shows regular decahedra with an average edge length of 67 ( 8 nm by using 5.0 g of PVP (890 equiv of gold concentration). Seven grams (1200 equiv) of PVP under the same conditions afforded smaller decahedra with an edge length of 48 ( 7 nm (80%) as well as some rounded and triangular particles (20%) (Figure 2c). Expanded SEM images (Figure 2b,d) reveal that particle shapes

are ideal decahedra regardless of PVP usage. More PVP addition yielded tinier decahedral particles, but the product contained a variety of different shapes as well. When 0.10 g (18 equiv) of PVP was used under the identical reaction conditions, colloidal particles and a small amount of aggregates were formed after the reaction, and the aggregates were easily removed by filtering the product solution. The resulting particles are nearly spherical as shown in Figure 3a, but a high-magnification image displays a regular icosahedral structure with slightly rounded edges (Figure 3b), in which each triangular face can be identified by the contrast. The TEM image in Figure 3c is a hexagonal projection of an icosahedron, and

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Figure 4. (a) SEM image of the mixture of truncated tetrahedra and octahedra. (b) SEM image of truncated tetrahedra under the optimal conditions. (c) SEM and (d) TEM images of a truncated tetrahedron. (Inset) ED pattern of a truncated tetrahedron along [111] zone axis. The bars represent 500 nm (a, b), and 200 nm (c, d).

the ED pattern of a particle shows a pseudo-six-fold symmetry, which matches well with the simulated pattern of an icosahedron standing along the [101] zone axis.23 The HRTEM image in Figure 3d represents a twin intersection with symmetric lattice fringes on neighboring faces without dislocations or parallel layers. The average diameter is measured as 94 ( 10 nm. Such icosahedral particles were investigated well in gas-phase experiments,24 but their colloidal forms with well-defined shapes were rarely observed.4b,25 A mixture of triangular shapes and octahedra was obtained as shown in Figure 4a when the PVP amount was reduced to 0.050 g (9 equiv of the gold precursor concentration). The optimized conditions gave triangular shapes as a major product >70% (Figure 4b). However, the SEM image of each particle reveals that the structure is not a triangular plate, but a truncated or partially developed tetrahedron. Figure 4c shows that the bottom triangle of a particle is larger than the top triangular face. The TEM image in Figure 4d displays a triangular projection with slightly rounded apexes, and ED pattern in the inset indicates a particle standing along normal to the {111} face. The average edge lengths of bottom and top triangles are 290 ( 40 and 210 ( 30 nm, respectively. The average edge size of tetrahedra is larger than those of decahedra and icosahedra, presumably due to less blocking of the particle surface by the surface capping reagent during the reaction. 3.2. Structures and Reaction Mechanisms of Gold Polyhedra. Gold polyhedral nanocrystals were checked by X-ray diffraction. Figure 5 exhibits only one peak assignable to {111} diffraction for each polyhedron, indicating that decahedral, icosahedral, and truncated tetrahedral structures are covered with {111} faces exclusively, and tend to stand along normal to the {111} planes. It is known that the relative surface energies in a face-centered cubic lattice are in the order of γ111 < γ100 < γ110 among the lowest index surfaces, due to distinct surface atom densities and coordination numbers with neighboring atoms.2 Accordingly, stable structures having {111} facets on the surface readily evolve during the particle growth.

Seo et al.

Figure 5. X-ray diffraction data of gold polyhedra.

Previous theoretical calculation results indicate that the most stable structure of gold nanoparticles is truncated decahedron in wide size ranges under vacuum conditions.26 Icosahedral and regular decahedral shapes are also more stable than other facecentered cubic structures, but they become less favorable with the increase of particle sizes. The structural total energy (Et) using macroscopic concepts as a guide is given by

Et(N) ) EBN + EσN + EγS

(1)

where N is the number of atoms, EB is the bulk energy per atom, Eσ is the structural strain energy per atom, Eγ is the average surface energy per unit area, and S is the surface area of the particles.11,27 When the particles are passivated with surface capping reagents, the interaction between surface atoms and capping molecules should be added in eq 1. If the capping reagent concentration is very high, decahedral seeds generated in the reaction mixture are effectively stabilized by PVP capping, and then the seeds are continuously grown to large decahedra without changing their structure. The internal strain of the decahedral shape is feasibly released by the splitting of the pentagonal axis as represented in Figure 1g. Consequently, regular decahedral particles are obtained at the end of the reaction. At relatively low PVP concentration, the internal surface energy of the crystals becomes more important than the interaction energy between surface atoms and PVP, and the particles prefer more rounded shapes for reducing the total surface area. The icosahedral structure is close to spherical and has stable {111} faces on the surface. The edge truncation of the icosahedron helps to diminish its structural strain energy. Decahedral and icosahedral nanocrystals are grown from the multiply twinned particles (MTPs). These structures are theoretically more stable than those of single-crystalline counterparts (cuboctahedra, truncated octahedra, etc.) within the small size ranges. However, the MTPs are chemically more reactive than the face-centered cubic structures because of high defect density on their surface.2,28 The Xia group has reported that Cl-

Shape Adjustment of Polyhedral Gold Nanocrystals SCHEME 1: Reaction Mechanism of the Formation of Gold Polyhedra

catalyzes the oxidative etching of silver and palladium MTPs by oxygen in air and generates single-crystalline nanoparticles.29 In our experiments, PVP can effectively protect the defects of MTPs against oxidative environments with Cl-/O2 at high PVP concentration, but etching of the particles by Cl-/O2 starts to occur below a certain threshold of PVP concentration (9 equiv of the gold concentration) to form single-crystalline structures (truncated tetrahedra and octahedra). The reduction rate of gold precursor also diminishes with a decrease of PVP concentration, which provides enough time to dissolve the MTPs during the reaction. In order to support this etching mechanism, 0.14 M of HCl (30 equiv of the gold concentration) was added to the reaction mixture during the synthesis of decahedra. The resulting product contained truncated tetrahedra (60%) and octahedra (10%) as well as decahedra (30%), although the original conditions yielded decahedral particles quantitatively. This indicates that the high Cl- concentration facilitates the dissolution of MTPs and the formation of single-crystalline particles even at high PVP concentration. In summary, changing the PVP concentration leads to the selective formation of decahedra, icosahedra, and truncated tetrahedra due to the relative structural energy difference and oxidative etching rate of the seeds as shown in Scheme 1. 3.3. Optical Spectra and Theoretical Simulations. UV/vis extinction spectra were measured for the structures of decahedra, icosahedra, and truncated tetrahedra in water. Each colloid was carefully washed with acid and water several times to guarantee the minimum amount of PVP layers adsorbed onto the particle surface. To ascertain the experimental optical spectra, DDA calculations were performed for extinction properties of the corresponding nanoparticle shapes. The ideal geometries were constructed with dipole points in the three-dimensional cubic lattice. The grid size, distance between interacting nearest neighbor dipoles, was 2 nm for decahedra and icosahedra. The icosahedral shape with a diameter of 94 nm was approximated to a sphere of an equal volume containing 33552 dipoles because of the structural analogy. For triangular plates5 and polyhedra,30 the tip sharpness is known to influence plasmon resonance peaks largely, and thus the “snipped” structures, in which the tip is cut off at the plane perpendicular to the pointing direction of each apex, are more close to the actual particle shapes. Similar tip snipping was also applied to our decahedral models with the cut of 5 nm from the apexes, and the resulting geometries with edge lengths of 88, 67, and 48 nm have 51297, 23547, and 8166 dipoles, respectively. Likewise, both truncated tetrahedra with edge lengths of 290 nm in

J. Phys. Chem. C, Vol. 112, No. 7, 2008 2473 the bottom and 230 nm in the top triangles and octahedra with a 220-nm edge length were snipped by 16 nm (Figure 4b). The grid sizes of the geometries were set to 5 nm in order to keep the number of dipoles manageable in DDA calculations. The resulting structures were approximated with 11660 and 41613 point dipoles. The extinction signals for all particles were averaged over 320 orientations with respect to the direction of incident light to consider random orientation of the nanoparticles in solution. For decahedral structures, the UV/vis extinction spectra exhibit two characteristic peaks for all samples, at 542 and 600 nm for 48-nm edge decahedra, 550 and 624 nm for 67-nm edge decahedra, and 550 and 664 nm for 88-nm edge decahedra, respectively. These extinctions are in excellent agreement with the peak positions calculated using the snipped decahedral models as shown in Figure 6a. A series of theoretical calculations reveal that the peaks at short wavelengths (500-600 nm) correspond to the polar dipolar plasmon mode, in which the fields near the two apexes along the five-fold axis are enhanced. The long wavelength peaks near 600-700 nm are associated with the azimuthal dipolar plasmon mode where the fields near the pentagonal edges are greatly enhanced. It is interesting that the two plasmon bands are red-shifted as the particle size becomes larger. This size dependency of the plasmon bands results from the retardation effect, in which the electromagnetic radiation induces phase shifts on opposite regions of the particle surface that are proportional to the ratio of the particle size to incident wavelength.12 In the 88-nm edge decahedra, a new band appears at 590 nm between the two dipolar resonances, although it is not very noticeable in the corresponding theoretical spectrum. The peak broadening and appearance of higher multipoles along the particle size increase are well-known phenomena for noble metal nanoparticles.12,31 In principle, theoretical extinction spectra generated by the DDA method are expected to show such trends because the radiative damping effect is offset by modeling the polarizabilty of point dipoles with the radiative reaction term included,32 and the phase retardation can be handled rightfully in the DDA method. However, it appears that the radiative damping for large nanoparticles is underestimated in DDA calculations, and for the 88-nm edge decahedra, a neighboring (less damped) azimuthal dipole peak at 664 nm overshadows the quadrupole mode significantly. Additional calculations for larger decahedra of 108- and 118-nm edge sizes clearly show distinguishable peaks for the quadrupole mode at 625 and 640 nm, respectively (Figure S1, Supporting Information). In the 118-nm edge decahedral model, the polar mode excitation exhibits the dipolar resonance at 560 nm as a single peak, but the azimuthal excitation shows the quadrupole mode at 640 nm as well as the dipolar resonance at 725 nm (Figure S2, Supporting Information). It confirms that the extinction at 590 nm between two dipolar peaks in 88-nm edge decahedra is assignable to the azimuthal quadrupole mode. All theoretical data for the decahedral structures are consistent with the Liz-Marza´n’s analysis using the BEM method.9b For icosahedra, the tip edges are truncated slightly as observed in parts b and c of Figure 3, which allows us to make the model for calculation as a sphere with the same 94-nm diameter. A theoretical extinction peak of a singlet at 560 nm is almost identical to the experimental data, as in Figure 6b. Although the icosahedral structure is based on the twin lattice, the optical spectrum shows no difference from that of single-crystalline spheres with the same volume, indicating that crystal structures are not largely related to plasmon resonances.

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Figure 6. UV-vis spectra (solid line) and theoretical calculations (dotted line) of gold (a) decahedra and (b) icosahedra. (c) UV-vis spectrum of truncated tetrahedra (solid line) and theoretical calculations of a truncated tetrahedron (dashed line) and an octahedron (dotted line).

For triangular prisms, the UV-vis extinctions are reported to be very sensitive to the particle size and tip sharpness. For instance, the red-most extinction peak of the triangular prism with a 100 nm edge size and a 16 nm thickness shifts from 770 to 600 nm by an edge snipping of 10 nm. Most triangular prisms in the literature have very broad plasmon resonances compared to theoretical predictions and thus have been analyzed as a mixture of different-sized prisms and side products.5,33 Our truncated tetrahedral sample in Figure 4b also contains ∼30% large octahedra and polygons as well as 70% truncated tetrahedra and exhibits broad plasmon resonances at 620 and 950 nm. For reasonable analysis, theoretical models of a truncated tetrahedron and an octahedron were chosen as representatives of the major and side products. The extinctions are estimated at 602 and 1051 nm (dash) for truncated tetrahedron, and at 562, 707, and 890 nm (dot) for octahedron (Figure 6c). The broad UV extinction peaks of the experimental spectrum can be interpreted by linear combination of the plasmon modes of these models. The broad peak at 950 nm corresponds to the mixture of in-plane dipolar plasmon resonances of the truncated tetrahedron and octahedron. It is noted that our truncated tetrahedral sample has its own size distribution of 14%, also leading to the band broadening. The absorption peak at 620 nm comes from the mixture of out-of-plane plasmon modes of truncated tetrahedron and octahedron as well as from spherical particles.

concentration, mainly due to decreased blocking of the particle surface by PVP from oxidative environments with Cl-/O2 in the reaction mixture. The resulting morphology of gold nanocrystals exhibits characteristic plasmon patterns in the UV-vis-NIR ranges, and these features were successfully interpreted by DDA calculations. This combination of synthesis and theoretical analysis may facilitate development of optimal metallic nanostructures for specific applications, such as biosensors, imaging, photothermal therapy, and photonic devices. In particular, surface enhanced Raman spectroscopy (SERS) using polyhedral gold nanocrystals could prove an effective tool for detecting analytes at extremely low concentrations. Similar polyhedra synthesis approaches of other face-centered cubic metals (Pd, Pt, Ni, etc.) are also in progress in order to study various surface (and shape-)-dependent catalytic reactions. Acknowledgment. This work was supported by the Korean Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2006-312-C00565) (to D.S., C.I.Y., and H.S.), the KRIBB Research Initiative program (to I.S.C.), and the Korean Science and Engineering Foundation (Grant No. R01-2006-000-10396-0) (to S.M.P. and S.R.). Supporting Information Available: Figures of extinction cross-sections. This material is available free of charge via the Internet at http://pubs.acs.org.

4. Conclusions Polyhedral shapes of gold nanocrystals could be tuned by altering PVP concentrations in a simple one-pot polyol process. At high PVP concentration, decahedral seeds were sufficiently stabilized to maintain their structure during the particle growth. Lower PVP concentration yielded icosahedra in order to reduce the total surface energy of the crystals. Single-crystalline truncated tetrahedra and octahedra were obtained at low PVP

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