Monodisperse Icosahedral Ag, Au, and Pd Nanoparticles: Size Control

Dec 10, 2008 - Eunyong Seo , Seo-Jin Ko , Sa Hoon Min , Jin Young Kim , and Byeong-Su Kim. Chemistry of Materials 2015 27 (13), 4789-4798. Abstract | ...
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Qingbo Zhang,† Jianping Xie,‡ Jinhua Yang,† and Jim Yang Lee†,‡,* †

Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, and ‡Singapore⫺MIT Alliance, 4 Engineering Drive 3, National University of Singapore, Singapore 117576

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aterials on the 1⫺20 nm length scale behave significantly differently from their bulk form, showing properties that are strongly dependent on shape, size, composition, and surface chemistry.1 The preparation of nanoparticles monodisperse in composition, shape, size, and crystal structure is important to scientific explorations and technology development. First and foremost, the unambiguous identification of the nanoparticle properties requires monodisperse nanoparticles.2,3 In addition, reliable application performance requires consistent properties which are only possible with monodisperse nanoparticles.4,5 Monodispersity also promotes self-assembly of the nanoparticles into superlattices, where the interaction between proximal particles in a well-ordered environment may give rise to new collective properties useful to some applications.6⫺19 While significant progress has been made over the years on the sizecontrolled synthesis of nanoparticles,2,19⫺23 the concurrent control of size and crystal structure in the same preparation, however, remains a challenge especially for nanoparticles with multiple twins. Multiply twinned particles (MTPs) with icosahedral morphology are found in many fcc noble metal nanoparticles.24 An icosahedron is made up of 20 face-sharing tetrahedrons. The icosahedral nanoparticles are therefore bounded by 20 triangular faces, and all of them are {111} planes of the fcc structure. The orderly arranged twins within each particle and the abundance of {111} planes and ridges formed by them on the nanoparticle surface are responsible for the observation of interesting electronic, optical, and catalytic properties.25,26 Although several groups have reported the synthesis of icosahedral metal nanoparticles, the parwww.acsnano.org

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Monodisperse Icosahedral Ag, Au, and Pd Nanoparticles: Size Control Strategy and Superlattice Formation

ABSTRACT This paper reports the synthesis and self-assembly of monodisperse icosahedral Ag, Au, and Pd

nanoparticles with tunable sizes. Ag nanoparticles were first prepared by a modified polyol method. The assynthesized Ag nanoparticles were all icosahedral multiply twinned particles (MTPs) with notable size variations. The variability was reduced to less than 5% after a digestive ripening post treatment. The nearly monodisperse Ag nanoparticles were then used in replacement reactions with Au and Pd precursors to produce monodisperse icosahedral Au and Pd nanoparticles. The size of the monodisperse icosahedral Ag, Au, and Pd nanoparticles could also be increased thereafter by seed-mediated growth. The closely matched geometrical attributes of these icosahedral nanoparticles allowed them to easily self-assemble into 3-D superlattices either in the solution phase or on a solid substrate. KEYWORDS: monodisperse · multiply twinned particles · nanoparticles · noble metal · self-assembly

ticles were formed amidst other morphologies, and there was significant polydispersity in the size distributions.27⫺34 The measured properties could therefore have been obfuscated by sample heterogeneity. Herein we report relatively simple procedures for the synthesis and self-assembly of monodisperse icosahedral Ag, Au, and Pd MTPs with tunable sizes, which are outlined in Scheme 1. In the first step, Ag nanoparticles with exclusively icosahedral morphology but a broad size distribution were prepared by a modified polyol process under reaction kinetics control. The size distribution of the Ag nanoparticles was then significantly narrowed without changing the icosahedral morphology by digestive ripening. Monodisperse Au and Pd icosahedral nanoparticles could then be synthesized by the replacement reaction between monodisperse Ag nanoparticles and Au or Pd precursor, respectively. The size of the nanoparticles could also be subsequently increased by seed-mediated growth while keeping the icosahedral geometry and size distribution unchanged. The closely matched sizes of these monodisperse MTPs

*Address correspondence to [email protected]. Received for review August 24, 2008 and accepted November 26, 2008. Published online December 10, 2008. 10.1021/nn800531q CCC: $40.75 © 2009 American Chemical Society

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ARTICLE Scheme 1. Procedure for preparing monodisperse Ag, Au, and Pd icosahedral nanoparticles and their 3-D superlattices.

allowed them to be easily assembled into 3-D superlattices in the solution phase or on a solid substrate. RESULTS AND DISCUSSION Synthesis of Icosahedral Ag Nanoparticles. Figure 1 shows the UV/vis absorption spectrum of Ag nanoparticles synthesized by 1,2-hexadecanediol reduction of Ag⫹ in 4-tert-butyl toluene at 200 °C, together with an inset of a digital photo of the nanoparticle solution. The absorption maxima at ⬃410 nm and the vivid yellowish brown solution are manifests of surface plasmon resonance (SPR), a characteristic feature of the Ag nanoparticles.35 The TEM image of the as-synthesized Ag nanoparticles in Figure 2A shows a relatively broad size distribution of 9.1 ⫾ 3.6 nm. Each particle in the TEM image has the characteristic varying contrast of MTPs. Although MTPs commonly manifest themselves as

Figure 1. UV/vis absorption spectrum of Ag icosahedral nanoparticles; the inset shows a digital picture of the Ag organosol.

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decahedrons and icosahedrons, the HRTEM characterization of the as-synthesized Ag nanoparticles found exclusively icosahedrons. For example, the randomly sampled nanoparticles in Figure 2B⫺D are indeed icosahedrons with different orientations as shown by the computer simulated images included as figure insets.29 More than 150 nanoparticles were counted by HRTEM, and none of them deviated from the icosahedral morphology. Hence the as-synthesized Ag nanoparticles are MTPs with exclusively icosahedral morphology. The predominance of Ag nanoparticles with exclusively icosahedral morphology came as a surprise since icosahedral MTPs are not thermodynamically stable structures for Ag nanoparticles in this size range based on internal strain considerations.24 On the other hand, it is known that kinetic factors are as influential as thermodynamic factors in determining the morphology of the product Ag nanoparticles.36 Hence the Ag nanoparticles in our case had to be a product of kinetic control. The detailed, atomistic level understanding of the formation of icosahedral Ag nanoparticles is not yet clear. However, the molecular dynamics (MD) study of Baletto et al. allowed us to draw some general inference regarding the formation process.37,38 The formation of Ag nanoparticles is likely to begin with the establishment of magic number clusters. Magic number clusters containing several to several hundreds of Ag atoms could exist in one of the three thermodynamically stable forms: truncated octahedral (TO) singlecrystalline nanoparticles, MTPs with decahedral morphology, or MTPs with icosahedral morphology. The most thermodynamically stable structure for the cluswww.acsnano.org

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ters varies with the number of atoms per particle and is decahedron for 75, 100, 146, and 192 atoms, icosahedron for 55, 147, 309, and 561 atoms, and truncated octahedron for 116, 201, 225, and 314 atoms. As the number of Au atoms per cluster increases with growth, the particles would undergo several successive structural transformations in order to adapt to the most thermodynamically stable morphology. The transformation rate depends on temperature, the nature of the prevailing capping agent, and the size of the particles. The final morphology of the particles is determined by the competition between structural transformation and layerby-layer growth. If, during the growth process where icosahedral nanoparticles are the most thermodynamically stable morphology, the time required for the deposition of an additional layer of atoms is less than the time needed for structural transformation into the next stable morphology, structural transformation would not occur. Instead, the particles would grow in a layerby-layer mode, preserving the icosahedral geometry of the underlying core and propaFigure 2. (A) TEM image of Ag icosahedral nanoparticles synthesized at 200 °C. (BⴚD) gating it to the final shape. HRTEM images of Ag icosahedral nanoparticles in different orientations: (B) along the MD simulation also predicted that decatwo-fold symmetry axis; (C) along the three-fold symmetry axis; (D) along the five-fold symmetry axis. Insets in the upper right corners of these figures are the FFT patterns of hedral MTPs and single-crystalline TO nanothe HRTEM images; insets in the bottom right corners of these figures are the scheparticles can also prevail in the final product matic illustrations of the icosahedrons in different orientations. if the layer-by-layer growth begins with decahedral or TO magic number clusters.37,38 The tuning of the morphology of Ag nano- pinned down when layer-by-layer growth commenced soon (within seconds) after the addition of the AgNO3 particles can then be accomplished by changing the solution to the reducing agent solution. On the other deposition rate of Ag atoms. We therefore varied the hand, the homogenization of reactant concentration concentration of the Ag precursor in polyol synthesis and temperature normally occurred over a much longer to investigate the effects of Ag⫹ on product morphology and as a test of this hypothesis. Figure 3A,B shows time scale. The nonuniformity in the reaction microenthe TEM and HRTEM images of Ag nanoparticle synthe- vironment is expected to affect the kinetics of layer-bylayer growth and the kinetics of structural transformasized with 10 mM AgNO3 solution. Both decahedral tion to different degrees, and hence nanoparticles with and icosahedral Ag nanoparticles proliferated in the different morphologies may be formed in the final product. By reducing the AgNO3 concentration to 5 mM, a mixture of icosahedral and decahedral MTPs and product. Another reason for the nonuniformity of nanosingle-crystalline TO Ag nanoparticles was formed (Fig- particle morphology in the product was the fluctuabilure 3C,D). Lowering the concentration of the Ag precur- ity of particle structures. The calculation by Baletto has shown that the energy barrier to transformation besor led to a general decrease in the Ag atom production rate, which shifted the balance between structural tween various structures is relatively low for Ag particles with a certain number of atoms.38 Consequently, transformation and layer-by-layer growth to different nanoparticles with these numbers of atoms could flucextents, forming a range of products of different mortuate between energetically similar structures during phologies. These results demonstrate that kinetic condifferent stages of growth because of entropy effects.36 trol can be a versatile and effective tool for tuning the The fluctuation then resulted in a mixture of nanopartimorphology of nanoparticles. cles with different morphologies to be formed at the A major reason for the formation of mixtures of end. The different microscopic environments and strucnanoparticles with different morphologies at low Ag⫹ concentrations was the lack of a globally homogeneous tural fluctuations of the particles also explain the observation of myriads of different Ag nanoparticle morpholreaction environment for each particle in the solution. ogies reported in the literature. In this study, however, It is known that the morphology of each particle was

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Narrowing the Size Distribution of Icosahedral Ag Nanoparticles. Although the as-synthesized Ag nanoparticles were uniform in shape (icosahedral), the size distribution of the nanoparticles was broad. This was a consequence of simultaneous nucleation and growth. It is known that the formation of monodisperse nanoparticles requires “separation of nucleation and growth”, for example, burst nucleation in a homogeneous solution within an extremely short period of time followed by slow growth.2 In our modified polyol process, the “hot-injection” technique was used to fulfill the “burst nucleation”. However, mass and heat transfer issues inherent in the mixing of solutions with different compositions and temperature are unfavorable to the formation of monodisperse nanoparticles. Although the size distribution may be narrowed by size-selective precipitation, the yield of such process is relatively low.5 Herein we demonstrate that the size distribution of icosahedral Ag nanoparticles can be significantly narrowed by a digestive ripening procedure while keeping the icosahedral morphology intact. Figure 3. (A) TEM image of the mixture of icosahedral and decahedral Ag MTPs. (B) HRFigure 4A shows the TEM image of Ag TEM image of the mixture of icosahedral and decahedral Ag MTPs. (C) TEM image of the nanoparticles after digestive ripening of mixture of icosahedral and decahedral Ag MTPs and single-crystalline TO nanoparticles. (D) HRTEM image of the mixture of icosahedral and decahedral Ag MTPs and single- icosahedral Ag nanoparticles synthesized at crystalline TO nanoparticles. 200 °C. The diameter of the Ag nanoparticles was 9.6 ⫾ 0.5 nm. This indicates that digeswe confirmed that it is experimentally possible to form tive ripening had increased the average size of the Ag exclusively Ag nanoparticles with icosahedral morphol- nanoparticles (initially 9.1 ⫾ 3.6nm) but narrowed the ogy under appropriate conditions. particle size distribution. The variations of contrast in Our results are interesting by comparison with the the TEM image and the characteristic lattice fringes in recent study of Yin et al., who prepared Ag nanopartithe HRTEM image (Figure 4B) confirm that all of the Ag nanoparticles were still icosahedrons after digestive ripcles by a similar polyol process except for the use of dichlorobenzene (DCB) as the solvent.39 All of the nano- ening. The improved monodispersity had to be brought about by interparticle transfer of Ag atoms during the particles there were single-crystalline nanoparticles with nearly spherical cubooctahedral morphology. This digestive ripening process. The increase in average parcould be due to the chlorine (from DCB)-catalyzed oxy- ticle size and the complete absence of other morphologies indicate that not only were there no new particles gen etching of the Ag twinning nuclei.40 Hence no formed in digestive ripening but indeed some particles MTP was observed in that study. had disappeared. The average size of the starting Ag nanoparticles was especially important in maintaining morphology conformity in digestive ripening. When icosahedral Ag nanoparticles with an average diameter of 13.1 nm were used as the starting material, some single-crystalline particles were found among the ⬃9.6 nm product nanoparticles of digestive ripening. This is an indication of the formation of some new nuclei not necessarily of the icosahedral morphology. The final size of Ag nanoparticles obtained here is larger than that of Ag nanoparticles prepared by Klabunde and coworkers using solvated metal atom dispersion (SMAD) followed by digestive ripening.41,42 One possible reason Figure 4. (A) TEM image of monodisperse Ag icosahedral nanoparticles. (B) is the use of different digestive ripening agents (dodeHRTEM image of an isolated Ag icosahedral nanoparticle. VOL. 3 ▪ NO. 1 ▪ ZHANG ET AL.

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3Ag(s) + AuCl4- f Au(s) + 3Ag+ + 4Cl-

(1)

2Ag(s) + Pd2+ f Pd(s) + 2Ag+

(2)

Meanwhile, the AuCl4⫺ or Pd2⫹ ions are reduced to Au or Pd atoms and deposited on the nanoparticle surface. Under appropriate preparation conditions, all of the Ag atoms in the starting nanoparticles could be replaced and the resultant Au or Pd nanoparticles would adopt the icosahedral MTP morphology. After the monodisperse Ag icosahedral nanoparticles were subjected to replacement reaction with HAuCl4, the color of the organosol changed from yellow to pink and an absorption maximum appeared in the UV/vis spectrum (Figure 5A) at ⬃530 nm, indicating that the Ag nanoparticles had been completely converted into Au nanoparticles.35 The energy-dispersive X-ray (EDX) analysis of the product nanoparticles (using samples ⬎500 particles each) detected only monometallic gold (Figure 5B). The absence of silver in the particles excludes the possibility of bimetallic alloy or www.acsnano.org

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cylamine in this study and dodecanethiol in the Klabunde study). In another paper from the Klabunde group, the authors confirmed that digestive ripening of Au nanoparticles with dodecylamine or dodecanethiol could lead to final particles with different sizes.21 Another reason is the different morphologies of the starting Ag nanoparticles (exclusively icosahedral here vs a mixture of Ag nanoparticle different morphologies usually prepared by SMAD). Synthesis of Monodisperse Au and Pd Icosahedral Nanoparticles. The fcc icosahedral nanoparticles are catalytically important not only because the entire exterior surface is bounded by {111} planes but also because of the preponderance of corners and edges where enhanced catalytic activity is often observed.43⫺45 Since most noble metal nanoparticles are industrially important catalysts, we opine that it is valuable to develop reliable techniques for the preparation of monodisperse icosahedral nanoparticles of the noble metals. Herein we will demonstrate a preparative route which is based on the replacement reaction between monodisperse icosahedral Ag nanoparticles and the noble Figure 5. (A) UV/vis absorption spectrum of Au icosahedral nanoparticles. The inmetal precursor salts, using Au and Pd as exset is the digital photo of the Au organosol. (B) EDX spectrum of the Au icosaheamples. Since the standard reduction potential of dral nanoparticles. (C) TEM image of monodisperse Au icosahedral nanoparticles. AuCl4⫺/Au (1.0 V vs standard hydrogen electrode, (D) HRTEM image of an isolated Au icosahedral nanoparticle. or SHE) or Pd2⫹/Pd (0.95 V vs SHE) is higher than core/shell nanoparticle formation that has been rethat of Ag⫹/Ag (0.80 V vs SHE), the silver atoms in the ported in other preparations.42,46,47 Figure 5C,D shows nanoparticles are oxidized to Ag⫹ by AuCl4⫺ or Pd2⫹ the TEM and HRTEM images of Au nanoparticles recovupon mixing the Ag nanoparticles with AuCl4⫺ or Pd2⫹ ered from the replacement reaction. The varying conin the solution. The replacement reaction between Ag trast in the TEM image and the lattice fringes in the HRnanoparticles and AuCl4⫺ or Pd2⫹ can be represented TEM image confirm conservation of the icosahedral by eqs 1 and 2. morphology in the replacement reaction. The TEM image shows a gold particle size distribution of 6.8 ⫾ 0.5 nm, and hence good monodispersity of the nanoparticles was also preserved. The gold nanoparticles were considerably smaller than the starting Ag nanoparticles. The reduction in particle size was caused by the decrease in the number of atoms per particle. The number of atoms in each Au nanoparticle was about onethird of that in the starting Ag nanoparticle because the stoichiometry of the replacement reaction requires that three Ag atoms be displaced for every Au atom deposited. The size of the Au nanoparticles was ⬃70% of the starting Ag nanoparticles, which is consistent with the value calculated from the stoichiometry of the replacement reaction (Supporting Information). The elevated temperature of reaction and the abundant presence of oleylamine in the solution were necessary for the formation of solid Au icosahedral nanoparticles. If the reaction was carried out at low temperatures, hollow nanoparticles could have been the principal product, as shown by Xia et al. Without the abundant presence of amine, AgCl would be formed and contaminated the Au nanoparticle product. VOL. 3 ▪ NO. 1 ▪ 139–148 ▪ 2009

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Monodisperse icosahedral Pd nanoparticles could also be prepared by the replacement reaction between monodisperse icosahedral Ag nanoparticles and the Pd precursor salt. Figure 6A shows the UV/vis absorbance spectrum and digital picture of the nanoparticles after the replacement reaction. The color of the organsol changed to black, and there was no absorption peak in the 300 to 800 nm spectral region. The black color of the organsol and the complete obliteration of Ag SPR are indications of the formation of Pd nanoparticles, which has no SPR in the visible region.48 The EDX spectrum of the particle (Figure 6B) confirmed the sole presence of monometallic Pd. The varying contrast in the TEM image of Figure 6C and the characteristic lattice fringes in the HRTEM image of Figure 6D are again evidence for the icosahedral morphology. TEM also measured a particle size distribution of 7.9 ⫾ 0.5 nm. The particle size distribution indicates good monodispersity, and the average size agrees well with the value calculated from the stoichiometry of the replacement reaction. Different from the replacement reaction between Ag nanoparticles and HAuCl4, Pd2⫹ was hydrophobiFigure 6. (A) UV/vis absorption spectrum of Pd icosahedral nanoparticles. The inset is the digital photo of the Pd nanoparticles in toluene. (B) EDX spec- zed by TOAB rather than oleylamine. This is because the presence of oleylamine would inhibit the trum of the Pd icosahedral nanoparticles. (C) TEM image of monodisperse Pd icosahedral nanoparticles. (D) HRTEM image of an isolated Pd icosahereplacement reaction between Ag nanoparticles dral nanoparticle. and Pd2⫹. It is hypothesized that complexation between Pd2⫹ and oleylamine had led to the decrease in the reduction potential of Pd2⫹. Size Manipulation of Monodisperse Icosahedral Nanoparticles. While high monodispersity of the nanoparticles allows the unambiguous identification of particle properties, the mapping of particle properties as a function of size requires a series of monodisperse nanoparticles of different sizes. The monodisperse Ag, Au, and Pd icosahedral nanoparticles prepared by the procedures described above had fixed and invariant sizes. However, by using the synthesized monodisperse icosahedral nanoparticles as seeds, icosahedral nanoparticles with larger sizes could be obtained through layer-by-layer deposition of new atoms onto the surface of existing particles. Figure 7A,C shows TEM images of the Ag nanoparticles after the deposition of 0.02 and 0.04 mmol of Ag atoms onto 0.01 mmol of icosahedral Ag nanoparticle seeds. TEM also measured particle size distribution of 13.8 ⫾ 0.5 and 16.4 ⫾ 0.5 nm, respectively. The icosahedral morphology was not affected by growth as the varying contrast, and characteristic lattice fringes could still Figure 7. TEM and HRTEM images of monodisperse Ag icosahedral nanoparticles synbe detected in TEM and HRTEM images, rethesized by seed-mediated growth: (A,B) addition of 0.002 mmol Ag atoms; (C,D) addispectively (Figure 7B,D). Hence the size of the tion of 0.004 mmol Ag atoms. VOL. 3 ▪ NO. 1 ▪ ZHANG ET AL.

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Ag nanoparticles could be increased while keeping morphology and particle size distribution intact. The final size of the nanoparticles was controllable by the molar ratio between Ag nanoparticles and the depositing Ag atoms. Similarly, monodisperse Au and Pd icosahedral nanoparticles could also be enlarged by the seed-mediated growth method. The temperature for the deposition of new Ag atoms in seed-mediated growth was lower than the temperature used in the polyol synthesis of the icosahedral Ag nanoparticles. The AgNO3 solution was also introduced in a dropwise manner. These were deliberate measures to lower the rate of formation of Ag atoms to prevent the formation of new nuclei. Indeed, by controlling the rate of addition of the AgNO3 solution in the seed-mediated growth at an appropriate level, it is possible (and easy) to satisfy the requirement of “separation of nucleation and growth” in sustained development of monodispersity.2 Figure 8. (A) TEM image of a 3-D superlattice of Ag nanoparticles assembled on the Self-Assembly of Monodisperse Icosahedral copper grid. (B) FFT pattern of the superlattice. (C) Electron diffraction pattern of the suNanoparticles. The narrow size distribution pre- perlattice. (D) TEM image of a two-layer superlattice. (E) Schematic representation of disposes the synthesized icosahedral nanothe two-layer superlattice. (F) SEM image of a superlattice formed in the solution. (G) High-magnification SEM image of the superlattice formed in the solution. particles to form 3-D superlattices by selfassembly. These 3-D superlattices could be potentially useful for studying the dipole⫺dipole inter- particle in the second layer was located above the “hollow” formed by three adjacent particles in the first actions between proximal particles and for fabricating layer. mesoscale materials or nanodevices with newfound In addition to forming superlattices on a solid subproperties.19,49⫺57 The nanoparticles prepared in this strate, the monodisperse icosahedral nanoparticles study could easily form 3-D superlattices either on a solid substrate or in the solution. These two approaches could also form superlattices directly in the solution. After aging the solution for 72 h, some superlattices were may be used to produce 3-D superlattices for different formed and settled to the bottom of the vial. Figure 8F applications. is the FESEM image of the superlattice formed as such, Figure 8A shows the TEM image of a typical superwith highly visible terraces and steps. The size of the sulattice of Ag icosahedral nanoparticles assembled on a perlattice could be as large as ⬃2 ␮m. The high magnicarbon-coated copper grid after slow solvent evaporafication SEM image (Figure 8G) shows excellent translation. Both the TEM images and the corresponding FFT pattern (Figure 8B) show a 3-D superlattice almost per- tional ordering of the particles in the superlattice. Besides Ag nanoparticles, the as-synthesized monodisfectly aligned according to the hexagonal closedpacked (hcp) structure. The electron diffraction pattern perse icosahedral Au and Pd nanoparticles could also of the superlattice (Figure 8C) shows a number of rings. form superlattices on the substrate and in the solution. This indicates that the nanoparticles comprising the su- Figures S1 and S2 in the Supporting Information are the perlattice did not have orientational ordering although TEM and FESEM images of superlattices formed by icosahedral Au and Pd nanoparticles, respectively. they exhibited excellent translational ordering. This is notably different from superlattices formed by singleCONCLUSIONS crystalline TO nanoparticles, which show orientational The synthesis and self-assembly of monodisperse ordering. The absence of orientational ordering was most likely caused by the large number of surface faces Ag, Au, and Pd icosahedral MTPs with tunable sizes per particle (20) and the inherent isotropy of the icosa- have been demonstrated. Ag nanoparticles with excluhedral geometry. Figure 8D is the TEM image of a super- sively icosahedral morphology but a broad size distribution were prepared by a modified polyol method unlattice comprising only two layers of particles, which der kinetic control. The size distribution of the Ag was formed by decreasing the concentration of Ag icosahedral nanoparticles was substantially narrowed nanoparticles in the solution. It shows that each nano-

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by a digestive ripening procedure which preserved the icosahedral morphology. The replacement reaction between Ag icosahedral nanoparticles and Au or Pd precursor salts was then used to produce monodisperse Au and Pd icosahedral nanoparticles. The size of the monodisperse icosahedral nanoparticles could be subsequently enlarged by seed-mediated growth while keeping the icosahedral morphology and the size distribution unchanged. The synthesized icosahedral

nanoparticles easily formed 3-D superlattices on solid substrates as well as in the solution because of their size and shape monodispersity. The successful preparation of monodisperse icosahedral MTPs and their assembly into well-ordered 3-D superlattices should be of interest to the study of the properties of isolated icosahedral particles as well as properties arising from the close coupling between proximal particles.

EXPERIMENTAL SECTION

emission SEM operating at 25 kV accelerating voltage. UV/vis spectra of the nanoparticles were collected by a Shimadzu 2450 UV/vis spectrophotometer at room temperature.

Preparation of Polydisperse Ag Icosahedral Nanoparticles. A solution of 300 mg of 1,2-hexadecanediol (90%, Aldrich) in 10 mL of 4-tert-butyl toluene (TBT, 99%, Aldrich) was heated to boiling or to predetermined temperature. One hundred milligrams of AgNO3 and 1 mL of oleylamine (70%, Aldrich) were dissolved in 6 mL of TBT. This mixture was injected into the hot TBT solution of 1,2-hexadecanediol under vigorous stirring by magnetic stir bars. After 5 min of stirring, the system was cooled down to room temperature. The Ag nanoparticles in the solution were precipitated by ethanol and washed thrice with ethanol to remove the free ligands, unreacted reactants, intermediates, and byproduct. Narrowing the Size Distribution of Ag Icosahedral Nanoparticles. The size distribution of the as-synthesized Ag nanoparticles was narrowed by a modified digestive ripening procedure reported in the literature.22,41,42 Briefly, 200 mg of Ag nanoparticles and 6.8 g of dodecylamine were mixed with 60 mL of TBT. The mixture was refluxed for 48 h in a nitrogen environment with magnetic bar stirring. The organsol was then cooled down, and the Ag nanoparticles were recovered by ethanol precipitation followed by re-dispersion into an organic solvent such as toluene or TBT. Preparation of Monodisperse Au and Pd Icosahedral Nanoparticles. Au and Pd icosahedral nanoparticles were prepared by the replacement reaction between Ag nanoparticles and the corresponding Au or Pd precursor salts. For the preparation of Au nanoparticles, 20 mL of 0.5 mM Ag organosol in toluene was heated to boiling. Then, 3.4 mL of 1 mM HAuCl4 solution in toluene containing 10 mg of dodecylamine was added dropwise to the Ag organosol. The mixture was refluxed for 20 min before it was cooled down. For the preparation of Pd nanoparticles, hydrophobized Pd2⫹ was first obtained by transferring Pd(NO3)2 from water to toluene with TOAB. Twenty milliliters of 0.5 mM Ag organosol in TBT was heated to 160 °C, and 10 mL of 10 mM Pd2⫹ solution in toluene was then introduced dropwise to the organosol. The mixture was heated for 4 h before cooling down. The assynthesized Au and Pd nanoparticles were collected by ethanol precipitation followed by re-dispersion in toluene. Manipulating the Size of the Icosahedral Nanoparticles. The size of the monodisperse icosahedral nanoparticles was increased by the seed-mediated growth method. The following is the procedure used to enlarge the size of the Ag nanoparticles, which is also applicable to the Au and Pd nanoparticles. A calculated amount of 1,2-hexadecanediol was added to 10 mL of 1 mM Ag nanoparticle solution in toluene, and the mixture was brought to boiling. Two milliliters or 4 mL of 10 mM AgNO3 toluene solution was added dropwise to the hot Ag organosol to form Ag nanoparticles with different sizes. The molar ratio of 1,2hexadecanediol to AgNO3 was fixed at 2:1. The mixture was refluxed for 15 min before cooling down. The as-synthesized nanoparticles were collected by ethanol precipitation and redispersed into toluene. Materials Characterization. TEM images were taken on a JEOL JEM 2010 operating at 200 kV accelerating voltage. HRTEM images were taken on a AEM 2010 TEM. An energy-dispersive X-ray (EDX) analyzer attached to the AEM 2010 TEM provided in situ determinations of the compositions of the as-synthesized nanoparticles. SEM images were taken on a JEOL JSM-6700F field

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Acknowledgment. We acknowledge the financial support from Ministry of Education Academic Research Grant R279-000260-112. Q.Z. and J.Y. acknowledge the National University of Singapore for their research scholarships. J.X. acknowledges the Singapore⫺MIT alliance for his research scholarship. Supporting Information Available: Estimation of the average diameter of monodisperse Au and Pd nanoparticles; additional TEM and FESEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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