Engineering Inorganic Hybrid Nanoparticles: Tuning Combination

Oct 16, 2008 - The morphologies of binary metal Pt/Au hybrid NPs were modulated by controllable attachment of Au nanoscale domains to Pt templates. Si...
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Langmuir 2008, 24, 13197-13202

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Engineering Inorganic Hybrid Nanoparticles: Tuning Combination Fashions of Gold, Platinum, and Iron Oxide Hai-Tao Zhang,*,† Jun Ding,† Gan-Moog Chow,† and Zhi-Li Dong‡ Department of Materials Science and Engineering, Faculty of Engineering, National UniVersity of Singapore, Singapore 117574, and School of Materials Science and Engineering, Nanyang Technological UniVersity, Nanyang AVenue, Singapore 639798 ReceiVed August 27, 2008. ReVised Manuscript ReceiVed September 12, 2008 Multistep colloidal chemical routes were employed to synthesize Pt/Au, Pt/iron oxide (IO), and Au/Pt/IO hybrid nanoparticles (NPs). The starting templates, Pt NPs, were synthesized by controlling the decomposition kinetics of platinum acetylacetonate in oleylamine. The morphologies of binary metal Pt/Au hybrid NPs were modulated by controllable attachment of Au nanoscale domains to Pt templates. Similarly, Pt/IO and Au/Pt/IO hybrid NPs were fabricated by the controllable attachment of Fe to the Pt or Pt/Au template NPs. The noble metal domains of as-prepared hybrid NPs had face center cubic crystal structures and did not alloy, as verified by high resolution transmission electron microscopy and X-ray diffraction spectrometry. X-ray diffraction spectrometry study indicates that the IO domains in the as-prepared NPs have a spinel structure. UV-vis study of binary metal Pt/Au hybrid NPs revealed that they have a characteristic plasmon resonance around 525 nm, while dumbbell-like Au/Pt/IO NPs had a plasmon resonance around 600 nm. Furthermore, magnetism study of the binary Pt-IO NPs clearly indicated that the interfacial interactions between Pt and IO domains could result in a shift of the blocking temperature.

Introduction Much progress in the fabrication of nanomaterials has been made after the classic talk of nanoscience and nanotechnology given by Feynman in 1959.1 As is well-known, solution-grown nanocrystals ranging from 1 to 100 nm are of great interest for not only fundamental research but also commercial applications. After intensive exploration, nanocrystals with different forms (spheres,2 cubes,3 wires,4 disks,5 and multibranches6) have been successfully fabricated by colloidal chemical methods up to now.7 * Corresponding author. E-mail: [email protected]. † National University of Singapore. ‡ Nanyang Technological University. (1) (a) Feynman, R. J. Microelectromech. Syst. 1992, 1, 60. (b) Sugimoto, T. Monodispersed Particles; Elsevier: Amsterdam, 2001. (c) Cushing, B. L.; Kolesnichenko, V. L.; O’Connor, C. J. Chem. ReV. 2004, 104, 3893. (d) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (e) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (2) (a) Murray, C. B.; Noms, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (b) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (c) Jin, R. C.; Cao, Y. W.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (d) Hyeon, T. Chem. Commun. 2003, 927. (e) Peng, X. G. AdV. Mater. 2003, 15, 459. (f) Sun, S. H.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (g) Murray, C. B.; Sun, S. H.; Gaschler, W.; Doyle, H.; Betley, T. A.; Kagan, C. R. IBM J. Res. DeV. 2001, 45, 47. (3) (a) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176. (b) Li, X. D.; Gao, H. S.; Murphy, C. J.; Gou, L. F. Nano Lett. 2004, 4, 1903. (c) Murphy, C. J.; San, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J. X.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (4) (a) Greene, L. E.; Law, M.; Goldberger, J.; Kim, F.; Johnson, J. C.; Zhang, Y. F.; Saykally, R. J.; Yang, P. D. Angew. Chem., Int. Ed. 2003, 42, 3031. (b) Vayssieres, L. AdV. Mater. 2003, 15, 464. (c) Law, M.; Goldberger, J.; Yang, P. D. Annu. ReV. Mater. Res. 2004, 34, 83. (5) (a) Puntes, V. F.; Zanchet, D.; Erdonmez, C. K.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 12874. (b) Maillard, M.; Giorgio, S.; Pileni, M. P. AdV. Mater. 2002, 14, 1084. (c) Chen, S. H.; Fan, Z. Y.; Carroll, D. L. J. Phys. Chem. B 2002, 106, 10777. (d) Zhang, H. T.; Wu, G.; Chen, X. H. Langmuir 2005, 21, 4281. (6) (a) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Nat. Mater. 2003, 2, 382. (b) Zhang, H. T.; Ding, J.; Chow, G. M. Langmuir 2008, 24, 375. (c) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700. (d) Storhoff, J. J.; Mirkin, C. A. Chem. ReV. 1999, 99, 1849. (e) Hao, E.; Bailey, R. C.; Schatz, G. C.; Hupp, J. T.; Li, S. Y. Nano Lett. 2004, 4, 327. (7) (a) Park, J.; Joo, J.; Kwon, S. G.; Jang, Y.; Hyeon, T. Angew. Chem., Int. Ed. 2007, 46, 4630. (b) Jun, Y. W.; Choi, J. S.; Cheon, J. Angew. Chem., Int. Ed. 2006, 45, 3414.

These nanocrystals can not only be used as “artificial atoms” to assemble periodically into nanocrystal superlattices with different crystal structures, but can also be used as “artificial molecules”. The “artificial molecules” are usually formed through controllable attachment of functional molecules or several nanoscale domains in a way of resembling from the organization of atoms.8,9 In addition, multi-component linear and nanoscale branched heterostructures and superlattices have been fabricated through cation-exchange or selectively grown by chemists recently.10,11 The fast development of controllable fabrication of nanoparticles (NPs) indicates that researchers will be able to design and fabricate much more complex hybrid NPs with multiple components and complicated geometries in the near future.9 For the time being, the promising direction in chemical synthesis of NPs is being expanded from single-component NPs to multicomponent hybrid NPs with discrete nanoscale domains (metal, oxide, chalcogenide, and phosphide) combined in controllable fashions.12 As to the seed-assistant synthesis of NPs, much attention has been paid to the effect of seeds on the reduction of metals and the effects of seeding with another material.13-15 Recently, conformal shape-controlled core-shell binary metal nanocrystals were formed through the epitaxial overgrowth of Pd on Pt due to the small lattice-mismatch.16 Because of their promising optical, catalyst and magnetic properties, gold, platinum, and iron oxides (8) (a) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature 2006, 439, 7072. (b) Perepichka, D. F.; Rosei, F. Angew. Chem., Int. Ed. 2007, 46, 2. (9) Cozzoli, P. D.; Pellegrino, T.; Manna, L. Chem. Soc. ReV. 2006, 35, 1195. (10) (a) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J.; Wang, L. W.; Alivisatos, A. P. Nature 2004, 430, 190. (b) Habas, H. E.; Yang, P. D.; Mokari, T. J. Am. Chem. Soc. 2008, 130, 3294. (11) (a) Robinson, R. D.; Sadtler, B.; Demchenko, D. O.; Erdonmez, C. K.; Wang, L. W.; Alivisatos, A. P. Science 2007, 317, 355. (b) Mokari, T.; Rothenberg, E.; Popov, I.; Costi, R.; Banin, U. Science 2004, 304, 1787. (12) Zeng, H.; Sun, S. H. AdV. Funct. Mater. 2008, 18, 391. (13) Murphy, C. J.; Jana, N. R. AdV. Mater. 2002, 14, 80. (14) Yong, K. T.; Sahoo, Y.; Choudhury, K. R.; Swihart, M. T.; Minter, J. R.; Prasad, P. N. Nano Lett. 2006, 6, 709. (15) Mokari, T.; Sztrum, C. G.; Salant, A.; Rabani, E.; Banin, U. Nat. Mater. 2005, 4, 855. (16) Habas, S. E.; Lee, H.; Radmilovic, V.; Somorjai, G. A.; Yang, P. D. Nat. Mater. 2007, 6, 692.

10.1021/la802805w CCC: $40.75  2008 American Chemical Society Published on Web 10/17/2008

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(IOs) have been studied extensively and are being used commercially in many areas. Core-shell and heterodimer Au-IO NPs were reported recently.17 Adding one more discrete catalytic domain to the optical-magnetic hybrid NPs and tuning the combination fashions are of great interest. Compared to that of single-component NPs, the morphology control and combination fashion modulation of multicomponent hybrid NPs are rarely explored and remain a great challenge. Here, we demonstrate the elaborate “bottom up” chemical synthesis of several binary and ternary hybrid NPs. This study focuses on the controllable growth/attachment of secondary metals to morphology-controlled Pt NPs to obtain morphology-tunable binary metal Pt-Au and binary Pt-IO hybrid NPs. Furthermore, the binary metal NPs were used as templates to fabricate much more complicated ternary hybrid NPs (Pt-Au-IO) through additional growth/attachment processes. Synergistically, the chemical and physical properties along with the tunable morphologies of hybrid NPs have been studied systematically.

Experimental Section Oleylamine (OA, >70%), oleic acid (90%), iron pentacarbonyl (Fe(CO)5, 99.99%), 1-octadecane (90%), HAuCl4 · 3H2O (99%), platinum (II) acetylacetonate (Pt(acac)2), toluene (99.8%), hexane (99.5%), and ethanol (99.5%) were purchased from Sigma-Aldrich Chemical Co. All NPs were synthesized using standard airless procedures and commercial reagents without purification. In this work, Pt NPs used as templates for the synthesis of more complex multicomponent NPs (Pt-Au, Pt-IO, and Pt-Au-IO) were synthesized by the thermal decomposition of Pt(acac)2 at 130-250 °C in OA.6b Pt-IO hybrid NPs were formed by the selective attachment of Fe upon template Pt NPs by the decomposition of Fe(CO)5 at 230-260 °C in octadecane and OA. Binary metal Pt-Au hybrid NPs were fabricated by the controllable attachment of Au to Pt NPs through the reduction of HAuCl4 in the mixture of OA and toluene. Au-Pt-IO ternary hybrid NPs were formed by the attachment of Au to Pt-IO NPs or the attachment of Fe to Pt-Au NPs. Then the particles were collected and dispersed in hexane for further characterization. Microstructure analysis of hybrid NPs was conducted using transmission electron microscopy (TEM; JEOL JEM 2010F and 210F) with an accelerating voltage of 200 kV. The magnetic properties were measured using a superconducting quantum interference device (SQUID; Quantum Design, USA). The crystal structures were analyzed by X-ray diffraction (XRD) spectrometry (Bruker D8 Advance, USA). The optical properties were studied by dispersing the as-prepared NPs into cyclohexane and using a UV-vis spectrophotometer (Shimadzu 1610).

Results and Discussion Chemical Synthesis of Pt-Au NPs. The Pt NPs with different morphologies were used as seeds for the morphology-controllable synthesis of binary metal Pt/Au hybrid NPs. TEM images of typical Pt NPs are shown in Figure S1 (see the Supporting Information). It is clear that they are uniform in morphology and size.6b As shown in Figure 1, peanut-, bud-, and grape-like as well as some novel forms of binary metal hybrid NPs could be prepared. Formation routines are shown in Scheme 1a. The XRD patterns (Figure S2) collected on binary metal NPs can be indexed to face-centered cubic (fcc) crystal structure. The diffraction peaks are split in the higher 2θ range (for example, Au220 and Pt220) owing to the relatively large lattice constant difference (4.08%) between Au and Pt. The result indicates that these two (17) (a) Shi, W. L.; Zeng, H.; Sahoo, Y.; Ohulchanskyy, T. Y.; Ding, Y.; Wang, Z. L.; Swihart, M.; Prasad, P. N. Nano Lett. 2006, 6, 875. (b) Xu, Z. C.; Hou, Y. L.; Sun, S. H. J. Am. Chem. Soc. 2007, 129, 8698.

Figure 1. Electron microscopy characterizations of morphologycontrollable binary Pt-Au metal NPs. (a,b,e) TEM images of peanutlike (Vii), bud-like (ix), and grape-like (xVi) NPs formed through the epitaxial growth of Au NPs onto the spherical Pt NPs or the terminal of branched Pt networks. (c) HRTEM image showing that a spherical Au NP could epitaxially grow on only one terminal of Pt multirod NPs to form bud-like binary metal Pt-Au hybrid NPs, and the hybrid NPs are single-crystalline. (d) TEM image of seed-network-like Pt-Au hybrid NPs (x) whose complexity of Pt domains were improved by a secondary growth of Pt onto the original Pt multirod domains. (f) TEM image indicating that the shape of Au domains could be tuned from sphere to peanut-like particles (xi) by one additional growth of Au on the Au domains. Insets: high-magnification TEM images of the typical corresponding NPs.

kinds of noble metals did not alloy. Unlike the Pt NPs that grew anisotropically below 150 °C in OA, the Au particles grew isotropically, even at room temperature in the mixture solution of OA and toluene. Au particle size increased with increasing regent HAuCl4, and the monodispersity increased with the increasing size (Figure S3). To understand the epitaxial nature of the interface in the bud-like Pt/Au NPs, a systematic highresolution TEM (HRTEM) imaging study was carried out. A typical HRTEM image shown in Figure 1c indicates that the hybrid binary metal NP is single-crystal and that Au grew selectively epitaxially upon only one terminal of multirod Pt NPs. The selective growth of Au particles upon multirod Pt NPs could be explained plausibly by the higher reactivity of the terminals.18 It should be pointed out that the terminals have higher reactivity compared to the rod body due to its larger curvature. Systematic study revealed that selective growth of Au upon the terminals of Pt NPs was also affected by their geometric structures. Au tended to grow epitaxially upon only one terminal of multirod Pt NPs. Once some Au atoms nucleated on one (18) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153.

Engineering Inorganic Hybrid Nanoparticles Scheme 1. Scheme of Hypothesized Multistep Pathways for the Formation of Hybrid NPsa

a (a) Sketch of the proposed mechanisms leading to the multi-component inorganic hybrid NPs. Bud-like Pt/Au NPs (ix) formed by attaching Au on one terminal of multirod Pt NPs (iii) could be used as a seed to synthesize Au/Pt/IO NPs with different forms (xii, xiii, xiV, xVi); the complex of Pt and Au domains in Pt/Au NPs (ix) could be improved by sequential growth of Pt and Au to form seed-network-like (x) and peanut-network-like (xi) NPs; grape-like Pt-(Au@IO) NPs (xVii) were synthesized by coating IO on the Au domains of grape-like Au-Pt NPs (viii) formed by growth of Au on all terminal of simple multirod Pt NPs (iii). (b) Schematic illustration of the formation mechanism of the ternary dumbbell-like Au-Pt-IO hybrid NPs (xV) by sequential growth of Au on heterodimer Pt/IO (iV) formed by attaching and oxidization of Fe on Pt NPs (i).

terminal of a multirod NP, the following Au atoms would tend to grow upon the formed Au seed at room temperature owing to the larger lattice mismatch (4.08%) between Au and Pt in order to minimize the overall system interface energy. On the other hand, Au tended to grow upon most terminals of advanced branched networks to form grape-like binary metal NPs (Figure S4) under relatively higher reaction temperature conditions. Unlike the overgrowth of Pd on Pt nanocubes or the anisotropic growth of Au along 〈111〉 directions to form nanorods with Pt nanocubes as seeds in aqueous solvents, our study revealed that the Au could only grow isotropically upon the terminals of branched networks to form grape-like NPs (Figure S4) or Au grew epitaxially upon one or more sites of spherical Pt NPs to form peanut-like NPs (Figure 1a). Such interesting behavior should arise from the larger lattice mismatch of Pt/Au (4.08%) compared to that of Pt/Pd (0.77%) and/or the effect of surfactants and solutions. The systematic study indicates that both the complexity of Pt domains and shape of Au domains of bud-like binary metal NPs could be tuned separately. Figure 1b shows a typical TEM image of typical bud-like Pt-Au NPs with Au domain size of 10.5 nm and Pt multirod domains formed by annealing Pt(acac)2 at 150 °C for 2 h. The complexity of Pt branched domains was improved greatly by annealing the hybrid NPs with Pt(acac)2 at 135 °C for 24 h in OA, as shown in Figure 1d. Our previous study indicated Pt would grow in the kinetic

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control stage at 135 °C in OA; therefore, the complexity of branched Pt domain increased with additional growth. Interestingly, the Au forms of binary metal NPs evolved from spheres to peanut-like particles (Figure 1f) after one more reduction of HAuCl4 upon the original Au domains. The Au domains of binary metal NPs (Figure 1d) served as nucleation seeds in this process. Owing to the relatively larger curvature of the terminals, which exhibit relatively higher reactivity, Au tended to nucleate and grow epitaxially on most uncovered terminals of branched Pt NPs (Figure 1d) at medium temperature (60 °C) to form grape-like NPs (Figure 1e). Therefore, many terminals of the branched networks were coated in order to minimize the overall surface energy. Chemical Synthesis of Pt-IO NPs. Pt NPs with different complexities could also be used as seeds for the decomposition of Fe(CO)5. Formation pathways of binary Pt/IO hybrid NPs are shown in Scheme 1b. As shown in Figure 2a, spherical Pt NPs were enwrapped partly by IO through an attachment and oxidation process to form binary heterodimer-like NPs. In addition, core-shell Pt@IO NPs (Figure S5) could also be synthesized with spherical Pt NPs as templates when the ratio of Fe(CO)5 to Pt(acac)2 was increased greatly. The result indicates that the Fe formed by the decomposition of Fe(CO)5 coated the Pt NPs partly and then coated the whole particles with increasing Fe. Furthermore, jelly-like Pt@IO NPs (Figure S5) with multirod Pt NPs as cores could also be formed. TEM study indicates that the jelly-like Pt@IO NPs are relatively monodispersed. The size of this kind of hybrid NP could be tuned through modulating the complexity of starting templates. Jelly-like hybrid NPs (Figure S6) with an average size of 55 nm were formed when Pt branched network NPs were used as templates. XRD patterns (Figure S7) collected on Pt NPs could be indexed to an fcc Pt (space group: Fm3m, ao ) 3.923 Å, JCPDS no. 04-0802), and Pt-IO NPs could be indexed as fcc Pt and spinel structures of magnetite (space group: Fd3m, ao ) 8.397 Å, JCPDS no. 82-1533). HRTEM study (Figure S6) reveals that the multirod Pt templates remained in their original form, and the coating oxides have a singlecrystal characteristic. Chemical Synthesis of Pt-Au-IO NPs. Pt/IO heterodimers (Figure 2a) could be used as templates to form dumbbell-like ternary hybrid Au-Pt-IO NPs (Figure 2b) through locally epitaxial growth of Au onto the exposed Pt domains. Owing to the relatively large lattice mismatch of Au and Pt (4.08%), the diffraction peaks of Au and Pt could be easily resolved in a higher diffraction angle range of the XRD pattern (Figure S7). TEM study (Figure S8) indicates that the every domain of the hybrid NPs is relatively monodispersed. Energy-dispersive X-ray (EDX) analysis (Figure S9) shows that the Au/Pt/Fe atom ratio of the dumbbell-like hybrid NPs is about 46:15:39. HRTEM analysis (Figure 2c) indicates that the (111) lattice fringes of the two noble metals are parallel to each other. It is clear that Au NPs grew epitaxially upon Pt NPs in order to minimize the unsaturated bonds on the interface. In other words, the interface energy of the hybrid nanoparicles was minimized. The mechanism for the formation of ternary hybrid NPs is shown in Scheme 1b. In addition, Pt-Au heterodimers (Figure S10) could also be formed by etching the IO domains of dumbbell-like IO-Pt-Au hybrid NPs. The result indicates that the formation of hybrid NPs is reversible, and simple hybrid NPs can be formed by etching one domain of complex hybrid NPs. Furthermore, nanoscale binary metal Pt-Au NPs could be used as seeds to synthesize more complex ternary Pt-Au-IO hybrid NPs with controllable morphologies. The bud-like Pt-Au NPs could be covered totally or partly by IO. The TEM study revealed that they could be coated totally when 0.4 mL of Fe(CO)5

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Figure 2. Electron microscopy characterizations of combination fashion-tunable ternary Au-Pt-IO hybrid NPs. (a) TEM image of heterodimer-like Pt-IO NPs; (b,c) TEM and HRTEM images of dumbbell-like Au-Pt-IO NPs; (d,e) TEM and HRTEM images of jelly-like (Au-Pt)@IO hybrid NPs; (f,g) TEM study indicates that either Pt or Au domains of the Pt-Au hybrid NPs could be covered selectively by IO; (h,i) TEM indicates that IO could be selectively attached to the terminals of Pt branched networks or Au NPs of grape-like Pt-Au hybrid NPs to form flower-like Pt-Au-IO hybrid NPs.

was allowed to decompose at 260 °C (Figure 2d). The XRD analysis (Figure S2) of naked and covered Au-Pt NPs revealed that both Pt and Au domains remained in the fcc crystal structure, and the shell was a cubic spinel structure of magnetite (IO). HRTEM images (Figure 2e) of the jelly-like ternary hybrid NPs show distinct lattice fringe patterns, indicating the single-crystal nature of encapsulated multirod Pt domains and the IO domains. IO was formed epitaxially to make its (111) lattice fringes parallel to the (111) lattice fringes of Au in order to minimize the overall interface energy. It will be of general interest for nanoscience if the different metal domains, whose lattice constants are close (mismatch 4.08%), could be selectively coated by Fe whose lattice constants are greatly different from these of Au or Pt. The systematic study revealed that selective epitaxial growth of IO upon the binary metal hybrid NPs was affected plausibly by both the size of the Au and the geometry structure of the Pt. The Au particles tended to be covered by IO when their size was relatively small (6.5 nm). On the other hand, the Pt domains tended to be coated under the reaction conditions in which the Au size was increased to 10.5 nm, the complexity of Pt multirod NPs was relatively high, and the precursor of IO was relatively absent (Fe(CO)5, 0.1 mL) at 250 °C. Flower-like Pt-Au-IO

Figure 3. UV-vis spectra of (a) Pt-Au binary metal NPs with different morphologies, and (b) heterodimer IO-Pt, dumbbell-like IO-Pt-Au ternary hybrid NPs, and heterodimer Pt-Au NPs obtained by etching IO of IO-Pt-Au ternary hybrid NPs.

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Figure 4. (a) Temperature dependence of magnetization measured under an applied field of 100 Oe. For the sake of presentation, the curves were normalized with respect to the value at the maximum of ZFC magnetization, MTB, for individual samples. (b-d) Normalized magnetization dependence of magnetic filed curves measured after ZFC (solid) and FC (hollow) processes for heterodimer, core-shell, and jelly-like Pt/IO hybrid NPs, respectively.

hybrid NPs (Figure 2h) were fabricated when the final annealing temperature was settled at 230 °C. TEM study indicates that both terminals of branched Pt nanostructures and exposed Au domains could be covered selectively. The grape-like binary metal hybrid NPs (Figure 1e) could also be used as seeds to synthesize another kind of flower-like Pt-(Au@IO) NPs (Figure 2i), in which the grape-like Au-Pt NPs served as bridges and Au domains were covered selectively by IO. The results reveal that the elemental iron tends to attach to the terminals owing to stronger reactivity, which arises from relatively larger curvature, at relatively low temperature (230 °C). On the other hand, the Fe tends to cover the bud-like Pt-Au hybrid NPs wholly at a relatively higher reaction temperature (260 °C). Such interesting behaviors should arise from the increasing formation speed of Fe. It is clear that the decomposition speed of Fe(CO)5 increases with increasing reaction temperature. Therefore, the morphology of ternary hybrid NPs could be modulated through tuning both the geometry of templates and the formation speed of Fe by controlling the decomposition temperature. Optical Properties. Surface plasmon resonance (SPR) properties of hybrid NPs with exposed Au were studied. The UV/vis spectra (Figure 3a) reveal that all binary metal (Pt-Au) NPs with exposed Au domains exhibit strong SPR absorption peaks around 525 nm, similar to the well-known characteristic of naked Au NPs.19a Although the refractive index (2.33) of Pt is high, Pt domains do not obviously affect the SPR peak of Au domains.

Wu’s study revealed that physical mixtures of Pt and Au have SPR peaks similar to those of pure Au NPs, while the solid solution bimetallic Pt/Au NPs do not exhibit significant SPR peaks.19b The results indicate that the Au domains maintained the same crystal structure as that of bulk Au, and the high refractive neighbors do not affect the SPR of Au domains. Compared to that of binary metal NPs, the SPR peak (Figure 3b) of dumbbelllike ternary Au-Pt-IO hybrid NPs is red-shifted to 560 nm. Heterodimers Au-Pt (Figure S10) obtained by dissolving the IO domains of the dumbbell-like Au-Pt-IO hybrid NPs exhibit the SPR peak around 525 nm, as shown in Figure 3b. The SPR absorption peak is close to those of binary Pt-Au NPs and Au NPs.19 It is well-known that SPR is sensitive to dielectric environments and affected by charge state. The results reveal that the SPR absorption of Au domains in ternary hybrid NPs is affected mainly by IO domains owing to their high refractive index (∼2.4) and the large difference in the carrier density between the metals and IO domains. A similar effect was also found in core-shell Au@Fe3O4 NPs and Au-PbS.17 Magnetic Properties. Anomalous ferromagnetism has been found in nanoscale materials of noble metal, Pt.5d,6b It would be very significant for foundational research if the combination (19) (a) Hostetler, M. J.; Wingate, J. E.; Zhong, C. J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17. (b) Wu, M. L.; Chen, D. H.; Huang, T. C. Chem. Mater. 2001, 13, 599.

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fashions of ferromagnetic noble metal NPs and magnetic oxides could be tuned systematically. The magnetic properties of the binary Pt/IO hybrid NPs with different combination fashions were studied using a SQUID. Figure 4a shows the temperature dependence of magnetization curves for the three typical kinds of binary Pt/IO NPs measured under an applied magnetic field of 100 Oe. All of them show superparamagnetism-like behavior at room temperature. The blocking temperatures (TB), at which the zero-field-cooling (ZFC) magnetization begins to drop and deviate from the field-cooling (FC) magnetization, are 80, 177, and 172 K for jelly-, core-shell-, and heterodimer-like binary Pt/IO NPs. TEM analysis (Figure 1) indicates that the IO size of jelly-like, core-shell-, and heterodimer-like binary Pt/IO NPs is 16 ( 1.2, 15.8 ( 0.9, and 13.6 ( 1.5 nm, respectively. EDX analysis indicates that atom ratios of Pt to Fe for them are 27:73, 7:93, and 28:72, respectively. Though the IO sizes of jelly-like and core-shell NPs are very close, the TB of jelly-like NPs is 97 K lower than that of core-shell NPs. Such an anomalous behavior should arise from the finite size effect and surface effect, considering that the branched Pt networks divide the IO domains and reduce magnetic ordering harder than that of spherical NPs. The TB of core-shell and heterodimer-like hybrid NPs are close although the IO volume (V) ratio of them is 1.4. If their magnetic anisotropy constants (K) are hypothesized to be the same, the TB ratio of core-shell to heterodimer hybrid NPs should be 1.4 using the equation K ) 25kBTB/V, where kB is Boltzmann’s constant. Compared with the previous reports of size-controllable IO NPs,20,21 similar TB values of core-shell and dumbbell-like binary Pt/IO NPs support the idea that the long-range magnetic ordering of IO in core-shell Pt@IO NPs was reduced and/or the fracture of the shell. Usually, size-effect and shape-effect magnetic properties of magnetic NPs are studied widely.20-23 Here, the effect of interfacial interaction on magnetic properties was systematically studied. Compared to the Fe atoms in bulk IO, Fe atoms at the Pt/IO interface have less nearest neighbors, and therefore the interatomic exchange coupling will reduce inevitably. Therefore, the uncompensated spins at the interface become canted and can

be saturated only under very high magnetic field.12 Figure 4b-d shows the magnetization dependence of field curves measured under the ZFC and FC processes with an applied field of 2 T. The exchange bias field for jelly-, core-shell- and heterodimerlike binary Pt/IO hybrid NPs is 270, 30, and 0 Oe, respectively. Such results indicate that the exchange interaction between Pt domains and IO domains of the hybrid NPs increases with increasing interface area.

(20) Park, J.; An, K. J.; Hwang, Y. S.; Park, J. G.; Noh, H. J.; Kim, J. Y.; Park, J. H.; Hwang, N. M.; Hyeon, T. Nat. Mater. 2004, 3, 891. (21) Park, J.; Lee, E.; Hwang, N. M.; Kang, M. S.; Kim, S. C.; Hwang, Y.; Park, J. G.; Noh, H. J.; Kini, J. Y.; Park, J. H.; Hyeon, T. Angew. Chem., Int. Ed. 2005, 44, 2872. (22) Song, O.; Zhang, Z. J. J. Am. Chem. Soc. 2004, 126, 6164. (23) Wang, Z. L.; Dai, Z. R.; Sun, S. H. AdV. Mater. 2000, 12, 1944.

Supporting Information Available: Experimental procedures for the synthesis of different kinds of NPs, TEM images, ED pattern, size distribution histograms, and EDX spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

Conclusions In summary, here we report the synthesis of multicomponent inorganic hybrid NPs (Au-Pt and Au-Pt-IO) with tunable morphologies, components, and combination fashions. The study indicates that multistep chemical synthesis of advanced hybrid NPs is affected not only by intrinsic crystal structures, but also by reaction parameters and geometry of discrete domains of template hybrid NPs. The successful synthesis of these trifunctional hybrid NPs provides a set of empirical guidelines for design and synthesis of increasingly complex hybrid nanomaterials with synergistically physical and chemical properties. The results indicate that simple hybrid NPs (two components) could be used as basic building blocks to form advanced hybrid NPs (three components). In addition, simple hybrid NPs could also be obtained by selectively etching one domain of advanced hybrid NPs. Magnetism study of the binary Pt-IO NPs clearly indicates that the interfacial interactions between Pt and IO could be modulated systematically by tuning the fashions of their interface. The results indicate that, synergistically, physiochemical properties can be obtained with the progress of chemical synthesis of NPs. These novel hybrid NPs are potential candidates for magnetic resonance imaging, biomolecule magnetic separation, sensors, catalysts, electrode materials, photoelectric and optical-magnetic materials, and building blocks for nanoscale devices. Acknowledgment. This work was supported by Singapore National Research Foundation (NRF). H.-T.Z. also acknowledges the financial support from the Department of Materials Science & Engineering at the National University of Singapore.

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