Coating Noble Metal Nanocrystals (Ag, Au, Pd, and Pt) on

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Coating Noble Metal Nanocrystals (Ag, Au, Pd, and Pt) on Polystyrene Spheres via Ultrasound Irradiation V. G. Pol, H. Grisaru, and A. Gedanken* Department of Chemistry and Kanbar Laboratory for Nanomaterials at the Bar-Ilan University Center for Advanced Materials and Nanotechnology, Bar-Ilan University, Ramat-Gan 52900, Israel Received October 14, 2004. In Final Form: February 8, 2005 The method of ultrasound irradiation is used for anchoring metallic nanocrystals (Ag, Au, Pd, and Pt) onto the surface of polystyrene spheres. In former studies, almost all the sonochemically prepared, coated metallic nanomaterials were formed as amorphous nanoparticles (Pol, V. G.; et al. Langmuir 2002, 18, 3352; Pol, V. G.; et al. Chem. Mater. 2003, 15, 1111; Zhong, Z. Y.; et al. Chem. Mater. 1999, 11 (9), 2350; Pol, V. G.; et al. Chem. Mater. 2003, 15, 1378), which were coated on various substrates (silica spheres, carbon spherules, titania, and alumina). On the other hand, the noble metal nanoparticles deposited on polystyrene spheres via ultrasound irradiation yielded nanocrystalline Ag, Au, Pd, and Pt particles on the surface of polystyrene as as-synthesized materials. The sonochemical mechanism is proposed based on chemical interactions between the particles.

Introduction The interest in nanocoatings relies mostly on the combination of the properties of the two (or more) materials involved, with emphasis on the fact that one of the materials (the shell) will determine the surface properties of the particles, while the other (the core) is completely encapsulated by the shell,5 so that although it does not contribute at all to the surface properties, it can be responsible for other (optical, catalytic, magnetic, etc.) properties5 of the system. Apart from this extremely important feature, it is also essential to take into account the possible interactions between the core and the shell, which may, in certain cases, determine the potential applications of the material. As a result of the unique combination5 of properties achieved by using core-shell geometry, there has been an increasing interest in creating nanomaterials and nanostructures with unique, complex properties. Coated nanomaterials provide a very high surface area and possess chemical and physical properties that are distinct from those of both the bulk phase and individual molecules. More than 50 vol % of the atoms are associated with grain boundaries6 or interfacial boundaries when the grain is sufficiently small (∼5 nm). Thus, a significant amount of the interfacial component between neighboring atoms associated with grain boundaries contributes to the physical properties of nanostructured materials. The main benefit from coating nanoparticles on flat or spherical surfaces is the avoidance of the spread of the nanoparticles to the environment, due to the bonding between the substrate and the coated nanoparticles. Thus, coated nanoparticles can become a safe way of moving nanomaterials across large distances. Supported catalysts are of special interest, for they allow for the fine dispersion * Corresponding author. E-mail: [email protected]. (1) Pol, V. G.; Srivastava, D. N.; Palchik, O.; Palchik, V.; Slifkin, M. A.; Weiss, A. M.; Gedanken, A. Langmuir 2002, 18, 3352. (2) Pol, V. G.; Gedanken, A.; Calderon-Moreno, J. Chem. Mater. 2003, 15, 1111. (3) Zhong, Z. Y.; Mastai, Y.; Koltypin, Y.; Zhao, Y. M.; Gedanken, A. Chem. Mater. 1999, 11 (9), 2350. (4) Pol, V. G.; Motiei, M.; Gedanken, A.; Calderon-Moreno, J.; Mastai,Y. Chem. Mater. 2003, 15, 1378. (5) Liz-Marzan, L. M.; Kamat, P. V. Nanoscale Materials; Kluwer Academic Publishers: Norwell, MA, 2003. (6) Leslie-Pelecky, D. L.; Rieke, R. D. Chem. Mater. 1996, 8, 1770.

and stabilization of small metallic particles,7 and they provide access to a much larger number of catalytically active atoms8 than in the corresponding bulk metal. A few methods that we will mention herein have already been applied to coating nanoparticles of noble metals on polystyrene (PS) substrates. Dokoutchaev et al. used an electrostatic deposition9 method for uniformly anchoring metal colloids (Pt, Pd, and Au) onto the surface of PS microspheres. Chen et al. reported on the preparation of platinum colloids10 on PS nanospheres and their catalytic properties in hydrogenation, and they also dispersed platinum colloids on PS nanospheres with surface-grafted poly(N-isopropylacrylamide) via the reduction of PtCl62by ethanol. The macroporous polymers11 with a functional group based on triisobutylphosphine sulfide are synthesized and characterized for the selective adsorption of gold and palladium. Five coordinating polymers are prepared from chloromethylated divinylbenzene PS, either by direct attachment of the phosphine sulfide to the polymer or through a spacer chain that is modified to introduce O and S heteroatoms. Hydrogenation of five-membered heterocycles of a polymer-supported palladium catalyst12 at normal temperature and pressure is reported by Guo et al. Well-dispersed Au/Pt bimetallic nanoparticles13 are generated on the microsphere surface via the in situ reduction of gold and platinum ions by radicals generated from the initiator, 2,2′-azobisisobutyronitrile. The positively charged polyethyleneimine (PEI) polyelectrolyte is adsorbed onto the surface of the (negatively charged) PS microspheres by both the electrostatic and the hydrogenbonding plasmon absorption of the gold core due to interparticle attraction.14 This is followed by adsorption (7) Auer, E.; Freund, A.; Pietsch, J.; Tacke, T. Appl. Catal. A 1998, 173, 259. (8) Caruso, R. A.; Antonietti, M. Chem. Mater. 2001, 13, 3272. (9) Dokoutchaev, A.; James, J. T.; Koene, S. C.; Pathak, S.; Prakash, G. K. S.; Thompson, M. E. Chem. Mater. 1999, 11 (9), 2389. (10) Chen, C. W.; Serizawa, T.; Akashi, M. Chem. Mater. 1999, 11 (5), 1381. (11) Sanchez, J. M.; Hidalgo, M.; Valiente, M.; Salvado, V. J. Polym. Sci., Part A: Polym. Chem. 2000, 38 (2), 269. (12) Guo, Z.; Feng, H.; Ma, H. C.; Kang, Q. X.; Yang, Z. W. Polym. Adv. Technol. 2004, 15, 100. (13) Chen, C. W.; Serizawa, T.; Akashi, M. Chem. Mater. 2002, 14, 2232.

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of the negatively charged Au colloidal nanoparticles (3 or 15 nm) into the PEI polymer via the amine group. In all these cases, the coatings were performed by impregnating the substrate in the precursor’s solution and conducting a reaction. Sonochemical methods qualify for the deposition task because of their ability to combine the synthesis of various nanomaterials and their deposition on various substrates in a single operation. The additional advantage of this method is the ability to control the particle size of the product by varying the concentration of the precursors in the solution. Power ultrasound affects chemical changes as a result of cavitation phenomena involving the formation, growth, and implosive collapse of bubbles in liquid.15 The sonochemical phenomena have been exploited for the deposition of several different nanoparticles on a variety of substrates. Metallic (e.g. Ag, Au, Ni), metal oxides (e.g., MgO), and highly magnetic (air-stable Fe) nanoparticles were uniformly deposited on a variety of substrates.1-4 None of the previous reports used PS spheres as a substrate. The only previous research where nanoparticles were coated on PS is that of Breen et al., who grew zinc sulfide16 films on carboxyl-modified PS microspheres (PS-CO2) through a sonochemical reaction conducted in an aqueous solution containing zinc acetate and sulfide, released through the hydrolysis of thioacetamide. We report herein on the sonochemical deposition of metallic nanoparticles (Ag, Au, Pd, and Pt) on PS spheres. The distinct difference between this report and previous reports on the deposition of these nanoparticles on silica spheres and other surfaces is that in all previous depositions the silver, gold, nickel and air-stable Fe nanoparticles were deposited on the substrate surface as amorphous products. Carrying out the same sonochemical reaction in the presence of PS spheres yielded Pt, Pd, Ag, and Au nanoparticles in their crystalline form. A mechanism explaining the difference between the deposition of these nanoparticles on ceramic and polymeric bodies is offered: The crystallinity of sonochemically coated Ag, Au, Pd, and Pt particles on the PS surface is detected by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) measurements, and the product morphologies are studied by low-resolution transmission electron microscopy (LR-TEM) measurements. Experimental Section Sonochemical Coating of Noble Metal Nanocrystals (Ag, Au, Pd, and Pt) on PS Spheres. Hydrogen tetrachloroaurate(III) (17 wt %; solution in dilute HCl), hydrogen hexachloroplatinate(IV) (8 wt %), silver nitrate, palladium nitrate hydrate, and 10 wt % of PS spheres (600 nm and 900 nm size) were dispersed in water. The PS was purchased from Aldrich and used as obtained. Doubly distilled water was used for the sonication. For gold deposition, 1 mL of PS spheres, 10 wt % dispersion in water, and 0.1 mL of chloroauric acid were added to 30 mL of distilled water in a sonication cell, which was attached to the sonicator horn under flowing argon. For all the reactions, argon gas was bubbled through the slurry for 2 h prior to sonication to expel dissolved oxygen/air. The sonication of the slurry with high-intensity ultrasound radiation was carried out for 45 min by direct immersion of the titanium horn (Sonics and Materials VCX600 sonifier, 20 kHz, 40 W/cm2) in a sonication cell. A calorimetric method17 was used to estimate the electroacoustic or energy transfer efficiency of the transducer to the solution. A 3-5 mL portion of 24 wt % aqueous ammonia was (14) Ji, T. H.; Lirtsman, V. G.; Avny, Y.; Davidov, D. Adv. Mater. 2001, 13, 1253. (15) Ultrasound: Its Chemical, Physical and Biological Effects; Suslick, K. S., Ed.; VCH: Germany, 1988. (16) Breen, M. L.; Dinsmore, A. D.; Pink, R. H.; Qadri, S. B.; Ratna, B. R. Langmuir 2001, 17, 903.

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Figure 1. XRD patterns of (a) Ag, (b) Au, (c) Pt, and (d) Pd nanoparticles deposited on PS spheres sonochemically. added dropwise during the sonication. The product was washed thoroughly (twice) with doubly distilled deoxygenated water, centrifuged at 9000 rpm, and then dried in a vacuum for 12 h. The vacuum-dried product of gold is termed gold nanoparticles deposited on PS spheres (GDP). The control reaction was carried out without ultrasound irradiation but by heating the reaction mixture at 90 °C. For Ag, Pt, and Pd deposition, an argon/hydrogen mixture (95:5) gas was purged during sonication for the reduction of AgNO3, Pd(NO3)2, and H2PtCl6 to form silver, Pd, and Pt metal, respectively. A total of 50 mg of AgNO3, Pd(NO3)2, and 0.2 mL of H2PtCl6 with 1 mL of PS spheres (10 wt % dispersion in water) were separately added to 30 mL of distilled water in a sonication cell, which was attached to the sonicator horn under a flowing Ar/H2 mixture. All other reaction parameter were kept the same as that of gold deposition. The vacuum-dried product of Ag is termed the silver nanoparticles deposited on PS spheres (SDP) sample. The vacuum-dried product of Pt is called platinum nanoparticles deposited on PS spheres (PDP). The vacuum-dried product of Pd is termed palladium nanoparticles deposited on PS spheres (PdDP). The XRD and LR-TEM measurements were conducted for GDP, SDP, PDP, and PdDP samples. The HRTEM and selected area energy dispersive X-ray analysis (SAEDS) were conducted for GDP and SDP samples to understand the presence of metal, their crystalline nature, and phase purity. Characterization. The XRD patterns of the product were measured with a Bruker AXS D* Advance powder X-ray diffractometer (using Cu KR ) 1.5418 radiation). The particle morphology and the nature of the noble metal nanoparticles’ adherence to PS spheres were studied with TEM on a JEOLJEM 100 SX microscope, working at an 80 kV accelerating voltage. A small quantity of the sample was added to ethanol and sonicated for 20 min in a vial by using a sonication bath. One or two drops of the nanoparticle solution were deposited on a carbon-coated copper grid. Keeping the grid on Whatman filter paper enabled the absorption of the droplet, dried in 5-10 min, and the excess solvent. The morphologies and nanostructure of the assynthesized products were further characterized with a JEOL2010 HRTEM model using an accelerating voltage of 200 kV. SAEDS of one individual particle was conducted using a JEOL2010 HRTEM model.

Results and Discussion 1. XRD. The XRD patterns (Figure 1) of sonochemically deposited (a) Ag, (b)Au, (c) Pd, and (d) Pt nanoparticles on PS spheres were measured to obtain information about the nature of the product, its crystal structure (the interatomic distance and angle), its purity, and so forth. The crystalline nature of the as-synthesized product (a) Ag [SDP] and (b) Au [GDP] nanoparticles deposited sonochemically on PS spheres is demonstrated by the diffraction peaks that match a face-centered cubic (fcc) phase of Au and Ag (PDF: 4-784). The peaks at 2Θ ) 38.18, 44.39, 64.58, and 77.54° are assigned as the (111),

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Figure 2. Low-resolution transmission electron micrographs with different magnifications of (a, b) bare PS spheres; the (c, d) SDP sample; the (e, f) GDP sample; (g, h) the platinum deposited on PS (PDP) sample; and (i, j) the palladium deposited on PS (PdDP) sample.

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Figure 3. HR-TEM of (a) the crystalline nature and interlayer spacing between the Au plane (111), (b) the interface between the coated gold nanoparticles and the PS sphere, (c) the ED obtained from the gold nanoparticles, (d) the crystalline nature of a Au nanoparticle on the surface of a PS (insert: FFT, masked FFT, and inverse FFT is provided for the marked area), surface showing an amorphous layer of oxidation, and (e) the crystalline nature of a Ag nanoparticle on the surface of PS (insert: FFT, masked FFT, and inverse FFT is provided for the marked area); the surface of coated silver shows an amorphous layer of oxidation.

(200), (220), and (311) reflection lines, respectively, of fcc Ag and Au particles. The crystalline nature of the assynthesized product (c), PDP, is demonstrated by the diffraction peaks that match the formation of a fcc phase of metallic platinum (PDF: 4-802). The peaks at 2Θ ) 39.76 and 46.24° are assigned as the (111) and (200) reflection lines, respectively, of fcc Pt particles. The crystalline nature of the as-synthesized product (d), PdDP, is demonstrated by the diffraction peaks that match the fcc phase of metallic palladium (PDF: 46-1043). The peaks

at 2Θ ) 40.12 and 46.65° are assigned as the (111) and (200) reflection lines, respectively, of fcc Pd particles. It is interesting to note that in all our previous publications1-4 it has been demonstrated that the sonochemically deposited Au or Ag on silica spheres and air-stable Fe on carbon spheres were amorphous in assynthesized materials. However, the noble metal nanoparticles deposited on PS spheres via ultrasound irradiation yielded nanocrystalline Ag, Au, Pd, and Pt particles on the surface of PS as the as-synthesized materials.

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area ED (Figure 3c) was measured for the Au particle depicted in Figure 3a. The obtained ED pattern is assigned to the respective planes of Au. Figure 3d demonstrates the fast Fourier transform (FFT) image of the coated Au particle, its masked FFT, and its inverse FFT, evidenced of the crystalline nature of coated gold nanoparticles. Similar evidence is offered for coated Ag nanoparticles: the FFT image of the coated Ag particle, its masked FFT, and its inverse FFT, shown in Figure 3e. The SAEDS results related to the GDP and SDP samples are presented in Figure 4a,b. The results of a 25 nm electron beam focused on the particle shown by an arrow in Figure 4a indicate the presence of carbon and Au. Figure 4b is a SAEDS depicted from picture 3e that shows the presence of carbon and Ag and the absence of any impurity. The Cu peak is depicted from the Cu grid used for HRTEM measurements. These results also confirm the presence of coated pure Ag and Au particles.

Figure 4. (a-b) SAEDS results of GDP and SDP samples.

Figure 2a,b shows the transmission electron micrographs of the two kinds of PS spheres employed in the current experiments. The particles have a narrow size distribution with a diameter of either 600 or 900 nm, with a perfect spherical shape and smooth surface that can be seen in the TEM pictures. Silver nanoparticles with an average size of ∼8 nm were deposited on the surface of the PS spheres with the aid of power ultrasound (Figure 2c,d). More than 95% of the coated gold nanoparticles have a diameter of approximately ∼6 nm that covers the surface of the PS spheres (Figure 2e,f). A nano-sized (∼10 nm) platinum deposition on the PS surfaces is demonstrated in Figure 2g,h. A few uncoated elongated particles are also observed. A TEM micrograph of the ∼10 nm palladium nanoparticles coated on PS substrates is shown in Figure 2i,j. This leads to the conclusion that ultrasonic radiation facilitates the deposition of nanocrystalline particles on PS substrates. Additional evidence for the crystalline nature of the SDP and GDP samples is provided by conducting HRTEM measurements for the SDP and GDP samples. Indeed, most of the coated Au and Ag particles showed interlayer spacing and an electron diffraction (ED) pattern that evidenced the crystalline nature of the particles. The image (Figure 3a) is recorded along the [111] zone. It illustrates the perfect arrangement of the atomic layers and the lack of defects. The measured distance between these (111) lattice planes is 0.234 nm, which is very close to the distance between the planes reported in the literature (0.2355 nm) for the fcc phase (PDF: 4-784) of Au. In Figure 3b, we focused on the interface between the coated gold nanoparticles and the PS sphere. The selected

Proposed Sonochemical Mechanism for the Deposition of Ag, Au, Pd, and Pt on PS Spheres The mechanism by which the Au, Ag, Pt, and Pd nanoparticles are bonded to the PS surface is related to the microjets and shock waves created near solid surfaces after the collapse of the bubble.17 These jets, which cause the sintering of micrometer-sized metallic particles,19 push the nanoparticles toward the PS surface at very high speeds. When nanoparticles hit the PS surface, sintering of the particles and/or interparticle collision between PS and Au, Ag, Pt, and Pd particles changes the surface morphology and reactivity, resulting finally in the coating of these particles. The control reaction for another GDP sample, which was carried out without ultrasound irradiation but by heating the reaction mixture at 90 °C yielded ∼50 nm gold particles. However, these particles did not coat the PS surfaces. The sonochemical mechanism for the deposition of gold and silver nanoparticles on silica spheres is discussed elsewhere.1,2 It is interesting to note that the products of many sonochemical reactions are in the form of amorphous nanoparticles. The reason for the amorphicity of the products is related to the high cooling rates (>1011 K/s) obtained during the collapse of the bubble, which does not allow the products to organize and crystallize. These high cooling rates result from the fast collapse that takes place in less than 1 ns.20 The mechanism of coating Au and Ag nanoparticles on silica spheres and PS spheres might be different, because they lead to the formation of amorphous and crystalline products, respectively. We believe that this is due to the differences in their surfaces and the chemical interactions between the particles. As we mentioned previously,1,2 chemical interactions between silica and Ag and/or gold lead to the formation of a chemical bond between SiO2 and Ag or Au due to the reactive surface silanols, which led to the production of an amorphous product after sonication. On the other hand, when we used the PS spheres as a substrate, chemical interactions between Ag or Au and PS are different. During the sonochemical reaction, PS spheres do not form chemical bonds but rather getting coated as a result of the sintering of the particles and/or interparticle collisions between PS and Au, Ag, Pt, and Pd, which leads only to the melting or softening of the polymer, and the metallic particles thus “dissolve” partially in the polymer. This leads to a better (17) Sivakumar, M.; Gedanken, A. Ultrason. Sonochem. 2004, 11, 373. (18) Suslick, K. S.; Choe, S. B.; Cichowlas, A. A.; Grinstaff, M. W. Nature 1991, 353, 414. (19) Barber, B. P.; Putterman, S. J. Nature 1991, 352, 414. (20) Suslick, K. S.; Price, G. J. Annu. Rev. Mater. Sci. 1999, 29, 295.

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heat exchange between the polymer and the metallic particle. Thus, the local temperature is raised and reaches the crystallization temperature. In the coating of the ceramics there is no softening of the outer ceramic layers. The contact time is shorter, which does not allow the local temperature to rise to that of the crystallization temperature. Another two possible explanations are as follows: (1) PS, unlike silica, is capable of plastic deformation at the impact site. This deformation carries away sufficient heat so that the cooling rate is not so high and, consequently, crystal growth can occur. The formation of amorphous products in sonochemistry is due to the fast

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cooling rates. In the current case this situation does not occur. (2) Another possibility is that somehow the PS is functioning, through chemical interaction, as a kind of templating site for initial deposition. Again, energy loss into the polymer matrix would allow for nanocrystalline formation. Remaining noble metal atoms in the impact site during the relatively short cooling time then use the templating interaction as a nucleating site for further growth. Acknowledgment. V.G.P. is grateful to Bar-Ilan University, Israel, for financial assistance. LA047465D