Pt Bimetallic Colloids on Polystyrene

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In Situ Formation of Au/Pt Bimetallic Colloids on Polystyrene Microspheres: Control of Particle Growth and Morphology† Chun-Wei Chen,*,‡ Takeshi Serizawa, and Mitsuru Akashi* Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan Received December 11, 2001. Revised Manuscript Received February 13, 2002

This paper reports in situ formation of Au/Pt bimetallic nanoparticles on the surface of polystyrene microspheres. This was accomplished by dispersion copolymerization of styrene and a poly(N-isopropylacrylamide) (PNIPAAm) macromonomer in ethanol-water media in the presence of HAuCl4 and H2PtCl6. The particle size and morphology of the polystyrene microspheres can be changed by varying the molar ratio of Au/Pt. The propagation of oligomer radicals and nucleation of polystyrene microspheres were controlled by the Au/Pt molar ratio. A monodisperse polystyrene microsphere, with polystyrene as the core and PNIPAAm as the corona, was obtained at the Au/Pt molar ratio of 2/8. Well-dispersed Au/Pt bimetallic nanoparticles were generated on the microsphere surface via in situ reduction of gold and platinum ions by radicals generated from the initiator, 2,2′-azobisisobutyronitrile (AIBN). The mean diameter of Au/Pt bimetallic colloids decreased with increasing Pt molar fraction. The nucleation and growth of the formed nuclei became controllable at higher Pt molar ratios owing to the lower reduction rate of gold ions and strong coordination affinity of PNIPAAm to the Pt atoms. The materials resulting from this study were characterized by transmission electron microscopy (TEM); atomic force microscopy (AFM); and FTIR, UVvisible, and X-ray photoelectron spectroscopy.

Introduction Nanoscale metal and semiconductor particles have attracted a great deal of attention because of their unique properties and potential applications in microelectronics,1,2 quantum dot lasers,3 chemical sensors,4 data storage,5 and catalysis.6 The formation of such nanoparticles necessarily requires stabilization to prevent agglomeration because of van der Waals forces. Consequently, a variety of methods have been developed dealing with the formation of metal nanoparticles on the surface of the matrix polymer7 or incorporation of the metal nanoparticles into the polymer matrix.8 The resulting materials can exhibit synergistic properties or combinations of properties, which go beyond those of † This paper is part XXXVI in the series of the study on Graft Copolymers Having Hydrophobic Backbone and Hydrophilic Branches. Part XXXV is as follows: Chen, M.-Q.; Kaneko, T.; Chen, C.-H.; Akashi, M. Chem. Lett. 2001, 1306. * To whom correspondence should be addressed. (C.W.C.) Phone: (732) 445-7092. E-mail: [email protected]. (M.A.) Phone: 8199-285-8323. E-mail: [email protected]. ‡ Current address: Department of Ceramic and Materials Engineering, Rutgers University, 607 Taylor Road, Piscataway, NJ 08854-8087. (1) (a) Schmid, G. Chem. Rev. 1992, 92, 1709. (b) Nanoparticles and Nanostructured Films; Fendler, J. H., Ed.; Wiley-VCH: Weinheim, Germany, 1998. (c) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18. (2) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunata, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (3) Alivisatos, A. P. Science 1996, 271, 933. (4) Emory, S. R.; Haskins, W. E.; Nie, S. J. Am. Chem. Soc. 1998, 120, 8009. (5) Sun, T.; Seff, K. Chem. Rev. 1994, 94, 857. (6) (a) Clusters and Colloids; Schmid, G., Ed.; VCH: Weinheim, Germany, 1994. (b) Lewis, L. N. Chem. Rev. 1993, 93, 2693.

single components. There are some reports on the growth and attachment of nanoparticles to the functionalized substrates.9 The self-assembled metal colloids have been used for surface-enhanced Raman spectroscopy (SERS),9a the construction of electrochemically active surfaces,10 and the photochemical conversion of solar energy.11 By using polymer microspheres as the substrates, the available surface area upon which the metal colloids may be anchored is much higher than that of the planar substrates. It is expected that the dense concentration of assemblies may enhance optical signal sensitivity for analytical applications and catalytic activity for electrochemical and photochemical processes. We previously demonstrated that platinum nanoparticles ranging in size from 1 to 2 nm could be immobilized on polystyrene microspheres with surfacegrafted poly(N-isopropyl-acrylamide) (PNIPAAm) via (7) (a) Spatz, J. P.; Mo¨ssmer, S.; Mo¨ller, M. Chem. Eur. J. 1996, 2, 1552. (b) Chen, C.-W.; Serizawa, T.; Akashi, M. Chem. Mater. 1999, 11, 1381. (c) Siiman, O.; Burshteyn, A. J. Phys. Chem. B 2000, 104, 9795. (d) Pathak, S.; Greci, M. T.; Kwong, R. C.; Mercado, K.; Prakash, G. K. S.; Olah, G.; Thompson, M. E. Chem. Mater. 2000, 12, 1985. (8) (a) Forster, S.; Antonietti, M. Adv. Mater. 1998, 10, 195. (b) Tsutsumi, K.; Funaki, Y.; Hirokawa, Y.; Hashimoto, T. Langmuir 1999, 15, 5299. (9) (a) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629. (b) Westcott, S. L.; Oldenbrug, S. J.; Lee, T. R.; Halas, N. J. Langmuir 1998, 14, 5396. (c) Schmitt, J.; Ma¨chtle, P.; Eck, D.; Mo¨hwald, H.; Helm, C. A. Langmuir 1999, 15, 3256. (10) Doron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313. (11) Dokoutchaev, A.; James, J. T.; Koene, S. C.; Pathak, S.; Prakash, G. K. S.; Thompson, M. E. Chem. Mater. 1999, 11, 2389.

10.1021/cm011634n CCC: $22.00 © 2002 American Chemical Society Published on Web 04/06/2002

In Situ Formation of Au/Pt Bimetallic Colloids

the reduction of ionic platinum by ethanol.7b The activity of the immobilized Pt colloids is even higher than that of PNIPAAm-stabilized Pt sol and stays practically unaffected on recycling seven times in aqueous hydrogenation of allyl alcohol. The surface-grafted PNIPAAm branches not only stabilize the platinum colloids by steric repulsion but also immobilize them on polystyrene microspheres. We have taken advantage of the reducing properties of radicals in the dispersion polymerization system to prepare silver colloids in situ on polystyrene microspheres.12 Silver ions are reduced both by the radicals directly generated from AIBN and by the oligomer radicals to yield free atoms. The particle sizes of both polystyrene microspheres and silver nanoparticles can be controlled by the initial AIBN, AgNO3, and PNIPAAm macromonomer concentrations. Here, we demonstrate the in situ formation of Au/Pt bimetallic colloids on the polystyrene microspheres in the dispersion polymerization of styrene and PNIPAAm macromonomer in ethanol-water media. The polymerstabilized Au/Pt bimetallic colloidal sols are more efficient catalysts than Pt monometallic colloids for both hydrogenation of olefins13 and visible light-induced hydrogen generation from water.14 The immobilized Au/Pt bimetallic colloids are potential candidates for active and recyclable catalysts. Moreover, the particle growth and morphology of both metal colloids and polymer latexes can be simply controlled by varying the molar ratio of Au/Pt. The coupling effects of coordination affinity of amide groups of PNIPAAm with metal ions and resulting metal atoms on the behavior of nucleation and growth of bimetallic colloids are presented. Experimental Section Materials. Styrene (Wako Pure Chemical Ind., Ltd.) was distilled under partial vacuum to remove inhibitor before use. 2,2′-Azobisisobutyronitrile (AIBN) from Wako was recrystallized from methanol. Chloroauric acid (HAuCl4, guaranteed reagent grade) and chloroplatinic acid (H2PtCl6‚6H2O, guaranteed reagent grade) were used as received from Nacalai Tesque, Inc. The mixture of distilled water and ethanol (Nacalai Tesque, Inc., purified by distillation) was used as a polymerization medium. The PNIPAAm macromonomer was prepared by the method reported in the previous paper.15 In the present study, molecular weight and molecular weight distribution of the PNIPAAm macromonomer were 5300 and 2.2, respectively, determined using gel permeation chromatography (GPC). The structure and composition are verified by 1H NMR spectroscopy in (methyl sulfoxide)-d6 with a JEOL GSX-400 spectrometer operating at 400 MHz. Metallized Polystyrene Microsphere Synthesis. Dispersion copolymerizations of styrene and PNIPAAm macromonomer were carried out batchwise in a glass tube in the presence of HAuCl4 and H2PtCl6 (see Table 1). The general procedure used to prepare all of the polystyrene microspheres with Au or Au/Pt colloids on their surfaces was as follows: Styrene (3.0 mmol), PNIPAAm macromonomer (0.5 g), AIBN (10.0 mg), and HAuCl4 and/or H2PtCl6 (0.006 M, total metal concentration) were added to aqueous ethanol mixture (70% ethanol, 5 mL). Each batch of copolymerization in a glass tube (12) (a) Chen, C.-W.; Chen, M.-Q.; Serizawa, T.; Akashi, M. Adv. Mater. 1998, 10, 1122. (b) Chen, C.-W.; Serizawa, T.; Akashi, M. Langmuir 1999, 15, 7998. (13) Chen, C.-W.; Akashi, M. Polym. Adv. Technol. 1999, 10, 127. (14) Toshima, N.; Yonezawa, T. Makromol. Chem., Macromol. Symp. 1992, 59, 281. (15) Chen, M.-Q.; Kishida, A.; Akashi, M. J. Polym. Sci., Part A: Polym. Chem. 1996, 34, 2213.

Chem. Mater., Vol. 14, No. 5, 2002 2233 Table 1. Particle Sizes and Distributions for the Microspheres and Bimetallic Au/Pt Colloidsa microspheresb exp AIBN PNIPAAm no. (mol %) Au/Pt (mol %) 1 2 3 4 5 6

2 2 2 2 2 2

10/0 8/2 6/4 5/5 4/6 2/8

3.0 3.0 3.0 3.0 3.0 3.0

Au/Pt colloidsb

Dn (nm)

PDI

dn (nm)

SD (nm)

1245 860 780 762 750 730

1.13 1.10 1.10 1.07 1.08 1.01

117.0 23.4 17.3 16.8 15.1 13.4

/ 6.7 6.6 5.8 4.3 3.7

a The number-average molecular weight of PNIPAAm was 5300 (confirmed by GPC). b The sizes and distributions of microspheres and Au/Pt bimetallic colloids were determined by TEM. Notation: PDI, polydispersity index; SD, standard deviation.

was repeatedly degassed by freeze-thaw cycles on a vacuum apparatus, sealed off, and then placed in an incubator at 60 °C for 24 h. The resulting microspheres were first dialyzed in distilled deionized water using a cellulose dialyzer tube to remove unreacted monomer, and then the latex particles were centrifuged and redispersed in water. The amount of residual metal in the supernatant was analyzed on an SAS-7500A (Seiko Instruments) atomic absorption spectrophotometer. Microsphere Characterization. Transmission electron microscopy and atomic force microscopy were used to determine the particle size. TEM images were obtained with a Hitachi H-700H microscope operation at an acceleration voltage of 150 kV at a magnification of 36 000 or 100 000. Specimens of the various metallized microspheres were prepared by slow evaporation of a drop of the appropriately diluted solution deposited onto a collodium-coated copper mesh grid, followed by carbon sputtering. The particle size (Dn) and polydispersity index (PDI ) Dw/Dn) of the polystyrene microsphere were measured from TEM images. Samples for atomic force microscopy (AFM) imaging were prepared by placing a drop of dispersion of polystyrene microspheres on freshly cleaved mica and allowing it to dry in the air. The samples thus prepared were imaged immediately after deposition as well as after several days without noticeable changes in appearance and average dimensions. AFM imaging was performed using a Nanoscope III-SPM system (Digital Instruments) with a J-type vertical engage piezoelectric scanner and operated in tapping mode in air. X-ray diffraction (XRD) measurements were taken as 2θ varied from 2∼75° using an automatic diffractometer (RADX, Rigaku International Co., Tokyo, Japan) with Cu KR radiation (λ ) 0.154 nm) and Ni filter generated at 45 kV and 25 mA. UV-visible measurements are performed with a JASCO model V-550 recording spectrophotometer, working in a spectral range between 200 and 800 nm. FTIR spectra are recorded in KBr pellets with a Shimadzu Fourier transform Nicolet spectroscope. Pressure was applied to the sample powder until the pellet was transparent. X-ray photoelectron spectra were obtained with a Shimadzu ESCA 1000 apparatus employing Mg KR radiation (1253.6 eV) and a pass energy of 31.5 eV. Peaks were referenced to carbon at 285.0 eV to account for sample charging. Good quality survey spectra were obtained with a single scan; core-line highresolution spectra were integrated over 5-10 scans depending on the intensity of the spectral region of interest. Percentages for each carbon environment quoted within the text are derived from peak areas in the high-resolution spectra.

Results and Discussion Metallized Microsphere Syntheses. We showed that well-dispersed silver colloids can be generated on the surface of polystyrene microspheres via in situ reduction of Ag+ by radicals in the dispersion polymerization system.12 In dispersion polymerization, the styrene monomer, PNIPAAm macromonomer, and silver

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nitrate are soluble in the continuous phase. As the polymerization proceeds, the monomer is converted into insoluble polymers and precipitation and coagulation of the polymers are observed in 30 min. After an induction period of 2 h, the color of the solution changed from white to yellow. However, this approach is not suitable for attachment of Pt colloids, because the coordination interaction between PNIPAAm and platinum ions are too strong for platinum ions to be reduced by radicals. Although the transparent solution became turbid in 30 min at 60 °C, the resulting microspheres did not change their color from white to dark gray, indicating that platinum colloids cannot be generated under these conditions. When polystyrene microspheres are separated from the reaction mixture by centrifugation, the supernatant was pale yellow and showed the absorption bands of platinum ions in 200-300 nm of the UVvisible spectrum.16 When the solution of PtCl62- and presynthesized polystyrene microspheres in water-ethanol at 90 °C was refluxed, the immobilization of Pt colloids on the surface of microspheres was observed.7b It is expected that the reducing ability of redicals under polymerization conditions is weaker than that of alcohol at refluxing temperature. Very recently, we found that platinum colloids stabilized by poly(vinylamine-co-Nvinylisobutyramide) copolymers by the alcohol reduction method because of the formation of strong polymermetal ion complex between the amine groups of poly(vinylamine) chains and platinum ions.17 Sodium borohydride, a fast reducing agent, is required to produce the platinum colloids in the presence of poly(vinylamineco-N-vinylisobutyramide). For the sample with HAuCl4 in the solution, the polystyrene microspheres showed a fast color change from white to dark red. Considering the high standard potential (E° )1.00 V, vs NHE) of AuCl4-/Au,18 one would expect that gold ions would be reduced easily by radicals. On the other hand, the coordination affinity of the stabilizing polymer for metal ions and atoms also plays an important role in the preparation of welldispersed metal colloids with respect to the nucleation and growth of the formed nuclei.14,19 Because the amide groups of PNIPAAm cannot form strong complexes with gold ions,20 the reduction rate of gold ions is relatively high. At the same time, the growth of the formed nuclei is hardly controlled because of the weak coordination ability of PNIPAAm to gold atoms. Consequently, the resulting Au colloids agglomerated to larger clusters and failed to be attached on the polystyrene microsphere surface. Toshima and co-workers studied the synthesis of Au/Pt bimetallic clusters using poly(N-vinyl-2-pyrrolidone) (PVP) as a stabilizing agent. They suggested that the weak coordination ability of PVP to gold atoms was responsible for the gold core formation.14 The X-ray diffraction (XRD) pattern using Cu KR radiation for polystyrene microspheres with Au clusters on their surfaces is shown in Figure 1. Three intense reflections (111, 200, 220) of face-centered cubic crystalline gold (16) Chen, C.-W.; Akashi, M. Langmuir 1997, 13, 6465. (17) Chen, C.-W.; Arai, K.; Yamamoto, K.; Serizawa, T.; Akashi, M. Macromol. Chem. Phys. 2000, 201, 2811. (18) Vanysek, P. In CRC Handbook of Chemistry and Physics, 71st ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1991. (19) Hirai, H. Makromol. Chem., Suppl. 1985, 14, 55. (20) Zhao, M.; Crooks, R. M. Chem. Mater. 1999, 11, 3379.

Chen et al.

Figure 1. X-ray diffraction pattern of polystyrene microspheres with Au monometallic particles on their surfaces.

were observed. These sharp reflections are attributed to the aggregated gold particles, as determined by TEM. In contrast, no obvious X-ray reflections were observed for silvered polystyrene microspheres, where individual silver nanoparticles were well-separated from their neighbors.12 Thompson et al. studied the morphology of silver particles in polyimide films by XRD and TEM.21 They also found that the XRD patterns suggest differing degrees of aggregation of silver particles. Well-dispersed Au/Pt bimetallic colloids can be attached on the surface of polystyrene microspheres by the in situ synthesis method. When AuCl4- and PtCl62ions are present in the polymerization solution simultaneously, the AuCl4- ions are reduced earlier than PtCl62- by radicals as expected by the standard potentials of the two metal ions. In the progress of the metalion-to-colloid reduction for PtCl62-, PtCl42- serves as the intermediate.16 The standard potentials of the corresponding half reactions are 0.68 and 0.755 V for PtCl62-/ PtCl42- and PtCl42-/Pt, respectively.18 The Au/Pt bimetallic colloids showed obviously increased induction periods compared with Au monometallic clusters. The reduction rate of AuCl4- ions is decreased upon addition of PtCl62- to the polymerization solutions. Toshima et al. reported a systematic study on the formation and structure of polymer-protected Pd/Pt, Au/Pd, and Au/ Pt bimetallic colloids by simultaneous alcohol reduction of two metal ions. They proposed a “geared step-cycled reduction” mechanism to explain the decrease in reduction rate of gold ions by the addition of platinum ions, in which case electrons flow from ethanol to AuCl4- via the redox couple of PtCl62-/PtCl42-/Pt. One would expect that the reduction of AuCl4- and PtCl62- ions by radicals would take place following a similar mechanism, where PtCl62- has an “electron mediating” effect to reduce AuCl4- to zerovalent Au atoms. Two results should be noted. First, the growth of formed nuclei became controllable with decreasing the reduction rate of AuCl4-. Second, the reduction of PtCl62- to Pt atoms by radicals became easier. Therefore, Au/Pt bimetallic colloids with controlled morphology can be formed in situ on the surface of polystyrene microspheres. The rate of ion reduction, the growth of formed nuclei, and the morphology of bimetallic colloids are strongly dependent on the molar ratio of Au/Pt. UV-visible spectroscopy was used to study the effect of the Au/Pt molar ratio on the morphology and structure of metallic colloids. In the case of Au monometallic particles, no obvious plasmon absorption band at ca. 530 nm for Au (21) Southward, R. E.; Thompson, D. W.; St. Clair, A. K. Chem. Mater. 1997, 9, 501.


nanoparticles was observed in the absorption spectrum of the polystyrene microsphere suspension because most of Au colloids agglomerated and precipitated on the bottom of the glass tube. At a Au/Pt molar ratio of 8/2, the broad absorption feature appears at around 530 nm. The aggregation of metal colloids causes a decrease in the intensity of the peak and also results in a long tail at the higher wavelength side of the peak.22 The intensity of the peak increases with increasing Pt molar fraction from 20 to 50%; and then decreases as the Pt molar fraction is over 60%. It is noteworthy that the plasmon absorption band also appears in the spectrum of metallized polystyrene microspheres at the molar ratio of Au/Pt ) 2/8. For PNIPAAm-stabilized Au/Pt bimetallic colloidal sols prepared by ethanol reduction, however, the plasmon absorption band disappeared in the spectra when the Pt molar fraction was over 60%.13 The phenomenon was explained by the core-shell alloy structure of the Au/Pt bimetallic colloids; that is, they have an Au core, and the Pt atoms are located on the surface of the Au core. Obviously, the Au/Pt bimetallic colloids attached on polystyrene microspheres did not possess the special core-shell structure. It is known that the size, shape, and structure of metal colloids are strongly dependent on the preparative conditions.23 Considering the successive reduction of AuCl4- and PtCl62- ions by radicals, we may interpret the structure of Au/Pt bimetallic nanoparticles in the microcluster model.24 Each Au/Pt bimetallic colloid particle consists of several microclusters, which have a modified Au core. The Pt atoms are deposited on the surface of the Au core but do not cover it completely. One would expect that the growth of bimetallic colloid nuclei would be controlled easily because of the strong coordination of PNIPAAm to the Pt atoms on the colloid surface. Above a platinum molar fraction of 50%, no aggregation of Au/Pt bimetallic colloids was observed, as confirmed by TEM. Particle Size and Morphology. The dependence of particle size and morphology on the molar ratio of Au/Pt was examined by TEM, and the results were summarized in Table 1. Figure 2a shows Au monometallic particles on polystyrene microspheres, where Au particles agglomerate to large clusters with a mean diameter of 117.0 nm and polystyrene microspheres have a large diameter and broad size distribution. In dispersion polymerization using macromonomers as the steric stabilizer, it is known that the final latex size increases with decreasing initiator concentration.15,25 This trend was also observed for the in situ synthesis of silvered polystyrene microspheres, although the radicals from AIBN also served as the reducing agent for the conversion of silver ions to free atoms.12b As discussed previously, gold ions were reduced by radicals in high speed at the beginning of the reaction. Consequently, the fraction of radicals to initiate the polymerization was decreased, and the resulting polystyrene microspheres had a larger mean diameter of 1250 nm. (22) Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763. (23) Dhas, N. A.; Gedanken, A. J. Mater. Chem. 1998, 8, 445. (24) Harada, M.; Asakura, K.; Toshima, N. J. Phys. Chem. 1994, 98, 2653. (25) Kawaguchi, S.; Winnik, M. A.; Ito, K. Macromolecules 1995, 28, 1159.

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Figure 2. TEM images of metallized polystyrene microspheres prepared according to the recipes in Table 1. The Au/ Pt molar ratios are 10/0 (a), 8/2 (b), 6/4 (c), 5/5 (d), 4/6 (e), and 2/8 (f), respectively.

As 20% Pt ions were added into the polymerization system, the reduction rate of gold ions was decreased, and the size of Au/Pt bimetallic colloids was 23.4 nm. At the same time, the final diameter of polystyrene microspheres was obviously smaller than that obtained in the presence of gold ions only (see Figure 2b). As reported in Table 1, both the mean diameter and the size distribution of the polystyrene microspheres decreased with increasing molar fraction of platinum. At the molar ratio of Au/Pt ) 2/8, monodisperse polystyrene microspheres were obtained with the smallest mean diameter of 730 nm (see Figure 2f). We interpret these results as the effect of the Au/Pt molar ratio on the propagation of oligomer radicals and nucleation of polystyrene microspheres. It is known that larger particles will be produced under conditions where phase separation is delayed to higher monomer conversions, leading to a smaller number of larger nuclei. This can be achieved either by the formation of lower molecular weight polymeric chains26 or by the increase in compat(26) Ray, B.; Mandal, B. M. Langmuir 1997, 13, 2191.

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ibility between the solvent and the polymer/stabilizer complex.27 The later is related to the effect of the medium composition on the effectiveness of the dispersion polymerization.28 For the in situ synthesis of metal colloids in dispersion polymerization, both of the radicals directly generated from AIBN and oligomer radicals are involved in the reduction of metal ions to free atoms.12 At higher molar ratios of Au/Pt, a large number of lower molecular weight oligomer radicals were consumed by the reduction of metal ions because of the high reduction rate of gold ions. The propagation of oligomer radicals would therefore be retarded, and resulting polymers of shorter chain lengths have greater solubility in the reaction medium. Because of the low concentration of high molecular weight chains in the medium, fewer polymerization nuclei are produced, which leads to larger but fewer final particles. The growth of metal particles also consumed some oligomer radicals, which also contributes to the formation of larger polystyrene microspheres. At a lower molar ratio of Au/Pt, on the contrary, fewer oligomer radicals were consumed. The oligomer radicals propagated smoothly to give larger molecular weight chains. One would expect that the nucleation of particles would occur at lower monomer conversions. Once the initial nucleation has occurred, the growth of the existing particles is favored compared to the formation of new particle nuclei.27 In this case, smaller polystyrene microspheres are formed with a narrow size distribution. As can be seen in Figure 2, the particle size and morphology of attached metal colloids were also strongly influenced by the metal composition. When the Pt molar fraction was lower than 50%, the size of the metal colloids was significantly larger, and some of them aggregated into clusters. For the reduction of metal ions by radicals and hydrated electrons, the primary metal particles are able to react with many radicals, leading to the accumulation of electrons.12,29 Thus, the metal ions reduction occurs at the surface of the cathodically polarized primary metal particles, and the particles grow smoothly. Above the Pt molar fraction of 50%, the average diameter of Au/Pt bimetallic colloids decreased with increasing Pt content and no aggregation was observed. In the case of Au/Pt ) 2/8, well-separated bimetallic colloids with an average diameter of 13.4 nm were observed on the surface of polystyrene microspheres. In a previous paper, we found the similar dependence of particle size on metal composition in the preparation of PNIPAAm-stabilized Au/Pt bimetallic colloidal sols.13 Ethanol served as the reducing agent, and the mean diameter of PNIPAAm-stabilized Au/Pt colloids was as small as 1.96 nm at the Au/Pt molar of 4/6. In the present work, the Au/Pt bimetallic colloids on the polystyrene microspheres have an average diameter from 13.4 to 16.8 nm as Au/Pt molar ratio varies from 2/8 to 5/5. The larger particle sizes may be due to the weak reducibility of radicals and hydrated electrons. Antonietti et al. reported that the size and morphology of metal colloids in block copolymer micelles were strongly dependent on the type of reducing agent.30,31

The use of a strong reducing agent obviously favors simultaneous nucleation at many sites, producing many smaller colloids per micelle. They also prepared the Au/ Pd bimetallic colloids with different metal ratios in micelles.31 It was found that the catalytic activity of Au/ Pd bimetallic colloids in the hydrogenation of cyclohexene is higher than that of Pd monometallic colloids and depends on the Au/Pd molar ratio. The phenomenon was explained by the core-shell structure of the metal colloids: Pd atoms are located on the surface of the cluster particles with Au cores. The size and morphology of polystyrene microspheres with or without Au/Pt bimetallic colloids on their surfaces were also evaluated by atomic force microscopy (AFM). The lateral sizes of the particles may be distorted because of the finite size of the AFM probe tip; therefore, the particle diameters were measured from the particles heights. Evaluation of the diameters of spherical polystyrene microspheres with the AFM heights agreed with that by TEM. The Au/Pt bimetallic particles were attached in a controlled, well-dispersed manner on the surface of polystyrene microspheres, whereas the surface of unmetallized polystyrene microspheres was smooth and featureless (see Figure 3). To determine the location of metal colloids, we also observed the thinsectioned metallized polystyrene microspheres by TEM. The Au/Pt bimetallic colloids were attached over the microsphere surface, and no metal particles were observed in the hydrophobic polystyrene cores, which agrees with the internal structure of silvered polystyrene microspheres obtained by in situ synthesis method.12 FTIR Spectroscopy. Figure 4a shows an FTIR spectrum of the unmetallized polystyrene microsphere, where the bands characteristic of the amide group of PNIPAAm, notably at 3410, 1655, and 1539 cm-1, are clearly observed. In Figure 4b, the spectrum of a metallized microsphere (Au/Pt ) 2/8) shows similar features to that of the unmetallized microsphere. However, the intensity of the 3410 cm-1 band due to the N-H stretching vibration decreased when the Au/Pt bimetallic colloids were attached on the microsphere surface. At the same time, the band of N-H bending vibration showed a blue shift to 1545 cm-1 upon metal colloid attachment. These observations provide evidence for the coordination interaction between the amide group of surface-grafted PNIPAAm chains and Au/Pt bimetallic colloids, which is consistent with the results obtained by XPS. X-ray Photoelectron Spectroscopy. The XPS data concerning unmetallized and metallized polystyrene microspheres are given in Table 2. The C 1s core level spectrum of polystyrene microspheres with Au/Pt bimetallic colloids on their surfaces is shown in Figure 5a. Deconvolution of the C 1s signal gave three peaks at 285.0, 286.3, and 287.9 eV. The first prominent peak that is due to the C-C/C-H bonds comprised 87.4% of the C 1s signal. The later two peaks have the same intensity of 6.3%, which are assigned to CsN and NsCdO environments of surface-grafted PNIPAAm

(27) Baines, F. L.; Dionisio, S.; Billingham, N. C.; Armes, S. P. Macromolecules 1996, 29, 3096. (28) Paine, A. J.; Luymes, W.; A. McNulty, J. Macromolecules 1990, 23, 3104. (29) Henglein, A. J. Phys. Chem. 1993, 97, 5457.

(30) Antonietti, M.; Wenz, E.; Bronstein, L. Adv. Mater. 1995, 7, 1000. (31) Seregina, M. V.; Bronstein, L. M.; Platonava, O. A.; Chernyshov, D. M.; Valetsky, P. M.; Hartmann, J.; Wenz, E.; Antonietti, M. Chem. Mater. 1997, 9, 923.


Figure 3. Tapping-mode AFM images (1 µm × 1 µm) of an unmetallized polystyrene microsphere (a) and a metallized microsphere at Au/Pt molar ratio of 2/8 (b), deposited on freshly cleaved mica.

Figure 4. FTIR spectra of the unmetallized polystyrene microsphere (a) and metallized polystyrene at Au/Pt molar ratio of 2/8 (b).

chains.7b The proportion of CsN or NsCdO component is considerably greater than its theoretical proportion of 4.3%, obtained from the mole fractions in the initial monomer feed and the structural formulas of the monomers. This information provides evidence for a core-corona structure of the metallized polystyrene

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microsphere. Similar results were obtained for XPS analysis of unmetallized microspheres and microspheres with gold monometallic particles on their surfaces. The PNIPAAm corona layer on the exterior provides the steric stabilization for polystyrene microspheres. The N 1s signal for the same polystyrene microsphere with attached Au/Pt bimetallic colloids (Au/Pt ) 2/8) can be deconvoluted into two peaks centering at 400.0 and 401.0 eV (see Figure 5b). The main peak at 400.0 eV is due to the free nitrogen atoms in the amide groups of PNIPAAm. The second one is due to the positive charged nitrogen atoms. The increase in binding energy of nitrogen indicates a partial decrease of the electronic density around the nitrogen atom, which is attributed to the coordination of amide groups of PNIPAAm to metal atoms on the Au/Pt bimetallic colloid surface. About 9.4% of amide groups are affected by the bimetallic colloids on the surface of polystyrene microspheres. For the unmetallized polystyrene microsphere, however, only one peak at 400.0 eV was observed in the N 1s XPS signal. There are no obvious differences between metallized and unmetallized microspheres for the O 1s signals, indicating that the CdO group is not affected by the bimetallic colloids. It is expected that the PNIPAAm chains would adsorb on the surface of Au/Pt bimetallic colloids though amide nitrogen atoms and attach them on the polystyrene microsphere surface. Polystyrene chains can be linked to the bimetallic colloids by the entanglements they make with the PNIPAAm loops on the colloid surface. The surface entanglement will restrict the lateral motions of the polymer chains and define an effective network for the attachment of Au/Pt bimetallic colloids on the polystyrene microsphere surface. Shull et al. explained the pinning of gold particles to the interface of polystyrene and poly(2-vinylpyridine) using the surface entanglement mechanism.32 In the case of Au monometallic particles (see Figure 5c), the coordinated nitrogen peak at 401.0 eV was not observed because the coordination affinity for Au to PNIPAAm was very weak.14 The Au 4f core level signal of polystyrene microspheres with Au monometallic particles on their surfaces is shown in Figure 6a. The spectrum clearly shows the presence of only one component of zerovalent gold. The Au 4f doublet is identified at 87.7 and 84.0 eV for Au 4f5/2 and Au 4f7/2, respectively. The binding energy at 84.0 eV for Au 4f7/2 and the spin-orbit splitting of 3.7 eV agree well with literature values.33 The Pt 4f core level spectrum for metallized polystyrene microspheres with Au/Pt molar ratios of 2/8 is shown in Figure 6b. The Pt 4f7/2 signal contains a predominant component at binding energy of 71.2 eV that is due to metallic Pt (Pt0).33 The low-intensity component at 72.6 eV may be attributed to the formation of platinum-oxide-like phase covering on platinum atoms. Nashner et al.34 reported the structural characterization of carbon-supported Pt-Ru bimetallic nanoparticles by X-ray absorption fine structure spectroscopy (32) Kunz, M. S.; Shull, K. R.; Kellock, A. J. J. Colloid Interface Sci. 1993, 156, 240. (33) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; Perkin-Elmer Corporation: Eden Prairie, MN, 1992. (34) Nashner, M. S.; Frenkel, A. I.; Akler, D. L.; Shapley, J. R.; Nuzzo, R. G. J. Phys. Chem. 1997, 101, 7760.

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Chen et al.

Table 2. XPS Data for the Unmetallized Polystyrene Microspheres and Microspheres with Au Monometallic and Au/Pt Bimetallic Colloids on Their Surfacesa C 1s

N 1s

N-C

NCdO

BE (eV) fwhm (eV) area (%)

285.0 1.9 85.4

286.5 1.4 7.3

287.9 1.4 7.3


285 1.7 87.0

286.4 1.2 6.5

287.9 1.2 6.5

PS-PNIPAAm-Au 400 1.9 100.0

532.0 2.4 100.0


285 1.7 87.4

286.3 1.1 6.3

287.9 1.1 6.3

PS-PNIPAAm-Au/Pt (2/8) 400.0 401.0 1.6 1.0 90.6 9.4

532.1 2.4 100.0

a

free

coordinated

Au 4f

C-C/C-H

PS-PNIPAAm 400.0 1.9 100.0

O 1s

Pt 4f7/2

4f7/2

4f5/2

84.0 1.8 54.0

87.7 1.8 46.0

Pt0

Pt2+

71.2 2.0 88.1

72.6 1.1 11.9

532.1 2.8 100.0

Binding energy (BE) was referenced by setting the C-C/C-H peak maximum in the C 1s spectrum to 285.0 eV.

Figure 6. High-resolution XPS spectra for metallized polystyrene microspheres: Au 4f region (a) for the sample of Au monometallic particles; Pt 4f region (b) for the sample of bimetallic colloids with Au/Pt molar ratio of 2/8.

Figure 5. High-resolution XPS spectra for metallized polystyrene microspheres: C 1s region (a) and N 1s region (b) for the sample of bimetallic colloids with Au/Pt molar ratio of 2/8; N 1s region (c) for the sample of Au monometallic particles.

(EXAFS). As evidenced by the evolution of EXAFS data, the oxidation of surface platinum is facile; either a chemisorbed oxide layer or a compound oxide species can be formed depending on the particle size and reaction conditions. No detectable Au 4f signals were observed in the core-line high-resolution spectrum in the spectral region from 95 to 75 eV. We interpret this result as the dual effects of lower Au molar fraction and preferential occupation of surface sites by the Pt layer. Conclusions The aim of the present work has been to explore a novel strategy for the attachment of Au/Pt bimetallic colloids on the polystyrene microsphere surface. In the dispersion copolymerization of styrene and a poly(Nisopropylacrylamide) macromonomer, the gold and plati-

num ions are reduced in situ by radicals. Well-dispersed Au/Pt bimetallic nanoparticles are attached on the surface of polystyrene microspheres, as confirmed by our TEM and AFM results. The mean diameter of polystyrene microspheres decreases with increasing Pt molar fraction. At the Au/Pt molar ratio of 2/8, monodisperse polystyrene microspheres are obtained. According to the XPS and FTIR measurements, the surface-grafted PNIPAAm chains not only serve as steric stabilizers to prevent the fluctuation of the polystyrene particles but adsorb the metal nanoparticles onto the surface of microspheres. The reduction rate of Au ions is decreased upon addition of Pt ions to the polymerization system. Each Au/Pt bimetallic colloid particle consists of several microclusters, which have a modified Au core. The Pt atoms are deposited on the surface of the Au core but do not cover it completely. It is noteworthy that Au/Pt bimetallic colloids have been widely used as the active catalyst for hydrogenation of olefins and visible light-induced hydrogen generation form water. When the metal colloids are attached on the polymer support, the catalyst will be separated easily from the reaction mixture by centrifu-


gation and recycled in the reaction. Ongoing research in our group focuses on evaluation of catalytic activity and stability of the Au/Pt bimetallic colloids attached on the polystyrene microsphere surface. Applications on both a laboratory and technical scale also seem feasible. Acknowledgment. C.-W.C. thanks the Ministry of Education, Science, Sports, and Culture, Japan for the scholarship. This work was financially supported in part by a Grant-in-Aid for Scientific Research in Priority

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Areas of New Polymers and Their Nano-Organized Systems (277/101266248) and a Grant-in-Aid for Scientific Research (10555326) from the Ministry of Education, Science, Sports, and Culture, Japan. Dr. M.-Q. Chen is thanked for his assistance with the microsphere synthesis and helpful discussion. We also want to thank Mr. T. Kakoi, Mr. W. Sakamoto, and Mr. Y. Ozono for the help with TEM, AFM, and XPS measurements. CM011634N

Pt Bimetallic Colloids on Polystyrene

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