Langmuir 2008, 24, 14203-14208
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Formation of Metal Nanoparticles on the Surface of Polymer Particles Incorporating Polysilane by UV Irradiation Toshiyuki Tamai,*,† Mitsuru Watanabe,† Yoshiro Hatanaka,† Hiroyuki Tsujiwaki,‡ Noboru Nishioka,‡ and Kimihiro Matsukawa† Osaka Municipal Technical Research Institute, 1-6-50 Morinomiya, Joto-ku, Osaka 536-8553, and Osaka Electro-Communication UniVersity, 18-8 Hatsucho, Neyagawa, Osaka 572-8530, Japan ReceiVed June 11, 2008. ReVised Manuscript ReceiVed October 1, 2008 Polystyrene particles incorporating poly(methylphenylsilane) (PMPS) were synthesized by miniemulsion polymerization. UV irradiation of the emulsion under air in the presence of metal salts such as HAuCl4 · 4H2O, AgNO3, and Na2PdCl4 led to the formation of metal nanoparticles on the surface of polymer particles; thus, metal nanoparticle/ polymer hybrid particles were obtained. The structures of the hybrid particles were confirmed by the surface plasmon resonance band and transmission electron microscopy images. The formation of metal nanoparticles depended on the functional groups and charge on the surface of the polymer particle. The metal nanoparticles were formed due to the reduction of metal ions, accompanied by the oxidation of PMPS. The interaction between the surface of the polymer particle and the metal ions plays an important role in the formation of the metal nanoparticle.
Introduction Metal nanoparticles have been studied intensively because they have unusual properties.1 However, it is often difficult to directly use noble metal particles in the industrial field, for instance, in the form of catalysts, because of their high tendency of agglomeration. In general, metal nanoparticles can be synthesized by the reduction of metal ions and then stabilized and supported by using surfactants,2 polymers,3 dendrimers,4 microgels,5 and polymer particles. Polymer particles exhibit a high surface-to-volume ratio, functional groups at the surface, and monodispersity, they have simple methods of preparation,6 * To whom correspondence should be addressed. E-mail: tositama@ omtri.city.osaka.jp. † Osaka Municipal Technical Research Institute. ‡ Osaka Electro-Communication University. (1) (a) Daniel, M.; Astruc, D. Chem. ReV. 2004, 104, 293. (b) Buruda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (c) Eustis, S.; El-Sayed, M. A. Chem. Soc. ReV. 2006, 35, 209. (2) Kuo, C.; Huang, M. H. Langmuir 2005, 21, 2012. (3) Sakai, T.; Alexandridis, P. Langmuir 2004, 20, 8426. (4) (a) Esumi, K.; Isono, R.; Yoshimura, T. Langmuir 2004, 20, 237. (b) Naka, K.; Itoh, H.; Chujo, Y. Nano Lett. 2002, 2, 1183. (5) (a) Hantzschel, N.; Zhang, F.; Eckert, F.; Pich, A.; Winnik, M. A. Langmuir 2007, 23, 10793. (b) Zhang, J; Shengqing, X.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126, 7908. (6) Antonietti, M.; Tauer, K. Macromol. Chem. Phys. 2003, 204, 207. (7) (a) Dokoutchaev, A.; James, J. T.; Koene, S. C.; Pathak, S.; Prakash, G. K. S.; Thompson, M. E. Chem. Mater. 1999, 11, 2389. (b) Pathak, S.; Greci, M. T.; Kwong, R. C.; Mercado, K.; Prakash, G. K. S.; Olah, G. A.; Thompson, M. E. Chem. Mater. 2000, 12, 1985. (8) Shi, W.; Sahoo, Y.; Swihart, M. T.; Prasad, P. N. Langmuir 2005, 21, 1610. (9) (a) Suzuki, D.; Kawaguchi, H. Langmuir 2005, 21, 8175. (b) Suzuki, D.; Kawaguchi, H. Langmuir 2005, 21, 12016. (c) Suzuki, D.; Kawaguchi, H. Langmuir 2006, 22, 3818. (10) Kim, H.; Daniels, E. S.; Dimonie, V. L.; Klein, A. J. Polym. Sci., Part A: Polym. Chem. 2007, 46, 912. (11) Wen, F.; Zhang, W.; Wei, G.; Wang, Y.; Zhang, J.; Zhang, M.; Shi, L. Chem. Mater. 2008, 20, 2144. (12) (a) Mei, Y.; Sharma, G.; Lu, Y.; Ballauf, M.; Drechsler, M.; Irrgang, T.; Kempe, R. Langmuir 2005, 21, 12229. (b) Lu, Y.; Mei, Y.; Drechsler, M.; Ballauf, M. Angew. Chem., Int. Ed. 2006, 45, 813. (c) Mei, Y.; Lu, Y.; Polzer, F.; Ballauf, M.; Drechsler, M. Chem. Mater. 2007, 19, 1062. (13) (a) Schuetz, P.; Caruso, F. Chem. Mater. 2004, 16, 3066. (b) Wang, Y.; Alexandra, S.; Angelatos, A. S.; Caruso, F. Chem. Mater. 2008, 20, 848. (14) Li, Y.; Boone, E.; El-Sayed, M. A. Langmuir 2002, 18, 4921. (15) Mayer, A. B. R.; Mark, J. E. Angew. Makromol. Chem. 1999, 268, 52. (16) Chen, C.; Serizawa, T.; Akashi, M. Chem. Mater. 1999, 11, 1381. (17) Mayer, A. B. R.; Grebner, W.; Wannemacher, R. J. Phys. Chem. B 2000, 104, 7278.
and they are suitable for supporting metal nanoparticles. The production of hybrid polymer particles by immobilizing metal nanoparticles on the surface of polymer particles has been studied intensely.7-18 Gold nanoparticle/polymer hybrid particles can be prepared by the adsorption of gold nanoparticles on the surface of polymer particles that have functional groups such as thiol and pyridine groups.7,8 In situ reduction of Au(III) ion by NaBH4 leads to the formation of gold nanoparticles on the surface of polymer particles that have amine and pyridine groups.9,11 These surface functional groups can be introduced by a variety of methods, and they strongly bind the gold nanoparticles to the polymer surface. Ballauf et al. reported that reduction of ions localized in polyelectrolyte brushes or microgels leads to the formation of metal nanoparticles.12 Caruso also reported the formation of metal nanoparticles on the polymer particle coated with polyelectrolyte multilayers.13 Palladium nanoparticles formed on polymer particles are expected to be used as catalysts,11,12,14-16 e.g., for the Suzuki coupling reaction.11,14 Gold nanoparticles, which exhibit a strong surface plasmon resonance band around 520 nm, can be used in imaging and sensing applications.1,8,9 It has been reported that gold nanoparticles, especially nanorods, are formed by UV irradiation of HAuCl4 in the presence of rodlike micelles of a cationic surfactant.19,20 It is easier to control the growth of metal nanoparticles in photochemical synthesis than in a process involving the addition of a reducing agent.19,20 We have already reported the preparation of gold nanoparticles by the photoreduction of Au(III) ions using poly(silane-acrylamide) block copolymers, which serve as reducing agents and protecting groups in their tetrahydrofuran solutions and polymer films.21,22 Polysilanes, which are characterized by an absorption band and fluorescence emission in the (18) Khan, M. A.; Perruchot, C.; Armes, S. P.; Randall, D. P. J. Mater. Chem. 2001, 11, 2363. (19) Kim, F.; Song, J. H.; Yang, P. J. Am. Chem. Soc. 2002, 124, 14316. (20) Esumi, K.; Matsuhisa, K.; Torigoe, K. Langmuir 1995, 11, 3285. (21) Matsukawa, K.; Matsuura, Y. Mater. Res. Soc. Symp. Proc. 2005, 847, 45. (22) (a) Nagayama, N.; Maeda, T.; Yokoyama, M. Chem. Lett. 1997, 397. (b) Fukushima, M.; Hamada, Y.; Tabei, E.; Aramata, M.; Mori, S.; Yamamoto, Y. Chem. Lett. 1998, 347. (c) Sanji, T.; Ogawa, Y.; Nakatsuka, Y.; Tanaka, M.; Sakurai, H. Chem. Lett. 2003, 32, 980. (d) Shankar, R.; Shahi, V. J. Organomet. Chem. 2008, 693, 307.
10.1021/la801809u CCC: $40.75 2008 American Chemical Society Published on Web 11/12/2008
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Tamai et al.
Scheme 1. Schematic Representation of the Preparation of Metal Nanoparticle/Polymer Hybrid Particles
Table 1. Polystyrene Emulsion Incorporating PMPS
emulsion
polymer (ratio of monomers a)
1a 1b 2a 2b 3a 3b 4
PS PS P[S-co-NIPAM] (90:10) P[S-co-NIPAM] (90:10) P[S-co-AAEM] (99:1) P[S-co-AAEM] (99:1) PS
[PMPS] (mol % vs surface diameter styrene) charge (nm) 1.9 1.9 1.9 1.9 1.9 1.9 0
anionic cationic anionic cationic anionic cationic anionic
b
169 181 256 118 139 131 170
a Ratio based on the number of moles of the monomers used for the polymerization. b Measured by DLS.
Experimental Section
UV region, are well-known photofunctional polymers.23a UV irradiation of polysilane causes Si-Si bond cleavage and produces a silyl radical, which can serve as a reducing agent.23 In this study, we synthesized polystyrene particles incorporating poly(methylphenylsilane) (PS/PMPS) by miniemulsion polymerization. It was expected that metal nanoparticles would be formed on the surface of PS/PMPS due to the photoreduction of the metal ion. We report the formation of polymer particles decorated with metal nanoparticles by UV irradiation of PS/ PMPS under air in the presence of metal salts (Scheme 1). An anionic or cationic charge that was derived from the charge of the surfactants and the radical initiators was present on the surface of the polymer particles.24 Functional groups, which are expected to coordinate to the metal ions, were also introduced on the surface of the polymer particles by the copolymerization of monomers. The surface charge and functional groups affected the formation of the metal nanoparticles. We discuss the effects of the interaction between the surface of the polymer particles and the metal ions during the formation of the metal nanoparticles. (23) (a) Miller, R. D. Chem. ReV. 1989, 89, 1359. (b) Steinmetz, M. G. Chem. ReV. 1995, 95, 1527. A photogenerated radical for a reducing agent was reported: (c) Itakura, T.; Torigoe, K.; Esumi, K. Langmuir 1995, 11, 4129. (d) Korchev, A. S.; Konovalova, T.; Cammarata, V.; Kispert, L.; Slaten, L.; Mills, G. Langmuir 2006, 22, 375. (e) Sakamoto, M.; Tachikawa, T.; Fujitsuka, M.; Majima, T. Chem. Mater. 2008, 20, 2060. (24) Watanabe, M.; Tamai, T. Langmuir 2007, 23, 3062.
Materials and Instruments. Styrene (Nacalai) and divinylbenzene (DVB, Nacalai) were passed through a neutral alumina column before use. N-Isopropylacrylamide (NIPAM; Kojin), acetoacetoxyethyl methacrylate (AAEM; Nacalai), HAuCl4 · 4H2O (Nacalai), AgNO3 (Nacalai), Na2PdCl4 (Aldrich), PdCl2 (Aldrich), HS-10 (Dai-ichi Kogyo Seiyaku, Scheme 1), K2S2O8 (KPS; Nacalai), 2,2′-azobis(2methylpropionamidine) dihydrochloride (AIBA; Wako), and hexadecane (Nacalai) were used as received. PMPS (Osaka Gas) was reprecipitated from 2-propanol/toluene (Mn ) 1.68 × 104, Mw/Mn ) 2.35). [(Methacryloyloxy)ethyl]dimethyldodecylammonium bromide was synthesized according to a procedure in the literature.25 Transmission electron microscopy (TEM) images were obtained using a JEOL model JEM-2100 microscope at an acceleration voltage of 100 kV. Specimens were prepared by dropping the aqueous dispersion onto a carbon-coated copper grid and allowing the water to evaporate. Particle size distributions of the emulsions were measured by dynamic light scattering (DLS) using an Otsuka Electronics DLS-6000HLC instrument. UV-vis absorption spectra were recorded on a JASCO V-560 spectrophotometer. The emulsions were diluted with distilled water to minimize light scattering caused by the particles, and then the UV-vis absorption spectra were measured at 25 °C using a 1 cm path length cell and an integrating sphere. Fluorescence spectra were measured at 25 °C by a Hitachi F-4500 spectrofluorometer using a 1 cm path length cell at an angle of 90° with respect to the direction of excitation light. Synthesis of Polystyrene Particles Incorporating PMPS. Polystyrene emulsions 1-4 were synthesized by miniemulsion polymerization (Table 1).26,27 In a typical experiment of anionic emulsion synthesis, a 100 mL flask equipped with a condenser and a mechanical stirrer was filled with styrene (4.6 g, 44.2 mmol), DVB (0.09 g, 2.2 mmol), PMPS (0.1 g, 1.9 mol % vs styrene; the number of moles of PMPS was calculated on the basis of the number of moles of methylphenylsilane as a constitutional repeating unit), hexadecane (0.36 g, 1.6 mmol, hydrophobe), HS-10 (0.05 g, 0.06 mmol, surfactant), and water (25 mL). PMPS was dissolved in a mixture of the monomers and the hydrophobe and then added to an aqueous surfactant solution. The mixture was stirred for 15 min and ultrasonicated (Branson 450 digital sonifier) for 9 min in an ice bath to obtain the miniemulsion. The miniemulsion was purged with nitrogen gas for 1 h and then heated to 80 °C. An aqueous solution (1 mL) of KPS (0.05 g, 0.19 mmol, radical initiator) and NaHCO3 (0.01 g, 0.13 mmol) was added to the flask, and the mixture was vigorously stirred at 80 °C for 18 h. The obtained emulsion 1a (19 wt % as estimated by gravimetric analysis) consisted of particles having a DLS diameter of 169 nm and a narrow size distribution (polydispersity index 280 nm) under air at room temperature for 60 min. The color of the mixture turned light gray due to the irradiation. A TEM image of the mixture showed the formation of palladium nanoparticles on the polymer particle surface (Figure 4).28 Gold and silver nanoparticle/polymer hybrid particles were synthesized in a similar manner using HAuCl4 · 4H2O and AgNO3, respectively (Table 2). The formation of gold and silver nanoparticles was monitored by observing the changes in the UV-vis absorption spectra originating from the surface plasmon resonance of the nanoparticles, as shown in Figure 2. The differential spectra are also shown in this figure for easy identification of the absorbance maxima.
Results and Discussion Preparation of Gold and Silver Nanoparticle/Polymer Hybrid Particles. Polystyrene emulsions incorporating PMPS (1a) were synthesized by miniemulsion polymerization (Table 1). Figure 1 shows the fluorescence emission spectrum of emulsion 1a. The spectrum can be attributed to the fluorescence emission of PMPS.23a The fluorescence disappeared (Supporting Information, Figure S2) when 1a was irradiated with a high-pressure Hg arc lamp (>280 nm). This showed that the photodecomposition of PMPS incorporated into the polymer particles occurred due to UV irradiation. An aqueous solution of HAuCl4 · 4H2O was added to 1a, and the mixture was stirred for 30 min in the dark under air at room temperature. It retained its milky white color. The mixture was then irradiated with the Hg lamp, and its color turned deep wine red. The UV-vis absorption spectra of the mixture are shown in Figure 2a. The absorption band around 560 nm is attributable to the surface plasmon resonance of gold nanoparticles.1a The differential spectrum is shown in Figure 2b. The intensity of the absorption band increased and the band maximum red-shifted (28) Evaluation of the catalytic activity of the palladium nanoparticles is currently under way. The emulsion of the hybrid particles 2a/Pd was purified by dialysis in a cellulose tube for a week. The hybrid particles were stable after the dialysis.
Figure 2. (a) UV-vis spectra of the UV-irradiated 1a/HAuCl4 · 4H2O, 0, 10, 30, 50, 70, and 90 min. (b) Same data, differential spectra. The spectrum of 0 min in (a) was used as the baseline. (c) UV-vis spectra of the UV-irradiated 1a/AgNO3, 0, 20, 40, 60, 80, 100, 130, and 160 min. (d) Same data, differential spectra.
with passage of irradiation time. UV irradiation induced the formation and growth of the gold nanoparticles. The TEM image (Figure 3a) indicated that gold nanoparticles with diameters in the range of ca. 10-20 nm (black particle) were immobilized on the surface of the polymer particle (gray particle). Polymer particles having a cationic charge (1b, 2b, 3b, Table 1), an N-isopropylamide group (2a, 2b), and an acetoacetoxy group (3a, 3b) were synthesized to study the effects of the charge and functional group of the polymer particle surface on the formation of metal nanoparticles as shown in Table 2. NIsopropylamide and acetoacetoxy groups are expected to
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Figure 3. TEM images of the metal nanoparticle/polymer hybrid particles: (a) 1a/HAuCl4 · 4H2O, (b) 1b/HAuCl4 · 4H2O, (c) 2a/HAuCl4 · 4H2O, (d) 2b/HAuCl4 · 4H2O, (e) 1a/AgNO3, (f) 1a.
coordinate to metal ions.12b,c,29 UV irradiation of cationic emulsion 1b that contained HAuCl4 · 4H2O afforded gold nanoparticle/polymer hybrid particles (Figure 3b). The UV irradiation of both anionic and cationic polymer particles having an N-isopropylamide group (2a, 2b) afforded gold nanoparticle/ polymer hybrid particles (Figure 3c,d). The surface charge and the functional groups of the polymer particles did not affect the formation of gold nanoparticles. Emulsion 1a containing AgNO3 was irradiated in a similar manner, and it turned light brown. The absorption band around 420 nm was attributable to the surface plasmon resonance of silver nanoparticles5b,17 (Figure 2c). The TEM image shows the formation of silver nanoparticle/polymer hybrid particles (Figure 3e). In contrast, the irradiation of cationic emulsion 1b that contained AgNO3 did not afford silver nanoparticle/polymer hybrid particles. Preparation of Palladium Nanoparticle/Polymer Hybrid Particles. Emulsion 1a containing Na2PdCl4 was irradiated, and it turned light gray. The TEM image shows a large number of spherical palladium particles (ca. 10 nm) as shown in Figure 3. In contrast, many small palladium nanoparticles (50 nm) by using conventional methods. Large palladium particles can be prepared by a seeding growth method: (a) Teranishi, T.; Miyake, M. Chem. Mater. 1998, 10, 594. (b) Lu, L.; Wang, H.; Xi, S.; Zhang, H. J. Mater. Chem. 2002, 12, 156. (c) Kim, J.; Chung, H.; Lee, R. Chem. Mater. 2006, 18, 4115. (32) Nikoobakht, B.; El-Sayed, M. A. Langmuir 2001, 17, 6368.
Metal Nanoparticle/Polymer Hybrid Particle Formation
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Table 2. Preparing Metal Nanoparticle/Polymer Hybrid Particles metal particles on the polymer surface
a
polystyrene particle (surface charge and coordinative group a)
metal salt
morphology
1a (anionic) 1b (cationic) 2a (anionic, NIPA) 2b (cationic, NIPA) 1a (anionic) 1b (cationic) 1a (anionic) 1b (cationic) 2a (anionic, NIPA) 2b (cationic, NIPA) 3a (anionic, AcAc) 3b (cationic, AcAc) 2a (anionic, NIPA) 2b (cationic, NIPA)
HAuCl4 · 4H2O HAuCl4 · 4H2O HAuCl4 · 4H2O HAuCl4 · 4H2O AgNO3 AgNO3 Na2PdCl4 Na2PdCl4 Na2PdCl4 Na2PdCl4 Na2PdCl4 Na2PdCl4 PdCl2 PdCl2
sphere sphere sphere sphere sphere b sphere sphere shapeless sphere shapeless sphere shapeless b
NIPA ) N-isopropylamide group. AcAc ) acetoacetoxy group.
b
size (nm)
TEM image
10-20 10-20 10-20 10-20 10-20
Figure 3a Figure 3b Figure 3c Figure 3d Figure 3e
