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Formation of Silver Nanoparticles Induced by Poly(2,6-dimethyl-1,4-phenylene oxide) Hoon Sik Kim,*,†,‡ Jae Hee Ryu,†,‡ Binoy Jose,‡ Byung Gwon Lee,† Byoung Sung Ahn,† and Yong Soo Kang*,‡ CFC Alternatives Research Center and Center for Facilitated Transport Membrane, Korea Institute of Science and Technology, 39-1 Hawolgokdong, Seongbukgu, Seoul 136-791, Korea Received May 7, 2001. In Final Form: July 5, 2001 Colloidal silver nanoparticles were easily obtained by reacting AgX (X ) BF4, PF6, SbF6, SO3CF3, ClO4, NO3) with poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) in a mixed solvent system of chloroform and methanol in the absence of UV irradiation. Rapid formation of colloidal silver nanoparticles was also observed when a silver salt was added to a chloroform solution containing PPO and 1-hexene. UV-vis spectra and transmission electron micrographs show that the colloidal silver particles formed from these methods are nanosized, stable, and uniformly distributed. PPO was found to be oxidized in the presence of a silver salt to give quinone and quinone derivatives which were analyzed by GC-Mass. A plausible mechanism for the formation of silver nanoparticles is proposed on the basis of the redox reaction of PPO in the presence of a silver salt.
Introduction Research on metal colloids is greatly stimulated due to the unique properties of nanoscopic materials in optical properties, catalytic activity, and magnetic properties which are different from bulk metals.1-13 Many studies on metal colloids have been reported in particular for silver, gold, and copper colloids. Investigation of the optical properties of silver nanoparticles is very interesting since the silver nanoparticles strongly absorb in the visible region due to surface plasmon resonance.14 Silver colloidal particles play important roles as substrates in studies of surface-enhanced Raman scattering15-17 and catalysis.9 A number of methods have been developed for preparing metal colloids, such as radiation chemical reduction,14,18,19 chemical reduction with or without stabilizing * To whom correspondence should be addressed. Tel: (+82-2)958-5855. Fax: (+82-2)958-5859. E-mail:
[email protected]. † CFC Alternatives Research Center. ‡ Center for Facilitated Transport Membrane. (1) Schmid, G. Chem. Rev. 1992, 92, 1709. (2) Hanamura, E. Phys. Rev. B 1988, 37, 1273. (3) Sun, T.; Seff, K. Chem. Rev. 1994, 94, 857. (4) Ozin, G. A. Adv. Mater. 1992, 4, 612. (5) Ershov, B. G.; Henglein, A. J. Phys. Chem. 1993, 97, 3434. (6) Yonezawa, Y.; Sato, T.; Ohno, M.; Hada, H. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1559. (7) Aihara, N.; Torigoe, K.; Esumi, K. Langmuir 1998, 14, 4945. (8) Sato, T.; Maeda, N.; Ohkoshi, H.; Yonezawa, Y. Bull. Chem. Soc. Jpn. 1994, 67, 3165. (9) Shiraishi, Y.; Toshima, N. J. Mol. Catal. A: Chem. 1999, 141, 187. (10) Duff, D. G.; Baiker, A. Langmuir 1993, 9, 2301. (11) Bright, R. B.; Musick, M. D.; Natan, M. J. Langmuir 1998, 14, 5695. (12) Wang, W.; Efrima, S.; Regev, O. Langmuir 1998, 14, 602. (13) Mateˇjka, P.; Vlcˇkova´, B.; Vohlidal, J.; Pancˇosˇka, P.; Baumruk, V. J. Phys. Chem. 1992, 96, 1361. (14) Kapoor, S. Langmuir 1998, 14, 1021. (15) Brandt, E. S.; Cotton, T. M. In Investigations of Surfaces and Interfaces-Part B, 2nd ed.; Rossiter, B. W., Baetzold, R. C., Eds.; John Wiley & Sons: New York, 1993; Vol. IXB, p 633. (16) Rivas, L.; Schchez-Cortes, S.; Garcia-Ramos, J. V.; Morcillo, G. Langmuir 2001, 17, 578. (17) Nickel, U.; Castell, A.; Po¨ppl, K.; Schneider, S. Langmuir 2000, 16, 9087. (18) Linnert, T.; Mulvaney, P.; Henglein, A.; Weller, K. J. Chem. Soc., Faraday Trans. 1990, 112, 4567.
polymers,20-22 chemical or photoreduction in reverse micelles,23 and thermal decomposition in organic solvents.9 We present here a simple and convenient method for preparing colloidal silver nanoparticles using poly(2,6dimethyl-1,4-phenylene oxide) (PPO) as a reducing agent. We also describe herein the effect of concentration of silver salt and oxygen on the colloidal silver formation. The colloidal silver nanoparticles formed were characterized by means of UV-vis spectroscopy and transmission electron microscopy (TEM). Experimental Section Materials. Poly(2,6-dimethyl-1,4-phenylene oxide), AgBF4, AgPF6, AgSbF6, AgSO3CF3, AgClO4, AgNO3, and 1-hexene were purchased from Aldrich and used as received. Chloroform and methanol were purchased from J. T. Baker and distilled before use. Preparation of Silver Nanoparticles. In a 20 mL vial, 0.3 mL of methanolic solution containing a silver salt (1.24-6.2 × 10-3 mmol) was added to a 0.1 wt % PPO solution in 10 mL of chloroform. Soon after the addition of a silver salt, silver nanoparticles started to form, which was manifested by a yellowish coloration of the solution. Silver nanoparticles were also obtained by mixing 0.3 mL of chloroform solution containing a silver salt (1.24 × 10-3 mmol) and 1-hexene (2.48 × 10-3 mmol) with 0.1 wt % PPO solution in 10 mL of chloroform. 1-Hexene was employed to increase the solubility of silver salts in chloroform by forming silver-olefin π-complexes. Characterization of Silver Nanoparticles. UV-vis spectra were measured with a Scinco UV S-2100 spectrophotometer using quartz cuvettes. Transmission electron micrographs were obtained from a Philips CM30 microscope operating at 300 kV. Samples were prepared by evaporating a solution of nanoparticles onto a 200 mesh copper grid, which was coated with a carbon support film. For IR measurements, the chloroform solution containing silver salt, 1-hexene, and PPO (AgBF4/1-hexene/PPO ) 1/2/1) was coated onto a 25 mm × 3 mm CaF2 window and (19) Yeung, S. A.; Hobson, R.; Biggs, S.; Grieser, F. J. Chem. Soc., Chem. Commun. 1993, 378. (20) Toshima, N.; Yonezawa, T.; Kushihashi, K. J. Chem. Soc., Faraday Trans. 1993, 89, 2537. (21) Liz-Marzan, L. M.; Lado-Tourino, I. Langmuir 1996, 12, 3585. (22) Liz-Marzan, L. M.; Philipse, A. P. J. Phys. Chem. 1995, 99, 15120. (23) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 12974.
10.1021/la010677f CCC: $20.00 © 2001 American Chemical Society Published on Web 08/24/2001
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Figure 1. Time evolution of the UV-vis spectra of the silver nanoparticles prepared from the AgBF4-PPO solution in chloroform/methanol: [AgBF4] ) 1.24 × 10-1 mM; 0.1 wt % PPO solution in chloroform; AgBF4/PPO ) 1/100.
Figure 3. TEM images and the corresponding histograms of silver nanoparticles obtained from the AgBF4-PPO solutions in various molar ratios of AgBF4/PPO: (a) 1/100, (b) 1/30, and (c) 1/20. Figure 2. UV-vis spectra of the silver nanoparticles prepared from the AgBF4-PPO solutions in chloroform/methanol in various molar ratios of AgBF4/PPO. dried under a vacuum. The coated window was placed in a specially designed gas cell together with an uncoated window.24 IR spectra were recorded on a Mattson Infinity spectrophotometer equipped with MCTA detector. The oxidation products of PPO were analyzed by GC-Mass (HP 6890-5973 MSD, HP-1, 50 m × 0.2 mm × 10 µm).
Results and Discussions Upon mixing AgBF4 in methanol with PPO in chloroform in a molar ratio of AgBF4/PPO ) 1/100, a clear yellow solution was obtained, and the intensity of the yellow coloration increased rapidly with time. The deep yellow color of the solution was maintained at least for 10 days and then became light red. Figure 1 shows the UV-vis spectra of the solution containing AgBF4 and PPO recorded at time intervals. As indicated by the yellowish coloration, a characteristic silver plasmon band appears at 420 nm. A shoulder band at around 400 nm is ascribed to the formation of diphenoquinone25 by the oxidation of PPO.26 The mechanism for the formation of silver nanoparticles and the oxidation of PPO will be discussed later. The effect of the molar ratio of AgBF4/PPO on the silver nanoparticle formation was investigated using UV-vis spectroscopy. Figure 2 shows the UV-vis spectra of the solutions with the molar ratios of AgBF4/PPO of 1/100, 1/30, and 1/20. The silver plasmon band shifts to a higher (24) Haber, J.; Wojciechowska, M. J. Catal. 1998, 110, 23. (25) Systematic name: 4-(3,5-dimethyl-4-oxo-2,5-cyclohexadienylidene)-2,6-dimethyl-2,5-cyclohexadienone. (26) Kende, A. S.; MacGregor, P. J. Am. Chem. Soc. 1961, 83, 4197.
wavelength with increasing molar ratio of AgBF4/PPO. As explained by Heard et al.27 and Henglein,28 the band shift to a higher wavelength can be attributed to the difference in the size and distribution of the particles. To confirm the correlation between silver plasmon band shift and particle size, TEM micrographs were taken on those three different colloid samples. The size distributions of silver particles were obtained by counting approximately 300 particles. The standard deviations were calculated based on the experimentally determined distributions. Figure 3 clearly shows that the particle size and distribution are dependent on the molar ratio of AgBF4/PPO. Upon increasing the molar ratio of AgBF4/PPO from 1/100 to 1/20, the mean particle sizes and the width of the distribution (standard deviation) increased from 9.2 to 20.0 nm and from 2.0 to 5.7 nm, respectively. These results strongly suggest that the band shift can be related to the changes in particle size and distribution. The change of the particle size and distribution with time was also monitored by TEM for the solution prepared by mixing AgBF4 in methanol with PPO in chloroform (AgBF4/PPO ) 1/100). Three samples were taken from the solution after 1, 3, and 10 days. TEM micrographs and histograms (Figure 4) show that all of the three samples contain similarly distributed silver particles with sizes of approximately 9-12 nm. The particle size and the distribution remain almost constant with time at least up to 10 days, demonstrating the stability of silver nanoparticles. (27) Heard, S. M.; Grieser, F.; Barraclough, C. G.; Sanders, J. V. J. Colloid Interface Sci. 1983, 93, 545. (28) Henglein, A. Chem. Mater. 1998, 10, 444.
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Figure 6. UV-vis spectra of the silver nanoparticles obtained from the AgBF4-PPO solutions (AgBF4/PPO ) 1/100) prepared under various conditions.
Figure 4. TEM images and the corresponding histograms of silver nanoparticles obtained from the AgBF4-PPO solution (AgBF4/PPO ) 1/100) stored at different times: (a) 1 day, (b) 3 days, and (c) 10 days.
Figure 5. Time evolution of the absorbance at λ ) 420 nm for the various solutions containing PPO and a silver salt (silver salt/PPO ) 1/100).
Colloidal silver nanoparticles were also obtained from different silver salts, AgX (X ) PF6, SbF6, SO3CF3, ClO4, NO3). As in the case of AgBF4, all of the colloidal silver nanoparticles prepared in a molar ratio of AgX/PPO ) 1/100 show characteristic silver plasmon bands centered at 420 nm. Figure 5 shows the time evolution of the absorbance at 420 nm for various silver salts. The rates of formation of colloidal silver nanoparticles are fast initially but become slower after 10 min for AgBF4, AgPF6, AgSbF6, AgSO3CF3, and AgClO4. On the other hand, AgNO3 shows a gradual increase of the absorbance at 420 nm. The slower rate of particle formation for AgNO3 can be attributed to the strong interaction between the silver
ion and NO3-.29 Unlike other anions with delocalized charges, NO3- is strongly coordinated to a silver ion through a chelation by two oxygen atoms and, therefore, the approach of oxygen atoms of PPO to silver ions becomes limited to a certain extent. Silver(I)-olefin π-complexes have also been employed in the preparation of silver nanoparticles to increase the solubility of silver salts in chloroform. Ag(1-hexene)X (X ) BF4, PF6, ClO4, SbF6, SO3CF3),29,30 which was prepared in situ from AgX with 1-hexene in chloroform, was added to a PPO solution in chloroform in a molar ratio of AgX/ 1-hexene/PPO ) 1/2/100. The resulting yellow solutions show UV-vis absorption bands at 420 nm, indicating the formation of silver nanoparticles. To study the influence of light and oxygen on the formation of colloidal silver nanoparticles, the solutions containing AgBF4 and PPO, AgBF4-PPO (AgBF4/PPO ) 1/100), were prepared under various conditions. Figure 6b is the UV-vis spectrum of the solution prepared under an atmospheric oxygen and stored under a room light for 15 min. Figure 6c is the UV-vis spectrum of the solution prepared under an atmospheric oxygen kept in complete darkness for 15 min. The similar band intensity at 420 nm for these two spectra indicates that the room light has a negligible effect on the formation of silver nanoparticles. However, as shown in Figure 6d, the UV-vis spectrum for the sample prepared under an argon atmosphere in a drybox shows a considerable decrease in the intensity of the band at 420 nm. The decrease of the band intensity at 420 nm implies that the formation of colloidal silver nanoparticles is affected by oxygen. To clearly see the effect of oxygen, the AgBF4-PPO solution was prepared and stored under 2 atm of oxygen for 15 min in a higher pressure glass reactor. The significant increase in the intensity of the band at 420 nm is shown in Figure 6a, demonstrating that the formation of silver nanoparticles is greatly accelerated in the presence of oxygen. It is reported that the photodegradation of PPO by laser flash photolysis gives absorption bands at about 400 nm.31 It is also known that PPO is in equilibrium with dimethylphenol and quinone derivatives in the presence of a Cu(II) salt with the simultaneous reduction of Cu(II) to Cu(I).32,33 For a better mechanistic insight into the (29) Sunderrajan, S.; Freeman, B. D.; Hall, C. K. Ind. Eng. Chem. Res. 1999, 38, 4051. (30) Quinn, H. W.; McIntyre, J. S.; Peterson, D. J. Can. J. Chem. 1965, 43, 2896. (31) Schneider, S.; Richter, F.; Brem, B. Polym. Degrad. Stab. 1998, 61, 453. (32) Bolon, D. A. J. Org. Chem. 1972, 37, 441.
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Figure 7. IR spectra of pure PPO and AgBF4-PPO films prepared from the AgBF4-PPO solution containing 1-hexene (AgBF4/1-hexene/PPO ) 1/2/1) stored at different times: (a) PPO, (b) AgBF4-PPO film after 1 h, and (c) AgBF4-PPO film after 10 days.
formation of colloidal silver nanoparticles induced by PPO, FT-IR study and GC-Mass analysis have been conducted for the solution prepared by mixing AgBF4 with PPO in chloroform in the presence of 1-hexene (AgBF4/1-hexene/ PPO ) 1/2/1). Figure 7a is the IR spectrum of pure PPO film. Spectra b and c in Figure 7 are the IR spectra of the films by evaporating the solutions stored for 1 h and 10 days after preparation, respectively. As shown in Figure 7c, a new peak corresponding to the carbonyl stretching band appears at 1651 cm-1, which is likely due to the formation of quinone or quinone derivatives from the oxidation of PPO. Among several oxidation products of PPO, 2,6-dimethylbenzoquinone (M+ ) 136) and diphenoquinone (M+ ) 240) have been identified in the GCMass analysis. From the IR and GC-Mass results, it is evident that the PPO oxidation to quinone derivatives is facilitated in the presence of a silver salt. On the basis of these results and observations by others,31-33 we propose a mechanism for the reduction of silver salts to silver nanoparticles in the presence of PPO as shown in Scheme 1. The oxygen atom of PPO can coordinate to silver ions through an electron donation to the vacant 5s orbital of the silver ion. Such an oxygen coordination to silver ions could cause the disorder of C-O bond strength. Subsequent C-O bond cleavage would result in the reduction of silver ions with the formation of quinone or quinone derivatives. The formation of silver nanoparticles is greatly accelerated in the presence of oxygen. It is likely that the interaction of benzylic radical species C and oxygen generates hydroperoxide species D,34 which in turn reacts with silver ions to produce silver nanoparticles.35 It is important to mention here that silver salts are widely (33) Baesjou, P. J.; Driessen, W. L.; Challa, G.; Reedijk, J. Macromolecules 1999, 32, 270. (34) Rivaton, A. Polym. Degrad. Stab. 1995, 49, 11.
used in the oxidation of hydroxy aromatic compounds to the corresponding quinoid systems.36 Conclusions Colloidal silver nanoparticles were conveniently synthesized from silver salts using PPO as a reducing agent. The investigation on the formation of silver nanoparticles using UV-vis spectroscopy and TEM shows that the silver colloids formed are nanosized, uniformly distributed, and stable at least for 10 days. Both particle size and the UV absorption band are strongly dependent on the molar ratio of AgBF4/PPO. On increasing the molar ratio of AgBF4/ PPO from 1/100 to 1/20, the particle size increases from 9.2 to 20.0 nm and the absorption band shifts to a higher wavelength. The rate of the formation of silver nanoparticles was greatly accelerated in the presence of oxygen. The oxidation of PPO to quinone and quinone derivatives was confirmed by FT-IR spectroscopy and GC-Mass analysis. Acknowledgment. The authors acknowledge the financial support from the Ministry of Science and Technology of Korea through the Creative Research Initiatives Program. LA010677F (35) Scott, G. Atmospheric Oxidation and Antioxidants; Elsevier: New York, 1965; Chapter 2. (36) Schafer, W.; Leute, R.; Schlude, H. Chem. Ber. 1971, 104, 3021.
