The Journal of
Physical Chemistry
0 Copyright 1995 by the American Chemical Society
VOLUME 99, NUMBER 2, JANUARY 12,1995
LETTERS Formation of Metal Particles in Aqueous Solutions by Reactions of Metal Complexes with Polymers L. Longenberger and G. Mills* Department of Chemistry, Auburn University, Auburn, Alabama 36849 Received: August 24, 1994; In Final Form: October 13, 1994@
Metal complexes of Au, Pd, and Ag are transformed into metal particles at room temperature in air-saturated aqueous solutions of poly(ethy1ene glycols) and poly(viny1 alcohols). Formation of Au particles is substantially faster in the case of poly(ethy1ene glycols). In several systems, an intermediate is observed at the early stages of the formation of Au particles; it decays in a few minutes to form metal particles. The stability of the resulting Au colloids as well as the rates of reaction are dependent on the polymer molar mass. The kinetic observations are explained under the assumption that Au(III) complexes initially bound to pseudocrown ether structures of the poly(ethy1ene glycols) are reduced by the macromolecules.
Introduction Considerable attention has been devoted to the unusual physical and chemical properties of small metal particles. 1-3 Changes in the optical properties and chemical reactivity of the crystallites have been correlated to size and surface effects. Variations in the surface plasmon resonance of colloidal Au and Ag particles have been used extensively to study these effect^.^-'^ Optical determinations with colloids are particularly suitable for reasons of simplicity; they also provide important information on size-dependent chemical properties of the particles.3s6s7910-12 Polymeric materials have been employed frequently as particle stabilizers in the chemical synthesis of metal colloids. The polymers interact with the small particles, preventing their agglomeration and pre~ipitation.'~A novel method for the preparation of metal particles is presented here, which is based on the reduction of Au(III), Ag(I), and Pd(II) complexes by reaction with poly(ethy1ene glycols) and poly(viny1 alcohols) in air-saturated aqueous solutions. This investigation is focused mainly on the formation of Au particles in solutions of poly@
Abstract published in Advance ACS Abstracts, December 15, 1994.
(ethylene glycols), where generation of the particles is most efficient. It was found that the rates of reaction increased significantly with increasing polymer molar mass. These results indicate that particle formation is faster with increasing binding of the AuC4- ions to pseudocrown ether cavities formed by the macromolecules,14because formation of the cavities increases with increasing molar mass of the p01ymers.l~
Experimental Section Poly(viny1 alcohols) with average molar masses (in g mol-') of 2.5 x 10" (PVA1) and 7.8 x lo4 (PVA2) as well as poly(ethylene glycol) 20000 (PEG PS20) were obtained from Polysciences, whereas AgN03, NaAuCk2H20, K2PdC4, and 18-crown-6 were from Aldrich. These materials were used as received. The Carbowax poly(ethy1ene glycols) were from Union Carbide, with average molar masses (in g mol-') of 1.45 x lo3 (PEG 1450), 3.35 x lo3 (PEG 3350), 8 x lo3 (PEG 8000), and 1.75 x lo4 (PEG 20M). These polymers were purified by repeated recrystallization from hot 2-propanol; the samples were dried and stored under vacuum. No peroxides or aldehydes were detected in these polymeric sample^.'^^'^ Titration experiments (under Ar) with solutions containing 3
0022-365419512099-0475$09.00/0 0 1995 American Chemical Society
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476 J. Phys. Chem., Vol. 99, No. 2, 1995
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(nm) Figure 1. Changes in the absorption spectra with time in an aqueous solution containing 1 x M NaAuC4 and 3 x M PEG 20M. x M of purified polymers yielded the following concentrations of basic groups: 6.7 x M (PEG 1450), 2.4 x M M (PEG 8000), and 9.7 x M (PEG 3350), 9.6 x (PEG 20M). Typically, a colloid was synthesized by adding 2.5 mL of a 2 x M NaAuC4 solution to approximately 40 mL of a polymer solution. The volume was adjusted to 50 mL with water while stirring; the final concentrations were 1 x M NaAuC4 and 3 x M PEG. Experiments were conducted at room temperature and without ambient light to avoid photoreactions of the gold complex.18a Water from a Millipore (Milli-Q) system was used in all preparations, all glassware was cleaned with aqua regia. UV-vis spectra were measured using the high-rate scan mode (1200 nm min-') of a Hitachi U-2000 spectrophotometer. A Nicolet 5PC FTIR instrument was used to collect infrared spectra. X-ray diffraction (XRD) was carried out with a Siemens D5000 powder diffractometer.
Figure 2. Absorption spectra of colloids prepared by the reaction of M metal salt with 3 x 1x M Carbowax polymers.
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Results
Figure 3. Infrared spectra of the products from solutions containing M PEG 20M (-) and 3 x 1x M NaAuCL and 3 x
Figure 1 shows absorption spectra obtained at different times after mixing AuC14- ions with PEG 20M. A fast process took place initially since an absorption band was detected at I 1 500 nm after mixing the reagents. The initial process was completed in about 8 min, yielding a broad absorption band centered at 525 nm. This band became more intense, and I,= shifted to shorter wavelengths in a slow second reaction step, to reach a value of 518 nm after about 8 h. Only minor increases in absorption occurred thereafter; the resulting red colloid was stable for several months. Red colloids were also formed in solutions with 1 x M AuC14- and 3 x M PVA1. Metal particles were slowly generated in about 6 days, with a concurrent shift of Imaxfrom 584 to 520 nm. A similar but slower colloid formation was obtained using PVA2. Presented in Figure 2 are the absorption spectra of Ag and Pd colloids prepared in solutions of 1 x M Agf or PdCL2ions with 3 x M PEG 20M. Formation of the turbid and unstable yellow Ag colloid (I,= = 437 nm) was completed in 20 days. In contrast, the reduction of the PdCh2- ions was finished in 28 h. The resulting metal particles were stable for several weeks and exhibited the typical structureless spectrum of colloidal Pd.6b As in the case of Au, formation of the Pd colloids proceeded slowly for 4 days in solutions of PVA1. Figure 2 also shows a comparison of the absorption spectra of Au colloids prepared with the PEG polymers 20M and 8000. Formation of colloidal Au in solutions with PEG 8000 took place in about 11 h after an induction period of 10 min. The
surface plasmon band of this colloid was more pronounced than that of the Au-PEG 20M colloid, with an absorption maximum at 532 nm. Also, an initial red shift of I,, (504-565 nm) followed by a blue shift to 532 nm occurred in the reaction with PEG 8000. Slow particle formation and unstable colloids with broader plasmon bands were obtained in solutions of PEG 3350 and PEG 1450. Identification of products generated from the reaction of Au complexes and PEG 20M was attempted using a solution containing 3 x M PEG 20M and 1 x M AuC14ions. A higher [AuC14-] was employed in this experiment in order to increase the concentration of reaction products. The resulting solution was separated from the precipitated metal and extracted with CHC13 to yield a highly concentrated red Au colloid. Condensation of the vapors produced by evaporation of this colloid yielded a waxy transparent material.19 The infrared spectrum of a sample supported on a NaCl window is presented in Figure 3. Included in this figure is the spectrum of a powder obtained in a blank experiment without complexes; the signals correspond to PEG 20M. The sharp bands of 20M at 2888 and 1107 cm-' appeared as broad and split bands in the case of the waxy product. The latter bands are similar to those of diethylene glycol,2oimplying that 20M is fragmented during the reaction with Au complexes. Furthermore, the spectrum of the waxy product in Figure 3 shows bands at 1600 and 1414 cm-' corresponding to carboxylate groups.21 Obvi-
M PEG 20M (---).
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PEG Molar Mass (g mol-' x lo3) Figure 4. Effect of the polymer molar mass on the logarithm of the initial rate of reaction. The solutions contained 1 x M NaAuC4 and 3 x M polymer. ously, the PEG polymers were oxidized in the reaction with the metal complexes, since these bands were absent in the spectrum of 20M. The high concentrations of polymer in our colloids hampered determinations of particle diameters by TEM. However, powders obtained by drying the concentrated Au colloid in CHC13 showed broad Au reflections in XRD experiments. An average diameter of 17 nm was estimated from the line width of the signals using the Schemer equation. Changes in optical density at the initial Amm of the colloids were used to follow the kinetics of the particle formation reaction. Efforts to fit the data with simple kinetic models were not successful. Thus, initial rates of reaction (Ro) were used, which are expressed as the change in optical density per min (AOD min-'). Figure 4 shows the dependence of log(R0) on the molar mass of the PEG polymers. Ro increased from 8 x AOD min-' to 4 x AOD min-' when the molar mass changed from 1.45 x lo3 to 1.75 x lo4 g mol-'. Also, Ro increased linearly with [PEG 20M] in the concentration range 2.4 x to 3 x M. Unlike the other linear Carbowax polymers, PEG 20M is a branched polymer containing an epoxide linking agent.17b A few experiments were carried out with the linear PEG PS20 polymer, which has a molar mass similar to that of PEG 20M. The initial rate was lower with PS20 (Ro = 3 x AOD min-'); the first step was followed by a slower second step lasting for about 2 days. The basic groups that are present in the PEG polymers exerted an additional influence on the initial rate. For example, Ro decreased to 2.4 x AOD min-' when solutions containing 3 x M PEG 20M were titrated with HCl or HNO3 to a pH of 5.5 prior to addition of the gold complex. On the other hand, the same value of Ro = 3.4 x AOD min-' was obtained using solutions of PVAl with or without 9.7 x M NaOH, since these polymers exhibited no buffering capacity. The effect of colloidal gold on the particle formation process was investigated by mixing 5 mL of the Au-PEG 20M colloid with a solution of this polymer, followed by addition of AuC4ions as described in the Experimental Section. A fast process was observed with Ro = 5.8 x AOD min-'. Formation of Au colloids in degassed solutions of 3 x lop3M PEG 8000 was fast, with no induction period and Ro = 3 x AOD min-', as compared with an induction period of 10 min and Ro AOD min-' of air-saturated solutions. = 2.3 x In several systems (solutions of PEG PS20, titrated PEG 20M, or at high [PEG 80001) the particle formation process was slow enough to allow for the detection of reaction intermediates. A weak absorption centered at 383 nm was observed after 1 min of reaction in these systems. Partial decay of this absorption occurred in the next 2 min; completion of this process took place in about 5 min. Simultaneously, the surface plasmon
resonance of the Au particles was formed at 1 > 500 nm, followed by a strengthening of the crystallite absorption band and shifts of .,1 Thus, the initial species are precursors of the metal particles. Au atoms, Au(1) complexes, and Au(III) chloro-oxygen complexes absorb at A < 300 nm.6a*22 AuC4ions bound to PEG were ruled out because we failed to detect the absorption at 383 nm in solutions with 0.1 M NaCl, where the AuC4- ions are stable toward hydrolysis reactions. Interestingly, transient Au clusters with an absorption band at about 410 nm that decay to form colloidal Au have been observed in the reduction of Au(CN)2- ions.6a This implies that the initial species may correspond to Au particles containing only a few metal atoms. Metal particles were not generated in the absence of the polymers, even when concentrated solutions of AuCL- were prepared. Only a weak shoulder at about 319 nm due to the ligand-to-metal charge transfer (LMCT) of the AuC4- ions?2b shown initially in Figure 1, was detected in aqueous solutions of the complexes. Au crystallites were not formed in solutions containing 3 x M ethylene glycol or diethyl ether, but precipitated particles were generated in solutions of 1%crown6. Unlike the processes in basic methanol," the rate of particle formation was the same in Pyrex or quartz containers.
Discussion The evidence gathered in this investigation indicates that PEG polymers can be used for the transformation of several metal complexes to small metallic particles. From the results of Figure 1 it appears that an efficient reduction of the metal ions and stabilization of the resulting metal particles are achieved in solutions containing Au(III) complexes and PEG 20M. Stable Pd colloids are also generated under similar conditions, whereas the polymer was less effective in the case of Ag (Figure 2). PVA polymers are good stabilizers of small Au and Pd particles but are less efficient initiators of the particle formation process when compared with PEG polymers of similar molar masses. The surface plasmon resonance of the resulting Au colloids is displaced to longer wavelengths with decreasing molar mass of the polymers. Thus, the stabilizing ability of PEG macromolecules is reduced as the polymer molar mass decreases. The infrared data of Figure 3 imply that PEG 20M is fragmented in the reaction with the Au complexes to yield oxidation products such as carboxylic acids. Also, the rate of particle formation increases as the molar mass of the PEG polymers increases (Figure 4) and with increasing polymer concentration. Fragmentation of the polymers to yield a mixture of oxidation products, and increases in the reaction rates with increasing molar mass and polymer concentration occur in the high-temperature oxidation of PEG polymer^.'^^.^^ Hence, our results are consistent with the idea that particle formation is initiated via oxidation of the macromolecules by the complexes. A reduction of the metal complexes by organic peroxides is inconsistent with the faster particle formation in the absence of air and with the absence of peroxides ([peroxides] < loV6M) in the purified PEG polymers. While basic groups present in the polymers accelerate the formation of particles, a larger acceleration is induced by increases in the PEG molar mass. For instance, the initial rate is 2.4 x AOD min-' when Au colloids are prepared using titrated PEG 20M, whereas the polymer with the fewest basic groups (PEG 1450) gave an initial rate that is 3 orders of magnitude lower (Ro = 7.9 x AOD min-'). The large increases in Ro with increasing PEG molar mass imply that the reaction is more efficient with increasing ratios of (-OCH2CH2-) to terminal OH groups. This is consistent with the fact
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478 J. Phys. Chem., Vol. 99, No. 2, 1995 that AuC14- ions bind to oxyethylene groups of PEG polymers when coiled macromolecules form cavities similar to those of crown ethers.I4 Binding of Ag(1) and Pd(I1) complexes by the cavities is likely to occur since these ions bind to crown ether molecules.24 Reduction of bound Au(II1) ions proceeds via oxidation of the oxyethylene groups by the metal center through the overall reaction (AuCl,-) -PEG
-.
Au(1)
+ 4C1- + 2H' + oxidation products (1)
where (AuCL-)-PEG represents AuCL- ions bound to the pseudocrown ether structures. Oxidation of the oxyethylene group through this reaction disrupts the pseudocrown ether structure. The resulting Au(1) species will then be free to migrate to other cavities where equilibrium 2 occurs, 3 A u ( I ) t 2Au
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(2)
followed by coalescence of metal atoms with formation of Au clusters and by cluster growth to yield small metal particles. We assume that Au atoms are stabilized from oxidative reactions when formed inside the cavities. For this reason, and because reaction 1 is faster when cavities are more abundant, the particle formation process is faster with increasing number of cavities. Formation of the cavities increases for PEG polymers with higher molar mass15 and with increasing concentration of the polymers, which explains the kinetic data of Figure 4. Further support to this interpretation is provided by the low rates of reaction for polymers that do not form pseudocrown ether structures, such as poly(viny1 alcohols). Blue shifts of the plasmon band occur during the formation of particles in all systems, which are preceded by red shifts of A, in cases where the formation of Au particles is slow. Similar optical effects have been reported before.11J8b Efforts to relate the red shifts with an increase in particle size failed because the correlation between size and A- applies for large particles (daverage = 20-120 nm) with a sharp plasmon but we only observed broad bands. It is known that broadening and red shifts of the plasmon band of Au particles occur when the metal particles agglomerate to form particle networks.25 Thus, the similar optical effects observed in our systems suggest that networks of Au crystallites are formed in the solutions containing polymers. The blue shifts of Am= noticed later on are due to coalescence of the agglomerated particles to form larger crystallites with sharper plasmon bands located at shorter wavelengths.
Acknowledgment. We are grateful to Union Carbide for a gift of Carbowax samples. We thank D. Stanbury and C. Shannon for helpful discussions and the U S . Navy for supporting L.L. through the CIVINS program. This work was supported by the Strategic Defense Initiative Organization's Office of Innovative Science and Technology (SDIORNI)
through Contract N60921-91-C-0078 with the Naval Surface Warfare Center.
References and Notes (1) Apell, S. P.; Giraldo, J.; Lundqvist, S. Phase Trans. 1990,24,577. (2) Lewis, L. N. Chem. Rev. 1993,93, 2693. (3) Henglein, A. J. Phys. Chem. 1993,97, 5457. (4)(a) Turkevich, J.; Garton, G.; Stevenson, P. C. J. Colloid Sci. Suppl. 1954,I, 26. (b) Heard, S. M.; Grieser, F.; Barraclough, C. G.; Sanders, J. V. J. Colloid Interface Sci. 1983,93, 545. (5) Kreibig, U. In Contributions of Cluster Physics to Materials Science and Technology;Davenas, J., Rabette, P. M., Eds.; NATO AS1 Ser. E, No. 104; Martinus Nijhof Publishers: Dordrecht, 1986; p 373. (6) (a) Mosseri, S.;Henglein, A,; Janata, E. J. Phys. Chem. 1989,93, 6791. (b) Michaelis, M.; Henglein, A. J. Phys. Chem. 1992,96, 6791. (c) Gutierrez, M.; Henglein, A. J. Phys. Chem. 1993,97, 11368. (7) (a) Meisel, D.; Mulac, W. A,; Matheson, M. S. J. Phys. Chem. 1981,85, 179. (b) Lee, P. C.; Meisel, D. J. Catal. 1981,70, 160. (8) (a) Platzer, 0.;Amblard, J.; Marignier, J. L.; Belloni, J. J. Phys. Chem. 1992,96, 2334. (b) Amblard, J.; Platzer, 0.;Ridard, J.; Belloni, J. J. Phys. Chem. 1992,96, 2341. (9) (a) Mostafavi, M.; Keghouche, N.; Delcourt, M. 0.; Belloni, J. Chem. Phys. Lett. 1990, 167, 193. (b) Mostafavi, M.; Keghouche, N.; Delcourt, M. 0. Chem. Phys. Lett. 1990,169, 81. (10) Huang, Z.-Y.; Mills, G.; Hajek, B. J. Phys. Chem. 1993,97, 11542. (1 1) Quinn, M.; Mills, G. J. Phys. Chem. 1994,98, 9840. (12) (a) Siiman, 0.; Hsu, W. P. J. Chem. SOC. Faraday Trans. 1 1986, 82, 851. (b) Siiman, 0.; Lepp, A.; Kerker, M. Chem. Phys. Lett. 1983, 100, 163. (13) Hirai, H.; Toshima, N. TailoredMetal Catalysts; Iwasawa, Y., Ed.; D. Reidel: Dordrecht, 1986; p 121. (14) Warshawsky, A,; Kalir, R.; Deshe, A.; Berkovitz, H.; Patchornik, A. J. Am. Chem. SOC. 1979,101, 4249. (15) Bailey, F. E.; Koleske, J. V. Alkylene Oxides and Their Polymers; Marcel Dekker: New York, 1991; p 161. (16) Peroxide and aldehyde measurements were performed iodometrically and with the 2,4-dinitrophenylhydrrazinereagent, respectively." The following pH values were determined for solutions with 3 x M of the purified polymers: 6 (PEG 1450). 6.9 (PEG 3350), 7.7 (PEG SOOO), and 10 (PEG 20M). A pH of about 5.5 was typical of PEG-free solutions. (17) (a) Hahner, U.; Habicher, W. D.; Schwetlick, K. In Polymer Stabilization Mechanisms and Applications; Billingham, N. C.; Wiles, D. M., Eds.; Elsevier: Essex, 1991; p 111. (b) Harris, J. M. J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1985,C25, 325. (18) (a) Torigoe, K.; Esumi, K. Langmuir 1992,8, 59 and references therein. (b) Ishizuka, H.; Tano, T.; Torigoe, K.; Esumi, K.; Meguro, K. Colloids Surf: 1992,63, 337. (19) The PEG polymers sublime under vacuum at T 5 60 O C , and the sublimation temperature decreases with decreasing molar mass. (20) Pachler, K. G. R.; Matlok, F.; Gremlich, H.-U. Merck FT-ZR Atlas; VCH: Weinheim, 1988. (21) Kemp, W. Organic Spectroscopy; Macmillan: Hong Kong, 1991; p 60. An additional broad band at above 3000 cm-', that is typical of hydrogen-bonded -C02H groups, was detected using a microporous polyethylene film (from 3M) as the support for the waxy product. T h ~ s band was not detected when NaCl was the support due to a reaction of the acid groups with NaCl to yield ionic carboxylates. (22) (a) Roulet, R.; Lan, N. Q.; Mason, W. R.; Fenske, G. P. Helu. Chim. Acta 1973,56, 2405. (b) Peck, J. A,; Tait, C. D.; Swanson, B. I.; Brown, G. E. Geochim. Cosmochim. Acta 1991, 55, 671. (23) Lloyd, W. G. J. Polym. Sci. A 1963,1, 2551. (24) (a) Borgarello, E.; Pelizzetti, E.; Lawless, D.; Serpone, N.; Meisel, D. J. Phys. Chem. 1990,94,5048. (b) Keep, A. K. Coord. Chem Rev. 1993, 127, 99. (25) (a) Quinten, M.; Schonauer, D.; Kreibig, U. Z. Phys. D 1989,12, 521. (b) Schonauer, D.; Quinten, M.; Kreibig, U. Z. Phys. D 1989, 12, 527. JP942273K
