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Preparation of Platinum Nanoparticles by Sonochemical Reduction of the Pt(II) Ion Yoshiteru Mizukoshi,*,† Ryuichiro Oshima,‡ Yasuaki Maeda,† and Yoshio Nagata‡ Department of Applied Materials Science, Faculty of Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Sakai, Osaka 599-8531, Japan, and Research Institute for Advanced Science and Technology, 1-2, Gakuen-cho, Sakai, Osaka 599-8570, Japan Received September 9, 1998. In Final Form: February 1, 1999 Platinum nanoparticles were prepared in an aqueous system using high-intensity ultrasound (200 kHz, 6 W cm-2). The particles formed in the presence of a surfactant (sodium dodecyl sulfate, SDS) were stable, homogeneously spherical, and relatively monodispersed with an average 2.6 nm diameter. The rate of formation of the platinum nanoparticles was 26.7 µM min-1 in the Pt(II)-SDS system. Reducing species generated near and/or in the hot bubbles, which were sonochemically induced in the media, would react with the PtCl42- complexes to form the platinum nanoparticles. Three kinds of the reducing species were proposed to be formed in the sonicated system: (a) radicals formed from the thermal decomposition of SDS at the interfacial region between the cavitation bubbles and bulk solution; (b) radicals formed via reactions of the hydroxyl radicals or hydrogen atoms with SDS; (c) hydrogen atoms. During the reduction of the Pt(II) ion, (b) and (c) ((b) > (c)) may be effective while (a) is only slightly effective, whereas in the cases of gold and palladium nanoparticles (a) was the main reductive species.
Introduction Metallic nanoparticles are quite attractive because of their physicochemical characteristics such as catalytic activity, optical properties, electronic properties, and magnetic properties.1 In particular, because of their high catalytic activity, the preparation methods of platinum nanoparticles and their characterizations have been widely investigated for a long time.2 The studies on the use of high-intensity ultrasound for the preparation of organic and inorganic materials have been on the increase. For example, the sonochemical preparations of amorphous iron,3 fullerenes,4 and latex particles5 have been reported. However, research focusing on the action of the reduction induced by ultrasound is still limited.6-13 Acoustic cavitation, namely the formation, growth, and implosive collapse of tiny bubbles in the * To whom correspondence should be addressed. † Osaka Prefecture University. ‡ Research Institute for Advanced Science and Technology. (1) Schmid, G., Ed. Colloids and Clusters; VHC Press: New York, 1995. (2) (a) Aika, K.; Ban, L. L.; Okura, I.; Namba, S.; Turkevich, J. J. Res. Inst. Catal., Hokkaido Univ. 1976, 24, 54. (b) Henglein, A.; Ershov, E. G.; Malow, M. J. Phys. Chem. 1995, 99, 14129. (3) Suslick, K. S.; Choe, S. B.; Chicowlas, A. A.; Grinstaff, M. W. Nature 1991, 353, 414. (4) Katoh, R.; Yanase, E.; Yokoi, H.; Usuba, S.; Kakudate, Y.; Fujishima, S. Ultrasonics Sonochem. 1998, 5, 37. (5) (a) Biggs, S.; Grieser, F. Macromolecules 1995, 28, 4877. (b) Copper, G.; Grieser, F.; Biggs, S. Chem. Austr. 1995, 62, 27. (6) Nagata, Y.; Watanabe, Y,; Fujita, S.; Dohmaru, T.; Taniguchi, S. J. Chem. Soc., Chem. Commun. 1992, 1620. (7) Nagata, Y.; Mizukoshi, Y.; Okitsu, K.; Maeda, Y. Radiat. Res. 1996, 146, 333. (8) Okitsu, K.; Bandow, H.; Maeda, Y.; Magata, Y. Chem. Mater. 1996, 8, 315. (9) Okitsu, K.; Mizukoshi, Y.; Bandow, H.; Maeda, Y.; Yamamoto, T.; Nagata, Y. Ultrasonics Sonochem. 1996, 3, 249. (10) Okitsu, K.; Mizukoshi, Y.; Bandow, H.; Yamamoto, T. A.; Nagata, Y.; Maeda, Y. J. Phys. Chem. B 1997, 101, 5470. (11) Yeung, S. A.; Hobson, R.; Biggs, S.; Grieser, F. J. Chem. Soc., Chem. Commun. 1993, 378. (12) Gutie´rrez, M.; Henglein, A. J. Phys. Chem. 1987, 91, 6687. (13) Mizukoshi, Y.; Okitsu, K.; Maeda, Y.; Yamamoto, T. A.; Oshima, R.; Nagata, Y. J. Phys. Chem. B 1997, 101, 7033.
medium is believed to play a major role in the preparation of these materials. To our knowledge, no other studies have been reported comparing the physicochemical properties of sonochemically prepared metallic particles with those made by other methods. In this paper, we reported that Pt(II) ions were sonochemically reduced to form platinum nanoparticles. In the sonicated system, three kinds of reducing species may be generated from the direct sonolyses of solvent water or solute sodium dodecyl sulfate (SDS) in and/or near hot bubbles, or the subsequent reaction between these radicals and SDS.7-10 The contributions of these species to the reduction were also discussed. With a comparison of the sonochemically prepared products to the radiochemically made ones, the characteristic properties of the reaction site induced by sonication are also evaluated. Experimental Section Reagent-grade K2PtCl4, NaI, KCl, and sodium dodecyl sulfate (SDS) were purchased from Wako Chemicals and used without further purification. 2-Methyl-2-propanol (tert-BuOH) from Wako Chemicals was purified by distillation. Argon (99.999% purity) was purchased from Osaka Sanso. Water was treated using a Millipore system (Milli-Q). The photoabsorption spectra of the metal ions and colloidal dispersions were measured by a spectrometer (Shimadzu UV3100). The determination of hydrogen was carried out using a gas chromatograph (Hewlett-Packard 5890 equipped with a thermal conductivity detector). A multiwave ultrasonic generator (Kaijyo 4021) and a barium titanate oscillator of 65 mm diameter were used for the ultrasonic irradiation and were operated at 200 kHz with an input power of 200 W. A sample solution of 60 mL was sonicated in a cylindrical glass vessel with a 50 mm inside diameter and total volume of 150 mL. The vessel had a sidearm with a silicone rubber septum for gas bubbling and sample extraction without exposing the sample to air. The vessel was mounted at a constant position relative to a nodal plane of the sound wave (3.75 mm: half the length of ultrasound wave from the oscillator). The bottom of the vessel was planar and was made as thin as possible (1 mm) because transmission of the ultrasonic waves decreases with
10.1021/la9812121 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/19/1999
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Figure 2. Change in the absorption spectra of a solution of 1 mM K2PtCl4 and 8 mM SDS with sonication time. (1.5 mL), the maximum absorption peaks of the Pt(II) complex shifted to 388 nm (4600 M-1 cm-1) and 332 nm (7600 M-1 cm-1) from 380.5 nm (46 M-1cm-1) and 317.5 nm (85 M-1cm-1), respectively, probably because of the ligand exchange reaction.
PtCl42- + 4I- f PtI42- + 4Cl-
Figure 1. Typical ultrasonic irradiation setup. V, valve; C, clamp; P, septum; I, water inlet; S, sample solution; O, oscillator; D, water outlet. increasing thickness of the bottom. The argon-purged aqueous solution of K2PtCl4 and SDS was sonicated. During the irradiation the vessel was closed. The sonication was carried out in a temperature-controlled water bath (about 20 °C). Figure 1 shows a typical sonication setup. For the evaluation of the intensity of the ultrasounds, water was sonicated and the rate of formation of hydroxyl radicals was measured. The measurements of the hydroxyl radicals were carried out by adding an Fe(II) aqueous solution to the irradiated water and the amount of Fe(III) ions ( ) 2194 M-1 cm-1, λ ) 304 nm at 298 K) formed from the H2O2 oxidation of Fe(II) was estimated by a spectrometric method.
H2O f H• + •OH
(1)
H• + H • f H 2
(2)
OH + •OH f H2O2
(3)
2Fe(II) + H2O2 f 2Fe(III) + 2OH-
(4)
•
From the yield of Fe(III) ions, the rate of formation of hydroxyl radicals was estimated at R(•OH) ) 2R(H2O2) ) R(Fe(III)) under argon. The rate of formation of hydroxyl radicals was estimated to be 25 µM min-1 under argon. However, reaction 5 did take place; therefore, some of the radicals formed were consumed by the reaction, so the net value of R(•OH) may be somewhat larger than the estimated value.
H• + •OH f H2O
(5)
As a comparative experiment involving the hydroxyl radical reaction, γ-ray irradiation was performed using 370 TBq 60Co. An N2O-saturated sample solution in a cylindrical glass vessel (35 mm inner diameter) was irradiated at a dose rate of 2.5 kGy h-1 (determined on the basis of the same formation rate of hydroxyl radicals as in the sonolysis). For the determination of the concentration of Pt(II), an improved colorimetric method using NaI was employed.14 When NaI solution (4.32 M, 0.15 mL) was added to the sonicated solution
(6)
At the same time, the platinum particles aggregated and it became possible to separate the aggregates by filtration through a membrane filter (0.2 µm pore size). By this procedure, Pt(II) ions can be spectrometrically determined without interference from the surface plasmon absorption of the platinum particles. The amount of zerovalent platinum formed by the irradiation was estimated from the reduced amount of Pt(II) ions. Specimens for transmission electron microscopy (TEM) were prepared by drying droplets of colloidal dispersions on a carbonsupported copper mesh in a vacuum. Observations were carried out with an electron microscope (JEOL JEM-200CX) operated at 200 keV. The sizes of more than 200 particles were measured on the enlarged micrographs to obtain a size distribution. Samples for the X-ray diffraction measurements were prepared by adding KCl to the colloidal dispersions, and the resulting Pt aggregates were filtered out, rinsed with pure water, and dried in a vacuum.
Results and Discussion Mechanism of Reduction of the Pt(II) Ion. Figure 2 shows the change in the absorption spectra of the K2PtCl4-SDS aqueous solution during the sonication under argon.15 The color of the solution changed from pale yellow to dark brown during the sonication. At the same time, the scattering of the light from a He-Ne laser was observed. During the sonication process, weak absorption peaks at 380.5 and 317.5 nm, characteristic of the PtCl42complex in aqueous solution, decreased and the nonstructured absorption band from the ultraviolet to the visible region assigned to platinum particles16 appeared. The dispersion systems of the obtained colloidal platinum were stable for more than several months. (14) Mizukoshi, Y.; Okitsu, K.; Bandow, H.; Nagata, Y.; Maeda, Y. Bunseki Kagaku 1996, 45, 327. (15) A K2PtCl4 aqueous solution was initially prepared as a stock solution and aged from several hours to several weeks. In dilute aqueous solution, PtCl42- complexes are transformed into mono- or diaquated complexes to a large degree via the following equations (e.g., a 1 mM solution of K2PtCl4 at equilibrium contains only 5% of PtCl42- with 53% of monoaquated and 42% of diaquated species at 25 °C) (Cotton, F. A.; Willkinson, G. Advanced Inorganic Chemistry, 5th ed.; John-Wiley: New York, 1988; p 919).
PtCl42- + H2O f PtCl3(H2O)- + Cl-
PtCl3(H2O)- + H2O f PtCl2(H2O)2 + Cl(16) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881.
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of the interior of the bubbles; however, it is still high enough for thermal decomposition of the solutes to take place. In addition, greater local hydroxyl radical concentrations in this region have been reported.18 (C) The bulk solution at ambient temperature. The reactions of solute molecules with hydrogen atoms or hydroxyl radicals take place here. In addition to reactions 1-3, the following reactions are proposed to explain the generation of reducing species, the reduction of noble-metal ions, and the aggregation of zerovalent-metal species into particles
Generation of reducing species:
Figure 3. Absorption spectra of a sonicated (0-60 min) and NaI-treated K2PtCl4 solution.
RH f R′ (radical a)
(7)
OH(H) + RH f R′′ (radical b) + H2O(H2)
(8)
Reduction of platinum ions: R′(or R′′) + Pt(II or I) f Pt(I or 0) + H+ + X (9) /2H2 + Pt(II or I) f Pt(I or 0) + H+
(10)
H• + Pt(II or I) f Pt(I or 0) + H+
(11)
1
Aggregation of zerovalent species: nPt(0) f Pt(0)n
Figure 4. Time profile of the concentration of the Pt(II) ion versus irradiation time. K2PtCl4, 1 mM; SDS, 8 mM. (O) sonication under Ar; (4) sonication under Ar, t-BuOH, 20 mM; (b) γ-radiation (dose rate: 2.5 kGy h-1).
Figure 3 shows the absorption spectra of the same solution employed in Figure 2 after the addition of NaI solution and subsequent filtration. The rate of reduction of Pt(II) ions, in other words, the rate of formation of platinum particles, could be estimated from the figure. Figure 4 shows the change in the concentrations of Pt(II) ions versus the sonication time. The rate of reduction of Pt(II) ions was found to be 26.7 µM min-1 and was slower than those of the Au and Pd ions (Au, 83 µM min-1; Pd, 140 µM min-1, these rates were measured under the same conditions employed in the case of Pt).7-8 The time dependence of the reduction of the Pt(II) ion by γ-radiolysis under N2O was also shown in the figure. The time profile of the concentration of the Pt(II) ion in the sonolysis and γ-radiolysis experiments were close to each other. This suggests that almost the same reactions occur in both cases. In sonochemistry, three kinds of reaction sites should be considered.17 (A) The interior of the collapsing cavitation bubbles. High temperatures (several thousand degrees) and high pressures (several hundred atmospheres) are generated here. Here, water vapor is pyrolyzed into hydrogen atoms and hydroxyl radicals (reaction 1). (B) The interfacial region between the cavitation bubbles and the bulk solution. The temperature is lower than that (17) (a) Mason, T. J., Ed. Advances in Sonochemistry; JAI Press: London, 1990; Vol. 1. (b) Mason, T. J., Ed. Advances in Sonochemistry; JAI Press: London, 1991; Vol. 2 (c) Mason, T. J., Ed. Advances in Sonochemistry; JAI Press: London, 1993; Vol. 3.
(12)
where RH and X denote SDS and the stable compound formed through the redox reaction 9, respectively. Reaction 7 is induced in region B by sonication (reaction, in region A is negligible because of the low vapor pressure of SDS). The zerovalent species of platinum generated through reactions 9-11 seem to be aggregated and form nanoparticles in reaction 12. In the sonicated system containing SDS, three kinds of reductants should be considered: radical a formed from the direct thermal decomposition of the surfactant in the interfacial region between the cavitation bubbles and bulk solution (reaction 7); radical b formed via reactions of hydroxyl radicals or hydrogen atoms with the surfactants (reaction 8); hydrogen atoms (formed via reaction 1). On the other hand, during γ-radiolysis, reducing radical a could not be produced because of the absence of the extreme conditions induced by cavitational collapse. Generated hydrated electrons during γ-radiolysis were scavenged by N2O and converted into hydroxyl radicals under an N2O atmosphere.
H2O f H• + •OH + eaq-
(13)
eaq- + N2O +H2O f N2 + •OH + OH-
(14)
During the preparation of the gold and palladium particles,7-8 the rates of the sonochemical reduction were much faster than those during γ-radiolysis. These comparative experiments show that reducing radicals a as well as radicals b and hydrogen atoms contributed to the reduction of the corresponding noble-metal ions in the sonochemical method. In the case of platinum, however, the same comparison suggests that reducing radicals a did not play an important role in the sonicated reduction as shown in Figure 4. When the K2PtCl4-SDS aqueous solution containing tert-BuOH was sonicated, the rate of reduction of the Pt(II) ion was significantly depressed from 26.7 µM min-1 (18) Gutie´rrez, M.; Henglein, A.; Ibanez, F. J. Phys. Chem. 1991, 95, 6044.
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Figure 6. XRD pattern of sonochemically prepared platinum nanoparticles. Dashed lines denote the peak positions of pure platinum for reference.
Figure 5. TEM photographs of platinum nanoparticles: (a) prepared by sonochemical method; (b) prepared by radiolytical method.
(without tert-BuOH) to 10.0 µM min-1 (Figure 4). tertBuOH, known as an effective radical scavenger, readily reacts with hydrogen atoms or hydroxyl radicals.
(CH3)3COH + •OH(H•) f •CH2(CH3)2COH + H2O(H2) (15) Under this condition, only a slight amount of radical b, which played the most important role in the reduction, was generated from SDS because of the scarcity of hydrogen atoms and hydroxyl radicals. These results support our proposals for the mechanism of reduction of Pt(II) ions. Reaction 10 appears to hardly occur based on the result that no change was observed when the short-period sonicated sample solution containing unreduced Pt(II) ions was left for a long time under a dilute hydrogen atmosphere formed by the sonolysis of water. Without SDS, the rate of reduction was very small. In this case, it was thought that the only reductants generated in the sonicated system are hydrogen atoms from the sonolysis of water molecules, and that their contribution to the reduction of the Pt(II) ion is not significant. In general, reducing species heterogeneously exist and they mostly accumulated in the vicinity of the collapsing bubbles. The lifetime of the cavitation bubble is said to be
on a microsecond order,19 which is a much longer time than that of the diffusion of noble-metal complexes in an aqueous system. Therefore, the reactions of all the noblemetal complexes used in this series of investigations (PdCl2, AuCl4-, and PtCl42-) would proceed under the same conditions. It was thought that the reaction rates are determined by the reactivities between the noble-metal complexes and reductants. It is reported that the redox potential of PdCl2/Pd0, AuCl4-/Au0, and PtCl42-/Pt0 are 0.99, 1.002, and 0.73 V, respectively.20 Radical a seemed to be less reactive and could not reduce PtCl42-, which has the lowest redox potential. As a result, the reduction rate of the Pt(II) ion would be slower than that of the Au(III) and Pd(II) ion. Characterization. During the reductive preparation of metal particles, the rate of reduction of noble-metal ions would affect the morphologies of the products, such as shape, diameter, and size distribution. As shown in Figure 4, the sonochemical and radiochemical preparation methods of platinum nanoparticles have no significant difference in the rate of reduction of Pt(II) ions. Both methods are very similar and comparable except for the occurrence of cavitational collapse induced by sonication. It is found that these methods are the most suitable cases for comparing the products in order to examine the ultrasonic characteristics. In Figure 5, the TEM micrographs of the sonochemically and radiochemically prepared platinum nanoparticles are shown. From these micrographs, the average diameters of the sonochemically and radiolytically prepared platinum particles were 2.6 ( 0.9 and 23.6 ( 7.7 nm, respectively. Pt particles prepared by the sonochemical method were very small and it was difficult to observe them in the bright field images of our TEM because of their low contrast. Therefore, dark field images were employed. The sonochemically induced Pt particles were nearly spherical in shape and well-dispersed. On the other hand, the radiolytic ones were not spherical, but irregularly flakelike-shaped with a diameter larger than the sonochemical ones. It was suggested that the morphologies of the particles probably depend on the ultrasonic effect. In both cases, the formation of zerovalent platinum proceeded at almost the same rates. It was thought that the ultrasound method might contribute to the aggregation process of the zerovalent platinum (reac(19) Suslick, K. S.; Hammerton, D. A.; Cline, R. E., Jr. J. Am. Chem. Soc. 1986, 108, 5641. Barber, B. P.; Putterman, S. J. Nature 1991, 352, 318. (20) Fujishima, A.; Aizawa, M.; Inoue, T. Denki Kagaku Sokuteihou; Gihodo Publ. Co.: Tokyo, 1984; Vol. 2.
Platinum Nanoparticles by Sonochemical Reduction
tion 12). For example, it was reported that collisions and subsequent sintering of the transition-metal particles were induced by the shock wave which was generated in the cavitational collapse process.21 However, detailed explanations cannot be given at the present time. Figure 6 shows the XRD pattern obtained from the platinum particles prepared by the sonochemical method. The positions of the diffraction peaks agreed with those (21) (a) Suslick, K. S. MRS Bull. 1995, 20, 29. (b) Suslick, K. S. Science 1990, 247, 1067.
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of pure platinum (shown in Figure 6 as dashed lines). This pattern shows that the particles consist of pure platinum crystallites. Acknowledgment. This study was financially supported by Special Coordination Funds for Promoting Science and Technology from the Japanese Science and Technology Agency. LA9812121
