Langmuir 2001, 17, 7717-7720
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Formation of Colloidal Gold Nanoparticles in an Ultrasonic Field: Control of Rate of Gold(III) Reduction and Size of Formed Gold Particles Kenji Okitsu,*,† Akihiko Yue,‡ Shuji Tanabe,‡ Hiroshige Matsumoto,‡ and Yoshihiro Yobiko§ Japan Science and Technology Corporation, 4-1-8 Hon-cho, Kawaguchi, Saitama 332-0012 Japan, Faculty of Engineering, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan, and Technology Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi, Osaka 594-1157, Japan Received March 19, 2001. In Final Form: June 27, 2001 Gold(III) (tetrachloroaurate(III)) was reduced in an aqueous solution containing only a small amount of 2-propanol to form colloidal gold nanoparticles in a standing wave system generated by a 200 kHz ultrasonic generator. The rates of gold(III) reduction and the sizes of the formed gold particles could be sonochemically controlled by controlling the irradiation parameters such as the temperature of the solution, the intensity of the ultrasound, and the positioning of the reactor. The size of gold particles strongly depended on the rate of gold(III) reduction, suggesting that the rate of gold(III) reduction affects the initial nucleation of the gold particles.
1. Introduction The focus on colloidal metal particles has been on their application in a wide range of industries because of their interesting physicochemical properties that are quite different from those of the bulk states. Since the time of Faraday, who first reported the formation of stable gold colloids in the presence of gelatin in the 19th century, a variety of preparative methods with reduction processes of metal ions have been developed.1-7 In recent years, the size of the produced colloidal particles could be controlled by selecting a suitable reducing agent and stabilizer added in the solution: Various types of stabilizers such as citrate,2 poly(N-vinyl-2-pyrrolidone),3 triphenylphosphine,4 disulfide,5 dendrimer,6 etc., were used to govern the particle nucleation and growth processes in addition to using the most basal parameters, such as the initial concentration * To whom correspondence may be addressed: Technology Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi, Osaka 594-1157, Japan. E-mail:
[email protected]. † Japan Science and Technology Corporation. ‡ Nagasaki University. § Technology Research Institute of Osaka Prefecture. (1) (a) Faraday, M. Philos. Trans. R. Soc. London 1857, 147, 145181. (b) Eck, D.; Helm, C. A.; Wagner, N. J.; Vaynberg, K. A. Langmuir 2001, 17, 957-960. (2) (a) Enustun, B. V.; Turkevich, J. J. Am. Chem. Soc. 1963, 85, 3317-3328. (b) Henglein, A.; Giersig, M. J. Phys. Chem. B 1999, 103, 9533-9539. (3) (a) Hirai, H.; Nakao, Y.; Toshima, N. Chem. Lett. 1978, 545-548. (b) Teranishi, T.; Miyake, M. Chem. Mater. 1999, 11, 3414-3416. (c) Han, M. Y.; Quek, C. H. Langmuir 2000, 16, 362-367. (4) (a) Schmid, G. Chem. Rev. 1992, 92, 1709-1727. (b) Schmid, G.; Pugin, R.; Sawitowski, T.; Simon, U.; Marler, B. Chem. Commun. 1999, 1303-1304. (5) (a) Teranishi, T.; Haga, M.; Shiozawa, Y.; Miyake, M. J. Am. Chem. Soc. 2000, 122, 4237-4238. (b) Yonezawa, T.; Yasui, K.; Kimizuka, N. Langmuir 2001, 17, 271-273. (6) (a) Zhao, M.; Crooks, R. M. Angew. Chem., Int. Ed. Engl. 1999, 38, 364-366. (b) Esumi, K.; Suzuki, A.; Yamahira, A.; Torigoe, K. Langmuir 2000, 16, 2604-2608. (7) (a) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; ElSayed, M. A. Science 1996, 272, 1924-1926. (b) Chen, S.; Kimura, K. Langmuir 1999, 15, 1075-1082. (c) Hodak, J. H.; Henglein, A.; Hartland, G. V. J. Phys. Chem. B 2000, 104, 9954-9965. (d) Liu, J.; Ong, W.; Roman, E.; Lynn, M. J.; Kaifer, A. E. Langmuir 2000, 16, 3000-3002. (e) Teranishi, T.; Hosoe, M.; Tanaka, T.; Miyake, M. J. Phys. Chem. B 1999, 103, 3818-3827.
of the starting materials.7 Although much research has been performed in the field of colloid science, there are only a few reports that stated a clear relationship between the rates of reduction of noble metal ions and the size of the formed metal particles, because many experimental difficulties exist in the solution systems.8 In this Letter, locally generated multibubbles with an extremely high temperature and high pressure were used for the preparation of gold nanoparticles. The reduction of gold(III) with sonochemically formed radical species was performed in an aqueous solution, and the reduction process was quantitatively monitored during ultrasonic irradiation. Previously, we reported that the size of sonochemically formed palladium metal particles could be readily changed in the range of nanometer order by changing the types and concentrations of the organic additives such as the alcohol and stabilizer.9 In this study, the rate of gold(III) reduction and the size of the formed gold particles were controlled by selecting the parameters involving the irradiation conditions and procedures. In particular, identical starting materials were used in each preparation to understand the net effect of the rate of gold(III) reduction on the size of the formed gold particles. 2. Experimental Section Ultrasonic irradiation was carried out with an ultrasonic generator (Kaijo type 4021, 200 kHz frequency) operating at 20200 W and in a water bath maintained at a constant temperature.10 The details of the irradiation setup and the characteristics (8) For example, there would be the following difficulties in (I) the control of the rate of reduction in conventional thermal reduction systems, in (II) the precise determination of the rate of reduction of metal ions because the rate was often too fast to trace the course of the reduction, and in (III) the quantitative analysis for the concentration of unreduced metal ions coexisting with the formed colloidal particles and their stabilizer in a solution. (9) (a) Okitsu, K.; Yue, A.; Tanabe, S.; Matsumoto, H. Chem. Mater. 2000, 12, 3006-3011. (b) Okitsu, K.; Bandow, H.; Maeda, Y.; Nagata, Y. Chem. Mater. 1996, 8, 315-317. (10) Under the optimum irradiation condition in the present standing wave system (4.0 mm distance from the surface of oscillator to the bottom plane of vessel, 20 °C temperature of solution, 1.43 W/cm2 absorbed power), the rate of H2O2 formation during the sonolysis of pure water was estimated by Fricke dosimetry to be ca. 10 µM/min in an argon atmosphere.
10.1021/la010414l CCC: $20.00 © 2001 American Chemical Society Published on Web 11/08/2001
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of the reaction vessel are described elsewhere.9 The bottom of the vessel was planar, 1 mm thick and 55 mm in diameter. An aqueous solution of NaAuCl4 (1.0 mM, 65 mL) was added to the vessel which was then purged with argon gas. As an accelerator for the gold(III) reduction, a small amount of 2-propanol was injected into the solution, corresponding to 20 mM. Various irradiation parameters were selected, such as the temperature of the solution, the intensity of the ultrasounds, and the distance between the reaction vessel and the oscillator. During the course of the irradiation, the vessel was closed from the atmosphere. The concentration of gold(III) during the irradiation was determined by a colorimetric method using NaBr reagent.11
3. Results and Discussion The sites of the sonochemical reactions are characterized by three different regions in an aqueous solution system: (1) the inside of the collapsing cavitation bubbles where the high temperatures (several thousands of degrees) and high pressures (hundreds of atmospheres) are produced,12 here water vapor is pyrolyzed into H atoms and OH radicals; (2) the interfacial region between the cavitation bubbles and the bulk solution where the temperature is lower than the inside of the cavitation bubbles but still high enough for thermal decomposition of the solutes to occur, in addition, the concentrations of local OH radicals were reported to be very high in this region;13 (3) the bulk solution at ambient temperature where the reactions of the solute molecules with OH radicals or H atoms, which escaped from the interfacial region, take place. It has also been reported that the sonochemical reduction of gold(III) readily occurs in an aqueous solution by the addition of various organic compounds.11 In the presence of 2-propanol, the following reactions of (1) to (4) proceed
H2O f •OH + •H
(1)
(CH3) 2CHOH + •OH (•H) f (CH3) 2C˙ OH + H2O (H2) (2) (CH3) 2CHOH f pyrolysis radicals
(3)
gold(III) + reducing radicals f f gold(0)
(4)
where eqs 1-3 indicate the formation of several types of reducing radicals (H atoms, (1-hydroxymethyl) ethyl radicals, pyrolysis radicals) and eq 4 shows the reduction of gold(III). The pyrolysis of water and 2-propanol molecules occurs in the hot cavitation bubbles and at their surface.14 In addition, the details of eq 4 could be indicated as follows
gold(III) + reducing radical f gold(II)
(5)
gold(II) + reducing radical f gold(I)
(6)
gold(I) + reducing radical f gold(0)
(7)
2gold(II) f gold(III) + gold(I)
(8)
where eqs 5, 6, and 7 correspond to the reduction of gold ions and eq 8 is the disproportionation, respectively. In these reactions, gold(II) and gold(I) ions were not deter(11) Nagata, Y.; Mizukoshi, Y.; Okitsu, K.; Maeda, Y. Radiat. Res. 1996, 146, 333-338. (12) Didenko, Y. T.; McNamara, W. B.; Suslick, K. S. J. Am. Chem. Soc. 1999, 121, 5817-5818. (13) Gutierrez, M. S.; Henglein, A.; Ibanez, F. J. Phys. Chem. 1991, 95, 6044-6047. (14) Misik, V.; Riesz, P. Ultrason. Sonochem. 1996, 3, 173-186.
Figure 1. Changes in the concentration of gold(III) during ultrasonic irradiation: (O) 3.5 mm, (0) 4.0 mm, and (4) 4.5 mm distance between reaction vessel and oscillator; 20 mM 2-propanol, argon atmosphere, 20 °C temperature, 1.43 W/cm2 power,
mined in the irradiated solution by the colorimetric method, attributing that the rates of eqs 6-8 are suggested to be considerably high compared with eq 5; eq 5 is suggested to be a rate-determing step. Therefore, it was considered that the rate of gold(0) formation would be almost equal to the rate of gold(III) reduction. Figure 1 shows the changes in the concentration of gold(III) during the ultrasonic irradiation. The reduction of gold(III) rapidly occurred due to the irradiation, but the progress of reduction instantly stopped when the irradiation was turned off. These results indicate that the sonochemical reaction via reducing species takes place only during the irradiation, and any direct reduction with alcohol molecules would be negligible. As seen in Figure 1, the rate of gold(III) reduction clearly changed by the positioning of the reaction vessel, i.e., the distance from ultrasound oscillator to the bottom plane of the reaction vessel. The rates of gold(III) reduction obtained under various irradiation conditions are summarized in Figure 2. Figure 2a shows the effects of the distance from the oscillator to the bottom of the vessel. The maximum rate of reduction was seen at the distance of ca. 3.8 mm, which was in good agreement with the half-length of the ultrasound (3.71 mm) used in the present study. This phenomenon could be considered that the sound is effectively transmitted into the reaction vessel when the node plane formed in the standing wave system was just located at the bottom of the reaction vessel.15 Figure 2b indicates the effect of the bulk solution temperature. It was found that the rates of reduction sensitively changed despite a very small change in the temperature from 2 to 40 °C. The rate increased with increasing the temperature up to 20 °C, but it decreased at higher temperature. This would be considered as follows: In the range of 2-20 °C, as the temperature of bulk solution increases, the water and alcohol molecules are more easily vaporized in the cavitation bubbles, resulting in the effective formation of reducing radicals which promote the reduction. On the other hand, when the temperature of the solution is further increased, the temperature generated in the collapsing bubble does not elevate due to the cushion effect which was caused by the incorporation of an excess amount of water and alcohol molecules into the bubble.16 Thus, the amounts of reducing radicals decrease under such a high-temperature solution. This speculation was supported by the report of Henglein et al.17 who clearly indicated the cushion effect, i.e., the (15) Henglein, A. Ultrasonics 1987, 25, 6-16.
Letters
Langmuir, Vol. 17, No. 25, 2001 7719 Table 1. Rate of Gold(III) Reduction and Average Size and Standard Deviation of Sonochemically Formed Gold Particles under Various Irradiation Conditions ratea (µM‚min-1) 37.0 49.2 49.5 50.2 60.4 75.8 87.5
parametersb 40 °C 4.5 mm 30 °C 2 °C 10 °C 3.5 mm 4.0 mm
av size ( SD (nm) 71.5 ( 19.0 71.8 ( 16.4 58.3 ( 16.1 40.3 ( 11.4 41.6 ( 10.1 33.3 ( 8.5 35.5 ( 8.0
a Initial rate of reduction. b Changed parameter was only indicated. Basal parameters are as follows: temperature of solution, 20 °C; distance between reaction vessel and oscillator, 4.0 mm; intensity of the ultrasound, 1.43 W/cm2.
Figure 2. Rates of gold(III) reduction as a function of (a) the distance between reaction vessel and oscillator, (b) the temperature of the bulk solution, and (c) the absorbed power of the ultrasound: (a) 20 °C, 1.43 W/cm2; (b) 4.0 mm, 1.43 W/cm2; (c) 20 °C, 4.0 mm.
relation between the vapor pressure of the solvent and the apparent temperature generated in the collapsing bubbles. The rates of reduction were also dependent on the intensity of the ultrasound (Figure 2c). The adsorbed ultrasonic power plotted as the abscissa was estimated by calorimetric measurements.18 It was recognized that (16) According to the “hot spot theory (Neppiras, E. A. Phys. Rep. 1980, 61, 159)”, the maximum temperature in the collapsing bubble, Tmax, could be roughly determined by the equation Tmax ) T0Pmax(γ 1)/P, where T0 is the temperature of sample solution, Pmax is the maximal pressure in the liquid at the moment of transient collapse, γ is the ratio of specific heats, Cp/Cv, and P is the sum of the vapor pressure of the solvent and the atmospheric gas pressure. When the temperature of the bulk solution increases, Tmax becomes lower because the value of P increases with increasing the amount of water vapor inside the bubble. In addition, the value of γ also decreases because the γ value of water is substantially lower than that of argon gas (e.g., γ ) 1.67 for argon and ca. 1.3 for water vapor). Consequently, the maximum temperature in the cavitation bubbles drops with increasing the amount of water vapor inside the bubble. (17) Buttner, J.; Gutierrez, M.; Henglein, A. J. Phys. Chem. 1991, 95, 1528-1530. (18) Kimura, T.; Sakamoto, T.; Leveque, J.-M.; Sohmiya, H.; Fujita, M.; Ikeda, S.; Ando, T. Ultrason. Sonochem. 1996, 3, 157-161.
the smaller the intensity of the ultrasound, the slower the rate of reduction and, furthermore, the threshold value of the intensity existed for the gold(III) reduction. No reduction proceeded using a conventional ultrasonic cleaner (Honda Electric Co., W-113, 28 kHz, 100 W, ca. 2 L bath volume). These results also mean that the gold(III) reduction requires sufficiently hot bubbles which cause pyrolysis of the water and 2-propanol molecules. The temperatures and/or numbers of bubbles would increase with the increasing intensity in the present irradiation system. Upon irradiation, the color of the solution changed from initial yellow to reddish or bluish purple. UV-vis spectral analyses showed that the plasmon absorption peak attributed to the colloidal gold metal particles was recognized at around 530 nm and the peak position also changed by the irradiation parameters. The sonochemically formed gold metal was characterized using transmission electron microscopy (TEM) spectroscopy.19 Spherical gold particles were observed in each preparation. Their average sizes were ca. 30-70 nm with a relatively narrow size distribution. Since no stabilizer was added to this preparation, the particle sizes were slightly large compared to those prepared in the presence of suitable stabilizers.11 The average size, standard deviation (SD) of the formed gold particles, and the rates of gold(III) reduction are summarized in Table 1. Despite the differential irradiation conditions from each other, it was clearly confirmed that the size of the particles decreased with the increase in the rate of reduction. On the basis of the results in Figure 2c, it is found that the rate of gold(III) reduction is closely related to the absorbed ultrasonic power. Therefore, the rates of reduction in Table 1 would correspond to the strength of the cavitation effect. It could be presumed that the strength of the shock wave increases with increasing the rate of gold(III) reduction taking into account the cavitation phenomena. If the shock wave affected the size of the formed gold particles, the size of the particles should become bigger at stronger shock wave. The opposite tendency was seen in the results of Table 1: the size of particles would not be affected by the strength of the shock wave which corresponds to the rate of reduction. The results seen in Table 1 suggested that (19) Specimens for TEM observation were prepared as follows: Alumina powders with a nominal particle size of 0.5 µm were added in the irradiated solution to immobilize the formed gold particles on the surface of the alumina powders. This procedure was carried out to avoid the aggregation of gold particles under the subsequent drying processes. The suspension containing gold particles immobilized on the alumina powders was washed with distilled water. The powders were then dispersed in water again, which was dropped onto a Cu grid coated with a colloidion film and dried in a vacuum. Observations were performed using a JEOL JEM-100S electron microscope at an acceleration voltage of 100 kV.
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the shock wave generated in the present irradiation system was extremely weak.20 Assuming the formation of monodispersed gold nanoparticles with a spherical shape, the material balance is shown as eq 9
(4/3) π (D/2)3F N ) CM
(9)
where D, F, N, C, and M are the diameter of the formed particle (m), the density (kg‚m-3), the number of particles (m-3), the initial concentration of the metal ions (mol‚m-3), and the molecular weight (kg‚mol-1), respectively. Thus, D can be expressed as follows
D ) (6CM/πFN)1/3
(10)
The total number of formed gold particles (N) is correlated to the number of formed nuclei, which should also correspond to the rate of gold(III) reduction (dC/dt). Therefore, eq 10 could be roughly expressed as follows
D ∝ (6CM/πF)1/3‚(dC/dt)-1/3
(11)
Equation 11 is in good agreement with the present experimental results that the size of the formed gold particles decreases with the increasing rate of reduction. This fact implies that the rate of gold(III) reduction strongly affects the nucleation process of the gold particles. The cavitation bubbles with an extremely high temperature and high pressure were locally generated by ultrasonic irradiation in solution. However, their size and lifetime were considerably small (several micrometers) and short (less than a sub-microsecond), respectively. Thus, the radical reactions that proceeded in the bulk solution could be regarded as a very mild condition at (20) As a preliminary experiment to estimate the strength of the shock wave, the effect of cavitation on an immersed aluminum foil was investigated. The result showed that no pit attributed to cavitation damage was observed, suggesting that the mechanical effect due to a shock wave would be relatively weak in the present 200 kHz sonication system.
around room temperature.21 Analogous to this, it could be considered that the formation of gold particles occurs in the bulk solution at such mild temperatures. Ultrasound is generally used to disperse or agglomerate of some particles or powders in solution by use of its mechanical effect (microjet impact and shock wave), but the behavior of colloidal nanoparticles under sonication have not yet been clarified. The mechanical effect due to a shock wave would be relatively weak in the present 200 kHz sonication system9a,20,22 because of the application of a high frequency in comparison with a 20 kHz horn-type ultrasonic generator. This speculation was supported by the previous experimental results,9b,11 in which various types of nanoparticles could be formed successfully without any aggregation by ultrasonic irradiation in the presence of stabilizers. This was also supported by the results in Table 1 that the size of the gold particles gradually increased with the decreasing rate of gold(III) reduction, although their experimental parameters are different. Taking into account the weak mechanical effect, we consider that the formation process of gold colloids in the present system is similar to that in a conventional preparation. It has been believed that the chemical effects of ultrasounds do not come from a direct interaction between the sound field and molecular species.23 When high frequency is used, the effect of the ultrasounds on the nucleation in the nanometer region might be quite small. In other words, the effect of ultrasonic cavitation on colloidal dispersion systems would be somewhat different in the size of the colloidal particles between nano- and micrometer ranges. LA010414L (21) There are two reasons why we regard radical reactions in the present study as a mild condition. First, gold(III) ions are nonvolatile and therefore would not be expected to enter the gas phase of the hot bubbles. Second, according to the Gibbs adsorption equation, gold(III) ions could not be accumulated at the interface of the bubbles. On the basis of these reasons, the reduction of gold(III) ions with reducing radicals, which escaped from the hot bubbles, proceeds in the bulk solution at ambient temperature. These radical reactions that occurred in the bulk solution are also similar to the radiation chemistry, in which radical reactions rapidly proceed at an ambient temperature. (22) Okitsu, K.; Yue, A.; Tanabe, S.; Matsumoto, H. In preparation. (23) (a) Mason, T. J.; Lorimer, J. P. Sonochemistry: Theory, Applications and Uses of Ultrasound in Chemistry; Ellis Horwood, Ltd.: Chichester, U.K., 1988. (b) Crum, L. A.; Mason, T. J.; Reisse, J. L.; Suslick, K. S. Sonochemistry and Sonoluminescence; Kluwer Academic Publishers: Dordrecht, 1999. (c) Thompson, L. H.; Doraiswamy. L. K. Ind. Eng. Chem. Res. 1999, 38, 1215-1249.
