20673
2005, 109, 20673-20675 Published on Web 10/15/2005
Sonochemical Synthesis of Gold Nanoparticles: Effects of Ultrasound Frequency Kenji Okitsu,*,† Muthupandian Ashokkumar,*,‡ and Franz Grieser‡ Graduate School of Engineering, Osaka Prefecture UniVersity, Gakuen-cho, Sakai, Osaka 599-8531, Japan, and Particulate Fluids Processing Centre, School of Chemistry, UniVersity of Melbourne, VIC 3010, Australia ReceiVed: September 1, 2005; In Final Form: September 23, 2005
The rate of sonochemical reduction of Au(III) to produce Au nanoparticles in aqueous solutions containing 1-propanol has been found to be strongly dependent upon the ultrasound frequency. The size and distribution of the Au nanoparticles produced can also be correlated with the rate of Au(III) reduction, which in turn is influenced by the applied frequency. Our results suggest that the rate of Au(III) reduction as well as the size distribution of Au particles are governed by the chemical effects of cavitation and are not significantly affected by the physical effects accompanying ultrasound-induced cavitation.
Introduction Sonochemical synthesis of various types of nanoparticles and nanostructured materials composed of noble metals,1-3 transition metals,4,5 semiconductors,6 carbon materials,7 and polymeric materials8 have received much attention in recent years. This is due to the unique reaction routes induced by acoustic cavitation in solution, which provides extreme conditions of transient high temperature and high pressure within the collapsing bubbles, shock wave generation, and radical formation.9 It is reported that a number of factors influence cavitation efficiency and the chemical and physical properties of the products. The dissolved gas, ultrasonic power and frequency, temperature of the bulk solution, and type of solvent are all important factors that control the yield and properties of the synthesized materials. To our knowledge, there is no comprehensive report addressing the frequency effects for the synthesis of nanoparticles and nanomaterials. This is because several reaction conditions are simulatenously changed (see results and discussion) when the applied frequency is changed. Therefore, to date it has been quite difficult to elucidate the effects of ultrasound frequency on sonochemical reaction rates and efficiencies. In this paper, the sonochemical reduction of Au(III) to produce Au nanoparticles was performed in the presence of a small amount of 1-propanol at different frequencies. The relationship between the ultrasound frequency and the rate of Au(III) reduction or the size of the formed Au nanoparticles are presented for the first time. In addition, we also discuss the mechanical effects of cavitation on the properties of sonochemically formed Au nanoparticles in aqueous solutions. Experimental Section Ultrasonic irradiation was carried out with two types of ultrasound irradiation systems: a horn type sonicator (Branson * Corresponding authors. E-mail:
[email protected].; masho@ unimelb.edu.au. † Osaka Prefecture University. ‡ University of Melbourne.
10.1021/jp0549374 CCC: $30.25
450-D, frequency: 20 kHz, diameter of Ti tip: 19 mm) and a standing wave sonication system with a series of transducers operating at different ultrasound frequencies (L-3 Communications ELAK Nautik GmbH, frequency: 213 kHz, 358 kHz, 647 kHz, 1062 kHz, diameter of oscillator: 55 mm). An aqueous solution of HAuCl4 (0.2 mM, 70 mL for 20 kHz, 200 mL for the others frequencies) was added to the reaction vessel and then purged with argon. As an accelerator for the Au(III) reduction, 1-propanol was injected into the solution. All experiments were performed with a constant ultrasonic power 0.1 ( 0.01 J s-1 mL-1 (0.1 ( 0.01 W mL-1), measured by calorimetry. During the irradiation, the solution temperature was maintained constant by circulating water at 21 ( 2°C through a jacket around the sonication cell. An argon atmosphere was maintained above the solution during irradiation. After irradiation, small volumes of the sonicated solutions were drawn from the cell. These solutions were added to a known volume of 1 mass% PVP solution to prevent clustering of the formed gold particles. The concentration of Au(III) during the irradiation was determined by a colorimetric method using NaBr reagent.10 The synthesized Au nanoparticles were analyzed using a transmission electron microscope (Philips CM10, TEM) at an acceleration voltage of 100 kV. Results and Discussion The sonochemical reduction of Au(III) ions has been studied extensively by several groups.2,10-12 As has been reported in the literature, the color of the Au(III) solution changed from an initial pale yellow to reddish purple with increasing time of ultrasonic irradiation. The UV-visible spectral analyses clearly indicated that a surface plasmon absorption attributed to the colloidal Au particles had emerged. This observation corresponds to the progress of the Au(III) reduction and the formation of Au nanoparticles. Figure 1 shows the initial rates of Au(III) reduction measured with five different frequencies. It can be readily recognized that the initial rates of reduction are significantly influenced by the frequencies. It can be observed in Figure 1 that, except for 20 kHz, the rate of reduction © 2005 American Chemical Society
20674 J. Phys. Chem. B, Vol. 109, No. 44, 2005
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Figure 1. Rate of Au(III) reduction as a function of ultrasound frequency. Au(III): 0.2 mM, 1-propanol: 20 mM, atmosphere: Ar, ultrasonic power: 0.1 W mL-1.
decreases with increasing frequency (213 kHz > 358 kHz > 647 kHz > 1062 kHz > 20 kHz). It has been reported2,10-12 that the sonochemical reduction of Au(III) in the presence of an organic additive proceeds via the following reactions (reactions 1-4),
H2O f •OH + •H
(1)
RH + •OH (•H) f •R + H2O (H2)
(2)
RH f pyrolysis radicals and unstable products
(3)
Au(III) + reducing species (•H, •R, etc.) f Au(0) (4) where RH denotes an organic additive. Reactions 1-3 indicate the sonochemical formation of the reducing radicals and reductants: (1) •H is formed from sonolysis of water, (2) •R and H2 are formed from the abstraction reaction of RH with •OH or •H, and (3) pyrolysis radicals and unstable products are formed via pyrolysis of RH and water. Finally, the reduction of Au(III) proceeds by the reaction with various reducing species and involves a number of complex reaction steps.12 Reactions 1-4 suggest that the rate of Au(III) reduction should strongly correlate with the amount of primary and secondary reducing radicals generated at different frequencies. Hence, the observed frequency trend in the rate of reduction of Au(III) ions can be expected to have a correlation with the cavitation efficiency at these frequencies. A change in the ultrasound frequency may affect one or many of the following factors: (1) the temperature and pressure inside the collapsing cavitation bubbles, (2) the number and distribution of bubbles, (3) the size and lifetime of bubbles, (4) the dynamics and symmetry (shape) of the bubble collapse, (5) the effect of 1-propanol on bubble temperature, secondary radical formation, etc. It is quite difficult to quantify the individual effects of these factors because they are interdependent, and it is almost impossible to control one factor without affecting the others. There are only a few reports that describe a reasonable and systematic relationship between the ultrasound frequency and the cavitation efficiency. Koda et al.13 reported the effect of frequency on the cavitation efficiency in the range of 19.5 kHz - 1.2 MHz, where KI oxidation and the Fricke reaction were used as a cavitation dosimetry. Their results showed that the highest efficiency was observed in the range of 96 kHz to 500 kHz. Hung and Hoffmann and Beckett and Hua also reported that the water sonolysis14 and degradation of CCl415 were relatively fast in the range of 205 kHz to 628 kHz compared to other frequencies. Our results compare reasonably well with
Figure 2. (a) TEM micrograph of Au nanoparticles synthesized after 120 min irradiation of 213 kHz ultrasound. (b) Histogram for the size distribution of Au nanoparticles.
these studies. The results shown in Figure 1 suggest that maximum amount of reducing radicals were generated at around 213 kHz in the presence of 1-propanol. It is possible to qualitatively account for the existence of an optimum frequency in the sonochemical reduction efficiency. As the frequency is increased, the number of cavitation bubbles can be expected to increase. This should in principle increase the number of primary and secondary radicals generated, which in turn would result in a linear increse in the amount of Au(III) reduced. On the other hand, at higher frequencies there may not be enough time for the accumulation of 1-propanol at the bubble/solution interface and for the evaporation of water and 1-propanol molecules to occur during the expansion cycle of the bubble. This would result in a decrease in the amount of active radicals. Furthermore, the size of the bubbles that produce the high local temperatures decreases with increasing frequency. Also, it has been reported in the literature that the type of cavitation is different at lower and higher frequencies.16 In the experimental setup used, the 20 kHz system generates predominantly transient cavitation whereas the systems used for higher frequencies generate stable cavitation. These multiple effects would result in a very complex frequency effect. Further investigation is in progress in order to quantify the frequency effects on these factors. While the effects of frequency on the sonochemical reactions have already been reported, the frequency dependence of the size of the metal colloids produced has not been reported elsewhere in the literature. Figure 2a shows a TEM micrograph of the Au nanoparticles synthesized in 20 mM 1-propanol solution at 213 kHz ultrasound. The average size and relative standard deviation of Au particles was 15.5 nm and 19.5%, respectively. It should be noted that the formation of nanoparticles with a relatively narrow distribution can be seen in Figure 2, although there is no effective stabilizer present during the sonochemical reduction of Au(III). Figure 3 shows the average size of the formed Au nanoparticles as a function of the ultrasound frequency. It can be observed in this figure that the ultrasound frequency significantly affects the size and size distribution of Au particles, indicating that the frequency is an important parameter that controls the size of the formed particles. In addition, the particle size can also be found to be inversely related to the rate of Au(III) reduction: the size of the particles decreased with an increase in the rate of reduction. The results presented in Figure 4 show that there is a direct correlation between the average size of the Au particles and the rate of Au(III) reduction. The size distribution of the particles that are produced also showed similar trend. These phenomena suggest that the nucleation process is
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Figure 3. Average size of the formed Au nanoparticles as a function of ultrasound frequency. Other experimental conditions are as in Figure 1 caption. Each error bar corresponds to the standard deviation of the size of Au nanoparticles.
Figure 4. Average size of the formed Au nanoparticles as a function of the rate of reduction of Au(III). Other experimental conditions are as in Figure 1 caption. Each error bar corresponds to the standard deviation of the size of Au nanoparticles.
important in determining the particle size, since the nucleation process is closely related to the rate of the reduction.10 To find out if fusion between Au nanoparticles has any influence in controlling the size of the particles formed at different frequencies, further experiments were carried out. Ultrasound is often used to disperse or agglomerate particles in solution by its mechanical effects, such as microjet impact and shock wave. Previously, Suslick and co-workers reported that transition metal particles such as Zn, Cr, Ni, and Mo with an average diameter of 5 to ∼10 µm were agglomerated/fused by 20 kHz ultrasonic irradiation.17,18 They revealed that high velocity collisions among the particles are induced by intense shock waves, and thus a neck between particles is formed from localized melting. In the case of Au nanoparticles in the diameter range of 15 nm to 25 nm, any fusion of nanoparticles was not observed during sonication at 213-1062 kHz frequencies. It is generally known that the strength of the mechanical (physical) effects associated with the collapse of microbubbles decreases with increasing ultrasound frequency, because the size of the cavitation bubbles decreases as the frequency increases. In the present study, it was found that the size of the formed Au nanoparticles were mainly dependent on the rate of Au(III) reduction and not on the frequency. If any mechanical effects, generated by the bubble collapse, had affected the growth of the particles, the size of the formed Au particles should have consistently increased with decreasing ultrasound frequency. By considering the results observed in our study, it can be proposed that the sonochemical nucleation and growth processes of metal
J. Phys. Chem. B, Vol. 109, No. 44, 2005 20675 clusters and nanoparticles are negligibly affected by the mechanical effects that are generated during cavitation. This is also supported by a previous report,10 where the effect of ultrasound intensity on the size of the formed gold particle was investigated and the mechanical effects were suggested to be insignificant in the case of nanoparticles exposed to relatively high-frequency ultrasound. It should also be noted that the observed frequency effect does not arise from a direct interaction between the sound field and any molecular species in solution. That is, the colloidal dispersions consisting of nanoparticles can be considered as a homogeneous system in a sonochemical process. In summary, frequency effects on the rate of reduction and the synthesis of nanoparticles have been identified for the first time. It was found that the rate of Au(III) reduction induced by cavitation was the highest at 213 kHz in the range of 20 to 1062 kHz. The size of the Au nanoparticles produced was closely related to the rate of sonochemical reduction of Au(III) ions. In addition, the physical effects, accompanying acoustic cavitation events, on the nucleation and growth processes in the nanometer-sized colloidal dispersion system were found to be insignificant. Acknowledgment. We acknowledge the support of the Australian Research Council. K.O. also acknowledges the support from the 21st century COE program of JSPS and Ministry of Education, Culture, Sport, Science and Technology in Japan. The authors also acknowledge the financial support from the Particulate Fluids Processing Centre (PFPC) and the University of Melbourne for awarding an international collaborative grant. References and Notes (1) Nagata, Y.; Watanabe, Y.; Fujita, S.; Dohmaru, T.; Taniguchi, S. J. Chem. Soc., Chem. Commun. 1992, 1620. (2) Yeung, S. A.; Hobson, R.; Biggs, S.; Grieser, F. J. Chem. Soc., Chem. Commun. 1993, 378. (3) Okitsu, K.; Bandow, H.; Maeda, Y.; Nagata, Y. Chem. Mater. 1996, 8, 315. (4) Suslick, K. S.; Choe, S.-B.; Cichowlas, A. A.; Grinstaff, M. W. Nature 1991, 353, 414. (5) (a) Dhas, N. A.; Gedanken, A. J. Phys. Chem. B 1997, 101, 9495. (b) Ramesh, S.; Koltypin, Y.; Prozorov, R.; Gedanken, A. Chem. Mater. 1997, 9, 546. (6) Hobson, R. A.; Mulvaney, P.; Grieser, F. J. Chem. Soc., Chem. Commun. 1994, 823. (7) Katoh, R.; Tasaka, Y.; Sekreta, E.; Yumura, M.; Ikazaki, F.; Kakudate, Y.; Fujiwara, S. Ultrasonics Sonochem. 1999, 6, 185. (8) Bradley, M.; Ashokkumar, M.; Grieser, F. J. Am. Chem. Soc. 2003, 125, 525. (9) Crum, L. A.; Mason, T. J.; Reisse, J.; Suslick, K. S. Eds. Sonochemistry and Sonoluminescence; Kluwer Publishers: Dordrecht, Netherlands, 1999. (10) Okitsu, K.; Yue, A.; Tanabe, S.; Matsumoto, H.; Yobiko, Y.; Yoo, Y. Bull. Chem. Soc. Jpn. 2002, 75, 2289. (11) Nagata, Y.; Mizukoshi, Y.; Okitsu, K.; Maeda, Y. Radiat. Res. 1996, 146, 333. (12) Caruso, R. A.; Ashokkumar, M.; Grieser, F. Langmuir 2002, 18, 7831. (13) Koda, S.; Kimura, T.; Kondo, T.; Mitome, H. Ultrasonic Sonochem. 2003, 10, 149. (14) Beckett, M. A.; Hua, I. J. Phys. Chem. A 2001, 105, 3796. (15) Hung, H.-M.; Hoffmann, M. R. J. Phys. Chem. A 1999, 103, 2734. (16) Tronson, R.; Ashokkumar, M.; Grieser, F. J. Phys. Chem. B 2002, 106, 11064. (17) Doktycz, S. J.; Suslick, K. S. Science 1990, 247, 1067. (18) Prozorov, T.; Prozorov, R.; Suslick, K. S. J. Am. Chem. Soc. 2004, 126, 13890.
