© Copyright 2007 American Chemical Society
OCTOBER 9, 2007 VOLUME 23, NUMBER 21
Letters Fine-Tuning Size of Gold Nanoparticles by Cooling during Reverse Micelle Synthesis Alexander B. Smetana,†,‡ Joanna Shaofen Wang,†,‡ John Boeckl,‡ Gail J. Brown,‡ and Chien M. Wai*,† Department of Chemistry, UniVersity of Idaho, Moscow, Idaho 83844-2343, and Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, Dayton, Ohio 45433-7707 ReceiVed April 27, 2007. In Final Form: July 18, 2007 By lowering the reaction temperature during metal ion reduction in a reverse micelle system, gold nanoparticle size can be subtly tuned from 6.6 to 2.2 nm in diameter. Under these reaction conditions, the water-to-surfactant ratio (W value) also plays an important role in controlling the particle size, enabling a wide range of products obtainable via a simple, quick, reproducible synthesis. Particle sizes were measured by HRTEM, and size trends were supported by UV-vis spectroscopy.
Introduction The past two decades have seen nanoparticle synthesis evolve from a struggle to the creation of a monodisperse colloid of one element1 to the creation of particles of many compositions2 and adjustable sizes3 with control over deposition into intricate selfassembled networks.4 One area that has yet to be fully realized is fine control of particle size while maintaining uniform chemical composition. Current synthesis conditions for creating particles of different sizes often include altering the chemical environment of the particle as well as the physical size of the nanoparticle.5 To truly be able to study these remarkable materials and understand the fundamental reasons behind their size-dependent * To whom correspondence should be addressed. E-mail: cwai@ uidaho.edu. Tel: 208-885-6787. Fax: 208-885-6173. † University of Idaho. ‡ Wright-Patterson Air Force Base.
properties, we must be able to prepare particles that differ from one another exclusively in particle diameter while the protecting environment of the particles remains the same. There exist effective yet harsh synthetic preparations for metal nanoparticles, including chemical vapor deposition (CVD)6 and laser ablation7 that form nanoparticles from bulk metals. Several wet chemical preparations are in use that reduce metal ions in a much milder bottom-up approach. These include the still widely used citrate reduction proposed by Turkevich in 1951 where the reducing agent and the stabilizer are the same and provide a simple, reproducible synthesis.1 Phase-transfer catalysts in a procedure developed by Brust et al. further refined this idea where smaller monodisperse particles were formed in a twophase reaction mixture where metal ions were reduced by NaBH4 in an aqueous phase and were taken into an organic phase where they came into contact with a strong capping agents dodecanethiol.8 More exotic methods of reduction such as
(1) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 58. (2) Sun, Y.; Rollins, H. W.; Guduru, R. Chem. Mater. 1999, 11, 7. (3) Fernandez, C. A.; Wai, C. M. Small 2006, 2, 1266. (4) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (5) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Chem. Mater. 2003, 15, 935.
(6) Smetana, A. B.; Klabunde, K. J.; Sorensen, C. M. J. Colloid Interface Sci. 2005, 284, 521. (7) Mafune, F.; Kohno, J.; Takeda, Y.; Kondow, T. J. Phys. Chem. B 2000, 104, 9111. (8) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 18, 801.
10.1021/la701229q CCC: $37.00 © 2007 American Chemical Society Published on Web 09/14/2007
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microwave,9 sonochemical,10 and even biological11 reduction of metal ions have been found to make quality products. Despite some excellent results from the methods above, the reverse micelle reaction scheme is the synthetic approach that is most capable of achieving the goal of fine tuning particle size. As numerous laboratories have shown, this approach lends itself to small alterations that can have a significant effect on the particle outcome.12-16 This includes control of the size of the reaction volume (water core),13 flexibility of the micelle interface by solvent manipulation,14 amount of aging allowed for particle growth,15 and adjusting the concentration and relative ratios of the various reactants.16 Yet even these parameters, when used in combination, often yield particles with only a limited number of size values, not a linear progression. Clearly another dimension of control must be induced for the particles during synthesis. This letter demonstrates that the size of gold nanoparticles can be adjusted quite efficiently by controlling the reaction temperature when conducted near the freezing point of water. Altering the temperature to influence the outcome of metal nanoparticles is not new. Many effective synthesis approaches to the creation of nanoparticles with a very narrow size distribution have been accomplished by increasing the temperature of the reaction system from 100 to 300 °C.17 Although this has been shown to aid the etching ability of the thiol ligands and thereby favor monodisperse particles, it is limited at such high temperatures in that it will result in only an equilibrium size of the particles, often termed “magic number” particles. Here we show that decreasing the temperature of a reverse micelle reaction system gives rise to progressively smaller particles and a novel degree of size control of dodecanethiol-coated gold particles. Reverse micelle reactions at temperatures lower than 20 °C have been carried out in very few cases18,19 and have not been adequately investigated to identify the exact effect. Here we intend to examine specifically the effect of lower-temperature conditions on the particle size of reverse micelle reactions and their effect on the reaction rate and quality of particles formed. The particle size window that we control here is not large; however, it is in the most important range currently studied. The properties of nanoparticles in this size regime change dramatically with the addition of a few atoms whereas particles with diameters greater than 15 nm often do not display the size-dependent characteristics for which metal nanoparticles are currently being studied. Research by Jana et al.20 demonstrates the ability to grow nanoparticles from 5 to 40 nm using a seed-growth method where additional layers of metal can be deposited on existing nanoparticle cores. In an opposing method, Mulvaney and LizMarzan show that particles g80 nm can be selectively eroded by application of Au3+, which oxidizes zero-valent gold atoms (9) Gao, F.; Lu, Q.; Komarneni, S. Chem. Mater. 2005, 17, 856. (10) Mizukosi, Y.; Okitsu, K.; Maeda, Y.; Yamamoto, T. A.; Oshima, R.; Nagata, Y. J. Phys. Chem. B 1997, 101, 7033. (11) Gardea-Torresdey, J. L.; Tiemann, K. J.; Gamez, G.; Dokken, K.; Tehuacanero, S.; Jose-Yacaman, M. J. Nanopart. Res. 1999, 1, 397. (12) Walsh, D.; Mann, S. Nature 1995, 377, 320. (13) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961. (14) Lopez-Quintela, M. A.; Rivas, J.; Blanco, M. C.; Tojo, B. C. Synthesis of Nanoparticles in Microemulsions. In Nanoscale Materials; Liz-Marzan, L. M., Kamat, P., Eds.; Kluwer Academic Publishers: Boston, 2003; Chapter 6, p 135. (15) Martin, J. E.; Wilcoxon, J. P.; Odinek, J.; Provencio, P. J. Phys. Chem. B 2000, 104, 9475. (16) Fleming, D. A.; Williams, M. E. Langmuir 2004, 20, 3021. (17) Hou, Y.; Kondoh, H.; Kogure, T.; Ohta, T. Chem. Mater. 2004, 16, 5149. (18) Hostetler, M. J.; Wingate, J. E.; Zhong, C.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17. (19) Lin, X. M.; Sorensen, C. M.; Klabunde, K. J.; Hadjipanayis, G. C. Langmuir 1998, 14, 7140. (20) Jana, N. R.; Gearheart, L.; Murphy, C. J. Langmuir 2001, 17, 6782.
Letters
from the surfaces of the particles.21 Although these results are impressive, the authors do not show the same fine adjustment in size that we can achieve by adjusting the parameters described in this letter. Experimental Methods Hydrogen tetrachloroaurate(III) trihydrate (99.99%), NaCNBH3 (95%), hexanes, toluene, and sodium bis(2-ethylhexyl)sulfosuccinate (AOT, 98%) were purchased from Aldrich and used as received. Distilled water was used for all aqueous solutions with no special treatment. Gold particles were prepared by reduction of gold ions suspended in AOT water-in-hexane microemulsions. Separate metal ion/ reducing agent solutions were prepared by dissolving 0.0178 g of AOT in 2 mL of hexane and adding an aqueous solution of 7.2 µL (water/surfactant ratio, W ) 10) of either the 0.9 M NaCNBH3 reducing agent or the 0.3 M Au3+ gold ion. The W value was manipulated by the amount of aqueous solution added. These micellar solutions were stirred for 1 h before reduction to equilibrate the reagents to the reaction temperature. The reaction temperature itself was manipulated by using a NaCl salt/ice bath, an ice bath, ambient temperature, and an oil bath to obtain temperature conditions of -15, 0, 20, and 40 °C, respectively. The reaction temperature was monitored using a thermocouple placed in the liquid solution before and after reduction. The microemulsion containing the reducing agent was added dropwise over a time span of 60 s to the gold ion-containing microemulsion under vigorous stirring. Dodecanethiol (100 µL) was added to the reaction immediately after all of the reducing agent had been added to the solution. This solution was then allowed to stir at the reaction temperature for 1 h. After this time, the gold particles were precipitated by adding a mixture of 8 mL of ethanol and 2 mL of methanol, followed by centrifugation. The supernatant was discarded, and the remaining particles were washed two more times with 5 mL of ethanol to remove AOT, spectator ions, and excess dodecanethiol. The particles were then resuspended in 2 mL of toluene. All procedures were conducted on the benchtop without the need for inert environments. TEM grids were prepared for imaging by depositing 5 µL of sample onto a carbon-coated copper grid purchased from Ted Pella. All micrographs were obtained on a Phillips CM200 FEG HRTEM operating at 200 keV. The average size of the gold nanoparticles was obtained from the TEM images by measuring the diameter of at least 200 particles using Image J Imaging software. UV-vis spectra were obtained on a Cary 500 Varian UV-vis spectrophotometer without dilution versus a background of pure toluene.
Results and Discussion The reverse micelle synthesis technique is a well-documented chemical reaction system for the synthesis of spherical nanoparticles.12-16,22-25 Its strength is that it is a simple procedure with relatively affordable chemicals. This procedure creates a unique nanoreactor in which reagents can be isolated from one another. The reduction of metal ions occurs when the reducing agent comes into contact with the metal ions, possible only after the successful collision of micelles containing each reagent. Because the highly reactive gold(0) atoms that are immediately formed are isolated from all but a few other reduced atoms, they form seed particles that can grow slowly and uniformly without significant agglomeration. To ensure a quality product, care must be taken during the synthesis process. The microemulsion (21) Rodriquez-Fernandez, J.; Perez-Juste, J.; Mulvaney, P.; Liz-Marzan, L. M. J. Phys. Chem. B 2005, 109, 14257. (22) Schwuger, M. J.; Stickdorn, K.; Schomacker, R. Chem. ReV. 1995, 95, 849. (23) Bagwe, R. P.; Khilar, K. C. Langmuir 2000, 16, 905. (24) Cason, J. P.; Miller, M. E.; Thompson, J. B.; Roberts, C. B. J. Phys. Chem. B 2001, 105, 2297. (25) Tojo, C.; Blanco, M. C.; Lopez-Quintela, M. A. Langmuir 1998, 14, 6835.
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Figure 1. Gold nanoparticles formed at different temperatures during the reduction stage at W ) 10: (a) -15, (b) 0, (c) 20, and (d) 40 °C.
containing the reducing agent must be added dropwise to a vigorously stirred solution over a certain time frame (1 min), followed by immediate addition of the capping agent. Under our reaction conditions, the addition of the dodecanethiol capping agent before reduction prohibits the reduction of Au3+ ions. This is most likely attributed to the complexation of gold ions with thiol. This is not observed in other reverse micelle synthesis and may be due to the milder nature of NaCNBH3 used in our preparation versus the more powerful, common reducing agent NaBH4. The quenching action of dodecanethiol in the reaction allows us to trap particles at their current sizes. The reaction conditions are manipulated such that the particle size can be anywhere from 2.2 to 6.6 nm when the reaction is quenched. Several laboratories have investigated metal reduction in reverse micelle reactions both chemically23,24 and theoretically.25 They all agree that the limiting step is the rate of exchange of reactants between micelles or kex. A successful fusion interaction between micelles must occur for this to take place, which occurs on average in 1 out of every 1000 collsions.25 This assumes that the diffusion of reactants and subsequent reduction are instantaneous with respect to micelle fusion. This process has been found to be on the order of 103-104 faster than kex and is an appropriate approximation.26 Although the parameters affecting kex such as solvent type, surfactant, and the addition of various cosolvents have been studied, temperature effects on the reaction rate have not been investigated. By lowering the temperature of the reaction, we have managed to slow down the relevant exchange in a very controllable manner. Coupled with the quenching effect of dodecanethiol on our reaction, we can trap the nanoparticles at very predictable stages in their growth. To emphasize the size difference between particles, the TEM images were adjusted with a locked aspect ratio to have the same magnification. At lower magnifications, particles created at temperatures of 0, 20, and 40 °C show long-range order in 2D arrays. The interparticle distance was found to be similar for each preparation with an average value of 1.9 ( 0.4 nm that is typical for dodecanethiol-capped nanoparticles. As can be seen from Figure 1 for the case of W ) 10, the product is spherical nanoparticles separated by a monolayer of dodecane on the surface of each particle. The standard deviation of the particles was small for all synthesis conditions, varying from 0.33 to 0.71 nm to display a narrow size distribution. A histogram for all temperature preparations at W ) 12 emphasizing the particle size range is provided as supplemental data. (26) Atik, S. S.; Thomas, J. K. J. Am. Chem. Soc. 1981, 103, 3543.
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Figure 2. Particles formed at W ) 5. Temperature during reduction are (a) -15, (b) 0, (c) 20, and (d) 40 °C.
For all W values, the particles formed at 40 °C were the most uniform whereas reactions at 0 and -15 °C gave products that contained a significant fraction of particles of the size shown. Along with these particles are some larger agglomerations and also some very small background particles that were difficult to define. As the temperature of the reaction falls below zero, the water cores of the micelles freeze and can be seen as white flakes on the bottom of the vial. These samples must be agitated before reaction, and it is highly doubtful that a standard micelle reaction takes place in solution at -15 °C. Although much slower, a color change does take place during the reaction, indicating the reduction of gold ions and the formation of nanoparticles. Despite the small particle size, the product still precipitates well with the ethanol/methanol mixture used. Above 20 °C, the increase in particle size with temperature also becomes smaller and was found not to be significant above 40 °C. Although we did find a relationship between the water core volume and particle size, this parameter also has its limits. At W values below 5, whether there is a true water core or merely a few encapsulated water molecules is debatable. Reactions at lower temperatures with this W value proved more difficult. For W values greater than 12, this method tends to produce more polydisperse, inconsistent particles. Probably as the cavity for reaction becomes very large, the isolated reaction environment is to some extent compromised, causing increased opportunity for agglomeration and growth. Within the W values of 5 and 12, fine tuning of Au nanoparticles can be achieved by lowering the temperature during reverse micelle synthesis. TEM pictures of the Au nanoparticles synthesized at different temperatures for W values of 8 and 12 are given in Supporting Information. The definite size of the nanoparticles was determined as shown previously by TEM microscopy. The size differences between particles can be accentuated by the UV-vis spectrographs of each sample. It has been shown previously that the wavelength of the peak maximum is characteristic of gold nanoparticles and is size-dependent and is red-shifted for larger particles.27 As can be seen from Figure 3, the peak maximum is red-shifted for reactions conducted at higher temperatures. This corresponds to larger particles, confirming the trend seen by TEM microscopy. There are several subtle points in our synthesis that lead to quality nanoparticles of different sizes. The addition of the micelles containing the reducing agent is conducted evenly over a period of 1 min. This requires seed particles to wait for additional material before growth, causing this to be an even and controlled (27) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668.
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Letters Table 1. Gold Nanoparticle Diametera for Each Reaction Condition (W Value vs Temperature) W value
5
8
10
12
-15 °C 0 °C 20 °C 40 °C
2.2 ( 0.71 3.3 ( 0.44 4.9 ( 0.46 5.2 ( 0.43
2.1 ( 0.58 4.0 ( 0.58 5.4 ( 0.43 6.1 ( 0.52
3.3 ( 0.44 5.6 ( 0.33 6.3 ( 0.37 6.4 ( 0.50
3.1 ( 0.47 5.6 ( 0.60 6.4 ( 0.51 6.6 ( 0.56
a
Figure 3. UV-vis spectra of gold particles prepared at W ) 10 at several different temperatures. The peak wavelengths are 519, 523, 527, and 530 nm for temperatures of -15, 0, 20, and 40 °C, respectively. Peak height was normalized to accentuate the peak maximum for each sample.
event. This is in opposition to Bagwe et al., in which the reducing agent is added all at once.23 Under their conditions, Bagwe has found that slower exchange reactions lead to larger particles. We find the opposite effect, but the reasoning in Bagwe’s case is that slower exchange leads to fewer nuclei and more material available for particle growth. In our situation, the slower reactions are quenched at the same time, leading to arrested growth and smaller particles compared to the growth and particle size resulting from faster reactions. It is evident from the data that fine tuning the gold nanoparticle size is achievable using this method. This is made possible by creating a nonequilibrium state that would otherwise favor the most stable particle size or a “magic number” of atoms as the product. If this were the case, then only selective sizes of particles would be obtainable. We also conclude that the low temperature used during these reactions is the key to this nonequilibrium process. Particle growth appears to be uniform in each case and quenched when dodecanethiol is added to the reaction mixture. As stated before, the addition of dodecanethiol to gold ions before the addition of the reducing agent prevents reduction and should also prevent further growth of the particles through the encapsulation of the metal core. The progressively lower
In nanometers.
temperatures likely reduce the number of collisions between micelles, thereby limiting the exchange of gold(0) for particle growth. This is seen experimentally as a faster color change during reduction at higher temperatures (instantaneous for 40 °C) as compared to reduction at lower temperatures (a color change does not take place until 20 s after the addition of the reducing agent). Also, the results for changing the W value support this theory. Smaller W values have smaller micelles, which consequently contain fewer gold ions in the water core. Given that the number of collisions between the micelles remains the same at a given temperature, fewer gold(0) atoms will be available for particle growth, resulting in smaller particles in the product. This is illustrated for each temperature in the horizontal rows of Table 1. Other laboratories have found no difference in particle size upon changing the W value.16 This is likely because their processes favor the most thermodynamically stable product either because of higher temperatures or longer reaction times or both, which is not the case for our preparation. In conclusion, we have developed a new facet of reverse micelle nanoparticle reactions allowing continuous tuning of gold nanoparticle size by controlling the temperature between -15 and 40 °C. We feel that this is due to the nature of the reaction that quenches the uniform growth of particles, possible only at low synthesis temperatures using the reverse micelle method. Acknowledgment. This work was supported by AFOSR (FA9550-06-1-0526). The support of the S. T. Li Foundation in the form of an Achievement in Science and Technology award to C.M.W. is acknowledged. We also thank Dr. Robert Wheeler of Universal Energy Systems for his help in obtaining electron microscope images. Supporting Information Available: TEM images for W ) 8 and 12 at all temperatures and histogram of particles for W ) 12 at all temperatures. This material is available free of charge via the Internet at http://pubs.acs.org. LA701229Q
