Metallic Nanoparticle Production Utilizing a Supercritical Carbon

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Metallic Nanoparticle Production Utilizing a Supercritical Carbon Dioxide Flow Process M. Chandler McLeod,† William F. Gale,‡ and Christopher B. Roberts*,† Department of Chemical Engineering, Auburn University, Auburn, Alabama 36849 and Materials Research and Education Center, Auburn University, Auburn, Alabama 36849 Received March 15, 2004. In Final Form: June 7, 2004 Metallic nanoparticles of palladium and silver ranging in size from 1 to 15 nm were produced entirely within carbon dioxide by spraying a carbon dioxide carrier solution containing CO2-soluble metal precursors into a CO2 receiving solution containing a reducing agent (NaBH(OAc)3 or H2) and fluorocarbon thiol stabilizing ligands. The process uses the benign solvent CO2 while also allowing for the production of nanoparticles with a limited number of chemical components. Particles were characterized by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS).

Introduction Nanoparticles have recently garnered significant interest as scientists try to understand and manipulate materials on the atomic level. Due to their small size and high surface area per unit volume, they provide a means of investigating fundamental phenomena on the nanoscale while also holding potential in applications such as catalysis, electronics, and optics. Nanoparticles have been produced using numerous synthesis techniques, one of which involves the use of reverse micelles as media for the production of monodisperse nanoparticles.1 In these systems, metal salts or metal functionalized surfactants can be solvated specifically within the reverse micelles’ polar water cores. A reducing agent can then be introduced into the microemulsion, which reacts with the metal salts or functionalized surfactants to form neutral metal atoms. The atoms then combine to form metal nanoparticles through a process of intermicellar collision and exchange.1-5 In this manner, the reverse micelle water core functions as a dynamic “nanoreactor” for the growth of nanoparticles until they grow to a final size determined by the thermophysical properties of the system and its ability to sterically stabilize the particles.6 While this method has been regularly investigated using alkane solvents and the surfactant AOT,1,7-9 researchers have recently expanded these techniques to include systems using perfluoropolyether surfactants in near-critical and supercritical carbon dioxide (sc-CO2) bulk solvent.10-12 CO2 provides benefits as an inexpensive, nontoxic, and non* Corresponding author. E-mail: [email protected]. Telephone: (334) 844-2036. Fax: (334) 844-2063. † Department of Chemical Engineering, Auburn University. ‡ Materials Research and Education Center, Auburn University. (1) Adair, J. H.; Li, T.; Kido, T.; Havey, K.; Moon, J.; Mecholsky, J.; Morrone, A.; Talham, D. R.; Ludwig, M. H.; Wang, L. Mater. Sci. Eng. 1998, 23, 139-242. (2) Hirai, T.; Sato, H.; Komasawa, I. Ind. Eng. Chem. Res. 1994, 33, 3262-3266. (3) Bagwe, R. P.; Khilar, K. C. Langmuir 1997, 13, 6432-6438. (4) Bagwe, R. P.; Khilar, K. C. Langmuir 2000, 16, 905-910. (5) Towey, T. F.; Khan-Lodhi, A.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1990, 86, 3757-3762. (6) Kitchens, C. L.; McLeod, M. C.; Roberts, C. B. J. Phys. Chem. B. 2003, 107, 11331-11338. (7) Pileni, M. P.; Lisiecki, I.; Motte, L.; Petit, C.; Cizeron, J.; Moumen, N.; Lixon, P. Prog. Colloid Polym. Sci. 1993, 93, 1-9. (8) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 1297412983. (9) Cason, J. P.; Miller, M. E.; Thompson, J. B.; Roberts, C. B. J. Phys. Chem. B. 2001, 105, 2297-2302.

flammable solvent with higher diffusivity and lower viscosity than conventional liquid solvents. But while compressed or supercritical CO2 supplants the need for dangerous alkane solvents, microemulsion nanoparticle synthesis is limited by small batch processing and would be challenging to implement on a large scale with CO2 because of the required high-pressure vessels. Production of nanoparticles in a CO2-based flow process could reduce the demands on large high-pressure vessels while simultaneously providing for spray coating of particles and collection of particles from the volatile solvent medium. Moreover, the feeble solvent nature of CO2 has so far limited the surfactants available for microemulsion formation. Hence, effective microemulsion-based nanoparticle synthesis has been limited to fluorinated surfactants such as perfluoropolyether (PFPE) surfactants and fluorinated analogues of conventional alkyl surfactants.10-13 In light of this, development of a new CO2-based process for metallic nanoparticle formation that does not require microemulsion formation would be an attractive variation from the conventional reverse micelle-based synthesis and stabilization routine. Sun et al.14,15 have recently experimented with the production of particles by rapidly expanding a CO2-carrier solution containing metal salts (e.g., AgNO3, NiCl2, and CoCl2) into a reductive, stabilizing/receiving solution consisting of NaBH4 reducing agent, Poly(N-vinyl-2-pyrrolidone) (PVP) polymer stabilizer, and ethanol solvent. In this case the metal salts were initially dissolved in the CO2 carrier solution using water-in-CO2 PFPE reverse micelles. The reduction reaction, particle formation, and final stabilization occur via rapid expansion of this carrier solution into the reductive receiving solution. This method of rapid expansion can provide for a more continuous production of particles using CO2. Once synthesized, nanoparticles require separation from the receiving etha(10) McLeod, M. C.; McHenry, R. S.; Beckman, E. J.; Roberts, C. B. J. Phys. Chem. B. 2003, 107, 2693-2700. (11) Holmes, J. D.; Bhargava, P. A.; Korgel, B. A.; Johnston, K. P. Langmuir 1999, 15, 6613-6615. (12) Ji, M.; Chen, X.; Wai, C. M.; Fulton, J. L. J. Am. Chem. Soc. 1999, 121, 2631-2632. (13) Dong, X.; Potter, D.; Erkey, C. Ind. Eng. Chem. Res. 2002, 41, 4489-4493. (14) Sun, Y.-P.; Atorngitjawat, P.; Meziani, M. J. Langmuir 2001, 17, 5707-5710. (15) Sun, Y. P.; Rollins, H. W.; Guduru, R. Chem. Mater. 1999, 11, 7-9.

10.1021/la0493262 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/20/2004

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nol solution by drying, cleaning of excess stabilizer, and/ or redispersal if they are to be used in further application. Conversely, Shah et al.16,17 have prepared metallic silver, iridium, and platinum nanoparticles via a batch process in supercritical carbon dioxide using CO2-soluble metal complexes rather than using reverse micelles as carriers for the metal precursors. Additionally, fluorinated thiols were used as stabilizing agents so that the particles could be stabilized within CO2 following reduction of the CO2soluble metal complex by hydrogen. The thiols function via the metal-sulfur interaction to form monolayers on the nanoparticle surface. This is necessary to prevent particle aggregation and to facilitate the dispersal of the particles into solvents that solvate the ligand tails.18 In an effort to address the concept of CO2-based continuous production of metal nanoparticles without the need for surfactant-based microemulsions for metal salt introduction, we have explored the concept of spraying a carrier solution into a receiving solution for metallic nanoparticles production using CO2 solvent as both the carrier and receiving solutions. Rather than employing a reverse micelle solution as the carrier for the metal salt, the method described here eliminates the use of surfactants by using CO2-soluble metal coordination complexes as a means of dissolving metal precursors into CO2. Instead of the 4-component reverse micelle microemulsion that contains water, CO2, surfactant (e.g., PFPE), and metal salt (AgNO3); the system is simplified to a 2-component system containing only CO2 and a CO2-soluble metal complex. Additionally, further employment of CO2 as the solvent for the receiving solution with added reducing agent and particle stabilizer has the potential for improved particle recovery in that CO2 circumvents difficulties associated with separating particles from liquid solutions. This process eliminates the difficulties associated with particle recovery as the CO2 receiving solution permits the particles to be easily separated from the solution by simply expanding the solvent away. Use of the nanoparticles in situations requiring surface coating would be facilitated by the fact that the particles could be spray-coated onto a surface. The experimental setup could even make possible the collection of particles by spraying supercritical CO2 solutions directly onto a surface, whereupon the CO2 would evaporate to leave only the stabilized nanoparticles. Experimental Section Materials. Palladium(II) hexafluoroacetylacetonate (Pd(hfac)), shown in Figure 1a, and acetone were purchased from Aldrich. Ag(hfpd) tetraglyme,19 where hfpd ) 1,1,1,5,5,5hexafluoropentane-2,4-dione, was generously provided by K. Morley and S. Howdle at the University of Nottingham and is shown in Figure 1b. The Ag(hfpd)’s high solubility in CO2 was demonstrated in their research using CO2 as a transport fluid for impregnation of the Ag coordination complex into porous substrates where it was subsequently reduced to form metal nanoparticles in the porous material.19 The fluorocarbon thiol 1H,1H,2H,2H-perfluorooctanethiol (C6F13C2H4SH) was obtained from Oakwood Products Inc. Hydrogen and carbon dioxide gas were obtained from BOC gases and HFE 7100 was obtained from 3M.

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Figure 1. Structure of the CO2-soluble (a) palladium and (b) silver complexes used in the CO2 spray production of fluorothiol coated silver nanoparticles.

Figure 2. Schematic diagram of experimental apparatus for continuous production of metallic nanoparticles using CO2 as the carrier/receiving fluid. Particle Formation. A schematic diagram of the experimental apparatus is shown in Figure 2. A custom-built, variablevolume piston/cylinder assembly, which has been described previously,20 was used to spray the metal complex into the receiving solution. In an experiment the sample side of this variable-volume cell was loaded with an appropriate amount of solid metal complex (i.e., Pd(hfac) or Ag(hfpd) tetraglyme) and then pressurized with CO2 at room temperature until a single phase was visually observed through the quartz window of the vessel. An Isco 260D high-pressure syringe pump was then used to pressurize the backside of the piston in the variable-volume cell so that a constant pressure was maintained. Meanwhile, the receiving solution was prepared by loading a 37-mL Jerguson gauge (a windowed high pressure vessel) with fluorinated thiol at 10 and 23 mM concentrations followed by the desired reducing agent. Initially solid NaBH(OAc)3 was used as the reducing agent at a concentration of 4 mM; however, because of NaBH(OAc)3’s low solubility in CO2, hydrogen gas was alternatively used by filling the vessel to 7 bar at 25 °C, corresponding to a concentration of 270 mM. Carbon dioxide was then added such that the pressure in the Jerguson gauge was between 110 and 270 bar with temperatures ranging from 25 to 130 °C as desired. The Jerguson gauge was situated in a temperature-controlled Thermolyne 30400 oven to achieve and maintain the temperature. Pressure was monitored in the Jerguson gauge using an Omega PX94510KGI amplified transducer. In this expansion/reduction process, the carbon dioxide serves as the bulk solvent and the hydrogen functions as a CO2-miscible reducing agent that reacts with the metal complex to produce metal nanoclusters and organic byproducts. The fluorinated thiols in our system act as CO2soluble ligands that stabilize and limit the growth of the nanoparticles once they are formed.

Results and Discussion (16) Shah, P. S.; Holmes, J. D.; Johnston, K. P.; Korgel, B. A. J. Phys. Chem. B. 2002, 106, 2545-2551. (17) Shah, P. S.; Husain, S.; Johnston, K. P.; Korgel, B. A. J. Phys. Chem. B. 2002, 106, 12178-12185. (18) Ulman, A. Self-Assembled Monolayers of Thiols; Academic Press: San Diego, 1998. (19) Morley, K. S.; Marr, P. C.; Webb, P. B.; Berry, A. R.; Allison, F. J.; Moldovan, G.; Brown, P. D.; Howdle, S. M. J. Mater. Chem. 2002, 12, 1898-1905.

Initially, an experiment was conducted in which a visually observed single phase solution of 0.7 wt % Ag(hfpd) tetraglyme in carbon dioxide solution at 275 bar and 25 °C was injected into a CO2 receiving solution at (20) Martin, T. M.; Lateef, A. A.; Thompson, J. B.; Roberts, C. B. J. Chem. Eng. Data 1999, 44, 11-15.

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Figure 3. Silver nanoparticles formed by expansion of CO2 solution carrying Ag(hfpd) tetraglyme into a receiving solution containing NaBH(OAc)3 reducing agent and fluorocarbon thiols as stabilizers. The particles were collected by (a) direct spray onto the TEM grid for characterization and (b) HFE drop in which the stabilized nanoparticles were redispersed after depressurizing the reaction vessel. Particle size distribution is shown in (c).

110 bar and 25 °C. The silver complex concentration in the Jerguson gauge vessel following injection was ca. 3.6 mM, while the receiving solution contained fluorinated thiol at a concentration of 10 mM and Na(OAc)3BH4 reducing agent at a concentration of 4 mM. Despite the

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fact that the reducing agent was not completely soluble in the carbon dioxide solution, expansion of the silver complex into the receiving solution still generated a mixture that turned from clear to yellow to ruby and lastly to maroon/purple in rapid succession. Particles were collected by spraying the solution out of a 750 µm tube onto a transmission electron microscopy (TEM) grid as well as by washing the residue from the Jerguson gauge with HFE liquid, and then placing a small drop on a TEM grid. Particles obtained by the spray technique are shown in Figure 3a, while particles collected from an HFE liquid drop are shown in Figure 3b. As can be seen by the size distribution in Figure 3c, the particles are relatively monodisperse with an average diameter of 4 nm and having a standard deviation of (42% (based on sizing of 446 particles from TEM images). Energy dispersive spectroscopy (EDS) work identified the particles as silver atoms present with fluorine and sulfur atoms, while the large nickel peak most likely results from the nickel mesh of the TEM grid. This analysis reveals the effectiveness of the fluorocarbon thiol molecule as a stabilizing ligand on the silver nanoparticle. Furthermore, the collection of particles by means of spraying the receiving solution onto a TEM grid demonstrates the potential spray coating of particles onto a surface for application or collection. Subsequently, palladium nanoparticles were synthesized by spraying a carrier solution of carbon dioxide and palladium(II) hexafluoroacetylacetonate (Pd(hfac)2) into the CO2/fluorinated thiol/H2 receiving solution at 270 bar and 130 °C. Hydrogen was used in this and all subsequent experiments as the reducing agent in order to address the incomplete solubility of NaBH(OAc)3 in CO2. To encourage a faster reduction by hydrogen, the receiving temperature was elevated to 130 °C; similarly, the pressure was increased to 270 bar to maintain significant CO2 density and solubility. The fluorinated thiol concentration was 17

Figure 4. EDS measurement of the silver nanoparticles formed by NaBH(OAc)3 reduction. The spectrum shows the presence of silver along with sulfur and fluorine, which indicates the presence of stabilizing fluorocarbon thiol.

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Figure 5. (a) Palladium produced by expanding a CO2/Pd(hfac) mixture into a CO2 receiving solution containing hydrogen reducing agent and fluorinated thiols as stabilizing agents. (b) The corresponding particle size distribution.

mM and the hydrogen concentration was 270 mM. In the experiment, a CO2 delivery solution at 310 bar and 30 °C containing 0.7 wt % pd(hfac)2 was sprayed such that ca. 2 mL was sprayed into the 37-mL Jerguson gauge. This CO2/Pd(hfac) addition corresponded to the insertion of Pd(hfac)2 such that the concentration of Pd(hfac) in the reaction view cell was 0.0255 mM. After introduction of the CO2/Palladium complex solution into the view cell, the final pressure was 280 bar. The receiving solution turned a light yellow following palladium complex addition into the reductive environment, indicating successful addition of the orange-colored palladium molecule. After ca. 5 min, the solution then turned a slightly darker color, indicating reduction of the palladium molecule. The solution was then allowed to cool to room temperature and the Jerguson gauge was washed with acetone. A small drop of solution was placed on a carbon-coated nickel TEM grid for transmission electron microscope analysis. TEM analysis, as shown in Figure 5a, indicates the successful formation of particles using hydrogen as the reducing agent and demonstrates relatively polydisperse particles, as seen in Figure 5b, with a mean size of 9 nm, 36% standard deviation (sizing of 91 particles). The polydispersity may be attributed to an altered thiol-metal effectiveness; however, the visible separation of the

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Figure 6. (a) Silver particles produced by expanding a CO2/Ag(hfpd) tetraglyme mixture into a CO2 receiving solution containing hydrogen reducing agent and fluorinated thiols stabilizing ligands. (b) The corresponding particle size distribution.

particle indicates that the particles are being stabilized from further agglomeration. Silver particles, shown in Figure 6a, were formed in the same manner using the highly CO2-soluble silver complex Ag(hfpd) tetraglyme, rather than the palladium molecule. In the experiment the carbon dioxide carrier solution in the variable volume cell was at 25 °C and 250 bar and contained 5.85 wt % Ag(hfpd) tetraglyme complex. Conditions in the Jerguson gauge prior to silver complex introduction were 220 bar and 120 °C while containing 23 mM fluorinated thiol and 270 mM hydrogen. The carrier solution was sprayed from the variable volume cell into the Jerguson gauge at ca. 1 mL/min and led to a darkening of the solution after a period of about 2 min. The Jerguson gauge was allowed to cool overnight, was then depressurized, and subsequently washed with an HFE liquid. This solution was used to prepare multiple TEM grids that show (see Figure 6) the particles packed in a selfassembled, organized arrangement. However, when the HFE nanoparticle mixture was examined by UV-vis analysis no absorptions indicative of silver nanoparticles were observed. This is attributed to the agglomeration of

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nanoparticles as seen on the TEM picture, which would alter the expected spectroscopic behavior of individual nanoparticles suspended within the fluid. Interestingly, the packing reveals the effectiveness of the thiol in stabilizing the particles from further agglomeration, as the particles are seen in a closely packed organized arrangement with well-defined separations. The particle size distribution, Figure 6b, reveals particles with a mean diameter of 3 nm and 30% standard deviation (sizing of 426 particles). These silver particles are noticeably more monodisperse than the palladium particles, a characteristic that may be due to a greater affinity between the silver and the thiol.

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These results demonstrate that nanoparticles can be prepared in a spray process in which the only solvent is carbon dioxide, a largely benign solvent. The spray process holds the potential for rapid production of metallic nanoparticles with a minimum of chemical constituents, requiring only solvent, metal precursor, reducing agent, and particle stabilizer. The supercritical and benign nature of carbon dioxide provides a means of facile particle separation by simply expanding the compressed solvent to leave only the stabilized nanoparticles. Additionally, as was demonstrated experimentally, the process could be refined to allow for the spray coating of particles onto a surface via rapid expansion of the carbon dioxide solution.

Summary Nanoparticles of silver and palladium were synthesized by spraying a carbon dioxide solution containing the metal precursor into a carbon dioxide receiving solution containing a reducing agent and a fluorinated thiol. Following reduction of the metal complex by the reducing agent, the fluorinated thiols function to sterically stabilize the metallic nanoparticles, as evidenced by TEM investigation.

Acknowledgment. Financial support from the Department of Energy, Basic Energy Sciences (DE-FG0201ER15255) and the National Science Foundation (CTS0207781) is gratefully acknowledged. We also thank Steven Howdle and Kelly Morley for their contribution of the CO2-soluble silver precursor. LA0493262