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Preparation of Silver Nanoparticles via Rapid Expansion of Water in Carbon Dioxide Microemulsion into Reductant Solution Ya-Ping Sun,* Pornpen Atorngitjawat, and Mohammed J. Meziani Department of Chemistry and Center for Advanced Engineering Fibers and Films, Howard L. Hunter Chemistry Laboratory, Clemson University, Clemson, South Carolina 29634-0973 Received February 26, 2001. In Final Form: June 21, 2001 Perfluoropolyether ammonium carboxylate stabilized water-in-CO2 microemulsion was used to dissolve silver nitrate salt for the RESOLV (Rapid Expansion of a Supercritical Solution into a Liquid SOLVent) process. The solution of reverse micelles with an aqueous silver nitrate core in CO2 was rapidly expanded into a room-temperature solution of sodium borohydride to form silver nanoparticles. The nanoparticles were characterized using optical, X-ray diffraction, and microscopy techniques. The nanoparticle properties were found to be dependent on the pre-expansion microemulsion conditions in CO2.
Introduction There has been significant recent interest in the use of supercritical CO2 for the preparation and processing of nanomaterials.1-7 Because of the solubility limitation with most solutes in supercritical CO2, surfactants containing both CO2-soluble and hydrophilic moieties are often added to supercritical CO2 to form reverse micelles.1,3-6 For example, Wai, Fulton, and co-workers used the microemulsion of water in supercritical CO2 with a fluorinated surfactant to synthesize silver and silver halide nanoparticles.3,4 The nanoparticles were collected via rapid expansion and characterized using optical and electron microscopy techniques.3,4 Similarly, Johnston and coworkers used the microemulsion to prepare nanoscale cadmium sulfide particles.5 These authors reported that a higher water-to-surfactant molar ratio for the microemulsion resulted in an increase in the nanoparticle sizes.5 Johnston and co-workers also prepared silver nanocrystals that were coated with fluorinated ligands for the dispersion in CO2 at moderate temperatures and pressures.6 These nanocrystals were again characterized using optical and electron microscopy techniques. We have developed a supercritical fluid rapid expansion8-10 method called RESOLV (Rapid Expansion of a Supercritical Solution into a Liquid SOLVent) for producing nanoscale semiconductor and metal particles.11-13 The (1) Supercritical Fluid Technology in Materials Science and Engineering: Synthesis, Properties, and Applications; Sun, Y.-P., Ed.; Marcel Dekker: New York, in press. (2) Watkins, J. J.; McCarthy, T. J. Chem. Mater. 1995, 7, 1991. (3) Ji, M.; Chen, X. Y.; Wai, C. M.; Fulton, J. L. J. Am. Chem. Soc. 1999, 121, 2631. (4) Ohde, H.; Rodriguez, J. M.; Ye, X. R.; Wai, C. M. Chem. Commun. 2000, 23, 2353. (5) Holmes, J. D.; Bhargava, P. A.; Korgel, B. A.; Johnston, K. P. Langmuir 1999, 15, 6613. (6) Shah, P. C.; Holmes, J. D.; Doty, R. C.; Johnston, K. P.; Korgel, B. A. J. Am. Chem. Soc. 2000, 122, 4245. (7) Cason, J. P.; Roberts, C. B. J. Phys. Chem. 2000, 104, 1217. (8) Petersen, R. C.; Matson, D. W.; Smith, R. D. J. Am. Chem. Soc. 1986, 108, 2100. (9) Eckert, C. A.; Knutson, B. L.; Debenedetti, P. G. Nature 1996, 383, 313. (10) Tom, J. W.; Debenedetti, P. G. J. Aerosol Sci. 1991, 22, 555. (11) Sun, Y.-P.; Rollins, H. W.; Guduru, R. Chem. Mater. 1999, 11, 7. (12) Sun, Y.-P.; Riggs, J. E.; Rollins, H. W.; Guduru, R. J. Phys. Chem. B 1999, 103, 77. (13) Sun, Y.-P.; Guduru, R.; Lin, F.; Whiteside, T. Ind. Eng. Chem. Res. 2000, 39, 4663.
nanoparticles thus obtained were small (less than 10 nm), with relatively narrow size distributions. In those experiments, supercritical solvents such as ammonia and THF were employed for the rapid expansion at relatively higher temperatures. In an effort to replace the organic solvents with CO2-based systems for RESOLV at ambient temperatures, we used a water-in-CO2 microemulsion to dissolve silver salt, similar to what was reported by Wai, Fulton, and co-workers3,4 and Johnston and co-workers.5,6 However, instead of in situ chemical reduction, the microemulsion with silver salt was rapidly expanded into a room-temperature solution for chemical reduction under ambient conditions, yielding silver nanoparticles. It was found that the nanoparticle properties were dependent on the pre-expansion microemulsion conditions. Experimental Section Materials. Silver nitrate (AgNO3) and sodium borohydride (NaBH4) were purchased from Aldrich. Spectroscopy or HPLC grade organic solvents were used as received. Poly(N-vinyl-2pyrrolidone) (PVP) of average molecular weight MW ≈ 360 000 was obtained from Sigma and used without further purification. Anhydrous ammonia and carbon dioxide (SCF grade) were purchased from Air Products. Perfluoropolyether carboxylic acid (PFPE-COOH, MW ≈ 655) was obtained from Ausimont. The conversion to the ammonium salt (perfluoropolyether ammonium carboxylate or PFPE-NH4) was accomplished via the neutralization reaction of PFPECOOH with aqueous ammonium hydroxide, followed by the removal of water and excess ammonia under a vacuum at 65 °C. The conversion was monitored and confirmed by results from NMR measurements. Measurements. The apparatus for the preparation of nanoparticles via RESOLV is illustrated in Figure 1. It consisted of a syringe pump for pressure generation and pressure maintenance during the rapid expansion process and a gauge for monitoring the system pressure. The heating unit consisted of a cylindrical solid copper block of high heat capacity in a tube furnace. The copper block was wrapped with a stainless steel tubing coil and inserted tightly into a stainless steel tube to ensure close contacts between the tubing coil and copper block for efficient heat transfer. The expansion nozzle was a fused silica capillary hosted in stainless steel tubing, which was inserted into the collection chamber containing a room-temperature solution. UV/vis absorption spectra were recorded on a computercontrolled Shimadzu UV-2101PC spectrophotometer. Powder X-ray diffraction measurements were carried out on a Scintag XDS-2000 powder diffraction system. Transmission electron
10.1021/la0103057 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/22/2001
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Langmuir, Vol. 17, No. 19, 2001
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Figure 1. A cartoon diagram of the RESOLV apparatus. microscopy (TEM) images were obtained on a Hitachi 7000 transmission electron microscope.
Results and Discussion Microemulsions of water in supercritical CO2 were prepared with PFPE-NH4 as the surfactant and used for the dissolution of silver cation. In a typical experiment, 0.25 mL of an aqueous AgNO3 solution (0.2 M) was added to a syringe pump, followed by the addition of PFPENH4 to the equivalent of a Wo (molar ratio of water vs surfactant) value of 5. The syringe pump was then loaded with CO2 to a pressure of 2000 psia, and the mixture in the syringe pump was stirred for 2 h to form reverse micelles with an aqueous AgNO3 core. The condition for the formation of reverse micelles was simulated in a highpressure optical cell. With either aqueous AgNO3 or aqueous Cu(NO3)2 as the micellar core, the microemulsion system in the optical cell appeared homogeneous. According to the absorption spectrum of the aqueous Cu2+ (centered at ∼740 nm), the Cu(NO3)2 salt was completely dissolved in the water/PFPE-NH4/CO2 microemulsion system. The reverse micellar solution of aqueous AgNO3/PFPENH4 in supercritical CO2 was rapidly expanded via a 50micron fused silica capillary nozzle into a room-temperature solution of NaBH4 in ethanol (Figure 1).11-13 The system pressure was maintained at 4000 psia during the rapid expansion. The silver nanoparticles produced in the rapid expansion/chemical reduction were protected from agglomeration by the presence of PVP polymer in the roomtemperature ethanol solution (5 mg/mL), forming a stable suspension. The absorption spectrum of the suspension shows the characteristic surface plasmon absorption of silver nanoparticles (a band peaking at ∼430 nm).14,15 For the analysis using TEM, the nanoparticle suspension was first diluted and then deposited via evaporating the solvent on a collodion film supported by a copper grid. Shown in Figure 2A is a TEM image of the silver nanoparticles, from which an average particle size of 7.8 nm and a size distribution standard deviation of 2.5 nm were obtained. A solid sample of the silver nanoparticles was also characterized using X-ray powder diffraction, which yielded a diffraction pattern that is typical of nanoscale silver particles (Figure 3).16 (14) Clarle´, K. P.; Schulze, W. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 350. (15) Kamat, P. V.; Flumiani, M.; Hartland, G. V. J. Phys. Chem. B 1998, 102, 3123. (16) Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedures; John Wiley & Sons: New York, 1959.
Figure 2. TEM images of the silver nanoparticles obtained via RESOLV with the microemulsions of Wo equal to 5 (A) and 20 (B).
Figure 3. X-ray powder diffraction pattern of the silver nanoparticles. The pattern for bulk silver in the JCPDS database is also shown for comparison.
It has been reported that silver nanoparticles can be synthesized via RESOLV using ammonia as the supercritical solvent for the rapid expansion.12 The results reported here for RESOLV with the microemulsion of aqueous solution in CO2 are in general similar to those from the rapid expansion of a supercritical ammonia
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solution. However, there are obvious advantages with the CO2-based system, including the environmentally benign nature of the solvent and the near-ambient system temperature. The latter will be important in the RESOLV processing of the metal salts that decompose at high temperatures. In addition, the pre-expansion conditions for the reverse micelles may be used to influence the properties of the produced nanoparticles. The average size of the silver nanoparticles is dependent on the Wo value of the reverse micellar solution in supercritical CO2. For example, reducing the amount of surfactant PFPE-NH4 in CO2 changed the Wo value of the microemulsion from 5 to 20, while the AgNO3 concentration in the aqueous core was kept constant. The rapid expansion of such an aqueous AgNO3/PFPE-NH4/ CO2 system with a higher Wo value of 20 into a roomtemperature solution of NaBH4 in ethanol again yielded silver nanoparticles. These nanoparticles and their stable suspension in ethanol (under the protection of PVP polymer) were also characterized using the optical and electron microscopy methods. While the absorption spectrum is similar to that of the silver nanoparticle suspension obtained from RESOLV with the microemulsion of Wo equal to 5, the TEM result shows significantly larger particles (Figure 2B), with an average particle size of 14.5 nm and a size distribution standard deviation of 5.8 nm. The trend is validated by the result with the microemulsion of an intermediate Wo value. The silver nanoparticles prepared via RESOLV with the microemulsion of Wo equal to 12 have an average particle size of 10.4 nm and a size distribution standard deviation of 3.8 nm. The Wo value determines the average size of the reverse micelles and, in this case, the contents of water and silver salt in the microemulsion in CO2. According to Zielinski et al.17 PFPE-NH4-based water-in-CO2 microemulsions of Wo equal to 11 and 31 contain water droplets of average radii of ∼2 and 3.5 nm, respectively. The droplets and droplet structure are little affected by experimental parameters such as the system pressure.17 If these results are used as a reference, the average sizes of the reverse micellar cores in our microemulsions are estimated to be
