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Synthesis of Gold Nanoparticles from an Organometallic Compound in Supercritical Carbon Dioxide Kunio Esumi,* Susumu Sarashina, and Tomokazu Yoshimura Department of Applied Chemistry and Institute of Colloid and Interface Science, Tokyo University of Science, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received March 6, 2004. In Final Form: May 6, 2004 This article presents the synthesis of gold nanoparticles in a single-phase supercritical fluid carbon dioxide solvent. The gold nanoparticles were formed by the reduction of triphenylphosphine gold(I) perfluorooctanoate with dimethylamineborane. Transmission electron microscopy, X-ray photoelectron spectroscopy, and UV-vis spectroscopy reveal the formation of gold nanoparticles of 1 nm in diameter. A high dispersion stability of the gold nanoparticles in supercritical carbon dioxide can be obtained by binding both triphenylphosphine and fluorocarbon ligands on the surface of the gold nanoparticles.
Introduction Metal nanoparticles are a very active field of research because of their potential applications including optical, electronic, and magnetic devices and catalysis.1 In particular, for gold, silver, and copper nanoparticles, it is very interesting to investigate their optical properties2-4 because they strongly absorb light in the visible region as a result of surface plasmon resonance. In addition, their peak wavelengths depend on the size, shape, and dispersion state of the metal nanoparticles. Recently, supercritical fluids as a new reaction medium have been extensively investigated to synthesize nanoparticles. For instance, supercritical carbon dioxide (scCO2) has been widely used as an ideal solvent because of its low cost, low toxicity, and readily accessible critical point. Until now, Ag,5,6 Pt,5 Pd,6 Ir,5 and Ag-Pd6 nanoparticles have been synthesized from their organo compounds in scCO2. However, one major limitation to the use of scCO2 is the poor solubility for the inorganic compounds. To overcome this disadvantage, water-inscCO2 microemulsions formed by using a certain surfactant allow inorganic compounds to be dispersed in this nonpolar fluid. Using water-in-scCO2 microemulsions, CdS,7 ZnS,8 AgS,9 and some metal nanoparticles10-12 have been prepared. In addition, although a synthesis of gold nanoparticles has been tried in the presence of ammonium carboxylate perfluoro ether in water-in-scCO2 microemulsions, no experimental data have been reported in detail.13 (1) Feldheim, D. L.; Foss, C. A., Jr. Metal Nanoparticles; Synthesis, Characterization, and Applications; Marcel Dekker: New York, 2002. (2) Schmid, G. Cluster and Colloids from Theory to Applications; VCH: Weinheim, 1994. (3) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley: New York, 1983. (4) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (5) Shah, P. S.; Husain, S.; Johnston, K. P.; Korgel, B. A. J. Phys. Chem. B 2001, 105, 9433. (6) Kameo, A.; Yoshimura, T.; Esumi, K. Colloids Surf. A 2003, 215, 181. (7) Holmes, J. D.; Bhargava, P. A.; Korgel, B. A.; Johnston, K. P. Langmuir 1999, 15, 6613. (8) Ohde, H.; Ohde, M.; Bailey, F.; Kim, H.; Wai, C. M. Nano Lett. 2002, 2, 721. (9) Ohde, H.; Hunt, F.; Wai, C. M. Chem. Mater. 2001, 13, 4130. (10) Kometani, N.; Toyoda, Y.; Yonezawa, Y. Chem. Lett. 2000, 682. (11) Ji, M.; Chen, X.; Wai, C. M.; Fulton, J. L. J. Am. Chem. Soc. 1999, 121, 2631. (12) Ohde, H.; Wai, C. M.; Kim, H.; Kim, J.; Ohde, M. J. Am. Chem. Soc. 2002, 124, 4540. (13) Yonezawa, Y.; Kometani, N. Chourinkai Saishin Gijutsu 2000, 4, 45.
Figure 1. Schematic diagram of the apparatus employed for the synthesis of gold nanoparticles in scCO2. Chart 1. Chemical Structure of TPAuFO
The synthesis of nanoparticles in scCO2 offers significant advantages over conventional liquid-phase systems including (i) being one of the most environmentally benign solvents available, (ii) the ability to recover particles easily, and (iii) the potential to synthesize the particles in the fluid and to subsequently conduct catalytic reactions using these particles. In this study, we report the synthesis of gold nanoparticles in a single phase of scCO2 using triphenylphosphine gold(I) perfluorooctanoate (TPauFO). The particles can be collected and redispersed in ethanol. As far as we know, this study is a first report of obtaining gold nanoparticles in single phase-scCO2. Experimental Section Materials. TPAuFO was synthesized according to a literature,14 and its structure was confirmed by 1H NMR, FTIR, and elemental analysis. All other chemicals were of reagent grade. The chemical structure of TPAuFO is shown in Chart 1. Preparation of Gold Nanoparticles. The reaction apparatus used in this study is schematically shown in Figure 1. (14) Appel, M.; Schlother, K.; Heidrich, J.; Beck, W. J. Organomet. Chem. 1987, 322, 77.
10.1021/la049415e CCC: $27.50 © 2004 American Chemical Society Published on Web 05/20/2004
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Figure 2. UV-vis spectra of TPAuFO in scCO2 before (a) and after (b) reduction with elapsed time. P ) 16 MPa; T ) 313 K; [TPAuFO] ) 0.2 mmol dm-3; [dimethylamineborane] ) 20 mmol dm-3.
Figure 3. TEM image and size distribution of gold nanoparticles synthesized in scCO2. P ) 16 MPa; T ) 313 K; [TPAuFO] ) 0.2 mmol dm-3; [dimethylamineborane] ) 20 mmol dm-3. The reaction in scCO2 fluid was performed in a fixed-volume stainless steel reactor (20 mL), whose interior was coated with Teflon. A view cell with two quartz windows (0.4-cm path length) connected to a UV-vis spectrophotometer (Hewlett-Packard 8453 A) was equipped with this system. In a typical experiment, 0.9-3.5 mg of TPAuFO was loaded into the vessel (20 mL), and then this system was closed. CO2 was introduced to the reactor by a CO2 delivery pump (SCF-Get, JASCO Co.) up to 16 MPa. Temperature was maintained at 313 K by the column oven. Pressure was controlled with a backpressure regulator (SCF-Bpq, JASCO Co.). The solution was stirred with an intelligent HPLC pump (PU-1586, JASCO Co.) for 0.5 h at constant pressure and temperature to generate supercritical state. Then, dimethylamineborane in ethanol (2 mol dm-3, 0.2 mL) was injected into the solution using a highpressure syringe pump (JASCO Co.), and the solution was stirred for 30 min. The gold nanoparticles were recovered by slowing depressurizing the high-pressure vessel and by adding ethanol to collect nanoparticles on the bottom of the vessel. Measurements. UV-vis absorption spectra of nanoparticles in scCO2 before and after reduction were recorded in situ on a Hewlett-Packard 8453 A diode array UV-vis spectrophotometer. The UV-vis spectra of redispersed nanoparticles in ethanol were also measured. The samples for transmission electron microscopy (TEM) were prepared by mounting a drop of the solution on a carbon-coated copper grid and by drying in a desiccator. The nanoparticles thus obtained were characterized with a Hitachi H-9000 NAR, operating at 200 kV and a direct magnification of 400 000×. The average particle size was determined by counting about 200 particles on micrographs. High-resolution X-ray photoelectron spectroscopy (XPS) spectra for samples were recorded using an ESCALAB MKII system (VG Scientific) with a monochromatic MG KR X-ray source (1253.6 eV). The binding energy was calibrated at the C(1s) peak energy (285 eV) and Si(2p) peak energy (103.5 eV). All peaks were
resolved by using a spectral processing program attached to the XPS operating system.
Results and Discussion Figure 2 shows UV-vis spectra of TPAuFO in scCO2 before and after reduction. Here, the concentrations of TPAuFO and dimethylamineborane are 0.2 and 2.0 mmol dm-3, respectively. In Figure 2a, TPAuFO (a white solid) shows an absorption band at around 220-280 nm, assigned to triphenylphosphine, and its intensity increases with the elapsed time and remains constant at above 10 min. On the other hand, a broad absorption band of TPAuFO in the visible region appears immediately by stirring in scCO2, but its absorption intensity decreases with the elapsed time. These results indicate that TPAuFO, a white solid, is dispersed in scCO2 at the early mixing time, scattering in the visible region, but when the elapsed time increases TPAuFO becomes dissolved accompanied by the increase in the absorption at 220280 nm. The solubility of TPAuFO in scCO2 determined by themethod just described was found to be about 0.36 mmol dm-3. After reduction of TPAuFO with dimethylamineborane, a broad band with two shoulders at about 310 and 420 nm15 appears in the region of 300-600 nm (Figure 2b). These features are different from those of nanometer-sized gold (>2 nm) having a plasmon band at about 520 nm. Accordingly, it is suggested that the gold nanoparticles synthesized in scCO2 are very small, probably less than 2 nm in diameter. Figure 3 shows a TEM image of gold nanoparticles and their size distribution. One can see that the average size (15) Negishi, Y.; Tsukuda, T. J. Am. Chem. Soc. 2003, 125, 4046.
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of the gold nanoparticles is 1.0 nm with a standard deviation of 0.3 nm. In addition, gold nanoparticles were also synthesized using different TPAuFO concentrations (0.1 and 0.3 mmol dm-3). The particle sizes of gold nanoparticles obtained are very similar to that using TPAuFO at 0.2 mmol dm-3, 1.1 ( 0.3 nm at 0.1 mmol dm-3, and 1.1 ( 0.2 nm at 0.3 mmol dm-3. Furthermore, it is interesting to note that the intensity of the absorption band at 428 nm, which is attributed to the gold nanoparticles, is proportionally increased with the TPAuFO concentration, indicating that the number of gold nanoparticles formed is proportionally increased with the TPAuFO under almost the same particle size of gold nanoparticles. In addition, it should be noted that the absorption spectra of gold nanoparticles in scCO2 remain unaltered during 2 h of stirring, indicating a stable dispersion of gold nanoparticles in scCO2. The particle formation reaction proceeds with dimethylamineborane aiding TPAuFO reduction to gold, followed by the formation of gold-gold bonds in nucleated clusters of gold atoms. Both triphenylphosphine and fluorocarbon ligands may adsorb to the gold nanoparticle surfaces, and their binding ligands establish an equilibrium during the reduction reaction which enables careful control of size. In addition, these ligands also provide steric stabilization by binding to the surface of the growing gold particles. Actually, in the water-in-scCO2 microemulsion, stable metal nanoparticles can be obtained in the presence of fluorocarbon surfactants6,11,12,16 that have a high affinity to the scCO2 medium. These studies also suggest that the fluorocarbon surfactants can operate as a stabilizing agent for the metal nanoparticles. To confirm how the ligands are adsorbed and interact at the surface of gold nanoparticles, the binding energies of phosphine in triphenylphosphine and fluorine in fluoocarbon chains were measured by XPS. It is found that the binding energies of both Au(4f7/2) (84.2 eV) and Au(4f5/2) (86.9 eV) are characteristic of Au(0)17 after the reduction of TPAuFO, while the binding energy of fluorine in TPAuFO at 690.0 eV shifts to 688.4 eV after the reduction. This shift (-1.6 eV) indicates that the fluorocarbon ligand binds the gold nanoparticles, and the reduction state of fluorine is expected. On the other hand, although two peaks of triphenylphosphine in TPAuFO of 2p1/2 (141.6 eV) and 2p3/2 (137.8 eV) are observed, the intensity of these two peaks becomes low after the reduction and a clear 2p peak at 133.0 eV appears. Because the binding energy of phosphine 2p in free triphenylphosphine is 130.9 eV,18 this shift (+2.1 eV) suggests that the triphenylphosphine ligand binds on the gold nanoparticles and the oxidation state of phosphine is expected. However, the XPS spectra of phosphine for the gold nanoparticles (16) McLeod, M. C.; McHenry, R. S.; Beckman, E. J.; Roberts, C. B. J. Phys. Chem. B 2003, 107, 2693. (17) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (18) Yamamoto, M.; Nakamoto, M. Chem. Lett. 2003, 452.
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Figure 4. Change in UV-vis spectra of gold nanoparticles redispersed in ethanol with standing time: (a) immediately, (b) 5 days, (c) 15 days. The gold nanoparticles used are synthesized at the same conditions as shown in Figure 1.
after washing with water disappear, whereas the spectra of fluorine for the gold nanoparticles are still unchanged. Thus, it is demonstrated that the fluorocarbon ligand binds strongly on the gold nanoparticles rather than the triphenylphosphine ligand. It is also interesting to evaluate the stability of the gold nanoparticles synthesized in scCO2 in organic solvents such as ethanol. Figure 4 shows UV-vis spectra of the gold nanoparticles redispersed in ethanol with standing time at room temperature. The spectrum of the gold nanoparticles redispersed immediately is very similar to that in scCO2, but the spectrum after 5 days exhibits a typical plasmon band at 520 nm. After 15 days, the plasmon band shifts from 520 to 600 nm. Thus, one can see that the gold nanoparticles redispersed in ethanol become aggregated with the standing time as a result of an insufficient affinity between the ligands adsorbing on the gold nanoparticles and ethanol molecules. Conclusions Gold nanoparticles have been synthesized by the reduction of TPAuFO with dimethylamineborane in a single-phase scCO2. TEM and XPS measurements reveal that gold nanoparticles having an average size of 1 nm are obtained, and a high dispersion stability of the nanoparticles is due to the binding of triphenylphosphine and fluorocarbon ligands on the nanoparticle surfaces. In addition, a dispersion stability of the gold nanoparticles redispersed in ethanol decreases with the standing time. This study also suggests that other metal nanoparticles with a narrow size distribution as well as a high dispersion stability can be synthesized in scCO2 using corresponding organometallic compounds with ligands acting as a stabilizer. Our research will progress in this direction. LA049415E
