Facile Synthesis of Silver Nanoparticles in CO2-Expanded Liquids

Mar 18, 2010 - Hsien-Te Hsieh, Wei-Kuo Chin* and Chung-Sung Tan. Department of ... Yi-Chen Huang , Clive H. Yen , Hsin-Wei Lin , Chung-Sung Tan...
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Facile Synthesis of Silver Nanoparticles in CO2-Expanded Liquids from Silver Isostearate Precursor Hsien-Te Hsieh, Wei-Kuo Chin,* and Chung-Sung Tan Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, Republic of China Received January 12, 2010. Revised Manuscript Received March 11, 2010 This approach provides a new technique to synthesize silver nanoparticles (AgNPs) using CO2-expanded liquids (CXLs) as the processing medium. A soluble form of silver carboxylate, silver isostearate (AgISt), was synthesized and characterized. The XRD and DSC analyses indicated that the methylated branched alky chains in AgISt exhibited a steric hindrance to impede the growth of layered structure of AgISt molecules, which led to the high solubility of AgISt in nonpolar solvents. By using AgISt as silver precursor, AgNPs of 2.64 ( 0.51 nm in diameter were synthesized in CO2expanded heptane with H2 as the reducing agent. The ATR-FTIR analysis showed that the produced AgNPs were capped with isostearic acid, which was derived from the reduction of AgISt. Hence, the isostearic acid capped AgNPs were well-dispersed in heptane to form a stable silver organosol.

Introduction The nontoxic, nonflammable, inexpensive, and abundant nature of carbon dioxide (CO2) has attracted great attention as an ideal processing medium in the fields of material science and nanotechnology.1-4 Noble metal nanoparticles, including silver and gold nanoparticles, have been synthesized through supercritical CO2 (sc-CO2) technologies such as the water-in-CO2 (w/c) microemulsions,5-7 rapid expansion of supercritical solution into a liquid solvent (RESOLV),8,9 sc-CO2 flow process,10 arrested precipitation,11,12 and other approaches.13-15 However, sc-CO2 is a poor solvent for many high molecular weight and polar compounds due to the low dielectric constant and polarizability per volume of CO2. Accordingly, CO2-philic fluorinated molecules including surfactants,5-9 capping ligands,10-14 and metal precursors10,15 are used to enhance the solubility of compounds in sc-CO2, although they are economically and environmentally unfavorable. Besides, a high process pressure (generally over 100 bar) is required to dissolve adequate amount of the fluorinated reagents in compressed CO2. *Corresponding author. Telephone: þ886-3-571-3721. Fax: þ886-3-5715408. E-mail: [email protected].

(1) Eckert, C. A.; Knutson, B. L.; Debenedetti, P. G. Nature 1996, 383, 313. (2) Holmes, J. D.; Lyons, D. M.; Ziegler, K. J. Chem.;Eur. J. 2003, 9, 2144. (3) Johnston, K. P.; Shah, P. S. Science 2004, 303, 482. (4) Shah, P. S.; Hanrath, T.; Johnston, K. P.; Korgel, B. A. J. Phys. Chem. B 2004, 108, 9574. (5) Ji, M.; Chen, X. Y.; Wai, C. M.; Fulton, J. L. J. Am. Chem. Soc. 1999, 121, 2631. (6) Ohde, H.; Hunt, F.; Wai, C. M. Chem. Mater. 2001, 13, 4130. (7) McLeod, M. C.; McHenry, R. S.; Beckman, E. J.; Roberts, C. B. J. Phys. Chem. B 2003, 107, 2693. (8) Sun, Y. P.; Atorngitjawat, P.; Meziani, M. J. Langmuir 2001, 17, 5707. (9) Meziani, M. J.; Pathak, P.; Beacham, F.; Allard, L. F.; Sun, Y. P. J. Supercrit. Fluids 2005, 34, 91. (10) McLeod, M. C.; Gale, W. F.; Roberts, C. B. Langmuir 2004, 20, 7078. (11) Shah, P. S.; Husain, S.; Johnston, K. P.; Korgel, B. A. J. Phys. Chem. B 2001, 105, 9433. (12) Shah, P. S.; Husain, S.; Johnston, K. P.; Korgel, B. A. J. Phys. Chem. B 2002, 106, 12178. (13) Fan, X.; McLeod, M. C.; Enick, R. M.; Roberts, C. B. Ind. Eng. Chem. Res. 2006, 45, 3343. (14) Moisan, S.; Martinez, V.; Weisbecker, P.; Cansell, F.; Mecking, S.; Aymonier, C. J. Am. Chem. Soc. 2007, 129, 10602. (15) Esumi, K.; Sarashina, S.; Yoshimura, T. Langmuir 2004, 20, 5189.

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CO2-expanded liquids (CXLs) form a new class of tunable solvents.16-18 CXLs are the mixtures where the compressed CO2 is dissolved into organic solvents accompanying volume expansion of the solutions. As compared to scCO2, CXLs are benefited by the milder operating pressure (tens of bar). It was reported that the volume expansion of organic solvents was higher than 500% by dissolving CO2 under mild pressure ranging from 40 to 70 bar.19 Therefore, the physicochemical properties of CXLs, including density, viscosity, solute diffusivity, and gas solubility, can be adjusted easily by dissolving various amount of CO2 into organic solvents.16,20-24 In addition, dissolving CO2 can reduce the solvating power of solvents and the precipitation of solutes is triggered in CO2-expanded liquids. On the basis of these phenomena, various methods such as gas antisolvent precipitation (GAS), precipitation with compressed antisolvent (PCA), supercritical antisolvent (SAS), solution enhanced dispersion by supercritical fluids (SEDS), and depressurization of an expanded liquid organic solution (DELOS) were adopted to precipitate fine particles composed of inorganic compounds, organic compounds, explosives, pharmaceuticals, and polymers.25-27 Recently, Roberts’ group reported the depositions of ligandcapped metal nanoparticles in CXLs to accomplish the uniform wide-area nanoparticle films and size-selection fractionation.28,29 In spite of these merits of tunable CXLs, however, applying CXLs (16) Jessop, P. G.; Subramaniam, B. Chem. Rev. 2007, 107, 2666. (17) Eckert, C. A.; Liotta, C. L.; Bush, D.; Brown, J. S.; Hallett, J. P. J. Phys. Chem. B 2004, 108, 18108. (18) Akien, G. R.; Poliakoff, M. Green Chem. 2009, 11, 1083. (19) Kordikowski, A.; Schenk, A. P.; VanNielen, R. M.; Peters, C. J. J. Supercrit. Fluids 1995, 8, 205. (20) Yin, J. Z.; Tan, C. S. Fluid Phase Equilib. 2006, 242, 111. (21) Lin, I. H.; Tan, C. S. J. Chem. Eng. Data 2008, 53, 1886. (22) Lin, I. H.; Tan, C. S. J. Supercrit. Fluids 2008, 46, 112. (23) Lopez-Castillo, Z. K.; Aki, S. N. V. K.; Stadtherr, M. A.; Brennecke, J. F. Ind. Eng. Chem. Res. 2008, 47, 570. (24) Xie, Z. Z.; Snavely, W. K.; Scurto, A. M.; Subramaniam, B. J. Chem. Eng. Data 2009, 54, 1633. (25) Jung, J.; Perrut, M. J. Supercrit. Fluids 2001, 20, 179. (26) Shariati, A.; Peters, C. J. Curr. Opin. Solid State Mat. Sci. 2003, 7, 371. (27) Yeo, S. D.; Kiran, E. J. Supercrit. Fluids 2005, 34, 287. (28) McLeod, M. C.; Anand, M.; Kitchens, C. L.; Roberts, C. B. Nano Lett. 2005, 5, 461. (29) McLeod, M. C.; Kitchens, C. L.; Roberts, C. B. Langmuir 2005, 21, 2414.

Published on Web 03/18/2010

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Article Scheme 1. Illustration of Synthesis of Silver Nanoparticles (AgNPs) in CO2-Expanded Heptane from Silver Isostearate (AgISt) Precursor with Hydrogen (H2) as the Reducing Gas

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Experimental Section Materials. Silver nitrate (99%, Showa), sodium hydroxide (97%, Sigma-Aldrich), stearic acid (95%, Sigma-Aldrich), isostearic acid (95%, TCI), and heptane (99%, Echo Chemical) were used as received without further purification. Triply distilled water (resistivity greater than 18 M cm) was produced by Millipore Milli-Q system. Carbon dioxide (99.5%) and hydrogen (99.98%) were purchased from Taiwan San Fu Gases Ltd. (Hazard: The flammable and explosive limits of hydrogen gas mixed with air are 4-75% and 18-59% in volume percentage. Hydrogen gas must be handled in well-ventilated condition.) Syntheses of Silver Isostearate (AgISt) and Silver Stearate (AgSt). AgISt was synthesized by the cation exchange

reaction of sodium salt of isostearic acid and silver nitrate.34 2.85 g of isostearic acid (10 mmol), 0.4 g of sodium hydroxide (10 mmol), and 100 mL triply distilled water were well-mixed at 70 °C for 30 min, then a clear solution containing sodium isostearate was formed. To this sodium isostearate solution, 100 mL of aqueous solution of silver nitrate (1.7 g, 10 mmol) was added dropwise. The AgISt powders yielded were collected, washed, and then dried under reduced pressure at 40 °C for 24 h. AgSt powders were also prepared by the same procedure using sodium salt of stearic acid and silver nitrate as reagents.

Synthesis of Silver Nanoparticles (AgNPs) in CO2-Expanded Heptane. A 50 mL glass vial containing AgISt/heptane

as process medium to synthesize metal nanoparticles has not been reported yet. Silver nanoparticles (AgNPs) have attracted considerable interest in many applications owning to their intrinsic size- and shape-dependent effects on antibacterial, catalytic, electronic, and optical properties. The direct synthesis of silver organosol is known to be a problem due to the poor solubility of silver ion in organic media. Consequently, the two-phase methods were adopted to prepare metal organosol through the phase transfer of the metal ions or presynthesized metal nanoparticles from an aqueous medium to an organic solvent.30,31 The disadvantages of the two-phase method were the necessity of toxic phase transfer agents such as tetraoctylammonium bromide or HCl, as well as the complicated process. In this study, a simple procedure was proposed to prepare silver organosol. AgNPs were synthesized in CO2-expanded heptane from silver precursor with hydrogen (H2) as reducing agent (Scheme 1). Instead of using water-soluble AgNO3, a hydrocarbonbased precursor, silver isostearate (AgISt), was employed as the silver ion source in the organic phase. It was reported that silver nanoparticles capped with isostearic acid were stably dispersed in compressed CO2.32,33 This suggests that the branched alky chains in AgISt precursor may counteract the antisolvent effect of CO2, as well as avoid the precipitation of AgISt from CO2-expanded liquids. Our method has an advantage over sc-CO2 based technology owing to the relatively low process pressure and the avoidable using of fluorinated reagents. Besides, the flammable and explosive H2 gas is diluted by a great quantity of CO2 to provide the fire suppression. To the best of our knowledge, no previous reports had demonstrated the direct synthesis of metal nanoparticles using CXLs as a reaction medium. (30) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc. Chem. Commun. 1994, 801. (31) Sarathy, K. V.; Raina, G.; Yadav, R. T.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1997, 101, 9876. (32) Bell, P. W.; Amand, M.; Fan, X.; Enick, R. M.; Roberts, C. B. Langmuir 2005, 21, 11608. (33) Anand, M.; Bell, P. W.; Fan, X.; Enick, R. M.; Roberts, C. B. J. Phys. Chem. B 2006, 110, 14693.

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solution (0.25 mM, 20 mL) was placed into a 150 mL stainless steel reactor. At 40 °C, the reactor was filled with H2 to 7 bar, followed by adding CO2 to build to the total pressure of 55 bar. Through the reduction reaction, AgNPs were formed in CO2-expanded heptane. Finally, a clear yellowish silver organosol was obtained by the depressurization of H2/CO2. Instead of using CO2expanded heptane, AgNPs were also synthesized in heptane in the presence of pure H2 under the pressures of 7 and 55 bar, respectively. Characterizations. The X-ray diffraction (XRD) pattern was recorded by a Rigaku Ultima IV X-ray diffractometer using Cu KR radiation operated at 40 kV and 200 mA. The 2θ angle was measured from 4° to 20° with a resolution of 0.05°. The differential scanning calorimeter (DSC, TA Instrument 2010) and the thermogravimetric analyzer (TGA, Perkin-Elmer TGA7) were used to evaluate the thermal properties of samples at a heating rate of 10 °C/min under N2 atmosphere. UV-visible spectrum was obtained using a Carry 50 Conc spectrophotometer in the range 280-800 nm, with a resolution of 2 nm. TEM image was performed on a JEOL JEM-2100 (HT) transmission electron microscope at 200 kV after placing a drop of silver organosol on a 200 mesh carbon-coated copper grid. ImageJ software was used to analyze the size distribution and average diameter of AgNPs. The attenuated total reflection Fourier transform infrared (ATR-FTIR) spectrum was performed by Perkin-Elmer Spectrum RXI FTIR spectrometer with 2 cm-1 resolution and 64 scans. The silver organosol was dried under reduced pressure at 40 °C to remove heptane, and the resulting black precipitate was used as the sample to examine.

Results and Discussion The solventless approaches have been developed to produce AgNPs through thermal decomposition of silver stearate (AgSt).34-36 If a wet process was applied, AgSt was not suitable to be used as silver precursor because of its low solubility in solvents.37,38 In this study, AgISt containing methylated branch alky chains was synthesized and found to have good (34) Abe, K.; Hanada, T.; Yoshida, Y.; Tanigaki, N.; Takiguchi, H.; Nagasawa, H.; Nakamoto, M.; Yamaguchi, T.; Yase, K. Thin Solid Films 1998, 329, 524. (35) Lee, S. J.; Han, S. W.; Choi, H. J.; Kim, K. J. Phys. Chem. B 2002, 106, 2892. (36) Yang, N. J.; Aoki, K.; Nagasawa, H. J. Phys. Chem. B 2004, 108, 15027. (37) Jacobson, C. A.; Holmes, A. J. Biol. Chem. 1916, 25, 29. (38) Malik, W. U.; Jain, A. K.; Jhamb, O. P. J. Chem. Soc. A 1971, 1514.

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Figure 1. XRD diffraction pattern of AgSt and AgISt in 2θ angle ranging from 4° to 20°. Various peaks are indexed as (0k0) reflections.

solubility in nonpolar organic solvents. Thus, AgISt precursors are the feasible candidates for the synthesis of AgNPs in the wet process. The structural differences between AgSt and AgISt were investigated by XRD, as shown in Figure 1. The XRD patterns reveals that AgSt exists a set of well-defined diffraction peaks, indexed as (0k0) planes, whereas AgISt showed only one major reflection, indexed as (030) plane. It is well-known that solid-state AgSt is an eight-membered-ring dimer composed of two Agþ ions bridged by two bidentate carboxylate groups of the stearate molecules.39,40 The dimers are stacked in an orderly manner one next to another through the intermolecular interactions of Ag-O bonds, and the straight long alky chains in AgSt are regularly extended to grow the preferred crystal-like layered structure35,41,42 resulting in a set of well-defined peaks in the XRD diffraction. In addition, the strong Ag-O bonds between dimers and the perfect layered structure lead to the poor solubility of AgSt in solvents. On the contrary, an AgISt dimer consists of four asymmetrical methylated branched alky chains. These branched chains not only exhibit steric barriers to the Ag-O bonding between AgISt dimers, but also inhibit the formation of regular layered crystal in AgISt. Hence, AgISt was found soluble in various nonpolar solvents including hexane, heptanes, toluene, and xylene. The schematic structures of AgSt and AgISt are depicted in Figure 2. The DSC and TGA traces of AgSt and AgISt are shown in Figure 3. In AgSt, two endothermic transitions at 125 °C (ΔH = 47 J/g) and 153 °C (ΔH = 35 J/g) represent the crystal-to-crystal transition and melting transition, respectively.40 However, a weak endothermic transition at 116 °C (ΔH = 3 J/g) is shown in AgISt, providing clear evidence of the less-regular layered structures of AgISt. TGA traces show that the residual mass fractions of AgSt and AgISt at 800 °C are 28.0 and 28.3 wt %, respectively, slightly higher than the mass fraction of metallic silver in C18H35O2Ag (27.6 wt %). In this report, the chemical reduction method was employed to synthesize AgNPs using AgISt and H2 as the precursor and reducing agent, respectively. Figure 4 shows the evolutions of (39) Tolochko, B. P.; Chernov, S. V.; Nikitenko, S. G.; Whitcomb, D. R. Nucl. Instrum. Methods Phys. Res. Sect. A-Accel. Spectrom. Dect. Assoc. Equip. 1998, 405, 428. (40) Binnemans, K.; Van Deun, R.; Thijs, B.; Vanwelkenhuysen, I.; Geuens, I. Chem. Mater. 2004, 16, 2021. (41) Lin, B.; Dong, J. S.; Whitcomb, D. R.; McCormick, A. V.; Davis, H. T. Langmuir 2004, 20, 9069. (42) Dong, J. S.; Whitcomb, D. R.; McCormick, A. V.; Davis, H. T. Langmuir 2007, 23, 7963.

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Figure 2. Schematic representation of the bilayer structures of (a) AgSt and (b) AgISt.

Figure 3. (a) DSC traces of AgSt and AgISt. The inset shows the enlarged portion of AgISt. (b) TGA traces of AgSt and AgISt.

UV-visible absorption of AgNPs-a (PH2 = 7 bar) and AgNPs-b (PH2 = 55 bar) in the range 280-800 nm. By increasing the pressure of H2 from 7 to 55 bar, the rate of reduction reaction to form AgNPs was significantly increased. The maximum absorbance values, λmax, of AgNPs-a and AgNPs-b appeared at 435 and 440 nm, while the reduction times were 60 and 10 min, respectively. Further increase the reduction time, no obvious changes in the absorbance of AgNPs were observed. The asymmetrical absorption bands of AgNPs-a and AgNPs-b implied that particles were generated with broad size distribution. TEM images and the corresponding particle size distribution histograms of AgNPs-a and AgNPs-b are shown in Figure 5. In TEM DOI: 10.1021/la100147c

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Figure 6. Evolution of UV-visible absorption spectra of AgNPs synthesized in CO2-expanded heptane. The inset represents the photograph of transparent AgISt/heptane solution and the resulting yellowish Ag organosol.

Figure 4. Evolution of UV-visible absorption spectra of (a) AgNPs-a (PH2 = 7 bar) and (b) AgNPs-b (PH2 = 55 bar).

Figure 5. TEM image and corresponding particle size distribution histogram of (a) AgNPs-a and (b) AgNPs-b. The scale bar represents 20 nm.

images, the broad size distribution of AgNPs is found. By counting 300 particles to estimate the average particle size, the average diameters of AgNPs-a and AgNPs-b were 3.63 ( 1.95 and 4.97 ( 2.60 nm, respectively. In order to enhance the reaction rate, as well as narrow the particle size distribution, compressed CO2 was employed to 10034 DOI: 10.1021/la100147c

expand the AgISt/heptane solution and the effect of CXLs on the synthesis of AgNPs was investigated. Figure 6 shows the evolution of UV-visible absorption spectra of AgNPs that were synthesized in CO2-expanded heptane with H2 at 7 bar, followed by adding CO2 to build up the total pressure of 55 bar. While the reduction time was 3 min, a broad absorption band with λmax at 437 nm was observed. As the reduction time progressed to 30 min, the λmax was blue-shifted to 412 nm and the symmetric intense absorbance was interpreted as the surface plasmon resonance of AgNPs. When the reduction time was further increased, no obvious change in the absorbance of AgNPs was found. As compared with AgNPs-a (Figure 4a), the formation of AgNPs in CO2-expanded heptane occurred at a faster reaction rate. It was reported that the viscosity of the alkane hydrocarbons mixture was decreased by adding CO2.43 The similar tendency was also found in other CO2-expanded organic solvents such as methanol, acetonitrile, and cyclohexane.44 Moreover, Tan’s group showed that the diffusivity of solutes as well as H2 solubility could be enhanced in CXLs.20-22 Bogel-Lukasik et al. reported that the hydrogenation rate of limonene in CXLs became faster compared to the pure H2 system without adding CO2.45 Therefore, the decrease of solution viscosity, increase of solute diffusivity, and higher H2 solubility in solvents were possible reasons to improve the mass transport in CO2-expanded heptane, as well as the formation rate of AgNPs. The photographs of AgISt/heptane solution and the resulting silver organosol are also shown in the inset of Figure 6. The clear yellowish silver organosol indicated that AgNPs were well-dispersed in heptane. TEM image and particle size distribution histogram of AgNPs formed in CO2-expanded heptane are illustrated in Figure 7. More uniform particles were synthesized in CO2-expanded heptane, and the average diameter of particle was 2.64 ( 0.51 nm. The selected area electron diffraction (SAED) pattern shown in Figure 7b demonstrated the four rings of (111), (200), (220), and (311) crystalline planes formed in AgNPs, which indicated the typical face-centered cubic (fcc) structure in metallic silver. In comparing with AgNPs-a and AgNPs-b, AgNPs synthesized in CO2-expanded heptane had smaller particle size and narrower size distribution. According to the crystal growth kinetics, more uniform nanoparticles were obtained upon the fast nucleation (43) Barrufet, M. A.; Salem, S. K. E.; Tantawy, M.; IglesiasSilva, G. A. J. Chem. Eng. Data 1996, 41, 436. (44) Li, H. P.; Maroncelli, M. J. Phys. Chem. B 2006, 110, 21189. (45) Bogel-Lukasik, E.; Fonseca, I.; Bogel-Lukasik, R.; Tarasenko, Y. A.; da Ponte, M. N.; Paiva, A.; Brunner, G. Green Chem. 2007, 9, 427.

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Figure 8. ATR-FTIR spectrum of (a) isostearic acid, (b) AgISt, and (c) AgNPs synthesized in CO2-expanded heptane.

Figure 7. (a) TEM image, (b) SAED pattern, and (c) corresponding particle size distribution histogram of the AgNPs synthesized in CO2-expanded heptane. The scale bar represents 20 nm.

and slow growth in synthetic process.46 The enhanced mass transport in CO2-expanded heptane might lead to the result of a large amount of nuclei formed simultaneously at the beginning stage of reaction. Accordingly, the synthesized AgNPs had relatively smaller particle size and narrower size distribution. The attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra of isostearic acid, AgISt, and AgNPs synthesized in CO2-expanded heptane are shown in Figure 8. The transmittances at 1702 and 928 cm-1 represented the CdO and out-of-plane O;H stretching bands of carboxylic acid group in isostearic acid, respectively. As the coordination between silver cation (Agþ) and carboxylate anion group (COO-) was proceeded to form AgISt, the transmittances of the CdO and O;H bands of carboxylic acid group in isostearic acid disappeared. Consequently, the new asymmetric (νas(COO-)) and symmetric stretching (νs(COO-)) bands of the silver carboxylate group in AgISt appeared at 1519 and 1393 cm-1, respectively. In AgNPs, the stretching bands at 1702 and 928 cm-1 were observed again due to the presence of the produced isostearic acid. Furthermore, the asymmetric (νas(COO-)) stretching was shifted to 1541 cm-1, indicating that the carboxylate groups of the produced isostearic acid were capped on the surface of AgNPs. Because of high (46) Peng, X. G.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343.

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surface area-to-volume ratio, the surfaces of naked nanoparticles possess a large free energy leading to the tendency of aggregation.47 Thus, the adsorption of isostearic acid on AgNPs not only provided the steric repulsion to limit the growth of particle size but also exhibited solvent-ligand interactions to disperse the AgNPs in CO2-expanded heptane without precipitation or aggregation. By contrast, the common ligand-capped metal nanoparticles, such as dodecanethiol-capped silver and gold nanoparticles, were deposited from CXLs because of the insufficient solvation strength of thiol molecules to overcome the interparticle attraction.28,29 Hence, the synthesized AgNPs had small size of 2.64 ( 0.51 nm and were well-dispersed in heptane. To the best of our knowledge, this report first demonstrated the direct synthesis of metal nanoparticles using CXLs as a reaction medium.

Conclusion In conclusion, the soluble hydrocarbon-based AgISt was employed as silver precursor to synthesize AgNPs in CO2expanded heptane under mild condition at 55 bar. The merit of using CO2-expanded heptane as the process medium was the efficient formation of AgNPs with smaller particle size and narrower size distribution. The isostearic acid derived from AgISt precursor was capped on the surfaces of AgNPs, resulting in the stable dispersion of AgNPs in heptane. Acknowledgment. This work was supported by the National Science Council of the Republic of China under grant NSC 952221-E-007-130-MY3 and NSC 96-2628-E-007-125-MY3. (47) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293.

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