Charge-Transfer Reactions of Silver Stearate-Coated Nanoparticles in

Voltammetry of silver stearate-stabilized nanoparticles in the cyclohexane/acetonitrile solution showed a reduction wave at −0.6 V and an oxidation ...
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Langmuir 2003, 19, 9904-9909

Charge-Transfer Reactions of Silver Stearate-Coated Nanoparticles in Suspensions Koichi Aoki,* Jingyian Chen, Nianjun Yang, and Hiroshi Nagasawa† Department of Applied Physics, Fukui University, 3-9-1 Bunkyo, Fukui-shi, 910-8507 Japan Received June 27, 2003. In Final Form: August 21, 2003 Voltammetry of silver stearate-stabilized nanoparticles in the cyclohexane/acetonitrile solution showed a reduction wave at -0.6 V and an oxidation wave at 1.1 V. From TEM, AFM, and UV-vis spectroscopy, the nanoparticles were deduced to be nearly monodispersed spherical particles 5 nm in diameter with the core-shell structure, which was composed of the core by Ag atoms and the shell by silver stearate. The oxidation and the reduction waves were caused by the silver atoms in the core and silver stearate in the shell, respectively. The voltammetric peak currents and the limiting currents at the rotating disk electrode were mostly controlled by diffusion of the nanoparticles in the solution. These currents allowed us to evaluate the ratio of the number of Ag atoms in the core to that in the silver stearate to be ca. 9.6, which agreed with the value obtained from gravimetry by the chemical oxidation. From this value and the diameter of the nanoparticles, we infer that one-third of the silver atoms on the particle surface should be bonded with stearate rather than a monolayer coating to form the shell.

Introduction Electrochemical properties of silver nanoparticles adsorbed on electrodes are different from those of bulk silver in that the nanoparticles increase in surface-enhanced Raman scattering,1 the hybridization detection of DNA,2 the catalytic reduction rates of cytochrome c and 2,6dichloroindophenol3 and of poly(phenylpyrrole) at a liquidliquid interface,4 and the heterogeneous rate constants.5 The electrochemical quantized capacitance, which has been observed at gold nanoparticles,6 has been found also at silver nanoparticles.7 These properties are believed to be ascribed to the compressed lattice constant as much as 9%,8 which decreases the melting point by as much as 700 °C.9 The extensive applicability of silver nanoparticles † Development Department 2, Development Center 2, Ebara Corp. * Corresponding author: Tel +81-776-278665; fax +81-776278494; e-mail [email protected].

(1) (a) Bright, R. M.; Musick, M. D.; Natan, M. J. Langmuir 1998, 14, 5695. (b) Zheng, J.; Li, X.; Gu, R.; Lu, T. J. Phys. Chem. B 2002, 106, 1019. (2) (a) Cai, H.; Xu, Y.; Zhu, N.; He, P.; Fang, Y. Analyst 2002, 127, 803. (b) Cai, H.; Wang, Y.; He, P.; Fang, Y. Anal. Chim. Acta 2002, 469, 165. (c) Wang, J.; Polsky, R.; Xu, D. Langmuir 2001, 17, 5739. (3) Sibbald, M. S.; Chumanov, G.; Cotton, T. M. J. Electroanal. Chem. 1997, 438, 179. (4) Johans, C.; Clohessy, J.; Fantini, S.; Kontturi, K.; Cunnane, V. J. Electrochem. Commun. 2002, 4, 227. (5) Bright, R. M.; Musick, M. D.; Natan, M. J. Langmuir 1998, 14, 5695. (6) (a) Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J.; Whetten, R. L.; Bigioni, T. P.; Guthrie, D. K.; First, P. N. J. Am. Chem. Soc. 1997, 119, 9279. (b) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098. (c) Chen, S.; Murray, R. W.; Feldberg, S. W. J. Phys. Chem. B 1998, 102, 9898. (d) Hicks, J. F.; Templeton, A. C.; Chen, S.; Sheran, K. M.; Jasti, R.; Murray, R. W.; Debord, J.; Schaaff, T. G.; Whetten, R. L. Anal. Chem. 1999, 71, 3703. (e) Chen, S.; Murray, R. W. Langmuir 1987, 3, 682. (f) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (g) Chen, S. J. Phys. Chem. 2000, 104, 663. (h) Chen, S. J. Am. Chem. Soc. 2002, 124, 5280. (7) Cheng, W.; Dong, S.; Wang, E. Electrochem. Commun. 2002, 4, 412. (8) (a) Wasserman, H. J.; Vermaak, J. S. Surf. Sci. 1970, 22, 164. (b) Apai, G.; Hamilton, J. F.; Stohr, J.; Thompson, A. Phys. Rev. Lett. 1979, 43, 165. (c) Montano, P. A.; Schulze, W.; Tesche, B.; Shenoy, G. K.; Morrison, T. I. Phys. Rev. B 1984, 30, 672. (d) Montano, P. A.; Purdum, H.; Shenoy, G. K.; Morrison, T. I.; Schulze, W. Surf. Sci. 1985, 156, 228.

has promoted the electrochemical synthesis at the aim of forming monodispersed nanoparticles.10 Surfaces of metal nanoparticles are necessarily protected with some kinds of surfactants; otherwise, nanoparticles might aggregate in solution or be stabilized on substrate in precipitate form. For example, gold nanoparticles are often coated with organic monolayer films such as organothiolate,11 organophosphine,12 and organoamine.13 Palladium nanoparticles are also coated with organic layers.14 Silver nanoparticles have been synthesized by stabilization with 3-mercaptopropionic acid,7 poly(phenylpyrrole),4 dodecanethiol,15 the hydrogenterminated Si(100) surface,16 iodide on electrode surfaces,3 fatty acids,17 citrate,1a,18 and ethylenediaminetetraacetic acid.1a,19 To exhibit specific properties of nanoparticles, the particle size should be close to monodispersed, as has (9) (a) Buffat, P.; Borel, J.-P. Phys. Rev. A 1976, 13, 2287. (b) Castro, T.; Reifenberger, R.; Choi, E.; Andres, R. P. Phys. Rev. B 1990, 42, 8548. (10) (a) Liu, H.; Favier, F.; Ng, K.; Zach, M. P.; Penner, R. M. Electrochim. Acta 2001, 47, 671. (b) Ng, K. H.; Liu, H.; Penner, R. M. Langmuir 2000, 16, 4016. (c) Rodriguez-Sanchez, L.; Blanco, M. C.; Lopez-Quintela, M. A. J. Phys. Chem. B 2000, 104, 9683. (d) Stiger, R. M.; Gorer, S.; Craft, B.; Penner, R. M. Langmuir 1999, 15, 790. (11) (a) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (b) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397. (c) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1997, 119, 12384. (d) Yamada, M.; Tamon, T.; Kubo, K.; Nishihara, H. Langmuir 2001, 17, 2363. (12) Schmid, G. Inorg. Synth. 1990, 27, 214. (13) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 882. (14) (a) Yamada, M.; Quiros, I.; Mizutani, J.; Kubo, K.; Nishihara, H. Phys. Chem. Chem. Phys. 2001, 3, 3377. (b) Quiros, I.; Yamada, M.; Kubo, K.; Mizutani, J.; Kurihara, M.; Nishihara, H. Langmuir 2002, 18, 1413. (15) Aslam, M.; Chaki, N. K.; Mulla, I. S.; Vijayamohanan, K. Appl. Surf. Sci. 2001, 182, 338. (16) Stiger, R. M.; Gorer, S.; Craft, B.; Penner, R. M. Langmuir 1999, 15, 790. (17) (a) Nagasawa, H.; Maruyama, M.; Komatsu, T.; Isoda, S.; Kobayashi, T. Phys. Status Solidi A 2002, 191, 67. (b) Kuwajima, S.; Okada, Y.; Yoshida, Y.; Abe, K.; Tanigaki, N.; Yamaguchi, T.; Nagasawa, H.; Sakurai, K.; Yase, K. Colloids Surf. A 2002, 197, 1. (c) Abe, K.; Hanada, T.; Yamaguchi, T.; Takiguchi, H.; Nagasawa, H.; Nakamoto, M.; Yase, K. Mol. Cryst. Liq. Cryst. 1998, 322, 173. (d) Abe, K.; Hanada, T.; Yoshida, Y.; Tanigaki, N.; Takiguchi, H.; Nagasawa, H.; Nakamoto, M.; Yamaguchi, T.; Yase, K. Thin Solid Films 1998, 327, 524. (18) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391. (19) Lee, N.-S.; Sheng, R.-S.; Morris, M. D.; Schopfer, L. M. J. Am. Chem. Soc. 1986, 108, 6179.

10.1021/la035144g CCC: $25.00 © 2003 American Chemical Society Published on Web 10/11/2003

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Figure 2. HRTEM micrograph of the silver stearate-coated nanoparticles. Figure 1. Illustration of structure of the silver stearate-coated nanoparticle predicted from the measurements of HRTEM, AFM, and UV-vis spectroscopy. The light gray circles are silver atoms in the core, while the black circles are the silver ions of stearate in the shell. The particle excluding the stearate acid group has the radius a. The diameter of silver atom is denoted by h.

been pointed out in the measurement of the quantized double layer capacitance.6a Nearly monodispersed particles have been synthesized with thermal decomposition of silver fatty acid salt at 250 °C.17c,d This method has another advantage in carrying out mass production of silver nanoparticles because of simple combination of the wet process and the low-temperature heating. The silver fatty acid salt nanoparticles thus synthesized exhibit the surface plasmon spectra, similar to the conventional thiol-stabilized silver nanoparticles. The spectra have been simulated well with Mie’s theory20 by use of the size obtained from high-resolution transmitted electron microscopy (HRTEM). The Ag lattice structure, especially the (111) surface, has been observed at the center of the nanoparticles through X-ray diffraction.17a Consequently, the central part is occupied with silver metal. The AFM image of the particles on a mica showed the flattened hemisphere which was surrounded with a protuberating ring, like a caldera, whereas that of the particles heated up to the decomposition temperature of the fatty acid showed only the flattened hemisphere.17a Thus, the particle seems to be composed of a core of silver crystal and a shell of fatty acid silver salt, as is illustrated in Figure 1. This shell obviously plays a part in protecting the particles against aggregation like a surfactant. This protection is responsible for catastrophic variation of electric conduction with temperature.17a The protective shell is predicted to exhibit the electrochemical behavior different from the core silver crystal because the silver atom of the carboxylate is partially charged by the dissociation to the carboxylic acid. Thus, it may be reduced electrochemically. In contrast, the oxidation may occur at the core, destroying the shell or passing the charge through the shell. This anticipation leads us to discriminating electrochemically the core silver atoms from the shell and hopefully to evaluate the amount of silver atoms in the shell and the core independently. On this expectation, we perform in this paper voltammetry of silver stearate-coated nanoparticles in organic suspensions. Experimental Section Chemicals. All the chemicals were of analytical grade. Acetonitrile was treated with molecular sieves 4A 1/8 (Wako, Tokyo) in order to remove water. Cyclohexane and tetra-nbutylammonium tetrafluoroborate (TBATFB) were used as received. The silver stearate-coated nanoparticles were synthesized with the technique previously reported.17a Silver stearate was synthesized by mixing the solutions of AgNO3, stearate acid, and NaOH and by rinsing and drying the product. (20) Kleemann, W. Z. Phys. 1968, 215, 113.

Spectroscopic and Microscopic Measurements. The particle core size was characterized using a 200 kV high-resolution transmission electron microscope (HRTEM, JEOL-2000) with a magnification of about 3 350 000. The samples were prepared by spreading a 100 mm3 drop of cyclohexane, including 1 µg of the nanoparticles, on a copper grid-supported amorphous carbon film and by evaporating cyclohexane from the thin film. UV spectrometry was performed with a UV-570 spectrometer (JASCO, Tokyo). Electrochemical Measurements. Electrochemical measurements were carried out a potentiostat (model 1112, Huso, Kawasaki) controlled by a computer at room temperature. The Pt disk electrode with a diameter of 1.6 mm was used as a working electrode. A Ag|AgxO was used as a reference electrode. The potential difference from the reference electrode of Ag|AgCl in 3 M NaCl was -0.055 V. A Pt coil was used as a counter electrode. The rotating disk electrode (RDE) was made of a Pt disk 3.5 mm in diameter. It was driven with RRDE-1 (Nikko Keisoku, Atsugi). The electrolyte solution was degassed with N2 gas for at least 20 min during the entire experimental procedure.

Results and Discussion Optical and Chemical Features of Nanoparticles. We attempted to dissolve the nanoparticles in hexane, toluene, benzene, nitrobenzene, benzonitrile, o-bromotoluene, cyclohexane, heptane, dodecane, dichloromethane, chloroform, carbon tetrachloride, methanol, ethanol, 1-butanol, acetone, acetonitrile, formic acid, butyric acid, triethylamine, and N,N-dimethylformamide. Only cyclohexane dissolved the silver stearate-coated nanoparticles. The nanoparticles could not be dissolved in acidic and basic aqueous solutions. Figure 2 shows a HRTEM micrograph of the silver stearate-coated nanoparticles. The nanoparticles were mostly spherical, and the average diameter is 4.7 ( 0.6 nm, where the error denotes the standard deviation evaluated by counting 250 particles. The size distribution was actually the same as that in Figure 3 of ref 17a. The minimum and the maximum diameters were 1 and 9 nm, respectively, and they were rarely found. The lattice structure was observed at the center of the nanoparticles, suggesting the presence of the silver crystal. The distance between the neighboring layers of the lattice is 0.24 nm, which is close to the separation, 0.236 nm, of the layers in Ag(111) crystal.21 Figure 3 shows the UV-vis spectrum of the silver stearate nanoparticles in cyclohexane solutions (curve a). Two clear bands were observed at 218 and 410 nm, of which absorbances varied linearly with the concentration of the nanoparticles, as shown in the inset of Figure 3. Therefore, they should result from the absorption of the nanoparticles. Figure 3 also shows the spectrum of silver strearate (curve b) dissolved in cyclohexane. A sharp band at 216 nm can be identified with the π-π* transition of the carboxyl group.22 Therefore, the band at 218 nm in curve a is attributed to the carboxyl moiety of silver (21) Barry, J. C. J. Microsc. 1998, 190, 267. (22) Lide, D. R., Editor-in-Chief; CRC Handbook of Chemistry and Physics, 82nd ed.; CRC Press: Boca Raton, FL, 2001-2002; pp 8-127.

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Figure 3. UV-vis spectra of (a) the silver stearate-coated nanoparticles and (b) silver stearate in cyclohexane solutions. The dots represent the best fit by use of the Mie equation for the diameter of 5 nm. The insert plot is the linear relationship between maximum absorbance at 218 (full circles) and 410 nm (open circles) with the concentration of the nanoparticles, where c0 ) 1.50 mg cm-3.

stearate on the particle. On the other hand, it is wellknown that colloidal silver nanoparticles exhibit absorption at the wavelength from 390 to 420 nm20 by Mie scattering. Hence, the band at 410 nm can be attributed to a property of the silver nanometals. We obtained the best fit spectrum (circles on curve a) for the diameter of 5 nm by use of the Mie equation. This value of the diameter is consistent with that estimated from the HRTEM photograph. This value may not include the stearic acid moiety because the Mie scattering responds only to the silver metal. To examine chemical reactivity of the nanoparticles, we attempted to oxidize the silver stearate nanoparticles with concentrated nitric acid, but failed. The silver stearate molecule did not react with concentrated nitric acid, either. When a silver wire is immersed in concentrated nitric acid, its surface becomes black owing to formation of silver oxides. When the nanoparticles including cyclohexane were mixed with the concentrated nitric acid in emulsion form, it showed no color change. Therefore, the nanoparticles are protected against the oxidation by concentrated nitric acid, probably because of the stearate coating on the surface. When an extra amount of acidic (1 M H2SO4) KMnO4 solution was mixed vigorously with the nanoparticles including cyclohexane at 40 °C, its purple color disappeared and black precipitation appeared. In contrast, mixing the suspension of silver stearate (not nanoparticles) in cyclohexane with the acidic KMnO4 solution did not show any color change. When a silver wire was inserted into the acidic KMnO4 solution, its surface soon became black owing to formation of insoluble silver sulfates. Consequently, the silver metal in the nanoparticles can be oxidized with acidic KMnO4 to yield silver sulfate. To determine the amount of the oxidized silver, the silver sulfate was substituted with chloride by adding NaCl until the black precipitate disappeared. The reaction follows:

10Ag(nanoparticle) + 2MnO4- + 6H+ + 5H2SO4 f 5Ag2SO4 + 2Mn2+ + 8H2O (1) Ag2SO4 + 2Cl- f 2AgCl + SO42-

(2)

AgCl was filtered, rinsed with methanol and with water, dried, and weighed to be 8.45 mg. This mass corresponds to 58.9 µmol of AgCl or 6.36 mg of Ag in 8.72 mg of

Figure 4. Cyclic voltammograms of 1.30 mg cm-3 silver stearate-coated nanoparticles in the mixed solvent of acetonitrile and cyclohexane (v/v 98/2) in 0.1 M TBATFB in (a) the wide from -0.8 to 1.3 V, (b) the positive (dashed), and (c) the negative (dashed) potential domains; voltammograms of (d) a silver wire and (e) silver stearate in the mixed solvent at the scan rate of 10 mV s-1.

nanoparticles. The weight ratio of the Ag metal in the nanoparticle is 0.73. This ratio is consistent with the value 0.70-0.76 estimated from size ratio by HRTEM.17a The remainder, 27% or 2.36 mg, should come from silver strearate. This value corresponds to 6.01 µmol. The molar ratio of the Ag metal to the silver strearate is 9.8. Voltammetry of Nanoparticles. Cyclohexane is not a good solvent for voltammetric measurements because of the low solubility of most salts, whereas acetonitrile dissolves salts. Cyclohexane mixed readily with acetonitrile by stirring the mixture for a few minutes. The mixture of 2% cyclohexane (for dissolving nanoparticle) and 98% acetonitrile (for dissolving TBATBF) was used for voltammetry. Figure 4 shows the cyclic voltammograms (curve a) of the nanoparticles in the mixed solution. The large oxidation wave, the small one, and the small reduction wave appeared at ca. 1.1, 0.1, and ca -0.6 V, respectively. To examine the effects of the reduction on the large anodic wave or those of the oxidation on the cathodic wave, we took voltammograms in the positive (from 0 to 1.4 V) and the negative (from -0.0 to -0.8 V) potential domains separately, as are shown in curves b and c, respectively. The wave at 1.1 V appeared even without the potential application at -0.7 V, and the wave at -0.6 V did as well without the potential application at 1.1 V. These waves at the second and the succeeding potential scans were very similar to those at the first scan. This observation indicates that (A) the oxidation at 1.1 V is independent of the reduction at -0.6 V and (B) the electrode reactions are not due to the adsorption but are controlled with a supply from the bulk solution. In contrast, the anodic peak at 0.1 V did not appear without the application of -0.7 V. Thus, this peak is due to the oxidation of the adsorbed products by the reduction at -0.7 V. A gray precipitate was found of the electrode surface after a long potential application at -0.7 V. The negative charge for the reduction current in the forward (from 0.0 to -0.7 V) and backward (-0.7 to 0.0 V) potential scans was almost equal to or slightly smaller than the positive charge of the anodic peak integrating from -0.0 to 0.2 V. This fact implies that the source of the anodic peak at 0.1 V is the reduction of the silver stearate moiety of the nanoparticles. A possible mechanism for the reduction and the reoxidation is

Ag(OOC(CH2)16CH3)(shell) + e- f Ag0(adsorbed) + CH3 (CH2)16COO- at -0.6 V Ag0(adsorbed) + e- f Ag+ at 0.1 V

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Langmuir, Vol. 19, No. 23, 2003 9907 Table 1. Values of nc/ns method

nc/ns

gravimetry slopes of Ip vs v1/2 slopes of Ip vs c/c0 slopes of IL vs w1/2

9.8 9.6 9.6 9.6

reaction rate constant is so large that current is controlled by diffusion, the peak current is expressed by

Ip ) 0.446nFc*A(DvnrF/RT)1/2 Figure 5. Concentration dependence of the peak currents, Ip, of the anodic wave (circles) at 1.1 V and the cathodic wave (triangles) at -0.6 V, observed at scan rate of 10 mV s-1, where c0 ) 0.43 mg cm-3.

Figure 6. Variations of the peak currents, Ip, of the anodic wave (circles) at 1.1 V and the cathodic wave (triangles) at -0.6 V with square roots of the potential scan rate in 1.30 mg cm-3 silver stearate-coated nanoparticles in the mixed solvent of acetonitrile and cyclohexane in 0.1 M TBATFB.

The anodic wave at 1.1 V is predicted to be the oxidation of Ag of the core to Ag+ in the mixed solution. To know properties of the larger anodic wave, voltammetry was performed by using a silver wire as a working electrode in the mixed solution. The voltammogram, curve d in Figure 4, shows that the current began to increase at 0.2 V owing to the oxidation of the silver wire. The oxidation potential of the nanoparticles (curve a at 1.1 V) much more positive than the oxidation of the silver wire implies that the silver metal in the core should be protected against the oxidation by the shell of silver stearate. We performed voltammetry of silver stearate molecule in the mixed solvent as a control experiment. The cathodic wave was found at -1.4 V, as is shown in curve e of Figure 4. The potential shift for the cathodic wave at -0.6 V of the nanoparticles from -1.4 V indicates that the reduction at -0.6 V is not due to the simple reduction of a moiety of silver stearate but is complicated by unstabilization of the silver stearate moiety by the closest neighboring other silver atoms. We plotted the cathodic and anodic currents against the concentrations of the nanoparticles, as is shown in Figure 5. The peak currents at the low concentrations were proportional to the concentrations. On the other hand, those at high concentrations (c > 8.5c0) did not vary with the concentration, irrespective of the anodic and the cathodic current, probably because of saturation of the nanoparticles in the mixed solvent. The proportionality as well as the saturation demonstrates that both the currents are ascribed to the nanoparticles. To examine the mass transport of the nanoparticles, we plotted the peak currents of the larger anodic and the cathodic peaks against the square root of the potential scan in Figure 6. Good proportionality was found, implying the diffusion control of the nanoparticles. If the electrode

(3)

where D is the diffusion coefficient of the nanoparticles, c* is its molar concentration, n is the number of electrons per particle relevant to the redox reaction, nr is the number of electrons (unity) of the electrode reduction of Ag+ + ef Ag, and the other variables have conventional meanings. The ratio of the slopes for the anodic peak to the cathodic peak in Figure 6 is 9.6. Values of c*, A, D, and nr should be common to those for the anodic and the cathodic currents because diffusion of the nanoparticles controls the mass transfer. Only the difference lies in n. The value of n for the oxidation, denoted as nc, may represent the number of silver atoms in the core per nanoparticle if all the silver atoms are simultaneously oxidized at one collision with the electrode. In contrast, the value of n for the reduction, denoted as ns, may stand for the number of silver stearate of the shell per nanoparticle. The ratio, nc/ns ) 9.6, is very close to the ratio, 9.8, obtained by the gravimetry for the chemical oxidation, as listed in Table 1. We try to estimate absolute values of peak currents. If we assume that the nanoparticle is a simple composite molecule of the core and the shell, the molar mass, M, can be expressed by the average:

M ) ncMAg + nsMSS

(4)

where MAg and MSS are the molar mass of silver and silver stearate. Dividing eq 4 by nc or ns gives

M/nc ) MAg + (ns/nc)MSS

(5)

M/ns ) (nc/ns)MAg + MSS

(6)

The values of M/nc and M/ns can be obtained from the known value of ns/nc in Table 1. When the nanoparticles with mass m dissolve in the solution of volume V, its molar concentration, c*, is expressed by m/MV. Then the anodic peak current can be rewritten as

(Ip)A ) 0.446(nc/M)(m/V)FA(DvnrF/RT)1/2

(7)

Similarly, the cathodic peak currents is given by

(Ip)C ) -0.446(ns/M)(m/V)FA(DvnrF/RT)1/2 (8) We estimated D from the Stokes-Einstein equation, D ) kBT/6πηa, where η is the viscosity of the solution. Since the mixed solution contained 98% acetonitrile, we assigned η to that value of acetonitrile (0.39 mPa s23). For the value of the radius a () 2.5 nm) of the silver portion of the nanoparticle (see Figure 1) obtained from the UV spectra and HRTEM photographs, we determined D ) 0.23 × 10-5 cm2 s-1. When known values of nc/M, ns/M, m, V, A, and D were inserted into eqs 7 and 8, we obtained (23) Lide, D. R., Editor-in-Chief; CRC Handbook of Chemistry and Physics, 82nd ed.; CRC Press: Boca Raton, FL, 2001-2002; pp 15-14.

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Figure 7. Illustration of electrode reaction mechanism for (i) the oxidation at 1.1 V and (ii) the reduction at -0.6 V.

(Ip)Av-1/2 ) 78.0 µA s1/2 V-1/2 and (Ip)Cv-1/2 ) -8.13 µA s1/2 V-1/2. In contrast, experimental values from Figure 6 are (Ip)Av-1/2 ) 77.1 µA s1/2 V-1/2 and (Ip)Cv-1/2 ) -8.0 µA V-1/2 s1/2. These values look consistent. Since eqs 7 and 8 are, however, valid only on the assumption of the reversible system, they should not provide accurate values of the slopes and will be discussed in the next section. From these electrochemical observations, we propose the following electrode reaction mechanism, as illustrated in Figure 7. The nanoparticle which diffuses toward the electrode surface from the bulk (i) is reduced at the silver stearate moiety of the shell to yield the deposition of silver metal on the electrode, according to the EC mechanism:

Ag(core)-Ag(OOC(CH2)16CH3)(shell) T Ag(core) - Ag+(shell) + CH3(CH2)16COO- (9) Ag(core)-Ag+(shell) + e- f Ag(core dispersed) + Ag0(adsorbed) (10) Since the charge of the cathodic wave was similar to the charge of the anodic peak at 0.1 V, the silver atoms of the core are not deposited on the electrode but are dispersed into the solution. The decrease in the overpotential (from -1.4 to -0.6 V) can be explained in terms of the less stabilization of the nanoparticle in reaction 9 owing to a number of the core silver atoms than that of silver stearate. On the other hand, the nanoparticle (ii) is oxidized at the core silver metal overcoming the electrochemical barrier (extra overpotential: 1.1 - 0.2 ) 0.9 V) of the silver stearate shell. The decrease in the barrier is ascribed not only to the distant charge transfer between the core and the electrode through the silver stearate layer but also to the partial positive charge at the Ag core through reaction 9. Rotating Disk Voltammetry. Variations of limiting currents at a rotating disk electrode (RDE) with the rotation rate can estimate accurately the diffusion current without assumption of charge-transfer rate constants. Figure 8 shows hydrodynamic voltammograms at the RDE. The almost steady-state current was obtained for the scan rates lower than 5 mV s-1 although the limiting currents were not on a plateau. The halfwave potential of the cathodic wave, (E1/2)C, ranged from -0.7 to -0.50 V, while that of the anodic wave, (E1/2)A, ranged from 1.0 to 1.2 V. As the rotation rate increased, the values of (E1/2)C and (E1/2)A shifted in the negative and the positive direction, respectively. Therefore, both charge-transfer reaction rates are slow. Figure 9 shows the dependence of the anodic limiting current, (IL)A, and the cathodic one, (IL)C, on the product

Aoki et al.

Figure 8. Hydrodynamic voltammograms of 2.2 mg cm-3 silver stearate-coated nanoparticles in the mixed solvent of acetonitrile and cyclohexane in 0.1 M TBATFB for the rotation rates (a) 900 and (b) 1600 rpm at the scan rate of 10 mV s-1.

Figure 9. Dependence of the anodic and the cathodic limiting currents at the RDE on the product of various values of ω1/2 and three values the weight concentration of silver stearate-coated nanoparticles for m/V ) 2.2 (circles), 4.4 (triangles), and (squares) 6.6 mg cm-3 in the mixed solvent of acetonitrile and cyclohexane in 0.1 M TBATFB at the scan rate of 5 mV s-1.

of the square root of rotation rate, ω1/2, and the weight concentration, m/V. The proportionality holds for both the oxidation and the reduction even if both values of ω1/2 and m/V were varied. Consequently, the current is sufficiently controlled by convective diffusion, as predicted by the Levich equation. When c* is replaced by m/VM in the Levich equation and n is substituted for nc and ns for the anodic and the cathodic current, respectively, we obtain

(IL)A ) 0.62nc(m/VM)FAD2/3υ-1/6ω1/2

(11)

(IL)C ) -0.62ns(m/VM)FAD2/3υ-1/6ω1/2

(12)

where υ is the kinematic viscosity of the solvent (acetonitrile), υ ) 0.39 mPa s/0.777 g cm-3. From the ratio of the slopes of the two lines in Figure 9, we evaluate nc/ns to be 9.6. This agrees with the values obtained by the other methods in Table 1. By evaluating nc/M and ns/M from the combination of nc/ns ) 9.6 and eqs 5 and 6, we obtained values of 0.62nc(m/VM)FAD2/3υ-1/6 and 0.62ns(m/VM)FAD2/3υ-1/6 in eqs 11 and 12 to be 0.571 and -0.0583 µA s1/2, respectively. On the other hand, values of the slope in Figure 9 are 0.528 and -0.055 µA s1/2. The theoretical values were larger than the experimental values by 8.1%. The discrepancy may be due to the overestimation of the diffusion coefficient or the radius because the estimation of the diffusion coefficient was based on the radius which did not include the domain of the stearic acid moiety. From the slope for (IL)A in Figure 9 and the known values of nc/M, m/V, A, and υ, we obtained the diffusion coefficient by use of eq 11. The new value of the diffusion coefficient is D′ ) 0.205 × 10-5 cm2 s-1, which leads to 2a′ ) 5.6 nm

Silver Stearate-Coated Nanoparticles

through use of the Stokes-Einstein equation. Consequently, the difference between the geometric radius and the diffusional radius, 0.3 ( ) (5.6 - 5.0)/2) nm, is the contribution of the stearic acid layer to diffusion. Since the length of the stearic acid is about 1.6 nm, the stearic acid moiety seems to creep on the silver surface, different from the image in Figure 1. This value suggests an image of a bent shape of stearic acid moiety caused by drag flow around the nanoparticle, as illustrated in Figure 7. We recalculated the slope of the plot of Ip vs v-1/2 in the voltammogram by use of the value of D′. Then we obtain (Ip)Av-1/2 ) 73.6 µA s1/2 V-1/2, which is slightly smaller than the experimental value (77.1 µA s1/2 V-1/2), implying a contribution of sluggish charge-transfer rate.24 We could not unfortunately determine nc, ns, and M although we could do nc/M and ns/M. A possible technique of determining nc and ns might be the bulk electrolysis. According to the previous derivation of the current-time curve for the bulk electrolysis,25 the chronoamperometric curve for a long time electrolysis is given by ln I(t) ) ln I(0) - [MI(0)/nFm]t, where the initial concentration was replaced by m/MV. The slope includes the ratio M/n. Therefore, we cannot determine each value of M, nc, or ns. Electrochemical techniques provide generally the total number of electrons transferred, i.e., nc*, but cannot discern between n and c* without knowing M. We estimate the structure of the shell from the value of nc/ns on the assumption that the arrangement of silver atoms in the nanoparticles is the same as in the bulk silver metal. The average volume of one silver atom is VAg ) MAg/dAgNA ) 0.017 nm3, where dAg is the density of bulk silver (10.5 g cm-3) and NA is Avogadro’s constant. Let the thickness of the shell be h, which does not contain stearic acid moiety (see Figure 1). Then the volumes of the core and the shell are (4π/3)(a - h)3 and (4π/3)[a3 - (a - h)3], respectively. The core includes (4π/3)(a - h)3/VAg silver (24) Matsuda, H.; Ayabe, Y. Z. Elektrochem. 1055, 59, 494. (25) Aoki, K.; Chen, J.; Ke, Q.; Armes, S. P.; Randall, D. P. Langmuir, in press.

Langmuir, Vol. 19, No. 23, 2003 9909

atoms, whereas the shell does (4π/3)[a3 - (a - h)3]/VAg silver atoms. The ratio should be equal to the value of nc/ns. Taking the ratio and calculating (a - h)3/[a3 - (a h)3] ) 9.6 leads to h/a ) 0.032 or h ) 0.080 nm for a ) 5.0/2 nm. If the silver atom were to be cubic, its side length might be VAg1/3 ) (0.017 nm3)1/3 ) 0.257 nm. Then the ratio, h/VAg1/3 ) 0.080/0.257 ) 0.33, gives the number of layers of the cubic silver atoms. The one-third indicates that all the outer silver atoms do not have stearate but that silver stearate occupy only one-third of the exposed surface. A possible structure of the shell surface is that a silver stearate molecule is located at the center of a honeycomb structure composed of six silver atoms. Conclusion The voltammetry of the silver stearate coated nanoparticles could detect successfully properties of the core and the shell independently. The nanoparticles were oxidized with a higher overpotential (1.0 V) than silver metal, whereas they were reduced with a less overpotential (0.8 V) than silver stearate. These overpotentials may be ascribed to the instability of the silver stearate moiety bound to the silver core and a potential barrier of the long length of the charge transfer through the shell. The most interesting result is to determine the ratio nc/ns to be 9.6 by means of gravimetry of the chemical oxidation, cyclic voltammetry, and voltammetry at the RDE. The determination is owing to the irreversible reactions of the oxidation in the core and of the reduction in the shell. The value of nc/ns images the structure of the shell, in which the one-third of the silver atoms on the surface are bound with stearic acid. The present technique of evaluating the ratio is useful generally for characterizing stability and reactivity of nanoparticles. Acknowledgment. This work was financially supported by Grants-in-Aid for Scientific Research (Grants 14340232, 14540556, and 15651043) from the Ministry of Education in Japan. LA035144G