Electrochemically Controlling the Size of Gold Nanoparticles (PDF ...

Journal of The Electrochemical Society, 153 共12兲 D193-D198 共2006兲

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0013-4651/2006/153共12兲/D193/6/$20.00 © The Electrochemical Society

Electrochemically Controlling the Size of Gold Nanoparticles Chien-Jung Huang,a,z Pin-Hsiang Chiu,b Yeong-Her Wang,b,c Kan-Lin Chen,d Jing-Jenn Linn,d and Cheng-Fu Yange a

Department of Applied Physics, National University of Kaohsiung, Nan-Tzu, Kaohsiung, Taiwan Institute of Electro-Optical Science and Engineering, cInstitute of Microelectronics, Department of Electrical Engineering, National Cheng-Kung University, Tainan, Taiwan d Department of Electronic Engineering, Fortune Institute of Technology, Kaohsiung, Taiwan e Department of Chemical and Materials Engineering, National University of Kaohsiung, Na-Tzu, Kaohsiung, Taiwan b

This work demonstrates the electrochemical synthesis of nanoscale gold particles using a surfactant solution. Tetradodecylammonium bromide 共TTAB兲 surfactant was applied to stabilize the gold clusters. Experimental results reveal that the size of the produced gold nanoparticles is controlled by the amount of TTAB surfactant, the current density, and the growth temperature. The size of the gold nanoparticles can be controlled in the range 58.3–8.3 nm. The particle size decreases as the amount of TTAB increases from 1 to 90 mg. The optimal current density in this study was 3 mA/cm2. The size of the produced nanoparticles increases linearly with the growth temperature from 25 to 60°C. The gold nanoparticles were observed by transmission electron microscopy, ultraviolet-visible spectrometry, and X-ray photoelectron spectroscopy. A mechanism for electrochemically controlling the size of the gold nanoparticles is presented. © 2006 The Electrochemical Society. 关DOI: 10.1149/1.2358103兴 All rights reserved. Manuscript submitted January 11, 2006; revised manuscript received July 11, 2006. Available electronically October 11, 2006.

Nanoscale materials are of great interest due to their unique optical, electrical, and magnetic properties. Extensive investigations of gold nanoparticles in biology, nonlinear optical switching, the formation of modified surfaces for surface-enhanced Raman scattering, immunoassay labeling, optical contrast agents, and catalysis revealed that the size and shape of the particles strongly determine their physical and chemical properties.1-6 Hence, controlling the particle size is very important. The electrochemical production of nanoparticles has been widely studied since the early work of Reetz et al. in 1994.7,8 Their studies indicated that size-selective nanosized transition metal particles could be prepared electrochemically using tetraalkylammonium salts as stabilizers of metal clusters in a nonaqueous medium. The electrochemical method has been demonstrated to be superior to other nanoparticle production approaches because of its lower processing temperature, modest equipment, ease of controlling the yield, low cost, and high quality.9-14 A recent study synthesized gold nanorods electrochemically by introducing a shapeinducing cosurfactant.15 Yin et al.16 developed a novel electrochemical technique for the size-controlled synthesis of spherical nanoparticles in poly共N-vinylpyrrolidone兲 solution. Bartlett et al.17 and Wiley et al.18 reported the deposition of metal using other electrochemical approaches. The authors’ research group developed the electrochemical method proposed herein to form crooked gold nanocrystals with a novel structure, using micelle templates formed from two surfactants with isopropanol addition.19,20 In addition, carefully controlling the amount of acetone solvent added to the solution of surfactants changes the shape of the gold nanoparticles from spherical to cubic.21 This work experimentally studies the synthesis of gold nanoparticles using a two-electrode electrochemical cell in surfactant solution, with special emphasis on the characteristics of the gold nanoparticles produced. The amount of surfactant, the current density, and the growth temperature are also investigated. Furthermore, a mechanism for controlling the size of gold nanoparticles using an electrochemical method is presented.

gold plate and platinum plates were cut to form the anode and the cathode, respectively. Both electrodes were initially cleaned using fine sandpaper and further cleaned using aqua regia and deionized water for 5 min each. They were then dried using nitrogen gas. Electrodes in the cell were spaced 5 mm apart and held in place using Teflon spacers. A growth solution of 0.08 M cetyltrimethylammonium bromide 共C19H42Br−N+, CTAB, Fluka, 98%兲 was prepared following the procedure presented in our previous work.19-21 The cationic CTAB surfactant served as the electrolyte. 3 mL of growth solution was put in a glass test tube. Then, a measured amount of powdered auxiliary surfactant 关tetradodecylammonium bromide 共C48H100Br−N+兲; TTAB, Fluka, 99%兴 was added to the tube, where it floated on the

Experimental The gold nanoparticles were prepared electrochemically using a simple two-electrode cell, with oxidation of the anode and reduction of the cathode. Figure 1 schematically depicts the electrochemical apparatus. The experimental cell was housed in a conventional 20 ⫻ 80 mm glass test tube. With dimensions of 30 ⫻ 10 ⫻ 0.5 mm, a

z

E-mail: [email protected]

Figure 1. 共Color online兲 Schematic diagram of the electrochemical apparatus for synthesizing gold nanoparticles.

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growth solution. The test tube was then sonicated 共ELMA T710DH, Germany兲 at 100 kHz for 5 min. The cationic TTAB surfactant, which also acts as an electrolyte, stabilized the resulting small particles. A series of samples of TTAB were made, with masses of 1–10, 30, 50, 70, and 90 mg, to determine whether the amount of TTAB has any effect on the particle size distribution. Applied voltage-controlled electrolysis was used throughout the process. Experiments were performed at three current densities 共1, 2, and 3 mA/cm2兲 and at three temperatures 共25, 40, and 60°C兲. Experimental runs were made under sonication at 100 kHz for 25 min per run. This work is a careful and systematic study of the effect of the surfactant, the applied voltage, and the growth temperature. All of these parameters must be considered to obtain synthetically useful results in a simple and reproducible manner. The microstructural characteristics of the produced nanoparticles were observed by field-emission-gun transmission electron microscopy 共FEG-TEM, Philips Tecnai G2 F20兲 at an accelerating voltage of 200 kV. TEM specimens for both microscopes were prepared by taking ⬃7 ␮L from the cell after an experimental run and mixing it with 2 mL of ethyl alcohol 共99.8%兲, and then putting a drop of the resulting mixture on a standard 200-mesh 3 mm copper grid coated with a thin carbon film 共Agar Scientific, England兲 and letting it dry in an electronic drying cabinet 共Jow Ruey Technical DRY-70, Taiwan兲 for 3 days at room temperature 共⬃25°C兲. The histograms of the distributions of particle diameters 共Di兲 for each sample were determined by making manual measurements of over 100 particles in the TEM images. The standard deviation 共␴兲 is given by the following relationship22 ␴=

再兺

Ⲑ

关N共Di − daverage兲2兴共N − 1兲2

冎

1/2

where daverage is the average diameter and N is the number of particles. The optical properties of the produced gold nanoparticles were evaluated using an ultraviolet visible 共UV/vis兲 spectrometer 共Hitachi 3310, Japan兲. X-ray photoelectron spectroscopy 共XPS, Escalab-210, VG Scientific兲 was employed to study the electron spectra and perform the elemental analysis. The sample for XPS characterization was prepared by drop-casting a single drop of growth solution onto a clean glass slide 共1 ⫻ 1 cm兲 and then drying it in air.

Figure 2. Transmission electron micrographs of gold nanoparticles prepared using different amounts of TTAB surfactant; 共a兲 1, 共b兲 10, 共c兲 30, 共d兲 50, 共e兲 70, and 共f兲 90 mg; scale bar represents 100 nm.

Results and Discussion Influence of the amount of surfactant on particle size.— In the electrochemical process, the bulk metal at the anode is oxidized to metal cations, which migrate to the cathode where reduction occurs with the formation of adatoms. Such clusters are trapped by the surfactant. The cationic TTAB 共C48H100Br−N+兲 surfactant is based on nitrogen atoms that carry the cationic charges and is adsorbed strongly onto most solid surfaces. This surfactant is seen as the electrolyte and a stabilizer in the growth solution, as shown in Fig. 1. The process was repeated using 1, 10, 30, 50, 70, and 90 mg of TTAB under otherwise identical conditions 共40°C, 2 mA/cm2兲 to determine whether the amount of TTAB surfactant affects particle size and optical properties. The TEM images of gold nanoparticles produced from the different amounts of TTAB, shown in Fig. 2a-f, all have the same scale bar. These TEM micrographs clearly show that the particle size can be controlled by the selection of the appropriate amount of TTAB. Figure 2f shows the smallest particles. Figure 3 presents a typical average diameter 共daverage兲 and standard deviation 共␴兲 of the gold nanoparticles. Statistical counting of more than 100 particles revealed that the average diameters of the gold nanoparticles were 58.3 ± 12.6, 44.6 ± 18.1, 36.7 ± 19.6, 25.3 ± 11.7, 17.2 ± 10.2, and 10.6 ± 6.5 nm. Notably, the particle size was inversely proportional to the amount of TTAB between 1 and 90 mg. Figure 4 shows the UV/vis absorption spectra data for solutions in experimental runs with various amounts of TTAB. Each sample appeared cloudy red to the human eye. Figure 4 shows that the gold

Figure 3. Histogram of the average diameter 共daverage兲 in the distribution with standard deviation 共␴兲, for gold nanoparticles formed by adding 共a兲 1, 共b兲 10, 共c兲 30, 共d兲 50, 共e兲 70, and 共f兲 90 mg of TTAB.



Figure 4. 共Color online兲 UV/vis absorption spectra of aqueous dispersions of gold nanoparticles with various amounts of TTAB and the dependence of the maximum SPR band on the amount of TTAB.

nanoparticles had a maximum absorbance band at around 540 nm, a value which agrees very well with the values in the literature, from 500 and 560 nm for the surface plasmon resonance 共SPR兲 band of spherical gold nanoparticles.23,24 The optical absorption and scattering of a spherical metal nanoparticle whose diameter is much smaller than the wavelength can be described using the quasistatic approximation. Based on the assumption that the surroundings are an isotropic matrix with dielectric constant ␧m, the actual absorption of light by the solution is given by the following equation25 Qabs =

3/2 18␲NV␧m ␧2 ⫻ ␭ 共␧1 + 2␧m兲2 + ␧22

关1兴

where Qabs is the absorption cross section of the particle, N is the number of nanoparticles per unit volume, V is the volume of nanoparticles, and ␭ is the wavelength of the absorbing radiation. ␧1 and ␧2 represent the real and imaginary parts of the material dielectric function, respectively 共␧ = ␧1 + i␧2兲. Use of Eq. 1 assumes that the spherical metal nanoparticles are far enough apart to scatter independently and that no multiple scattering occurs. The condition for maximum absorption of the SPR band is given by the following expression ␧1 = − 2␧m

关2兴

provided that damping is weak. The metal is taken to have the simple dielectric function ␧1 = ␧⬁ −

␭2 ␭2p

关3兴

and the peak position of the SPR band obeys 2 ␭position = ␭2p共␧⬁ + 2␧m兲

关4兴

where ␧⬁ is the high frequency value of the dielectric function. The value of ␧⬁ 共about 13.2 for gold兲 is determined by all of the transitions within the metal at UV and higher frequencies. ␭p is the plasma wavelength of the bulk metal and is given by ␭2p = 4␲2c2m␧0 /ne2

关5兴

where n is the concentration of free electrons in the metal, m is the effective mass of the conduction electrons, c is the velocity of light in vacuum, e is the electron charge, and ␧0 is the vacuum permittivity. As is clear from Eq. 4, the peak position of the SPR band is sensitive to the solvent dielectric constant 共␧m兲 and the plasma wavelength of the bulk metal 共␭p兲. ␭p varies inversely with the square root of n. The most important parameter that affects ␭p is n,

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which is directly related to the particle size26 and the simple phenomenological description of the adsorbate effect.27 When small nanoparticles are either positively or negatively charged, this is sensitively reflected as either a decrease or an increase in n because the particle volume is small.28,29 Additionally, some molecules such as thiols, surfactants, or polar solvents are intimately bound to the particle surface, and they may provide or withdraw additional electron density at the interface.30-32 Equation 4 clearly states that changes in the particle charge density and solution dielectric constant alter the plasma frequency and can cause a shift in the peak position of the SPR band. In this study, the inset in Fig. 4 plots the relationship between the peak position of the maximum absorption SPR band for various amounts of surfactant TTAB and average particle size. The peak position of the SPR band decreases with a blue shifting from 540 to 522 nm as the particle size decreases from 58.3 to 10.6 nm. The peak with the shortest SPR band is observed for particles produced by 90 mg of TTAB, indicating the presence of particles of the minimum size. For very small particles, the SPR absorption peak is broadened so much that it disappears, which is consistent with other reports.33,34 Furthermore, the intensity of the SPR band decreased as the amount of TTAB increased. This relation is explained by Eq. 1, which states that the intensity of the SPR band depends strongly on N and V. During an electrochemical process, the bulk gold at the anode is oxidized to yield gold cations, which then migrate to the cathode where reduction yields gold adatoms. These gold adatoms are trapped by the surfactant to form gold nanoparticles. Surfactant molecules form a bilayer structure around the gold nanoparticles, in which the inner layer is bound to the gold surface via the surfactant headgroups.35 Therefore, the surfactant is usually considered to be a stabilizer or a micelle-template, which controls the size of the gold nanoparticles 共Fig. 1兲. The surfactant around the cathode gradually changed in color from off-white at the beginning of the reaction to a ruby red at the end of the reaction; the red substance diffused slowly from the cathode surface to the surfactant solution during electrolysis. The appearance of the red color reveals the formation of gold nanoparticles. Under the given experimental conditions, the amount of gold nanoparticles decreases as the amount of TTAB increases. Deposition of gold ions on the cathode surface decreased as the amount of TTAB increased, because the number of gold ions decreased from the anode to the cathode. The large amount of surfactant is well known to reduce the conductivity of the electrolyte, reducing the rate of deposition. Therefore, increasing the amount of TTAB reduced the gold deposition rate on the cathode and considerably reduced the formation of gold nanoparticles. The role of TTAB surfactant in the electrochemical reaction was considered as follows. First, under fixed conditions of current density, time, and electrode area in the electrochemical reaction, the electrolyte 共surfactant兲 concentration influenced the amount of gold oxidized/ reduced 共number of adatoms兲. Second, the amount of TTAB also influenced the particle size. In this work, although the particle size decreased as the TTAB amount increased, the amount of gold oxidized/reduced decreased as the amount of TTAB increased because of the drop in deposition rate. In this study, more TTAB surfactant synthesized smaller particles 共Fig. 2兲 and fewer gold nanoparticles. However, the intensity of the SPR band is determined by the larger amount of TTAB, which is responsible for the lower concentration of smaller gold nanoparticles. Figure 5 shows the XPS analysis of gold nanoparticles produced using various amounts of TTAB to elucidate their composition. The XPS spectra exhibited no material other than gold or the surfactant. Figure 5 shows the spectra of Au 4f, Br 3d, C 1s, N 1s, and O 1s core levels of gold nanoparticles and the surfactant. Additionally, a slight Si 2p core level spectrum appears at a binding energy of about 103 eV, because the samples are prepared on a glass slide. The 284.5 eV binding energy is a C 1s core level peak, from the tail of the surfactant hydrophobic group. The Br 3d5/2 and Br 3d3/2 core level spectra peaks are at 186.8 and 180.2 eV, respectively, indicating the presence of the hydrophilic portion of the surfactant. The O


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Figure 5. 共Color online兲 XPS electron spectra of gold nanoparticles for various amounts of TTAB surfactant; inset also shows a typical XPS spectrum of the Au-4f core level.

1s core level peak is observed at 531.3 eV. An N 1s peak from the front of the surfactant hydrophobic group is observed at a binding energy of 420.2 eV. Figure 5 shows a magnified Au 4f core-level spectrum with various amounts of TTAB surfactant. The Au 4f core-level can be characterized by two pairs of peaks associated with Au 4f7/2 and Au 4f5/2 spin-orbit coupling. These are characteristic values for element Au共0兲. Au is in 0 valence. The results also clearly show that the binding energy of the Au 4f peak can be changed by altering the amount of TTAB surfactant. When the amount of TTAB surfactant is 10 mg, as in image 共a兲, the binding energies of Au 4f7/2 and Au 4f5/2 are 84.0 and 87.8 eV, respectively. When the amount of the TTAB surfactant is increased from 30 to 90 mg, the binding energies of Au 4f7/2 and Au 4f5/2 peaks change slightly from 84.2 to 84.5 and 88.0 to 88.3 eV, respectively. Notably, the binding energies of Au 4f7/2 and Au 4f5/2 peaks increased with the amount of TTAB surfactant. The difference between the binding energies of the Au 4f7/2 and Au 4f5/2 peaks for all samples is 3.8 eV, a value similar to that of bulk gold. The typical Au 4f7/2 and Au 4f5/2 peaks for the bulk gold binding energy are at 83.9 and 87.6 eV, respectively,36 probably because of a shift in the Fermi level as the particles become smaller. Such shifts have been reported elsewhere.37-39 Controlling the size of particles by changing the current density.— During the operation of our electrochemical cell, bulk gold is ionized and then travels as gold ions from the anode to the cathode 共anode-oxidation/cathode-reduction兲. The reduction of the gold ions at the cathode affords so-called adatoms. These adatoms aggregate in the presence of the surfactants to form the nanoparticles. The current flows through the electrolyte as moving gold ions. Thus, the current density is the current flow per cross-sectional area normal to the flow of current. The rate of removal of gold ions can also be described in terms of current density. The current density is, however, determined by the applied electric field. In this study, to determine whether the current density has any effect upon the size of gold nanoparticle, a series of preparations was carried out using current densities varying from 1 to 3 mA/cm2. At the highest current density of 3 mA/cm2, the cathode was strongly gold-colored and so was electroplated in gold. At 1 mA/cm2, the cathode was its original color; the color of the solution had changed only slightly. At a given amount of surfactant and temperature, a change in current density alone visibly affected the color of the reacted solution. Ex-

Figure 6. Transmission electron micrographs of gold nanoparticles prepared at current densities of 共a兲 1, 共b兲 2, and 共c兲 3 mA/cm2; scale bar represents 100 nm.

perimental runs were performed at current densities of 1, 2, and 3 mA/cm2 to test the effect of current density on the production of gold nanoparticles. The results show that the particle size decreased as the current density increased, as revealed by TEM analysis 共Fig. 6兲. The average diameters of the gold nanoparticles were 53.7 ± 17.2, 32.1 ± 11.1, and 12.2 ± 7.3 nm, respectively. Therefore, smaller particles are formed at higher current density. Figure 7 shows the UV/vis absorption spectra obtained in this series of experiments. The peak of the SPR band is blueshifted from 538 to 523 nm as the particle size decreases from 58.3 to 10.6 nm, as shown in the inset in Fig. 7. This relationship between the current density and the particle size from the free energy of formation of a gold cluster, ⌬G共N兲, has two terms, as follows40 ⌬G共N兲 = − Nze兩␩兩 + ␾共N兲

关6兴

where N is the number of ions in the gold cluster, ␩ is the overpotential, z is the ion charge, e is the charge of the electron, and ␾ is the electrostatic potential. The first term is related to the number, N, of gold ions that move from the solution to the crystal phase on the surface of the cathode and the second term is related to the increase in the surface energy associated with the creation of the surface of a gold cluster. This increase in the surface energy equals the difference between the binding energies of the N bulk gold ions and those of the N gold ions arranged on the surface of the gold crystal. Both terms in Eq. 6 are functions of the size of the gold cluster, N. The size of the critical nucleus in two dimensions is given by



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Figure 7. 共Color online兲 UV/vis absorption spectra of aqueous dispersion of gold nanoparticles produced at different current densities.

Nc =

bs␧2 共ze␩兲2

关7兴

where Nc is the number of atoms in the gold cluster, b is the factor that relates the surface area S of the nucleus to its perimeter P 共b = P2 /4S; b = ␲ for a circular nucleus兲, s is the area occupied by one atom on the surface of the gold nucleus, and ␧ is the edge energy. Therefore, Nc strongly depends on the overpotential and is inversely proportional to ␩2. The critical radius of the surface nucleus rc is a function of the overpotential rc =

s␧ ze␩

关8兴

Therefore, rc is inversely proportional to the overpotential 共␩兲, and the electron transfer overpotential may be defined as the rate of change of electrode potential with the current associated with the limiting rate of electron transfer across the phase boundary between the electrically conducting electrode and the ionic conducting solution.41,42 The overpotential is directly related to the current density. Both experimental and theoretical results show that changing the current density in the electrochemical cell controls the size of the gold nanoparticles, revealing that a larger current density is associated with smaller particles. Effect of growth temperature on particle size.— Finally, the possible effect of growth temperature on the size of surfactantstabilized gold colloids was experimentally studied. At a given amount of TTAB surfactant, neither the current density nor the growth temperature visibly changed the color of the solution, but they did affect the cathode. At high temperature, the cathode was more heavily gold-plated. However, a more detailed study showed a moderate and significant positive correlation between the temperature and the size of the produced particles. Process temperatures of 25, 40, and 60°C 共30 mg, 2 mA/cm2兲 were applied. The results showed that the particle size increases with the growth temperature, as shown by TEM analysis and shown in Fig. 8. Figure 8 shows the TEM images of gold nanoparticles produced at growth temperatures of 25, 40, and 60°C, which yielded respective particles of sizes 8.3 ± 2.2, 34.5 ± 12.3, and 53.8 ± 18.3 nm. Figure 9 presents UV/vis absorption spectra of gold nanoparticles grown at various temperatures. The inset in Fig. 9 plots the relationship between the peak position of the maximally absorbing SPR band at different growth temperatures and average particle sizes. The peak position of the SPR band decreases as it is redshifted from 521 to 538 nm as the particle size is increased from 8.3 to 53.8 nm, and the temperature falls

Figure 8. Transmission electron micrographs of gold nanoparticles prepared at growth temperatures of 共a兲 25, 共b兲 40, and 共c兲 60°C; scale bar represents 100 nm.

from 25 to 60°C. These results show that the particle size can also be controlled by the growth temperature, to which it is proportional. The results can be explained by the higher diffusion and migration rates of surfactant intermediates at higher temperatures, the inhibition of the decrease in the viscosity of

Figure 9. 共Color online兲 UV/vis absorption spectra of aqueous dispersions of gold nanoparticles produced at different growth temperatures.


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Table I. Properties of gold nanoparticles synthesized under various conditions. Sample TTAB amount 共mg兲 1 10 30 50 70 90 Current density 共mA/cm2兲 1 2 3 Temperature 共°C兲 25 40 60

Size 共nm兲

SPR 共nm兲

58.3 44.6 36.7 25.3 17.2 10.6

542 536 530 528 524 522

53.7 32.1 12.2

538 530 523

8.3 34.5 53.8

521 530 538

the medium, which reduces the overpotential, and other changes.43 Other factors, such as the higher rate of desorption of the surfactant stabilizer from the particle surface at higher temperatures, may also play a role. Table I presents the basic characteristics of gold nanoparticles synthesized under various conditions. The experimental results presented above clearly demonstrate the fabrication of good-quality gold nanoparticles using a simple electrochemical cell with a stabilizer that is composed of surfactant. The size of the produced nanoparticles is well controlled by varying the amount of TTAB surfactant, the current density, and the growth temperature. Conclusions This study reports the electrochemical production of gold nanoparticles in surfactant solution. The size of the particles is controlled by varying the amount of TTAB surfactant, the current density, and the growth temperature. Changing the amount of the TTAB most strongly affected the particle size, and the SPR band is blueshifted from 540 to 522 nm as the particle size decreases from 58.3 to 10.6 nm. The peak of the SPR band is redshifted from 521 to 538 nm as the particle size increases from 8.3 to 53.8 nm and the temperature falls from 25 to 60°C. For the range of variables studied, the optimal parameters were a current density of 3 mA/cm3 for a cell solution with 30 mg of TTAB and a growth temperature of 25°C, which produced the smallest nanoparticles of size diameter 8.3 nm. Various characterizations, such as by UV/vis absorption spectroscopy, TEM, and XPS, indicated that the produced gold nanoparticles are of good and stable character. The electrochemical method is confirmed to be appropriate for the low-cost, low-temperature, and rapid fabrication of gold nanoparticles. Acknowledgment This work was partially supported by the National Science Council 共NSC兲 of Taiwan under contract no. NSC 94-2215-E-390-001. The authors thank the Department of Mechanical Engineering, Southern Taiwan University of Technology 共STUT兲, Taiwan, for their support in TEM sample preparation.

National University of Kaohsiung assisted in meeting the publication costs of this article.

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