Formation Process of Two-Dimensional Networked Gold Nanowires

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Langmuir 2004, 20, 7837-7843

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Formation Process of Two-Dimensional Networked Gold Nanowires by Citrate Reduction of AuCl4- and the Shape Stabilization Lihua Pei, Koichi Mori,† and Motonari Adachi* Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611 0011, Japan Received March 21, 2004. In Final Form: June 10, 2004 Gold nanowires with a two-dimensional (2-D) network structure were formed by citrate reduction of AuCl4- with a low concentration of citrate. The structure change during the growth processes was observed by transmission electron microscopy (TEM) and the variation in concentrations of gold species in the aqueous solution was monitored by UV-vis spectra and Inductively Coupled Argon Plasma Emission Spectrophotometer (ICAP). The formation of 2-D gold nanowires was induced by the small amount of reducing agent because the preliminary gold nanoparticles formed by reduction of AuCl4- were thermodynamically unstable in the aqueous solution due to the insufficient capping of citrate. One of the key points of nanowire formation is the preferential adsorption of AuCl4- instead of citrate ions on the surface of the preliminary gold particles, which results in an attracting force between gold nanoparticles.30 We propose a hit-to-stick-to-fusion model, in which gold nanoparticles adhere by the attraction force and stick together, causing selective deposition of reduced gold metallic species on the concave surface of the two sticking particles, followed by fusion into nanowires. Nanowires then connect with each other, forming a network structure. The evidence obtained from TEM observation of transformation from gold nanowires on a TEM grid to large nanoparticles by hydrogen gas reduction and time-resolved measurements of gold ions suggest that gold ions not only are crucial for the growth of gold nanowires but also play an important role in stabilizing the shape of gold nanowires during the formation process. This method for synthesizing 2-D gold nanowires is simple and relatively easy application to the synthesis of other metallic nanowires such as silver or platinum is expected.

I. Introduction Morphological control of nanomaterials has attracted a great deal of attention because their optical, electronic and magnetic properties are strongly dependent on the size and shape of the particles.1-4 There have been intensive efforts to synthesize ordered 1-D metallic nanowires using various methods such as electrochemical5 and photochemical6,7 reduction in aqueous surfactant media, templating from porous alumina,8,9 polycarbonate membrane10 or carbon nanotube,11,12 wet chemical synthesis based on a seed-mediated growth mechanism,13,14 * Corresponding author: Motonari Adachi, Phone: 81-774-383518, Fax: 81-774-38-3405, E-mail: [email protected]. † AIR WATER Inc. (1) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881. (2) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289. (3) Zhang, Z.; Sun, X.; Dresselhaus, M. S.; Ying, J. Y. Phys. Rev. B 2000, 61, 4850. (4) Bockrath, M.; Liang, W.; Bozovic, D.; Hafner, J. H.; Lieber, C. M.; Tinkham, M.; Park, H. Science 2001, 291, 283. (5) Yu, Y. Y.; Chang, S. S.; Lee, C. L.; Wang, C. R. C. Phys. Chem. B 1997, 101, 6661; Chang, S. S.; Shih, C. W.; Chen, C. D.; Lai, W. C.; Wang, C. R. C. Langmuir 1999, 15, 701. (6) Esumi, K.; Matsuhisa, K.; Torigoe, K. Langmuir 1995, 11, 3285. (7) Kim, F.; Song, J. H.; Yang, P. J. Am. Chem. Soc. 2002, 124, 14316. (8) Martin, B. R.; Dermody, D. J.; Reiss, B. D.; Fang, M. M.; Lyon, L. A.; Natan, M. J.; Mallouk, T. E. Adv. Mater. 1999, 11, 1021. (9) Van der Zande, B. M.; Bohmer, I. M. R.; Fokkink, L. G. J.; Schonenberger, C. Langmuir 2000, 16, 451. (10) Cepak, V. M.; Martin, C. R. J. Phys. Chem. B 1998, 102, 9985. (11) Govindaraj, A.; Satishkumar, B. C.; Nath, M.; Rao, C. N. R. Chem. Mater. 2000, 12, 202. (12) Fullam, S.; Cottell, D.; Rensmo, H.; Fitzmauice, D. Adv. Mater. 2000, 12, 1430. (13) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065; Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 617; Jana, N. R.; Gearheart, L.; Murphy, C. J. Adv. Mater. 2001, 13, 1389. Gao, J.; Bender, C. M.; Murphy, C. J. Langmuir 2003, 19, 9065.

and solution-phase method based on capping reagents.15 Recently, Murphy et al.13 succeeded in formation of single crystalline metal nanorods with high aspect ratio with a multistep seed-mediated method by using CTAB as the directing agent. They also examined dependence of the gold nanorod aspect ratio on the nature of the directing surfactant in aqueous solution, and presented that the length of the surfactant tail is critical for producing gold nanorods.13 El-Sayed et al.14 elaborately elucidated the growth mechanism of gold nanorods prepared by seedmediated growth method and electrochemical method and studied the crystal structures of the gold nanorods using high-resolution TEM. Except for these methods, Penner et al.16 developed an electrochemical step edge decoration (ESED) technique for the preparation of long (>500 µm) nanowires composed of noble or coinage metals including nickel, copper, silver and gold. Recently, organization of nanoparticles into nanowires with a networked structure and investigation of the growth process have been actively studied owing to the potential uses in microelectronics, optoelectronics, nanoscale electronic devices, and other fields. For example, Pileni et al.17,18 have prepared a network structure of copper nanowires covering a two-dimensional (2-D) space uniformly in a reverse micellar system. Barnard et al.19 (14) Wang, Z. L.; Mohamed, M. B.; Link, S.; El-Sayed, M. A. Surf. Sc. 1999, 440, L809; Nikoobakht, B.; El-Sayed, M. A. Langmuir 2001, 17, 6368; Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (15) Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2(2), 165; Sun, Y.; Xia, Y. Adv. Mater. 2002, 14, 833. (16) Walter, E. C.; Murray, B. J.; Favier, F.; Kaltenpoth, G.; Grunze, M.; Penner, R. M. J. Phys. Chem. 2002, 106, 11407. (17) Lisiecki, I.; Billoudet, F.; Pileni, M. P. J. Phys. Chem. 1996, 100, 4160. (18) Pileni, M. P. Langmuir 1997, 32666. (19) Barnard, J. A.; Fujiwara, H.; Inturi, V. R.; Jarratt, J. D.; Scharf, T. W.; Westona, J. L. Appl. Phys. Lett. 1996, 69, 2758.

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reported that nanostructured continuous networks of Fe were prepared by sputter deposition onto the surface of nanochannel alumina. Chen et al. 20 fabricated both networked gold nanostructured particles and twisted gold nanorods suspended in water via a laser ablation technique. Mafune´ et al. 21 also reported that the network of gold nanoparticles was formed in a sodium dodecyl sulfate (SDS) solution by irradiation of an intense pulsed laser on gold nanoparticles. Wang et al. 22 found that gold nanoparticles were crosslinked covalently by chemical oxidation into a stable nanoparticle ensemble with chainlike structures. Chang et al. 23 found that silver nanoparticles spontaneously organized into chainlike structures in supercritical water. Although these studies succeeded in fabricating 2-D nanostructures, a full understanding of the formation mechanism of the metallic nanowire network still requires studies on the formation process in a more simplified reaction system. Therefore, it will be a significant challenge to simplify the synthesis route and clarify the growth mechanism of the nanowire network structure. In our research group, we have developed a simple chemical reduction method for the fabrication of gold nanowires with a 2-D network structure.24,25 By carefully controlling the ratio of the concentration of the reducing reagent to Au ions in the citrate reduction of AuCl4-, gold nanowires were formed and connected to a network structure uniformly covering a 2-D space. Furthermore, the electric resistance of a film dipcoated with gold nanowires synthesized by this method was estimated to be around 1 ohm/cm, which was 1 order magnitude lower than that of the commercially available transparent electric conductive glass.24 This result provides great expectations for the application of such metallic networked nanowires to microelectronic devices. The formation of gold sol via citrate reduction of AuCl4- has been extensively studied by a number of researchers, yet these investigations were mainly focused on the fabrication of uniform gold nanoparticles.26,27 Until now, to our knowledge, there has been no report on synthesizing gold nanowires using this simple chemical reduction method. Since the reaction system only contains the gold precursor and reducing agent, it is helpful to have an insight into the growth process of the gold nanowire network. In the present work, the authors have focused on the formation mechanism of the 2-D gold nanowire network and the wire shape stabilization related to the presence of gold ions in the aqueous solution. The 2-D network structure definitely implies that gold nanowires could cover a 2-dimensional space uniformly on a TEM grid or on making a 2-dimensional film of a nanosize thickness. The structure change during the growth processes was observed by transmission electron microscopy (TEM) and the variation in concentrations of gold species in the aqueous solution was monitored by UV-vis spectra and Inductively Coupled Argon Plasma Emission Spectrophotometer (ICAP). It was found that gold ions not only (20) Chen, C. D.; Yeh, Y. T.; Wang, C. R. C. J. Phys. Chem. Solids 2001, 62, 1587. (21) Mafune´, F.; Kohno, J.; takeda, Y.; Kondow, T. J. Phys. Chem. B 2003, 107, 12589. (22) Wang, T.; Zhang, D.; Xu, W.; Li, S.; Zhu, D. Langmuir 2002, 18, 8655. (23) Chang, J. Y.; Chang, J. J.; Lo, B.; Tzing, S.; Lin,. Y. Chem. Phys. Lett. 2003, 379, 261. (24) Adachi, M.; Mori, K.; Sato, Y.; Pei, L. J. Chem. Eng. Jpn. 2004, 37(5), 604. (25) Pei, L.; Mori, K.; Adachi, M. Chem. Lett. 2004, 33(3), 324. (26) Turkevich, J.; Stevenson, P. C.; Hillier, J. J. Discuss. Faraday Soc. 1951, 11, 55. (27) Frens, G. Nature Phys. Sci. 1973, 241, 20.

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were crucial for the growth of gold nanowires but also played an important role in stabilizing the shape of gold nanowires during the formation process. The importance of gold ions in the fabrication and stabilization of 2-D gold nanowire networks observed in this study opens up new possibilities and considerations for the shape control of anisotropic metallic nanoparticles. II. Experimental Section Chemicals. Sodium tetrachloroaurate (NaAuCl4‚2H2O), trisodium citrate, and hydrobromic acid (HBr) were purchased from Nacalai Tesque. Water used throughout the experiments was purified by a MilliQ system (Millipore). All the chemicals were analytical grade and used without further purification. Synthesis of Gold Nanowires. The experimental method was a modification of conventional citrate reduction of AuCl4in water.26,27 In a typical synthesis, 200 mL of aqueous solution of sodium tetrachloroaurate (NaAuCl4‚2H2O, 0.25 mM) was stirred in a temperature-controlled oil bath until the solution was warmed to the desired temperature (80 °C). Then, a controlled amount of trisodium citrate was added rapidly into this solution. After the initiation of the reaction, the solution was taken out at specific time intervals for UV-vis spectroscopy and TEM observation. To investigate the influence of citrate concentration on the 2-D gold nanowires, the molar concentration ratio of citrate/ NaAuCl4 (denoted as R) was varied from 0.1 to 2.7. The experiments could reproducibly accomplish by carefully cleaning the glassware, controlling the temperature and fixing the stirring rate. Characterization. The shape of the sample was observed by transmission electron microscopy (TEM, JEOL JEM 200CX) operating at 200 kV. To investigate the real state of the solution at different reaction stages, the TEM images were obtained from the solution without any purification or centrifugation. For the TEM observation, samples were obtained by dropping 2-µL solutions onto carbon-coated copper grids placed on a filter paper for rapid removal of the liquid. The UV-vis spectra of the colloidal solutions were recorded on a Shimadzu UV-2200 spectrometer. Time-resolved Measurement of Concentration of Gold Species. The concentration of gold ions in the solution phase was determined by a colorimetric method using HBr acid.28 This was carried out by the following procedure: 3 µL 48% HBr acid was injected into 5-mL aliquots taken from the reaction flask, followed by centrifugation for 15 min at 6000 rpm to remove the large gold particles. The supernatant layer was used to measure the UV-vis absorbance. The concentration of gold ions was calculated from the peak absorbance of AuBr4- at 381 nm, which was formed in an anion exchange reaction between Cl- and Br-. The whole concentration of gold species in the same supernatant was measured using an Inductively Coupled Argon Plasma Emission Spectrophotometer (ICAP-500, Thermo Electron Co.) device. Then the concentration of metallic gold in the solution was calculated from the difference between concentration of whole gold species and gold ions.

III. Results (1) Formation of Gold Nanowires with a 2-D Network Structure. Figure 1 shows typical TEM images of 2-D gold nanowire networks prepared at a citrate concentration ratio (R) of 0.2. It can be seen that gold nanowires were formed together with some tiny particles at a reaction time of 30 min (Figure 1a). When the reaction proceeded to about 60 min, the gold nanowires were connected in a network structure, covering a 2-D space uniformly (Figure 1b). The 2-D gold network extended to a surface area of several square micrometers, or even larger. The average diameter of the nanowires was approximately 11 nm as shown in a high-resolution TEM (HRTEM) image of one part of the gold nanowires (Figure 1c). This network structure is rigid enough to survive ultrasonic vibration in water after high-speed centrifuga(28) Handbook of Analytical Chemistry (Japanese); Maruzen: 1991.

Formation Process of 2-D Networked Gold Nanowires

Figure 1. TEM images of 2-D gold nanowire network obtained at R ) 0.2: (a) reaction time of 30 min, (b) reaction time of 60 min, (c) HRTEM of nanowire network of one part of (b), (d) electron diffraction of (c).

Figure 2. UV-vis spectra corresponded to the TEM images of 2-D gold nanowire network shown in Figures 1(a) and (b).

tion. The bond between two particles combined by deposition of metallic gold on the concave surface is shown in the inset of Figure 1c. A lattice image of two combined particles shows different orientation, indicating that two particles collided and joined. The electron diffraction of the gold nanowire (Figure 1d) shows scattering points corresponding to (111), (200), (220), (311), and (420) of gold crystalline facets, confirming that gold nanowires include many small particles that have random and independent orientation. These findings clearly show evidence that gold polycrystalline nanowires are formed by tightly fused nanoparticles. Figure 2 shows UV-vis absorption spectra taken from the solutions corresponding to the sample shown in Figure 1a and 1b. It should be noted that almost-flat absorbance curves with a board peak were observed from 500 to 900 nm. The flat adsorption pattern corresponded to a blue suspended solution in the reaction process. We have not observed the spectra over 900 nm due to the limitation of our UV-vis spectrometer. However, C. R. C. Wang et al.20

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reported that the corresponding absorption spectrum of suspended, networked Au nanostructures exhibited a long rising feature from visible to near-IR region. Their spectra below the range of 900 nm were quite similar to our findings. Generally, oblate metal particles (e.g., rods and wires) show two plasmon absorption peaks with energies that are characteristic of the long and short axes of these systems.5 When a single nanorod is formed, one peak corresponding to the short axes appears at about 520 nm, while the other corresponding to the long axes appears at about 600-1300 nm depending on the aspect ratio.5,20 However, in the present case gold nanowires form a network structure, which shows a flat absorption pattern with a board peak observed from 500 nm to 900 nm. These broad spectra may be understood by the superposition of the longitudinal resonance of gold nanorods with various aspect ratios at relevant wavelengths. This finding was also observed for a network structure in recent report by Wang et al.20 Comparison of the optical spectra with the corresponding TEM morphology reveals that the appearance of the broad absorbance band is actually mirrored by the appearance of the nanowires of the 2-D network structure. (2) Influence of Concentration of Citrate on the 2-D Gold Nanowires Formation Process. To investigate the influence of concentration of citrate on the 2-D gold nanowire formation process, the molar concentration ratio of citrate/NaAuCl4 (denoted as R) was varied from 0.1 to 2.7 and the reaction was monitored for 60 min. The results showed that only spherical particles were obtained as the final product in the range of R larger than 0.4. However, at R ) 0.3 or 0.2, gold nanowires were formed and connected in 2-D network structures. There was no particle or wire formation observed for R ) 0.1, inferring that this amount of citrate is not sufficient to induce the reduction of AuCl4-. Since great difference occurs between the concentration ratio R ) 0.3 and 0.4, the reactions under these two conditions were separately monitored for 180 min. Figure 3a shows a typical example of the variation of solution color when R was controlled at 0.4. Upon the addition of sodium citrate, the initial light yellow solution became colorless, indicating the reduction of AuCl4-. Then the solution turned light blue, dark blue, and finally wine red after about 40 min, as shown in Figure 3a. Thereafter, this wine-red color remained constant over several hours, implying the formation of stable gold nanoparticles. However, at R ) 0.3, the blue color was maintained for 180 min or longer. Figure 3b and 3c show the UV-vis absorption spectra taken from the solutions at different reaction stages for R ) 0.4 and 0.3, respectively. It can be seen that at reaction time of 1 min under R ) 0.4, there was no absorption from 500 to 900 nm, corresponding to the colorless solution shown in Figure 3a. Then, a flat absorption pattern, which is assigned to the 2-D gold nanowire network formation, appears during the early reaction process, that is, at 6 and 20 min. However, when the solution turned to wine red after 40 min, the flat spectrum disappeared and the plasmon band blue shifted to a shorter wavelength with a narrow peak exhibited at about 547 nm. The corresponding TEM images for R ) 0.4 show that at a reaction time of 6 min, gold nanowires extended from a large spherical aggregate (Figure 4a). After 20 min, large aggregates disappeared and nanowires only covered a 2-D space uniformly (Figure 4b). However, the nanowires broke and aggregated into nanoparticles after 40 min with larger diameters than the preliminary nanoparticles (Figure 4c). No change of the particle morphology was observed after

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Figure 4. TEM images of 2-D gold nanowire network obtained at R ) 0.4 with reaction time of (a) 6, (b) 20, and (c) 40 min, respectively. Image (d) is a sample of (a) after washing in water.

Figure 3. Solution color for R ) 0.4 (a) and UV-vis absorption spectra of the reaction solutions at different reaction stages. (b) R ) 0.4, reaction time of 0, 1, 6, 20, and 40 min; (c) R ) 0.3, reaction time of 0, 3, 10, 30, 60, and180 min.

the larger particles were formed. The large aggregates observed in the initial reaction stage (Figure 4a) were considered to be formed by aggregation of tiny gold particles, because they could be separated by a watersoluble thiol compound in the suspended solution, and the large aggregates disappeared.24 Figure 4d is an image of a nanowire of 6 min reaction after washing with water, indicating only nanowires were recovered and the tiny particles could be removed by water. This has been discussed in our previous paper.24 In the case of R ) 0.3, the solution showed a blue color until 180 min, which corresponded to a flat absorption pattern similar to the initial stage of R ) 0.2, as shown in Figure 3c. Figure 5 shows TEM images of samples prepared at R ) 0.3 with various reaction times. Figure 5a was obtained from the solution with 10 min reaction, and shows gold nanowires covering a 2-dimensional space more uniformly than those of R ) 0.4. With an increase of reaction time to 60 min, part of the gold nanowires

became thicker and straighter with diameter of approximately 43 nm (Figure 5b). We attribute this change of wire shape both to the Ostwald ripening at the expense of small gold particles and to the deposition of newly formed metallic gold by the reduction of gold ions, because gold ions still existed in the solution, as shown later. The nanowire network remained after 180 min of reaction (Figure 5c), which corresponded to the flat absorption spectrum. The comparison between R ) 0.4 and 0.3 suggests that 2-D gold nanowires can be formed in the early stage of reaction in a wide range of citrate concentration ratio (R) larger than 0.4, because blue suspended solution was always observed in the early reaction stages in the present experimental conditions, that is, 0.2 < R < 2.7. However, they are not stable and break down quickly at high R values. The reason for the wire shape breakdown will be further discussed in the next section. Based on the above investigation, gold nanowires with high conversion yield must be prepared by choosing an appropriate concentration ratio of citrate to maintain the shape stability at a longer reaction time. (3) Monitoring of the Concentration of Gold Species During the Reaction Process. Figure 6 shows the time-resolved measurement of the concentration of gold species in the aqueous solution for R ) 0.4 and R ) 0.3. The three curves represent the concentrations of total gold (total gold species in the aqueous solutions detected by ICAP), gold ion, and metallic gold (difference between the front two items) in the supernatant layer after centrifugation. The concentrations of total gold and gold ions both decreased with time. Comparing with the UV absorbance observation, the concentration of metallic gold increased when no absorbance was exhibited in Figure 3. However, when the absorbance began to appear from 500 to 900 nm with a flat curve, the concentration of metallic gold in the upper supernatant layer decreased. The initial reaction stage, in which no absorbance appeared, should be considered as nucleation and formation of small particles. The diameters of these particles are considered to be as smaller than 1.7 nm since they do not exhibit surface plusmon absorption around 530 nm.29 After the initial reaction stage, the concentration of metallic gold

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Figure 5. TEM images of 2-D gold nanowire network obtained at R ) 0.3 with reaction time of (a) 10, (b) 60, and (c) 180 min, respectively.

Figure 6. Time-resolved concentrations of gold species in the solution phase. The concentrations of AuCl4- (denoted as [AuCl4-]) and whole gold (denoted as [Au]) were measured with HBr method and ICAP method, respectively; the difference between the above two represents the concentration of metallic gold presented in the supernatant layer after centrifugation.

suddenly decreased, contributing to the growth of particles and formation of nanowires. From the TEM observation, it was confirmed that gold nanowires were formed gradually at this stage, under a low concentration range of citrate. Here, the following reaction scheme should be noted: (1) The rate of reduction by citrate under a low concentration was relatively slow and a small number nuclei were formed. In case of a high citrate concentration, that is, high R, a large number of nuclei were formed. (2) After the nucleation stage, a large amount of AuCl4(29) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R L. J. Phys. Chem. B 1997, 101, 3706.

remained in the solution and was reduced to metallic gold, resulting in the formation of nanoparticles. Under a low R condition, relatively large-sized particles formed, whereas under a high R condition, very small particles were formed, because the number of nuclei determines the particle size. (3) As the reaction proceeds, gold nanoparticles collided due to the attraction force caused by adsorption of AuCl4- and stuck together. According to the force measurement by Zukoski et al.30 between gold surfaces and gold particle by Atomic Force Microscopy (AFM), AuCl4- is adsorbed in a noncharged state and can be preferentially bound onto the gold surface even in the presence of excess citrate ions, resulting in an attractive interaction. Details will be discussed later. AuCl4- was gradually consumed by reduction to metallic gold, adsorbed on the gold surface, and preferentially deposited on the concave surface. This resulted in the formation and growth of gold nanowires, that is, enlargement in length and diameter, and formation of a 2-D network structure. When gold ions were completely consumed, gold nanowire was transformed to gold nanoparticles. When R ) 0.4, breakdown of gold nanowires occurred after 40 min, as shown in Figure 4c and Figure 6a. However, for R ) 0.3, the reaction was much slower and a certain amount of gold ions still remained after 180 min. UV-vis spectra and TEM observations showed that formation of 2-D gold nanowires always corresponds to the presence of large amount of AuCl4- in the solution and the breakdown of the wire shape always coincided with the disappearance of the AuCl4-, regardless of variation in the experimental conditions. We further investigated the influence of the presence of AuCl4- on the formation process of the gold nanowires by the following experiment: a 100-mL volume of aqueous solution of NaAuCl4 (0.25 mM) was added to 100 mL of the preceding reaction solution after 10 min under R ) 0.4, where R was reduced to approximately 0.2 due to this addition. Unlike the previous case, that is, where change of solution color from blue to wine red occurred after 40 min at R ) 0.4, this solution remained blue until 20 h. UV absorbance and TEM observation confirmed that a nanowire network was formed and was stable in the solution. This result can be understood from the view that relatively excessive AuCl4in the solution played an important role in maintaining the wire shape. To verify the decisive effect of AuCl4- on the shape stability, a TEM grid with as-synthesized nanowires was treated with H2 gas at room temperature, then observed again. Figure 7 shows that the beautiful nanowires (Figure 7a) broke after H2 treatment and larger particles were formed (Figure 7b). This result indicates (30) Biggs, S.; Mulvaney, P.; Zukoski, C. F.; Grieser, F. J. Am. Chem. Soc. 1994, 116, 9150; Wall, J. F.; Grieser, F.; Zukoski, C. F. J. Chem. Soc., Faraday Trans. 1997, 93(22), 4017. (31) Tan, C. K.; Newberry, V.; Webb, T. R.; McAuliffe, C. A. J. Chem. Soc., Dalton Trans. 1987, 1299.

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Figure 7. TEM images of as-synthesized gold nanowires (a) and after treatment with H2 (b). Sample (a) was prepared at R ) 0.3 for 10 min. The TEM grid of (b) was obtained by putting the TEM grid (a) in atmosphere of H2 gas for 1 h at room temperature.

that the gold ions capped on the surface of gold nanowires were reduced by H2,31 resulting in a breakdown of the nanowire shape. Therefore, it was confirmed that gold ions played an important role in stabilizing the shape of gold nanowires during the formation process. IV. Discussion The chemical reduction of AuCl4- with sodium citrate has been conventionally used for synthesis of gold nanoparticles (known as the Frens method).26,27 Here, sodium citrate not only acts as a reducing agent but also as a capping agent in the reduction reaction. The formed particles are stable in the aqueous solution due to the adsorption of citrate ions on the surface of the particles, which can build up a strong repelling layer, thus preventing the particles from coming into close contact. Therefore, a large amount of sodium citrate was generally used in order to get stable gold particles. However, in this study, gold nanowires with a 2-D network structure were formed with a very low concentration of citrate in the reduction reaction. The formation mechanism of these 2-D gold nanowires can be schematically depicted as shown in Figure 8. There are three important factors that have critical impacts on the formation of the gold nanowires. The first one is an insufficient amount of sodium citrate. When the molar

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ratio of the sodium citrate/NaAuCl4 was reduced to the range from 0.2 to 0.4, the preliminary gold nanoparticles formed by the chemical reduction of AuCl4- were considered to be thermodynamically unstable because of both the small size and the insufficient capping of citrate. Particles in such an unstable state have been reported to show a tendency to undergo fusion into wire-like structures.32,33 Kiely et al.32 found that the smaller ones among the alkyl-thiol stabilized bimodal nanoparticles (4.5 and 7.8 nm) were connected to form a semi-continuous network on a TEM grid after exposure to air for several months. They attributed the fusion of gold nanoparticles to the lower stability of the smaller particles, which are truncated cuboctahedra exposing both the {111} and {100} surface facets. Yonezawa et al.33 also reported that their unstable 2.7-nm-sized gold particles displayed automatic fusion into larger particles in the 1-D assemblies, where bromide ions surrounding gold nanoparticles were considered to accelerate the fusion of the particles to form the wire-like structures. The second factor is competitive adsorption of AuCl4- and citrate ions on the surface of the preliminary gold nanoparticles. As reported by Zukoski et al.,30 AuCl4can be preferentially bound onto the gold surface even in the presence of excess citrate ions, resulting in an attractive interaction. In their AFM measurement of the forces between a gold sphere and a gold plate in aqueous solutions, a repulsive interaction was observed in trisodium citrate solution because of the adsorption of these anions at the gold/water interface. However, the observed interaction force in gold (III) chloride solution was always attractive, that is, in the case of only gold (III) chloride without citrate and also in the competitive adsorption of gold (III) chloride and citrate ions. Addition of gold (III) chloride to the AFM cell after the preadsorption of citrate anions caused the force of interaction to change from a repulsive force to an attractive one. Furthermore, the diffuse layer potential of the gold surface decreases with the adsorbed negative ion of AuCl4- when charged citrate ions are displaced. Thus, they inferred that AuCl4- does not adsorb as a charged ion, but as either a complex counterion or as AuCl3 with one of the Cl- ligand displaced.

Figure 8. Schematic illustration of the formation process of the 2-D gold nanowire network by citrate reduction of AuCl4- (a) and preferential deposition of metallic gold on the concave surface of the two particles when they adhere together (b).

Formation Process of 2-D Networked Gold Nanowires

Also, adsorption of AuCl4- on the gold surfaces reduces the surface charge, increasing the van der Waals attractive forces between the gold surfaces, resulting in attractive interaction. Finally, another important finding obtained from the AFM measurement by Biggs et al.,30 was that the gold surfaces were observed to jump toward contact when approaching around 10 nm, driven by the large van der Waals attraction forces. This would substantially facilitate the random hit and stick of gold nanoparticles toward fusion into 1-D nanowires, which subsequently form a wirelike network structure. Thus, we can hypothesize the formation of gold nanowires as follows: First, gold ions are partly reduced to metallic gold by citrate and make gold nuclei at the initial reaction stage. The nuclei become nanoparticles by gathering reduced metallic gold surrounding the nuclei. The colorless solution and absence of plasmon absorbance in the range of 500-900 nm indicate that gold nanoparticles are presumed to be smaller than 1.7 nm. These primary nanoparticles may act as seeds for growth of nanoparticles and formation of nanowires. Second, reduction of the remaining AuCl4- leads to the growth of primary nanoparticles. Nanoparticles randomly move by Brownian motion and approach adjacent particles within 10 nm, attaching to other particles driven by the van der Waals attractive force. Thus, following the jump in model, these nanoparticles hit and stick together very rapidly, followed by deposition of newly formed gold atoms onto the concave regions of the connected particles through capillary phenomenon based on Kelvin equation, that is, a kind of localized Oswald ripening. It is considered that the evolution of the network starts with single wires at the early reaction stage and ends up in a network as the reaction proceeds. Third, the relatively excessive AuCl4(32) Kiely, C. J.; Fink, J.; Brust, M.; Bethell, D.; Schiffrin, D. J. Nature 1998, 396, 444. (33) Yonezawa, T.; Onoue, S.; Kimizuka, N. Chem. Lett. 2002, 12, 1172.

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capped on the surface of nanowire networks maintains their shapes. It should be noted that when the AuCl4- had disappeared by the reactions at certain concentration of citrate (i.e., R ) 0.4), the nanowires broke and aggregated into thermodynamically stable gold nanoparticles. During this process, it was seen that the gold ions played an important role in stabilizing the gold nanowire shape based on the repulsion between anions. When gold ions disappeared due to reduction with citrate, nanowires were transformed to large nanoparticles. V. Conclusion In conclusion, gold nanowires with a 2-D network structure were formed by citrate reduction of AuCl4- with a low concentration of citrate. It was shown that gold ions played an important role in forming and stabilizing the shape of gold nanowires. One of the key points of this process was the preferential adsorption of AuCl4- on the surface of the preliminary gold particles instead of citrate ions, which resulted in an attracting force between gold nanoparticles. This method for synthesizing 2-D gold nanowires is simple and is expected to be applicable to the synthesis of other metallic nanowires such as silver or platinum. In addition, the importance of gold ions for the fabrication and stabilization of the 2-D gold nanowire networks observed in this study opens up new possibilities and considerations for the shape control of anisotropic metallic nanoparticles. Acknowledgment. This work was supported by a Grant in Aid for Scientific Research from the Ministry of Education, Culture, Science, Sports and Technology of Japan. The authors gratefully acknowledge Professor S. Isoda and Dr. Y. Murata for assistance with the TEM observation. Professor T. Yonezawa is also appreciated for helpful discussion. LA049262V