A Facile Room-Temperature Synthesis of Gold Nanowires by Oxalate

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A Facile Room-Temperature Synthesis of Gold Nanowires by Oxalate Reduction Method S. Navaladian, C. M. Janet, B. Viswanathan, T. K. Varadarajan, and R. P. Viswanath* National Centre for Catalysis Research, Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India ReceiVed: June 9, 2007; In Final Form: July 10, 2007

Polycrystalline gold nanowires were synthesized by a simple chemical reduction method using potassium oxalate as the reducing and capping agent along with polyvinylpyrrolidone (PVP) as the co-capping agent at room temperature. Different molar concentration ratios of HAuCl4 and potassium oxalate were studied for the synthesis of Au nanowires. Kinetics studies for Au colloid formation were carried out using UV-vis spectroscopic technique for the various concentration ratios of the reactants. As the concentration of oxalate was increased, the rate of reduction as well as particle growth was found to increase. XRD and TEM analyses revealed the formation of polycrystalline Au nanowires with a preferential growth of the {111} facet. In the absence of PVP, no Au colloid was formed. Studies revealed that the formation of the Au nanowire was due to the bridging nature of the oxalate dianion. The formation of Pt nanowires further supported the role of oxalate when the same method was adopted for the synthesis of Pt nanowires without using PVP. A plausible mechanism was proposed for the formation of Au nanowires explaining the role of oxalate.

Introduction Distinctive features of the current trend in the area of nanoresearch mainly focus on the synthesis of colloidal particles of controlled size, shape, and surface chemistry. These particles are interesting as building blocks of larger superstructures, which have versatile applications in electronics, photonics, catalysis, chemical sensing, and other areas.1 It has been known for a long time that monodisperse collections of spherical and polyhedral nanoparticles can give rise to well-ordered crystals of 2- and 3-D arrays.2 On the other hand, less symmetrical polydispersed particles and linkers can give rise to more complex and interesting superstructures, both on the nanoscale3 and on larger length scales.4 If one can adapt similar methodologies for the fabrication of metal nanowires without using any template, it will be beneficial for the demand of a simple roomtemperature synthesis of metal nanowires. Because of excellent electrical and thermal conductivity and mechanical properties, gold nanowire is noted for its promising technological applications as an interconnect in nanoelectronics, as a light emitting diode5 in optoelectronics, and as a glucose sensor. However, these functions are strongly dependent on crystallographic and morphological characteristics. Precise control over the relative position and orientation of the nanocomponents is required in such systems to obtain useful properties. Perfect, straight, single crystalline rods and wires can be synthesized by various methods such as wet chemical synthesis based on a seed mediated growth mechanism,6 solution phase methods based on capping agents7 or a reverse micellar8 or polyol process,9 template based methods using nanoporous solid templates such as alumina membranes,10 polycarbonate films,11 carbon nanotubes,12 MCM-4113 and SBA-15,14 electrochemical deposition15 and step edge decoration methods,16 photochemical methods,17 chemical oxidation methods,18 selfassembly in supercritical conditions,19 and top down methods based on nanolithography and laser ablation.20 When sodium * Corresponding author. E-mail: [email protected].

citrate, lysine, and 2-mercapto succinic acid were utilized as reducing and capping agents, they were reported to form Au nanowires21,23 under certain conditions. Murphy et al.22 have provided insight on the nature of the directing surfactant on the aspect ratio of the rods or wires formed in a seed mediated growth process. Bottom up self-organization of self-assembled nanoentities is an attractive approach for creating a bridge between macroscopic systems and nanoscale dimensions that many modern technologies demand. Even though many methods are known, the mechanism of formation of Au nanowires is different due to factors such as rate of reduction, nature, and the concentration of the capping agents as well as reducing agents and reaction conditions such as the pH and temperature of the reaction medium. Although these studies succeeded in fabricating 1- and 2-D nanostructures, a full understanding of the mechanism of formation of the metallic nanowire network still requires studies on 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 this paper, we will demonstrate a simple chemical reduction method for the fabrication of gold nanowires with a 2-D network structure. By carefully controlling the ratio of the concentration of the reducing agent to Au ions in the chemical reduction of AuCl4-, gold 2-D networks of nanowires were formed. A symmetric bidentate oxalate molecule has been utilized as the reducing and capping agent along with polyvinyl pyrrolidone (PVP) for the first time in the formation of Au nanowires. Because of the preferential orientation of the {111} plane and the defect driven twinning observed in the as-synthesized Au nanowire, this material will be a promising candidate in the area of catalysis. Formation of the wire morphology has been further supported for Pt also using potassium oxalate as a reducing and capping agent without the use of PVP even though the kinetics in this case was sluggish.

10.1021/jp0744782 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/31/2007

Facile Room-Temperature Synthesis of Gold Nanowires

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Experimental Procedures Synthesis of Au Colloids. In a typical procedure, 0.2 g of PVP (Mw ≈ 40 000, Sigma-Aldrich) was added to the 20 mL 1 mM HAuCl4 (Sigma-Aldrich, 99.5% purity) solution in a beaker and stirred for 10 min to dissolve PVP completely. Then, a 0.5 M K2C2O4 (Merck, 99% purity) solution was added. All the glassware and magnetic pellets were washed with aqua regia (3:1 HCl/HNO3) before the reaction. Different molar concentration ratios of HAuCl4 to potassium oxalate were studied, and these were 1:1, 1:2, 1:5, 1:7, and 1:10. After a few minutes of addition of potassium oxalate, decolorization of the yellow HAuCl4 solution was observed. The appearance of a pink solution followed by blue was observed in the case of 1:5, 1:7, and 1:10, whereas the solution was pink and violet for 1:1 and 1:2, respectively. After 2 h of the preparation, the colloids were centrifuged at 10 000 rpm for 10 min with a large amount of deionized water. Preparation of Pt Colloids. To the 20 mL 2 mM H2PtCl6 (Sigma-Aldrich, 99.5% purity) solution, 8 mL of the 50 mM potassium oxalate (Merck, 99% purity) solution was added and stirred for 1 h. After 24 h, the color of the H2PtCl6 solution completely disappeared and became colorless. Some gold bronze colored threads were observed in the solution. After 10 days, this clear solution was dried and analyzed by SEM, XRD, TEM, and EDAX. The solution was allowed to further age at room temperature, and a black colloid was observed after 35 days. This black colloid was analyzed through TEM after washing by centrifugation. After centrifugation, the residue was dispersed in ethanol and utilized for TEM analysis by drop casting on a carbon coated copper grid followed by drying at room temperature under a low vacuum. UV-vis absorption studies were carried out to monitor the growth of the Au nanoparticles in the solution by recording the surface plasmon resonance (SPR) bands of the Au colloid during the course of reduction and particle growth. TEM and HRTEM analyses were carried out using a Philips CM12 transmission electron microscope working at a 100 kV accelerating voltage and JEOL-3010 transmission electron microscope working at 300 kV, respectively. UV-vis spectra were recorded using a Jasco V-530 spectrophotometer. Powder X-ray diffraction patterns of Au nanoparticulates were recorded in a Shimadzu XD-D1 diffractometer using Ni filtered Cu KR radiation (λ ) 1.5406 Å). SEM images were recorded using a JSM-6330F scanning electron microscope operating at 20 kV. Results and Discussion UV-vis spectra of the Au nanocolloids formed under various concentration ratios of AuCl4- and potassium oxalate are shown in Figure 1. In the case of concentration ratio of 1:1, the surface plasmon band is observed around 552 nm24 as shown in Figure 1b. Tailing may be due to the formation of spherical particles along with some amount of anisotropic particles. The corresponding HRTEM images (see Figure 2a) show the presence of spherical particles of a size range of 15-40 nm and some triangular anisotropic particles. The UV-vis spectrum corresponding to the Au colloid with the 1:2 concentration shows two different peaks at 538 and 1090 nm (see Figure 1c). The corresponding TEM image shows the formation of anisotropic particles such as hexagonal and triangular plates.25 The hexatwinned nanosheet of size of around 250 nm is observed in the TEM image, and the twin boundaries present in it originate from the center of the plate. Self-assembly of the particles is also evident from the images. When the concentration ratio is increased to 1:5, a broad SPR band is observed as expected for

Figure 1. UV-vis spectra of Au colloids prepared with various concentration ratios of HAuCl4 and K2C2O4. (a) 1:5 (without PVP), (b) 1:1, (c) 1:2, (d) 1:5, (e) 1:7, and (f) 1:10.

Figure 2. (a and b) HRTEM images of Au nanoparticles formed with 1:1 and 1:2 concentration ratios of HAuCl4 and K2C2O4, respectively. (c and d) TEM and HRTEM images of Au nanowires formed from 1:5 concentration ratio.

the anisotropic particles or self-assembly of the nanoparticles.23,26 From the TEM image (see Figure 2c), it is clear that a chain-like 2-D network of the Au nanowires of diameters around 15-20 nm is formed. The corresponding HRTEM image in Figure 2d shows a part of these nanowires, which gives the conformation that many particles of different sizes and shapes are fused together. In addition, exposed facets of this wire are found to be aligned in different directions.27 The polycrystalline nature of the wires is evident from this, and it further supports the fact that the wires are formed at the expense of particles. There is no clear SPR band in the absence of PVP for the 1:5 concentration ratio (Figure 1a). This indicates that no stable colloid is formed in the absence of PVP and hence proves the significance of PVP. A red residue, which sticks to the walls of the reaction vessel, only is observed when no PVP is used. However, in the case of the 1:7 concentration ratio, the formation of a 2-D network of Au nanowires of thicknesses in the range of 10-20 nm is observed. These nanowires are branched and lengthy in nature as shown in Figure 3a. The corresponding SAED spot pattern shown implies single crystalline metallic Au. EDAX also confirms that the network is made out of Au metal (see Figure S8 in Supporting Information). However, the

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Figure 3. (a and b) TEM and HRTEM images of Au nanowires formed with a 1:7 concentration ratio of HAuCl4 and K2C2O4. (c and d) HRTEM images of short Au wires formed with a 1:10 concentration ratio. (Inset shows SAED pattern recorded at a portion of the wire.)

corresponding HRTEM image in Figure 3b shows the chainlike branched nanowires with twin boundaries, and therefore, these wires are polycrystalline. The spot pattern observed could be due to the focusing of the electron beam to any one particle that constitutes the wire. The powder XRD pattern of the Au colloid corresponding to a concentration ratio of 1:7 shows peaks attributed to the fcc system of Au metal, and the corresponding d values match with the JCPDS file no. 04-0784 (see Figure S9 in Supporting Information). When the intensity ratios of the peaks are compared with that of bulk Au, the intensity of the {111} facet is found to be more predominant than normally is expected. The texture coefficient is calculated using eq 128

C(hkl)i )

I(hkl)i 1 / Io(hkl)i n

I(hkl)n

∑nIo(hkl)n

(1)

where C(hkl) is the texture coefficient of the facet {hkl}, I(hkl) is the intensity of the {hkl} reflection of the sample under analysis, Io(hkl) is the intensity of the {hkl} reflection of a polycrystalline bulk sample, and n is the number of the reflections taken into account. By using this equation, the preferential growth of the facets can be understood. C(hkl) is expected to be unity for the facet, which does not have preferential growth. If it is higher than unity, it is a preferentially grown facet. C(hkl) values of different facets of the Au nanocolloid and average crystallite sizes are given in Figure 4. The average crystallite sizes of Au nanoparticulates have been calculated from each reflection of the XRD pattern using Scherrer’s equation.29 From the calculation, it is found that the {111} and {222} facets have C(hkl) values greater than unity, and the values are 1.5 and 1.18, respectively. This shows that the {111} facet of the Au nanowires is parallel to the surface of the substrate.30 In addition, the peaks corresponding to {111} and {222} show less broadening than the other facets as reflected from the crystallite sizes derived from the corresponding peaks. Similar to C(hkl), the average crystallite size calculated for each peak deviates from each other, particularly {111} and {222}. This also indicates the preferential growth of {111} in the Au nanowire. The values of average crystallite sizes reveal that the longitudinal direction of the wire

Figure 4. Correlation of the (a) texture coefficients and (b) crystallite sizes calculated from powder XRD pattern with the corresponding facets of Au nanowires synthesized with a 1:7 concentration ratio of the HAuCl4 and K2C2O4 solution.

mainly contains the {111} facet but the growth along the {111} direction is not continuous. It is because of the fact that the average crystallite size with respect to the {111} facet is 11.3. If the growth of the {111} facet is continuous, the expected crystallite size will be as high as can be seen in TEM micrographs. This average crystallite size is attributed to the particles that constitute the wire and indicates that the wire is polycrystalline. Moreover, it is important to note that the plots of texture coefficients and crystallite size against different facets show the same trend. Thus, XRD results support the observation that the lattice fringes of the {111} facet of the Au nanowire lie in different directions as is seen from the HRTEM micrograph in Figure 2d. UV-vis spectra of Au colloids corresponding to concentration ratios of 1:5, 1:7 and 1:10 show broad bands starting from 520 nm to the near-IR region (see Figure 1). This shows the anisotropic nature of the Au nanoparticles present in the colloid. HRTEM images of Au nanoparticles formed in the case of the 1:10 concentration ratio show the presence of poorly textured short wires of a diameter of approximately 20 nm. The core shell-like featureless bend and clumsy networks of wires of Au are clearly seen in Figure 3c,d. The reduction of Au (III) by potassium oxalate at room temperature is facile due to the favorable reduction potentials of HAuCl4 (E°([AuCl4]-/Au,4Cl-) ) +0.93 V)31 and potassium oxalate (E°(2CO2/C2O42-) ) -0.49 V)31 as given in eq 233

2HAuCl4 + 3K2C2O4 f 2AuV + 6CO2v + 2HCl + 6KCl (2) Kinetics of particle growth has been monitored using the SPR intensity change in the UV-vis spectra for each concentration ratio (see Figures S1-S5 in the Supporting Information). Initially, the solution has an absorbance at 310 nm due to HAuCl4. This peak disappears after the addition of the potassium oxalate solution, indicating the consumption of AuCl4-. The solution becomes colorless due to the reduction of Au (III) to

Facile Room-Temperature Synthesis of Gold Nanowires

Figure 5. (a) Correlation between saturation time of SPR band at λmax and molar concentration ratios of HAuCl4 and K2C2O4 used for the growth of Au nanoparticulates. (b) Normalized plot showing the change in the intensity of the SPR band with respect to time. Normalization was achieved by dividing each peak intensity (A) by the saturation intensity value (As).

Au metal. As the saturation of Au atoms is attained, the formation of nuclei takes place. Then, the peak around 555 nm appears, and its intensity increases with time. This implies the growth of Au nanoparticles and Ostwald ripening. As it reaches some particular size, restricted growth may take place to form anisotropic shapes such as triangular and polygonal plates and wires. These anisotropic structures may give rise to the SPR band at a higher wavelength around 848 nm. Unlike other concentration ratios, 1:2 shows the appearance of a SPR band corresponding to longitudinal absorption (1090 nm) of light by Au nanoparticles along with the SPR band corresponding to the transverse absorption (538 nm). In the case of other concentration ratios such as 1:5, 1:7 and 1:10, the appearance of the SPR band in the higher wavelength region observed only after the formation of a particular amount of spherical particles. This shows the restricted growth of Au nanoparticles in particular directions to yield anisotropic nanostructures such as the triangular and polygonal plates. This restricted growth may be due to the arresting of particular facets by capping or adsorption of oxalate at the 1:2 concentration ratio. At higher concentrations, the adsorption of oxalate on other facets also is possible due to the excess concentration of oxalate. As a result, a smaller amount of anisotropic particles is formed, and the degree of self-assembly is high. To understand the rate of growth of the spherical particles, the saturation intensity of SPR bands corresponding to transverse absorption is plotted against time and is shown in Figure 5a.

J. Phys. Chem. C, Vol. 111, No. 38, 2007 14153 This reveals that as the concentration ratio increases, the rate of the reduction also increases. Consequently, the rate of particle growth also is high. However, Figure 5b shows the correlation of the change in the SPR intensity for each scan, which has been normalized with the highest SPR intensity (saturation) against time. This gives an idea about the change in the rate of particle growth with respect to time. In the plot, an initial slight increase in each curve corresponds to the beginning of the formation of Au nanoparticles. A further steep increase is due to the growth of the particles. However, in the case of a 1:1 concentration ratio, the nucleation as well as the growth was slower than all other concentration ratios. This may be due to the deficiency of oxalate in the solution, which delays or reduces the saturation point of the Au growth species. This effect is more pronounced only in the case of the 1:1 and 1:2 concentration ratios. In the case of the 1:1 and 1:2 concentration ratios, the rate of reduction of Au (III) is slow, and heterogeneous nucleation may be more predominant so that both the 1:1 and the 1:2 concentration ratios result in highly polydispersed particles. There is a faster reduction of Au (III) in the case of the 1:10 concentration ratio, and therefore, in this case, short Au nanowires are formed. This is because shape control under certain kinds of capping agents with faster particle growth is difficult. Formation of the polycrystalline nanowires under the influence of the oxalate has not been properly understood or discussed in the literature so far. If PVP is not used, no stable Au colloid is formed. Of course, PVP is a good capping agent for Au nanoparticles.34 Being a high molecular weight polymer, PVP controls the rate of reduction of Au (III) by reducing the contact between AuCl4- and oxalate by steric hindrance. PVP reduces the rate of particle growth also for the same reason. However, the oxalate concentration affects the morphology of the Au nanoparticles. Also, PVP is not known for assisting the growth of Au nanowires at room temperature. As per the experimental results, it is clear that oxalate plays a major role in the formation of Au nanowires. In addition, the concentration of oxalate also affects the length and texture of the Au wires formed. To understand the nature of oxalate in directing the morphology of the Au nanoparticulates, the synthesis of Pt nanoparticles also was attempted by the same method with a 1:10 concentration of H2PtCl6 and potassium oxalate in the absence of PVP. However, after 35 days, the formation of a black colloid appeared. Even though the reduction potentials of Au and Pt are closer, the reduction of Pt (IV) to Pt (0) by oxalate is slower than that of Au. Standard reduction potentials31of Pt are E°([PtCl6]2-/[PtCl4]2-,2Cl-) ) 0.68 V and E°([PtCl4]2-/Pt,4Cl-) ) 0.755 V. The Pt reduction reaction is given in eq 3

H2PtCl6 + 2K2C2O4 f PtV + 4CO2v + 2HCl + 4KCl (3) This difference in the rate of reduction of Au and Pt is mainly due to the poor coordination chemistry of gold.35 Pt (II) forms oxalate complexes with the oxalate anion K2[Pt(C2O4)2], a square planar compound,36 which exhibits a rod-like morphology in SEM and TEM images shown in Figure 6 a,b. The formation of gold bronze thread-like crystals in the solution confirms the formaton of K2[Pt(C2O4)2. These crystals are the stacking of the platinum oxalate polymer and Pt ion within the complex is bonding via the partially filled dz2 orbital, which allows for metallic conduction properties.37 The wires in the SEM image exist as bundles and the thickness of the wires varies from 100 nm to micrometers. From the TEM image, the thickness of the wire is found to be around 100 nm, and an aspect ratio of ≈11 is observed. The corresponding EDAX spectrum shows the

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Figure 6. (a) SEM and (b) TEM images of K2[Pt(C2O4)2] complex analyzed after 10 days of reaction of H2PtCl6 with a K2C2O4 1:10 molar ratio. In SEM, the image scale bar is 3 µm.

presence of C, O, K, and Pt along with Cu, which comes from the carbon coated Cu grid. This reveals the formation of an intermediate compound. The powder XRD pattern of these intermediate wires matches neither with Pt metal nor with potassium oxalate. This may be presumably concluded as being K2[Pt(C2O4)2]. These species are present as molecular wires (Figure S11 in the Supporting Information), which is similar to that reported in the literature.37 TEM images of the black Pt colloid formed after 35 days show the formation of 2-D networks of Pt nanowires of a thickness approximately in the range of 15-20 nm (see Figure S13 in the Supporting Information). The SAED pattern shows the polycrystalline nature of the Pt nanowires formed. Decomposition of the oxalato platinum complex takes place due to the favorable reduction

Navaladian et al. potentials, and then particles of Pt must have been formed in the colloid. This reduction process is slow due to the considerable stability of K2[Pt(C2O4)2]. Since excess of the oxalate dianion was present in the solution, Pt nanoparticles must have been formed. This is an evidence for the formation of wires due to the presence of the oxalate dianion. Fu et al.38 reported the formation of Pt nanowires when hydrogen gas was used as a reducing agent for Pt (IV) with oxalate as a capping agent. It was reported that when PVP or citrates were used as capping agents, only the Pt particles were formed without any signicant formation of wires.38 In this current study for Pt, oxalate was employed as a reducing as well as a capping agent. The current results along with the already available reports reveal that the formation of wire is due to the presence of oxalate. This is due to the symmetric bidentate capping action of oxalate over the surface of the Pt nanoparticles. From the results of Pt nanowire formation, it is understandable that it is the same case as with Au wire formation, although the reduction kinetics is different for both Au and Pt. The carboxylic (-COO-) groups present in citrate act as the capping sites that cap the surface of the nanoparticles.39 In the same way, the oxalate also can cap the Au nanoparticles. Murphy et al.40 reported the self-assembly of Au nanorods using the adipate dianion in neutral or slightly basic medium. The adipate dianion possesses two terminal carboxylic groups, which can bridge the two nanorods to self-assemble. In the same way, oxalate also can bridge the nanoparticle to self-assemble. The plausible mechanism is given in Scheme 1. In the first step, the reduction, formation of nuclei, particle growth, and capping of Au nanoparticles by PVP and oxalate dianion are represented. During Au nanoparticle growth, due to the bidendate nature of the oxalate, it may bridge the Au nanoparticles through a sideon fashion as shown in the second step. Since the size of the oxalate is smaller, the fusion of the self-assembled particles may take place through the stacking of Au atoms in between bridged particles as shown in the third step. If this process continues, the formation of the polycrystalline Au wire takes place. The stable geometry of the oxalate dianion is given in the schematic mechanism.41 Adsorption of other ionic species such as Cl-, K+, and H+ on the Au surface was avoided for clarity. When the concentration of oxalate becomes high, the extent of capping also will be high. At the same time, the rate of reduction of Au (III) and the particle growth also increase. In such a condition, the short aggregates consisting of larger particles are formed due to a high degree of self-assembly and faster reduction as observed in the case of the 1:10 concentration ratio. The formation of featureless short wires in the case of the 1:10

SCHEME 1: Plausible Mechanism for the Formation of Au Nanowires with Potassium Oxalate as a Reducing and Capping Agent in the Presence of PVP (Co-capping Agent)

Facile Room-Temperature Synthesis of Gold Nanowires concentration ratio may arise due to the paucity of Au atoms that fill the gap of the two particles to form the nanowires. This paucity of Au atoms in the growth solution may occur due to the faster growth of the Au nanoparticles. The formation of nanoplates and triangles may be due to the adsorption of oxalate on the particular facet of the nuclei or small particles generated during the growth of the nanoparticles.42 Since some of the facets of small particles or nuclei are arrested by capping the oxalate, the unarrested facets may grow to form such anisotropic particles. Recently, Lee et al.43 reported the utilization of a citrate reduction method for the synthesis of Au nanowires by quenching the reaction by reducing the temperature of the reaction vessel. In the absence of quenching, the Au nanoparticles are formed from the intermediate Au nanowire. In the literature, there is a report on the formation of Au nanowires network with a deficient amount of citrate. It has been proposed by Pei et al.23 that the adsorbed AuCl4- on the Au clusters renders them attractive to each other to form nanowire networks. However, the removal of AuCl4- leads to the breakage of nanowires and the formation of spherical nanoparticles from the nanowires. In general, the citrate reduction method required some heat for reduction to occur. However, the oxalate reduction method yields stable Au nanowires at room temperature itself in the presence of PVP, and resulting Au nanowires are stable. The same method cannot be directly adopted for the synthesis of silver nanowires due to the formation of silver oxalate, which is a water-insoluble white solid prepared by the reaction of Ag+ with oxalate dianion. The decomposition of silver oxalate requires a temperature of 140 °C to yield silver nanoparticles in the presence of capping agents in inert atmosphere as reported by our group.44 Conclusion The synthesis of polycrystalline Au nanowire 2-D networks by a simple chemical reduction method using an oxalate dianoin as the reducing and capping agent at room temperature was demonstrated without the use of a template or a regular surfactant such as CTAB or a seed. Kinetics studies based on the UV-vis spectra revealed that the oxalate concentration is directly proportional to the reduction of Au (III). The lower concentration ratios of oxalate and HAuCl4, namely, 1:1 and 1:2, gave rise to only spherical and anisotropic particles such as plates and triangles, respectively. Other concentration ratios of 1:5 and 1:7 show the formation of polycrystalline Au 2-D networks, whereas the 1:10 ratio shows short Au nanowires and aggregates. To understand the role of oxalate, the same method was adopted for the reduction as well as capping of Pt. Analysis of intermediates showed the formation of micro- and nanorods of K2[Pt(C2O4)2] molecular wires that have been known to be used for applications such as 1-D conductivity and anisotropic optical properties. The formation of Pt nanowires was observed only in the presence of oxalate. It was proven that the bridging nature of the oxalate dianion is responsible for the formation of nanowires of Au and Pt in the solution phase synthesis. The fine Au nanowires fabricated by this oxalate reduction method are proposed to have high potentials for catalysis as well as electrochemical sensor applications because the catalytically more active {111} facet is predominantly present. Acknowledgment. A research grant from CSIR, New Delhi is acknowledged with thanks. The authors acknowledge the DST Nanotechnology Centre, IIT Madras for HRTEM measurements. Supporting Information Available: UV-vis spectra corresponding to kinetics studies for 1:1, 1:2, 1:5, 1:7, and 1:10

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