Photochemical formation of electrically conductive silver nanowires on polymer scaffolds

Journal of Colloid and Interface Science 344 (2010) 334–342

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Photochemical formation of electrically conductive silver nanowires on polymer scaffolds Subrata Kundu *, David Huitink, Ke Wang, Hong Liang * Materials Science & Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123, USA

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Article history: Received 27 October 2009 Accepted 6 January 2010 Available online 11 January 2010 Keywords: UV-photoirradiation Silver Nanowires PVA Polymer

a b s t r a c t A photochemical method has been exploited for the synthesis of electrically conductive silver (Ag) nanowires in a polymer solution in the presence of negatively charged Au seed particles. The synthesis was completed within 8 min of UV-photoirradiation in ambient conditions. The nanowires were fabricated on a PVA template having an average diameter of 135 ± 20 nm and a length of 10–20 lm. The current– voltage (I–V) characterization showed that the PVA–Ag nanowires were continuous, having Ohmic behavior with low contact resistance. Results indicate that the PVA acted as a reducing agent, stabilizing agent, and a template for the nucleation and growth of Ag nanowires. The Ag deposition was highly selective and on the PVA only. Our research indicated that the PVA–Ag nanowires might be useful as interconnects in nanoscale integrated circuitry, functional nanodevices, and in optoelectronics. Ó 2010 Elsevier Inc. All rights reserved.

1. Introduction In the past few years, the synthesis and properties of metal nanoparticles (NPs) has received much attention due to their promising optical [1], electrical [2], magnetic [3] and thermal [4] properties. The high surface-to-volume ratio of NPs results in dramatic changes in their properties in comparison to their bulk metal. Among the different noble metals studied so far, silver NPs have been paid considerable interest because of their potential applications in optics [1], catalysis [5], microelectronics [2], and surface enhanced Raman scattering [6] studies. Recently, synthesis and characterization of one-dimensional (1-D) nanowires has attracted much attention due to their promising electrical, optical, and biological properties. Nanowires serve as a unique tool in nanoelectronics for nanodevice fabrication. Among the different metal nanowires, silver nanowires have been shown particular interest because bulk silver exhibits superior electrical and thermal conductivity vs. other metals, which might lead to its use as interconnections in sophisticated nanodevices [7]. Various reports have been published for the synthesis of well-organized single crystal 1-D nanowires of silver [8–10]. The synthesis of silver nanowires proceeds via three main steps. Initially, the dissolution of silver salt in appropriate solvent; then the reduction of the salt in the presence of appropriate reducing agent, and finally the stabilization of the particles in the presence of appropriate stabilizing agent or templates to form the desired nanowires. The template-based synthetic routes include molecular * Corresponding authors. Fax: +1 979 845 3081. E-mail addresses: [email protected] (S. Kundu), [email protected] (H. Liang). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.01.004

sieves [11], polymer templates [12], block copolymers [13], DNAmolecule templates [14], micelle-copolymer gel templates [15], etc. Radhakrishnan and coworkers synthesized Ag NPs on a polymer template at 110 °C for 1 h [16]. Peng and coworkers synthesized silver nanowires on a zeolite template [9]. Wang et al. synthesized silver nanowires exploiting photochemical approach [10]. Long silver nanowires were synthesized by Liu et al. [17] using a film casting method in gelatin at room temperature. Thin films of silver nanowires were synthesized by Zhang et al. on glass walls by mild chemical reduction in aqueous solutions of poly (methacrylic acid) [18]. There are several other method also reported for the synthesis of silver nanowire [19,20]. However, most of the methods find major drawbacks due to inhomogeneous growth, presence of excess reducing agent with the nanowire, short length of the nanowire and long reaction processing time. Recently, polymer based syntheses of silver NPs have been shown great interest due to their unique properties. Zhang and coworkers synthesized mixtures of 1-D nanorods and nanowires using a microwave polyol method [21]. Kong and Jang synthesized Ag NPs embedded in polymer nanofibers using a radical-mediated dispersion polymerization reaction [22]. Lee and coworkers synthesized Ag NPs in PVP by c-irradiation techniques [23]. Saraf and coworkers synthesized Ag NPs using the microwave heating method [24]. Ultraviolet-irradiation techniques have been used to synthesize metal NPs like Ag [5], Au [25], Pd [26] and semi-conductor wires [27] at significantly higher speed compared to conventional thermal convection methods. Recently, El Khoury et al. reported that UV-irradiation technique could offers a simple and efficient method for the purification of gold nanorods from excess thiol surfactants in the solution [28]. To date, there have been no

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reports for the in situ synthesis of 1-D Ag nanowires by 8 min of UV-photoirradiation on PVA scaffolds in the presence of negatively charged Au seed particles. Here, we put forward a one-step in situ non-micellar method to fabricate Ag nanowires. The Ag nanowires were synthesized by mixing AgNO3 solution with PVA in the presence of negatively charged Au seed particles under 8 min of UV-photoirradiation. The novelty lies in the in situ synthesis of Ag nanowires in a polymer template rather than low molecular weight amphiphiles or general surfactant systems that do not yield the nanowires. The synthesized nanowires were found to be electrically conductive and the process is simple, fast, and straightforward.

tion. The particles in this solution were used as seeds within 2–5 h after preparation. The UV–Vis spectrum showed an absorption band maximum at 507 nm as shown in Fig. 1A. The average particle size measured from a transmission electron micrograph was 5 ± 0.3 nm (Fig. 1B and C). The citrate serves only as capping agent since it cannot reduce gold salt at room temperature (25 °C).

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2. Experimental 2.1. Reagents Hydrogen tetrachloroaurate tri-hydrate (HAuCl43H2O, 99.9%), sodium borohydride (NaBH4), and tri-sodium citrate dihydrate (Na3C6H5O72H2O) were purchased from Sigma–Aldrich and used as received. Silver nitrate (AgNO3) was also purchased from Sigma and used as received. Poly (vinyl alcohol) (PVA) with different molecular weights of 10–12 kDa, 30–70 kDa and 126–180 kDa were purchased from Sigma–Aldrich and used without further purification. De-ionized (DI) water was used for all synthesis experiments. 2.2. Instruments The ultraviolet–visible (UV–Vis) absorption spectra were recorded in a Hitachi (model U-4100) UV–Vis–NIR spectrophotometer equipped with a 1 cm quartz cuvette holder for liquid samples. A high resolution transmission electron microscope (HR-TEM, JEOL JEM-2010) was used at an accelerating voltage of 200 kV. The energy dispersive X-ray spectrum (EDS) was recorded with the instrument connected with the HR-TEM during TEM experiments. The XRD analysis was conducted at a scanning rate 0.020 S1 in the 2h range 35–85° using a Rigaku Dmax cA X-ray diffractometer with Cu–Ka radiation (k = 0.154178 nm). The X-ray photoelectron spectroscopy (XPS) analysis was carried out using a Kratos Axis Ultra Imaging X-ray photoelectron spectrometer with monochromatic Al–Ka line (1486.7 eV). The incident X-ray beam was normal to the sample surface and the detector was 45° away from the incident direction. The analysis spot on the sample was 0.4 mm by 0.7 mm. An atomic force microscope (AFM) probe was used for scanning the surfaces for the conductivity study made by a multimode AFM instrument from Pacific Nanotechnology Inc. The tip was coated with a double layer of chromium and platinum–iridium approximately 25 nm thick on both sides of the cantilever. The tip side coating enhances the conductivity of the tip and allows electrical contacts. The detector side coating enhances the reflectivity of the laser beam by a factor of 2 and prevents light from interfering within the cantilever. The bending of the cantilever due to stress is less than 3.5% of the cantilever length. A xenon lamp source (Newport Corporation) having frequency between 47 and 63 Hz was used for UV-photoirradiation.

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2.3. Synthesis of citrate-capped negatively charged gold seed particles A 20 ml aqueous solution containing 2.5  104 M HAuCl4 and 2.5  104 M tri-sodium citrate was prepared in a flask. Next, 0.6 ml of ice-cold 0.1 M NaBH4 solution was added to the solution all at once with continuous stirring. The solution turned pink immediately after adding NaBH4, indicating particle forma-

Fig. 1. UV–Vis and transmission electron microscopy (TEM) images of Au seed particles: (A) the UV–Vis spectrum of the Au seed particles having a plasmon absorption band at 507 nm. The inset shows the pink colored Au seed solution; (B) low magnified and (C) high magnified TEM images of Au seed particles. The average size of the seed particles is 5 ± 0.3 nm. (For interpretation of color mentioned in this figure legend the reader is referred to the web version of the article.)

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2.4. Synthesis of Ag nanowires on PVA using UV-photoirradiation A stock solution of 102 M AgNO3 solution was made and kept in the dark for protection against sunlight. Poly (vinyl alcohol) (PVA) solution (1% by weight, 126–180 kDa) was prepared by dissolving PVA in DI water and stirring overnight. In a typical synthesis, 6 ml of PVA solution was mixed with 0.4 ml of 102 M silver nitrate solution. Subsequently, 0.1 ml of Au seed solution was added to the PVA and silver nitrate mixture. The resulting solution mixture was then irradiated under UV-light for about 8 min. The UV-irradiation was done by keeping the UV-light source 8 cm away from the sample glass vials. The color of the resulting solution changed to light yellowish after complete UV-irradiation. The solution was found to be stable for more than six months under ambient conditions without change in any optical properties. 2.5. Preparation of samples for TEM, EDS, XRD, XPS and I–V studies The synthesized Ag nanowires on PVA solution were characterized using TEM, EDS, XPS, XRD analysis and from current–voltage (I–V) measurements. The samples for TEM and EDS were prepared by placing a drop of the corresponding Ag nanowire solution onto a carbon-coated Cu grid followed by slow evaporation of solvent at ambient condition. For XPS and XRD analysis, glass slides were used as the substrates for the thin film preparation. The glass slides were cleaned thoroughly in acetone, ethanol and iso-propanol and sonicated for about 15 min. The cleaned substrates were covered with the Ag nanowire solution and dried in a vacuum chamber. After the first layer was deposited, subsequent layers were deposited by repeatedly adding more Ag nanowire solution and drying. Final samples were obtained after 8–10 depositions and then analyzed using XPS and XRD techniques. The I–V measurements were performed using an AFM conductive probe scanning the substrate in a contact mode. The AFM probe was in contact with the nanowire and an electrical potential was applied through the nanowire and the AFM probe. The applied electrical potentials through the nanowire and the AFM probe were increased from 250 lV to 250 lV using 10 lV steps of 6s duration and maintaining for 3s at each prescribed measurement. For each potential, the current flow through the nanowire was recorded by the LabView program of the AFM computer with various applied voltages.

molecular weight. A fully hydrolyzed PVA with a high molecular weight possesses high water resistance and high tensile strength [29]. PVA is widely used as a lubricant and for film forming, emulsifying and different polymerization reactions. In the last few years the PVA has been used widely as a template and a capping agent for NP syntheses [30,31]. Fig. 2 shows the UV–Vis absorption spectrum of the reaction mixture at different stages of the synthesis process. An aqueous colorless solution of AgNO3 has no specific absorption band in the visible region (curve A, Fig. 2). An aqueous solution of PVA (1% by weight) has no specific absorption band in the visible region above 200 nm due to absence of any aromatic moiety (curve B, Fig. 2). A mixture of silver nitrate and PVA shows a new absorption band indicated with curve C, Fig. 2. This absorption band shows a broad low intensity spectrum from the range 240–350 nm due to formation of PVA–Ag+ complex. After the addition of Au seed to the PVA–Ag+ complex mixture and UV-photoirradiation for 10 min, the solution color changed from colorless to light yellow indicating the formation of Ag NPs. The absorption peak for the PVA–Ag+ complex fully disappeared and a sharp peak at 415 nm appeared due to the surface plasmon resonance of Ag NPs (curve D, Fig. 2) [24,32]. When the PVA–Ag+ mixture was photo-irradiated for a long period of time (>60 min) without any seed particles, a broad surface plasmon band was generated as indicated by curve E in Fig. 2. This absorption band contains absorption maxima at 437 nm with a small hump at 343 nm. The inset of Fig. 2 shows the yellowish colored Ag nanowire solution. Fig. 3A shows the UV–Vis absorption spectra for the successive formation of Ag nanowire with different photoirradiation times. With the increasing UV-irradiation, the absorption intensity also increases and reaches a maximum after 8 min (curve A–G, Fig. 3A) of UV-irradiation, confirming the completion of the reaction. The kmax value was found almost unchanged for the entire synthesis indicating the uniformity of the Ag nanowires. Fig. 3B is the ln (abs) vs. time (T) plot for the formation of Ag nanowires. The plot shows linear correlation having a rate constant value (k) of 1.73  101 min1 with respect to the Ag nanowire formation. The standard deviation and correlation coefficient for this measurement were 0.05571 and 0.99096,

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3. Results and discussion

Ag nanowires were synthesized using UV-photoirradiation techniques in the presence of Au seed particles using PVA as stabilizing agent. The important point is that the polymer itself acts as a template for the formation of Ag nanowire as well as a stabilizing agent. The syntheses of the Ag nanowires are completed within 8 min of UV-photoirradiation which is significantly faster than any other previous reports of Ag nanowire synthesis [11–16]. The process does not need any harsh reducing conditions or low molecular weight amphiphiles like surfactants, etc. that do not produce the desired nanowires. Fig. 1A shows the UV–Vis spectrum of the Au seed particles. The citrate-stabilized negatively charged Au seed particles show a plasmon absorption band at 507 nm. The inset shows the pink colored Au seed solution. Fig. 1B (low magnification) and 1C (high magnification) are the corresponding transmission electron microscopy (TEM) images. The average size of the seed particles measured from TEM images is 5 ± 0.3 nm. PVA is a well known organic polymer and is mostly used as a water soluble organic binder whose properties depend on its

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Wavelength (nm) Fig. 2. UV–Vis absorption spectrum of the reaction mixtures at different stages of the synthesis process. (A) Absorption spectrum of aqueous AgNO3 solution; (B) absorption spectrum of aqueous PVA (1% by weight) solution; (C) absorption spectrum of the mixture of silver nitrate and PVA solution due to formation of PVA– Ag+ complex; (D) surface plasmon resonance (SPR) band for Ag nanowires with an absorption maximum at 415 nm; (E) SPR band of Ag NPs when synthesized in the absence of Au seed particles, shows two absorption maxima, one at 437 nm and the other at 343 nm. The inset shows the yellowish color Ag nanowire solution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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part of the samples. The red arrows in Fig. 4E and F show some non-uniformity of the nanowire structure. Fig. 5 shows the four different TEM images at other reduction conditions. When an aqueous solution of AgNO3 is irradiated for more than 2 h using UV-light in the absence of PVA or Au seed only large Ag clusters are formed with no specific shapes as shown in Fig. 5A and B is the TEM image of the particles when the synthesis was done in the absence of Au seed particles corresponding to curve E in Fig. 2. Particles were formed but had no specific shapes. The average diameter of the particles is 200 ± 50 nm. The inset of Fig. 5B shows the corresponding high magnification image. Fig. 5C and D show the images of the Ag NPs formed with different molecular weight PVA’s. Fig. 5C is the image when we used 10– 12 kDa PVA whereas Fig. 5D is the image when we used 30–70 kDa PVA. Here the average diameter of the particles is 70 ± 20 nm. The particles formed anisotropic shapes with few spherical ones. 3.3. Energy dispersive X-ray spectroscopy (EDS) analysis

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3.4. X-ray diffraction (XRD) analysis

Standard Deviation = 0.05571 Correlation coefficient (R) = 0.99096 2

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The chemical composition of the PVA–Ag nanowires was obtained using energy dispersive X-ray spectroscopy (EDS) during TEM as shown in Fig. 6. The EDS spectrum consists of the elements Ag, C, Cu, Cr and Cl. The Ag peak came from the Ag nanowire, the C and Cu peaks came from the carbon-coated Cu TEM grid and the Cr peak came from the sample holder used for TEM. The small Cl peak probably came from the Au seed solution, as HAuCl4 was used as a Au(III) precursor.

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Time (min) Fig. 3. (A) UV–Vis absorption spectra for the successive formation of Ag nanowire with different photoirradiation times from 1 to 8 min (curve A–G). (B) The ln (abs) vs. time (T) plot for the formation of Ag nanowire having rate constant value (k) = 1.73  101 min1.

respectively. The synthesized Ag nanowire solution is remarkably stable with no sign of change in color or oxide formation after six months of storage under ambient conditions at room temperature.

3.2. Transmission electron microscopy (TEM) analysis Figs. 4 and 5 show the transmission electron microscopy (TEM) images of the PVA–Ag nanostructures at different reaction conditions. Fig. 4A and B show the low magnification TEM images of the PVA–Ag nanowires from the same sample. It is clear that the nanowires are several microns long and wrapped with each other. The average diameter of the nanowires is 135 ± 20 nm. Some individual NPs were also found in the solution as labeled with red circles and arrows. The inset of B shows the corresponding higher magnification image. Fig. 4C shows Ag nanowires from another part of the sample. Here all the nanowires were crosslinked with each other during the sample preparation. Fig. 4D shows an image of a single Ag nanowire at low magnification. Fig. 4E and F show the TEM image is single nanowire at higher magnification. The insets of Fig. 4D and F show the corresponding selective area electron diffraction (SAED) pattern of the individual Ag nanowire and confirms that the nanowires are single crystalline in nature. Another inset of Fig. 4F shows the HR-TEM of a single Ag wires from another

The X-ray diffraction pattern of the Ag nanowires is shown in Fig. 7. The diffraction peaks originated from the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) planes of Ag nanowires (JCPDS card number 4-0783). These diffraction peaks confirm the face centered cubic (fcc) structure of the Ag nanowires [33]. All the diffraction peaks are sharp with the (1 1 1) diffraction peak having the highest intensity. The relative intensity in diffraction pattern are mostly depends upon the crystal structure and the lattice orientation of crystallites. According to Wang [34] the shape of any fcc nanocrystals is measured by the ratio of growth rates along the h1 0 0i and h1 1 1i directions. Here, PVA acts as a template and stabilizer for the Ag nanowire synthesis, so the selective interaction between PVA and different crystal facets of Ag nanowires affects the reaction rate at a specific direction and produced the desired Ag nanowires. Hu et al. [35] also observed similar types of X-ray diffraction pattern for their synthesis of Ag nanowires using acetaldehyde reduction. From the SAED analysis they assigned that the growth rate along the h2 1 1i direction with nanowire axis [35]. Although, in our study we have not fully confirmed about the growth direction of the nanowires and the details crystal orientation will be discuss in future research. 3.5. XPS analysis Fig. 8 shows the overall survey and Ag (3d) XPS spectrum of PVA–Ag films on a glass substrate. The survey spectrum in Fig. 8A contains expected characteristic peaks of O (1s) at 529.27 eV, C (1s) at 281.17 eV and Ag (3d). The Ag (3d) peaks are a doublet which arises due to spin–orbit coupling (3d5/2 and 3d3/2) and are shown in Fig. 8B. The binding energies for the Ag 3d5/2 and Ag 3d3/2 peaks are 368.05 eV and 374.20 eV respectively. These peaks at these specific binding energy values confirm the formation of Ag nanowires [36]. There are no oxide formation peaks observed from the image, indicating high stability of the Ag nanowires. The O and C peaks

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Fig. 4. Transmission electron microscopy (TEM) images of the PVA–Ag nanowires at different magnification. (A) and (B) are the low magnification TEM images of the PVA–Ag nanowires with average diameter of 135 ± 20 nm. The inset of (B) shows the corresponding higher magnification image; (C) the TEM image of Ag nanowires from another part of the sample shows crosslinking between the nanowires; (D) the TEM image of a single Ag nanowire at low magnification; (E) and (F) the TEM image is single nanowire at higher magnification. The insets of (D) and (F) show the corresponding selective area electron diffraction (SAED) pattern. Another inset of (F) shows the HR-TEM of a Ag wires from another part of the samples.

came from the PVA used for the synthesis and stabilization of the Ag nanowires in our reaction. 3.6. Study with other reaction parameters We have tested the effects of PVA concentration, effects of AgNO3 concentrations, amount of Au seed addition, UV-photoirra-

diation time, etc. The Ag nanowires formed at the particular concentration given in the experimental section. When the AgNO3 concentration is high (P103 M), the Ag particles were formed but mostly aggregated with each other. When the AgNO3 concentration is low (6106 M), no Ag particles were formed in the experimental time scale. We examined the effects of varying the concentrations of PVA. Ag nanowires formed only at high

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Fig. 5. The TEM images at different reaction conditions: (A) micron-size Ag clusters formed when an aqueous solution of AgNO3 was irradiated for more than 2 h using UVlight in the absence of PVA or Au seed. (B) The TEM image of the Ag particles when synthesis was done in absence of Au seed particles. The average diameter of the particles are 200 ± 50 nm. The inset shows the corresponding higher magnification image. (C) and (D) show the TEM images of the Ag NPs formed with different molecular weight PVA’s like 10–12 kDa and 30–70 kDa. The average diameters of the particles is 70 ± 20 nm.

Fig. 6. The energy dispersive X-ray spectrum (EDS) of the PVA–Ag nanowires.

concentrations (P1%) of PVA. In other cases (60.01%) mostly spherical particles formed with few wires. We have examined the reaction with different molecular weight PVA’s like 10– 12 kDa, 30–70 kDa and 126–180 kDa and obtained nanowires only with 126–180 kDa. For other low molecular weight PVA’s mostly

Fig. 7. The X-ray diffraction (XRD) patterns of the PVA–Ag nanowires. The diffraction peaks are assigned from the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) planes of Ag nanowires.

anisotropic particles formed with few wires in some cases (as shown in Fig. 5C and D). This is probably due to the longer chain

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not find any hysteresis from which we conclude that the Ag nanowires are continuous structure. In our present study, the overall circuit resistance for the linear fit I–V curve is 9962.3 Ohm including the surface resistance. This resistance is an upper bound on the resistance of the Ag nanowire itself as nanowire-substrate contact resistance might be significant. The bulk resistivity of Ag metal is 15.87 nX m. So the resistance value for the nanowires is significantly higher than the bulk Ag value as described above. We measured the conductivity of Ag nanowires right after the experiments. After 3 months of aging, we found almost the same resistances in both cases. The PVA–Ag nanowires are highly stable and these properties are desirable for device fabrication. More details of the electrical characterization of these nanowires will be discussed in the future.

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length polymers having a higher tendency to form foam or fiberlike molecular template than the shorter chain length polymers. We have varied the amount of Au seed. When the Au seed concentration is high, the Ag wires formed much faster but the wires were not uniform in shape. When the Au seed concentration is low, the reaction takes a longer time to produce the nanowires. We have varied the UV-irradiation time and conclude that 8 min irradiation is sufficient for the formation of nanowires. All of these control experiments give us a generalized idea about the reaction process. 3.7. Conductivity (current, I vs. voltage, V) measurement The synthesized PVA–Ag nanowires were deposited on a substrate to measure their electrical conductivity using an AFM probe. We constructed an I–V curve with an external applied potential in order to measure the current. The details of the conductivity measurement have been discussed elsewhere [37,38]. It is well known that Ag metal has a good electrical conductivity. The spherical Ag NPs did not show any reasonable conductivity due to the significant gap between Ag particles when placed on a substrate. However, when the Ag NPs aligned together to form a continuous wire, they exhibit reasonable conductivity depending on the sample as well as other experimental parameters. Fig. 9 shows the corresponding I–V curve of a PVA–Ag nanowire. From the I–V curve, it is shown that the current increases linearly with the applied potential following the Ohm’s law. This indicates good contact of the Ag nanowire on the substrate. In our measurements we did

As discussed earlier, at higher concentrations, the PVA acts as a chain or fiber-like template for materials synthesis. The formation of Ag nanowires on PVA was attributed to the direct redox reaction between PVA and Ag+. This was proven by the fact that there was no other reducing agent present in our reaction mixture. The formation of a PVA–Ag nanowire proceeds via two steps. In the first step, Ag+ forms a complex with PVA, which is evident from the UV–Vis absorption spectrum in curve C, Fig. 2. In the second step, after photoirradiation, the hydroxyl groups present in PVA generate radical species which act as reducing agents to reduce Ag+ to Ag(0), which continues to grow on the PVA chain. The UV-photoirradiation of Ag salt itself did not produce Ag nanowires in the absence of PVA as shown in Fig. 5A. Therefore, we believe that PVA acts both as a reducing agent and a template for the growth of Ag nanowires. The formation of wire-like PVA template at higher concentration of polymer was already reported [30,31]. Hu et al. reported the use of PVA as a nonchelating agent and molecular template for the formation of sub-micron SnO2 nanorods [31]. They explained that the hydroxyl groups in PVA can form hydrogen/covalent bonds initially when PVA and DI water are mixed together and stirred overnight to get a transparent and homogeneous mixture. The PVA eventually formed a wire-like molecular template. Once they mixed in Sn4+ cations and added the mixed solution into an autoclave, the Sn4+ began to precipitate and grow on the PVA template, generating layer/wire-like SnO2 microrods [31]. Here in our study, during photoirradiation, the hydroxyl groups present in PVA produce hydrated electron or radical species that reduce the Ag+ to Ag(0). From spectroscopic studies it has been proved that photolysis of hydroxylic compounds generates hydrated electrons [39–41]. Other well known hydroxylic compounds such as TX-100 [32], ascorbic acid [39], benzophenone [40] and 2,7-dihydroxynaphthalene (2,7-DHN) [41]. have been used for the reduction of metal ions. Xia’s group reported earlier that ethylene glycol, also having hydroxyl groups was used for the synthesis of Ag nanowires in the presence of PVP at high temperature (185 °C) [42]. Deoxyribonucleic acid (DNA) has hydroxyl groups in its deoxyribose sugar unit and acts as a reducing agent under UV-photoirradiation [25]. It was reported that PVA could not act as a reducing agent without any irradiation but worked as a good reducing agent under photoirradiation by generating radical species [43]. Mills’ group reported earlier on the formation of metallic particles during photoirradiation in the presence of PVA [44]. Using our method, we believe that radical species or solvated electrons formed during the photoirradiation of PVA are responsible for the reduction of Ag+ to Ag(0). Once the Ag(0) is formed, it grows successively in the PVA template to form the Ag nanowires. During UV-photoirradiation both the nucleation and growth processes took place at the PVA solution interface. So the question arises, what is the specific role of Au seed particles for the

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formation of Ag nanowires? In our study, we used citrate-capped negatively charged Au seed particles which affect the synthesis in three different ways. First, in the presence of Au seed particles, Ag+ first adsorbed both on PVA and on Au seed particles. Second, the electron transfer took place from PVA to Ag+ via the Au seed particles. Finally, Au seed enhanced the reaction time to complete within 8 min of UV-photoirradiation. Therefore, Au seed not only acts as a catalyst during electron transfer process but also as a reaction mediator and accelerates the reaction during the electron transfer process. The hydroxyl groups of PVA coordinate with and stabilize the Ag particles along the PVA chain to produce the desired nanowires. In the absence of Au seed particles, the forma-

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tion of Ag NPs takes longer and results in particles with undefined shapes as shown in Fig. 5B. This control experiment indirectly proved that the electron transfer took place via the Au seed particles, which increased the rate of the reaction. Initially, PVA–Ag complex formed in the solution and adsorbed onto the negatively charged Au seed particles. Upon UV-photoirradiation the Ag+ ions were reduced and deposited on the Au seed or on the PVA and growth took place along the PVA template to form the nanowires. Similar types of Au seed assisted synthesis of Ag nanocubes were reported earlier by Kundu et al. in the presence of polystyrene sulfonate (PSS) solution under microwave heating [24]. They explained that Ag+ ions deposited on the Au seed or on the PSS and

Fig. 9. The current (I)–voltage (V) plot of PVA–Ag nanowire. From the plot, the resistance of the linear fit I–V curve is 9962.300286 Ohm.

Scheme 1. The schematic presentation for the formation of nanowire and other shaped Ag NPs.

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growth takes place in specific directions guided by PSS to produce the nanocubes. Here in our study, both the Au seed and the PVA also helped for the formation and growth of Ag nanowires. Furthermore, the Au seed particles accelerated the reaction by participating during the electron transfer process. The formation of Ag nanowires and other shaped particles are schematically shown in Scheme 1. The electrical characterization signifies that the Ag nanowires are conductive and behave Ohmically. We believe that this process for the synthesis of conductive Ag nanowires within 8 min of UV-photoirradiation is faster than any other reported methods. The details of the mechanism at the atomic scale for the wire synthesis are not fully understood and further study is required. The present process might be valuable for the fabrication of other mono-metallic/bi-metallic nanowires in desirable time scales. 4. Conclusions In summary, we have described a one-step in situ process for the synthesis of electrically conductive Ag nanowires on PVA in the presence of negatively charged Au seed particles under 8 min of UV-photoirradiation. The synthesized nanowires are 10–20 microns long, continuous and behave Ohmically. The process is very simple, fast, and straightforward. The PVA acts both as a reducing agent and a stabilizing one. It directs the growth of the particles along the polymeric chain/template and produces the nanowires. This process might lead to a quick manufacturing process for the synthesis of composite nanowires for future applications in nanoelectronics. Acknowledgments This research was in part sponsored by the NSF-05,06,082 and 05,35,578; the Department of Mechanical Engineering, Texas A&M University; and the Texas Engineering Experiments Station. We wish to acknowledge Mr. Sean Lau from Texas A&M University for proofreading the manuscript. Supports for TEM and EDS by Dr. Zhiping Luo at the Microscopy Imaging Center (MIC), Texas A&M University, were greatly appreciated. References [1] D.D. Evanoff, G. Chumanoy, Chem. Phys. Chem. 6 (2005) 1221. [2] A. Heilmann, A. Kiesow, M. Gruner, U. Kreibig, Thin Solid Films 343 (1999) 175.

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