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

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Journal of Colloid and Interface Science

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Photochemical formation of electrically conductive silver nanowireson polymer scaffolds

Subrata Kundu *, David Huitink, Ke Wang, Hong Liang *

Materials Science & Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123, USA

a r t i c l e i n f o

Article history:Received 27 October 2009Accepted 6 January 2010Available online 11 January 2010

Keywords:UV-photoirradiationSilverNanowiresPVAPolymer

0021-9797/$ - see front matter � 2010 Elsevier Inc. Adoi:10.1016/j.jcis.2010.01.004

* Corresponding authors. Fax: +1 979 845 3081.E-mail addresses: skundu@tamu.edu (S. Kundu), h

a b s t r a c t

A photochemical method has been exploited for the synthesis of electrically conductive silver (Ag) nano-wires in a polymer solution in the presence of negatively charged Au seed particles. The synthesis wascompleted within 8 min of UV-photoirradiation in ambient conditions. The nanowires were fabricatedon 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 Ohmicbehavior with low contact resistance. Results indicate that the PVA acted as a reducing agent, stabilizingagent, and a template for the nucleation and growth of Ag nanowires. The Ag deposition was highly selec-tive and on the PVA only. Our research indicated that the PVA–Ag nanowires might be useful as intercon-nects in nanoscale integrated circuitry, functional nanodevices, and in optoelectronics.

� 2010 Elsevier Inc. All rights reserved.

1. Introduction

In the past few years, the synthesis and properties of metalnanoparticles (NPs) has received much attention due to theirpromising optical [1], electrical [2], magnetic [3] and thermal [4]properties. The high surface-to-volume ratio of NPs results in dra-matic changes in their properties in comparison to their bulkmetal. Among the different noble metals studied so far, silverNPs have been paid considerable interest because of their potentialapplications in optics [1], catalysis [5], microelectronics [2], andsurface enhanced Raman scattering [6] studies. Recently, synthesisand characterization of one-dimensional (1-D) nanowires hasattracted much attention due to their promising electrical, optical,and biological properties. Nanowires serve as a unique tool innanoelectronics for nanodevice fabrication. Among the differentmetal nanowires, silver nanowires have been shown particularinterest because bulk silver exhibits superior electrical and thermalconductivity vs. other metals, which might lead to its use as inter-connections in sophisticated nanodevices [7].

Various reports have been published for the synthesis ofwell-organized single crystal 1-D nanowires of silver [8–10]. Thesynthesis of silver nanowires proceeds via three main steps. Ini-tially, the dissolution of silver salt in appropriate solvent; thenthe reduction of the salt in the presence of appropriate reducingagent, and finally the stabilization of the particles in the presenceof appropriate stabilizing agent or templates to form the desirednanowires. The template-based synthetic routes include molecular

ll rights reserved.

liang@tamu.edu (H. Liang).

sieves [11], polymer templates [12], block copolymers [13], DNA-molecule templates [14], micelle-copolymer gel templates [15],etc. Radhakrishnan and coworkers synthesized Ag NPs on a poly-mer template at 110 �C for 1 h [16]. Peng and coworkers synthe-sized 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. Thinfilms of silver nanowires were synthesized by Zhang et al. on glasswalls by mild chemical reduction in aqueous solutions of poly(methacrylic acid) [18]. There are several other method also re-ported for the synthesis of silver nanowire [19,20]. However, mostof the methods find major drawbacks due to inhomogeneousgrowth, 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 beenshown great interest due to their unique properties. Zhang andcoworkers synthesized mixtures of 1-D nanorods and nanowiresusing a microwave polyol method [21]. Kong and Jang synthesizedAg NPs embedded in polymer nanofibers using a radical-mediateddispersion polymerization reaction [22]. Lee and coworkers syn-thesized Ag NPs in PVP by c-irradiation techniques [23]. Sarafand coworkers synthesized Ag NPs using the microwave heatingmethod [24]. Ultraviolet-irradiation techniques have been usedto synthesize metal NPs like Ag [5], Au [25], Pd [26] and semi-con-ductor wires [27] at significantly higher speed compared to con-ventional thermal convection methods. Recently, El Khoury et al.reported that UV-irradiation technique could offers a simple andefficient method for the purification of gold nanorods from excessthiol surfactants in the solution [28]. To date, there have been no

1.0

SPR of Au seed Particles

A

S. Kundu et al. / Journal of Colloid and Interface Science 344 (2010) 334–342 335

reports for the in situ synthesis of 1-D Ag nanowires by 8 min ofUV-photoirradiation on PVA scaffolds in the presence of negativelycharged Au seed particles.

Here, we put forward a one-step in situ non-micellar method tofabricate Ag nanowires. The Ag nanowires were synthesized bymixing AgNO3 solution with PVA in the presence of negativelycharged Au seed particles under 8 min of UV-photoirradiation.The novelty lies in the in situ synthesis of Ag nanowires in a poly-mer template rather than low molecular weight amphiphiles orgeneral surfactant systems that do not yield the nanowires. Thesynthesized nanowires were found to be electrically conductiveand the process is simple, fast, and straightforward.

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Wavelength (nm)

Au seed solution

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Fig. 1. UV–Vis and transmission electron microscopy (TEM) images of Au seedparticles: (A) the UV–Vis spectrum of the Au seed particles having a plasmonabsorption 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 averagesize of the seed particles is 5 ± 0.3 nm. (For interpretation of color mentioned in thisfigure legend the reader is referred to the web version of the article.)

2. Experimental

2.1. Reagents

Hydrogen tetrachloroaurate tri-hydrate (HAuCl4�3H2O, 99.9%),sodium borohydride (NaBH4), and tri-sodium citrate dihydrate(Na3C6H5O7�2H2O) were purchased from Sigma–Aldrich and usedas received. Silver nitrate (AgNO3) was also purchased from Sigmaand used as received. Poly (vinyl alcohol) (PVA) with differentmolecular weights of 10–12 kDa, 30–70 kDa and 126–180 kDawere purchased from Sigma–Aldrich and used without furtherpurification. De-ionized (DI) water was used for all synthesisexperiments.

2.2. Instruments

The ultraviolet–visible (UV–Vis) absorption spectra were re-corded in a Hitachi (model U-4100) UV–Vis–NIR spectrophotom-eter equipped with a 1 cm quartz cuvette holder for liquidsamples. A high resolution transmission electron microscope(HR-TEM, JEOL JEM-2010) was used at an accelerating voltageof 200 kV. The energy dispersive X-ray spectrum (EDS) was re-corded with the instrument connected with the HR-TEM duringTEM experiments. The XRD analysis was conducted at a scanningrate 0.020 S�1 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 carriedout using a Kratos Axis Ultra Imaging X-ray photoelectron spec-trometer with monochromatic Al–Ka line (1486.7 eV). The inci-dent X-ray beam was normal to the sample surface and thedetector was 45� away from the incident direction. The analysisspot on the sample was 0.4 mm by 0.7 mm. An atomic forcemicroscope (AFM) probe was used for scanning the surfaces forthe conductivity study made by a multimode AFM instrumentfrom Pacific Nanotechnology Inc. The tip was coated with a dou-ble layer of chromium and platinum–iridium approximately25 nm thick on both sides of the cantilever. The tip side coatingenhances the conductivity of the tip and allows electrical con-tacts. The detector side coating enhances the reflectivity of thelaser beam by a factor of 2 and prevents light from interferingwithin the cantilever. The bending of the cantilever due to stressis less than 3.5% of the cantilever length. A xenon lamp source(Newport Corporation) having frequency between 47 and 63 Hzwas used for UV-photoirradiation.

2.3. Synthesis of citrate-capped negatively charged gold seed particles

A 20 ml aqueous solution containing 2.5 � 10�4 M HAuCl4 and2.5 � 10�4 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 solu-tion all at once with continuous stirring. The solution turnedpink immediately after adding NaBH4, indicating particle forma-

tion. The particles in this solution were used as seeds within2–5 h after preparation. The UV–Vis spectrum showed an absorp-tion band maximum at �507 nm as shown in Fig. 1A. The aver-age particle size measured from a transmission electronmicrograph was 5 ± 0.3 nm (Fig. 1B and C). The citrate servesonly as capping agent since it cannot reduce gold salt at roomtemperature (25 �C).

336 S. Kundu et al. / Journal of Colloid and Interface Science 344 (2010) 334–342

2.4. Synthesis of Ag nanowires on PVA using UV-photoirradiation

A stock solution of 10�2 M AgNO3 solution was made and keptin the dark for protection against sunlight. Poly (vinyl alcohol)(PVA) solution (1% by weight, 126–180 kDa) was prepared by dis-solving PVA in DI water and stirring overnight. In a typical synthe-sis, 6 ml of PVA solution was mixed with 0.4 ml of 10�2 M silvernitrate solution. Subsequently, 0.1 ml of Au seed solution wasadded to the PVA and silver nitrate mixture. The resulting solutionmixture was then irradiated under UV-light for about 8 min. TheUV-irradiation was done by keeping the UV-light source 8 cm awayfrom the sample glass vials. The color of the resulting solutionchanged to light yellowish after complete UV-irradiation. The solu-tion was found to be stable for more than six months under ambi-ent conditions without change in any optical properties.

2.5 A = Only AgNO3 (10-2 M)

2.5. Preparation of samples for TEM, EDS, XRD, XPS and I–V studies

The synthesized Ag nanowires on PVA solution were character-ized using TEM, EDS, XPS, XRD analysis and from current–voltage(I–V) measurements. The samples for TEM and EDS were preparedby placing a drop of the corresponding Ag nanowire solution onto acarbon-coated Cu grid followed by slow evaporation of solvent atambient condition. For XPS and XRD analysis, glass slides wereused as the substrates for the thin film preparation. The glass slideswere cleaned thoroughly in acetone, ethanol and iso-propanol andsonicated for about 15 min. The cleaned substrates were coveredwith the Ag nanowire solution and dried in a vacuum chamber.After the first layer was deposited, subsequent layers were depos-ited by repeatedly adding more Ag nanowire solution and drying.Final samples were obtained after 8–10 depositions and then ana-lyzed using XPS and XRD techniques. The I–V measurements wereperformed using an AFM conductive probe scanning the substratein a contact mode. The AFM probe was in contact with the nano-wire and an electrical potential was applied through the nanowireand the AFM probe. The applied electrical potentials through thenanowire and the AFM probe were increased from �250 lV to250 lV using 10 lV steps of 6s duration and maintaining for 3sat each prescribed measurement. For each potential, the currentflow through the nanowire was recorded by the LabView programof the AFM computer with various applied voltages.

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B = Only PVA (1%) C = Mixture of AgNO3 (10-2 M) + PVA (1%)

D = PVA-Ag nanowire

B

D

C EA

E = Ag NPs without Au seed

Fig. 2. UV–Vis absorption spectrum of the reaction mixtures at different stages ofthe synthesis process. (A) Absorption spectrum of aqueous AgNO3 solution; (B)absorption spectrum of aqueous PVA (1% by weight) solution; (C) absorptionspectrum 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 anabsorption maximum at �415 nm; (E) SPR band of Ag NPs when synthesized in theabsence of Au seed particles, shows two absorption maxima, one at 437 nm and theother at 343 nm. The inset shows the yellowish color Ag nanowire solution. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)

3. Results and discussion

3.1. UV–Vis spectroscopy study

Ag nanowires were synthesized using UV-photoirradiationtechniques in the presence of Au seed particles using PVA as stabi-lizing agent. The important point is that the polymer itself acts as atemplate for the formation of Ag nanowire as well as a stabilizingagent. The syntheses of the Ag nanowires are completed within8 min of UV-photoirradiation which is significantly faster thanany other previous reports of Ag nanowire synthesis [11–16]. Theprocess does not need any harsh reducing conditions or low molec-ular weight amphiphiles like surfactants, etc. that do not producethe desired nanowires.

Fig. 1A shows the UV–Vis spectrum of the Au seed particles. Thecitrate-stabilized negatively charged Au seed particles show a plas-mon absorption band at �507 nm. The inset shows the pink col-ored Au seed solution. Fig. 1B (low magnification) and 1C (highmagnification) are the corresponding transmission electronmicroscopy (TEM) images. The average size of the seed particlesmeasured from TEM images is 5 ± 0.3 nm.

PVA is a well known organic polymer and is mostly used as awater soluble organic binder whose properties depend on its

molecular weight. A fully hydrolyzed PVA with a high molecularweight possesses high water resistance and high tensile strength[29]. PVA is widely used as a lubricant and for film forming, emul-sifying and different polymerization reactions. In the last few yearsthe PVA has been used widely as a template and a capping agentfor NP syntheses [30,31]. Fig. 2 shows the UV–Vis absorption spec-trum of the reaction mixture at different stages of the synthesisprocess. An aqueous colorless solution of AgNO3 has no specificabsorption band in the visible region (curve A, Fig. 2). An aqueoussolution of PVA (1% by weight) has no specific absorption band inthe visible region above 200 nm due to absence of any aromaticmoiety (curve B, Fig. 2). A mixture of silver nitrate and PVA showsa new absorption band indicated with curve C, Fig. 2. This absorp-tion band shows a broad low intensity spectrum from the range240–350 nm due to formation of PVA–Ag+ complex. After the addi-tion of Au seed to the PVA–Ag+ complex mixture and UV-photoir-radiation for 10 min, the solution color changed from colorless tolight yellow indicating the formation of Ag NPs. The absorptionpeak for the PVA–Ag+ complex fully disappeared and a sharp peakat �415 nm appeared due to the surface plasmon resonance of AgNPs (curve D, Fig. 2) [24,32].

When the PVA–Ag+ mixture was photo-irradiated for a long per-iod of time (>60 min) without any seed particles, a broad surfaceplasmon band was generated as indicated by curve E in Fig. 2. Thisabsorption band contains absorption maxima at 437 nm with asmall hump at 343 nm. The inset of Fig. 2 shows the yellowish col-ored Ag nanowire solution. Fig. 3A shows the UV–Vis absorptionspectra for the successive formation of Ag nanowire with differentphotoirradiation times. With the increasing UV-irradiation, theabsorption intensity also increases and reaches a maximum after8 min (curve A–G, Fig. 3A) of UV-irradiation, confirming the com-pletion of the reaction. The kmax value was found almost un-changed for the entire synthesis indicating the uniformity of theAg nanowires. Fig. 3B is the ln (abs) vs. time (T) plot for the forma-tion of Ag nanowires. The plot shows linear correlation having arate constant value (k) of 1.73 � 10�1 min�1 with respect to theAg nanowire formation. The standard deviation and correlationcoefficient for this measurement were 0.05571 and �0.99096,

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Standard Deviation = 0.05571Correlation coefficient (R) = 0.99096

Rate constant (k) = 1.73 x 10-1 min-1ln (abs) vs Time (T) plot

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A

Fig. 3. (A) UV–Vis absorption spectra for the successive formation of Ag nanowirewith 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 � 10�1 min�1.

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respectively. The synthesized Ag nanowire solution is remarkablystable with no sign of change in color or oxide formation aftersix months of storage under ambient conditions at roomtemperature.

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 condi-tions. Fig. 4A and B show the low magnification TEM images ofthe PVA–Ag nanowires from the same sample. It is clear that thenanowires are several microns long and wrapped with each other.The average diameter of the nanowires is 135 ± 20 nm. Some indi-vidual NPs were also found in the solution as labeled with red cir-cles and arrows. The inset of B shows the corresponding highermagnification image. Fig. 4C shows Ag nanowires from anotherpart of the sample. Here all the nanowires were crosslinked witheach other during the sample preparation. Fig. 4D shows an imageof a single Ag nanowire at low magnification. Fig. 4E and F showthe TEM image is single nanowire at higher magnification. The in-sets of Fig. 4D and F show the corresponding selective area electrondiffraction (SAED) pattern of the individual Ag nanowire and con-firms that the nanowires are single crystalline in nature. Anotherinset of Fig. 4F shows the HR-TEM of a single Ag wires from another

part of the samples. The red arrows in Fig. 4E and F show somenon-uniformity of the nanowire structure.

Fig. 5 shows the four different TEM images at other reductionconditions. When an aqueous solution of AgNO3 is irradiated formore than 2 h using UV-light in the absence of PVA or Au seed onlylarge Ag clusters are formed with no specific shapes as shown inFig. 5A and B is the TEM image of the particles when the synthesiswas done in the absence of Au seed particles corresponding tocurve E in Fig. 2. Particles were formed but had no specific shapes.The average diameter of the particles is 200 ± 50 nm. The inset ofFig. 5B shows the corresponding high magnification image.Fig. 5C and D show the images of the Ag NPs formed with differentmolecular weight PVA’s. Fig. 5C is the image when we used 10–12 kDa PVA whereas Fig. 5D is the image when we used30–70 kDa PVA. Here the average diameter of the particles is�70 ± 20 nm. The particles formed anisotropic shapes with fewspherical ones.

3.3. Energy dispersive X-ray spectroscopy (EDS) analysis

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

3.4. X-ray diffraction (XRD) analysis

The X-ray diffraction pattern of the Ag nanowires is shown inFig. 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 cardnumber 4-0783). These diffraction peaks confirm the face centeredcubic (fcc) structure of the Ag nanowires [33]. All the diffractionpeaks are sharp with the (1 1 1) diffraction peak having the highestintensity. The relative intensity in diffraction pattern are mostlydepends upon the crystal structure and the lattice orientation ofcrystallites. According to Wang [34] the shape of any fcc nanocrys-tals is measured by the ratio of growth rates along the h1 0 0i andh1 1 1i directions. Here, PVA acts as a template and stabilizer forthe Ag nanowire synthesis, so the selective interaction betweenPVA and different crystal facets of Ag nanowires affects the reac-tion rate at a specific direction and produced the desired Ag nano-wires. Hu et al. [35] also observed similar types of X-ray diffractionpattern for their synthesis of Ag nanowires using acetaldehydereduction. From the SAED analysis they assigned that the growthrate along the h2 1 1i direction with nanowire axis [35]. Although,in our study we have not fully confirmed about the growth direc-tion of the nanowires and the details crystal orientation will be dis-cuss in future research.

3.5. XPS analysis

Fig. 8 shows the overall survey and Ag (3d) XPS spectrum ofPVA–Ag films on a glass substrate. The survey spectrum inFig. 8A contains expected characteristic peaks of O (1s) at529.27 eV, C (1s) at 281.17 eV and Ag (3d). The Ag (3d) peaks area 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 andAg 3d3/2 peaks are 368.05 eV and 374.20 eV respectively. Thesepeaks at these specific binding energy values confirm the formationof 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

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–Agnanowires 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 partof 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 athigher 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 wiresfrom another part of the samples.

338 S. Kundu et al. / Journal of Colloid and Interface Science 344 (2010) 334–342

came from the PVA used for the synthesis and stabilization of theAg nanowires in our reaction.

3.6. Study with other reaction parameters

We have tested the effects of PVA concentration, effects ofAgNO3 concentrations, amount of Au seed addition, UV-photoirra-

diation time, etc. The Ag nanowires formed at the particular con-centration given in the experimental section. When the AgNO3

concentration is high (P10�3 M), the Ag particles were formedbut mostly aggregated with each other. When the AgNO3 concen-tration is low (610�6 M), no Ag particles were formed in the exper-imental time scale. We examined the effects of varying theconcentrations of PVA. Ag nanowires formed only at high

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 UV-light 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 particlesare 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 weightPVA’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.

Fig. 7. The X-ray diffraction (XRD) patterns of the PVA–Ag nanowires. Thediffraction 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.

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

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

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XPS survey @ Ag nanowire

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Fig. 8. The X-ray photoelectron spectrum (XPS) of PVA–Ag nanowires. (A) Theoverall survey and (B) the spectrum of Ag (3d) on glass substrate.

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length polymers having a higher tendency to form foam or fiber-like molecular template than the shorter chain length polymers.We have varied the amount of Au seed. When the Au seed concen-tration is high, the Ag wires formed much faster but the wires werenot uniform in shape. When the Au seed concentration is low, thereaction takes a longer time to produce the nanowires. We havevaried the UV-irradiation time and conclude that 8 min irradiationis sufficient for the formation of nanowires. All of these controlexperiments 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 sub-strate to measure their electrical conductivity using an AFM probe.We constructed an I–V curve with an external applied potential inorder to measure the current. The details of the conductivity mea-surement have been discussed elsewhere [37,38]. It is well knownthat Ag metal has a good electrical conductivity. The spherical AgNPs did not show any reasonable conductivity due to the signifi-cant gap between Ag particles when placed on a substrate. How-ever, when the Ag NPs aligned together to form a continuouswire, they exhibit reasonable conductivity depending on the sam-ple as well as other experimental parameters. Fig. 9 shows the cor-responding I–V curve of a PVA–Ag nanowire. From the I–V curve, itis shown that the current increases linearly with the applied po-tential following the Ohm’s law. This indicates good contact ofthe Ag nanowire on the substrate. In our measurements we did

not find any hysteresis from which we conclude that the Ag nano-wires are continuous structure. In our present study, the overallcircuit resistance for the linear fit I–V curve is 9962.3 Ohm includ-ing the surface resistance. This resistance is an upper bound on theresistance of the Ag nanowire itself as nanowire-substrate contactresistance might be significant. The bulk resistivity of Ag metal is15.87 nX m. So the resistance value for the nanowires is signifi-cantly higher than the bulk Ag value as described above.

We measured the conductivity of Ag nanowires right after theexperiments. After 3 months of aging, we found almost the sameresistances in both cases. The PVA–Ag nanowires are highly stableand these properties are desirable for device fabrication. More de-tails of the electrical characterization of these nanowires will bediscussed in the future.

3.8. Reaction mechanisms of PVA–Ag nanowires synthesis

As discussed earlier, at higher concentrations, the PVA acts as achain or fiber-like template for materials synthesis. The formationof Ag nanowires on PVA was attributed to the direct redox reactionbetween PVA and Ag+. This was proven by the fact that there wasno other reducing agent present in our reaction mixture. The for-mation of a PVA–Ag nanowire proceeds via two steps. In the firststep, Ag+ forms a complex with PVA, which is evident from theUV–Vis absorption spectrum in curve C, Fig. 2. In the second step,after photoirradiation, the hydroxyl groups present in PVA gener-ate radical species which act as reducing agents to reduce Ag+ toAg(0), which continues to grow on the PVA chain. The UV-photoir-radiation of Ag salt itself did not produce Ag nanowires in the ab-sence of PVA as shown in Fig. 5A. Therefore, we believe that PVAacts both as a reducing agent and a template for the growth ofAg nanowires. The formation of wire-like PVA template at higherconcentration of polymer was already reported [30,31]. Hu et al.reported the use of PVA as a nonchelating agent and moleculartemplate for the formation of sub-micron SnO2 nanorods [31].They explained that the hydroxyl groups in PVA can form hydro-gen/covalent bonds initially when PVA and DI water are mixed to-gether and stirred overnight to get a transparent and homogeneousmixture. The PVA eventually formed a wire-like molecular tem-plate. Once they mixed in Sn4+ cations and added the mixed solu-tion into an autoclave, the Sn4+ began to precipitate and grow onthe PVA template, generating layer/wire-like SnO2 microrods[31]. Here in our study, during photoirradiation, the hydroxylgroups present in PVA produce hydrated electron or radical speciesthat reduce the Ag+ to Ag(0). From spectroscopic studies it has beenproved that photolysis of hydroxylic compounds generates hy-drated electrons [39–41]. Other well known hydroxylic com-pounds such as TX-100 [32], ascorbic acid [39], benzophenone[40] and 2,7-dihydroxynaphthalene (2,7-DHN) [41]. have beenused for the reduction of metal ions. Xia’s group reported earlierthat ethylene glycol, also having hydroxyl groups was used forthe synthesis of Ag nanowires in the presence of PVP at high tem-perature (185 �C) [42]. Deoxyribonucleic acid (DNA) has hydroxylgroups in its deoxyribose sugar unit and acts as a reducing agentunder UV-photoirradiation [25]. It was reported that PVA couldnot act as a reducing agent without any irradiation but workedas a good reducing agent under photoirradiation by generating rad-ical species [43]. Mills’ group reported earlier on the formation ofmetallic particles during photoirradiation in the presence of PVA[44]. Using our method, we believe that radical species or solvatedelectrons formed during the photoirradiation of PVA are responsi-ble for the reduction of Ag+ to Ag(0). Once the Ag(0) is formed, itgrows successively in the PVA template to form the Ag nanowires.During UV-photoirradiation both the nucleation and growth pro-cesses took place at the PVA solution interface. So the questionarises, what is the specific role of Au seed particles for the

S. Kundu et al. / Journal of Colloid and Interface Science 344 (2010) 334–342 341

formation of Ag nanowires? In our study, we used citrate-cappednegatively charged Au seed particles which affect the synthesisin 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 seedparticles. Finally, Au seed enhanced the reaction time to completewithin 8 min of UV-photoirradiation. Therefore, Au seed not onlyacts as a catalyst during electron transfer process but also as a reac-tion mediator and accelerates the reaction during the electrontransfer process. The hydroxyl groups of PVA coordinate withand stabilize the Ag particles along the PVA chain to produce thedesired nanowires. In the absence of Au seed particles, the forma-

Fig. 9. The current (I)–voltage (V) plot of PVA–Ag nanowire. From the p

Scheme 1. The schematic presentation for the form

tion of Ag NPs takes longer and results in particles with undefinedshapes as shown in Fig. 5B. This control experiment indirectlyproved that the electron transfer took place via the Au seed parti-cles, which increased the rate of the reaction. Initially, PVA–Agcomplex formed in the solution and adsorbed onto the negativelycharged Au seed particles. Upon UV-photoirradiation the Ag+ ionswere reduced and deposited on the Au seed or on the PVA andgrowth took place along the PVA template to form the nanowires.Similar types of Au seed assisted synthesis of Ag nanocubes werereported earlier by Kundu et al. in the presence of polystyrenesulfonate (PSS) solution under microwave heating [24]. They ex-plained that Ag+ ions deposited on the Au seed or on the PSS and

lot, the resistance of the linear fit I–V curve is 9962.300286 Ohm.

ation of nanowire and other shaped Ag NPs.

342 S. Kundu et al. / Journal of Colloid and Interface Science 344 (2010) 334–342

growth takes place in specific directions guided by PSS to producethe nanocubes. Here in our study, both the Au seed and the PVAalso helped for the formation and growth of Ag nanowires. Further-more, the Au seed particles accelerated the reaction by participat-ing during the electron transfer process. The formation of Agnanowires and other shaped particles are schematically shown inScheme 1. The electrical characterization signifies that the Agnanowires are conductive and behave Ohmically. We believe thatthis process for the synthesis of conductive Ag nanowires within8 min of UV-photoirradiation is faster than any other reportedmethods. The details of the mechanism at the atomic scale forthe wire synthesis are not fully understood and further study is re-quired. The present process might be valuable for the fabrication ofother mono-metallic/bi-metallic nanowires in desirable timescales.

4. Conclusions

In summary, we have described a one-step in situ process forthe synthesis of electrically conductive Ag nanowires on PVA inthe presence of negatively charged Au seed particles under 8 minof UV-photoirradiation. The synthesized nanowires are 10–20 mi-crons long, continuous and behave Ohmically. The process is verysimple, fast, and straightforward. The PVA acts both as a reducingagent and a stabilizing one. It directs the growth of the particlesalong the polymeric chain/template and produces the nanowires.This process might lead to a quick manufacturing process for thesynthesis of composite nanowires for future applications innanoelectronics.

Acknowledgments

This research was in part sponsored by the NSF-05,06,082 and05,35,578; the Department of Mechanical Engineering, Texas A&MUniversity; and the Texas Engineering Experiments Station. Wewish to acknowledge Mr. Sean Lau from Texas A&M Universityfor proofreading the manuscript. Supports for TEM and EDS byDr. Zhiping Luo at the Microscopy Imaging Center (MIC), TexasA&M University, were greatly appreciated.

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