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Particuology xxx (2012) xxx–xxx

Contents lists available at SciVerse ScienceDirect

Particuology

jo ur n al hom ep age: www.elsev ier .com/ locate /par t ic

apid synthesis and growth of silver nanowires induced by vanadium trioxidearticles

aitao Fu, Xiaohong Yang, Aibing Yu, Xuchuan Jiang ∗

chool of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia

r t i c l e i n f o

rticle history:eceived 2 May 2012eceived in revised form 5 June 2012ccepted 12 June 2012

eywords:g nanowires

a b s t r a c t

This study demonstrates a novel approach for rapid synthesis of silver (Ag) nanowires induced by vana-dium trioxide (V2O3) particles in aqueous solution at room temperature. Silver nanowires have anaverage diameter of 20 nm and length up to a few micrometers by parametric optimization. The micro-structure of the silver nanowires was characterized by TEM, HRTEM, SEM, and XRD techniques. Theoptical property of the as-prepared product was measured by ultraviolet–visible (UV–vis) spectroscopy.The possible growth mechanism of Ag nanowires via oriented attachment of Ag nanocrystals was dis-

riented attachmentanadium trioxide particlesoom-temperature synthesis

cussed. The present approach shows several unique features such as rapid (a few minutes), reproducibleand high-yield reaction with no need of any modifiers. V2O3 rods were reported for the first time tobe used for synthesis of silver nanowires, playing multiple roles as reducing agent, template, and cata-lyst. The silver nanowires produced are promising for optical applications (e.g., SERS) due to their roughsurface.

© 2012 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of

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. Introduction

Many efforts have been devoted to controlled synthesis andssembly of metal nanowires because of their potential functions interconnects or active components in fabricating nanodevicesCui, Wei, Park, & Lieber, 2001; Hu, Odom, & Lieber, 1999; Xia et al.,003; Zach, Ng, & Penner, 2000). In particular, one-dimensional1D) silver nanostructures have become one of the most important

embers of these functional metal nanowires. Silver nanowiresttract increasing attention due to their unique optical, electronic,nd physicochemical properties. With proper shape and size con-rol, they can be used in diverse applications including opticalevices (Fedutik, Temnov, Woggon, Ustinovich, & Artemyev, 2007),urface enhanced Raman spectroscopy (SERS) (McFarland, Young,ieringer, & van Duyne, 2005; Shanmukh et al., 2006), and sensors

Roy et al., 2009).Various methods have been developed to synthesize Ag

anowires to make use of their beneficial properties for large-cale application. These methods can be divided into two main

Please cite this article in press as: Fu, H., et al. Rapid synthesis and groParticuology (2012), http://dx.doi.org/10.1016/j.partic.2012.06.006

ategories, physical methods, e.g., electron beam induced lithog-aphy (Chaney, Shanmukh, Dluhy, & Zhao, 2005; Kondo &akayanagi, 1997; Kramer, Birk, Jorritsma, & Schonenberger, 1995;

∗ Corresponding author.E-mail address: xcjiang@unsw.edu.au (X. Jiang).

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674-2001/$ – see front matter © 2012 Chinese Society of Particuology and Institute of Process Ettp://dx.doi.org/10.1016/j.partic.2012.06.006

Sciences. Published by Elsevier B.V. All rights reserved.

ilvis-Cividjian, Hagen, Kruit, Stam, & Groen, 2003), thermal vaporeposition (Chen, Furusho, & Mori, 2009), and chemical methods,.g., photochemical (Eisele et al., 2010; Zhao, Zhu, & Chen, 2003),lectrochemical (Fang et al., 2009; Lin, Chen, Chen, & Cheng, 2008;eppler & Janek, 2007), and wet-chemical methods (Murphy & Jana,002), which have been proved to be powerful in shape control.owever, limitations exist in the above methods. Many of theseethods are restricted by critical experimental conditions such as

igh reaction temperature (>100 ◦C), long reaction time (tens toundreds minutes or longer), and requirement for specified surfaceodifiers (surfactants or polymers). For example, in the electron

eam induced lithography method (Chaney et al., 2005; Kondo &akayanagi, 1997; Kramer et al., 1995; Silvis-Cividjian et al., 2003),nergetic beams of photons, ions, and electrons have to be used,hile thermal vapor deposition method (Chen et al., 2009) needs

oth high vacuum and high temperature (as high as 1173 K). Sim-larly, surface modifiers are needed in chemical methods, such asitric acid as capping agent in photochemical method (Eisele et al.,010; Zhao et al., 2003), cetyltrimethyl ammonium bromide (CTAB)nd tetradecylammonium bromide (TDAB) as surfactants in elec-rolyte in electrochemical method (Fang et al., 2009; Lin et al., 2008;eppler & Janek, 2007), and poly(vinylpyrrolidone) (PVP) as tem-

wth of silver nanowires induced by vanadium trioxide particles.

late in wet-chemical method (Murphy & Jana, 2002; Qin, Park,uang, & Mirkin, 2005; Sun & Xia, 2002; Sun, Tao, Chen, Herricks,

Xia, 2004). These limitations may impede the development offficient and cost-saving strategies for making silver nanowires

ngineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

ARTICLE IN PRESSG ModelPARTIC-453; No. of Pages 13

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fficiently, and with high yield and reproducibility. Therefore, aacile, efficient, and low-cost method needs to be explored to meethese challenges.

Recently, wet-chemical synthesis was extensively studied,howing that silver nanoparticles can be prepared at low temper-tures in aqueous solutions with low cost, especially in our group.or example, Jiang, Chen, Chen, Xiong, and Yu (2011) and Jiang,eng, and Yu (2006) demonstrated that triangular and circular sil-er nanoplates (2.3 nm in thickness) could be successfully madey a self-seeding co-reduction method in aqueous solution. Thisethod does not need external seeds, organic solvents, and can

e carried out at room temperature. Other examples of synthe-is of silver nanoparticles can be found in the literature. Yuan,hang, and Zhu (2011) suggested that silver nano/microparticlesith different morphologies could be prepared by electrosynthesisith an H2O–oleic acid or an H2O–glycerol mixed solvent (volume

atio 1:1) as the electrolytic medium and AgNO3 as the suppor-ing electrolyte. The silver nanoparticles exhibited good catalyticroperty in the reduction of methyl orange and methylene blue.hang, Qi, et al. (2004) reported that Ag nanowire thin films cane synthesized by a wet chemical method at room temperature inqueous solution. The formation of products was influenced by theoncentration of the polymer (poly(methacrylic acid) (PMAA)), pHnd glass wall of the reactor. Jana, Gearheart, and Murphy (2001)btained silver nanorods with different aspect ratios from nearlypherical 4 nm silver nanoparticles by a seed-mediated growthpproach in a rod-like micellar medium. Another example of theet-chemical method is presented by Caswell, Bender, and Murphy

2003) who synthesised silver nanowires in an aqueous solution, inhe absence of seeds and surfactant. However, these methods couldardly avoid temperatures above 100 ◦C, requirement of surfaceodifiers, and long reaction time.The orientated attachment mechanisms of nanowires during

ynthesis are commonly reported in metal oxides. For instance,alder and Ravishankar (2007) demonstrated that ultrathin sin-le crystalline Au nanowires could be obtained by adding ascorbiccid to a mixture containing gold nanoparticles and aging at roomemperature for an extended period. It was demonstrated that theormation of Au wires was due to the preferential removal of theapping agent of amine from the Au{1 1 1} planes of the nanopar-icles followed by fusion. SnO2 nanowires were obtained from annsemble of 0D quantum dots through assistance of oleylaminen alcohols. They found that {0 1 0} is the most common attach-

ent plane in the nanowires (Xu, Zhuang, & Wang, 2008). Similarrowth mechanism also took place in CeO2 nanoflowers (Zhout al., 2008), MnO2 nanowires (Portehault, Cassaignon, Baudrin, &olivet, 2007), etc. For silver nanoparticles, some low-dimensionaltructures formed by oriented attachment such as nanorods (Sunt al., 2011) and dendrites (Wen et al., 2006) were discussed.owever, there have been but few reports on the formation androwth mechanism of Ag nanowires via orientated attachment.nderstanding the mechanism will be useful for shape-controlled

ynthesis of nanowires.In this paper we demonstrate a rapid wet-chemical approach,

n which V2O3 particles are introduced for the first time to gen-rate silver nanowires in aqueous solution at room temperature.his method shows several advantages, such as simple synthe-is system, room-temperature and rapid (within a few minutes),nd no need of seeds and external additives. By this approach,ilver ions can be quickly reduced by V2O3 (in a few minutes),ollowed by nucleation and growth on the surface of V2O3 par-

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icles (Fig. 1). In the whole process, V2O3 particles play multipleoles, including acting as reducing agent, template and catalyst. Thearticle characteristics (morphology, size, and composition) areharacterized by various advanced techniques such as transmission

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xx (2012) xxx–xxx

lectron microscopy (TEM), high resolution TEM (HRTEM), energy-ispersive X-ray spectroscopy (EDS), scanning electron microscopySEM), X-ray diffraction (XRD), and X-ray photoelectron spec-roscopy (XPS). The possible formation and growth mechanism ofilver nanowires through oriented attachment is also discussed.inally, the unique optical property of such Ag nanowires will bevaluated.

. Experimental work

.1. Materials

Ethylene glycol (reagent plus, ≥99%), sodium ortho-anadate (Na3VO4, ≥90%), HCl (32%), sodium dodecylulfate (SDS), cetyltrimethyl ammonium bromide (CTAB),oly(vinylpyrrolidone) (PVP) (Mr = 55,000), AgNO3 (analyticaleagent), HNO3 (70%), and NaOH (analytical reagent) were pur-hased from Sigma–Aldrich and Ajax Finechem Pty. Ltd. Pureater was purchased from Rowater, Australia. All the chemicalsere directly used without any further treatment.

.2. Synthesis

.2.1. Preparation of V2O3 particlesIn a typical synthesis, Na3VO4 was dissolved in ethylene glycol

ith a concentration of 0.15 M in a flask and stirred for ∼30 minntil Na3VO4 was totally dissolved, while the color of the solutionhanged to light yellow. After heating the yellow solution for a fewours, a blue–purple precipitate was formed. The flask was thenllowed to cool to room temperature. The products were washedith ethanol and then separated by centrifugation. Finally, V2O3articles were obtained by calcining the precipitate at 500 ◦C in aitrogen (N2) atmosphere for 2 h.

.2.2. Preparation of Ag nanowires0.5 mL 0.01 M AgNO3 solution was rapidly dropped into 10 mL

ransparent suspension containing 0.2 mmol of the as-preparedanadium trioxide (V2O3). The transparent suspension thenecame slightly brown though still transparent. Within 1 min, blackotton-like precipitate was quickly formed and deposited naturallyn the bottom. This precipitate could be redispersed into water byonication and the suspension was transparent again. Finally, theroduct was separated by centrifugation, and then rinsed severalimes using pure water.

.3. Characterizations

The morphology of the samples was investigated by JEOL 1400icroscope (TEM) and Philips CM200 FEG microscope (HRTEM),

perated at an accelerating voltage of 100 kV and 200 kV respec-ively. Scanning electron microscopy (SEM) was performed for

icrostructure on a Hitachi 900 microscope operated at 20 kV.owder X-ray diffraction (XRD) pattern for sample compositionas recorded on a Philip MPD diffractometer with Cu K� radi-

tion. UV–vis absorption spectra were obtained for tracking theormation and growth of silver nanowires on a Varian Cary 5 UV-IS-NIR spectrophotometer. X-ray photoemission spectroscopic

XPS) analysis was conducted for the confirmation of redox with Physical Electronics PHI 5000 Versaprobe spectrometer with Al

wth of silver nanowires induced by vanadium trioxide particles.

� radiation (1486 eV). The angle between the crystalline plane andhe analyzer was 45◦. Analysis of the spectra was performed usinghe Physical Electronics Multipak software package. The solutionH was obtained on a Sartorius PB-10 pH Metre.

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. Results and discussion

.1. V2O3 particles

The morphology and composition of the V2O3 prepared forynthesis of Ag nanowires were characterized by SEM and XRDechniques, as shown in Fig. 1. The SEM image of the as-prepared2O3 particles shown in Fig. 1(a) indicates that the particles areomposed of rods with diameters of ∼200 nm and length of 1 �m.ig. 1(b) shows an XRD pattern of the rhombohedra phase of V2O3JCPDS 00-034-0187). In this study, V2O3 rods will play importantoles in synthesis of Ag nanowires.

.2. Structure and morphology of the prepared Ag nanowires

To characterize the microstructure of the as-produced samples,EM (HRTEM), SEM, XRD, and EDS techniques were employed inhis work. Fig. 2(a) shows a SEM image of the as-prepared silveranowires, which suggests that the product mainly contained rel-tively uniform nanowires with diameter of ∼20 nm and length upo a few micrometers. The composition of the product was iden-ified by XRD technique. Fig. 2(b) shows that all silver nanowiresere metallic elemental silver (JCPDS file No. 04-783). Notably, the

ntensity ratios between Ag(2 0 0) and (1 1 1), and between (2 2 0)nd (1 1 1) peaks were smaller than the conventional values (0.14s. 0.4) and (0.12 vs. 0.25) based on the standard JCPDS card. Thesebnormal intensity ratios indicated that silver nanowires had abun-ant {1 1 1} facets. The small diffraction peak centered at 35.9◦

ight be attributed to 4H silver rather than face-centered-cubicfcc) silver structure (Shen et al., 2009). To confirm the purity ofhe nanowires, EDS for a single nanowire was carried out as shownn Fig. 2(c) that no other elements and phases appeared (except foropper element originated from TEM copper grid), indicating thathe nanowires were composed of pure silver. In addition, to removeOx, various methods will be used, e.g., centrifugation, membraneeparation and pH adjusting. Their effectiveness needs more worko be performed in the future.

Detailed TEM images of the as-prepared Ag nanowires are pre-ented in Fig. 3. Fig. 3(a) shows that the Ag nanowires were straight

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n shape, while some were formed with a bent structure similar tohose generated via the glycothermal method, as observed in ourecent study (Jiang, Xiong, Chen, Chen, & Yu, 2011b; Jiang, Xiong,ian, et al., 2011). Fig. 3(b) shows a high-magnification image of

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ern of the as-synthesized V2O3.

he thicker nanowires, indicating that the thicker nanowire wasomposed of several single nanowires aggregated side by side inhe longitudinal direction. The results further prove the uniformiameters of the nanowires. Fig. 3(c) shows a single silver nanowireith rough surface, which might be formed by a mechanism to

e described in a later section. The corresponding selected arealectron diffraction (SAED) pattern is shown in Fig. 3(d), in whichhree sets of crystalline planes could be indexed: {2 2 0}, {3 1 1}nd {2 2 2} respectively. Weak diffraction spots were found amonghe dominant spots, indicating that the nanowires probably con-ained some stacking faults during their formation (Long, Ustin, &o, 1999; Shen et al., 2009; Zhang, Feng, et al., 2004). HRTEM tech-ique was used to further identify the crystal structure, indicatinghat the domain crystalline plane was Ag{1 1 1} with lattice spacef 2.35 A (Fig. 3(e)). The growth direction of the silver nanowiresas clearly perpendicular to the {1 1 1} planes.

.3. Effect of experimental parameters

Since the synthesis rapidly occurred in a simple system at roomemperature, the morphology of the product was mainly affectedy two parameters: molar ratio of silver to vanadium oxide andoncentration of AgNO3. To obtain shape and size controlled silveranowires, the experimental parameters were optimized in thisork as demonstrated below.

.3.1. Molar ratio of silver to vanadium oxideThis molar ratio is one of the critical influence factors in the

hape and size controlled synthesis. The molar ratio was adjustedrom 0.05:1, through 0.3:1, 0.5:1, 5:1, 10:1, to 100:1, while otherarameters were kept constant ([V2O3] = 0.2 mM). The TEM images

n Fig. 4 show silver nanoparticles with different shapes andizes formed at different molar ratios. At low molar ratio (0.05:1,ig. 4(a)), instead of wires, spherical particles were formed on theurface of V2O3 (marked by arrows). Nanowires appeared at theatio of 0.3:1 (Fig. 4(b)), though the nanowires were so soft thathey could intertwine on the rod-like V2O3. The inset in Fig. 4(b)hows an enlarged TEM image of the V2O3 rod intertwined by Aganowires. With increasing molar ratio, relatively uniform silver

wth of silver nanowires induced by vanadium trioxide particles.

anowires with diameter of ∼20 nm could be collected through wide range, from 0.5:1 to 5:1 (Fig. 4(c)–(d)). When the rationcreased to 10:1, branch-like particles were formed (Fig. 4(e)).hrough careful observation, the formation of branch-like particles

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Fig. 2. (a) SEM image of as-prepared Ag nanowires; (b) XRD pattern of the Ag nanowires; (c) EDS spectrum of a single Ag nanowire.

Fig. 3. (a) A TEM image of the as-prepared Ag nanowires; (b) an enlarged TEM image of the red area in (a); (c) a TEM image of a single nanowire; (d) SAED pattern taken froman individual nanowire; (e) HRTEM image of the nanowires clearly showing the {1 1 1} lattice fringes with a spacing distance of ∼2.35 A. (For interpretation of the referencesto color in this figure legend, the reader is referred to the web version of this article.)

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ig. 4. TEM images of silver products obtained under different molar ratios of silverre 2 �m. (For interpretation of the references to color in the text, the reader is refe

as caused by the aggregation among the individual nanowires.ith the ratio further increasing to 100:1, the appearance of

iverse morphologies including thin film structure was observed,hich might result from the rapid nucleation and growth of theanowires (Fig. 4(f)) due to such high ratio. That is, the molar ratiof silver to vanadium oxide greatly affects the morphology and sizef the obtained silver nanoparticles. The suitable ratios of silver toanadium oxide for generation silver nanowires are 0.5:1–5:1.

.3.2. Concentration of AgNO3The concentration of reactant(s) can also affect the morphology

nd size of the Ag nanowires. The TEM images in Fig. 5 illus-rate that the shape and size of silver nanoparticles were heavilyffected by the concentration of silver nitrate. At low concentra-ion (0.2 mM of AgNO3), relatively uniform nanowires could bebtained (Fig. 5(a)). When concentration increased to 1 mM, the

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anowires were rougher and softer (Fig. 5(b)). Considering the for-ation mechanism of the nanowires (see the following section),

he rough and soft nanowires were probably caused by orientatedttachment of non-uniform silver nanoparticles generated at an

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nadium: (a) 0.05:1, (b) 0.3:1, (c) 0.5:1, (d) 5:1, (e) 10:1, and (f) 100:1. All scale barso the web version of this article.)

arly stage. High concentration of Ag+ could accelerate the rate ofucleation and growth of the initial Ag nanoparticles, and furtherake them non-uniform. Higher concentrations of 2–3 mM would

vidently not be suitable for generating 1D silver nanostructure.he particles were of radial morphology, as shown in Fig. 5(c) andd), especially in the inset SEM image of Fig. 5(d). The concentrationf silver nitrate, with its important role in the formation and growthf silver nanowires, is another crucial impact factor in shape andize control.

.4. Formation mechanism

The formation of the nanowires is fairly rapid. The UV–vis spec-ra shown in Fig. 6 could be used to prove the rapid formation and torack the growth process. The spectra show that there was a smallbsorption peak at ∼ 620 nm for V2O3 suspension, and another peak

wth of silver nanowires induced by vanadium trioxide particles.

t ∼300 nm for blank AgNO3 solution. After adding AgNO3 solutiono V2O3 suspension, the plasmon resonance immediately appearedround 350 nm, which was assigned to the Ag nanowires. Inarticular, the plasmon resonance band was nearly kept the same

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Fig. 5. TEM images of silver nanoparticles obtained under different concentrations of silvto vanadium was maintained as 5:1. The inset in (d) is a SEM image showing the radial pa

Fig. 6. Time-dependent UV–vis spectra for tracking the formation and growth ofAg nanowires, along with blank references of AgNO3 solution and V2O3 suspension(scanning interval step is ∼3 min).

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V3+/V4+ = −0.337 V and

t 350 nm (tested for ∼30 min), suggesting that the formation of theilver nanowires was completed within a short time. We proposehat the rapid formation is inextricably linked with the formation

echanism.The reaction of the formation of the nanowires is obviously

typical solid–liquid surface reducing reaction. Via our study, itas found that the formation process of the nanowires should

e divided into four parts: adsorption, reduction, desorption, and

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usion. E

er nitrate: (a) 0.2 mM, (b) 1 mM, (c) 2 mM, and (d) 3 mM. The molar ratio of silverrticles.

.4.1. AdsorptionThe Ag+ was reduced on the surface of V2O3 particles, due to

he high redox potential. To study the surface characteristics of2O3, the pH of the V2O3 suspension was measured before and afterdding Ag+. It was found that the pH of V2O3 suspension (4.30) wasower than that of DI-water (6.86), implying that the V2O3 particles

ight have combined with hydroxyl of water to result in releas-ng hydrogen ions from water. Thus, the surface of V2O3 particles

as negatively charged. When Ag+ was added into this system, theegatively charged V2O3 particles adsorbed positively charged Ag+

y electrostatic attraction. And then, Ag+ was reduced on the sur-ace. Electrostatic attraction evidently accelerates the adsorptionrocess.

To verify the surface property of the V2O3 particles and investi-ate the effects of pH on the formation of Ag nanowires, the pHf the V2O3 suspension was adjusted to 1, 7 and 10 by addingNO3 and NaOH solutions, and then Ag+ was added into the sus-ension. Unfortunately, no precipitate was found in the systemhen the pH of the suspension equaled 1, since the V2O3 particlesere dissolved in such an acidic system. Evidently the appear-

nce time of the precipitates was extended with the decrease ofH, indicating that a proper basic system favored the rapid for-ation of Ag nanowires. The corresponding HRTEM images of the

s-prepared Ag nanowires are displayed in Fig. 7, showing clearlyhe difference of the nanowires at pH = 4 (original pH) (Fig. 7(A)),

(Fig. 7(B)) and 10 (Fig. 7(D)). With increasing pH, some smallanoparticles appeared together with Ag nanowires. XRD patternshown in Fig. 7(D) confirmed the small particles to be Ag(VO3)H2O,hen pH was higher.

wth of silver nanowires induced by vanadium trioxide particles.

0V4+/V5+ = −0.991 V) to Ag+ (E0

Ag+/Ag0 = 0.799 V). The XPS result

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Fig. 7. TEM images of the nanowires prepared at different pH: (A) pH = 4 (original pH); (BpH: (a) pH = 10; (b) pH = 7 and (c) pH = 4 (original pH). � represents Ag, while � correspon

Fig. 8. XPS spectrum of vanadium oxides showing the redox reaction happening inthe formation of silver nanowires, in which the peak of V2p3/2 (516.2 eV) could bea

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cttw(wnooent amounts of CTAB lead to different formation process. For lowmolar ratio of CTAB (0.1 M), no significant precipitate appeared,

ssigned to V+4, while V2p3/2 (517.5 eV) to V5+.

Fig. 8) reveals that the product mainly contained pentavalentanadium accompanied by a small amount of tetravalent vanadiumndicating that most of the trivalent vanadium was transformedo pentavalent vanadium (Silversmit, Depla, Poelman, Marin, & De

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ryse, 2004). Such high redox potential provides huge energy dropo accelerate the reduction reaction.

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) pH = 7; (C) pH = 10; and (D) XRD patterns of the nanowires prepared at differentds to Ag(VO3)H2O.

.4.3. DesorptionThe desorption of Ag nanoparticles is probably due to the fol-

owing reason. The high reaction rate between Ag+ and V2O3 andhe low ionization potential of Ag+ (EAg = 7.6 eV) (Lide, 2003) mayasily make Ag particles grow larger (de Dios, Barroso, Tojo, Blanco,

López-Quintela, 2005), and then the neutral Ag particles with-ut electrostatic attraction moved away from the surface of V2O3articles and get free in water. That is also the reason why theg nanowires can separate from V2O3 particles, instead of formingg@VOx core–shell structures. However, some of the nanoparticlesay still attach onto the surface of V2O3 particles by van der Waals

orces. The nanowires formed on the base of such Ag nanoparticlesill wrap around V2O3 rods (Fig. 4(b)) or form a radial structure.

.4.4. FusionDue to the rapid reaction, it is hard to capture the growth pro-

ess of the Ag nanowires. Therefore, a growth inhibitor is requiredo terminate the rapid formation process of silver nanowires. Inhis study, several surfactants (CTAB and SDS) and polymer (PVP)ere tested as capping agents. 0.1 mL and 0.5 mL 0.1 M additives

molar ratios of additives to Ag+ were 2:1 and 10:1, respectively)ere added into the suspension, followed by adding Ag+. Due to theegatively charged surface of V2O3 particles (proved in the previ-us section), cationic surfactant cations ([CTA]+) may first absorbn the surface to impede the absorption of Ag+. Moreover, differ-

wth of silver nanowires induced by vanadium trioxide particles.

hough some precipitate could still be found after centrifugation.he corresponding TEM image is shown in Fig. 9(a), indicating that

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ig. 9. TEM images of the effects of various molar ratios of surfactants on the foDS:Ag+ = 5:1; (e) PVP:Ag+ = 2:1 and (f) PVP:Ag+ = 5:1.

o nanowires were formed other than rather spherical nanostruc-ures. In this system, cation with large molecular weight woulde preferentially absorbed on the surface of V2O3 particles, whichould prevent the absorption of Ag+. The white particles easily dis-ppeared when exposed to sunlight. Therefore, it is proposed thathe particles in Fig. 9(a) might be AgBr which probably came fromhe reaction between Br− and Ag+. For high molar ratio of CTAB0.1 M), white precipitate AgBr immediately appeared after addingg+ into the suspension due to high concentration of Br− (Fig. 9(b)).hat is, the CTAB is not suitable for shape control of silver nanowiresn our proposed system, though others used it for shape/size controlf silver nanoparticles (Wei et al., 2005).

For SDS, due to the anionic group in the molecule, the function oftopping or slowing down the reaction is not obvious, since the neg-tively charged molecules are difficult to absorb on the negativelyharged surface of V2O3 particles. Fig. 9(c) shows the Ag nanowiresormed when 0.1 mL 0.1 M SDS (SDS:Ag+ = 2) was added, which isimilar to Fig. 5(c), indicating that the low molar ratio of SDS hasittle effect on the formation of Ag nanowires. However, for high

olar ratio of SDS, SDS molecules can adsorb on the freshly formedg nanoparticles, due to their long molecular chains. Therefore, thehain-like structure can be observed from Fig. 9(c) and (d).

The effects of PVP are different from those of CTAB and SDS.urthermore, differences also appeared in high and low molaratios of PVP (Fig. 9(e) and (f)). At a low molar ratio of PVP, theanowires and nanospheres coexisted. With increasing molar ratio,nly nanospheres were observed, indicating that PVP had success-ully stopped the growth of silver nanowires. The neutral moleculeVP would adsorb on the surfaces of both V2O3 particles and theresh Ag nanoparticles, due to the influence of charge. The effectf stopping the growth of Ag nanowire was obvious for high molaratio of PVP. Therefore, high molar ratio of PVP (PVP:Ag+ = 5:1) wassed to stop the growth of Ag nanowires.

PVP molecules can be adsorbed not only on the surface of

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2O3 to slow down the reaction rate, but also on the surface ofhe freshly formed Ag nanoparticles to stop the further growth ofg nanowires. Fig. 10(a)–(d) shows the TEM images taken from

he samples prepared at different time when PVP (PVP:Ag+ = 5:1)

t(tt

n of Ag nanowires: (a) CTAB:Ag+ = 2:1; (b) CTAB:Ag+ = 5:1; (c) SDS:Ag+ = 2:1; (d)

as added into the reaction system. At the beginning of the reac-ion, spherical nanoparticles with size of ∼20 nm were formedFig. 10(a)). With increased time, the adsorption of PVP moleculesn the surface of the nanoparticles would somehow stop their fur-her growth (Fig. 10(b)). After a few seconds, the length of theanowires increased obviously, while the diameter was kept con-tant (Fig. 10(c)). Over ∼1 min, almost all nanowires were formedFig. 10(d)). Fig. 10(e) shows the UV-Vis spectra of the Ag nanopar-icles stopped by PVP during the formation of Ag nanowires, andig. 10(f) displays the change of position of maxim absorption withrowth time, gradually shifting from 470 to 350 nm within 90 s,ndicating that the shape of the Ag nanoparticles transformed fromphere to wires.

In fact, PVP has recently been used for shape and size controln some anisotropic growth of silver nanostructures. For exam-le, Sun and Xia (2002) demonstrated that PVP, as a coordinationeagent, can help to obtain uniform silver nanowires in a ethyl-ne glycol solution. They proposed that the PVP macromoleculesight coil around the nanowires as they grow along their longi-

udinal directions. It is generally accepted that this coordinationeagent kinetically controls the growth rates of various faces of aetal through selective adsorption and desorption on these planes.

similar situation can also be found in our previous study (Jiang,iong, Chen, et al., 2011; Jiang, Xiong, Tian, et al., 2011). Here, theole of PVP is different from the previous study due to the differentrowth mechanism of nanowires. In fact, in both systems, the PVPolecules will coil around the initial nanoparticles (the seeds in

olyol-thermal system), thus leading to the failure of fusion amongg nanospheres in our case, and kinetic growth in polyol-thermalystem. This role of PVP in our case also confirms to some degreehe growth mechanism of Ag nanowires.

Fig. 11 shows TEM images of a forming nanowire with thessistance of PVP. At the initial stage, some small silver nanopar-icles (10–20 nm in diameter) were formed (Fig. 11(a)), which

wth of silver nanowires induced by vanadium trioxide particles.

hen aggregated and fused together to form a chain-like nanowireFig. 11(b)). To further identify the chain-like structure, the HRTEMechnique was employed. Fig. 11(c) shows the joint portion ofwo nanoparticles, revealing that the chain was formed by a

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ig. 10. The effect of PVP on the growth of silver nanowires at different growth tanoparticles stopped by PVP at from 0 s (corresponding to Fig. 10(a)) to 90 s (corrend growth time.

ead-to-tail oriented growth through lattice match of the Ag{1 1 1}lanes. These nanowires were actually single-crystal rods whichere made of well-ordered parallel (1 1 1) planes. Such aggrega-

ion of nanoparticles may lower the surface energy in the whole

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ystem. In addition, the concavo-convex surface of the nanowireslso confirmed the head-to-tail oriented growth of the individualanoparticles (Zhu, Zhu, & Xiang, 2009). Jiang, Zeng, Chen, and Yu

uoF

ig. 11. (a) TEM image of tiny Ag nanoparticles showing the aggregation and formation of nith rough surface; and (c) HRTEM image of the joint section showing the lattice matche

(a) at the initial stage, (b) 20 s, (c) 40 s, and (d) 90 s; while (e) UV spectra of theing to Fig. 10(d)); (f) relationship between peak position of maximum absorption

2011) suggested that particle shape and the force among particlesre the two factors which substantially affect the self-assemblyf particles. In our case, the single-crystal rods were able to ful-l the condition of particle shape, though the driving force is yet

wth of silver nanowires induced by vanadium trioxide particles.

nknown. Normally, fusion happens between the planes whichwn high surface energy, thus lowering the whole system energy.or fcc metals, the {1 1 1} planes are most stable, which means

anowire; (b) a magnified TEM image showing the chain-like structure of nanowiresd {1 1 1} planes.

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he lowest surface energy, then followed by the {1 0 0} and {1 1 0}lanes. Here, we do believe that the fusion by Ag{1 1 1} planes plays

key role in the formation of silver nanowires, though the exactechanism calls for further investigation.Similar scenario of oriented attachment has appeared in other

ilver nanostructures such as nanorods and dendrites. Sun et al.2011) reported that Ag nanorods could be obtained in a mag-etic field, in which the formation of nanorods was driven bywinned defects. Wen et al. (2006) synthesized Ag nanodendritesith the aid of Zn microparticle suspension, demonstrating thatiffusion and oriented attachment of silver clusters along differentirections (e.g., 〈1 0 0〉 and 〈1 1 1〉) in alternation led to the forma-ion of Ag nanodendrites. However, both above methods hardlyed the particles to forming longer Ag nanowires. Formation byriented attachment can also be found in other nanostructures,.g., Au nanowires (Halder & Ravishankar, 2007), SnO2 nanowiresXu et al., 2008), CeO2 nanoflowers (Zhou et al., 2008), and MnO2anowires (Portehault et al., 2007), although different conditionsere employed.

.5. The role of V2O3 in the formation of Ag nanowires

In the proposed synthesis strategy, V2O3 particles obviouslylay important roles in the formation of Ag nanowires, as will beiscussed in detail in this section.

.5.1. As a reducing agentAs well-known, the reducing agents used to synthesize Ag

anowires, could be roughly classified into strong and weak. Forhe strong reducing agents (e.g., NaBH4, ascorbic acid), they lead toapid growth at low temperature but not to uniform size. There-ore, they are always used with capping agent (trisodium citrate,VP, or other surfactants) to slow down the growth rate, to fur-her terminate kinetic growth. For example, Jana et al. (2001) usedscorbic acid and NaBH4 as reducing agents, CTAB as micellar tem-late, and sodium citrate as stabilizer, to prepare Ag nanorods andanowires in water at room temperature. NaOH was also used todjust the pH of the system. By using this method, aspect ratio coulde controlled by the adjustment of pH. Another typical example ofreparation by a strong reducing agent was reported by Zhang, Qi,t al. (2004). In their study, Ag nanowires were prepared by a wethemical method in aqueous solution at room temperature, usingscorbic acid as a reducing agent, and PMAA as a shape controller.rom these examples, we may conclude that the preparation of Aganowires by a strong reducing agent is always carried out in com-licated systems in which capping agents need to be added to slowown the growth rate. In contrast, the low reaction rate driven by

weak reducing agent (e.g., PVP and ethylene glycol) can lead toniform growth, but accompanied by long reaction time and highemperature. Sun and Xia (2002) demonstrated that Ag nanowiresould be obtained by a glycol-thermal method under assistance ofVP at 160 ◦C for 60 min. In their study, ethylene glycol acted as

reducing agent and a solvent, while PVP acted as a coordinatinggent. In fact, some of the weak reducing agents also act as cappinggents such as trisodium citrate and PVP. For instance, Caswell et al.2003) synthesized Ag nanowires in trisodium citrate solution withroper pH. In addition, Washio, Xiong, Yin, and Xia (2006) reportedhat Ag nanoplates could be obtained via reduction of Ag+ by PVPt 60 ◦C for 21 h. In both cases, trisodium citrate and PVP actedespectively as reducing agent and capping agent. In a word, rapid

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eduction is not favored to obtain well shape-controlled nanopar-icles, and either strong reducing agents need addictives to slowown reaction rate or weak reducing agents need long reactionime and higher temperatures.

(ete

xx (2012) xxx–xxx

In our case, although V2O3 is actually a strong reducing agent tog+, both uniform morphology and rapid growth can be achievedimultaneously, which is attributed to the formation mechanismf the Ag nanowires. That is, the initial Ag nanorods were rapidlyormed by strong reducing agent V2O3, followed by fusion betweenheir {1 1 1} planes. In addition, the surface of V2O3 rods changedignificantly after reduction (Fig. S1).

.5.2. As a templateBased on the previous discussion, the formation of the small

g nanorods undergoes three steps: adsorption, reduction, andesorption. Therefore, the surface property of V2O3 particles needso be investigated. The pH of the V2O3 suspension would reflecthe surface property. It was found that the pH was lower than thatf Di-water. The pH (∼4.30) of V2O3 suspension before adding Ag+

olution reveals that some H+ ions may have been released fromater (see Eq. (1) below). The surface of V2O3 would be negativeue to the weak bonding with the hydroxyl group. The negativelyharged sites on the V2O3 particles would adsorb Ag+ ions, so aso enhance the surface redox reaction, which has positive influ-nce on nucleation and growth of silver nanocrystals. After Aganorods formed, the fusion process among {1 1 1} planes of vari-us Ag nanorods appeared, resulting in the formation of nanowiresy lowering the surface energy (Jiang, Xiong, Chen, et al., 2011;iang, Xiong, Tian, et al., 2011; Liu, Wehmschulte, Lian, & Burba,006; Lu, Kobayashi, Kikkawa, Tawa, & Ozaki, 2006; Zhang, Qi, et al.,004).

The redox reaction between V2O3 and Ag+ proceeds rapidly, ashown by the instantaneous decrease of pH from 4.30 to 3.83 afterhe addition of Ag+. The pH drop was probably caused by furthereleasing H+ during the redox process, as might be speculated by Eq.2) below, in which the relative species [VO3(H2O)]− was supposedrom the XRD pattern (Fig. 7(D)) and consistent with the referenceLivage, 1991). When OH− was added into the system, Eq. (1) would

ove to the right due to the reaction between H+ and OH−, whichould lead to the increase of (V2O3)x(OH−)y. Eq. (2) would thenove to the right as well, resulting in the increase of [VO3(H2O)]−.

he peaks of AgVO3(H2O) (Fig. 7(D)) became more obvious withH. That is also why the reaction rate increased with pH.

H2O + xV2O3 → (V2O3)x(OH−)y + yH+ (1)

5x − y)H2O + 6xAg+ + (V2O3)x(OH−)y → 4xAg0

+ 2xAgVO3(H2O) + (6x − y)H+ (2)

Apparently, the template (V2O3 particles) in our case is differentrom those used to assist the growth of nanowires. The templateselping growth of nanowires belong mainly to two types: “hard”nd “soft”. Soft templates mean that their structure is flexible andunable in solution, such as micelles, reverse micelles, and DNA

olecule networks. A good example is presented by Pileni ando-workers (Pileni, 2003; Maillard, Giorgio, & Pileni, 2003) whoemonstrated that Ag and Cu nanoparticles could be prepared viaeverse micelles (AOT/isooctane/water reverse micelles) as a “soft”emplate. They found that the Ag nanodisk size could be tunedetween 30 nm and 100 nm by changing the ratio of reducing agenthydrazine) to Ag+. Another example of such template was pre-ented by Braun, Eichen, Sivan, and Ben-Yoseph (1998), who usedNA as a “soft” template to prepare Ag nanowires, 12 mm long and00 nm wide. Different from the “soft” template, “hard” templates

wth of silver nanowires induced by vanadium trioxide particles.

mesoporous materials, alumina membranes, carbon nanotubestc.) can normally give uniform size, and are stable and can restricthe formation and growth of metallic nanoparticles under differ-nt conditions. Yu, Peng, Zhai, Li, and Wei (2008) demonstrated that

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B) TEM image of Ag nanowires prepared by glycol-thermal approach.

ong and continuous silver nanowires within multi-walled carbonanotubes (MWCNTs) were formed after irradiating the suspensionf MWCNTs-sliver nitrate solution in the presence of 2-propanalue to radiation-induced reduction of silver ions. The Ag nanowiresould be as long as 2 mm, with well-oriented structure. Anotherxample of the “hard” template is presented by Zong et al. (2004),ho reported that transparent Ag nanowires arrays embedded in

nodic alumina membrane could be prepared by a template-basedpproach combining ac electrodeposition and subsequent etchingf substrate. After the ac electrodeposition, the anodic aluminahannels were filled with Ag nanowires. However, in our case, theole of template for V2O3 is not associated with shape formation,ut nucleation. With V2O3 in solution, the rough surfaces of V2O3articles (Fig. S1) provide not only absorption sites but also nuclea-ion sites, which leads to heterogeneous nucleation that has beenroved to more easily obtain precipitate (Ag nanoparticles) than inomogeneous system. This is probably another reason for the rapid

ormation of Ag nanowires.

.5.3. As a catalyst or “microelectrode”Besides the two basic roles of V2O3 particles discussed above, we

ropose that V2O3 particles also play a role on the formation of theire-like structure, and on the high reaction rate (the reaction com-letes within a few minutes). Actually, the formation of nanowires

s a typical catalytic process for V2O3 (absorption, reaction, andesorption). In addition, Dupuis, Abu Haija, Richter, Kuhlenbeck,nd Freund (2003) reported that V2O3 (0 0 0 1) film can be pre-ared on Au{1 1 1} planes by evaporation of metallic vanadium inn oxygen atmosphere, followed by annealing at 700 K. Due to theimilar lattice structure of Au and Ag, we propose that the {0 0 0 1}urface of our V2O3 particles could also induce the generation ofg{1 1 1} to result in the initial rods composed of large area {1 1 1}lane with large surface energy. To reduce such surface energy, theusion between the Ag{1 1 1} planes occurred, as confirmed by theRTEM observation (Fig. 11(c)). Moreover, Michaelis, Henglein, andulvaney (1994) demonstrated that the silver could be immedi-

tely formed and deposited on Pt particle surface, but not when theore particles were absent. A similar situation may occur in our case.hat is, the V2O3 particles can also act as “microelectrodes”, whichmmediately reduce silver and make the Ag nanorods deposit on2O3 particles surface.

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Combining the two examples, we deduce that the quick reac-ion rate and the formation of the regular shape by simple redoxeaction without any surface modifier are associated with V2O3articles in this system. However, this role still needs further

otbs

ires obtained by glycol-thermal approach (b) (Jiang, Xiong, Tian, et al., 2011); and

onfirmation. Nonetheless, the V2O3 particles are of significantmportance and are first reported in the formation of silveranowires.

In addition, V2O3 is antiferromagnetic (Kosuge, Takada, & Kachi,963), with a Néel temperature of 168 K. For antiferromagneticaterials, when temperature is higher than Néel temperature, the

ntiferromagnet shows the same magnetic behavior as paramag-et. Without external magnetic field, paramagnet does not showny magnetism in macroscopic perspective. On the other hand, Ags diamagnetic when external magnetic field exists. In our case, noxternal magnetic field was used, and thus the magnetism of V2O3as little effect on the formation of Ag nanowires.

.6. Optical property of the Ag nanowires

The optical property of silver nanoparticles is structure-ependent. Fig. 12(A) shows the UV–vis absorption spectrum ofhe chain-like silver nanowires (curve (a)), where only one broadlasmon resonance peak centered at ∼353 nm was observed. Inomparison, the Ag nanowires generated via a high-temperaturelycol-thermal method (Jiang, Xiong, Tian, et al., 2011) displayedwo plasmon resonance peaks. An intense peak centered at383 nm (curve (b)) could be assigned to the transverse plasmon

esonance, while a shoulder located at ∼353 nm is the same as theeak position in curve (a). In both cases the longitudinal plasmonesonance was not significant in the UV–vis spectra, as is consis-ent with observations in previous studies (Caswell et al., 2003;un, Gates, Mayers, & Xia, 2002a).

Generally speaking, gold and silver nanorods and nanowiresave two principal plasmon bands. The band at short wavelengthsorresponds to absorption and scattering of light along the shortxis of the nanorod/nanowire (transverse plasmon band), and theand at long wavelengths corresponds to absorption and scatteringf light along the long axis of the nanorod (longitudinal plasmonand). The plasmon bands are tunable from the visible to the near-

R with the nanorod aspect ratio (Murphy, Sau, Gole, & Orendorff,005). However, the band usually disappeared for silver nanowiresith large aspect ratios (Caswell et al., 2003; Sun, Gates, et al., 2002;

un, Yin, Mayers, Herricks, & Xia, 2002). This result may be ratio-alized by considering that long silver nanowires usually do notonform to any uniform aspect ratio since they can easily bend

wth of silver nanowires induced by vanadium trioxide particles.

r twist in solution, thus significantly decreasing the intensity ofhe longitudinal plasmon band leading to the disappearance of theand. In some cases, transverse plasmon band may appear with ahoulder (the peak located at ∼350 nm in curve (b) of Fig. 12(A)).

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he shoulder should be assigned to bulk Ag (Sun, Gates, et al., 2002;un, Yin, et al., 2002).

The discrepancy of the UV spectra is probably due to the struc-ures of two types of nanowires. Fig. 12(B) displays the morphologyf the nanowires prepared by high-temperature method. Com-ared to the nanowires generated by our method, the nanowiresormed via oriented attachment show rough surface, imperfectrystallization (Fig. 3(c) and (e)), and most importantly “soft” struc-ure due to high aspect ratio; while the Ag nanowires generated byhe high-temperature glycol-thermal approach have smooth sur-ace, twinning structures, and “tough” structure (Jiang, Xiong, Tian,t al., 2011). The “soft” structure made the nanowires bend eas-ly and coil together, which might lead to similar optical property

ith bulk Ag (one broaden peak at 353 nm). For the Ag nanowiresenerated by the high-temperature method, the “tough” structurelways made the nanowires “straight”. Therefore, these nanowireshow typical UV-vis absorption spectrum of Ag nanowires (twoeaks at 353 and 383 nm). In addition, besides the bend structurehe aggregation of several nanowires along the longitudinal direc-ion (Fig. 3(b)) might also lead to varied “apparent sizes” furthereading to the broadened peak for curve (a) in Fig. 12(A).

. Conclusions

We have demonstrated a facile but effective method for the syn-hesis of Ag nanowires with uniform diameter and reproducibilitynder ambient conditions. By this method, Ag nanowires can beapidly prepared in aqueous solution at room temperature. Theormation of the Ag nanowires undergoes four steps: adsorption,eduction, desorption, and fusion, assisted by V2O3 particles whichre used for the first time in the synthesis of silver nanowires,isplaying multiple functions such as a reducing agent, a tem-late and a catalyst. The silver nanowires show optical propertiesifferent from those prepared by the glycol-thermal method, prob-bly due to the rough surfaces. The proposed method shows manydvantages, e.g., cost saving and rapid synthesis. However, we needo point that the removal of V2O3 particles from product seems

little difficult, and more work needs to be performed in theuture. In addition, without considering separation, we proposehat the product of VxOy–Ag nanocomposites can be obtained underppropriate control, and would be useful for gas sensors (Schlecht,use, Raible, Vossmeyer, & Burghard, 2004) or catalysts (Xue, Chen,hang, Auroux, & Shen, 2010).

cknowledgments

We gratefully acknowledge the financial support of the Australiaesearch Council (ARC) Discovery Projects. The authors acknowl-dge access to the UNSW and USyd node of the Australianicroscopy & Microanalysis Research Facility (AMMRF).

ppendix A. Supplementary data

Supplementary data associated with this article can beound, in the online version, at http://dx.doi.org/10.1016/j.partic.012.06.006.

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