Colloids and Surfaces A: Physicochem. Eng. Aspects 254 (2005) 57–61

A simple method for preparation of silver dendrites

Xuelu Gaoa, Guohua Gua, Zhengshui Hua,∗, Yu Guoa, Xun Fua, Jinming Songb

a Qingdao University of Science and Technology, Qingdao 266042, PR Chinab Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, PR China

Received 18 July 2004; accepted 5 November 2004Available online 28 December 2004

Abstract

Single crystal silver dendrites have been synthesized by heterogeneous reduction of AgNO3 in toluene solutions of a triblock copolymer(EO)20–(PO)70–(EO)20. The triblock copolymer stabilized system exhibits high stability, which makes sample handling and analysis simple.By this method, the concentration of Ag in the colloidal system can be as high as 8.5 g/L. TEM, XRD, IR, and UV–vis measurements areused to characterize the resulting hybrid systems.©

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2004 Elsevier B.V. All rights reserved.

eywords:Colloid; Copolymer; Nanostructures; Crystal growth; Optical properties

. Introduction

Nanosized materials exhibit surprising and novel phe-omena linked to nanometer dimensions, which make themreatly differ from their bulk counterparts and becomeromising candidates for a wide variety of technological ap-lications in active catalysts[1–3], magnetic materials[4,5],onlinear optical materials[6–9], etc. So nanosized transitionetals have been the subject of much intensive research over

he past few decades, especially in recent years.It is well known that catalytic reactivity depends on the

ize and shape of the metal nanoparticles, and therefore theynthesis of well-controlled shapes and sizes of colloidal par-icles could be critical for their application. A conventionalethod to get relevant metal nanoparticles is reducing metal

alt in a suitable solvent, in which a suitable surfactant is usedo control the growth of the metal particles and to make theolloids stable.

Metal nanoparticles were usually prepared in water10–15]or other polar solvents such as alcohol[16,17]. Com-only used hydrophilic polymer surfactants involve polyvi-

and polyvinylpyrrolidone (PVP)[13,15,16]. Silver nanorodwere prepared by electro-reduction of AgNO3 in aqueous solution in the presence of PEG[14], and silver dendrites weprepared by the same technique in the presence of DNA[18].

Ultraviolet irradiation has been used for reduction tenique to synthesize single-crystal silver nanorods anddrites from AgNO3 solution, in which PVA was used asprotecting agent[19]. Experiments indicated that the cocentration of both AgNO3 and PVA has a significant effeon the formation and growth of these novel nanostructIn addition, ultrasonic also has been proved to be helpfthe process of silver dendritic nanostructure growth[20,21].

Fewer references reported their preparation in non-psolvents, which was known to be a more complex probVorobyova et al.[22] reported that silver colloidal dispesions in decane were synthesized by an interphaseaction between silver complex with quaternary ammonsalt and sodium borodydride, which were dissolved incane and water, accordingly. Ag+ ions were transferred froaqueous solution to decane usingN,N,N-tridecyl(3-aza-3decyltridecan)ammonium iodide as the phase transfer re

ylalcohol (PVA)[10–13], polyethylene glycol (PEG)[14],

∗ Corresponding author. Tel.: +86 532 4022787.E-mail address:zhangwjbj@163.com (Z. Hu).

and reduced with aqueous sodium borohydride. Brust et al.[23,24]prepared and characterized solutions of 1–3 nm goldparticles. In their experiments, AuCl4

− was transferred fromaqueous solution to toluene using tetraoctylammonium bro-

d.

927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserveoi:10.1016/j.colsurfa.2004.11.009

58 X. Gao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 254 (2005) 57–61

mide as the phase-transfer reagent and reduced with aque-ous sodium borohydride in the presence of dodecanethiol(C12H25SH). Alkanethiol-derived nanosize silver clusterswere synthesized by the similar way. The essence of theseprocedures is solvent extraction.

The past decade has seen considerable progress in the de-velopment of synthetic strategies to prepare block copoly-mers of various architecture, solubility, and functionality.Metal colloidal dispersions prepared in non-polar solventsin the presence of amphiphilic block copolymers (ABCs),such as polystyrene-b-poly(ethylene oxide) (PS-b-PEO)[25],polystyrene-b-poly(4-vinylpyridine) (PS-b-PVP) [26,27],have also been reported. Platonova et al.[27] prepared cobaltcolloids in toluene in the presence of PS-b-PVP. Rutnako-rnpituk et al.[28] prepared cobalt nanoparticle dispersionsin toluene in the presence of poly[dimethylsiloxane-b-(3-cy-anopropyl) methylsiloxane-bdimethylsiloxane] (PDMS–PC-PMS–PDMS) triblock copolymer by similar technique.These techniques are effective in the preparation of metalcolloidal dispersions, but most of ABCs are difficult to be syn-thesized and no industrial products are available, which holdback their application. Moreover, the in situ concentration ofmetal colloids is very low, e.g. in the order of 0.1–100 mg/L[26], making metal colloids too expensive for the industrialapplication.

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The excessive NaBH4 could be separated by centrifugation atlast.

Before their X-ray powder diffraction (XRD) and Fouriertransform infrared (FT-IR) measurements, samples were pu-rified by washing the resulting colloids in distilled water threetimes via sonication, followed by centrifugating and remov-ing of the water layer. For XRD measurement, toluene wasevaporated from the already purified colloids to get silverpowders.

A JEM-200EX (JEOL) transmission electron microscope(TEM) was used to observe the sizes and morphologies ofthe silver nanocrystallites. XRD pattern was recorded onRigaku D/Max-r A X-ray diffractometer using Cu K� ra-diation (40 kV, 100 mA). IR spectra were recorded on BrukerVector-22 FT-IR spectrophotometer.

UV–vis spectra were recorded between 300 and 700 nmusing an Agilent 6010 UV–vis spectrophotometer. Spectrawere obtained by measuring the freshly prepared silver col-loids in a quartz cell with a path length of 1 cm. The experi-mental data were corrected for the background absorption ofthe toluene solution of copolymer with the same concentra-tion of the silver colloids.

3. Results and discussion

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Triblock copolymer (EO)m–(PO)n–(EO)m is one of theew commercial ABCs, and has been used in many fi29–31]. In this paper, a simple method for the preparatioilver dendrites has been reported. The experiment is bn the reduction of AgNO3 in toluene in the presence

riblock copolymer (EO)20–(PO)70–(EO)20. By this methodhe concentration of Ag in the colloidal system can be ass 8.5 g/L. Transmission electron microscopy, powder Xiffraction, and FT-IR spectroscopic analysis have been

o characterize the silver nanocyrstallites. This methoelatively simple and no complicated equipment or speeagents are needed.

. Experimental

Triblock copolymer of poly(oxyethylene)–poly(oxyprylene)–poly(oxyethylene) [(EO)20–(PO)70–(EO)20] wasurchased from Aldrich, the overall molecular weight

he sample wasMw = 5800, the polydispersity index ww/Mn = 1.018. The other materials were used as receivDesired amounts of AgNO3 and triblock copolymer wer

ixed in toluene. The mixture was vigorously stirred foays to make AgNO3 thoroughly disperse into the solutiofter that, the AgNO3 was reduced with nubbly NaBH4.he solution color gradually changed towards black, iating the formation of silver dendrites. Typical reacime was 5 h. The temperature throughout the whole exments was controlled at 20◦C. It was remarkable that tholloidal product was so stable that the system coulde destroyed even if it was centrifuged at 3000 rpm for

The typical TEM images of the products obtained byucing the solution containing AgNO3 and copolymer arhown inFig. 1. It is apparent that the dendritic structuresormed by the growth of the nanocrystallites through speirections.

The experiments indicated that the concentrationgNO3 (CAgNO3) played a significant role in the formatind growth of the silver nanoparticles. If the concentratio

he copolymer was kept atCcopolymer= 1× 10−3 mol/L, thereere some free nanoparticles sized about 30−100 nm in di-meter left in the colloidal solution whenCAgNO3 was lower

han 1× 10−2 mol/L except the loose and tiny dendrites,ig. 1(a). Higher concentration of AgNO3 was obviously

avorable for the aggregation and growth into the dentructures of silver cluster. WhenCAgNO3 was higher tha× 10−2 mol/L, their TEM images (Fig. 1(b)–(d)) showed

hat the dendrites had already grown into network. It wasbserved in the experiments that the viscosity of the sy

ncreased with the increase of the concentration of AgN3,ndicating more dendrites were formed.

The concentration of copolymer also played anortant role in the formation of silver dendrites. Txperiments showed that whenCcopolymer decreased t× 10−4 mol/L, the concentration of dispersed AgNO3 couldot achieve 4× 10−3 mol/L during the same even longtirred time, though the mole ratio of copolymer to AgN3as the same as whenCcopolymer= 1× 10−3 mol/L andAgNO3 = 2× 10−2 mol/L. We found that better silver derites could be formed from 1× 10−3 mol/L copolymer

oluene solution when the mole ratio of copolymer to AgN3

X. Gao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 254 (2005) 57–61 59

Fig. 1. TEM images of the silver dendrites. The insert in (d) is corresponding electron diffraction pattern of silver dendrites.Ccopolymer= 1× 10−3 mol/L, and(a)CAgNO3 = 1× 10−2 mol/L; (b) CAgNO3 = 2× 10−2 mol/L; (c) CAgNO3 = 4× 10−2 mol/L; (d) CAgNO3 = 8× 10−2 mol/L.

was at the range of 1:20 to 1:80 as shown inFig. 1(b)–(d).When the mole ratio of copolymer to AgNO3 was lower than1:80, AgNO3 could not be dispersed into toluene completelyeven if the time was prolonged, which might indicate thatAgNO3 was saturated.

A typical XRD pattern of the as-prepared silver dendriticnanoparticles (seeFig. 2, where the concentrations of AgNO3and copolymer were 4× 10−2 and 1× 10−3 mol/L, respec-tively) showed that the presented diffraction peaks could beindexed as (1 1 1), (2 0 0), (2 2 0) planes of the face-centeredcubic (fcc) silver phase. The average original particle size ofthe sample obtained from the XRD pattern by the Sherrerformula was about 35 nm. The insert inFig. 1(d) showed atypical electron diffraction pattern indicating that these silverdendrites were single crystals.

UV–vis absorption spectra have been proved to be quitesensitive to the formation of silver colloids since silver par-ticles exhibit an intense absorption peak due to the surface

plasmon excitation.Fig. 3presents the absorption spectra ofnewly prepared silver nanoparticles. Spectra (a)–(d) in it cor-respond to compositions of (a)–(d) shown inFig. 1, respec-tively. With the decrease of silver concentration, all of thespectra indicate the tendency of continuous narrowing and

Fig. 2. The XRD pattern of silver dendrites.

60 X. Gao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 254 (2005) 57–61

Fig. 3. UV–vis spectra of the newly prepared silver colloids: the mole ra-tio of copolymer to AgNO3 was (a) 1:10, (b) 1:20, (c) 1:40, and (d) 1:80,respectively. The spectra were normalized and did not reflect the absoluteabsorption.

shift of absorption maximum towards shorter wavelengths.This can be attributed to a temporarily decreasing particlesize or, alternatively, to a decrease of intercollodial interac-tions[26]. Higher silver concentration may result in the ag-gregation of smaller particles to form bigger ones in certaindirections, which is of advantage to the growth of dendrites

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but makes the system thermodynamically unstable. It is wellknown that the size and shape of particles may influence theposition and the width of the plasmon resonance. The rela-tively broad band obtained for the low silver concentration isowing to the fact that its particle diameter has relatively largedispersion range[22], which can be observed in correspond-ing TEM image.

Fig. 4 is the FT-IR spectra of the bare triblock copolymer(a) and the nanoparticles (b) prepared in the present work.In the spectrum of the bare triblock copolymer, the mainbands can be assigned individually as follows. The band at2971 cm−1 is attributed to the asymmetric CH stretching vi-bration of CH3; 2869 cm−1 is assigned to symmetric CHstretching vibration of CH2; and the strong absorption at1109 cm−1 is attributed to the CO C stretching vibration.The IR spectrum of the particles is similar with that of thecopolymer, which indicates that there are no strong reactionsbetween particles and the copolymer. It probably can be in-ferred that the copolymer makes the colloidal particles stablethrough static electricity gravitation and/or steric hindranceeffect.

4. Conclusion

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ig. 4. Comparison of the FT-IR spectra of (a) the bare triblock copolymernd (b) nanoparticles prepared in the present work.

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Catal.

In this paper, a chemical reduction method wasessfully used for preparation of silver dendrites at 2◦Criblock copolymer (EO)20–(PO)70–(EO)20 as a protectingent. It was found that the concentrations of both AgN3nd copolymer played an important role in the formaf nanocrystallites. The advantages of the method are

he process is simple, the resulting system is stablehe concentration of silver in the resulting system canomparatively higher than that in silver colloids prepay other methods. These might provide more opportun

or feasible mass production of nanostructure silver denolloids.

cknowledgments

We are indebted to Mrs. Jingyi Yang for her endean recording TEM images for our products. We also th

r. Lei Chen and Haitao Zhu for their useful discussioany thanks also to the reviewers who made very us

omments and suggestions. Supported by the Natural Scoundation of Shangdong province (No. L2000B01).

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