INSTITUTE OF PHYSICS PUBLISHING NANOTECHNOLOGY

Nanotechnology 17 (2006) 4019–4024 doi:10.1088/0957-4484/17/16/004

Synthesis of silver nanoparticles using thepolyol process and the influence ofprecursor injectionDongjo Kim, Sunho Jeong and Jooho Moon

Department of Materials Science and Engineering, Yonsei University, 134 Shinchon-dongSeodaemun-gu, Seoul 120-749, Korea

E-mail: jmoon@yonsei.ac.kr

Received 20 May 2006, in final form 29 June 2006Published 14 July 2006Online at stacks.iop.org/Nano/17/4019

AbstractSpherical silver nanoparticles with various sizes and standard deviations weresynthesized by the polyol process. Two different synthesis methods werecompared in order to investigate the influence of reaction parameters on theresulting particle size and its distribution. In the precursor heating method,wherein a solution containing silver nitrate was heated to the reactiontemperature, the ramping rate was determined to be a critical parameteraffecting the particle size. In contrast, in the precursor injection method, inwhich a silver nitrate aqueous solution was injected into hot ethylene glycol,because of rapid nucleation, the injection rate and the reaction temperaturewere important factors in terms of reducing the particle size and attainingmonodispersity. Silver nanoparticles with a size of 17 ± 2 nm were obtainedat an injection rate of 2.5 ml s−1 and a reaction temperature of 100 ◦C.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Metal nanoparticles play important roles in many differentareas. For example, they can serve as a model system toexperimentally probe the effects of quantum confinement onelectronic, magnetic, and other related properties [1]. Theyhave also been widely exploited for use in photography,catalysis, biological labelling, photonics, optoelectronics,information storage, surface-enhanced Raman scattering(SERS), and the formulation of magnetic ferrofluids [2–5].The intrinsic properties of metal nanoparticles are mainlydetermined by their size, shape, composition, crystallinity, andstructure. In principle, any one of these parameters could becontrolled to fine-tune the properties of metal nanoparticles [6].

Among the various metal nanoparticles, silver nanoparti-cles have been widely investigated because they exhibit un-usual optical, electronic, and chemical properties, dependingon their size and shape, thus opening many possibilities withrespect to technological applications [7–9]. During the lasttwo decades, many synthesis methods have been reported forthe preparation of silver nanoparticles with tailor-made size,shape, and size controllability.

In general, silver nanoparticles can be produced byvarious methods including the chemical reduction of silverions with [10] or without [11] stabilizing agents, thermaldecomposition in organic solvents [12], and chemical andphotoreduction in reverse micelles [13–15]. Using thesemethods, silver nanoparticles with spherical, octahedral,tetrahedral, hexagonal, cubic, wire, coaxial cable, triangularprism, disc, triangular mark, belt, and shell shapes have beenmanufactured [16–22]. All these advances have promoted thescientific knowledge on the nature of nanomaterials.

The synthesis method used in the present study isthe so-called polyol method, which is well suited for thepreparation of nano-sized metal or oxide particles of variousshapes [23, 24]. In particular, the synthesis of sphericalsilver nanoparticles with high monodispersity from the polyolmethod has been reported in various studies [25–29]. Thegeneral polyol process involves the dissolution of a protectingagent or stabilizer in a polyol medium. The required silverprecursor is then added to this solution. Although thesynthesis process of monodisperse silver nanoparticles is wellestablished, little is known about the influence of precursorinjection during the polyol synthesis. In this paper, we have

0957-4484/06/164019+06$30.00 © 2006 IOP Publishing Ltd Printed in the UK 4019

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Figure 1. XRD diffraction patterns of the synthesized silvernanoparticles at a temperature of 120 ◦C and an injection rate of1 ml s−1 for 30 min by the injection method (0.25 mol l−1 AgNO3

and 0.027 mol l−1 PVP).

synthesized silver nanoparticles by either heating the precursorsolution to the reaction temperature or injecting an aqueoussilver salt solution into hot ethylene glycol. The influencesof the synthesis conditions including the reaction temperature,concentration of reactants, and injection rate were examinedfor both methods.

2. Experimental details

2.1. Synthesis of silver particles by heating the precursorsolution

Silver nitrate (99.9%, Aldrich), used as a precursor ofAg, was dissolved in ethylene glycol (EG, 99.9%, Aldrich)together with polyvinylpyrrolidone (PVP, Mw = 10 000,Aldrich), which protects the synthesized silver nanoparticlesfrom agglomeration. This solution was stirred vigorously in areactor fitted with a reflux condenser, followed by heating theprecursor solution at temperatures of 100–150 ◦C, at a constantheating rate of 1–7.5 ◦C min−1. The reaction was maintainedfor 30 min at each reaction temperature. After the reactionwas completed, the solution was cooled to room temperature,and the silver particles were separated from the liquid bycentrifugation and then repeatedly washed with ethanol. Theresulting particles were dried at room temperature.

2.2. Synthesis of silver particles by injecting precursorsolution

All the chemicals used in the precursor injection method areidentical to those described above, except for the injection ofsilver nitrate aqueous solution. First, PVP was completelydissolved in ethylene glycol and this solution was heated tothe reaction temperature. Deionized water containing 40 wt%silver nitrate was injected into a hot PVP–ethylene glycolsolution maintained at the reaction temperature. The particlerecovery procedures are also identical to those of for theprevious method.

1.5

1.0

0.5

0.0300 400 500 600 700

Wavelength (nm)

Abs

orba

nce

Time Increasing

0 min10 min

20 min30 min40 min

50 min

Figure 2. UV–vis spectra during the formation of silvernanoparticles at a temperature of 120 ◦C and an injection rate of1 ml s−1 with time intervals of 10 min after precursor injection.(0.25 mol l−1 AgNO3 and 0.027 mol l−1 PVP).

2.3. Apparatus

The injection of the precursor was carried out using asyringe pump (KDS200, Kd Scientific) at a constant rate.The synthesized silver nanoparticles were characterized byscanning electron microscopy (SEM, JEOL JSM-6500). Themean particle size of silver nanoparticles and the standarddeviation of particle population were determined from imageanalyses of SEM micrographs of each particle. Thex-ray diffraction (XRD) patterns of silver nanoparticles wererecorded using a D/max-Rint 2000 (Rigaku), and ultraviolet–visible (UV–vis) spectroscopy was conducted for the samplesextracted in the course of the reaction using a quartz cuvette(6030-UV, Hellma) by a UV–vis spectrophotometer (JASCOV-570).

3. Results and discussion

The x-ray diffraction patterns and UV–vis spectroscopy revealthat the synthesized silver particles have good crystallinity andhigh purity, despite the silver nanoparticles being prepared bythe reduction of a precursor at low temperature. The x-raydiffraction pattern of typical silver nanoparticles synthesizedby the precursor injection method under the conditions of a1 ml s−1 injection rate and 120 ◦C reaction temperature for30 min is presented in figure 1. The x-ray diffraction patternsof all synthesized silver particles in this study are almostidentical and reveal high crystallinity. The spectroscopicanalysis results of the synthesized silver nanoparticles, whosereaction conditions corresponded with those of the samplesused for the XRD analysis, are shown in figure 2 as a functionof time at intervals of 10 min. The silver particles weregenerated gradually, as is clearly indicated by the increasingintensity of the surface plasmon band at ∼420 nm in the UV–vis spectra. Furthermore, we observed no significant changein the absorption peak intensities within 10–20 min afterthe precursor was injected. This implies that the nucleationand growth reactions reach completion within such a timeperiod in which the specific reaction kinetics depend on

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Synthesis of silver nanoparticles using the polyol process and the influence of precursor injection

Table 1. Synthesis conditions for the preparation of silver particles using the method of heating the dissolved solution.

Heating Mean StandardAgNO3 PVP Temperature Time rate size deviation Figure

Sample (mol l−1) (mol l−1) (◦C) (min) (◦C min−1) (nm) (nm) ID

1 0.25 0.027 120 30 1 42 14 3(a)2 0.25 0.027 120 30 5 27 7 3(b)3 0.25 0.027 120 30 7.5 18 4 3(c)4 0.25 0.027 100 30 1 27 10 4(a)5 0.25 0.027 150 30 1 33 10 4(b)6 0.004 0.002 120 30 1 36 11 4(c)7 0.016 0.093 120 30 1 37 13 4(d)

the silver nitrate precursor concentrations. Based on theseobservations, we selected a reaction time of 30 min for furthersyntheses to assure complete reaction from silver ions to silvernanoparticles.

3.1. Synthesized silver particles by heating the precursordissolved solution

It is well known that the morphology and size distributionof metallic particles produced by the reduction of metallicsalts in solution depends on various reaction conditionssuch as temperature, time, concentration, molar ratio ofmetallic salt/reducing agent, mode and order of addition ofreagents, presence and type of protective agents, degree andtype of agitation, and whether nucleation is homogeneousor heterogeneous [27]. The synthesis conditions used forproducing the Ag particles by heating the precursor solutionare summarized in table 1. The mean particle size and thestandard deviation of the resulting Ag particles obtained at eachset of synthesis conditions are also presented, together withfigure number identifications for the corresponding particles.

The reaction scheme for producing fine and monodispersesilver particles using the polyol process involves the followingsuccessive reactions: reduction of the soluble silver nitrateby ethylene glycol, nucleation of metallic silver, and growthof individual nuclei in the presence of a protective agent,PVP. The fully reacted particle sizes synthesized from thepolyol process depended strongly on the ramping rate of theprecursor solution to the reaction temperature; at a lowerheating rate larger particles were generated, most likely due toa slower nucleation rate, while at a higher rate faster nucleationproduced smaller-sized particles. This influence on the Agparticles is shown in figure 3. At a heating rate of 1 ◦C min−1,the mean size of silver particles was 42 nm (figure 3(a) ofsample 1), and increasing the heating rate to 7.5 ◦C min−1

yielded smaller and more monodisperse particles with a meansize of 18 nm (figure 3(c) of sample 3). The particle sizedependence on the reaction temperature and the precursorconcentration of the reactant are also illustrated in figure 4.The particle size of the silver decreased slightly when thereaction temperature was decreased from 150 to 100 ◦C (fromfigures 4(a) to (b)). The particles prepared at concentrationsof 0.004 and 0.016 M exhibited almost identical sizes of∼36 nm (figures 4(c) and (d)). These findings indicate thatthe particle size dependence on temperature and concentrationis negligible; rather, the heating rate is a more critical factorin determining the particle size in the method of heating theprecursor solution.

Figure 3. SEM images of the synthesized silver nanoparticles by themethod of heating the dissolved solution at different heating rates of(a) 1 ◦C min−1, (b) 5 ◦C min−1 and (c) 7.5 ◦C min−1 (details intable 1).

3.2. Synthesized silver particles using precursor injectionmethod

In order to obtain monodisperse metal particles, generally,rapid nucleation in a short period of time is important; thatis, almost all ionic species have to be reduced rapidly tometallic species simultaneously, followed by conversion tostable nuclei so as to be grown [28]. In the method of heating aprecursor solution, however, both nucleation and growth can

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Table 2. Synthesis conditions for the preparation of silver particles using the injection method.

Injection StandardAgNO3 PVP Temperature Time rate Mean size deviation Figure

Sample (mol l−1) (mol l−1) (◦C) (min) (ml s−1) (nm) (nm) ID

8 0.25 0.027 100 30 1 19 3 5(a)9 0.25 0.027 120 30 1 36 8 5(b)

10 0.25 0.027 150 30 1 54 13 5(c)11 0.25 0.027 100 30 2.5 17 2 6(a)12 0.25 0.027 120 30 0.2 — — 6(b)

Figure 4. SEM images of the synthesized silver nanoparticles by the method of heating the dissolved solution at different temperatures of(a) 100 ◦C and (b) 150 ◦C and at different precursor concentrations of (c) 0.004 mol l−1 AgNO3, 0.002 mol l−1 PVP and(d) 0.016 mol l−1 AgNO3, 0.093 mol l−1 PVP (details in table 1).

proceed gradually with increasing temperature. As such, itis difficult to synthesize particles with high monodispersity.Therefore, the rapid injection of silver nitrate aqueous solutioninto ethylene glycol maintained at the reaction temperaturewould guarantee a short burst of nucleation after which thenuclei would continue to grow without additional nucleation,thus ensuring monodispersity.

Upon addition of the silver nitrate aqueous solution to hotethylene glycol, the Ag+ species are reduced to metallic silver.The concentration of metallic silver in solution increases,reaching the supersaturation conditions and finally the criticalconcentration to nucleate. Spontaneous nucleation then takesplace very rapidly and many nuclei are formed in a shorttime, lowering the silver concentration below the nucleationand supersaturation levels into the saturation concentrationregion. After a short period of nucleation, the nuclei grow bythe deposition of metallic silver until the system reaches thesaturation concentration. At the end of the growth period, allthe metal particles have grown at almost the same rate and thesystem exhibits a narrow particle size distribution.

Table 2 summarizes the synthesis conditions, each ofwhich yielded silver nanoparticles of different size andmorphological features, using the precursor injection method.

The size of the resulting silver particles was strongly dependentupon the reaction temperature and the injection rate. Theparticle size dependence on the reaction temperature, whichis defined as the temperature of ethylene glycol when theprecursor solution is injected, is exhibited in figure 5. Ondecreasing the reaction temperature from 150 to 100 ◦C, atan injection rate of 1 ml s−1, the mean size of the final silverparticles was reduced from 54 nm (figure 4(c) of sample 10) to19 nm (figure 4(a) of sample 8). This temperature dependenceon particle size can be explained as follows. Because ofthe relatively high temperature used in the synthesis of silverparticles, the Brownian motion and mobility of surface atomsincrease. This enhances the probability of particle collision,adhesion, and subsequent coalescence. However, PVP isadded to protect the particles from agglomeration. Particlecoalescence is the means by which the system tries to attainthermodynamic equilibrium by reducing its total surface area.

The injection rate also exhibits a significant influenceon the size and the morphology of the synthesized silvernanoparticles. The silver particles from the more rapidinjection rate of 2.5 ml s−1 are shown in figure 6(a) (sample11). A further increase in the injection rate yielded silverparticles with a slightly reduced size of 17 nm. However,

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Synthesis of silver nanoparticles using the polyol process and the influence of precursor injection

Figure 5. SEM images of the synthesized silver nanoparticles by theinjection method at different temperatures of (a) 100 ◦C, (b) 120 ◦Cand (c) 150 ◦C (details in table 2).

the silver particles obtained at an injection rate of 0.2 ml s−1

are polydisperse, with a size ranging from tens of nanometresto tens of micrometres. The slower injection rate results inlarger silver particles with a broader distribution in size andshape. It is believed that the slower injection rate conditioninduces a long period of nucleation. Upon precursor injection,a few nuclei form and stable nuclei grow over the time thatsilver nitrate is continuously injected into the ethylene glycol.Some of these injected silver ions are converted to nuclei andsome are used for growing pre-existing nuclei. Eventually,polydispese particles with a larger size are obtained. Basedon these observations, we found that careful control ofthe precursor injection rate and the reaction temperate isimportant in terms of obtaining silver nanoparticles with highermonodispersity in the polyol method.

3.3. Comparison between two methods

Figure 7 presents a schematic illustration of the silvernanoparticle synthesis methods of precursor heating andprecursor injection. The notable difference between the twomethods is the period of reduction of Ag+ species. Whenthe precursor dissolved solution is heated, the temperature of

Figure 6. SEM images of the synthesized silver nanoparticles atdifferent conditions: (a) a temperature of 100 ◦C and an injection rateof 2.5 ml s−1; (b) a temperature of 120 ◦C and an injection rate of0.2 ml s−1 (details in table 2).

the reactants increases gradually and the reduction of Ag+also proceeds slowly, followed by the concentration of Ag0

approaching a critical concentration for nucleation. Whennucleation occurs, some Ag0 species convert to nuclei andsome Ag+ reduce to Ag0 continuously; that is, the nucleationstep is maintained for a relatively long period of time. The longperiod of the nucleation step yields synthesized particles thatare larger-sized and polydispersive. For the injection method,in contrast, Ag+ species dissolved in water (or other solvents)reduce to Ag0 rapidly upon injection into the hot solution.As a result, the solution reaches a critical concentration fornucleation, and consequently the nucleation occurs in a shortperiod of time. Therefore, the injection of precursor is a criticalstep for synthesizing nanoparticles with a reduced size and anarrow size distribution.

4. Conclusions

Spherical silver nanoparticles with a controllable size and highmonodispersity were synthesized by the polyol method. Twodifferent synthesis methods for producing the Ag nanoparticleswere compared in terms of particle size and monodispersity.Silver nanoparticles with a size of 18 ± 4 nm were obtainedat a reaction temperature of 120 ◦C and a heating rate of7.5 ◦C min−1 in the precursor heating method, where theheating rate was a critical parameter affecting particle size.In the precursor injection method, on the other hand, theinjection rate and reaction temperature were important factorsfor producing uniform-sized Ag with a reduced size. Silvernanoparitlces with a size of 17 ± 2 nm were obtained atan injection rate of 2.5 ml s−1 and a reaction temperatureof 100 ◦C. The injection of the precursor solution into ahot solution is an effective means to induce rapid nucleation

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Figure 7. Schematic illustration of the two synthesis methods: heating the precursor dissolved solution and injection of the precursor.

in a short period of time, ensuring the fabrication ofsilver nanoparticles with a smaller size and a narrower sizedistribution.

Acknowledgment

This work was supported by the National Research Laboratory(NRL) Program of the Korea Science and EngineeringFoundation.

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