Effects of Reaction Conditions on the Properties of SphericalSilver Powders Synthesized by Reduction of an OrganometallicCompound

YING-JUNG CHIANG,1 SEA-FUE WANG,1,3,4 CHUN-AN LU,2

and HONG-CHING LIN2

1.—Department of Materials and Mineral Resources Engineering, National Taipei University ofTechnology, 1, Sec. 3, Chung-Hsiao E. Rd., Taipei 106, Taiwan, ROC. 2.—Material and ChemicalResearch Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan, ROC.3.—e-mail: seafuewang@yahoo.com. 4.—e-mail: sfwang@ntut.edu.tw

Silver powders were synthesized by reducing a silver organometallic com-pound, silver 2-ethylhexanoate, with di-n-octylamine. The effects of prepara-tion conditions on the characteristics of the powders were investigated. Silverpowders prepared from silver 2-ethylhexanoate and di-n-octylamine in theratio 2:1 (MA21) at 150�C for 3 h had the best characteristics (average particlesize 277 nm, narrow particle-size distribution, high tap density of 4.0 g/cm3),and were also obtained in high yield (98%). Use of an excessive amount of di-n-octylamine resulted in intense thermolysis and a low yield of silver powders ofirregular morphology with a wide particle-size distribution. As the proportionof silver 2-ethylhexanoate was increased, the silver powders obtained had abimodal particle-size distribution and a relatively low tap density. Silver filmsseemed to have high resistivity when the temperature used for synthesis ofthe silver powders was too low or reaction time was insufficient. The electricalresistivities of silver films prepared from MA21 powders and sintered at 300�Cand 500�C for 30 min were 3.8 9 10�6 X cm and 2.3 9 10�6 X cm, respec-tively, close to that of bulk silver.

Key words: Silver, decomposition of organometallic compound, electricalconductivity

INTRODUCTION

Both silver and copper have good electrical conduc-tivity, but silver is more resistant to oxidization underambient conditions.1 Silver has better thermal andelectrical conductivity than other metals.2–4 Silverparticles, especially nanometer-scale,also have uniqueoptical5,6 and catalytic7,8 properties and antibacterialability.9 Silver metal used in common electrical andelectronic applications, including radio-frequencyidentification (RFID), multilayer ceramic capacitors(MLCCs), solar cells, printedcircuit boards, membrane(MB) circuit boards, and many thick film componentsincorporated in electronic devices, is fabricated from

conductive pastes and inks.10–18 Silver powder is acritical component of these pastes and inks in theproduction of electronic devices with good perfor-mance.

A variety of physical and chemical methods hasbeen used to prepare silver particles; examplesinclude milling,19,20 atomization, thermal decompo-sition,21,22 an electrochemical process,8 and wetchemical reduction processes.23–27 Many chemicalreduction methods are based on a large volume ofsolution containing water-soluble silver salt pre-cursors, a reducing agent, a protective agent, and adispersion agent. Several methods have recentlybeen developed for preparation of spherical ultra-fine sliver particles coated with organic cappingagents, for example alkanethiolates, carboxylates,and alkylamines.28–31 Kashiwagi and coworkers(Received December 31, 2013; accepted May 29, 2014)

Journal of ELECTRONIC MATERIALS

DOI: 10.1007/s11664-014-3270-7� 2014 TMS

investigated how thermolysis of long-chain silveralkyl carboxylates in the presence of differentalkylamines, without use of a solvent, could be usedto produce spherical silver nanoparticles.22,30,31 Thesize of the silver nanoparticles depended on alkylchain length and the structure of the silvercarboxylate.

In our previous study,32 uniform submicronspherical silver particles of high tap density weresuccessfully synthesized by use of a simple inex-pensive method—heating an alkyl carboxylate,silver 2-ethylhexanoate, and an alkylamine, di-n-octylamine, at 150�C for 3 h. On the basis of theresults from that study, optimization of the pre-cursors and reaction conditions were investigatedfurther in this study. The effects of reaction condi-tions (including reduction temperature, reactiontime, and ratio of silver 2-ethylhexanoate to di-n-octylamine) on the physical characteristics (tapdensity, particle morphology, particle size, andparticle-size distribution) and electrical perfor-mance of the synthesized silver powders wereinvestigated and are discussed here.

EXPERIMENTAL

The organometallic precursor, silver 2-ethylhex-anoate (C7H15COOAg), was prepared by the methodused in the previous study.32 All silver powderswere synthesized from silver 2-ethylhexanoate anddi-n-octylamine (C16H35N, 99%; Alfa Aesar, UK). Avariety of reaction conditions—mole ratios of silver2-ethylhexanoate to di-n-octylamine (2:3, 1:1, 4:3,2:1, 8:3, 4:1, and 6:1), reaction temperature(130–170�C), and reaction time (1–4 h)—wereinvestigated for preparation of the silver powders.Silver 2-ethylhexanoate and di-n-octylamine at dif-ferent molar ratios were mixed in a flask at 150�Cfor 3 h to produce submicron silver particles. Xylenewas then added to the suspension in the flask, andthe contents were stirred at 80�C for 1 h then cooledto room temperature. The silver particles were thenwashed clean with fresh xylene and dried by rotaryevaporation. The powders were classified on thebasis of the silver 2-ethylhexanoate-to-di-n-octyl-amine ratio as MA23, MA11, MA43, etc.

The yields of the silver powders obtained by use ofdifferent reaction conditions were determined onthe basis of the weight of reduced silver, aftercleaning with fresh xylene and drying by rotaryevaporation, compared with the nominal weight ofsilver in the silver 2-ethylhexanoate. All tap densi-ties of the silver powders were measured with a tapdensity tester (ETD-1020, Electrolab, India) at 250taps per min (USP 2), by use of American Society forTesting and Materials (ASTM) test methods. Vari-ations in morphology and particle size of the silverparticles were investigated by use of a field-emissionscanning electron microscope (SEM; S4500, Hitachi,Japan). The particle-size distribution was deter-mined from scanning electron micrographs in

accordance with ASTM Standard E 20, in which thesize of particles were determined from the length ofthe line bisecting the area of the particle image, andall particles were measured in the same direction.Thermogravimetric analysis (TGA) of the silverparticles was performed with a thermogravimetricanalyzer (Perkin–Elmer, USA), in air, at a heatingrate of 10�/min, to determine organic residue con-tent. To minimize experimental errors, silver pow-der sample size was always >100 mg.

The electrical performance of the silver powderswas characterized by use of thick-film spiral silverpatterns. Thick-film pastes consisting of the sub-micron silver powders, ethyl cellulose, and ethyleneglycol were mixed, deagglomerated by use of a high-speed mixer (THINKY, USA), then passed througha triple-roller grinder (EXAKT, Germany) to facili-tate breakdown of pigment agglomerates. To ensureprintability of the paste, the solid content of thesilver pastes was controlled at 80 wt.%, on the basisof results reported elsewhere.32 To confirm the dis-persity of the silver particles in the organic vehicleafter passage through the triple-roller grinder, thesilver paste was diluted with xylene and the distri-bution of silver particle size was characterized byuse of a dynamic light-scattering particle size ana-lyzer (LB-550, HORIBA, Japan).

The silver pastes were screen-printed on to glasssubstrates as spiral patterns and then thermallytreated at different temperatures (300, 400, or500�C) for 30 min in air. The thermal treatment wasperformed in a tunnel oven with three heatingzones, in which the samples were heated to thereaction temperatures in 5 min, kept at these tem-peratures for 30 min, and restored to room tem-perature in another 5 min. The dimensions of thescreen-printed patterns were: 100 cm spiral length,1.0 mm width, and 8–15 lm thickness. A Keithley2400 multimeter with a four-point probe was usedto measure the bulk resistance of the fired silverpatterns. The resistivity of the silver films was cal-culated by use of the equation: q = (RÆwÆt)/l, in whichR is the resistance of the spiral circuits, and w, t,and l the real line width, thickness, and length,respectively.

RESULTS AND DISCUSSION

Effects of Synthesis Temperature

Figure 1 shows the morphology of the silverpowders prepared from silver 2-ethylhexanoate anddi-n-octylamine at a mole ratio of 2:1 and reacted attemperatures from 130�C to 170�C for 3 h in air.

Characteristics of the silver powders, includingaverage particle size, d90/d10 ratio obtained fromSEM, tap density, and batch yield, are listed inTable I. As the temperature of synthesis wasincreased from 130�C to 170�C the particle size ofthe powders increased slightly, from 243 nm to295 nm, and powder morphology, based on SEMobservation, became smoother. The d90/d10, ratio

Chiang, Wang, Lu, and Lin

determined from the micrographs, revealed that theparticle-size distribution of the powders correlatedstrongly with the temperature of synthesis. Pow-ders prepared at 130�C had a wide particle-sizedistribution (large d90/d10 ratio) that became nar-rower as the temperature of synthesis wasincreased. The narrowest particle-size distributionwas observed for powders synthesized at 150�C. Asthe temperature of synthesis was increased to 160�Cand 170�C, differences between particle-size distri-bution became trivial; the distributions, however,seemed much wider than those of the powders pre-pared at 150�C.

As found in a previous study,32 organic residuesconsisting mostly of silver 2-ethylhexanoate and asmall amount of di-n-octylamine were attached tothe silver particles. Organic residues adsorbed bythe powder surfaces were less than 1.2 wt.%,depending on the conditions used for synthesis.Figure 2 shows the TGA curves of MA21 silverpowders prepared at different temperatures for 3 h.The curves contained two major regions of weightloss, those below and above approximately 200�C.The primary weight loss, occurring in the temper-ature range 150–200�C, was caused by decomposi-tion of residual silver 2-ethylhexanoate, which wassignificantly accelerated by the presence of di-n-octylamine. Minor weight loss occurred in the200–350�C range, because of thermal decompositionof di-n-octylamine. Total weight loss was 0.8 wt.%and 0.3 wt.% for MA21 powders synthesized at130�C and at 140�C and higher, respectively.

Tap density, an indicator of the packing ability ofsilver powder, is crucial to thick-film application,because it significantly affects the electrical con-ductivity and thermal conductivity of the fired films.As shown in Table I, the silver powders had hightap densities ranging from 3.3 g/cm3 to 4.0 g/cm3,depending on the temperature used for synthesis.The tap density of the MA21 silver powdersincreased with increasing temperature of synthesis

from 3.3 g/cm3 at 130�C to a maximum of 4.0 g/cm3

at 150�C and 160�C, then decreased to 3.5 g/cm3 at170�C. In addition to tap density, yield, i.e. per-centage recovery of silver from the precursors, isalso important, because it affects manufacturingcost. As listed in Table I, yields of the MA21 pow-ders prepared at 130�C and 140�C were 36% and53%, respectively. As the temperature of synthesiswas increased to >150�C, the yield of MA21 pow-ders increased to >96%. Overall, the MA21 silverpowders prepared at 150�C had the best powdercharacteristics, and were obtained in high yield(98%). As indicated by the results, synthesis of thesilver powders must be precisely controlled within anarrow temperature range to guarantee a highyield.

Effects of Reaction Time

MA21 powders synthesized at a reaction temper-ature of 150�C for different times were examined tostudy the effects of reaction time on the character-istics of the silver powders. The relatively modestyield and tap density (51% and 3.3 g/cm3) of pow-ders synthesized at 150�C for 1 h testified to theinsufficiency of the 1-h reaction time for reduction ofsilver 2-ethylhexanoate by di-n-octylamine at thistemperature. When the reaction time was increasedto 2 h, the yield of MA21 silver powder increased to95% and its tap density reached 4.0 g/cm3, the samevalues as for those reacted for 3 or 4 h.

Figure 3 shows scanning electron micrographs ofMA21 silver powders prepared at 150�C for differ-ent times. The average particle size of the powders,determined from the SEM, increased slightly from253 nm to 279 nm as reaction time was increasedfrom 1 h to 4 h. After reaction for 1 h, powders wereobserved to contain a few granular and plate-likeparticles. As reaction time was increased, the MA21powders became nearly spherical with smooth sur-faces. When synthesis was performed at 150�C for

Fig. 1. SEM of silver powders prepared from silver 2-ethylhexanoate and di-n-octylamine at a ratio of 2:1 with reaction at 130�C (a) and(e), 140�C (b) and (f), 150�C (c) and (g), and 170�C (d) and (h) for 3 h.

Effects of Reaction Conditions on the Properties of Spherical Silver Powders Synthesizedby Reduction of an Organometallic Compound

3 h and 4 h, uniform, highly crystalline powderswere usually obtained. The corresponding d90/d10

ratios, determined from the micrographs, are listedin Table I to indicate variations in particle-sizedistributions. It is evident that powders reacted for3 h had the narrowest particle-size distribution.

Also listed in Table I are the weight lossesdetermined from the TGA curves of the MA21 silverpowders prepared at 150�C for different durationsafter heating to 400�C. The total weight loss ofpowders synthesized at 150�C for 1 h was approxi-mately 0.7 wt.%. The weight loss decreased withreaction time until 3 h was reached. For powdersprepared at 150�C for 3 h and 4 h the weight losswas similar, 0.3 wt.%. It is apparent the silverpowders contain more organic residues on the par-ticle surfaces if produced with insufficient reactiontime. A minimum reaction time of 3 h is needed tominimize adsorption of organic residues and furnishuniform and near spherical powders.

Effects of the Molar Ratio of Silver2-Ethylhexanoate to Di-n-octylamine

To understand the effects of the precursors, silver2-ethylhexanoate and di-n-octylamine in molarratios ranging from 2:3 to 6:1 were used to synthe-size the silver powders. All experiments were con-ducted using the optimum synthesis temperature(150�C) and reaction time (3 h), determined earlier.The ratios of silver 2-ethylhexanoate to di-n-octyl-amine used in this study are listed in Table I. Forthe precursor mixture with the largest di-n-octyl-amine content (silver 2-ethylhexanoate-to-di-n-octylamine ratio 2:3), the yield of silver powder(MA23) was only 38%, i.e. only slightly more thanone third of the silver in the precursors was recov-ered. As the silver 2-ethylhexanoate content of theprecursor mixture was increased (greater silver

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Fig. 2. TGA curves of the silver powders prepared from silver2-ethylhexanoate and di-n-octylamine at a ratio of 2:1 at differenttemperatures for 3 h.

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Chiang, Wang, Lu, and Lin

2-ethylhexanoate-to-di-n-octylamine ratio), the yieldof silver powder increased. The yield was increasedsubstantially, to 98%, when the silver 2-ethylhex-anoate-to-di-n-octylamine ratio was 2:1 (MA21).When the silver 2-ethylhexanoate content of theprecursor mixtures was increased further, the yieldfluctuated slightly but stayed in the range 95–98%.

The corresponding d90/d10 ratios derived fromSEM of powders prepared from different ratios ofsilver 2-ethylhexanoate to di-n-octylamine at 150�Cfor 3 h are shown in Table I. The characteristics ofthe silver powder seemed to be affected by the silver2-ethylhexanoate and di-n-octylamine content of theprecursor mixture during the synthesis. When thesilver 2-ethylhexanoate-to-di-n-octylamine ratiowas 2:3 (MA23), the reduction reaction furnishedparticles with irregular morphology, and granular,polyhedral, and plate-like shapes were observed.

The average particle size of the MA23 powders was244 nm, with a wide particle-size distribution (larged90/d10 value). It is evident that adding an excessiveamount of the reducing agent (di-n-octylamine)induced an intense thermolysis reaction and createda thermally unstable environment which, in turn,generated silver powders with irregular morphologyand wide particle-size distribution in low yield. Asthe di-n-octylamine content of the precursorsdecreased to the MA11 and MA43 levels, the silverpowders obtained had uniform particles with agranular morphology, and the average particle sizerose slightly to 280 nm and 329 nm, respectively.The MA43 silver powders had the largest particlesize obtained in this study. When the silver 2-eth-ylhexanoate-to-di-n-octylamine ratio was 2:1(MA21), the silver powders had a narrow particle-size distribution, and granular particles with anaverage size of 277 nm were obtained. As the silver2-ethylhexanoate content of the precursors wasincreased, the silver powders generated, includingMA83, MA41, and MA61, had a bimodal particle-size distribution with larger particles ranging from300 nm to 550 nm and smaller particles rangingfrom 50 nm to 150 nm. The number of smaller silverparticles increased with silver 2-ethylhexanoatecontent, reducing the average particle sizes of theMA83, MA41, and MA61 powders to 234 nm,209 nm, and 194 nm, respectively. Although parti-cle size and d90/d10 value varied with silver 2-eth-ylhexanoate-to-di-n-octylamine ratio from 2:1 to 6:1in the precursors, silver powders with nearlyspherical morphology (MA21, MA83, MA41, andMA61) were still obtained. Figure 4 shows theaverage particle sizes, d25 and d75, determined fromSEM, after use of different molar ratios of silver2-ethylhexanoate to di-n-octylamine. It is apparentthat average particle size reached a peak of 329 nm,for MA43 silver powders synthesized at 150�C for3 h in air. As the precursor content deviated fromthat used to prepare MA43, the resulting average

Fig. 3. SEM of silver powders prepared from silver 2-ethylhexanoate and di-n-octylamine at a ratio of 2:1 and reacted at 150�C for 1 h (a) and(e), 2 h (b) and (f), 3 h (c) and (g), and 4 h, (d) and (h).

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Fig. 4. Average particle size, d25, and d75 for silver powders pre-pared from different molar ratios of silver 2-ethylhexanoate and di-n-octylamine at 150�C for 3 h.

Effects of Reaction Conditions on the Properties of Spherical Silver Powders Synthesizedby Reduction of an Organometallic Compound

silver particle size decreased dramatically. Figure 4also indicates that the particle-size distributioncontinued to broaden as the mole ratio of silver2-ethylhexanoate to di-n-octylamine went beyond 2.

Tap density tends to be affected by such powdercharacteristics as morphology, particle size, andparticle-size distribution. MA21 silver powders withuniform and nearly spherical particles had thehighest tap density of 4.0 g/cm3. As the precursorswere formulated to increase the weight of di-n-octylamine as reducing agent, the tap densities ofthe MA43, MA11, and MA23 powders decreased to3.6, 3.6, and 3.3 g/cm3, respectively, because of thepresence of particles with irregular morphology.Increasing the silver 2-ethylhexanoate content ofthe precursors reduced the tap densities of thepowders, probably because of agglomeration of thesmaller silver particles in powders with a bimodalparticle-size distribution. Accordingly, the tap den-sities of powders MA83, MA41, and MA61 were 3.8,3.6, and 3.5 g/cm3, respectively.

The TGA results listed in Table I show totalweight loss was 0.2–0.7% for silver powders pre-pared from silver 2-ethylhexanoate and di-n-octyl-amine in different molar ratios at 150�C for 3 hwhen heated to 400�C. Total weight loss was greaterfor powders MA41 and MA61 because of the pre-sence of small particles with larger surface areawhich absorb more organic matter. Overall, it canbe concluded that, in terms of physical powdercharacteristics, the desired granular silver particlesof uniform size distribution can be synthesized byuse of silver 2-ethylhexanoate and di-n-octylaminein 2:1 molar ratio at 150�C for 3 h.

Electrical Characterization

To evaluate the electrical performance of the sub-micron silver powders prepared in this study forthick film applications, conductive pastes with80 wt.% solid loading, consisting of selected silverpowders, ethyl cellulose, and ethylene glycol, wereprepared. The dispersity of the submicron silverpowders in the pastes was estimated on the basis ofthe silver particle size value obtained by conductinglight-scattering measurement on methanol-dilutedpastes. As shown in Table I, the average sizes of thesilver particles in the pastes, ranging from 244 nmto 419 nm, are obviously larger than those deter-mined from SEM of the as-synthesized powders;this might be because of slight agglomeration ofsilver particles during paste preparation. However,the extent of agglomeration was still far fromcausing blockage of screen and stayed well withinthe limits for thick film applications.

All silver pastes were screen-printed on to glasssubstrates to form spiral patterns and were subse-quently sintered at 300�C, 400�C, or 500�C for30 min in air. Figure 5 shows the resistivities of thesilver films prepared from the MA21 silver powderssynthesized at different temperatures and with

different reaction times. After sintering at 300�C for30 min, all the silver films had electrical resistivitylower than 1 9 10�5 X cm. The resistivity of silverfilms prepared from the MA21 powders synthesizedat 130�C for 3 h and 150�C for 1 h were larger thanthose of other silver films, because of the low tapdensity of the silver powders used. Even when thesintering temperature was increased to 500�C,resistivity was still high compared with that of filmsprepared from the other powders. The results implythe films tended to have high resistivity if the silverpowders used were synthesized at a temperaturewhich was too low and reaction time was insuffi-cient. In this study, the lowest electrical resistivityof approximately 3.8 9 10�6 X cm was observed forfilms prepared from MA21 silver powders synthe-sized at 150�C for 3 h and 170�C for 3 h.

The measured resistivities of the silver filmsprepared from the silver powders synthesized fromsilver 2-ethylhexanoate and di-n-octylamine at dif-ferent molar ratios are listed in Table I. The highestresistivity was observed for silver film preparedfrom MA23 powders with the lowest tap density,synthesized from precursor containing the highestdi-n-octylamine content. The silver films preparedfrom the MA41 and MA61 powders had good con-ductivity, even though the tap density was not asgood as for MA21 powders. This might be becausethe agglomerates of the MA41 and MA61 powders,caused by adsorption of organic residues, were bro-ken down by the triple-roller grinder before screenprinting. The resistivities of the MA21, MA41, andMA61 silver films after sintering at 300�C for30 min were less than 3.8 9 10�6 X cm. When sin-tering was performed at 500�C for 30 min, theresistivities of these silver films dropped to as low as2.1 9 10�6 X cm, approximately 1.3 times that forbulk silver.

300 400 5000.0

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Res

istiv

ity (

ohm

-cm

)

Temperature (oC)

130oC3h 150oC1h 150oC2h 150oC3h 170oC3h

Fig. 5. Resistivities of silver films prepared from the silver powderssynthesized at different temperatures and sintered at 300�C, 400�C,and 500�C for 30 min.

Chiang, Wang, Lu, and Lin

CONCLUSION

The physical characteristics and electrical perfor-mance of silver powders were highly dependent onthe temperature used for reduction, reaction time,and ratio of silver 2-ethylhexanoate to di-n-octyl-amine in the precursor. MA21 silver powders pre-pared at 150�C for 3 h had an average particle size of277 nm, the narrowest particle-size distribution, theoptimum tap density of 4.0 g/cm3, and approximately0.3 wt.% organic residue adsorbed on their surfaces.They were also obtained in high yield (98%). Silverfilms tended to have high resistivity if the silverpowders used were synthesized at a temperaturewhich was too low or if reaction time was insufficient.Silver films prepared from MA21, MA41, and MA61powders synthesized at 150�C for 3 h had electricalresistivity less than 3.8 9 10�6 X cm after sinteringat 300�C; after sintering at 500�C, electrical resis-tivity dropped to approximately 1.3 times that ofbulk silver (1.59 9 10�6 X cm).

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Effects of Reaction Conditions on the Properties of Spherical Silver Powders Synthesizedby Reduction of an Organometallic Compound