vesar

a, S

sity,

y, Do

ised f

silicout

ctrolup to 17.5 nm, due to an enhanced particle surface electric charge, thereby inhibiting particle growth. Meanwhile, a further increase

production of advanced ceramics. According to theBogush and Zukoskis model, which describes the par-ticle growth of silica in sol-precipitation, stable silica

with the surfactant, which provides the driving force forparticle aggregation [5,6]. The use of an anionic surfac-tant enables us to produce nano-sized particles of silica

Ceramics International

www.elsevier.com/locate/ceramintthe TEOS concentration, amount of seed particles, andadmixture concentration.particles are formed and grown via the aggregation ofprimary particles that are,nucleated in a supersaturatedsolution [13]. Therefore, the particle size and distri-bution change signicantly according to the solvent,solution pH, and reactant and catalyst concentrations,all of which aect the nucleation and aggregation of theprimary particles. For example, with uniform seed par-ticles of silica in the sol-precipitation [4], a multi-modalparticle size distribution can be obtained by modifyingthe particle nucleation and growth processes via varying

and its mechanism is also suggested based on primaryparticle aggregation [7].The potential of electrolyte additives, which are

highly eective in controlling particle growth, has notyet received signicant attention in the synthesis ofnano-sized particles via sol-precipitation. Accordingly,the current study investigates the potential applicationof mono-valent electrolyte additives for nano-particlesynthesis in sol-precipitation at a high concentration ofTEOS in relation to the practical application of theparticle size, while cesium chloride had the least.# 2003 Elsevier Ltd and Techna S.r.l. All rights reserved.

Keywords: Nano particles; Electrolyte admixtures; Zetapotential; Sol-precipitation

1. Introduction

Controlling the particle size and distribution in sol-precipitation continues to be a major concern in the

Surfactants that form micell in the solution are oftenapplied to the sol-precipitation process to control thesize and uniformity of silica particles [57]. A uniformparticle size originates from the molecular interactionparticle charge, thereby promoting particle growth. Among thein the additive concentration (above the optimal concentration) signicantly increased the particle size, due to neutralization of the

present additives, sodium iodine had the most eect on reducing theEect of electrolyte additisilica p

Sung-Soo Kima, Hyun-Seok KimaDepartment of Chemical Engineering, KRRC, Kyunghee Univer

bDepartment of Chemical Engineering, Joongang Universit

Received 3 December 2002; received in rev

Abstract

The eect of mono-valent electrolyte additives on nanosizedinvestigated. Without additives, the particle size varied from ab

ammonia concentrations. The addition of a small amount of eleon sol-precipitated nanoticles

un Geon Kimb, Woo-Sik Kima,*

Yongin Kiheung Seochun 1, Kyungki-Do, 449-701, South Korea

ngjak-ku Heuksuk-dong 221, Seoul, 156-756, South Korea

orm 5 March 2003; accepted 9 April 2003

a particles, synthesized via the sol-precipitation method, was35 nm to hundreds of nm depending on the TEOS, water, and

ytes (below the optimal concentration) reduced the particle size30 (2004) 171175Initially, a 350 ml mixture of de-ionized water,methanol (Junsei, ACS grade), and ammonia (Showa,

0272-8842/03/$30.00 # 2003 Elsevier Ltd and Techna S.r.l. All rights reserved.doi:10.1016/S0272-8842(03)00085-3

* Corresponding author. Tel.: +82-31-201-2576; fax: +82-31-202-

1946.

E-mail address: wskim@khu.ac.kr (W.S. Kim).sol-precipitation method.

2. Experimental

Japan), and zeta potentiometry.

particles in the product suspension were highly mono-

eects of the surface conductivity and double layerand 0.448 mol/l of ammonia.

nternFig. 1. Typical TEM image of nano-particles of silicon oxide synthe-

sized by sol-precipitation at 0.224 mol/l of TEOS, 4.48 mol/l of water,3. Results and discussion

3.1. Eect of reaction conditions

Based on the particle formation mechanism in the sol-precipitation process [2,3], a huge number of primaryparticles is rst nucleated in the initial high super-saturated solution. Then, the primary particles arerapidly aggregated to form stable particles, which growwith the further aggregation of primary particles. Afterthis particle induction period, any further primary par-ticles generated under supersaturation are consumed forthe growth of stable particles. As such, the resultingdispersed in size and spherical in shape, as shown inFig. 1.However, if the generation of primary particles by

supersaturation exceeds the consumption of primaryparticles for the growth of stable particles during sol-precipitation, new stable particles are spontaneouslyformed by the self-aggregation of the extra primaryparticles, resulting in a multi-modal distribution of par-ticle sizes in the product suspension [8]. Yet, in the pre-sent study, the mono-dispersity of the particle size wasmaintained even over a wide range of TEOS, H2O, andNH3 concentrations, implying that the population ofstable particles formed during the induction period wassucient to consume the primary particles nucleatedafter the induction period. Furthermore, the initialsupersaturation level (supersaturation level duringinduction period) was critical in determining the particlesize of the product suspension in the sol-precipitation,as the higher particle formation with a higher initialsupersaturation resulted in a smaller particle size in theproduct suspension. Therefore, as shown in Fig. 2, theincreased particle size with increased water, TEOS andNH3 concentrations may have been due to the initialnucleation at a lower supersaturation level resultingfrom the promotion of the hydrolysis and condensationreactions, as demonstrated in a previous study [911]. Itis interesting to note that the particle size was almostindependent of the H2O concentration over a widerange of values, due to the extreme excess of H2O in thehydrolysis reaction. Meanwhile, the TEOS and NH3concentrations had a linear eect on improving theparticle size because the hydrolysis and condensationreactions were proportionally facilitated.

3.2. Eect of admixtures

Under xed reaction conditions for TEOS, NH3, andH2O, as shown in Fig. 3, the particle size was clearlyreduced by the addition of a small amount of electro-lytes. However, a further increase in the electrolyteaddition caused a signicant increase in the particle size.For the mono-valent electrolytes NaOH, NaCl, NaI,KCl, CsCl, and CsI, the optimal electrolyte concen-tration for minimizing the particle size occurred around1.0105 to 1.0104 mol/l, and the minimum particlesize at this optimal concentration varied between 30.0and 17.5 nm. Furthermore, the particle size trend rela-tive to the electrolyte concentration was similar for allthe electrolytes.The addition of electrolytes above the optimal con-

centration reduced the repulsive interaction between theparticles by neutralizing the particle charge, therebypromoting particle aggregation and particle growth.Conversely, the addition of electrolytes below the opti-mal concentration increased the particle charge due to

ational 30 (2004) 171175ACS grade) was loaded in a semi-batch reactor, then100 ml of a TEOS (Fluka, ACS grade) reactant solutionwas fed into the reactor at a constant ow rate of 3.33ml/min. The concentrations of TEOS, H2O, and NH3 inthe reactor were changed from 0.02 to 1.0 mol/l, 1.12 to22.5 mol/l, and 0.224 to 1.8 mol/l, respectively. Tomodify the sol-precipitation process, mono-valent elec-trolytes of NaOH, NaCl, NaI, KCl, CsCl, and CsI wereadded to the mixture initially loaded in the reactor. Theelectrolyte concentration in the reactor was variedbetween 106 and 103 mol/l. In the present experiment,the above concentrations indicate values based on thenal mixture volume in the reactor.After feeding, the product suspension was con-

tinuously agitated for 48 to complete the reaction in thereactor. Thereafter, the product suspension was ana-lyzed using a particle size analyzer (Zetasizer, Malvern,UK), transmission electron microscopy (H-600, Hitachi,

172 S.-S. Kim et al. / Ceramics I

n. (a)

l of T

0.224 mol/l of TEOS and 4.48 mol/l of water.Fig. 2. Eect of reaction conditions on particle size of sol-precipitatio

mol/l of ammonia, (b) variation of water concentration with 0.224 mol/variation of TEOS concentration with 4.48 mol/l of water and 0.448

EOS and 0.448 mol/l of ammonia, and (c) variation of ammonia withrelaxation, thereby reducing the particle size [1214].Among the electrolytes used in the current study, ionswith a higher electronegativity had a stronger eect onthe particle size. As such, Na+ had more eect onreducing the particle size than K+ and Cs+, while Cl

and I had more eect on reducing the particle size thanOH. Therefore, the maximum particle size reduction(from 34.3 to 17.5 nm) was achieved with NaI at2.0105 mol/l, while the minimum (from 34.3 to 29.5nm) was achieved with CsCl at 9.0105 mol/l. It is alsointeresting to note that the optimal electrolyte con-centrations for the maximum particle size reductionwere consistent with the maximum electric charge ofthe particles observed in previous studies in a diluteelectrolyte solution [1214].

To conrm the additive eect on particle size, thezeta potential of the particles relative to the electrolyteconcentration was investigated based on 34.3 nm silicaparticles prepared under xed reaction conditions forTEOS, NH3, and H2O without an additive. As shown inFig. 4, the negative zeta potential of the particlesincreased with the electrolyte concentration until theoptimal concentration, then reduced with a furtherincrease in the electrolyte concentration above the opti-mal concentration. Among the electrolytes tested, NaIhad the most eect on the particle zeta potential, whileCsCl had the least, as also observed in the additive eecton the particle size. Accordingly, this result suggeststhat the eect of electrolytes on reducing the particlesize of silica in sol-precipitation originates from the

S.-S. Kim et al. / Ceramics International 30 (2004) 171175 173

chloride and iodine anions with common cation of sodium.Fig. 3. Eect of electrolyte additives on particle size of silicon oxide in sol-precipitation under xed reaction conditions of 0.224mol/l of TEOS, 4.48mol/l of

water, and 0.448 mol/l of ammonia. (a) variation of hydroxide, chloride, and iodine anions with common cation of sodium, (b) variation of sodium,

potassium, and cesium cations with common anion of chloride, and (c) variation of sodium and cesium cations with common anion of iodine.

Fig. 4. Eect of electrolyte additives on zetapotential of particles prepared under xed reaction conditions of 0.224 mol/l of TEOS, 4.48 mol/l of

water, and 0.448 mol/l of ammonia without additive. (a) variation of sodium and cesium cations with common anion of chloride and (b) variation of174 S.-S. Kim et al. / Ceramics International 30 (2004) 171175

modication of the particle charge, thereby preventingaggregation for particle growth.

4. Conclusion

In the sol-precipitation method, the particle size ofsilicon oxide was found to vary signicantly withchanges in the reaction conditions and the addition ofelectrolytes. As the particle size is predominantlydetermined by the particle nucleation in the initialinduction period, the particle size was enhanced whenincreasing the TEOS and NH3 concentrations, whichpromoted the TEOS hydrolysis and condensationreactions.The eect of electrolytic additives on the particle size

was found to originate from the modication of theparticle charge, as particle growth is achieved viaaggregation of the primary particles. Below the optimalelectrolyte concentration, which maximized the particlezeta potential and minimized the particle size, the par-ticle charge was improved by the addition of the elec-trolyte, resulting in a reduced particle size. However,above the optimal concentration, the particle size wasenhanced with an increase in the electrolyte concen-tration, as particle aggregation was promoted viadepression of the particle charge due to the adsorption

Acknowledgements

This research was nancially supported by KOSEF(R01-1999-000-00272-0) in 19992001.

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Effect of electrolyte additives on sol-precipitated nano silica particlesIntroductionExperimentalResults and discussionEffect of reaction conditionsEffect of admixtures

ConclusionAcknowledgementsReferences