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Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 473–481

Functionally modified core-shell silica nanoparticles byone-pot synthesis

Michele Marini a, Behzad Pourabbas b,∗, Francesco Pilati a, Paola Fabbri a

a Dipartimento di Ingegneria dei Materiali e dell’Ambiente, Universita di Modena e Reggio Emilia,Via Vignolese 905A, 41100 Modena, Italy

b Nanostructured Materials Research Center, Sahand University of Technology, Tabriz 51335-1996, Iran

Received 21 August 2007; received in revised form 10 October 2007; accepted 18 November 2007Available online 23 November 2007

bstract

In this work functionally modified silica nanoparticles were synthesized in base-catalyzed reaction of the elementary molecules: TEOS andinyltriethoxysilane (VTEOS) using ethanol as the solvent. The main feature of the developed method is the addition of the modifier molecule,TEOS, after appearance of the first primary particles in the reaction mixture. A core-shell structure was detected for the final particles having

arbon-rich phase in the shell and silicon-rich phase in the core. The effect of TEOS/VTEOS molar ratio, solvent, reaction temperature and timeas investigated on the particle size and morphology of the particles. The results showed that TEOS/VTEOS molar ratio of 90/10 and, reaction

emperature of 40 ◦C using ethanol as the solvent yield the desired core-shell structure after 30 min of reaction time. 2007 Elsevier B.V. All rights reserved.

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eywords: TEOS; VTEOS; Silica nanoparticles; Core-shell; Modification

. Introduction

Silica nanoparticles can easily be synthesized by Stoberrotocol [1] or its modified version [2] and are also avail-ble commercially in different particle sizes. However, itsydrophilic surface characteristic has to be modified prior toolymer grafting or preparation of silica/polymer nanocompos-tes. In most of the cases, a pre-modification reaction is carriedut using the surface –OH groups existing as silanol groups withifferent modifying molecules.

Trialkoxysilane compounds have widely been used for pre-odification of the silica nanoparticles creating new and suitable

unctional groups for further reactions including polymer-zation reactions. For instance, polyimide grafting has beenchieved using aminopropyltriethoxysilane in a work done byuji et al. [3]. In this case, amine functionality has intro-

uced by condensation–hydrolysis reactions of ethoxysilaneroups of the modifier molecule and surface –OH groupsn the silica nanoparticles. There is also other possibility

∗ Corresponding author. Tel.: +98 411 4249612; fax: +98 411 4249610.E-mail address: pourabas@sut.ac.ir (B. Pourabbas).

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927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2007.11.022

o introduce polymerization initiators like azo or peroxideroups using the created functional groups [4] by subse-uent reactions of the already attached functional group oro successive in situ polymerizations to cover the particlesith different shells [5]. Vinyltriethoxysilane (VTEOS) [6–8],ethacroyloxypropyltriethoxysilane [9], methyltrimethoxysi-

ane [10] and �-glycidyloxypropyltrimethoxysilane [11] arexamples of modifier molecues or coupling agents which haveeen used for surface modification or reaction modification ofhe silica nanoparticles for grafting purposes or preparation ofolymer/silica nanocomposites.

Modifier molecules which are not essentially of trialkoxysi-ane type have also been used for surface modification ofhe silica nanoparticles such as epichlorohydrin [12], tolylene-,4-diisocyanate [12,13] oleic acid [14] and stearic acid15].

It must be noted that in the approach of using surface hydroxylroups of the already synthesized silica nanoparticles, the sur-ace –OH groups have different chemical moieties and thus

ifferent reactivity. Only the isolated hydroxyl groups are avail-ble for subsequent reactions, by alkoxysilane molecules forxample, and the others are stabilized in hydrogen bonds [13].oreover, by using alkoxysilane modifier molecules, there is

4 hysicochem. Eng. Aspects 317 (2008) 473–481

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74 M. Marini et al. / Colloids and Surfaces A: P

lways a possibility of side reactions such as self-condensationithout reacting on the surface. Furthermore, modification of

lready synthesized or commercial silica nanoparticles addsdditional step to the preparation process of the polymer/silicaanocomposites.

Therefore, one-pot synthesis (vs. post-modification) ofhe modified and functional silica-nanoparticles could be anlternative approach in order to reduce the steps of thereparation and remove the disadvantages associated with theormer approach [16]. Base-catalyzed hydrolysis–condensationf TEOS/VTEOS mixture has been reported previously by Xund co-workers [17] leading to spherical particles with theverage diameter of 350 nm. The particles were then used foroly(styrene-divinylbenzene) grafting with an application ashromatography column packing materials. Although they havenvestigated some properties of the obtained silica nanoparticlesike specific surface area, porosity of the particles and, carbonontent of the hybrid silica nanoparticles as well as SEM microg-aphy however, they did not focus on the detailed structure ofhe particles or parameters affecting the characteristics of thenal particles. In an another work, acid-catalyzed sol–gel reac-

ion of TEOS/VTEOS mixture has been studied by Posset etl., with the purpose of using the obtained gel as coating mate-ials for glass and PMMA substrates [18] therefore, sphericalodified silica nanoparticles was not the main purpose of theork.The intention of the present work was to develop a one-pot

ethod for the synthesis of modified silica nanoparticles startingrom fundamental molecules instead of using already synthe-ized silica nanoparticles. For this purpose, TEOS which haseen the starting molecule for the synthesis of silica nanoparti-les in Stober reaction [1,2,19] and, VTEOS, as the modifierolecule were used. The outline for the synthesis is repre-

ented in Scheme 1. The expected core-shell structure (as ishown in Scheme 1) was confirmed by TEM with carbon-ich shell and the core, exclusively being made up of siliconnd oxygen elements, as confirmed by EDS microanalysis.ther instrumental methods including SEM, FTIR and TGAere used to further structural characterization of the obtainedanoparticles. Parameters like the molar ratio of TEOS/VTEOS,eaction temperature and time as well as the solvent werenvestigated and found to be effective on the final proper-ies of the modified silica nanoparticles. Interpretation of theifferent features of the reaction and the effective reactionarameters has been made according to the available mech-nisms for particle formation and growth during the Stobereaction.

. Experimental

.1. Materials

Ethanol (95%), methanol (98%), toluene (97%), vinyltri-

thoxysilane (VTEOS, 99%), ammonium hydroxide solutionNH4OH, 28%), and tetraethoxysilane (TEOS, 99%) were pur-hased from Aldrich chemical company and used as receivedithout any further purification.

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cheme 1. Representative strategy used for synthesis of core-shell modifiedilica nanoparticles.

.2. Synthesis of the vinyl modified silica nanoparticles

Solution consisting of NH4OH (0.9 M) and water (9.5 M) in00 ml of EtOH was firstly prepared. To this solution at 40 ◦C,1:1 (w/w) solution of TEOS (0.25 M final concentration) intOH was added suddenly with vigorous stirring. Stirring wasontinued for 30 min. During this time, VTEOS was addedfter the reaction mixture started to become turbid (after aboutmin). The amount of VTEOS was so that to obtain samplesith different TEOS/VTEOS molar ratios including 95/5, 90/10,5/15 and 80/20. Finally, the white precipitates was separatedy centrifugation at 4000 rpm for 20 min and then washed withater and EtOH for several times. The final product was dried

or 12 h under reduced pressure with a yield of 91–96 percentepending on TEOS/VTEOS molar ratios used. Unmodified sil-ca nanoparticles were also prepared by this procedure withoutsing VTEOS, for comparison. The procedure also repeated forreparation of different products by changing the reaction tem-erature from 40 ◦C to room temperature, reaction time from

0 min to 1.5 h and, the solvent from EtOH to MeOH in order toerify the reaction parameters’ effect on the particles. The molaratio of TEOS/VTEOS was 90/10 in all the latter mentionednvestigations.

: Physicochem. Eng. Aspects 317 (2008) 473–481 475

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.3. Characterization

TEM analyses were performed with a JEOL JEM 2010 trans-ission electron microscope operating at 200 keV. The samplesere prepared by dipping 150-mesh carbon coated copper grids

nto dispersion solution of the nanoparticles in water. EDS exper-ments were carried out by an Oxford INCA 100 EDS mountedn the TEM instrument with 15 nm detection size.

SEM analyses were performed with an environmental scan-ing electron microscope (ESEM) Quanta-200 under mediumacuum without any metallization of the samples. Accelera-ion voltage of 2.5 keV was used and images were recordedt 15,000× of magnification. The samples were prepared byispersing the nanoparticles in water, dropping and drying theolution on SEM stabs.

Fourier-transform infrared (FTIR) spectroscopy wasmployed to investigate the variation in surface chemicaltructure of the nanoparticles. The spectra were recorded inBr pellets on a Bruker Vertex 70 instrument.Thermogravimetric analyses (TGA) were carried out using

TA TGA 7 instrument under oxygen streaming atmosphere atheating rate of 20 ◦C min−1 between room temperature and

00 ◦C.

. Results and discussions

.1. Synthesis of the vinyl modified silica nanoparticles

In the present work, Stober reaction of a mixture of differ-nt alkoxysilane molecules, TEOS and VTEOS, was utilized inrder to one-pot synthesis of the vinyl functionalized modifiedilica nanoparticles as is shown representatively in Scheme 1. Itas expected to three ethoxysilane groups of VTEOS to incorpo-

ate in Si–O–Si network formation along with the ethoxysilaneroups of TEOS. Consequently, the vinyl group of the attachedTEOS would functionalize the surface of the silicate nanopar-

icles making them suitable for subsequent surface reactionsncluding in situ polymerization. However, the first attemptshowed that no any sol–gel particles obtain when VTEOS andEOS had been introduced both at the start of the reaction. Con-ersely, in the case of adding VTEOS after the time that the firstucleuses have been formed by TEOS, and the reaction mixtureas turned to turbid, the sol–gel process went to completionroducing the modified silica nanoparticles. In order to discussbout the possible reasons for the observed phenomena, knowl-dge about the reactions and the mechanisms which are alreadyroposed for particle formation and growth in the course of thetober reaction is necessary. Therefore, we leave this until the

atter parts of the text where a brief summary on the availableechanisms is provided.Fig. 1 shows the TEM images at two different magnification

evels for the modified silica nanoparticles prepared by usingEOS/VTEOS = 90/10 molar ratio. As can be seen, the particles

re in core-shell structure with silicon-rich core and carbon-richomposition for the shell as was confirmed by EDS microanaly-is. The majority of the cores were detected having a diameter inhe range of 220–240 nm with a narrow size distribution as it can

imto

EOS/VTEOS molar ratio of 90/10: (a) lower magnification and (b) higheragnification.

e seen in Fig. 1a. The thickness of the shell was estimated to be8–22 nm. Although Fig. 1 shows multiple cores being settlednside the envelop of the organic shell however, single core-hell nanoparticles were also detected in TEM images whichs shown in Fig. 2a. In order to comparison, TEM image of thenmodified silica nanoparticles, prepared by the same procedureithout using VTEOS, is provided in Fig. 2b. The particle size of

he unmodified silica nanoparticles is about the same size of theore in the modified silica core-shell nanoparticles however, ast will be discussed later, our studies showed that parameters like

olar ratio of VTEOS with respect to TEOS, solvent, reactionemperature and time are effective on the size and morphologyf the core-shell nanoparticles.

476 M. Marini et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 473–481

Fig. 2. TEM images for (a) the modified silica nanoparticles prepared by molarratio of TEOS/VTEOS = 90/10 and (b) unmodified silica nanoparticles preparedby the same method without using VTEOS.

Fig. 3. ESEM image of the modified silica nanoparticles prepared byTv

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Fig. 4. FTIR spectra of the modified silica nanoparticles

EOS/VTEOS molar ratio of 90/10. The image is obtained under mediumacuum, without any metallization or using silver adhesive.

The modified silica nanoparticles were also characterizedy environmental scanning electron microscopy (ESEM) underedium vacuum without using any metallization or applying

ilver paste in order to save the morphology and agglomerationtate of the nanoparticles. The result is shown in Fig. 3. Theodified silica nanoparticles can nicely be seen on the ESEM

tab. They are completely spherical, nearly uniform in size and,ithout aggregation.Moreover, FTIR spectrum shown in Fig. 4 provided proofs

or the existence of vinyl functional groups in the preparedilica nanoparticles. In addition to Si–O–Si vibration bandst 1101, 945 and 795 cm−1 there is a peak at 1635 cm−1

ttributed to vinylic C C vibrations [6]. Therefore, based onhe characterizations made above, the evidences show success-ul synthesis of the vinyl modified silica nanoparticles withistinct core-shell structure in the case of using 90/10 molaratio of TEOS/VTEOS in the described method of synthe-

is. The effect of reaction parameters on the structure androductivity of the method are discussed in the following sec-ions.

prepared by 90/10 molar ratio of TEOS/VTEOS.

: Physicochem. Eng. Aspects 317 (2008) 473–481 477

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.2. The effect of TEOS/VTEOS molar ratio

In order to verify the effect of TEOS/VTEOS molar ratio,ifferent samples were prepared by the same procedure givenn Section 2.2 by changing TEOS/VTEOS molar ratio. There-ore, additional samples by using 95/5, 85/15 and 80/20 molaratios were prepared and compared with the sample of 90/10hich was discussed in the preceding section. Fig. 5(a)–(c) showEM images for the particles obtained by 95/5, 85/15 and 80/20olar ratios, respectively. By comparison of the TEM images it

ecomes clear that the thickness of the organic shell increasesy increasing VTEOS concentration. In the case of 95/5 molaratio, the particle average diameter is about 600 nm without anyrganic shell detectable around the particle (Fig. 5(a)). How-ver, when the molar ratio of TEOS/VTEOS has been changedo 85/15 (Fig. 5(b)), a thick but irregular organic shell is formed.n this case, the core average diameter is about 200 nm. Accord-ngly, the shape of the core-shell structure changes deeply whenhe relative molar ratio of VTEOS increases further, as is evi-enced by TEM image for 80/20 sample shown in Fig. 5(c). Inhe latter case, the particles are completely far from a true core-hell structure showing multiple cores with diameters of about0–40 nm formed inside an irregularly shaped organic phase.

In general, it can be concluded that increasing the relativeolar ratio of VTEOS with respect to TEOS, impose to major

ffects. Firstly, the thickness of the shell increases and its shapehanges by increasing VTEOS relative molar ratio. Secondly, theize of the core decreases by increasing the relative molar ratio ofTEOS. A discussion is provided for the observed effect of theEOS/VTEOS molar ratio in the last parts of the papers basedn the available mechanisms for silica nanoparticle formationnd growth models developed for Stober reaction.

By the fact that using 90/10 molar ratio of TEOS/VTEOSields the modified silica nanoparticles with desirable and dis-inct morphological aspects, further studies were conducted bysing this molar ratio of the reactants.

.3. The effect of temperature, time and solvent

By changing the solvent from EtOH to MeOH, (all other con-itions remaining unchanged) very small particles were obtainedFig. 6) for TEOS/VTEOS molar ratio of 90/10. Particles areery small, in the range of 30–40 nm however, it is difficult toonsider them as modified silica nanoparticles with core-shelltructure. In addition, there are big agglomerations as can beeen in Fig. 6b. The results are in close similarity with the TEMmages reported by Xu et al. for TEOS and dimethyldiethoxysi-ane (DDS) hybrid sol prepared in MeOH. They have detectedwo mass-fractal regions for the sol according to SAXS and TEMesults [20]. Obtaining smaller particles when using MeOH ashe solvent is a well-known phenomenon in sol–gel prepara-ion of silica nanoparticles and the results is in agreement withhose obtained by other investigators [20–22]. Recently, it has

een investigated that using different alcohols as solvent, thearticle sizes of the silica nanoparticles obtained from base-atalyzed reaction of different alkoxysilane molecules increasesrom MeOH to EtOH to PrOH [22]. The polarity of the alcohol Fig. 5. TEM images of the particles obtained by different molar ratios of

TEOS/VTEOS: (a) 95/5, (b) 85/15 and (c) 80/20.

478 M. Marini et al. / Colloids and Surfaces A: Physic

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By using geometrical formulas and data from thermogravi-metric analysis (TGA), it is possible to calculate some propertiesof the modified silica nanoparticles however, due to the welldefined core-shell structure which obtained only for sample

ig. 6. TEM images showing the obtained particles in MeOH as the solvent: (a)igher magnification and (b) lower magnification.

hanges the kinetics of the hydrolyses–condensation reactionsf the alkoxy groups leading to different size nanoparticles inifferent solvents [20,21].

At prolonged reaction time, 1.5 h instead of 30 min (keep-ng the other conditions unchanged); a wider polydispersity wasbtained for the particles as can be seen in Fig. 7. Particles asmall as 30 nm and as big as 600 nm are detectable in the Fig. 7reaction product of TEOS/VTEOS molar ratio of 90/10).

In exploring the reaction temperature effect, increasing theeaction temperature (from 40 ◦C to higher temperatures) wasot considered because of the oxidation and/or auto polymer-zation probability of the vinyl groups. However, the reactionarried out at room temperature did not lead to any sol–gel

anoparticles within 30 min possibly due to slow kinetics ofhe condensation–hydrolysis reactions of the alkoxysilyl groupst room temperature.

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ochem. Eng. Aspects 317 (2008) 473–481

As a result, using TEOS/VTEOS molar ratio of 90/10 and therocedure provided in Section 2.2, core-shell silica nanoparticlesith organic-rich shell and Si–O–Si network in the core cane obtained. Applying any changes in the relative molar ratiof the reactant alkoxysilane compounds, solvent, reaction timer temperature, results in undesirable changes in the core-shelltructure and/or productivity of the reaction.

.4. Structural specifications and TGA of the modifiedilica nanoparticles

ig. 7. TEM images showing the particles obtained at room temperature: (a)ower magnification and (b) higher magnification.

M. Marini et al. / Colloids and Surfaces A: Phy

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ig. 8. TGA curves between room temperature and 700 ◦C, heating rate of0 ◦C/min under O2 atmosphere for (a) unmodified silica nanoparticles and,b) modified nanoparticles obtained by 90/10 molar ratio of TEOS/VTEOS.

repared by 90/10 molar ratio of TEOS/VTEOS (in conditionsescribed in Section 2.2); the calculations were possible only forhis sample. Fig. 8 shows the TGA curves for unmodified sil-ca nanoparticles and 90/10 sample. These were obtained underxygen atmosphere between room temperature and 700 ◦C ateating rate of 20 ◦C min−1. The main decomposition peakhich is attributed to decomposition of the organic portion of

he modified nanoparticles starts at 293 ◦C and continues gen-ly up to 623 ◦C. The weight loss percentage associated withhis peak is 5.3 wt.% (Fig. 8b). Unmodified silica nanoparti-les (Fig. 8a), show no any significant weight loss peak so thatetween 293 and 623 ◦C the weight lost is only 0.48 wt.%. Thebserved TGA behavior is in general agreement with the resultsbtained by Zaharescu et al. for VTEOS gel obtained by acid-atalyzed reaction [23]. If we consider that the core is compactnd non-porous, as is shown by Xu and co-workers [17], andiving a density of 2.2 g/ml [24] to the core, ρcore, density of thehell, ρshell, can be obtained using Eq. (1).

shell =(

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here �wshell and �wcore are the weight ratios for the shellnd core respectively, obtained from TGA and, Vcore/Vshell ishe volume ratio of core to shell which can be calculated usingore radii and shell thickness in TEM images. Density of thehell ρshell was obtained to be 0.19 g/ml, and it means that thehell is actually far less dense than the core and, it should beonsidered a very low density material indeed.

The relationship between specific surface area (SSA) andarticle diameter d, can be expressed with Eq. (2) [19].

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SA =

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For 90/10 sample, taking the average diameter of the cores d,o be 230 nm (from TEM images) and the density of the cores

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sicochem. Eng. Aspects 317 (2008) 473–481 479

.2 g/ml [24], the SSA value can be calculated from Eq. (2) to be1.86 m2/g. This is in good agreement with the value obtainedy Xu from BET measurements (12.27 m2/g) [17].

The surface density of vinyl groups dV, can be calculatedsing Eq. (3):

V(�mol/m2) = �wshell/�wcore

Mw,VSSA× 106 (3)

here Mw,V is the molar weigh of the vinyl groups.Using Eq. (3) and the data for �wshell/�wcore from TGA

xperiments and SSA value obtained from Eq. (2), the densityf vinyl groups dV was calculated to be 177 �mol/m2 which ismazingly a high value for the density of vinyl groups on theurface of the silica nanoparticles.

.5. Mechanistic aspects of the core-shell structureormation

.5.1. An overview on the available modelsThe kinetics and mechanisms for the formation of monodis-

ersed silica nanoparticles in the course of the sol–gel reactionf TEOS has been the subject of many investigations. Scheme 2hows the base catalyzed mechanism for the hydrolysis reac-ion, which is an SN2 type, and the two-step condensation

echanism, proposed by Iler [25] and Keefer [26]. Accordingo Scheme 2, steric and inductive effects imposed by organicubstituents are expected to change the reaction kinetics of therganic-substituted alkoxysilanes with respect to TEOS, gener-lly.

In order to illustrate mechanisms involved in the forma-ion of monodispersed silica nanoparticles from hydrolysis–ondensation reactions of TEOS, due to the fast and complicatedeactions and that after early stages of the reaction, colloidalolid particles separate out from the sol; available instrumentalechniques were not able so far, to provide a comprehensiveicture of the mechanism [20]. However, projections of theechanism have been established by several investigators based

n indirect experimental data and evidences. One of the com-only reviewed mechanisms being supported by instrumental

ata, is the aggregation model proposed by Bogush and Zukoski27,28] improved later by Boukari and Harris by using SAXS29–31].

On the other hand, there are a few numbers of reports avail-ble in the literature discussing about the base-catalyzed Stobereaction of mixed alkoxysilanes, studying the particle growth32] and size [33] and, the reaction kinetics [20,34] of mixed-recursor silane systems. For the mixed-precursor system, it haseen shown that the complexity arise not only for statisticallyossible cross reactions of different silanes but also for changeshat happens in the reaction kinetics of the individual silaneompounds when the system changes from single-precursor toixed-precursor system [20,32].In general, the overall aspects of the proposed mechanisms

or the mixed precursor system can be summarized as following:

a) In basic solution, the kinetics of the hydrolysis reactionchange due to the presence of the second and organic-

480 M. Marini et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 473–481

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substituted alkoxysilane so that the rate of hydrolysis forTEOS become faster in the mixed-precursor system.

b) The rate of condensation reactions decreases for TEOS inthe mixed-precursor system.

c) A cross condensation between the hydrolysis products ofthe two different silanes is possible, and it occurs mainly onthe surface of the particles.

d) The mechanism for particle formation and growth remainsgenerally the same as what is proposed for TEOS singleprecursor system: formation of primary particles by agglom-eration of partially hydrolyzed TEOS species, adsorptionand condensation of the partially hydrolyzed monomers anddensification of the particles by condensation reactions.

e) The organic-substituted silane molecules participate in thecourse of the particle formation and growth however,they impose different effects including lower tendency toagglomeration and lower stability of the colloidal particles.

.5.2. Interpretation of the observed phenomena in ourork based on the available models

It is known that there is an induction period before formationf the first primary particles and the concentration of partiallyydrolyzed monomers increases during this induction, periodefore particle formation [30,35]. This is the most importanteature of our work in which VTEOS was added to the reac-ion mixture when (around 5 min) primary particles had beenormed by TEOS only. It is also very important that we did notbtain any sol–gel product starting by TEOS and VTEOS at theeginning of the reaction. This is in contrast to what is reportedy Xu and co-workers [17], obtaining silica nanoparticles afterh of reaction time at very low concentration of basic catalyst.lthough, they have not reported any TEM images to verify the

ore-shell nanoparticle formation, we believe that the main rea-

on for different results is the using different procedures. Basedn the outlined mechanism above, it seems that contribution ofTEOS, when it is present at the early times of the reaction,

n hydrolysis–condensation reactions (of TEOS) prevents parti-

acee

nds in basic solutions [20]. R is OEt for TEOS and CH CH2 for VTEOS.

le growth, reasonably, because of its un-hydrolysable organicarts. On the other hand, in this case, agglomeration and furtherondensation of the partially hydrolyzed or co-condensate prod-cts become impossible due to non-polar moiety of the VTEOS.herefore, the reaction products will never form spherical col-

oidal particles in agreement to item (e) above.If VTEOS is added later, when the primary particles have

een formed by hydrolysis–condensation reactions of TEOSour procedure), the usual mechanism was followed for par-icle formation and growth. At the time of adding VTEOS,pecies present in the reaction mixture are: the growing particles,ydrolysis product of TEOS which can contribute to parti-le growing mechanism, unreacted TEOS and, recently addedTEOS molecules. Therefore, in order to VTEOS become

ncorporated into the particle growth mechanism, it must beydrolyzed first, which is a slow reaction in basic media and then,he concentration of its hydrolysis product must reach to a certainoncentration to compete with TEOS and its hydrolyzed prod-cts. It is clear that during this time, particle growing continueso produce bigger particles however, after insertion of VTEOSpecies onto the surface of the particles, the growing stops. Theime-scale of the proposed process depends on VTEOS initialoncentration. The reason for this conclusion is the observationhat average core size of the particles depends on VTEOS rel-tive molar ratio: the higher VTEOS relative molar ratio, themaller core size. It must be considered also that presence ofifferent alkoxysilanes modifies the reaction kinetics, hydroly-is and condensation reactions of the individual alkoxysilanesitems (a) and (b) above]. This can account for the formationf bigger particles when VTEOS molar ratio is very low (sam-le prepared by 95/5 molar ratio of TEOS/VTEOS, Fig. 6a).hese particles are quiet bigger than those were obtained in

he absence of VTEOS, Fig. 2b). In the presence of small

mount of VTEOS, faster hydrolysis of TEOS increases theoncentration of the partially hydrolyzed TEOS species andventually, more frequently adsorption of them by the alreadyxist and growing particles. On the other hand, there is no enough

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TEOS in the reaction mixture to block the particle growingrocess.

Finally, although the observations were in agreement withhe theories, it should be noted that the suggested series ofvents proposed for the formation of the core-shell structureere based on the facts and experimental results observed in

his work combined with the general mechanistic models pro-osed and developed by other investigators. Therefore, manyxperimental evidences should be collected and many detailshould be elucidated by future works in order to consolidate,efuse or reform any of the proposed explanations.

. Summary

Functionally modified silica nanoparticles were synthesizedy one-pot method and addition of VTEOS to the already pro-ressing base-catalyzed hydrolysis–condensation reactions ofEOS. The core-shell structure of the nanoparticles havingarbon-rich shell and silicon and oxygen-rich core was con-rmed by TEM images. Investigations showed that the molaratio of TEOS/VTEOS play a significant role in morphology ofhe core-shell structure, core size and, shell thickness. The shellas characterized to be a low density material with a calculatedensity of 0.19 g/ml typical for porous materials. Surface den-ity of the vinyl groups was estimated using TGA data to be17 �mol/m2. In addition to TEOS/VTEOS molar ratio, factorsncluding solvent, temperature and reaction time were investi-ated to alter the core-shell structure, the size and shell thicknessn the final product. Correlation between the observed experi-

ental results with the mechanistic views available for Stobereaction of the single and mixed-precursor systems showedgood qualitative agreement between theory and experiment

ndings. One-pot synthesis of the functionally modified silicaanoparticles has a cost-reducing effect on industrial prepara-ion of polymer grafted silica nanoparticles and polymer/silicaanocomposites of different kinds. Moreover, the high densityf the surface functionality obtained in this work, which is dif-cult to be obtained by post-modification methods, ensures theecure and effective interactions of the silica nanoparticles withany other reactive molecules with a wide range of properties

nd applications.

eferences

[1] W. Stober, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62.

[

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