Preparation of silica particles utilizing the sol-gel and the emulsion-gel processes

COLLOIDS AND ELSEVIER

Colloids and Surfaces A: Physicochemicaland Engineering Aspects99 (1995) 79 88

A

SURFACES

Preparation of silica particles utilizing the sol-gel and the emulsion-gel processes Ritva Lindberg a,1, Johan Sj6blom a,,, G6ran Sundholm b a University of Bergen, Department of Chemistry, Allegaten 41, N-5007 Bergen, Norway b Helsinki University of Technology, Laboratory of Physical Chemistry and Electrochemistry, Kemistintie 1, FIN-02150 Espoo, Finland Received 10 October 1994; accepted 6 February 1995

Abstract

The preparation of silica particles by means of the sol-gel or emulsion-gel techniques has been studied. In the case of precipitation from homogeneous solutions, we have studied the whole solution phase in the system tetraethyl orthosilicate-ethanol-aqueous ammonia and have mapped the region for monodisperse particles. These particles have a size of 70-640 nm. Their size and monodispersity are controlled by the composition of the system. Utilizing the emulsion-gel technique it is possible to increase the particle size to some tens of micrometers, but the polydispersity increases. In the emulsion-gel process the best results were obtained for small water contents (below 15 wt.%) and low amounts of surfactant (less than 0.5 wt.%) and for considerably longer reaction times than for the conventional sol-gel process.

Keywords: Emulsion-gel process; Monodispersity; Silica particles; Sol-gel process

1. Introduction

In 1968, St6ber et al. [ 1 ] introduced a method for the preparation of monodisperse silica particles from aqueous alcohol solutions of silicon alkoxides containing ammonia. After this pioneering work, many studies have been performed in this area [ 2 - 1 0 ] . Three reactions are generally used to describe the hydrolysis and condensation of silicon alkoxides [ 11,12] Hydrolysis

--=Si--OR + H20 ~--=Si-OH + R O H

(1)

* Corresponding author. t Permanent address: Helsinki University of Technology, Laboratory of Physical Chemistry and Electrochemistry, Kemistintie 1, FIN-02150 Espoo, Finland. 0927-7757/95/$09.50 © 1995Elsevier Science B.V. All rights reserved SSDI 0927-7757(95)03117-0

Alcohol condensation

~Si--OR + ~Si-OH =--Si-O-Si--= + R O H

(2)

Water condensation

=Si--OH + ~Si-OH~ ~Si-O-Si~- + H20

(3)

where R is an alkyl group, C~H2x+I; in the case of tetraethyl orthosilicate, R is C2H5. Depending on the reaction environment, the condensation might result in either the formation of a three-dimensional gel network or of single, in most cases monodisperse, particles. In the following we will only discuss reactions at high pH values resulting in particle formation. Strber et al. [ 1] found that the size of the silica particles that can be obtained in tetraethyl orthosilicate (TEOS)-ethanol-water mixtures varies from 50 nm to about 1 ~tm. The largest average diameter

80

R. Lindberg et al./Colloids Surfaces A: Physicochem. Eng. Aspects 99 (1995) 79 88

is about 700 nm, with a narrow distribution [ 11 ]. The final particle size depends on the type of silicon alkoxide and alcohol. Particles prepared in methanol solutions are the smallest, while the particle size increases with increasing chain length of the alcohol. The particle size distribution also becomes broader when using longer-chain alcohols as solvents. The particle size is further increased when increasing the hydrocarbon chain length of the particle precursor [ 1]. With TEOS, the particle size is found to increase with increasing concentrations of water and ammonia up to about 7 M and 2 M, respectively, after which the effect is reversed [1,3,10 12]. The final particle size is usually reached within 3 10 h of the reaction being started [2,3]. The particle size tends to decrease and its standard deviation increases when the TEOS concentration is increased. Normally less than 5% of the particles deviate more than 8% from the average size [ 11 ]. Tan et al. [13] studied the effect of temperature on the size of the silica particles. They found that a decrease in temperature gave monodisperse particles from T E O S - w a t e r ~ t h a n o l solutions with a size of about 2 gin. According to Bogush and Zukoski [4], the presence of an electrolyte, such as NaC1, increased the final particle size. By increasing the NaC1 concentration from 0 to 1 0 - 4 M the particle size increased from about 340 nm to 710 nm. Bogush et al. [3] have described a seeded growth technique for preparing larger particles. In this technique a seed suspension is precipitated utilizing the St6ber reaction. When the reaction is complete, TEOS and water are added to the seed suspension in a 1:2 mole ratio. The drawback with this technique is that if the added amount of TEOS exceeds a critical value, which depends on initial particle size, a second population of particles will appear. With this technique, it is possible to prepare more monodisperse particles and increase their mass fraction in the sol, but the size of the monodisperse particles does not exceed 1 Itm. Most of these studies have been limited to rather narrow ratios of the participating constituents, TEOS water(ammonia) ethanol. In this study, we focus on reaction conditions, which involve more

or less the whole solution phase in the corresponding three-component diagram, for different amounts of ammonia. In addition to this, our goals are to prepare silica particles in excess of 1 Itm by utilizing the emulsion gel process. This process is similar to emulsion polymerization and is, as such, a development of the microemulsion-gel process reported on earlier [14,15].

2. Experimental 2.1. Reagents In the sol-gel process, tetraethyl orthosilicate (98%; Fluka Chemica), absolute ethanol (A/S Vinmonopolet), 1-propanol (puriss. p.a.; Fluka AG), isopropanol (technical grade; A/S Vinmonopolet), 1-butanol (p.a.; Riedel-deHa~n), ammonia (25% p.a.; Merck) and sodium chloride, nitrate and sulphate (p.a. Merck) were all used without further purification. Water was distilled and ion exchanged (Seralpur pro 90 CN; less than 0.1 ItS cm 1). In the emulsion-gel process, the following reagents were used: tetraethyl orthosilicate (98%; Aldrich), ammonia (25% p.a.; Merck), decane, heptane, cyclohexane, toluene, dodecane and hexadecane (98%; Aldrich, Sigma or Merck), nonionic surfactants Berol 26 (polyoxyethylene(4) nonylphenyl 6ther), Berol 160 (polyoxyethylene(6) dodecyl ether) and Berol 267 (polyoxyethylene(8) nonylphenyl ether) from Berol Nobel AB. Water was distilled and ion exchanged (Milli-Q, less than 18 M ~ cm).

2.2. Experimental procedures Homogeneous samples were prepared according to the following procedure. Alcohol, water and ammonia were weighed into a glass vessel'equipped with a cap, and were mixed by shaking. TEOS was added and mixing was started immediately.

R. Lindberg et al./Colloids Surfaces A." Physicochem. Eng. Aspects 99 (1995) 79-88

Samples were mixed usually for 8 h with ultrasound (Bandelin sonorex super R K 102 H). In some cases, samples were mixed with a mechanical stirrer or by hand. Salts were added as electrolyte solutions. All experiments were conducted at room temperature (22°C), unless otherwise stated. The seeded growth technique was undertaken as in Ref. [-3] with the exception that only TEOS was added instead of a mixture of water and TEOS, and that the addition was made after 1 h not 8 h. Emulsions were prepared according to several different methods. In the first method, the surfactant and water were weighed into a beaker. Samples were mixed with use of a magnetic stirrer, and TEOS, or a mixture of TEOS and hydrocarbon, was added. Emulsification with ultrasound then followed. During this procedure, ammonia was added. Alternatively, surfactant and hydrocarbon were weighed into a beaker and water was added. The samples were emulsified with a Heidolph emulsifier (9600 13 500 rev min -1 for 2-20 min depending on the sample) and then T E O S was added and the samples were stirred either with use of the emulsifier or with ultrasound. If the pH of water was not adjusted initially, ammonia was added before stirring. Specimens for electron microscopy (SEM; J E O L JSM-6400 or Philips 505 instruments) were prepared by placing a drop of solution on a specimen plate. The solvent was allowed to evaporate at room temperature. Specimens from the emulsion samples were made either from the aqueous phase of the separated emulsion or from ethanol or acetone solutions obtained by washing the emulsion sample. The oil phase was removed from the separated emulsion and the aqueous phase was extracted with hexane or petroleum ether, and washed with water and ethanol or with decane, water and acetone. Samples were coated with a mixture of gold and platinum or just with gold (Polaron ES100 or Polaron E5300; 20 mA; 2.5 min; argon gas atmosphere). Particles were photographed with a Polaroid camera. The size of the particles was calculated from the SEM pictures using an average of 90 particles in most cases. If the particles were aggregated, only an estimation of particle size could be made.

81

3. Results and discussion

3.1. Solution-based systems TEOS-wate~ethanol systems Figs. 1 4 display the influence of different ratios of water, ethanol, TEOS and ammonia inside the original solution phase on particle size, mono-

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R. Lindberg et aL/Colloids Surfaces A: Physicochem. Eng. Aspects 99 (1995) 79-88

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by StOber et al. [ 1 ] and Bogush et al. [3]. StOber et al. [ 1] found no effect of TEOS concentration on the final particle size. In contrast to Bogush et al. [3] who reported larger particles for increasing TEOS concentrations, van Helden et al. [2] found that the particle size decreased. Our results are in good agreement with those of Bogush et al. although the highest TEOS concentrations render a decrease in the particle size. The maximum particle size depends on the ammonia concentration and the water-to-TEOS mole ratio, as shown in Fig. 5. When the water-to-TEOS mole ratio is about 50, the particle size is at a maximum depending, however, slightly on the ammonia concentration (Fig. 6). Several workers [3-5,10] have reported a maximum particle size at a corresponding ratio of about 40. As seen in Fig. 7 and also reported by several workers [1,3,10], the particle size seems to increase with increasing concentration of ammonia. The particle size passes through a maximum and for a constant water-to-TEOS mole ratio this maximum is reached at an ammonia concentration of about 0.9 M. The polydispersity in particle size increases, given a fix amount of ammonia-to-water, as the concentration of TEOS increases and, given a fix ratio of TEOS-to-ammonia, as the water concentration is decreased. According to van Helden et al. [2] the particles become more irregular and the

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dispersity and aggregation tendencies of the silica particles prepared. In the ethanol solutions, the particle size distribution was in the range from 30 to 1000 nm, and the monodisperse particles were in the range 70-640 nm. The particle size increased with increasing amount of water up to 20 wt.% (about 9 M) in the ethanol solutions. The water amounts are somewhat higher than those reported

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Fig. 4. Location of samples in the phase diagram for TEOS ethanol water for an ammonia concentration of 0.4 M. The polydisperse area is unconfirmed but probably exists.

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Fig. 7. Particle size vs. ammonia concentration in the system TEOS-ethanol-water. (O) 0.18 M TEOS; H20/TEOS , 11.6. (El) 0.17M TEOS; H20/TEOS, 23.1. (O) 0.16M TEOS; H20/TEOS, 40.8. size distribution wider as the TEOS concentration exceeds 0.2 M. However, our results indicate that it is possible to obtain monodisperse silica particles from suspensions containing up to 0.6 M TEOS. Hence, the crucial parameters seem to be the water and ammonia concentrations. The region of the solution phase (Figs• 1-4) in which the monodisperse particles could be prepared is reduced pronouncedly with increasing ammonia concentration. The highest and the lowest ammonia concentrations together with either high T E O S

83

concentrations or low water concentrations will easily produce irregular silica-based aggregates. The most commonly invoked mechanism for particle formation from supersaturated solutions is that of L a M e r and Dinegar [16,17]. They proposed that in a precipitation reaction, uniformity results from a short nucleation period followed by particle growth controlled by the diffusion of molecules to the surface of the nuclei. However, this mechanism will obviously not account for all cases. Several workers have noticed that nucleation continues throughout the period of growth [3,5,10,18]. Furthermore, electron micrographs have confirmed the coexistence of large spherical aggregates with rough surfaces consisting of a large number of tiny particles which are apparently taking part in an aggregation process [ 11 ]. In the present study, we observed a slight tendency for aggregation during the first 10 min. After this period of time, discrete but rather polydisperse particles could be observed. The sphericity and monodispersity of the silica particles were found to increase with time for up to 8 h. However, after 24 h, no further change in the properties of the particles could be observed. • The mean particle size also shows an increase during the first 4 h, after which it slightly decreases with a simultaneous increase in the monodispersity of the size. Fig. 8 shows that the smaller particles are less monodisperse, less spherical, and possess a higher surface roughness. The larger spherical particles prepared with higher concentrations of ammonia and water are almost perfect in shape with a smooth surface and a low polydispersity. This is in good agreement with the results of van Blaadern and Vrij [6]. In addition, Bogush et al. [3,10] have found that bimodal final particle size distributions are common near the m a x i m u m particle size occurring at higher TEOS and ammonia concentrations. We also found bimodal particle size distributions in some samples, with an average particle size around 400-500 nm and modest TEOS and ammonia concentrations. Effect of temperature, mixing, salt addition and the use of seeded growth technique Only a few experiments was carried out at reduced temperatures (4 and - 1 5 ° C) and no clear

84

R. Lindberg et al./Colloids Surfaces A." Physicochem. Eng. Aspects"99 (1995) 79 88

effect on the particle size could be observed. At reduced temperatures, the mode of stirring of the sample had a clear impact on the particle size. In contrast to the room temperature conditions, where monodisperse particles seem to be produced regardless of the stirring rate, the preparation at 15 °C needs vigorous stirring in order to prevent particle aggregation. Addition of sodium chloride (2-20 x 10 -3 M) resulted in a slight increase in particle size, but in most cases the particles just aggregated. A similar tendency was observed with other salts tested. This might be due to an excess of electrolyte. The use of the seeded growth technique has only a minor effect on the particle size and monodispersity. -

(a)

The use o f other alcohols as solvents

When other alcohols like propanol, isopropanol or 1-butanol were used, the particle size increased slightly, but, simultaneously, the polydispersity increased and aggregates were formed in almost all tests experiments in which the condensation took place under constant mixing. The situation was the same when a mixture of ethanol and some other alcohol was used instead of pure alcohols. The largest particle sizes were a few micrometers, but in most cases the size was below 1 p.m. 3.2. Colloidal systems

(b)

(c)

General aspects

The idea behind the use of different kinds of colloidal matrices is to produce particle shapes that cannot normally be obtained by means of precipitation from supersaturated solutions. The microemulsion technique has been widely employed for the preparation of nanosized particles in the range 5 10 nm [14,15,19-22]. It is obvious that this technique can be complementary to the precipitation technique with regard to the production of extremely small particles. For the production of organic polymers, i.e. latex particles, emulsion polymerization has been widely used Fig. 8. SEM pictures of silica particles. (a) 0.19 M TEOS, 0.4 M NH3, 2.2 M H 2 0 , bar 100 nm. (b) 0.17 M TEOS, 0.36 M NH3, 4.1 M H20, bar 1 gm. (c) 0.19 M TEOS, 2.4M NH3, 8.9 M H20, bar 1 gm.

R. Lindberg et al./Colloids Surfaces A: Physicochem. Eng. Aspects 99 (1995) 79-88

[23-28]. Depending on the experimental conditions, monodisperse latex particles of the order of 500 nm are generally produced. For the preparation of monodisperse inorganic particles, the emulsion technique has rarely been utilized [29-31]. Our model system consists of a non-ionic surfactant, water and oil, where we, under the correct conditions, can produce very stable water-oil (w/o) emulsions [32,33]. It is quite obvious that we need to start from an oil-continuous emulsion as matrix. An alkaline aqueous phase is initially dispersed into the hydrocarbon medium and stabilized by an appropriate non-ionic surfactant of the alkylphenylethoxylatetype. When the TEOS is mixed into the continuous hydrocarbon phase, the TEOS molecules will not have any diffusion restrictions. As a result, they will penetrate the surfactant.membrane surrounding the aqueous droplets and hydrolysis will take place as a consequence of the contact with the aqueous phase. The Si(OH)x(OCHzCHz)y molecule is polar and hence has a natural tendency to accumulate at the w/o interface. When hydrolysis proceeds, the SiOH-based molecule will become water soluble and hence the condensation will take place inside the aqueous droplet. Thus a properly designed w/o emulsion should guarantee that the aqueous cavities restrict the growth of the silica particles given a low level of droplet-droplet interaction in the initial emulsion. In order to restrict the interaction between added reactants, reaction products and the emulsifier we have chosen a non-ionic surfactant as stabilizer. Using a w/o emulsion as a reaction medium for particle preparation is, of course, based on the idea that each droplet serve as a nucleus for particle growth. The formed particles should, in their shape, size and size distribution, reflect the original colloidal matrix. However, when the final particles are analyzed, it is noted in most cases that the density of each particle cannot be explained if no mass transport between the different droplets is assumed to have taken place. An assumed, homogeneous distribution of the precursor molecules over all the emulsion droplets would, in most cases, produce less dense material, although the size and shape is that of the original emulsion. The background to the mass transport may be manyfold. It is

85

well-known that the emulsion droplets undergo both flocculation and coalescence, which will give different nuclei the possibility of aggregating into much denser particles. On the other hand, coagulation of the primarily formed nuclei can also lay the basis for particles that one normally observes. In the case of the emulsion gel process where tetraethyl orthosilicate is used as the precursor molecule, one can consider a continuous diffusion of the TEOS molecules from the oil phase to the nuclei, and in this way the final amount of molecules in the precipitated particle can be qualitatively determined. Emulsions with high water contents

Table 1 summarizes some of the results of particle formation in these emulsions, when the water content exceeds 40% by weight. There seems to be a clear tendency that these samples undergo gelation after some time. The procedure is that during the emulsification process (ultrasound) small particles are produced. Since the amount of TEOS is high, the concentration of particles is accordingly high. When the ultrasound treatment is stopped, the aggregation of small particles will proceed, and larger, more irregular structures are formed. An additional parameter influencing the original emulsion is the amount of ethanol released during the hydrolysis step. It is well known that the addition of ethanol to a w/o emulsion will result in a destabilization. Such a destabilization will promote the formation of irregular aggregates. Emulsions with lower water contents

Table 2 gives a summary of particle formation in these systems. For water contents exceeding 15 wt.%, much of the same features as described above for water contents above 40 wt.% is repeated. There seems to be an initial particle formation, but the particles will not remain stable when stored. Hence different kinds of structures or aggregates will form. The increase in ammonia concentration of the water phase from 1 to 6 M causes the formation of more flocculated precipitates. This is most probably due to faster hydrolysis and condensation reactions followed by faster ethanol formation and destabilization of the emulsion. For the lowest water and surfactant contents, the particles

R. Lindberg et al./Colloids Surfaces A." Physicochem. Eng. Aspects 99 (1995) 79 88

86

Table 1 Emulsions with high water content a

HzO/TEOS

H20 (%)

TEOS (%)

Surfactant (%)

69.8 74.9 89.4 70.3 75.2 89.5 46.8

29.7 24.6 10.1 29.3 24.3 10.0 6.9a

0.5 b 0.5 b 0.5 b 0.5* 0.5 ~ 0.5 ~ 1.9*

25 33 96 26 33 96 77

45.7 *

1.1f

1.7~

379

Observation

mole ratio Clear gel Turbid liquid Clear solution Clear gel Turbid gel Clear solution Two phases: the lower is heavily precipitated, the upper is a clear solution; small (below 2 gm) aggregated particles Two phases: the lower is an emulsion, the upper is a clear solution; irregular particles, aggregates

" Ammonia concentration in the water phase is 1 M. b Berol 160. Berol 267. d The remaining 44.4% is decane. e 4.4 M N H 3 . f The remaining 51.5% is heptane.

Table 2 Emulsions with low water content H20 (%)

TEOS (%)

Oil (%)

Surfactant (%)

H20/TEOS mole ratio

NH 3 (M) a

Observation

10.8 14.0 28.0 27.6

4.3 1.7 6.9 5.7

84.4 b 83.8 b 63.2b 64.8 b

0.5 e 0.5 e 1.9e 1.9f

26 65 46 52

1.0 4.4 0.5 1.2

37.6

5.9

54.6 b

1.9f

73

1.2

2.7

6.6

90.6 b

0.1 f

4

3.2

5.6

6.9

87.4 b

0.1 f

7

3.2

5.4

15.1

79.3 b

0.2 f

4

1.3

5.7

14.9

79.3 c

0.1 f

4

1.1

5.9

14.8

79.1 a

0.2 f

4

1.3

Two phases, irregular particles, a few spheres below 1 gm Two phases, a few spheres, net structure Precipitate, spheres, aggregates, net structure Two phases, light precipitate in lower phase, spheres below 1 ~tm, aggregates Two phases, lower turbid, upper clear, spheres below 1 gm, aggregates Heavy precipitate in lower phase, clear upper phase, irregular particles, a few spheres, aggregates 1 10 gm Heavy precipitate in lower phase, clear upper phase, spheres, aggregates 1 2 gm Emulsion in lower phase, clear upper phase, spheres, irregular particles below 1 8 gm Emulsion in lower phase, clear upper phase, spheres, irregular particles 1 8 lam Emulsion in lower phase, clear upper phase, spheres at irregular particles 1-5 p.m

a Concentration in water phase. b Decane. c Toluene. a Cyclohexane. e Berol 267. f Berol 26.

R. Lindberg et al./Colloids Surfaces A." Physicochem. Eng. Aspects 99 (1995) 79-88

produced seem to undergo slight flocculation/ aggregation which is not so extensive as for the higher water contents. With lower TEOS contents (approximately 7 wt.%) the particles were spherical in shape, while higher TEOS contents (approximately 15 wt.%) tended to produce particles with irregular shapes. Replacing decane with toluene or cyclohexane did not cause any changes in particle size or shape. With ageing of the samples (1 week) the particle size is increased to 10-40 gm. The particles are spherical but monodispersity is not achieved (Fig. 9). Sherif and Shuy [31] have prepared yttriumstabilized zirconia particles and they report that the particle size depends on the stirring rate during hydrolysis. In general, the slower the stirring, the larger the particles. Especially when ultrasound was applied during the hydrolysis step, submicrometer particles formed. In our experiments, all the samples were stirred either with ultrasound or with a rotor using high speeds. This is probably one reason why the particle size tends to be small. Sherif and Shuy [31] also reported that in order to obtain spherical particles, a relatively stable emulsion has to be formed during hydrolysis. Irregular particles may be due to a fast destabilization of the emulsion. In our study, the emulsions

87

were destabilized within less than 2 h in most cases. We could not see any changes in the particle size for decreasing surfactant contents, although Sherif and Shuy [31] found that the more surfactant used, the smaller the particles. It should be mentioned that Sherif and Shuy [31] obtained spherical particles when they used acetates, but granules when they used alkoxides as starting materials.

4. Conclusions As has been already shown by several workers the hydrolysis and condensation of alkoxide compounds by using the sol-gel process in order to obtain uniform oxide particles are very useful. Monodisperse spherical silica particles from about 50 to 700 nm can be easily prepared. The drawback of this technique is that it is not possible to prepare monodisperse particles larger than about 1 pm. The size of the particles can be controlled with added water and ammonia in the T E O S - e t h a n o l water (ammonia) system. The area of the phase diagram where the monodisperse particles are formed depends mainly on the ammonia concentration of the system. Under the experimental conditions adopted in the emulsion study, small regular particles were initially formed. When these systems were inspected by means of optical microscopy most of them looked monodisperse and there were no tendencies to aggregation. However, when analyzed by means of electron microscopy, there are clear signs of agglomeration/aggregation, especially at the highest water content, i.e. above 20 wt.%. When the water content was kept low (below 15 wt.%) the systems produced more individual particles, although their size and shapes could not be fully controlled and brought to uniformity. With the ageing of emulsion samples, the size of the spherical particles is increased to 10 40 gm, but the monodispersity is not achieved.

100 pm Fig. 9. SEM picture of silica particles from an emulsion containing 14.9% H20, 7.1% TEOS, 77.9% decane and 0.1% surfactant. H20/TEOS mole ratio, 24; reaction time, 1 week; bar, 100/am.

Acknowledgements The Neste Oy Foundation is gratefully acknowledged for financial support. R.L. would also like

88

R. Lindberg et al./Colloids Surfaces A: Physicochem. Eng. Aspects 99 (1995) 79 88

to thank Nordisk Forskerutdanningsakademi (NorFa) for financing her stay in Bergen.

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