A novel method for synthesis of silica nanoparticles

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Abstract

A sequential method has been used, for the first time, to prepare monodisperse and uniform-size silica nanoparticles using ultrasonication by sol–gel process. The silica particles were obtained by hydrolysis of tetraethyl orthosilicate (TEOS) in ethanol medium and a detailed study was carried out on the effect of different reagents on particle sizes. Various-sized particles in the range 20–460 nm were synthesized. The reagents ammonia (2.8–28 mol L−1), ethanol (1–8 mol L−1), water (3–14 mol L−1), and TEOS (0.012–0.12 mol L−1) were used and particle size was examined under scanning electron microscopy and transmission electron microscopy. In addition to the above observations, the effect of temperature on particle size was studied. The results obtained in the present study are in agreement with the results observed for the electronic absorption behavior of silica particles, which was measured by UV–vis spectrophotometry.

Introduction

Silica nanoparticles occupy a prominent position in scientific research, because of their easy preparation and their wide uses in various industrial applications, such as catalysis, pigments, pharmacy, electronic and thin film substrates, electronic and thermal insulators, and humidity sensors [1]. The quality of some of these products is highly dependent on the size and size distribution of these particles.

Stober et al. [2], in 1968, reported a pioneering method for the synthesis of spherical and monodisperse silica nanoparticles from aqueous alcohol solutions of silicon alkoxides in the presence of ammonia as a catalyst, and different sizes of silica nanoparticles were prepared ranging from 50 nm to 1 μm with a narrow size distribution. The size of particles 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 longer-chain alcohols are used as solvents. After this, a large number of studies were conducted in this area [3], [4], [5], [6], [7], [8], [9], [10], [11]. In the present study, two main types of reactions are involved: (i) silanol groups are formed by hydrolysis and (ii) siloxane bridges are formed by a condensation polymerization reaction: Hydrolysis: Sisingle bond(OR)4 + H2O ⇄ Sisingle bond(OH)4 + 4Rsingle bondOH, Condensation: 2Sisingle bond(OH)4 → 2(Sisingle bondOsingle bondSi) + 4H2O.

The condensation rate depends on reaction conditions which might result in the formation either of a three-dimensional network or in formation of single monodisperse particles [12]. A seeded growth technique has described by Bogush et al. [3] for the preparation of larger particles. In this technique a seed suspension is precipitated utilizing a Stober reaction. When the reaction is completed, TEOS and water are added to the seed suspension in a 1:2 mole ratio. The drawback of this technique is that, if the amount of TEOS exceeds a critical value, a second population of particles will appear. Using this technique, it is possible to prepare more monodisperse particles and increase their mass fraction in the sol, but with this method, it is not possible to increase the size of the monodisperse particles beyond 1 μm. The effect of electrolyte on size of silica nanoparticles was described by Bogush and Zukoski [5], and in their study, they reported that when the electrolyte (NaCl) concentration was increased from 0 to 10−4 M, the particle size increased from 340 to 710 nm.

Huang and co-workers have reported that sonication during a reaction could significantly increase the yield of carbodiimide-mediated amidations [13]. In view of this, in the present study, we have determined the effect of each reagent on particle size in addition to the effect of temperature on ultrasonication. To our knowledge, this is the first report on a sequential addition method for preparation of silica particles by a sol–gel process.

Section snippets

Reagents

Tetraethyl orthosilicate (TEOS) (99.99%, Aldrich), ethanol (99.99%, Aldrich), and ammonium hydroxide (28%, Wako) were used without any further purification. Milli-Q water (18.2 Ω) was used throughout the experiment.

Characterization

Silica particles were prepared using a sol–gel process with a sequential addition technique in an ultrasonication bath. Various types of experiments were conducted in which the concentration of one reagent was fixed and the concentration of other reagents were changed one by one.

Results and discussion

According to Bogush and Zukoski [5], five parameters play an important role in the size and size distribution of silica nanoparticles: (i) concentration of TEOS, (ii) concentration of ammonia, (iii) concentration of water, (iv) effect of alcohol, and (v) reaction temperature. In the present study, a systematic study was carried out by a sol–gel process using a sequential addition method and the results are discussed. The main parameters and their effects on particle size are summarized in Table

Conclusions

The present method is a simple and convenient method compared to other existing methods such as batch and semibatch methods. So far, we have studied all the parameters that affect the size of silica particles. Experiments on the effect of ultrasonication power on the size of silica particles are in progress. The influence of multiparametric optimization in sequential manner will be taken up.

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