솔-젤법을 이용한 은 나노입자가 함유된 구형 실리카 입자의 제조

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

화학공학의 이론과 응용 제8권 제1호 2002년

신승일, 오철, 오성근

한양대학교 화학공학과

Preparation of Spherical Silica Nanoparticles Doped with Silver Nanoparticles

using Sol-Gel Method

Seung-Il Shin, Chul Oh, Seong-Geun Oh

Department of Chemical Engineering,  Hanyang University

Introduction

Nanoscale metal and semiconductor particles are of current interest because they mark a material transition range between quantum and bulk properties [1]. With decreasing particle size, bulk properties are lost as the continuum of electronic states becomes discrete (i.e. quantum size effect) and as the fraction of surface atoms becomes large [2]. The electronic and magnetic properties of metallic nanoparticles and nanoclusters show new properties, [3] which can be utilized in novel applications in areas that range from nonlinear optical switching, and catalysis, to high-density information storage, for example [4].

In recent years, sol-gel synthesis of nanocomposites containing nanoparticles of noble metals (Au, Ag, Pt and Pd) in oxide matrices (SiO2, TiO2, V2O5 and WO3) has been rapidly developed [5-8]. Many authors have studied the preparation and properties of coating films of these materials containing Ag and Pt nanoparticles[9, 10]. They have shown that such coatings are suitable for nonlinear optical applications.

Surface coating or surface modification of a nanometer semiconductor and metal particles offers a new challenge to synthesis. By changing the thickness of the shell and particles size, the overlap of the wave functions and the band gap can be changed [11]. Chang et al. have reported that composite coatings on the nanoscale can be readily achieved with a variety of morphological features[12]. Silica particles with CdS cores of different morphology were prepared by Chang et al. The physical parameters of each material as well as the interface between the "hetero-structures" will have a great influence on the optical properties.

A wide variety of methods have been used to preparation such as small clusters, including the use of hydrosols, [13] irradiation, [14] reduction and stabilization, [15] microemulsion and reverse micelle systems [16]. Adair et al. prepared nanocomposites composed of SiO2 with Ag nanometer size cores inside a microemulsion reaction matrix [17]. The average size of nanocomposites ranged from 12.5 to 30 nm depending upon the experimental conditions. The process was performed in conjunction with hydrolysis of organometallic precursors such as tetraethoxysilane (TEOS), followed by condensation in water nanodroplets to form a coating on the nanosize metal within reverse micelle structure.

Up to now, microemulsions have been employed as a reaction medium for the preparation of particles [18, 19]. But emulsions have not been widely used to prepare particles due to their intrinsic instability. In our previous study, we reported about the preparation of spherical silica particles in O/W/O multiple emulsion at various conditions [20]. It was found that the emulsion stability is the most important factor in the shape of particles. In this study, silver nanoparticles-dopped silica particles were prepared by sol-gel reaction in water-in-oil emulsion. The aqueous silver colloidal solution was used as water phase in emulsion. The polymers were added to stabilize the emulsion structure during the formation of silica particles containing Ag nanoparticles. The morphology of silica particles containing silver nanoparticles was investigated through FE-SEM and the chemical composition of particles was analyzed through the EDS.

Experimental

Materials

Tetraethyl orthosilicate (TEOS) as a precursor, hydroxypropyl cellulose (HPC, MW 370000) as a stabilizer of emulsion, polyoxyethylene(20) sorbitan monolaurate (Tween 20), AgNO3, Na3citrate□2H2O and FeSO4□7H2O were obtained from Aldrich Chemical Company. N-decyl alcohol (Sigma) as an external phase of emulsion, and sorbitan monooleate (Span 80, Sigma) as an emulsifer to form W/O emulsion, Polyethylene glycol (PEG, Junsei, MW 20000), NH4OH (Acros, 25%,) as a catalyst were used as received. Technical grade ethanol and Milli-Q water were used in all the preparations.

Preparation of the spherical silica containing silver nanoparticles

The silver colloid solution was prepared by Carey Lea method (as adapted by Frens and Overbeek ) [21]. The general procedure used to prepare silica spheres containing Ag nanoparticles is outlined in Figure 1. In the modified St□ber method, silver colloid solution was dispersed into n-decyl alcohol (containing HPC and Span 80) to form silver colloid solution-in-decyl alcohol emulsion. The certain amount of NH4OH was added into silver colloid solution to catalyze the formation of silica particles. To stabilize the silver colloid solution during the formation of silica particles, PEG and Tween 20 were also added into colloid solution. After the temperature was stabilized to 40oC, TEOS was rapidly injected into decyl alcohol phase in emulsion. Then, TEOS will diffuse into the water/oil interface in emulsion and the hydrolysis of TEOS would occur herein. The hydrolyzed TEOS would penetrate into water droplets and particles were formed by condensation inside the droplets of silver colloid solution. The reaction was continued for varying period time. After the reaction, the particles were collected and washed with water and ethanol, then dried at 80 oC for 24 hours.

Morphology of Particles

The size, and morphology of the resulting composite particles were determined by Field emission scanning electron microscopy (FE-SEM, Jeol JSA 840A) and UV-visible absorption spectroscopy (SCINCO S-2150) was used to investigate the particle size of silver nanoparticles in aqueous colloid solution. For the FE-SEM studies, samples were coated with platinum by sputtering for 10 minutes at 15mA. Photomicrographs were obtained operating at 15 KV, a working distance 15 mm at 5,000 magnifications. For the TEM pictures of colloid solutions, samples were prepared by adding drops of freshly prepared cluster solution on a carbon film supported on a Cu grid. The UV spectroscopy was also used to study the variation of the size of silver nanoparticles in water by adding NH4OH and Tween 20.

Results & Discussion

The TEOS molecules, which were added into the decyl alcohol in W/O emulsion, would penetrate the surfactant layer surrounding the aqueous phase and then hydrolysis would occur as aresult of the contact with the aqueous phase. As hydrolysis proceeds, the Si-OH based molecules will become water soluble, and hence go slowly into the water region of emulsion. The gel network by condensation would take place herein. As a result, the insoluble hydrated silica particles would be formed in the water droplets. Also the colloidal silver particles dispersed in the water phase would be entrapped inside the silica particles [22].

The hydolysis of TEOS can be catalyzed by HCl or NH4OH. Acid hydrolysis, contrary to alkaline catalyst, generates ethanol in the system quickly[23]. It has been shown that ethanol can greatly influence the stability of emulsion. In the previous study, we found that it was very difficult to prepare the spherical silica particles in emulsion system due to the fast breakage of emulsion structure when HCl was used as catalyst. Even though the NH4OH was used to catalyze the hydrolysis of TEOS, the emulsion structure became unstable as the reaction proceeded due to the formation of ethanol. To enhance the emulsion stability, the oil soluble polymer HPC was added into the decyl alcohol. It was impossible to obtain spherical silica particles without adding the HPC in our system.

It has been reported by Esquena and Solans that the size of silica particles formed in emulsion was controlled by coalescence [24]. The faster the coalescence of emulsion droplets, the larger silica particle size. This means that tuning coalescence by addition of small concentration of polymer can have a great effect on the particle size.

The silver was not detected by EDS analysis of the surface of silica particles in sample 1 because the silver particles were completely entrapped inside the silica particles. The silver particles at the surface of silica particles might be completely removed during the washing process with water and ethanol. The composite particles were partially dissolved in 50 wt% HF aqueous solution.

In conclusion, the spherical silica particles doped with silver nanoparticles were prepared in aqueous silver colloid solution-in-decyl alcohol emulsion by sol-gel reaction of TEOS. The stability of emulsion during the sol-gel reaction was crucial factor for the formation of spherical silica particles. Without adding HPC into the decyl alcohol in emulsion, the spherical particles were not formed in our system. Also it was found that the high viscosity of aqueous droplets hindered the formation of spherical particles. This might be due to the hindrance of the movements of hydrolyzed TEOS molecules in highly viscous medium. The amount of silver nanoparticles in silver colloid solution did not influence the morphology of silica particles. Thus, the amount of silver in silica particles can be controlled easily. By using the preparation technique of silica particles doped with silver nanoparticles proposed in this study, it might be possible to develop the particles of high third order nonlinear optical susceptibility.

Acknowledgement

We acknowledge support for this work from Center for Ultramicrochemical Process Systems sponsored by KOSEF.

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Figure 1. Processing steps for the preparation of silica particles containing silver nanoparticles.