계면활성제 수용액에서 Ag 나노입자 제조

신승일, 오성근, 김대욱*

한양대학교 화학공학과, (주)삼성 SDI*

Preparation and Stabilization of Silver Nanoparticles

in the Aqueous Surfactant Solution

Seung-Il Shin, Seong-Geun Oh, Dae-Wook Kim*

Department of Chemical Engineering, Hanyang University,

Samsung SDI Co.,Ltd.*

INTRODUCTION

Synthesis of nanoparticles is an emerging field in material science, chemistry and chemical engineering. Nanoparticles exhibit novel and hybrid properties different from molecular and bulk solid state limit and find various applications in areas such as microelectronic devices, catalysis, liquid and solid composites, non-linear optics and so on. Colloidal sols of dispersed particles (metals, semiconductors, polymers) have attracted considerable interest due to their photochemical, photocatalytic, and photovoltaic properties.

Surfactants show several types of organized assemblies which provide specific size, geometrical control and stabilization to particulate assemblies formed within the organized surfactant assemblies. Among the host surfactant assemblies that are available for the formation of nanoparticle species may be mentioned the following ; the aqueous micellar solution, reverse micelle, microemulsion, vesicle, monolayer, Langmuir-Blodgett films and bilayer lipid membranes.

Depending on the nature of the prepared materials, two types of adsorption may be considered ; hydrophobic particles with the hydrophobic parts or hydrophilic particles with the hydrophilic parts of the surfactant. Furthermore, according to the charge of the particle and surfactant head groups show two kinds of binding behavior. In the case of ionic surfactant of opposite charge to the particle, the hydrophilic head groups of surfactant bind to the particle surfaces, which become hydrophobic. Such surfactant-covered particles cannot be kept in an aqueous environment, unless a double layers of surfactant molecules is absorbed, which is difficult to achieve with very small colloidal particles. With ionic surfactant of the same charge as the particle surface, the binding of the surfactant head groups is not occurred.

In this study, we prepared positively charged Ag colloid in the absence and presence of surfactant. The particle size distribution and UV-absorption of Ag colloid was investigated. Furthermore, the stabilization model of Ag colloid was suggested.

EXPERIMENTAL

NP9 [poly-oxyethylene (9) nonyl phenol ether] was kindly provided from Ilchil chemicals (Korea). Tween20 [polyoxyethylene (20) sorbitan monolaurate, Aldrich], SDS (sodium dodecyl sulfate, Aldrich, 99.5%), CTAB (cethyl trimethyl ammonium bromide, Acros, 99%+ TLC grade) was used as received as surfactant. De-ionized water (electrical resistivity 18.2□) was used to form microemulsions. AgNO3(silver nitrate, Aldrich, 99.9%) as starting material and N2H4·1H2O (hydrazine monohydrate, Aldrich, 98%) as a reduction agent were used as received to prepare Ag particles.

Silver colloid are prepared by reduction of AgNO3 solution (0.05M) using hydrazine solution (0.1M) in the presence and absence of surfactant, according to

N2H4 + 4Ag+ + 4OH- → 4Ag0 + 4H2O + N2

First, selected surfactant aqueous solution were added in flask. Then, hydrazine solution was added and mixed homogeneously. Sencondly, AgNO3 was added and the reaction was proceeded. All experiment was performed at thermostatic water bath and prepared silver colloid also was kept same condition.

The morphology of particles formed in aqueous surfactant solution was studied by Field Emission Scanning Electron Microscopy (FE-SEM, Jeol model JSM-6340F). Samples were coated with gold by sputtering for 1min. Photomicrographs were obtained operating at 15kV, a working distance of 9 mm at 100,000 magnification. UV-visible spectra were measured with a Shimadzu UV-2101PC spectrophotometer. Dynamic light scattering measurements were performed with Malvern Zetamaster instrument at the wavelength 514.5nm of Ar-Ne at 90o to measure the particle size of Ag particle.

RESULTS AND DISCUSSION

In the case of a pure aqueous phase reduction, the prepared Ag particle was immediately precipitated and slowly formed very large agglomerate with gray color because of their hydrophobic nature. Otherwise silver colloid prepared in the presence of a surfactant was relatively good stability compare with sample prepared aqueous water phase.

It is well known that the Ag particle charged positively in aqueous solution. In the case of anionic surfactant, SDS we could observed very good stability in spite of + charge of Ag particle and didnt observed any precipitation.

This result caused by hydrophobic bonding of anionic surfactant on the adsorption state of +charged hydrophobic particle surface as shown in Fig. 1. This phenomena can occurs when the combination of mutual attraction between hydrophobic groups of the surfactant molecules and their tendency to escape from an aqueous environment becomes large to permit them to adsorb onto the solid adsorbent by aggregating their chains. Adsorption of surfactant molecules from the liquid phase onto or adjacent to other surfactant molecules already adsorbed on the solid adsorbent also may occur by this mechanism.

But in the case of CTAB, because of short chain length(about 1.8nm) and electrical repulsion between particle and hydrophilic head groups very few surfactant molecules only can adsorb the particle surface. So they cant function enough stabilizing effect to disperse the particles.

Among the tested surfactant, tween20 also show good stability by steric effect because of their adsorption of hydrophobic groups to particle surface and strong hydration of large hydrophilic groups as shown in Fig. 2.

groups of surfactant to particle surface takes place and it is allow that the polyoxyethylene chain form the steric barrier. Because the highly hydrated polyoxyethylene chain extends into the aqueous phase in the form of coils which given an effective barrier against aggregation so long as the chains are long enough. In the case of nonionic surfactant, there is no attraction between particle and hydrophilic groups of surfactant. Furthermore, because of hydrophobic property of Ag particles, adsorption of hydrophobic

Otherwise, in the case of NP9 system, poor colloidal stability was observed. We guess that this phenomenom was caused by difference in surfactant structure. It is the reason Tween20 has the sufficient oxyethylene groups to provide steric hindrance. Otherwise NP9 dont provide effective steric barrier to disperse the Ag particle because of insufficient polyoxyethylene quantity.

Particle size and size distribution was obtained using dynamic light scattering technique. The measurement was performed after 6hours from starting reaction. Ag particle prepared in the presence of tween20 and SDS showed small size and narrow size distribution as shown in Fig. 3. This size distribution corresponded relatively with stability of colloid according to surfactant species. It is apparent that unstability of colloid was caused by lack of protection effect of surfactant against aggregation and continuous growth of prepared particle.

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Fig. 1. Schematic diagram of size control and                 stabilization of Ag particles by adsorption of             anionic surfactant on the surface of particles

Fig. 2. Schematic diagram of stabiliaztion effect of       hydrophobic particles by surfactant with           polyoxyethylene chain in aqueous solution.

Fig. 3. Particle size distributions of silver particles prepared in various surfactants as stabilizers

(a) SDS (b) Tween20 (c) NP9 (d) CTAB