다양한 안정화제에 의한 수용액에서 은 나노입자의 안정화

남동헌, 신승일, 오성근

한양대학교 화학공학과

Stabilization of Silver Nanoparticles

in Aqueous Solution by Various Stabilizers

Dong-Hun Nam, Seung-Il Shin, Seong-Geun Oh

Department of Chemical Engineering, Hanyang University

INTRODUCTION

The preparation of particles with nanometer size is an interesting field. Because of their small size, these crystallites exhibit novel material properties which largely differ from the bulk properties. The chemical reactivity small metallic particles is strongly dependent on particle size.

Silver is the metal most commonly used, but preparation of monodisperse silver colloids is difficult. It is possible to form silver colloids by chemically reducing silver ions in solution(reduction can also be achieved using ultrasound or radiation). The most remarkable feature of metal nanoclusters is that their chemical and physical properties are quite different from those of bulk metals and metal atoms.

Silver can be rapidly produced by a strong reducing agent, such as hydrazine. This tends produce a long nucleation period, resulting in particles of varying size. Slower rates of silver production can be achieved using organic reducing agents but this also produces colloids containing particles of differing sizes. The commonly used citrate reduction, for example, produces a mixture of particle sizes and shapes. These colloids give a good surface enhanced Raman spectroscopy (SERS) enhancement but one of the advantages of using a colloid, that the surface is well defined, is lost. It is also notable that greater enhancements of Raman activity can be produced using particles of a narrowed size range so monodisperse suspensions may have the potential to be better substrates. Hydrazine reduction can produce small particles that are fairly monodisperse but the reaction is difficult to control to generate larger particles. A more reproducible method of generating silver colloids of a predetermined size is to create very small particles with a strong reducing agent and to use these as nuclei to grow larger ones with a weaker reducing agent.

Colloidal dispersions of metal nanoclusters have been prepared chemically by reduction of the corresponding metal salts and physically by pulverization of the metallic mass. The chemical procedure is superior to the physical one in regard to small size and narrow size distribution of nanoclusters as well as stability and mass-productivity of nanoclusters. The colloidal dispersions of metal nanoclusters are stabilized by citrate ion, linear polymers, organic ligands and surfactants. Surfactants show several types of organized assemblies which provide specific size, geometrical control and stabilization to particulate assemblies formed within the organized surfactant assemblies.

Our choice of silver metal is based on several factors. Firstly, it is the most frequently studied cheapest noble metal in this field. Secondly, it finds topical importance in the practical world, viz., photography, catalysis, analysis, etc. Thirdly, it has a narrow intense plasmon surface enhanced Raman spectroscopy, chemical absorption band in the visible region which is very much susceptible to surface/interface effects.

In this study, we prepared Ag colloid in aqueous solution by various stabilizers. The particle size distribution, morphology and UV-absorption of Ag colloid was investigated. Furthermore, the stabilization model of Ag colloid was suggested.

EXPERIMENTAL

Sodium Citrate Dihydrate(HOC(CO2Na)(CH2CO2Na)2·2H2O, Aldrich, 99+%), Tween20(polyoxyethylene (20) sorbitan monolaurate, Aldrich), SDS(sodium dodecyl sulfate, Aldrich, 99.5%) and Polyvinylpyrrolidone((C6H9NO)n, Kanto chemicals, Japan, average M.W.:40000) as stabilizing agent, Silver nitrate (AgNO3 , Kojima chemicals, Japan, 99.9%) as starting material and hydrazine monohydrate(N2H4·H2O, Aldrich, 98%) as a reduction agent were used to prepare Ag nanoparticles. Water was treated using a Millipore system(Milli-Q) twice.

Silver colloid are prepared by reduction of AgNO3 solution using hydrazine solution in the presence and absence of stabilizer, 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 and size of particles formed in surfactant solutions were investigated by Field Emission Scanning Electron Microscopy (FE-SEM, Jeol model JSM-6340F) and transmission electron microscopy (TEM, Jeol-model JEM-2000EXⅡ). A drop of the dispersed samples was placed on Cu grids (200 mesh) covered with a carbon film. Photomicrographs were obtained operating at 200 kV with a magnification of 100,000.  UV-visible spectra of prepared samples were taken in aqueous solvent with a SCINCO S-2150 spectrophotometer. Particle size distribution was calculated by image analysis over more than 100 counts.

RESULTS AND DISCUSSION

Colloidal silver particles in the nanometer size range were synthesized in aqueous solutions, by reduction of AgNO3 with various stabilizer(surfactant, polymer etc.). The stabilizer/metal ratio has an important effect on the extent of the reduction nanoparticles: large stabilizer concentrations are necessary for a complete dispersion.

Metal nanoparticles have mainly been studied because of their unique optical properties as especially nanoparticles of the noble metals copper, silver, and gold have a broad absorption band in the visible region of the electromagnetic spectrum. Solutions of these metal nanoparticles show a very intense color, which is absent for the bulk material as well as for the atoms. Their origin is attributed to the collective oscillation of the free conductive electrons induced by an interacting electromagnetic field. These resonances are also denoted as surface plasmons. With regard to the optical properties of the dispersions, the silver plasmon absorption band is shifted to larger wavelengths, probably due to the presence of free silver ions and to surface modification by adsorbed stabilizer molecules.

Finally, we can point out that Particle size and size distribution was obtained using image analysis technique. Ag particles prepared in the presence of Tween 20 and SDS showed a small and narrow size. The Ag particles prepared in the presence of Tween 20 showed the smallest particle size with 10-15 nm and showed a narrow size distribution. In case of SDS solution, the size of particles was around 25-30 nm. The size of Ag particle prepared in the presence of SDS was confirmed using TEM as shown in figure 1. Morphology of particle showed sphere like shapes. This size distribution corresponded relatively with the stability of colloid according to stabilizer species. It is apparent that instability of colloid was caused by lack of protection effect of surfactant against aggregation and continuous growth of prepared particle in solution.

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(a)                             (b)

Fig.1. TEM pictures of silver particles prepared in surfactant stabilizers

(a) Tween 20         (b) SDS

Fig.2 SEM pictures of silver nano-sized powders obtained by citrate stabilizer.