Colloids and Surfaces A: Physicochem. Eng. Aspects 257–258 (2005) 273–276

Synthesis and characterization of Ag and Ag–SiO2 nanoparticles

Young Hwan Kim, Don Keun Lee, Young Soo Kang∗

Department of Chemistry, Pukyong National University, 599-1 Daeyon 3-Dong, Namgu, Pusan 608-737, Republic of Korea

Received 10 October 2003; accepted 11 July 2004Available online 22 January 2005

Abstract

Ag-oleate complex was prepared by the reaction of AgNO3 with sodium oleate in water solution. The thermal decomposition of preparedAg-oleate complex was analyzed with thermogravimetric analysis (TGA) and the crystallization process was also observed with X-raydiffraction (XRD). Ag nanoparticles have been prepared by thermal decomposition using electric furnace called autoclave at 300◦C for about4 h. TEM images of the particles showed two-dimensional assembly of particles with diameter of 8.0± 1.3 nm, demonstrating the uniformityof these nanoparticles. Energy-dispersive X-ray (EDX) spectrum and XRD peaks of the nanoparticles showed the highly crystalline nature ofthe Ag structures.

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Ag–SiO2 nanoparticles were fabricated by the well-known Stober method. Results show a dense coverage of silica nanospheres o00 nm size with homogeneously arranged Ag nanoparticles deposited on SiO2 nanoparticles. Ag–SiO2 nanoparticles were investigated wEM images, EDX spectrum, UV–vis spectrophotometer.2004 Elsevier B.V. All rights reserved.

eywords:Ag; Ag–SiO2; Thermal decomposition

. Introduction

The synthesis of nanosized particle is a growing researcheld in chemical science, in accordance with the extensiveevelopment of nanotechnology[1–3]. The size-inducedroperties of nanoparticles enable the development of newpplications or the addition of flexibility to existing systems

n many areas, such as catalysis, optics, microelectronics ando on[4–7]. In this study, Ag and Ag–SiO2 nanoparticles ap-lied to life environment are studied. In many other studies,g nanoparticle is synthesized by using various methods suchs chemical method (by borohydride, hydrazine at room tem-erature or by alcohol at elevated temperature),�-irradiation,ush-pull method and photochemical method[8–10]. In thisaper, Ag nanoparticles are synthesized by using thermalecomposition method. Ag–SiO2 nanoparticles are synthe-ized by using well-known Stober method at low temperatureoute for coating SiO2 nanoparticles with Ag nanoparticles.

∗ Corresponding author. Tel.: +82 51 6206379; fax: +82 51 6288147.E-mail address:yskang@pknu.ac.kr (Y.S. Kang).

This synthesis was done completely without extereducing agents or media, adhesive aids or functionalagents.

2. Experimental

2.1. Synthesis of Ag nanoparticle

The aqueous solutions contained 1 M silver nit(AgNO3) and 1 M sodium oleate were stirred at 20◦C for 2 h.After filtering and drying, it was transferred into the Pytube and immediately heating to 295◦C at 2◦C/min (0.3 Torr)for 2 h and then cooled at room temperature. MonodispeAg nanoparticles were obtained. Structural characterizof the product was done with TEM, selected area difftion (SAD) and X-ray diffraction (XRD). Optical absorptiof Ag nanoparticles was identified with UV–vis spectroptometer. The decomposition of Ag-oleate complex waslyzed with thermogravimetric analysis (TGA) and the crtallization process was observed with XRD.

927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2004.07.035

274 Y.H. Kim et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 257–258 (2005) 273–276

2.2. Synthesis of Ag–SiO2 nanoparticle

Silica nanoparticles were synthesized according to thewell-known Stober method by hydrolysis and condensationof tetraethylorthosilicate (TEOS) in a mixture of ethanolwith water, using ammonia as catalyst to initiate the reaction.The size of silica nanoparticles is controlled by the molarratio of TEOS, water and ammonia. The synthesis startswith mixing and stirring of the components, requires about2 h reaction time and is finished by centrifugation. Theseparated products were dried at temperature up to 100◦Cfor 2 h [11]. For the synthesis of Ag–SiO2 nanoparticles, Agnanoparticles prepared by above method were added to silicasolution. After the mixed solution was stirred for 4 h at roomtemperature, it was filtered and dried at room temperaturefor 2 h. Finally, Ag–SiO2 nanoparticles were obtained. Theprepared Ag–SiO2 nanoparticles were characterized withTEM, EDS and UV–vis spectrophotometer.

2.3. Apparatus

The structure of Ag and Ag–SiO2 nanoparticle studiedwith XRD (Philips, X’Pert-MPD system) and analysis ofelement was confirmed by TEM-EDX. Ag and Ag–SiO2nanoparticle size were determined with TEM (JEOL,J

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Fig. 2. X-ray diffraction patterns (Cu K� radiation) of silver-oleate complex(A), aging of silver-oleate complex at 300◦C (B).

the pristine silver-oleate complex has been observed in theXRD analysis. This indicates that the complex is amorphous.However, peaks of silver have been observed in the agingof silver-oleate complex at 300◦C. Peaks are broadened dueto the nanocrystalline nature of silver.Fig. 2(B) shows threepeaks at 2θ values of 38.2◦, 44.5◦ and 64.5◦ corresponding to(1 1 1), (2 0 0) and (2 2 0) planes of silver, respectively. No im-purity peak is observed in the XRD spectrum.Fig. 3is TEMimages of monodisperse nanocrystallite of silver. Most ofthe silver particles are spherical. The mean size of silver nan-oclusters was determined as 8.0± 1.3 nm. This shows that thesilver nanoparticles have very narrow size distribution.Fig. 4shows the EDX spectra of silver nanoparticles excited by anelectron beam (200 kV). Peaks for the elements of Ag andCu were observed together. The Cu peaks are due to the 400

Fig. 3. TEM image of silver nanoclusters. The average particle size is de-termined as 8.0± 1.3 nm.

EM-2010).

. Result and discussion

.1. Ag nanoparticle

The decomposition path of the silver-oleate complextudied by TGA analysis.Fig. 1shows weight loss for silveleate complex during heat treatment under air flow. Atrong endothermic peak was observed at 295◦ C. The peas related to the evaporation of oleate molecules by theomposition of silver-oleate complex. In the case of CThe weight loss occurred form 220, revealing that CTolecules were decomposed thermally[12]. Fig. 2illustrates

he XRD pattern of silver-oleate complex (A) and agingilver-oleate complex at 300◦C (B). No peak of silver in

ig. 1. TGA/DTG curve of silver-oleate complex during heat treatmener air flow.

Y.H. Kim et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 257–258 (2005) 273–276 275

Fig. 4. Energy-dispersive X-ray spectrum of silver nanocrystallites.

mesh copper grid. There is no impurity atom in the nanopar-ticles except silver atom. Accordingly, from the EDX spectrawe could confirm that the nanoparticles in TEM images arepure silver particles.

3.2. Ag–SiO2 nanoparticle

Fig. 5 shows TEM image of Ag–SiO2 nanoparticleswhose SiO2 nanoparticle size is about 200 nm and Agnanoparticle size is below 10 nm. TEM image showsSiO2 nanoparticles were homogeneously impregnatedwith Ag nanoparticles.Fig. 6 shows UV–vis spectrum ofAg nanoparticle (λmax= 410 nm) dispersed in isooctanesolution. Ag nanoparticles display an optical absorptionband at 410 nm (∼3 eV), typical of absorption at metallicAg nanoparticle, due to the surface plasmon resonance(SPR) [13]. The colloidal suspensions of silver particles

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Fig. 6. UV–vis spectra of Ag nanoparticle (λmax= 410 nm) and Ag–SiO2nanoparticle (λmax= 420 nm).

were a bright yellow-greenish color, due to the intense bandsaround the excitation of SPR[14]. Although the conductionand valence bands of semiconductors are separated by awell-defined band gap, metal nanoparticles have close-lyingbands and electrons move quite freely. The free electronsgive rise to a surface plasmon absorption band on the surfaceof metal particles, which depends on both the particle sizeand chemical surroundings. This absorption band is a littleblue-shifted compared with the plasmon absorption bandof silver colloids prepared by citrate reduction method(λmax= 434 nm)[15]. It is caused that silver nanoparticleshave narrower size distribution and smaller diameter thansilver nanoparticles prepared by citrate reduction method.

UV–vis spectrum of Ag–SiO2 nanoparticle is also shownin Fig. 6. UV–vis spectrum of Ag–SiO2 nanoparticle showsabsorption band at 420 nm whereas that of Ag nanoparticlesprepared is shown at 410 nm. This absorption band is ratherbroad and red-shifted compared with the plasmon absorptionband of Ag nanoparticle. As discussed earlier, the positionand shape of the plasmon absorption of silver nanoparti-cles are strongly dependent on the particle size, dielectricmedium, and surface-adsorbed species. This is concerned

ig. 5. TEM image of Ag–SiO2 nanoparticle; SiO2 nanoparticles sizeetermined as about 200 nm and Ag nanoparticles size is below 10 n

Fig. 7. Energy-dispersive X-ray spectrum of Ag–SiO2 nanoparticles.

276 Y.H. Kim et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 257–258 (2005) 273–276

with red-shifted absorption band of Ag–SiO2 [16,17]. EDXspectrum ofFig. 7shows that this product consists of Ag andSi components.

4. Conclusion

A new synthetic method has been discovered to producemonodisperse silver nanoclusters using thermal decompo-sition of Ag-oleate complex, which was prepared by thereaction of AgNO3 with sodium oleate in water solution.By TGA measurement, thermal decomposition of Ag-oleatecomplex was observed at 295◦C. TEM image shows thatmonodisperse silver nanoparticles (8.0± 1.3 nm) are packedin a highly organized manner. This work is easily extendedto other metals and to alloys of two or more metals. Andthis method can be easily increased scale up for industrialpurpose. Silica homogeneously coated with silver nanoparti-cles prepared by using well-known Stober method is carriedout at room temperature. This method is well suited forpreparing metal nanoparticle coatings on monodispersesilica nanoparticle without the chemical aids usually appliedto achieve particulate coatings. The size of impregnated Agnanoparticles of SiO2 in this process is similar to that of pureAg nanoparticles prepared by thermal decomposition ofAg-oleate complex. Andλ value at 420 nm of Ag–SiOn iclesp ex.

Acknowledgment

This work was financially supported by Korean Ministryof Science and Technology as Specific Research and De-velopment Program on New Functional Chemical Materials,2003.

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