Direct Deposition of Silver Nanoparticles on Silica Nanoparticles in Aqueous Suspensions

T. Zidki1, H. Cohen2 and D. Meyerstein1,2

1 Chemistry Department, Ben-Gurion University of the Negev, Beer Sheva, Israel. Fax: 972-8-6472943; Tel: 972-8-6472737. 2 Department of Biological Chemistry, Ariel University Center of Samaria, Ariel, Israel.

Abstract

The deposition of silver nanoparticles on silica nanoparticles in aqueous media is described. These nanocomposites can be dispersed in water at very high concentrations (up to 70 wt. %). Moreover, these nanocomposites do not contain any bridging molecule which usually aids the attachment of the silver nanoparticles to the silica surface. However these organic bridging molecules might interfere in possible catalytic reactions.

Introduction

Metal nanoparticles (NPs) are the subject of intense studies over the last decade1-5. Metal NPs attract scientific attention due to their special surface active nature as expressed in their chemical6,7, physical8-11, optical5,12-14 and other properties. These properties enable the use of metal NPs in many applications e.g. catalysis15-18, in special electronic devices19-21, and in green chemistry21,22.

Metal NPs have a poor stability in suspensions, therefore their concentrations are limited, at least in water (in most cases the metal precursor concentration is of the order of milli-molars2,23-25). Large concentrations of silica NPs can be dispersed in aqueous solutions. Thus it is suggested that deposition of metal NPs on silica NPs can be used to increase the concentration of metal NPs appreciably. Several studies demonstrated that the deposition of metal NPs on silica NPs is facilitated by attaching it to silica NPs to which bridging molecules such as APS (Amino-Propyl-Silanol)26-28 or MPS (Mercapto-Propyl-Silanol)29 were previously attached. Alternatively metal NPs stabilized by anions, e.g. citrate, are adsorbed on silica to which a positively charged polymer is also adsorbed30. When the composite NPs are used in chemical reactions, e.g. in catalysis, the bridging molecules might play a "non-innocent" role, i.e. to react with the reagents and thus possibly yielding undesirable side products and shortening the stability of the composite NPs. In this study we developed a synthetic procedure to produce nanocomposites (NCs) composed of silver metal deposited directly as small "islands" on the surface silica NPs. This method yields the NC product without any linker or stabilizer molecule. Furthermore, these NCs can be dispersed in water at very high concentrations.

Experimental

Materials

All the chemicals were of A.R. grade and were used without further purification. The water used was deionized and was further purified by a Barnstead EasyPure RoDi water purification system with a final resistance of > 5 MΩ/cm. The silica NPs powder used was Cabosil M-5 from Riedel-de Haën.

Instrumentation

Ultrasonic dispersions of the suspensions were done using a MRC ultrasonic cleaner model DC150H. Centrifugation was carried out using a Sorvall RC 6 centrifuge equipped with a SLA 1500 rotor. UV-Vis measurements were carried out using an Agilent Diode Array spectrophotometer model 8453, which enables measurements in the range of 190-1100 nm and a resolution of less then ± 2 nm. TEM analyses were performed using a Tecnai 12 G2 TWIN TEM (FEI) acc. Volt. 200 kV of Philips Company. The NP suspensions were dried on special grids. The grids were Formvar/Carbon, 300 mesh, Copper from Ted Pella.

Synthesis of the silver on silica NCs

Silica powder (Cabosil M-5) was dispersed in a given volume (V) of water to give 2.25 wt% of silica. The mixture was sonicated until it became a homogeneous white suspension. A concentrated silver perchlorate aqueous solution was added drop-wise to the stirred silica suspension to form 1.25 mM of silver. The suspension was stirred for five hours so that the silver ions will be homogenously dispersed on the silica particles. Then a reducing reagent was added. 1.25 mM of NaBH4 ice-cold aqueous solution was added very slowly to the latter suspension. Color changes were observed (white of the silica particles → pale yellow → orange → dark brown). The suspension was centrifuged (10,000 rpm or 15,800 g) until all the solid was precipitated. The very dark precipitate was collected and a little portion of the supernatant was added to it so that the suspension was concentrated by about 60 fold (dependent on the batch). The concentrated suspension was sonicated until all the solid precipitate disappeared yielding a dark homogenous-like suspension. The resultant suspension is stable for weeks, without any observed precipitation. In case of precipitation, agitation or short sonication is sufficient to re-disperse the NPs in the suspension.

Characterization of the NCs

Fig. 1 shows the UV-Vis spectrum of the NCs. It can be seen from Fig. 1 that the plasmon band is very narrow which indicates a narrow size distribution31. From the TEM micrograph, Fig. 2, one can see that indeed the size distribution of the silver NPs is narrow with an average size of 15 ± 2 nm. It can also be seen from Fig. 2 that the silica NPs are very small chain-like aggregates. It is of interest to point out that there are no free silver NPs, i.e. all of the silver NPs are deposited as small "islands" on the silica NPs surface.

Fig. 1 UV-Vis spectrum of the SiO2-Ag NCs. The data presented is normalized to the original concentration of the suspension and to 1 cm optical path.

Fig. 2 TEM micrograph of SiO2-Ag NPs.

Conclusions

This study paper presents a synthetic route to creating very stable highly concentrated suspensions of silver NPs with a narrow size distribution. The silver NPs are supported on silica NPs with no bridging molecule. The advantage of not using a bridging molecule is that no undesirable side reactions will occur between the bridging molecule and any reagent present in suspension.

Acknowledgements

This study was supported in part by a grant from the Budgeting and Planning Committee of The Council of Higher Education and the Israel Atomic Energy Commission. D.M. wishes to express his thanks to Mrs. Irene Evens for her ongoing interest and support.

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