Synthesis, Characterization, and Utilization of Single Crystalline Nanoparticles of Silver
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ABSTRACT This paper describes a procedure based on the Tollens' process for preparing crystalline nanoparticles of silver with well-controlled, uniform sizes. The starting reagents are similar to those commonly used for electroless deposition of silver and are commercially available from the Peacock Laboratories (Philadelphia, PA). Only under appropriate conditions, mixing of these reagents was able to generate stable dispersions of silver colloids, rather than thin films of silver coated on surfaces of objects immersed in the solution (including the inner surface of the container). We have demonstrated the capability and feasibility of this method by forming stable dispersions of highly monodispersed, single crystalline colloids of silver with dimensions in the range of 20-50 nm.
INTRODUCTION Nanoparticles have been intensively investigated because of their unique electronic and optical properties that are substantially different from those of the bulk materials [1]. A lot of
efforts have been, in particular, devoted to the synthesis and characterization of stable colloidal dispersions of silver, gold, and other noble metals [2]. A rich variety of methods have been developed in the past two decades for preparing these colloidal particles; notable examples include condensation from the vapor phase, chemical reduction or photoreduction in solutions or reverse micelles, chemical vapor deposition on solid substrates, and thermal decomposition in solvents or polymer matrixes [3]. The solutions of these colloidal particles usually display a very intense color, which can be attributed to the collective oscillation of conduction electrons induced by an electromagnetic field. It has been shown that the size, morphology, stability, and properties (chemical or physical) of these colloidal particles have a strong dependence on the specificity of the preparation method and the experimental conditions [4]. Here we would like to describe a simple and convenient method for preparing crystalline nanoparticles of silver with uniform sizes and well-defined morphologies. The fundamental reaction step of this approach is the Tollens' reaction: Ag(NH 3 )2+(aq) + RCHO (aq)
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1- Ag (s) + RCOO- (aq)
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Mat. Res. Soc. Symp. Proc. Vol. 581 © 2000 Materials Research Society
The final colloidal dispersions were stable in air, for at least 4-5 months. To demonstrate the versatility of this method, silver nanoparticles were also prepared (using a similar procedure) as coatings on mesoscale silica beads. Previous studies have indicated that such hybrid materials may find a broad range of uses in developing nonlinear optical devices, static electronic shielding coatings, and catalysts [5].
EXPERIMENTAL A commercial kit (HE-300, Peacock Laboratories) that is commonly used for electroless deposition of silver films was employed to produce silver nanoparticles in the form of a stable aqueous dispersion. This kit consists of three separated solutions: (A) silver solution (silver nitrate), (B) activator solution (sodium hydroxide), and (C) re
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