Uniform spherical colloidal palladium particles by reduction of solid complex precursors
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A procedure is described that yields uniform spherical colloidal palladium particles of modal diameters ranging between 0.1 and 0.7 yam. The process consists of two stages. First a monodispersed solid composite precursor is precipitated in solutions containing PdCl 2 , urea, and a nonionic surfactant. The resulting particles are then reduced in aqueous media, either by hydrazine or ascorbic acid in the presence of a protective agent. No change in particle shape occurred during the transformation to pure metals.
I. INTRODUCTION It has been widely recognized that in many applications the properties and the reproducibility of products can be greatly improved if dispersions of uniform particles are used directly or as starting materials.1 For example, optical, magnetic, catalytic, sintering, and other characteristics of powders are greatly affected by the size and shape of finely dispersed matter.2'3 In recent years much progress has been made in the preparation of such "monodispersed" systems and in their uses.4"6 Such procedures yielded many inorganic compounds of simple or mixed composition, as well as pure elements, including metals. Various methods for the preparation of colloidal palladium dispersions in aqueous solutions produced mostly very small spherical or spheroidal particles.7"13 One approach to generate uniform larger metal particles is to first precipitate finely dispersed solid precursors, which may be subsequently reduced to pure metals.14 Depending on the properties of the latter, the transformation can be carried out either in aqueous solutions using an appropriate reducing agent or by contacting the original powders with hydrogen gas at elevated temperatures. Such methods were successfully employed to produce uniform colloidal metals including copper,15 iron,16'17 and ruthenium.18 This work describes the preparation of spherical palladium particles of narrow size distributions with modal radii ranging between 0.1 and 0.7 /mm. The precursor dispersion was obtained by aging aqueous solutions of palladium chloride and urea at elevated temperatures (75-90 °C). The uniformity of the sogenerated dispersions was enhanced by the addition of a nonionic surfactant. The resulting composite solids were then reduced in aqueous dispersions either by hydrazine or ascorbic acid. The aggregation of the metallic par2404
http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 9, Sep 1994
Downloaded: 25 Mar 2015
ticles during the transformation process could be prevented by the addition of polyvinylpyrrolidone. II. EXPERIMENTAL Palladium chloride (PdCl2, Alfa, 99.9%), urea (Mallinckrodt, analytical reagent), Triton X-405 (Rohm & Haas), hydrazine (Fisher, reagent grade), L-ascorbic acid (Fisher, certified ACS), and polyvinylpyrrolidone (PVP, Aldrich, special grade) were used as obtained. The stock solution of PdCl2 was prepared by addition of a small amount of HC1 (1 cm 3 of cone. HC1 per 100 cm 3 ) in order to accelerate the dissolution of the salt. All solutions were filtered through 0.2 /mm Nuclepore membranes
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