Preparation of silver spheres by aggregation of nanosize subunits
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Dispersed uniform spherical silver particles were prepared in the absence of a protective colloid by rapidly mixing concentrated isoascorbic acid and silver-polyamine complex solutions. By varying the nature of the amine, temperature, concentration of reactants, silver/amine molar ratio, and the nature of the silver salt, it was possible to tailor the size of the resulting metallic particles in a wide range (80 nm to 1.3 m). The silver spheres were formed by aggregation of nanosize subunits, the presence of which was detected by both electron microscopy and x-ray diffraction. Due to its simplicity, high metal concentration, and the absence of polymeric dispersants, the described method represents an advantageous route to manufacture cost-effectively dispersed uniform silver particles for electronic applications.
I. INTRODUCTION
While the applications of silver nanoparticles in surface-enhanced Raman spectroscopy, medicine, and biology are mostly responsible for the strong recent interest in this metal, larger colloidal silver particles of submicrometer and micrometer size are of even greater practical importance. Indeed, large quantities of submicrometer and micrometer-sized particles with various shapes are used to build conductive elements in plasma display panels,1–3 multilayer ceramic capacitors,4 solar cells,5 printed circuit boards,6 and many other thick film components7 incorporated in most electronic devices surrounding us. The technological progress in these applications depends increasingly on the ability to control the size, shape, and internal structure of the particles. Highly dispersed uniform spherical silver particles are particularly important for the electronic industry, as they provide distinct advantages. For example, in the manufacturing of plasma display panels, silver spheres with smooth surface allow a better photolithographic patterning.8 Furthermore, the superior packing of such particles favors the formation of compact “green” structures that yield more continuous conductive sintered layers.9 Many silver powders presently used in electronics are generated by processes using high-molecular-weight polymers as dispersants and contain residual organics, which can interfere with their sintering. Therefore, the methods capable of generating well-dispersed, uniform, large
spherical silver particles without polymers as protective colloids are particularly attractive. Fine silver particles have been prepared by various methods including the reduction of silver salts in solutions9–13 or reverse micelles systems,14 photoreduction,15 and thermolysis.16 The precipitation in homogeneous solutions is by far the most versatile approach due to the broad range of solvents available and the large variety of reductants, dispersants, and complexing agents.17–19 Still, there are very few methods capable of generating large silver spheres in the absence of dispersants, and those already reported20 use low metal concentration and undesirable chemicals. In this study, we show that complexes formed by silver w
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