Preparing Monodisperse Metal Powders in Micrometer and Submicrometer Sizes by the Polyol Process

Abstract

One of the newer tendencies in materials science has been to tailor-make classical products (long associated with old applications) with controlled properties for special uses, especially in high technology. Preparing dispersed systems in which all particles have nearly uniform size (monodisperse solids) is a typical example. This goal can be achieved in some cases through cleverly controlled particle growth from a liquid medium. Examples of such preparations include gold colloids prepared by Zsigmondy and later by Turkevich et al., sulfur sols obtained by LaMer, metal oxides and hydrous oxides prepared by Matijević et al., silica, etc. These dispersions have been used either to check theories of colloid science, or to a lesser extent, for industrial purposes. In the case of fine metal particles, a uniform size distribution associated with a low degree of agglomeration, and sometimes the spherical shape, appear as particularly convenient characteristics for certain applications. The production of conductive inks or pastes for electronic materials and for the preparation of conductive paints are particularly good examples.In so-called thick film technology, conductive inks and pastes are screen printed on a ceramic substrate in order to form, after firing, a conductive film with a thickness less than 10 μm. This technique is, for instance, used to form the network in hybrid integrated circuits or the internal electrodes of multilayer ceramic capacitors.Metallic powders in thick film compositions are usually precious metals (Au, Ag, Pt, Pd), their mixtures, or alloys. Cheaper metals such as copper or nickel are tested and may be potential substitutes for precious metals in different specific applications. Powders for thick film composition are mainly obtained through chemical precipitation from aqueous or organic solutions, which yield high purity powders. Modification of precipitation parameters (such as the nature and the concentration of the starting metallic compound and of the reducing agent, reaction temperature, viscosity of the medium) and the addition of additives and surfactants, can often be used to control particle size and agglomeration.Over the past few years, we have developed a new process for preparing finely divided metal powders of easily reducible metals (such as precious metals and copper) or less reducible metals (such as cobalt, nickel, cadmium, or lead) by precipitation in liquid polyols. This reaction will be used as an example in order to discuss the mechanism of formation of uniform micrometer and submicrometer size metal particles by precipitation reactions.

Faceting of Nanocrystals during Chemical Transformation: From Solid Silver Spheres to Hollow Gold Octahedra

“…20 We also notice that the nanocrystals synthesized in this case have average sizes much smaller than those produced using the regular polyol process which typically uses ethylene glycol as solvent as well as reducing agent, and polyvinylpyrrolidone (PVP) as surfactant. 21,22 This suggests stronger attachment of oleylamine than PVP to the silver nanocrystal surface.Performing a replacement reaction on Ag nanocrystals with dissolved Au 3+ leads to formation of hollow gold nanocrystals with slightly broadened size distribution, as shown in Fig 1B. Contrast is sensitive to mass-thickness in conventional TEM images; consequently, hollow particles appear lighter at their center in images. Energy Dispersive X-ray analysis (EDX) on clusters of Au hollow nanocrystals typically does not give rise to a silver signal, implying that the replacement reaction proceeds essentially to completion and that Ag is present at <5%, if at all, in these nanocrystals.…”

“…20 We also notice that the nanocrystals synthesized in this case have average sizes much smaller than those produced using the regular polyol process which typically uses ethylene glycol as solvent as well as reducing agent, and polyvinylpyrrolidone (PVP) as surfactant. 21,22 This suggests stronger attachment of oleylamine than PVP to the silver nanocrystal surface.…”

Interactive effect of agitation rate and oxidative etching on growth mechanisms of silver nanowires during polyol process

“…[31] In the case of silver nanostructures, EG, AgNO 3 and PVP serve as the polyol, salt precursor and polymeric capping agent, respectively. In a typical synthesis, 10 mL of EG was heated at 160 C for one hour.…”

Synthesis and magnetorheology of suspensions of submicron-sized cobalt particles with tunable particle size

“…Reduction of metallic ions in a liquid polyol has been extensively used for the preparation of spherical particles of magnetic and non-magnetic materials (Fiévet 2000, Fiévet et al 1989, Sun et al 2005, Viau et al 1996. We used this method to synthesize spherical cobalt particles with different average particle diameter.…”