Sonochemical deposition and characterization of nanophasic amorphous nickel on silica microspheres

Nanophasic, amorphous clusters of elemental nickel in the size range 10-15 nm have been deposited on submicrospheres of amorphous silica by the sonication of a suspension containing nickel tetracarbonyl and silica submicrospheres in Decalin by a high-intensity ultrasound radiation. The nickel-coated silica spheres have been characterized by X-ray, TEM, SEM/EDXA, BET nitrogen adsorption, dynamic Light scattering, IR spectroscopy, and magnetic susceptibility measurements. The as-deposited amorphous clusters transform to a polycrystalline, nanophasic, fee nickel on heating in an inert atmosphere of argon at a temperature of 400 degrees C. Nitrogen adsorption measurements showed that the amorphous nickel with a high surface area undergoes a loss in surface area on crystallization. Particle size determinations by dynamic light scattering suggested the agglomeration between nickel-coated spheres to be less than that for uncoated spheres. Scanning and transmission electron microscopic investigations revealed the silica spheres coated with polycrystalline nickel to be connected by larger aggregates of nickel (30-40 nm), forming a neck. As-deposited amorphous nickel showed a superparamagnetic behavior, while the polycrystalline nickel on silica was found to be ferromagnetic. FT-IR investigations showed a significant change in the surface silanol composition for the coated and uncoated silica. Ultrasound-driven cavitation desorbs the adsorbed water on silica, making the free silanols available for reaction with nickel species. A positively charged nickel species thus formed could constitute a nucleating site for further aggregation of nickel. An alternate mechanism for the interaction of nickel clusters with the silica surface is proposed, wherein ultrasound irradiation results in the dehydrative condensation of hydrogen-bonded silanols to form siloxane Links followed by the formation of a bond between nickel and the bridging oxygen of the siloxane links.