SONOCHEMICAL DEPOSITION AND CHARACTERIZATION OF NANOPHASIC AMORPHOUS NICKEL ON SILICA MICROSPHERES

Nanophasic, amorphous clusters of elemental nickel in the size range 1 0-15 nm have been deposited on submicrospheres of amorphous silica by the sonication of a suspension containing nickel tetracarbonyl and sil ica submicrospheres in Decalin by a high-intensity ultrasound radiatio n. The nickel-coated silica spheres have been characterized by X-ray, TEM, SEM/EDXA, BET nitrogen adsorption, dynamic Light scattering, IR s pectroscopy, and magnetic susceptibility measurements. The as-deposite d amorphous clusters transform to a polycrystalline, nanophasic, fee n ickel on heating in an inert atmosphere of argon at a temperature of 4 00 degrees C. Nitrogen adsorption measurements showed that the amorpho us nickel with a high surface area undergoes a loss in surface area on crystallization. Particle size determinations by dynamic light scatte ring suggested the agglomeration between nickel-coated spheres to be l ess than that for uncoated spheres. Scanning and transmission electron microscopic investigations revealed the silica spheres coated with po lycrystalline nickel to be connected by larger aggregates of nickel (3 0-40 nm), forming a neck. As-deposited amorphous nickel showed a super paramagnetic behavior, while the polycrystalline nickel on silica was found to be ferromagnetic. FT-IR investigations showed a significant c hange in the surface silanol composition for the coated and uncoated s ilica. Ultrasound-driven cavitation desorbs the adsorbed water on sili ca, making the free silanols available for reaction with nickel specie s. A positively charged nickel species thus formed could constitute a nucleating site for further aggregation of nickel. An alternate mechan ism for the interaction of nickel clusters with the silica surface is proposed, wherein ultrasound irradiation results in the dehydrative co ndensation 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.