The 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 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. This is followed by the formation of a bond between nickel and the bridging
oxygen of the siloxane group. The surface topography and the adhesion of amorphous and polycrystalline nickel nanoparticles on the surface of silica submicrospheres (200–250 nm) were probed by atomic force microscopy (AFM), and reported in another publication [45]. Probe areas down to 50 Â 50 nm in dimension
were scanned on a single submicrosphere immobilized in a thermoplastic resin
bed. Amorphous nickel particles formed by the aggregation of nickel clusters were
soft, experienced poor adhesion to the silica, and caused huge tip-induced particle
movements; but polycrystalline nickel nanoparticles in the size range 20–30 nm
were hard and adhered strongly to the silica. The stronger adhesion of polycrystalline nickel is explained in terms of a silicate-type impurity phase formed in the
nickel–silica interface during crystallization.
Ramesh has extended this study to another magnetic metal, Co. Ferromagnetic
cobalt nanoparticles of a size similar to 10 nm well adhered to hard silica microspheres (225–250 nm) were synthesized by the sonochemical decomposition of a
volatile organic precursor, cobalt nitrosyl carbonyl [Co(CO) 3 NO], in a suspension
of silica in decalin, followed by crystallization of the resultant amorphous product
[46]. Silica spheres carrying ferromagnetic cobalt nanocrystals were deposited on a
single crystalline silicon [100] substrate by spin coating. The two-dimensional organization of the magnetic microspheres on silicon and the adhesion of cobalt
nanoparticles on the surface of microspherical silica have been examined by scanning electron microscopy and atomic force microscopy (AFM), respectively. Twodimensional arrays of hard spherical particles carrying a nanoprobe hold potential
as scanning tip arrays (STA) in force microscopy. Though rigid single molecules of
proteins were originally envisioned as suitable probes, the authors suggested the
possibility that magnetic nanoparticles could also fit the criteria.
Sonochemistry was also used to deposit Ni on amorphous and crystalline alumina [47]. The study concluded: (1) that amorphous alumina can provide a great
number of active sites for reaction with nickel, and can yield a good coating effect
in which most of nickel adheres tightly to the alumina surface, while in the case of
the crystallized alumina as substrate, most of the nickel particles are distributed in
the free space among the alumina submicrospheres. (2) As compared to the unadhered nickel, the adhered nickel has a strong interaction with the alumina core,
which can retard the crystallization of elemental nickel and, conversely, promote
the formation of the spinel phase NiAl 2 O 4 . (3) The first stage of the interaction
between the nickel and the alumina may be through the isolated hydroxy groups
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
122
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. This is followed by the formation of a bond between nickel and the bridging
oxygen of the siloxane group. The surface topography and the adhesion of amorphous and polycrystalline nickel nanoparticles on the surface of silica submicrospheres (200–250 nm) were probed by atomic force microscopy (AFM), and reported in another publication [45]. Probe areas down to 50 Â 50 nm in dimension
were scanned on a single submicrosphere immobilized in a thermoplastic resin
bed. Amorphous nickel particles formed by the aggregation of nickel clusters were
soft, experienced poor adhesion to the silica, and caused huge tip-induced particle
movements; but polycrystalline nickel nanoparticles in the size range 20–30 nm
were hard and adhered strongly to the silica. The stronger adhesion of polycrystalline nickel is explained in terms of a silicate-type impurity phase formed in the
nickel–silica interface during crystallization.
Ramesh has extended this study to another magnetic metal, Co. Ferromagnetic
cobalt nanoparticles of a size similar to 10 nm well adhered to hard silica microspheres (225–250 nm) were synthesized by the sonochemical decomposition of a
volatile organic precursor, cobalt nitrosyl carbonyl [Co(CO) 3 NO], in a suspension
of silica in decalin, followed by crystallization of the resultant amorphous product
[46]. Silica spheres carrying ferromagnetic cobalt nanocrystals were deposited on a
single crystalline silicon [100] substrate by spin coating. The two-dimensional organization of the magnetic microspheres on silicon and the adhesion of cobalt
nanoparticles on the surface of microspherical silica have been examined by scanning electron microscopy and atomic force microscopy (AFM), respectively. Twodimensional arrays of hard spherical particles carrying a nanoprobe hold potential
as scanning tip arrays (STA) in force microscopy. Though rigid single molecules of
proteins were originally envisioned as suitable probes, the authors suggested the
possibility that magnetic nanoparticles could also fit the criteria.
Sonochemistry was also used to deposit Ni on amorphous and crystalline alumina [47]. The study concluded: (1) that amorphous alumina can provide a great
number of active sites for reaction with nickel, and can yield a good coating effect
in which most of nickel adheres tightly to the alumina surface, while in the case of
the crystallized alumina as substrate, most of the nickel particles are distributed in
the free space among the alumina submicrospheres. (2) As compared to the unadhered nickel, the adhered nickel has a strong interaction with the alumina core,
which can retard the crystallization of elemental nickel and, conversely, promote
the formation of the spinel phase NiAl 2 O 4 . (3) The first stage of the interaction
between the nickel and the alumina may be through the isolated hydroxy groups
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
122