reducing the thickness of the coated layer, as well as the amount of silver found
between silica particles rather than on their surface.
Two questions were specifically investigated in these projects. First, the mechanism by which the deposited particles reach and are anchored to the host surface.
Secondly, what is the mechanism by which the nanoparticles adhere to the surface,
and are not removed, despite the severe stirring caused by the ultrasonic waves?
Our answer to the first question involves the formation of shock waves and microjets created as after-effects to the collapse of the bubble [1a]. These effects always
result when a bubble collapses near a solid surface. These jets, according to our
interpretation, push the ultrafine particles towards the solid sphere at very high
speeds, and are also known to cause the melting of colliding bodies and their sintering [52]. Once these nanoparticles collide with the surface, chemical bonds or
weak interactions keep them on the surface in most cases. AFM studies have
demonstrated [45] that the interaction becomes much stronger when the amorphous deposited particles are annealed at their crystallization temperature. The
cantilever could scratch and move the particles only for the as-prepared products.
Once they were annealed they were unaffected by the cantilever pushing power.
Ulman and coworkers have described in a few papers [53–55] a method by
which sonochemically prepared nanoparticles are deposited on a flat surface, usually a silicon wafer. They describe their approach as a ‘‘plug and play’’, in which
sonochemically synthesized amorphous Fe 2 O 3 nanoparticles are incorporated onto
device-quality Si wafers. After annealing the amorphous Fe 2 O 3 nanoparticles they
change their properties from super-paramagnetic to soft ferromagnetic. The samples exhibit multiple light emissions with wavelengths that are crucial for optical
fiber communications.
Later, they demonstrate [55] that sonochemically synthesized Fe 2 O 3 nanoparticles are introduced onto Si from an alcohol suspension. On annealing this sample in ultra-high vacuum, the oxygen atoms change the bonding partner from Fe
to Si and desorb as SiO at 750
C. This results in the formation of nanoparticles of
Fe on the surface and exhibits ferromagnetic behavior. Deposition of a thin layer
(2 nm) of Si onto the sample containing the metallic Fe nanoparticles followed by
annealing at 560
C leads to optically active Si.
Papadimitrakopoulos and coworkers reported on transparent Si/SiOx nanocomposite films, spontaneously adsorbed on glass or quartz substrates from their colloidal suspensions via a sonication-assisted oxidation process [56]. Individual
nanosilicon particles (ca. 20 nm) appear to cover a significant part of the substrate
along with agglomerates of the order of 50–80 nm in thickness. Kinetic studies
indicate a rapid initial adsorption that slows down significantly after 3 h.
6.1.1.5 Sonochemical Synthesis of a Polymer-Metal Composite
The use of ultrasound radiation for polymerizing various monomers was reviewed
in [1a]. Here we will discuss how ultrasound waves have been used successfully
been to embed ultrafine metallic particles in a polymeric matrix. The first report
was by Wizel and coworkers [57]. They used ultrasound radiation to prepare a
composite material made of polymethylacrylate and amorphous iron nanoparticles.
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
124
between silica particles rather than on their surface.
Two questions were specifically investigated in these projects. First, the mechanism by which the deposited particles reach and are anchored to the host surface.
Secondly, what is the mechanism by which the nanoparticles adhere to the surface,
and are not removed, despite the severe stirring caused by the ultrasonic waves?
Our answer to the first question involves the formation of shock waves and microjets created as after-effects to the collapse of the bubble [1a]. These effects always
result when a bubble collapses near a solid surface. These jets, according to our
interpretation, push the ultrafine particles towards the solid sphere at very high
speeds, and are also known to cause the melting of colliding bodies and their sintering [52]. Once these nanoparticles collide with the surface, chemical bonds or
weak interactions keep them on the surface in most cases. AFM studies have
demonstrated [45] that the interaction becomes much stronger when the amorphous deposited particles are annealed at their crystallization temperature. The
cantilever could scratch and move the particles only for the as-prepared products.
Once they were annealed they were unaffected by the cantilever pushing power.
Ulman and coworkers have described in a few papers [53–55] a method by
which sonochemically prepared nanoparticles are deposited on a flat surface, usually a silicon wafer. They describe their approach as a ‘‘plug and play’’, in which
sonochemically synthesized amorphous Fe 2 O 3 nanoparticles are incorporated onto
device-quality Si wafers. After annealing the amorphous Fe 2 O 3 nanoparticles they
change their properties from super-paramagnetic to soft ferromagnetic. The samples exhibit multiple light emissions with wavelengths that are crucial for optical
fiber communications.
Later, they demonstrate [55] that sonochemically synthesized Fe 2 O 3 nanoparticles are introduced onto Si from an alcohol suspension. On annealing this sample in ultra-high vacuum, the oxygen atoms change the bonding partner from Fe
to Si and desorb as SiO at 750
C. This results in the formation of nanoparticles of
Fe on the surface and exhibits ferromagnetic behavior. Deposition of a thin layer
(2 nm) of Si onto the sample containing the metallic Fe nanoparticles followed by
annealing at 560
C leads to optically active Si.
Papadimitrakopoulos and coworkers reported on transparent Si/SiOx nanocomposite films, spontaneously adsorbed on glass or quartz substrates from their colloidal suspensions via a sonication-assisted oxidation process [56]. Individual
nanosilicon particles (ca. 20 nm) appear to cover a significant part of the substrate
along with agglomerates of the order of 50–80 nm in thickness. Kinetic studies
indicate a rapid initial adsorption that slows down significantly after 3 h.
6.1.1.5 Sonochemical Synthesis of a Polymer-Metal Composite
The use of ultrasound radiation for polymerizing various monomers was reviewed
in [1a]. Here we will discuss how ultrasound waves have been used successfully
been to embed ultrafine metallic particles in a polymeric matrix. The first report
was by Wizel and coworkers [57]. They used ultrasound radiation to prepare a
composite material made of polymethylacrylate and amorphous iron nanoparticles.
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
124