tance of the reaction vessel from the oscillator. For example, the rates of reduction
under several atmospheres were in the order: CH 4 ¼ CO 2 < N 2 < Ne < He <
Ar < Kr, where no reduction proceeded under the CH 4 and CO 2 atmospheres. It
was clearly seen that the rates of reduction were influenced by the cavitation phenomenon. Upon irradiation, colloidal gold particles having a surface plasmon absorption were formed, although in the absence of any stabilizers for the gold particles. It was found by TEM observation that the average size of the formed gold
particles varied from 30 to 120 nm dependent upon the irradiation parameters.
The size of the gold particles correlated to the initial rate of gold (III) reduction; the
higher the rate of reduction, the smaller the particles.
The magnetic metals were also prepared by a method [25] based on the rapid
expansion of supercritical fluid solutions (RESS) coupled with chemical reduction
to produce nickel, cobalt, iron, and iron oxide nanoparticles of reasonably narrow
size distribution. Under the protection of a polymer stabilization agent, the largely
amorphous metal nanoparticles form stable suspensions in room-temperature solvents.
A nonmetallic element, silicon, was prepared sonochemically by reducing tetraethyl orthosilicate (TEOS) with a colloidal solution of sodium. The product was
obtained as 2–5 nm sized, highly aggregated particles. The silicon exhibited a luminescence similar to that of porous silicon. This procedure is suggested as a
general sonochemical reduction leading to the formation of metallic nanoparticles
[26].
6.1.1.2 Sonochemical Synthesis of Metallic Colloids
Although a few of the above-mentioned metallic nanoparticles were formed as
colloidal solutions, and since this review is centered on their synthesis, they were
included in the previous section. This section is devoted to the unique properties
of metallic colloidal solutions prepared sonochemically. A recent survey by Grieser
deals with the sonochemical formation of metallic colloids [27]. In the abovementioned examples noble metal nanoparticles (e.g., Au, Pd, Ag) are obtained by
sonicating aqueous solutions of the corresponding salts in the presence of a surfactant, which largely stabilizes the naked colloid. Likewise, a colloidal solution of
metallic iron particles (8 nm, average size) has been obtained by sonolysis of
Fe(CO) 5 in the presence of oleic acid [28]. Smaller particles are obtained with poly(vinylpyrrolidine), although in both cases the iron was amorphous and exhibited
high magnetism. The sonochemical preparation of magnetic fluids has also been
described by other authors [29, 30]. The systems include: (i) a cobalt colloidal solution in decalin stabilized by oleic acid, (ii) a colloidal dispersion of amorphous
metallic iron in a polymeric matrix, and (iii) a Fe 2 O 3 colloidal solution in hexadecane stabilized by oleic acid. For the cobalt colloidal solution uniform acicularshaped particles were further obtained by an aging process in air (Figure 6.1). The
figure demonstrates the conversion of 5–10 nm sized Co particles in a 1 mm sized
acicular particle after 1 month of aging under ambient conditions.
Elongated copper nanoparticles were prepared by sonicating the abovementioned precursor, copper hydrazine carboxylate, in an aqueous solution con6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
118
under several atmospheres were in the order: CH 4 ¼ CO 2 < N 2 < Ne < He <
Ar < Kr, where no reduction proceeded under the CH 4 and CO 2 atmospheres. It
was clearly seen that the rates of reduction were influenced by the cavitation phenomenon. Upon irradiation, colloidal gold particles having a surface plasmon absorption were formed, although in the absence of any stabilizers for the gold particles. It was found by TEM observation that the average size of the formed gold
particles varied from 30 to 120 nm dependent upon the irradiation parameters.
The size of the gold particles correlated to the initial rate of gold (III) reduction; the
higher the rate of reduction, the smaller the particles.
The magnetic metals were also prepared by a method [25] based on the rapid
expansion of supercritical fluid solutions (RESS) coupled with chemical reduction
to produce nickel, cobalt, iron, and iron oxide nanoparticles of reasonably narrow
size distribution. Under the protection of a polymer stabilization agent, the largely
amorphous metal nanoparticles form stable suspensions in room-temperature solvents.
A nonmetallic element, silicon, was prepared sonochemically by reducing tetraethyl orthosilicate (TEOS) with a colloidal solution of sodium. The product was
obtained as 2–5 nm sized, highly aggregated particles. The silicon exhibited a luminescence similar to that of porous silicon. This procedure is suggested as a
general sonochemical reduction leading to the formation of metallic nanoparticles
[26].
6.1.1.2 Sonochemical Synthesis of Metallic Colloids
Although a few of the above-mentioned metallic nanoparticles were formed as
colloidal solutions, and since this review is centered on their synthesis, they were
included in the previous section. This section is devoted to the unique properties
of metallic colloidal solutions prepared sonochemically. A recent survey by Grieser
deals with the sonochemical formation of metallic colloids [27]. In the abovementioned examples noble metal nanoparticles (e.g., Au, Pd, Ag) are obtained by
sonicating aqueous solutions of the corresponding salts in the presence of a surfactant, which largely stabilizes the naked colloid. Likewise, a colloidal solution of
metallic iron particles (8 nm, average size) has been obtained by sonolysis of
Fe(CO) 5 in the presence of oleic acid [28]. Smaller particles are obtained with poly(vinylpyrrolidine), although in both cases the iron was amorphous and exhibited
high magnetism. The sonochemical preparation of magnetic fluids has also been
described by other authors [29, 30]. The systems include: (i) a cobalt colloidal solution in decalin stabilized by oleic acid, (ii) a colloidal dispersion of amorphous
metallic iron in a polymeric matrix, and (iii) a Fe 2 O 3 colloidal solution in hexadecane stabilized by oleic acid. For the cobalt colloidal solution uniform acicularshaped particles were further obtained by an aging process in air (Figure 6.1). The
figure demonstrates the conversion of 5–10 nm sized Co particles in a 1 mm sized
acicular particle after 1 month of aging under ambient conditions.
Elongated copper nanoparticles were prepared by sonicating the abovementioned precursor, copper hydrazine carboxylate, in an aqueous solution con6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
118