organic additives with OH radicals and H atoms, and (iii) reduction by radicals
formed from pyrolysis of the additives at the interfacial region between cavitation
bubbles and the bulk solution. The reduction of Ag (I) and Pt (II) mainly proceeds
through reaction pathway (ii). For Pd (II) and Au (III), the reductions mainly proceed through reaction pathway (iii). The reduction of Rh (III) was not achieved
under the same conditions; however, on addition of sodium formate, reduction
occurred and the preparation of Rh particles was successful. In another study [18]
platinum nanoparticles were prepared sonochemically in an aqueous system in the
presence of a surfactant (sodium dodecyl sulfate, SDS). The particles were stable,
homogeneously spherical, and relatively monodispersed with an average diameter
of 2.6 nm. Reducing species generated near and/or in the hot bubbles would react
with the PtCl 4
À2 complexes to form the platinum nanoparticles. Three kinds of
reducing species were proposed. Amorphous silver nanoparticles of 20 nm size
have been prepared [19] by the sonochemical reduction of an aqueous silver nitrate
solution in an atmosphere of argon–hydrogen. The sonochemical reduction occurs
through the generation of hydrogen radicals during the sonication process.
The role of the surfactant was further studied in sonochemical reduction [20, 21]
processes of Pt (IV) ions in water. It was investigated in the presence of various
kinds of surfactants such as sodium dodecylsulfate (SDS) and sodium dodecylbenzenesulfonate (DBS) as anionic surfactants, polyethylene glycol monostearate
(PEG-MS) as non-ionic, and dodecyltrimethylammonium chloride (DTAC) and
bromide (DTAB) as cationic surfactants. An improved colorimetric determination
reveals that Pt (IV) ion is reduced to zero-valent metal in two steps: step (1) Pt (IV)
ion to Pt (II) ion, and step (2) Pt (II) ion to Pt (0), and after the completion of step
(1), step (2) sets in.
Another Japanese group [22] reported on the preparation of Pd and Pt by sonochemical reduction of solutions containing H 2 PtCl 6 or K 2 PdCl 4 . The effect of atmospheric gas on the particle size distribution was investigated. Average diameters
and standard deviations of the Pd particles prepared under Ar (Pd/Ar) and N 2
(Pd/N 2 ) were found to be 3.6 G 0.7 nm (Pd/Ar) and 2.0 G 0.3 nm (Pd/N 2 ). A
smaller and sharper distribution of the particle size was observed for the Pd particles formed under a N 2 atmosphere. In the case of Pt, a smaller and sharper distribution of the particle size was observed for the particles prepared under a Xe
atmosphere. The importance of Xe can be explained in terms of a hot-spot temperature created by acoustic cavitation. Nanosized gold particles were also prepared
from gold (III) (tetrachloroaurate (III)) [23]. The Au (III) was reduced in an aqueous solution containing only a small amount of 2-propanol to form colloidal gold
nanoparticles. The rates of gold (III) reduction and the sizes of the gold particles
formed could be sonochemically controlled by controlling the irradiation parameters such as the temperature of the solution, the intensity of the ultrasound, and
the positioning of the reactor. The size of the gold particles depended strongly on
the rate of gold (III) reduction, suggesting that this rate affects the initial nucleation of the gold particles. A similar study was published recently by Okitsu et al.
[24]. The rates of gold (III) reduction were strongly dependent on the atmosphere,
the temperature of the bulk solution, the intensity of the ultrasound, and the dis6.1 Sonochemistry 117
formed from pyrolysis of the additives at the interfacial region between cavitation
bubbles and the bulk solution. The reduction of Ag (I) and Pt (II) mainly proceeds
through reaction pathway (ii). For Pd (II) and Au (III), the reductions mainly proceed through reaction pathway (iii). The reduction of Rh (III) was not achieved
under the same conditions; however, on addition of sodium formate, reduction
occurred and the preparation of Rh particles was successful. In another study [18]
platinum nanoparticles were prepared sonochemically in an aqueous system in the
presence of a surfactant (sodium dodecyl sulfate, SDS). The particles were stable,
homogeneously spherical, and relatively monodispersed with an average diameter
of 2.6 nm. Reducing species generated near and/or in the hot bubbles would react
with the PtCl 4
À2 complexes to form the platinum nanoparticles. Three kinds of
reducing species were proposed. Amorphous silver nanoparticles of 20 nm size
have been prepared [19] by the sonochemical reduction of an aqueous silver nitrate
solution in an atmosphere of argon–hydrogen. The sonochemical reduction occurs
through the generation of hydrogen radicals during the sonication process.
The role of the surfactant was further studied in sonochemical reduction [20, 21]
processes of Pt (IV) ions in water. It was investigated in the presence of various
kinds of surfactants such as sodium dodecylsulfate (SDS) and sodium dodecylbenzenesulfonate (DBS) as anionic surfactants, polyethylene glycol monostearate
(PEG-MS) as non-ionic, and dodecyltrimethylammonium chloride (DTAC) and
bromide (DTAB) as cationic surfactants. An improved colorimetric determination
reveals that Pt (IV) ion is reduced to zero-valent metal in two steps: step (1) Pt (IV)
ion to Pt (II) ion, and step (2) Pt (II) ion to Pt (0), and after the completion of step
(1), step (2) sets in.
Another Japanese group [22] reported on the preparation of Pd and Pt by sonochemical reduction of solutions containing H 2 PtCl 6 or K 2 PdCl 4 . The effect of atmospheric gas on the particle size distribution was investigated. Average diameters
and standard deviations of the Pd particles prepared under Ar (Pd/Ar) and N 2
(Pd/N 2 ) were found to be 3.6 G 0.7 nm (Pd/Ar) and 2.0 G 0.3 nm (Pd/N 2 ). A
smaller and sharper distribution of the particle size was observed for the Pd particles formed under a N 2 atmosphere. In the case of Pt, a smaller and sharper distribution of the particle size was observed for the particles prepared under a Xe
atmosphere. The importance of Xe can be explained in terms of a hot-spot temperature created by acoustic cavitation. Nanosized gold particles were also prepared
from gold (III) (tetrachloroaurate (III)) [23]. The Au (III) was reduced in an aqueous solution containing only a small amount of 2-propanol to form colloidal gold
nanoparticles. The rates of gold (III) reduction and the sizes of the gold particles
formed could be sonochemically controlled by controlling the irradiation parameters such as the temperature of the solution, the intensity of the ultrasound, and
the positioning of the reactor. The size of the gold particles depended strongly on
the rate of gold (III) reduction, suggesting that this rate affects the initial nucleation of the gold particles. A similar study was published recently by Okitsu et al.
[24]. The rates of gold (III) reduction were strongly dependent on the atmosphere,
the temperature of the bulk solution, the intensity of the ultrasound, and the dis6.1 Sonochemistry 117