posited from an aqueous solution of AgNO 3 using NTA as a complex agent. The
shape of the Ag nanoparticles could be modified as a function of the concentration
of AgNO 3 and NTA in the deposition solutions. In the second paper [152], X-ray
amorphous silver nanoparticles were prepared by a pulse sonoelectrochemical
method from an aqueous solution of AgBr in the presence of gelatin. Finally, it was
shown that PbSe [153] nanoparticles, ca. 12 nm, could be prepared by a pulse sonoelectrochemical technique from an aqueous solution of sodium selenosulfate
and lead acetate. Most recently, Zhu et al. [154] utilized a sonoelectrochemical
route for the preparation of uniform silver nanowires of a single crystalline nature.
Finally, in a very recent publication, silver nanoparticles were used as a model case
to study the processes involved in sonoelectrochemical synthesis of nanoparticles
[155]. The authors propose that sonoelectrochemical synthesis is highly affected by
the formation of a suspensive electrode. This new model, the suspensive electrode,
is based on the concept that the nanoparticles suspended in solution gain the
sonoelectrode potential and thus act as part of the electrode. That is to say, during
sonoelectrochemical synthesis, a suspension of charged nanosized particles is
formed in solution and acts as part of the sonoelectrode.
Hodes et al. [156] demonstrated that the sonoelectrochemical technique could
be used to synthesize closed fullerene-like structures of MoS 2 at room temperature
by electrodeposition from a thiomolybdate solution. It should by noted that either
electrodeposition or ultrasonic irradiation alone results in X-ray amorphous MoS 2
products, but the combination of both gives well-crystallized, closed structures of
MoS 2 and MoS 2 nanotubes. A mechanistic study to explain the formation of the
closed curved MoS 2 structures shows that in the first step amorphous MoS 2 is
formed by electrodeposition onto the sonic probe cathode. At this point the deposit
could be spheroidal or planar. Crystallization of the amorphous MoS 2 , which normally requires high temperatures occurs in the collapse of the cavitation bubbles.
The formation of MoS 2 nanotubes occurs if the bubble collapse occurs at the
electrodeposit that is still on the electrode surface. In this case, the effect of the
bubble collapse is asymmetric, leading to an asymmetric-shaped product, namely
the nanotube.
In conclusion, the combination of electrochemical and sonic processes provides
many experimental variables which should allow control of particle size and shape,
and will probably be applicable to the formation of closed structures of other
layered compounds that can be prepared by electrochemical (and quite likely also
chemical) techniques.
6.3
Microwave Heating
Microwave heating (MWH) is the second method for the fabrication of nanomaterials that will be discussed in this chapter. Microwaves are electromagnetic
radiation, whose wavelengths lie in the range 1 mm to 1 m (frequency range 0.3 to
300 GHz). A large part of the microwave spectrum is used for communication
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
152
shape of the Ag nanoparticles could be modified as a function of the concentration
of AgNO 3 and NTA in the deposition solutions. In the second paper [152], X-ray
amorphous silver nanoparticles were prepared by a pulse sonoelectrochemical
method from an aqueous solution of AgBr in the presence of gelatin. Finally, it was
shown that PbSe [153] nanoparticles, ca. 12 nm, could be prepared by a pulse sonoelectrochemical technique from an aqueous solution of sodium selenosulfate
and lead acetate. Most recently, Zhu et al. [154] utilized a sonoelectrochemical
route for the preparation of uniform silver nanowires of a single crystalline nature.
Finally, in a very recent publication, silver nanoparticles were used as a model case
to study the processes involved in sonoelectrochemical synthesis of nanoparticles
[155]. The authors propose that sonoelectrochemical synthesis is highly affected by
the formation of a suspensive electrode. This new model, the suspensive electrode,
is based on the concept that the nanoparticles suspended in solution gain the
sonoelectrode potential and thus act as part of the electrode. That is to say, during
sonoelectrochemical synthesis, a suspension of charged nanosized particles is
formed in solution and acts as part of the sonoelectrode.
Hodes et al. [156] demonstrated that the sonoelectrochemical technique could
be used to synthesize closed fullerene-like structures of MoS 2 at room temperature
by electrodeposition from a thiomolybdate solution. It should by noted that either
electrodeposition or ultrasonic irradiation alone results in X-ray amorphous MoS 2
products, but the combination of both gives well-crystallized, closed structures of
MoS 2 and MoS 2 nanotubes. A mechanistic study to explain the formation of the
closed curved MoS 2 structures shows that in the first step amorphous MoS 2 is
formed by electrodeposition onto the sonic probe cathode. At this point the deposit
could be spheroidal or planar. Crystallization of the amorphous MoS 2 , which normally requires high temperatures occurs in the collapse of the cavitation bubbles.
The formation of MoS 2 nanotubes occurs if the bubble collapse occurs at the
electrodeposit that is still on the electrode surface. In this case, the effect of the
bubble collapse is asymmetric, leading to an asymmetric-shaped product, namely
the nanotube.
In conclusion, the combination of electrochemical and sonic processes provides
many experimental variables which should allow control of particle size and shape,
and will probably be applicable to the formation of closed structures of other
layered compounds that can be prepared by electrochemical (and quite likely also
chemical) techniques.
6.3
Microwave Heating
Microwave heating (MWH) is the second method for the fabrication of nanomaterials that will be discussed in this chapter. Microwaves are electromagnetic
radiation, whose wavelengths lie in the range 1 mm to 1 m (frequency range 0.3 to
300 GHz). A large part of the microwave spectrum is used for communication
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
152