realized that iron oxide was the most popular material to be synthesized by this
method.
The first report was by Komarneni and coworkers [179] using a microwavehydrothermal process to catalyze the synthesis of crystalline oxides such as TiO 2 ,
ZrO 2 and Fe 2 O 3 , and binary oxides such as KNbO 3 and BaTiO 3 . The importance of
this work was that this technique led to fine powders of these materials. The effect
of different parameters, such as concentration of chemical species, time and temperature, on the crystallization kinetics of the above phases has been investigated
under microwave-hydrothermal conditions using microwaves of 2.45 GHz frequency.
The second work was by Dong et al. [180] who report on the preparation of
submicron uniform a-Fe 2 O 3 by microwave-induced hydrolysis of ferric salts with
concurrent dissociation of urea. Although it was not conducted in a domestic MW
oven, it is still on our list of references. A mixed solution of FeC 3 and CO(NH 2 ) 2
was placed in an Erlenmeyer flask equipped with a reflux condenser. This flask was
inserted into a cylindrical resonant cavity. After microwave irradiation (2.4 GHZ,
500 W) for a certain period of time, the flask was moved to a homothermal water
bath (at 94
C) to be aged. Uniform particles of approximately spherical shape with
mean diameter 75 nm were obtained. An almost identical paper was published
a little later by Li and Wei [181]. Fe(NO 3 ) 3 was the iron source instead of the chloride [180]. They conducted the reaction at different pH values. In this way they
obtained a larger variety of shapes, including acicular cubic and spherical particles,
all in the nanometer range.
Palchik [182] obtained nanosized amorphous iron oxide (Fe 2 O 3 ) by the pyrolysis of iron pentacarbonyl, Fe(CO) 5 , in a modified domestic microwave oven in
refluxing chlorobenzene as solvent under air. The reaction time was 20 min. Separate particles of iron oxide, 2–3 nm in diameter, were obtained together with aggregated spheres with a diameter of 25–40 nm. Differential scanning calorimetry
measurements showed an amorphous/crystalline phase transition at about 250
C.
A study of microwave effects on the formation of nanoparticles in the hydrolysis
reaction of FeCl 3 with NaH 2 PO 4 to get spindle-type colloidal hematite particles
under microwave radiation, was reported by Han and coworkers [183]. They found
that the reaction rate increased greatly, and the reaction conditions, for example,
the acidity of the solution, concentration ratio of the components, and the microwave radiation time, had important effects on the nanoparticle formation and their
morphology. They discussed the roles of microwaves and the concentration of
H 2 PO 4
À during the hydrolysis process.
Nanometer-sized quasicubic and spindle a-Fe 2 O 3 particles were prepared by microwave heating from Fe
þ3 salt solutions [184]. The obtained a-Fe 2 O 3 particles
formed by MWH have smaller size and more uniform distribution than with conventional heating. Inorganic ions such as H
þ , OH
À and NaF, were found to affect
the precipitating rate of spindle a-Fe 2 O 3 and accelerate the hydrolysis of ferric ions.
Another research project leading to the formation of amorphous nanoparticles
was undertaken by Zhu’s group [185]. The product was synthesized by microwave
irradiation heating of an aqueous solution containing ferric chloride, polyethylene
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
158
method.
The first report was by Komarneni and coworkers [179] using a microwavehydrothermal process to catalyze the synthesis of crystalline oxides such as TiO 2 ,
ZrO 2 and Fe 2 O 3 , and binary oxides such as KNbO 3 and BaTiO 3 . The importance of
this work was that this technique led to fine powders of these materials. The effect
of different parameters, such as concentration of chemical species, time and temperature, on the crystallization kinetics of the above phases has been investigated
under microwave-hydrothermal conditions using microwaves of 2.45 GHz frequency.
The second work was by Dong et al. [180] who report on the preparation of
submicron uniform a-Fe 2 O 3 by microwave-induced hydrolysis of ferric salts with
concurrent dissociation of urea. Although it was not conducted in a domestic MW
oven, it is still on our list of references. A mixed solution of FeC 3 and CO(NH 2 ) 2
was placed in an Erlenmeyer flask equipped with a reflux condenser. This flask was
inserted into a cylindrical resonant cavity. After microwave irradiation (2.4 GHZ,
500 W) for a certain period of time, the flask was moved to a homothermal water
bath (at 94
C) to be aged. Uniform particles of approximately spherical shape with
mean diameter 75 nm were obtained. An almost identical paper was published
a little later by Li and Wei [181]. Fe(NO 3 ) 3 was the iron source instead of the chloride [180]. They conducted the reaction at different pH values. In this way they
obtained a larger variety of shapes, including acicular cubic and spherical particles,
all in the nanometer range.
Palchik [182] obtained nanosized amorphous iron oxide (Fe 2 O 3 ) by the pyrolysis of iron pentacarbonyl, Fe(CO) 5 , in a modified domestic microwave oven in
refluxing chlorobenzene as solvent under air. The reaction time was 20 min. Separate particles of iron oxide, 2–3 nm in diameter, were obtained together with aggregated spheres with a diameter of 25–40 nm. Differential scanning calorimetry
measurements showed an amorphous/crystalline phase transition at about 250
C.
A study of microwave effects on the formation of nanoparticles in the hydrolysis
reaction of FeCl 3 with NaH 2 PO 4 to get spindle-type colloidal hematite particles
under microwave radiation, was reported by Han and coworkers [183]. They found
that the reaction rate increased greatly, and the reaction conditions, for example,
the acidity of the solution, concentration ratio of the components, and the microwave radiation time, had important effects on the nanoparticle formation and their
morphology. They discussed the roles of microwaves and the concentration of
H 2 PO 4
À during the hydrolysis process.
Nanometer-sized quasicubic and spindle a-Fe 2 O 3 particles were prepared by microwave heating from Fe
þ3 salt solutions [184]. The obtained a-Fe 2 O 3 particles
formed by MWH have smaller size and more uniform distribution than with conventional heating. Inorganic ions such as H
þ , OH
À and NaF, were found to affect
the precipitating rate of spindle a-Fe 2 O 3 and accelerate the hydrolysis of ferric ions.
Another research project leading to the formation of amorphous nanoparticles
was undertaken by Zhu’s group [185]. The product was synthesized by microwave
irradiation heating of an aqueous solution containing ferric chloride, polyethylene
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
158