these particles are inert in the ambient atmosphere. Consequently, the RAMO
(French acronym of Reactor Autoclave MicroOnde) system appears to provide an
efficient source of energy for the rapid production of inert powders of iron, magnetite, and iron–magnetite composites.
Although most of the synthetic work related to iron oxide nanoparticles has
concentrated, as we have demonstrated, on Fe 2 O 3 , and especially on the a, hematite phase, a few papers also reported on the preparation of magnetite, Fe 3 O 4 .
Submicron-sized (0.15–0.2 mm) spherical agglomerates of magnetite (Fe 3 O 4 ) powders have been prepared successfully by microwave hydrothermal (MH) reaction of
ferrous sulfate and sodium hydroxide in the temperature range 90–200
C [190].
This work is closely related to the report of [190]. The Mo ¨ssbauer spectra of these
powders indicated that stoichiometric Fe 3 O 4 particles are obtained only when the
molar ratio of Fe/NaOH b 0.133 is maintained in the solution. It is observed that
the Fe/NaOH ratio is an important parameter for the controlled oxidation of ferrous salts in alkaline media under MH conditions to produce stoichiometric Fe 3 O 4 .
Furthermore, the kinetics of MH synthesis are one order faster than the reported
conventional hydrothermal (CH) synthesis. The value of the saturation magnetization M ¼ 70 emu g
À1 is obtained in the case of stoichiometric Fe 3 O 4 . However,
when ferric salt is treated in an alkaline medium, single-phase a-Fe 2 O 3 is obtained
under the MH conditions of 200
C (194 psi).
Nanometric ferrites were also prepared by the MWH method. Komarneni
[191] has reported on the synthesis of technologically important ferrites such
as ZnFe 2 O 4 , NiFe 2 O 4 , MnFe 2 O 4 , and CoFe 2 O 4 by using novel microwavehydrothermal processing. The precursors, nitrates of zinc, nickel, manganese, or
cobalt, were mixed with ferric nitrate and neutralized with ammonia to a specific
pH. Nanophase ferrites with high surface areas, in the range of 72–247 m
2 g
À1 ,
have been synthesized in a matter of a few minutes at temperatures as low as
164
C. The rapid synthesis of nanophase ferrites via an acceleration of reaction
rates under microwave-hydrothermal conditions is expected to lead to energy
saving.
Other oxides have also been prepared by the MWH methods. Ceria, CeO 2 , titania, TiO 2 , and zirconia, ZrO 2 , and the tin oxides were the most popular. Zhu’s
group [134] prepared nanometer-sized CeO 2 by two methods; sonochemical, and
MWH. The precursors were aqueous solutions containing (NH 4 ) 2 Ce(NO 3 ) 6 , hexamethylenetetramine and poly (ethylene glycol) (PEG M w ¼ 19,000).
The influence of microwave power and reaction time on the formation of CeO 2
nanoparticles was investigated. When the microwave power is in the range 10% to
40%, the as-prepared CeO 2 nanoparticles are of similar size and morphology. Violent ‘bump’ boiling of the solvent occurs when the power is greater than 50%.
When the reaction time was less than 5 min, no turbidity was observed and the
solution remained transparent. After exposure to microwave irradiation for 8 min
the solution became a turbid yellow, indicating the formation of the product. After
10 min, the reaction was complete, and the yield was as high as 90%. If the reaction time was prolonged to 30 min or even longer, the yield did not increase further, and the size and morphology of the CeO 2 nanoparticles remained almost
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
160
(French acronym of Reactor Autoclave MicroOnde) system appears to provide an
efficient source of energy for the rapid production of inert powders of iron, magnetite, and iron–magnetite composites.
Although most of the synthetic work related to iron oxide nanoparticles has
concentrated, as we have demonstrated, on Fe 2 O 3 , and especially on the a, hematite phase, a few papers also reported on the preparation of magnetite, Fe 3 O 4 .
Submicron-sized (0.15–0.2 mm) spherical agglomerates of magnetite (Fe 3 O 4 ) powders have been prepared successfully by microwave hydrothermal (MH) reaction of
ferrous sulfate and sodium hydroxide in the temperature range 90–200
C [190].
This work is closely related to the report of [190]. The Mo ¨ssbauer spectra of these
powders indicated that stoichiometric Fe 3 O 4 particles are obtained only when the
molar ratio of Fe/NaOH b 0.133 is maintained in the solution. It is observed that
the Fe/NaOH ratio is an important parameter for the controlled oxidation of ferrous salts in alkaline media under MH conditions to produce stoichiometric Fe 3 O 4 .
Furthermore, the kinetics of MH synthesis are one order faster than the reported
conventional hydrothermal (CH) synthesis. The value of the saturation magnetization M ¼ 70 emu g
À1 is obtained in the case of stoichiometric Fe 3 O 4 . However,
when ferric salt is treated in an alkaline medium, single-phase a-Fe 2 O 3 is obtained
under the MH conditions of 200
C (194 psi).
Nanometric ferrites were also prepared by the MWH method. Komarneni
[191] has reported on the synthesis of technologically important ferrites such
as ZnFe 2 O 4 , NiFe 2 O 4 , MnFe 2 O 4 , and CoFe 2 O 4 by using novel microwavehydrothermal processing. The precursors, nitrates of zinc, nickel, manganese, or
cobalt, were mixed with ferric nitrate and neutralized with ammonia to a specific
pH. Nanophase ferrites with high surface areas, in the range of 72–247 m
2 g
À1 ,
have been synthesized in a matter of a few minutes at temperatures as low as
164
C. The rapid synthesis of nanophase ferrites via an acceleration of reaction
rates under microwave-hydrothermal conditions is expected to lead to energy
saving.
Other oxides have also been prepared by the MWH methods. Ceria, CeO 2 , titania, TiO 2 , and zirconia, ZrO 2 , and the tin oxides were the most popular. Zhu’s
group [134] prepared nanometer-sized CeO 2 by two methods; sonochemical, and
MWH. The precursors were aqueous solutions containing (NH 4 ) 2 Ce(NO 3 ) 6 , hexamethylenetetramine and poly (ethylene glycol) (PEG M w ¼ 19,000).
The influence of microwave power and reaction time on the formation of CeO 2
nanoparticles was investigated. When the microwave power is in the range 10% to
40%, the as-prepared CeO 2 nanoparticles are of similar size and morphology. Violent ‘bump’ boiling of the solvent occurs when the power is greater than 50%.
When the reaction time was less than 5 min, no turbidity was observed and the
solution remained transparent. After exposure to microwave irradiation for 8 min
the solution became a turbid yellow, indicating the formation of the product. After
10 min, the reaction was complete, and the yield was as high as 90%. If the reaction time was prolonged to 30 min or even longer, the yield did not increase further, and the size and morphology of the CeO 2 nanoparticles remained almost
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
160