the cooling rates obtained during the cavitational collapse are estimated to be
greater than 2 Â 10
9 K s
À1 . That is the reason why amorphous iron oxide, Fe 2 O 3 ,
can be prepared by sonicating Fe(CO) 5 in a decalin solution under air without
adding any glass former.
The characterization of an amorphous material is difficult because it lacks characteristic XRD diffractions, so that Auger or Mo ¨ssbauer spectroscopies are preferred, along with other conventional analytical assays such as spot test and iodometric titration [71]. The Fe 2 O 3 nanoparticles are converted to crystalline Fe 3 O 4
nanoparticles when heated to 420
C under vacuum or when heated to the same
temperature under a nitrogen atmosphere. The magnetization of pure amorphous Fe 2 O 3 at room temperature is very low (< 1:5 emu g
À1 ) and it crystallizes at
268
C.
Ultrafine powders of Cr 2 O 3 and Mn 2 O 3 have been prepared at room temperature
by the sonochemical reduction of aqueous solutions containing (NH 4 ) 2 Cr 2 O 7 and
KMnO 4 , respectively [75]. The yield of the sonochemical reduction has been enhanced by raising the reaction temperature or by using a 0.1 M aqueous solution of
ethanol. The amorphous powders are nanosized (50–200 nm), and the surface area
varies from 35 to 48 m
2 g
À1 . The crystallization of amorphous Mn 2 O 3 and Cr 2 O 3
could be achieved by heating them at 600 and 900 K, for 4 h, respectively.
Ultrasound irradiation of a slurry of Mo(CO) 6 in decalin for 3 h under ambient
air produces blue-colored, Mo 2 O 5 Á2H 2 O [76]. FT-IR analysis of this material reveals
the existence of MobO and MoaO bonds as well as hydrogen-bonded and coordinated water molecules. The amount of water molecules was determined by thermogravimetric analysis. Characterization using powder X-ray diffraction (XRD)
and transmission electron microscopy (TEM) with selected area electron diffraction
(SAED) shows the amorphous nature of the blue product. The TEM picture shows
that the blue oxide is composed of spongy platelet nanoparticles (20 nm in diameter). Heating the initial blue powder at 300
C for 2 days under an oxygen, hydrogen, and nitrogen atmosphere yields X-ray crystalline MoO 3 , MoO 2 , and a mixture
of MoO 3 and MoO 2 , respectively. X-ray photoelectron spectroscopy (XPS), along
with the potentiometric titration analysis of the blue oxide, confirms the formation
of pentavalent molybdenum oxide. UV–visible absorption studies of the blue
product demonstrate that the characteristic absorption of the Mo(V) (d1 – cation)
oxide system and the Mo ions probably consist of two types of coordination symmetry (Td and Oh). Electron paramagnetic resonance (EPR) experimental results
revealed an unusual doublet pattern, which is ascribed to superhyperfine coupling
of pentavalent molybdenum with a proton of coordinated water. The nanostructured amorphous pentavalent molybdenum oxide (blue oxide) thus formed has
also been successfully deposited ultrasonically on Stober’s silica microspheres
(250 nm). The TEM images of silica-supported blue oxide reveal uniform distribution and the strong adhering nature of the blue oxide. FT-IR spectroscopy illustrated the structural changes that occur when the amorphous SiO 2 is coated sonochemically with the blue oxide.
The sonochemical oxidation of molybdenum carbonyl would not occur in the
bubble, due to its low vapor pressure. Therefore, the sonochemical reaction of
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
130
greater than 2 Â 10
9 K s
À1 . That is the reason why amorphous iron oxide, Fe 2 O 3 ,
can be prepared by sonicating Fe(CO) 5 in a decalin solution under air without
adding any glass former.
The characterization of an amorphous material is difficult because it lacks characteristic XRD diffractions, so that Auger or Mo ¨ssbauer spectroscopies are preferred, along with other conventional analytical assays such as spot test and iodometric titration [71]. The Fe 2 O 3 nanoparticles are converted to crystalline Fe 3 O 4
nanoparticles when heated to 420
C under vacuum or when heated to the same
temperature under a nitrogen atmosphere. The magnetization of pure amorphous Fe 2 O 3 at room temperature is very low (< 1:5 emu g
À1 ) and it crystallizes at
268
C.
Ultrafine powders of Cr 2 O 3 and Mn 2 O 3 have been prepared at room temperature
by the sonochemical reduction of aqueous solutions containing (NH 4 ) 2 Cr 2 O 7 and
KMnO 4 , respectively [75]. The yield of the sonochemical reduction has been enhanced by raising the reaction temperature or by using a 0.1 M aqueous solution of
ethanol. The amorphous powders are nanosized (50–200 nm), and the surface area
varies from 35 to 48 m
2 g
À1 . The crystallization of amorphous Mn 2 O 3 and Cr 2 O 3
could be achieved by heating them at 600 and 900 K, for 4 h, respectively.
Ultrasound irradiation of a slurry of Mo(CO) 6 in decalin for 3 h under ambient
air produces blue-colored, Mo 2 O 5 Á2H 2 O [76]. FT-IR analysis of this material reveals
the existence of MobO and MoaO bonds as well as hydrogen-bonded and coordinated water molecules. The amount of water molecules was determined by thermogravimetric analysis. Characterization using powder X-ray diffraction (XRD)
and transmission electron microscopy (TEM) with selected area electron diffraction
(SAED) shows the amorphous nature of the blue product. The TEM picture shows
that the blue oxide is composed of spongy platelet nanoparticles (20 nm in diameter). Heating the initial blue powder at 300
C for 2 days under an oxygen, hydrogen, and nitrogen atmosphere yields X-ray crystalline MoO 3 , MoO 2 , and a mixture
of MoO 3 and MoO 2 , respectively. X-ray photoelectron spectroscopy (XPS), along
with the potentiometric titration analysis of the blue oxide, confirms the formation
of pentavalent molybdenum oxide. UV–visible absorption studies of the blue
product demonstrate that the characteristic absorption of the Mo(V) (d1 – cation)
oxide system and the Mo ions probably consist of two types of coordination symmetry (Td and Oh). Electron paramagnetic resonance (EPR) experimental results
revealed an unusual doublet pattern, which is ascribed to superhyperfine coupling
of pentavalent molybdenum with a proton of coordinated water. The nanostructured amorphous pentavalent molybdenum oxide (blue oxide) thus formed has
also been successfully deposited ultrasonically on Stober’s silica microspheres
(250 nm). The TEM images of silica-supported blue oxide reveal uniform distribution and the strong adhering nature of the blue oxide. FT-IR spectroscopy illustrated the structural changes that occur when the amorphous SiO 2 is coated sonochemically with the blue oxide.
The sonochemical oxidation of molybdenum carbonyl would not occur in the
bubble, due to its low vapor pressure. Therefore, the sonochemical reaction of
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
130