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as Ni and Co, which do not form any network structure, like silicates. The lack of a network-forming multivalent bond is the probable reason why no stable mesoporous oxides have been synthesized for divalent elements yet. Octadecylamine has been used as the organic structure-directing agent to prepare Ni and Co oxides. A relatively better surface area has been obtained for the Co oxide, but in Ni, the surface area was less. The porous (solvent-extracted) cobalt oxide has been used as a catalyst in the oxidation reaction of cyclo-hexane in mild conditions. The catalyst has shown relatively better conversion of cyclohexane into cyclohexanone and cyclohexanol than the nanostructured cobalt oxide catalyst of a regular structure.

Gasgnier and co-workers [65] reported the ultrasound effects on metallic (Fe and Cr), iron sesquioxides (alpha-, gamma-Fe2O3), calcite, copper, lead, and manganese oxides as powders. Different kinds of materials, such as powders, were submitted to ultrasound after mixing in water, dode-cane, or dilute acetic acid (5%). After treatment, first, a decrease of particle sizes, and second, the formation of unexpected and unknown compounds were observed on a mesoscopic scale (CaO from CaCO3, for example). For iron and iron oxides, it was found that the magnetic properties (susceptibility and effective magnetic moment) were changed slightly. A shift in the Morin temperature transition for a-Fe2O3 was attributed to the formation of impurities. For inorganic oxides (Cu2O, PbO, Pb3O4, and Mn3O4) sonicated in dilute acetic acid, the formation of metallic acetates is easily achieved, and it is rapid (10 s for PbO).

Zhong and co-workers [66, 67] coated molybdenum oxide on submicrospheres of amorphous alumina and crystalline alumina, using a sonochemical method. Blue oxide of molybdenum was found to be formed on the alumina surface, while on amorphous alumina, the presence of an isolated tetrahedrally coordinated Mo oxide species was confirmed. The surface area of the alumina-coated Mo oxide is about 11 times larger than that of the bare alumina. An explanation for this change is offered. Amorphous tungsten oxide was prepared by ultrasound irradiation of a solution of tungsten hexacarbonyl W(CO)6 in diphenylmethane (DPhM) in the presence of an Ar (80%)-02(20%) gaseous mixture at 90 °C. Heating this amorphous powder at 550 °C under Ar yields snowflake-like dendritic particles consisting of a mixture of monoclinic and orthorhombic WO2 crystals (Fig. 14). Annealing of the as-prepared product in Ar at 1000 °C causes the formation of a WO2-WO3 mixture containing nanorods (around 50 nm in diameter) and packs of these nanorods. Heating the product in air for 3 h leads to triclinic WO3 crystal formation, with a basic size of 50-70 nm [68].

ZrO2 • «H2O nanopowders were prepared by the ultra-sonication approach at room temperature [69]. Photoluminescent properties of the as-prepared ZrO2 nanoparticles were investigated [Fig. 15(A)]. The use of ultrasound has proven to dramatically reduce the temperature, and makes the reaction conditions very easy to maintain during the preparation. The PL spectra obtained from the as-prepared nano-ZrO2 samples excited with an excitation wavelength of 254 nm show fluorescence emission. Although the detailed PL mechanism for the nano-ZrO2 is still under research, the emission appearing at the short-wavelength excitation

Figure 14. Transmission electron micrograph of WO2 crystal showing dendrite morphology. Heating the amorphous powder at 550 °C under Ar yields snowflake-like dendritic particles consisting of a mixture of monoclinic and orthorhombic WO2 crystals. Reprinted with permission from [68], Yu. Koltypin et al., J. Mater. Chem. 12, 1107 (2002). © 2002, Royal Society of Chemistry.

Figure 14. Transmission electron micrograph of WO2 crystal showing dendrite morphology. Heating the amorphous powder at 550 °C under Ar yields snowflake-like dendritic particles consisting of a mixture of monoclinic and orthorhombic WO2 crystals. Reprinted with permission from [68], Yu. Koltypin et al., J. Mater. Chem. 12, 1107 (2002). © 2002, Royal Society of Chemistry.

has been attributed to the near band-edge transitions. In Figure 15(B), the emissions have been attributed to the involvement of midgap trap states, such as surface defects, and oxygen vacancies arising out of the sonochemical synthesis [70-71]. Cerium oxide (CeO2) nanoparticles were prepared sonochemically, using cerium nitrate and azodi-carbonamide as starting materials, and ethylenediamine or tetraalkylammonium hydroxide as additives. The additives have a strong effect on the particle size and particle size distribution. CeO2 nanoparticles with a small particle size and a narrow particle size distribution are obtained with the addition of additives, while highly agglomerated CeO2 nanoparticles are obtained in the absence of additives. Monodispersed CeO2 nanoparticles with a mean particle size of ca. 3.3 nm are obtained when tetramethylammonium hydroxide (TMAOH) is used as the additive and the molar ratio of cerium nitrate/azodicarbonamine/TMAOH is 1/1/1. Blue shifts of the absorption peak and the absorption edges of the products are observed in the UV-vis absorption spectra as a result of the quantum size effect [72].

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