Mesoporous Solids

Another closely related field that has been developed by Gedanken and co-workers is the sonochemical coating of submicron ceramic spheres (silica, alumina, and zirconia) by a large variety of nanoparticles. This was done by synthesizing the ceramic spheres by conventional techniques (like the Stobber method for silica particles). The spheres were then introduced in the sonication bath, mixed with the solution of the precursor, and the ultrasonic radiation was passed through the solution for a predetermined time. In this way, nanoparticles of metals (Ni and Co, for example), metal oxides (Fe2O3, Mo2O5), rare earth oxides (Eu2O3, Tb2O3), semiconductors (CdS, ZnS), and Mo2C were deposited on a ceramic surface.

Gedanken and co-workers recently developed meso-porous (MSP) materials, and used ultrasound radiation in the deposition of amorphous nanomaterials into the mesopores. They demonstrated that the sonochemical technique can be employed for the synthesis of mesoporous metal oxides. The sonochemical method reduced the time period required for such synthesis by manifold, and also produced more stable structures. Excellent results were obtained with silica, titania, yittria-stabilized zirconia (YSZ), and Fe2O3 [100].

Further, a sonochemical method was used to prepare MSP silica [101], MSP YSZ (yittria-stabilized zirconia), and other MSP materials. The sonochemical products were shown to have thicker walls than those synthesized by the conventional methods. MSP titania, prepared under sono-chemical conditions, has the highest surface area reported so far. In addition, sonochemistry has been used for the insertion of amorphous nanoparticles into the mesopores: Mo2O5 into the mesopores of MCM-41 (MSP silica) [103], and amorphous Fe2O3 [104] in the mesopores of MSP titania. Five physical methods were used to prove that the amorphous nanoparticles are indeed anchored onto the inner walls of the channels. The amount of Mo2O5 that was inserted in the mesopores was 45% by weight. An attempt to increase this amount showed that the excess is deposited outside the mesopores.

The sonochemical preparation of air-stable iron nano-particles having a very high magnetization has been reported. Iron nanoparticles are pyrophoric, and protecting them against oxidation is a challenge. However, the sonication of Fe(CO)5 in diphenylmethane results in iron nanoparticles coated by a polymer. Further annealing of the sample yields the air-stable product [105]. The characterization of the product and the stability studies are based on Mossbauer spectroscopy, XRD, and magnetization measurements. Although some efforts in materials science are still directed toward developing new methods for the fabrication of nanomaterials, more attention is directed these days to the control of the size and shape of the nanoparticles. Various research groups have demonstrated over the years that the control of the particle size is quite easy when using sonochemistry. It is accomplished simply by the variation of the concentration of the precursor in the irradiated solution. The more dilute the solution, the smaller are the particles. The shapes of the products of the sonochemical process are less predictable. A major factor is the presence or absence of a surfactant. We can just mention that shapes such as Olympic nanorings (BaFe12O19) [72], nanocylinders (GaOOH) [106], nanotubes (TiO2) [107], nested inorganic fullerenes (Ti2O) [108], and spheres were among the shapes of the sonochemical products.

Chen et al. developed a simple method for the preparation of Pd particles in-situ within the mesoporous silica host by soaking and subsequent irradiation by ultrasound at room temperature. The straightforward process yields composite samples containing Pd nanoparticles, with a mean size of 5-6 nm diameter, located within the pores of the meso-porous silica host [109].

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