(98g) Assembling of Nanoparticles Using Ice Crystals

Various inorganic metal oxides can be synthesized through the sol-gel method. It is known that fibers of such metal oxides can be synthesized by freezing their parent hydrogels ?unidirectionally.? We found that fibers obtained through this method are generally porous, which indicates the possibility of using them as adsorbents and/or catalysts. Moreover, we showed that the diameters of the fibers, as well as the sizes of the pores within them can be controlled separately, and such controlling can be conducted simultaneously by simply adjusting preparation conditions. In this unidirectional freezing method, thoroughly aged hydrogels were usually used as the precursor in order to obtain long and strong fibers. However, we found that by applying this method to hydrosols or freshly gelled hydrogels, gels with unique morphologies, such as lamellar sheets, flat fibers and microhoneycombs, can also be obtained. We also found that the morphology which appears during unidirectional freezing can be controlled by adjusting the firmness of the precursor sol or gel. As this method practically uses ice crystals as the template, we named this method the ?ice template method.? Microhoneycombs obtained through the ice template method have micrometer sized straight channels, and the thicknesses of the walls which form the channels are only up to a few micrometers. Therefore, the pressure drop when fluids are passed through them is extremely low even though the diffusion path lengths within the porous walls are extremely short, a feature that cannot be obtained in packed columns. In this work, we attempted to widen the range of applicability of the ice template method. We applied the ice template method to mixtures of silica hydrogels and various nanoparticles. We found that composite microhoneycombs with morphologies quite similar to those synthesized through the conventional ice template method can be obtained. This means that the ?modified? ice template method can also be applied to materials which cannot be obtained through sol-gel transition, as long as they are combined with sol-gel derivable materials such as silica. We believe this is a promising method to provide microhoneycombs which can be used as adsorbents and/or catalysts in various microdevices.