Micromolding of Nanoparticles Using Unidirectionally Formed Ice Crystals

Abstract

The handling of nanoparticles is quite difficult, and still remains as a challenging topic for researchers as well as engineers. One method to enable fairly easy handling of this material is to mold it into microstructures which can be easily handled. Nanoparticles can be molded quite easily to have a spherical morphology. However, if the resulting spheres are large, the diffusion paths within them will tend to become long, and therefore generally, it will become very difficult to utilize the unique properties of the nanoparticles included within them. On the other hand, if the resulting spheres are small, a severe pressure drop will occur when fluids are passed through a column packed with them. Therefore, it is necessary to develop a method to mold such nanoparticles to have a morphology which has short diffusion paths, and which do not cause significant pressure drops during usage.

One ideal morphology is a microfiber which has a diameter in the μm range. Another ideal morphology is a monolithic microhoneycomb having channels which sizes are also in the μm range. However, it is very difficult, if not impossible, to mold nanoparticles to have such morphologies using conventional methods.

Recently in our laboratory, we found that silica hydrogels can be molded into fibers or into a monolithic microhoneycomb by freezing them unidirectionally. As ice crystals which grow within the hydrogels during freezing act as the template, we named this method the ice templating method. In this work, we applied this method to silica hydrogels including nanoparticles. We found that the nanoparticles could be molded into microfibers and monolithic microhoneycombs using this newly developed freezing method. The properties of the obtained microstructures were also investigated in detail.