|M.Sc Student||Liora Omer|
|Subject||The Study of Meso-Phase Formation Mechanisms by Cryo-TEM|
|Department||Department of Nanoscience and Nanotechnology||Supervisors||Professor Emeritus Talmon Yeshayahu|
|Full Professor Daniella Goldfarb|
|Full Thesis text|
Self-assembled molecular structures can serve as structure-directing agents in the fabrication of inorganic nanomaterials. We studied in parallel two different systems of self-assembled organic structures, serving in the formation of inorganic structures. We used several techniques of transmission and scanning electron microscopy (TEM and SEM) to elucidate the mesostructures in the two systems we studied. In addition, we conducted preliminary electron paramagnetic resonance (EPR) experiments to extend our understanding to the molecular scale.
In one system we studied the structural evolution during the formation of large-pore cubic Ia3d silica-based mesoporous materials, synthesized with Pluroinc P123 and butanol as structure directing agents (KIT-6 mesoporous material). We used cryogenic high resolution SEM (cryo-HRSEM) and freeze-fracture-replication TEM (FFR-TEM). Typically a silica precursor is added to acid-catalyzed solution of Pluronic P123 and butanol. The latter serves as a co-solute, which can be added either at the beginning of the reaction, or after precipitation and the formation of a hexagonal phase. In this study we focused on the structural evolution from the hexagonal phase to the final cubic phase. Cryo-HRSEM and FFR-TEM images showed that from the hexagonal phase a perforated layer (PL) phase is formed, which later evolves into a bicontinuous structure. The final cubic phase forms within the layers, maintaining their orientation. We suggest a formation mechanism involving cylinder merging for the hexagonal to PL transition. Upon additional polymerization of the silica, the structure relaxes into the stable bicontinuous cubic phase.
We also studied lithocholic acid (LCA) nanotubes, self-assembled in alkaline solution, as structure-directing agents in the process of electroless deposition of copper. We examined several chelating agents for the copper ions, and found hydrogen tartrate (HT) to be the most effective for our procedure. Henceforth, we used HT as the chelating agent, and examined the influence of several parameters on the final inorganic structures: higher copper concentration, type of copper salt, and dialysis medium. The higher copper concentration reaction produced stable 1-dimensional (1-D) structures. However, the uniformity of the “template” LCA nanotubes was lost. When the copper source was copper chloride, we obtained slightly better uniformity of the 1-D structures than with copper sulfate. We also found that the 1-D structures are better dispersed in NaCl solution than in distilled water. By preliminary CW-EPR experiments we confirmed binding of the copper ions to the LCA nanotubes, which promotes the deposition on the nanotubes surfaces upon reduction of the copper.