The Formation Mechanism of Mesoporous Materials studied by EPR Spectroscopy and Cryo-TEM.

摘要

Ordered mesoporous materials raised a wide interest in the scientific community due to their unique structural properties which encompasses nanosize ordered channels. These materials have potential applications in diverse technological fields such as catalysis, membranes, microelectronics and sensors. The formation of these materials is initiated by the interaction of micelle of surfactant molecules with precursors of an inorganic oxide, usually silica, which further polymerizes to form solids with a well defined pore structure and amorphous walls.;The aim of this study is to explore the details of this intriguing reaction mechanism on two types of materials, hexagonal and cubic, prepared with Pluronic block-copolymers as surfactants. The Pluronic micelles are characterized by a hydrophobic polypropylene oxide core and a hydrophilic polyethylene oxide corona. Examples of questions we address are: How does a homogeneous micellar solution transforms into an ordered phase? What kinds of interactions are responsible to this transformation?;Our work focused on processes that take place on two levels, the molecular one and the mesoscale, which were investigated by combining electron paramagnetic resonance (EPR) and cryogenic transmission electron microscopy (cryo-TEM) techniques. The molecular level studies combine in-situ and freeze quench EPR spectroscopic techniques applied to nitroxide spin-probes introduced (at minute quantities) into the reaction mixtures. The nitroxide radicals serve as paramagnetic probes in the reaction mixture. They do not react or affect the reaction product but they sense in-situ the changes that occur in their environment during the reaction. The EPR spectrum provides information regarding the dynamic of the probe, which is affected by its surrounding, and the polarity of its environment. Fine structural details, such as the distribution of water and additives within the micelle, can be obtained from Electron Spin Echo Envelope Modulation (ESEEM) techniques that measure weak superhyperfine interactions of an electron spin with nearby nuclear spins. Double Electron Electron Resonance (DEER), which measure the interaction between electron spins in the range of 1.5--7 nm, can track changes in micelle size and aggregation numbers. The evolution of the microstructures during the reaction was studied by cryo-TEM, through collaboration with Prof. Ishii Talmon from the Technion, Haifa, Israel.;Using these methods the formation mechanisms of hexagonal SBA-15 material and the cubic KIT-6 material were explored. These showed that the formation of these mesoporous materials starts with the penetration of silicate ions into the corona of the micelles. This is driven either by charge matching or hydrogen bonding, mediated by the anions present in solution. The hydrolysis and condensation of the silicate ions in the corona region removes water from the corona and causes change in the micellar curvature. This leads to rearrangement of the original micellar morphology, mainly lengthening the micelles, followed by condensation of the silicate-covered micelles into ordered phase. This then transforms into an ordered hexagonal phase in the case of SBA-15, whereas in the case of the cubic KIT-6, the hexagonal phase transforms into the cubic phase. The transformation occurs within the presence of butanol added either at the beginning or after the formation of the hexagonal phase.;We showed that DEER can be used to study the properties of various nanostructures in solution, specifically their volume. The feasibility of the method was initially tested on micelles of Pluronic block copolymers. After establishing the methodology we have applied DEER as a tool to follow the evolution of the solution nanostructures during the formation of KIT-6, a new stage in the reaction mechanism was detected which involves an increase in the hydrophobic core and a decrease in the aggregation number in the first ten minutes of the reaction. This was not observed from the Cryo-TEM and other EPR methods.;In addition to the above, within the cooperation with Prof. Ron Naaman from Weizmann Institute, we study the organization of organic monolayers on GaAs.;In conclusion, we can state that in this thesis we succeeded to tackle the complexity of the formation mechanism of mesoporous materials at two different length scales. Our research gave a full picture on the formation of mesoporous materials and thus gave a better synthetic control of these materials.