New Type Of Porous Material And Self-assembly Of Hybrid Materials Synthesis And Characterization

Organic-inorganic self-assembled ordered mesoporous materials have become a hot interdisciplinary spot of chemistry, physics and materials science. In the last 16 years, mesoporous materials with various compositions, morphologies, pore structures, pore sizes and functions have been successfully synthesized, and their formation mechanism has been carefully investigated. However, the practical application of mesoporous materials is seldom reported. Therefore, there are still big challenges in the in-depth understanding of their self-assembly mechanisms and further design and synthesis of novel functional mesoporous materials, as well as their application research.Mesoporous silica is the most well known among all mesoporous materials. Due to the plain physical and chemical properties of amorphous silica itself, the fabrication of mesoporous materials with functional compositions is a main task in this area. Mesoporous silica can serve as a good substrate for further modification, or a hard template to produce mesoporous replicas. Meanwhile, the experience accumulated in the synthesis of mesoporous silica is beneficial to guide the preparation of mesoporous materials with other compositions. In this dissertation, by taking mesoporous silica as our starting point, we have prepared several novel non-silica mesoporous materials and Si-based organic/inorganic hybrid crystals via changing inorganic precursors, chemical transformation of silica frameworks, or replication of porous silica templates, and carried out careful investigation on the organic-inorganic self-assembly mechanisms.In chapter 2, periodic mesoporous organosilica (PMO) materials with ultra large mesopores and face centered cubic mesostructures were prepared via a low temperature synthesis, using ethylene bridged organosilica precursors and organic swelling agents. The pore size of～15 nm is the largest among PMO materials ever reported. The influence of reaction temperature and acidity was carefully investigated. The pore size was expanded by decreasing temperature, and low acidity benefited the formation of ordered mesostructure at relatively low temperatures. In the case of using mixture of organosilica and silica precursors, the shrinkage of unit cell and pore size was found when the ratio of organosilica ones was increased. A continuous structural transformation from mono-dispersed organosilica hollow nano-spheres to PMO materials was observed when the ratio of organosilica precursors to organic templates was increased, and a packing of preformed spherical composite micelles was proposed for the formation mechanism of PMO materials with ultra large cage-like mesopores.In chapter 3, organosilica precursors with longer alkyl bridging groups, octylene and hexylene groups, were applied and non-porous, self-assembled bridged organosilica crystals were obtained. The careful characterization and theoretical calculation revealed that cyclic dimers from completely hydrolyzed precursors served as building blocks to construct the crystals via hydrogen bonds between silanol groups and van der Waals interactions among carbon chains. Alkyl chains adopted all-trans conformation in the crystals. In previous literature reports, such bridged organosilica crystals were obtained in the case that organic groups had strong interactions, while only amorphous hybrids were formed when flexible long alkyl chains were used as bridging groups. It was found in our case that the decreasing of the sol-gel reaction rate through addition of surfactants, or decreasing of precursor concentrations or reaction temperatures, was critical to the formation of alkyl bridged hybrid crystals. This research adds new understandings to the sol-gel process of organosilica materials and may provide guidance to the synthesis of Si-based hybrid crystals with other functional bridging groups.In chapter 4, mesoporous polycrystalline silicon materials with high surface area and pore volume were prepared by a magnesiothermic reduction of mesoporous silica precursor SBA-15. It was found that the Si-MgO composites after reduction constituted an ordered mesostructure in which Mg substituted Si in the original silica frameworks, while Si nanocrystals were expelled into and filled original mesopores. The mesopores were generated by the removal of MgO. Si nanocrystals of～5nm in diameter, as well as little silicon oxides due to incomplete reaction or post oxidation formed the inorganic framework. The mesoporous silicon had a relatively strong photoluminescent band centered at 550 run. Silica precursors with other structures were also applied in the same reaction, and porous silicon materials having different morphologies and pore structures were obtained.In chapter 5, two different kinds of porous carbon based materials were synthesized by replication of porous silica precursors. In the first part, carbon foam materials with hierarchical pores, including macro-, meso and micropores, were prepared using macroporous silica foams as templates and sucrose as carbon source. The macropores of silica foams were well preserved. The removal of silica frameworks generated uniform mesopores of～5 nm in width, and the carbon frameworks provided micropores. Such hierarchically porous carbon with interconnected pores at different length scales may become good substrates for the loading of different guest molecules. In the second part, a novel mesoporous carbon/titania composite photocatalyst was produced by loading of titania nanoparticles into the pores of rod-like mesoporous carbon materials. It had a much better performance in the degradation of organic dyes with high concentration compared to commercial titania photocatalyst P25, due to the combination of strong adsorption capacity of mesoporous carbon to organic molecules and excellent photocatalytic activity of titania nanocrystal.