Silicone Surfactants Directing For Hierarchical Porous Materials

Porous materials attract a great deal of attention because of their large specific surface area and pore volume, tunable pore size and controllable structures and compositions, which make them suitable for wide applications in catalysis, separation, biomedicine and optoelectronic devices. Synthesizing of the porous materials with hierarchical structures provides new families of the porous materials and leads to understanding the natural biomineralization process. The co-operative self-assembly of organic molecules and inorganic precursor into well-defined architectures is crucial for fabricating porous materials. Block copolymers are widely used in the synthesis of porous materials due to their large variety of self-assembly phase behaviors.In this work, silicone surfactants were used as the tectonic unit to build up hierarchical structures. Silicone surfactants are amphiphilic copolymers with hydrophobic polydimethylsiloxane(PDMS) chains. Our approach to the simultaneous formation of porous structure was to use its particular properties(1) for integrating the template and silica source in PDMS hydrophobic groups, which could be oxidized at high temperatures and converted to silica; and(2) PDMS segments were very flexible, and the PDMS chains possessed good compatibility with other hydrophobic segments, which led to the particular phase behavior to create materials with complex structures. In this work, the self-assemble behavior and the structural transition mechanisms of the silicone surfactants in the fabrication V of the hierarchical porous materials were investigated.In Chapter 1, the relevant literatures and research background were reviewed, and the significance of this topic and research strategies were discussed.In Chapter 2, a novel pathway was reported to synthesize ordered mesoporous silica@carbon hierarchical complex by the co-assembly of silicone surfactants with the hydrophilic polymer oligomer. The ethylene oxide(PEO) segment of the silicone surfactants could interact with the hydroxyl group of phenolic resols by hydrogen bonding interactions in the aqueous solution. During hydrothermal treatment under alkaline condition, the phenolic resols cross-linked to form resins. During pyrolysis, the phenolic resins further cross-linked and were decomposed to form the carbon frameworks, and the PDMS chains were oxidized to form mesopores and nano-sized silica, resulting in silica@carbon mesoporous materials. In particular, the silica was positioned in the pores in uniform nano-size in this mesoporous complex. Lithium charge and discharge performance showed that the silica nanoparticles can be reduced to silicon during the first discharge procedure, thereby increasing the lithium capacity of the carbon material.In Chapter 3, a novel silica nanosphere with hierarchical porous structures was carried out by using comb-like silicone surfactant as the template and tetraethyl orthosilicate(TEOS) as the silica source. Comb-like silicone surfactants could be self-assembled to form bilayer vesicles in aqueous solution. Collision of the vesicle was caused by Brownian motion and Van der Waals force, forming more complex secondary structure. TEOS was used to copy the aggregation structure of the vesicles. Furthermore, hierarchical porous nanospheres could be obtained by high temperature removal of surfactants: the first level- silica nanospheres with mesopores; the second level- internal vesicle aggregation shaped cavity; and the third level- silica particles obtained by the decomposition of PDMS. It has been found that(i) the size of the silica sphere was reduced with increasing of the synthesis temperatures due to the fusion rate of small vesicles was accelerated by increasing of temperature;(ii) the size of the silica sphere was decreased by decreasing surfactants concentration because the rate of effective collisions of the vesicles and the number of the molecules that self assemble into a single vesicle was lessened; and(iii) the sphere size and wall thickness were increased by the increasing of TEOS.In Chapter 4, hierarchical porous silica clusters with special structure, an assembly of small onion-like vesicles were synthesized in one-pot. Comb-like silicone surfactants and amphiphilic block copolymers P123 were used as template and co-template, respectively. The special structure of the clusters was formed by the tetrahedral closely packing of spherical-like vesicles, resulting in a m Fd 3 structure. This packing structure possessed the lowest surface energy. The hardness of the particles was the key factor for the formation of this structure. It had been found that the proper P123 addition, p H value and temperature make the vesicles have the most appropriate degree of hardness in order to the occurrence of a soft ball assembly.(i) The amount of P123 not only affected the hydrophobicity of the micelles, but also the hardness of the vesicles. The lamellar structure could not be formed with too little P123 addition; however, vesicles could not reach the proper hardness with too much P123 addition, due to the rigidity of the C-C chain and C-O chain of P123;(ii) the hydrolysis/condensation rates of the TEOS were affected by the p H value. Silica clusters with m Fd 3 structure couldn’t be formed with the low hydrolysis/condensation rates of the TEOS; and(iii) the hydrophobicity of the micelles and the hydrolysis/condensation rates of TEOS were influenced by the synthesis temperature; the hierarchical silica clusters could only be obtained at suitable temperatures.