Synthesis And Formation Mechanism Of Hierarchically Structured Mesoporous Silica

Mesoporous materials are a type of porous materials with ordered pores on the range of 2-50 nanometers, which have attracted great attention due to their wide applications to adsorption, separation, catalysis, electrode materials, optoelectronic devices, chemical sensors and nonlinear optical mater, and so on, due to their high surface area, ordered mesopores and tunable pore distribution. Hierarchically structured mesoporous silica has attracted great attention due to their potential application and their relationship to the biomineralization and biomimetic chemistry. To achieve the hierarchically structured mesoporous silica, simultaneous control of the structural parameters on the nanometer scale and the morphology on the micrometer scale are required. It is still a challenge to synthesize the hierarchically structured mesoporous silica for the materials science. In the past decades, on the basis of the interaction between the amphiphilic organic molecules and the inorganic species in the static hydrothermal system, only simple structure and morphology could be obtained, because it was difficult to control the mesophase at the different region and the morphology on the micrometer scale. Therefore, the hierarchical structure and complex morphology were difficult to prepare and rarely reported in the static hydrothermal system. Inspired by the biomineralization of the natural organism, which behaves dynamic process of time and space, we brought the idea of "dynamic process" into the synthesis of mesoporous silica to fabricate silica-based novel meso-structures, complex morphology and hierarchical structure by exploring the new synthetic system. The meso-structured silica were characterized by nitrogen sorption analyzer, power X-ray diffraction (XRD), small angle X-ray scattering (SAXS), scanning electron microscope (SEM), transmission electron microscope (TEM), etc. There are four main parts in this thesis.1) Anionic surfactant-templated mesoporous silica (AMS) spheres with radially oriented mesoporesMesoporous silica spheres with radially oriented mesopores were synthesized using an anionic surfactant as the template through the co-structure directing route. The spherical size and mesostructure can be finely tuned by changing the pH value of the synthetic system in the range of 8.8 to 6.4. In addition, when the pH value was decreased to 5.8, instead of spheres, anisotropic morphologies such as elliptical, peanutlike and trifurcate particles were obtained, exhibiting core/shell structure due to the different orientations of the mesopores in the core and the shell of the particles. When the pH value was further decreased, lamellar mesophase would be form. It is proposed that the evolution of the morphologies and mesostructures of the products templated by anionic surfactants strongly depend on the pH value of the synthetic system, and the formation process undergoes a re-alignment self-assembly mechanism (several hours). Firstly, clusters of organic-inorganic composite aggregate to form larger organic/inorganic hybrid with a special shape. Subsequently, the hybrid re-alignments to form preferential ordered structure (for example, radial mesochannel) to minimize its micelle free energy dependent on its shape.2) Synthesis of silica hollow spheres with ordered and radially oriented mesochannels by cavitation bubble templated methodUsing anionic surfactant as templates, ordered, robust and dispersed mesoporous silica hollow spheres with radially oriented mesochannels were synthesized with the aid of ultrasonic irradiation. The diameter of the hollow spheres was mostly in the range of 100-500 nm. The hollow spheres possessed thin and uniform shell with the thickness of 35-40 nm, and the shell with radially oriented mesopores exhibited well-ordered structure. By controlling the sonochemical processing time, pH value and additional reagent, the shell thickness, mesostructure (hexagonal, radial, or disordered), and shape of the inner cavity (hexagonal or spherical shape) of the hollow spheres could be facilely tuned. The obtained mesoporous silica possessed uniform and short meso-channels, which were beneficial to the diffusion of the molecules in the mesopores. It was proved that the high reaction velocity in the Knoevenagel condensation.The formation process of the hollow spheres with radially mesostructure was proposed of the cavitation bubble as the template of the hollow structure. Subsequently, a relatively slow cooperative realignment process of the silica/surfactant hybrid mesophase in this anionic surfactant templating system formed the ordered radial mesopores. Firstly, the bubbles generated by ultrasonic irradiation were surrounded by the silica/surfactant hybrid to form hollow structure. During the succedent reaction, the initially formed disordered silica/surfactant hybrid mesophases in the shell underwent a structural transformation and finally formed radially oriented mesochannels. The cavitation bubbles were in situ templates for hollow structure, which possessed environment-benign, process-facile and low cost advantages compared with polymer spheres, emulsion and carbon spheres as the consumed templates.3) Synthesis of radiolaria-like, hierarchically structured mesoporous silica hollow spheres by emulsion-based kinetic self-assemblyRadiolaria-like, hierarchically structured mesoporous silica hollow spheres with long, radially protruding and multicellular structured spines were fabricated through a kinetic self-organization process. This hierarchically structured silica is self-assembled by a unique emulsion-templating method, in which the acidified anionic surfactant N-lauroylsarcosine sodium (Sar-Na) acts as an oil phase. In this way, the surfactants can be pre-stored as the oil phase. Under the succedent experimental procedure, the pre-stored surfactants were gradually released and meanwhile were involved in the co-assembly with silica precursors to form mesostructured silica. The kinetic processes of deformation of oil droplets, release of surfactants, and co-assembly of surfactants and silica precursors result in the complex organization of the hierarchically mesostructured silica. This kinetic method achieved the pre-store and release process of the surfactant, which was related to the growth mechanism of morphological control of unicellular organisms, for instance, the radiolaria. This method would be expected to produce other complex morphologies and hierarchical structures.4) Synthesis of hierarchically nanoporous single crystal mesoporous silica from kinetic template of mesomorphous PAA-CTA complexes Hierarchically nanoporous single crystal mesoporous silica was fabricated for the first time by using mesomorphous polyelectrolyte-surfactant complexes (PAA-CTA) as kinetic template. Through the synergy of two kinetic self-assembly processes of silica-surfactant micelles co-assembly and polymer phase separation, the long range order through the mesomorphous polyelectrolyte-surfactant complexes was reserved and evolved into single crystal Pm3n mesostructured silica, and the domains of phase-separated polyelectrolyte chains served as the template for secondary nanopores within the single crystal SBA-1. Interestingly and importantly, the presence of large amount of secondary nanopores did not disturb the long-range order of mesostructure of the mesoporous silica particles, which would possess both the functions of crystal-like regularity and high diffusion efficiency of hierarchical pores. This kinetic templating mechanism would be generally applicable in fabrication of other hierarchically structured single crystal materials and may be related to the formation mechanism of complicated biominerals.