Electron Microscopy Towards Novel Mesocrystals: A Structure And Mechanism Study

Crystal is an object with translational symmetry. The atomic arrangement plays a crycial role for the properties of crystal and can be solved by crystallographic method. During the past two decades,the development of novel mesocrystals have been at the forefront of the development of materials science,and therefore they are expected to find applications in fields of catalysis,separation,sensor,drug delivery,and electronics,etc. However,the mesocrystals are considered as“cavity crystals”,where the pores or organic templates have long?range?ordering while the frameworks are amorphous silicas. It is very hard to determine the structure of the SMCs only by X?ray diffraction (XRD) analysis,as there are only a few broad peaks because the ordering of the material is very sensitive and could have various fluctuations,and sometimes the peaks can overlap. Besides,the SMCs are always too small to perform the single?crystal XRD analysis. It is very difficult to solve mesostructure by the conventional characterizations for atomistic crystals. Thus the structural characterization of mesocrystals is strongly rely on transmission electron microscopy (TEM).In chapter 2,cage?type structure,two?dimensional (2D) cylindrical hexagonal (C),bicontinuous gyroid (G),bicontinuous diamond (D) and lamellar (L) mesophases have been obtained by using amino acid derived anionic amphiphilic molecules N?stearoyl?L?glutamic acid (C18GluA) as the template,3?aminopropyl?trimethoxysilane (APS) as CSDA and tetraethyl orthosilicate (TEOS) as the silica source,in the presence of nonionic co?surfactant C16(EO)10 (Brij?56). The mesophases changed in an order that cage?type?C?intergrowth of C and D?intergrowth of C and G?D?G?L,with the increase of the Brij?56/C18GluA molar ratio. The mesophases tranformation was explained by the organic/inorganic interface curvature change caused by the hydrophobicity and penetration effect of Brij?56. The structural relationship with the surface curvature analysis of the mesophases were investigated, furthermore,the observed intergrowth structures have been studied in detail. Besides,by using electron crystallographic method,the actual g parameters of D and G obtained in this system were determined and compared.In Chapter 3,we found that a novel hollow sphere with remarkable polyhedral crystal-like hollow morphology and a highly ordered double diamond bicontinuous mesoporous shell has been formed in a unique crystal growth system. Vesicle with a low-curvature lamellar structure was first formed by the self-assembling of amino acid derived amphiphilic carboxylic acid in the presence of a non-ionic surfactant,which followed a structural transformation to a highly ordered crystalline mesoporous shell with double diamond structure by maintaining a hollow cavity. The formation of the reversed multiply twinned particle (MTP) led to the icosahedral/decahedral hollow shape,and the sphere achieved the lowest total free energy by maximizing the spherical surface coverage.In Chapter 4,we analysed the structure and formation mechanism of a DNA- silica complex which synthesised based on the cooperative effects of N–trimethoxylsilylpropyl–N,N,N–trimethylammonium chloride (TMAPS). We found the first example of DNA liquid crystal silica mineralization platelet-like crystal with a extremely rare DNA 2D–square packing. By using TEM,it has been found that the hexagonal morphology of the crystal platelets with 2D-square symmetry composed of domains structures which arranged by 60o and the 2D-square structure was transformed from 2D-hexagonal structure. We speculated that the p4mm structure with a small interaxial separation was formed at higher concentrations by the formation of an electrostatic“zipper”with interactions between the negatively charged strands and positively charged grooves of opposing molecules.In Chapter 5, anionic surfactant templated mesoporous silica nanospheres with different mesostructure and particle size were synthesized by anionic surfactant, co-structure directing agent and silica source in the presence of nonionic surfactant. The formation mechanism of the nanospheres were investigated at different reaction periods. Besides, silica hollow spheres with different shell porosity were simply synthesised with micelle and emulsion dual templating route by using oleic acid as template and co-structure directing agent. The mesoporous silica hollow spheres with high porosity could be achieved by adding a moderate amount of ethanol in the OA synthesis system, depending on the co-surfactant effect of ethanol that changes the curvature of micelles. The formation of the silica hollow spheres has been studied in detail with reaction time.