| With the rapid development of modern technology, study of materials at the nano-scale has become extremely important in material science. Nanoporous materials have been attracted increasingly attention because of their superior characteristics, such as large surface area, high porosity, low density, high permeability, and excellent high adsorption performance. These extraordinary characteristics can result in potential applications in biological sensors, drug delivery, gas separation, energy storage and fuel cell technology, adsorbents, catalysts, and photonics. Moreover, nanoporous materials can be used as templates to fabricate novel nanomaterials that cannot be obtained by other methods. As a consequence, porous materials have become a globally focused research topic. Deep understanding of their formation mechanism and the structure-property relationship is vital for the "tailored" synthesis of nanoporous materials with controlled structures and desired functions. Comprehensive characterization of the nano-structure is an important link and key bridge in the process of "design and synthesis—characterization—property and application", which is a general flow chart in the study of various nanomaterials.In this dissertation, we mainly focus on the synthesis of novel nanoporous materials. Importantly, we employ the state-of-the-art electron tomography (ET) technique to understand the 3D architectures of the synthesized porous materials, such as complicated concentric circular (CC) mesostructures, hierarchical helical (HH) mesostructures, hard-sphere packing mesoporous materials and ordered macroporous siliceous foams (MOSFs). The comprehensive ET characterizations have allowed us to uniquely determine the true structural information of various porous materials, through which, their formation mechanisms have been clarified. Our studies have provided a solid foundation for future design and manufacture of controllable nanoporous materials with desired functionalities.In Chapter 2, a series of mesoporous materials with both crystal symmetry and/or unconventional helical symmetry were prepared by using ionic surfactant as structure-directing agent and perfluorooctanoic acid (PFOA) as co-templates, including CC and HH mesostructures. Because the traditional transmission electron microscopy (TEM) images provide only a 2D dimensional projection of a 3D object, many fine structural features are overlapped along the electron beam direction. Moreover, both the closed helical (CH) and CC hexagonal mesostructures consist of non-conventional crystal symmetry, and the difference of their structural features is very small (in the range of several nanometers). This has limited the determination of the CC mesostructures for a long time. In Section 2.1, we applied the ET technique to obtain tomographic slices with a thickness of less than 1 nm from three orthogonal planes, which eliminate the thickness effect and reflect the true structural characteristics. Through using these ultrathin slices, the complex CC hexagonal mesostructure can be successfully differentiated from its CH counterpart. The pitch and the chirality are the two most significant parameters for the analysis of a helix. How to exactly and effectively obtain these two parameters is very challenge. The HH mesostructure, not only have traditional 2D hexagonal symmetry, but also two different helical operations (the internal twist and the external spiral), thus helical parameters are difficult to be determined and the detailed structural analysis is rarely reported. In Section 2.2, the fine structure of a complicated HH rod has been successfully solved by ET. By making use of the tilt series and tomogram slices, the pitch and handedness of both internal and external helices are directly determined. Moreover, the origin of helical fringes in HH mesostructures and the distribution of fringes as a function of the helical parameters have been clearly elucidated. This is the first report using the ET method to solve a complex structure with both conventional crystal symmetries and unusual geometrical configurations, which provides a new method for the characterization of complex porous materials.In Chapter 3, an intriguing evolution from a simple internal helix to a HH mesostructure or a complicated screw-like and CC mesostructure was successfully determined. Based on the structural characterization described in Chapter 2, we demonstrate a topological helix-coil transition between the internal and external helices to reveal the origin of the HH mesostructure and the relationship between the straight helical and HH rods. Moreover, the boundary condition of the helix-coil transition is clarified to explain the formation of complex helical structures in detail. It is proposed that the final structural characteristics are governed by the balance between the decrease in the surface free energy and the maintenance of the hexagonal packing in one individual rod. This success has opened new opportunities in determining the true structural characteristics of complex porous architectures, thus paving a way to systematically explore their formation mechanisms and structural transitions.Helical and CC mesostructured materials with pure silica composition have been successfully synthesized, however, there is no report on CC periodic mesoporous organosilicas (PMO) architecture and only one publication on chiral PMOs up to now. PMOs with uniform distribution of functional organic groups and tunable hydrophilic/hydrophobic organic framework, have much more virtues and potential applications compared with pure inorganic silica framework. After understanding the structural evolution of helical mesostructures (Chapter 3), we used ethylene bridged organosilica precursors instead of inorganic silica source, to successfully prepare PMOs with chiral and CC mesostructures through similar process in Chapter 4. By increasing the amount of PFOA, a pore architecture transition of PMO materials from hexagonal-arrayed, straight longitudinal channels to helical and CC mesostructures was achieved. Such a transition has not been observed previously in PMO materials. Our discovery is helpful in understanding the supra-molecular cooperative assembly of hybrid materials and their structural and morphological evolutions, which are important in the future applications of PMO materials.On the basis of studying the effect of PFOA on the self-assembly of ionic surfactant in the basic condition, we further investigated the interaction between PFOA and non-ionic block copolymer surfactant in the acid solution in Chapter 5. By adjusting the molar ratio of PFOA and block copolymer, a structural transition from a highly ordered 2D hexagonal mesostructure with a rod-like morphology to multi-lamellar vesicles with sharp edges was achieved. Importantly, an intriguing intermediate structure transition state at the end of hexagonally mesostructured rods was firstly observed in our systematic study, which provides key evidence in understanding the structural transition mechanism. It is suggested that PFOA molecules with rigid fluorocarbon chains interact with PEO through hydrogen bonding, which will modify the hydrophilic/hydrophobic volume ratio and, in turn, induce the structural transition. It is anticipated that our understandings as well as the use of block copolymers and fluorinated surfactant mixed templates will lead to the fabrication of novel porous materials.In Chapter 6, the packing structures of ordered, layered siliceous foams at the nanoscale have been successfully determined by ET. In general, the self-assembled inorganic/organic composite vesicles behave like nanobubbles, which form aggregates first in large areas, and then experience a fusion of adjacent vesicles into supra-structures along with the condensation and dehydration of silica species. The MOSFs adopt an ordered 2D hexagonal arrangement in single-layered areas, regular honeycomb patterns for double-layered samples, and polyhedric cells similar to the Weaire-Phelan structure in multilayer areas. All three packing modes follow the principle of minimizing surface area, which is well understood in soap foams at the macroscopic scale. Our contribution extends this principle to nano-scale, which will be important in the design and synthesis of novel macroporous and/or foam-like materials with versatile applications.In Chapter 7, we utilized a series of advanced characterization methods, such as synchrotron radiation small-angle X-ray scattering (SAXS), electron crystallography and ET, to comprehensively analyze an ordered hard-sphere packing (HSP) silica mesostructure with plate-type hexagonal morphology and bimodal pores. High-resolution SAXS result showed that HSP silica with Fm3m symmetry is in fact an intergrowth of cubic and hexagonal close-packing structures and the intensity ratio of main diffraction peaks is different from other conventional mesostructure with the same symmetry. The 3D visualization of the pore structure reconstructed by electrostatic potential maps demonstrates the existence and positions of the mesopores, tetrahedral and octahedral sites in the lattice, which confirms the HSP formation mechanism proposed previously. We further utilize ET to directly observe and accurately determine the sizes of mesopores and octahedral interspaces, which agrees well with the nitrogen sorption result. The existence of interspaces induces the interior structural difference which has been detected by SAXS and validated by small-angle X-ray diffraction simulations. In addition, we propose that the unique plate-like hexagonal morphology is formed by the close packing of preformed spherical composite micelles in the layer-by-layer manner. This work explains the bimodal pore distribution and accurately confirmed the existence of the packing interspaces and their sizes, in further support of the HSP formation mechanism. Moreover, this study shows the power of comprehensively using various characterization methods on solving the complex nano-scale structures.
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