Extensive Inspection of an Unconventional Mesoporous Silica Material at All Length-Scales

The structure of an unconventional mesoporous material, formed by the packing of silica coated spherical micelles as hard spheres, has been uniquely determined through a series of advanced characterization techniques. The synchrotron-based small-angle X-ray scattering (SAXS) analysis confirms that the bulk material assembled via the hard sphere packing (HSP) route exhibits a strong 200 reflection and a relatively weaker 111 reflection, which is the first example in all reported mesostructured materials with the same symmetry. At the morphological macroscale, high-resolution scanning electron microscopy (SEM) images directly show that the hexagonal platelike micrometer-sized particles consist of nanospheres (20 nm in diameters) in a close packing mode. The intrinsic pore structure of calcined HSP material has been reconstructed using both electron crystallography (EC) and electron tomography (ET) techniques, which can be simply viewed as a face-centered cubic (fcc) packing of monodispersed hollow silica nanospheres. The EC technique provides a three-dimensional visualization of the pore organization and demonstrates the existence and crystallographic positions of the cagelike mesopores, octahedral and tetrahedral cavities. The ET method directly and accurately determines the sizes of the mesopores and octahedral cavity and offers nanometer-scale structural information at any given local area, which cannot be obtained by conventional transmission electron microscopy (TEM). To our knowledge, this is the first time that the EC and ET techniques are simultaneously employed and provide complementary information for the mesostructure determination. More importantly, the structural details collected from the synchrotron SAXS, high resolution SEM, EC and ET techniques are consistent and support the HSP mechanism, different from the well-understood liquid crystal templating or cooperative self-assembly pathways. The complex pore structure and the existence of octahedral and tetrahedral cavities are responsible for the unusual indexation of the SAXS, which is further validated by the structural simulation. Our work provides both a comparative and comprehensive case study to show the strength and limitation of individual techniques and demonstrates the need for the careful characterization of novel structures by a selection of complementary, state-of-the-art methods which provide selective structural information at different length scales.