Mesoporous silicas, SBA-15, KIT-6, FDU-12 and SBA-16, have been used as hard templates, allowing crystal growth of transition metal oxides, such as Cr₂O₃, Co₃O₄, WO₃, NiO, CeO₂, In₂O₃, Fe₂O₃, MnO₂, etc, inside the pores. After removing the silica templates, the final products are porous crystals of metal oxides with various morphologies depending on the pore systems in the templates. The materials have a strong application background. For example, they might be developed into self-supported nanoscale catalysts, advanced materials for fuel cells and Li-batteries. Synthesis, structural characterization and the most recent progress of these materials are reviewed. Three most popular synthesis methods, namely the surface functionalization method, the evaporation method and the solid-liquid method, are discussed. In addition to X-ray powder diffraction and nitrogen adsorption/desorption methods, electron microscopy, including transmission electron microscopic imaging, electron diffraction and energy dispersive X-ray spectroscopy techniques, is a powerful tool in studies of these new nanomaterials. Some examples of electron microscopic studies of crystal structures, defects, crystal orientations and crystal growth mechanisms with possible confinement effect of nanopores are demonstrated. Future developments of these materials are also discussed.

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