Structural study of highly ordered mesoporous silica thin films and replicated Pt nanowires by high-resolution scanning electron microscopy (HRSEM)

We have investigated the structures of mesostructured (before calcination) and mesoporous (after calcination) silica thin films and replicated Pt nanowires by high-resolution scanning electron microscopy (HRSEM). Highly ordered lamellar, two-dimensional (2D) hexagonal, and three-dimensional (3D) hexagonal mesostructured/mesoporous silica thin films were synthesized by adjusting the concentration of triblock copolymer (EO₂₀PO₇₀EO ₂₀; P123) used as a structure-directing agent. Direct HRSEM observation of these mesostructured/mesoporous silica thin films demonstrated that they have inherent external structures on the top surfaces: (1) the lamellar mesostructured silica film exhibits a smooth plane; (2) the 2D hexagonal mesoporous silica film exhibits curved stripes; (3) the 3D hexagonal mesoporous silica film exhibits 2D porous arrangements (e.g. 6-fold symmetry and pseudo 4-fold symmetry). The removal of P123 by calcination had no effect on the surface structure but caused an anisotropic contraction, as found from cross-sectional images. Methods for the formation of the 2D hexagonal mesoporous film affected the orientation of mesochannels. Pt replicates, electrodeposited in the mesochannels of the 2D hexagonal mesoporous silica films, exhibited new structures, i.e., S-shaped, Y-shaped, and swirling-shaped nanowires with ellipsoidal cross section. Direct HRSEM observation of the mesostructured/ mesoporous silica thin films on the top surface and cross section provides detailed information on the external and internal structures, which is significant and useful for various applications of these thin films.