Fabrication of nanomaterials using porous templates
Fabrication and characterization of different nanomaterials by using porous anodic aluminum oxide (AAO) templates were studied. Amorphous carbon nanotubes were prepared by casting thin films of polyacrylonitrile (PAN) and polystyrene-block-polyacrylonitrile (PS-b-PAN) within a AAO membrane followed by pyrolysis. Raman and wide angle X-ray diffraction (WAXD) measurements indicate that the carbon nanotubes are of low crystallinity. When diblock copolymers of PS-b-PAN were used, it was found that, nanopores were created within the nanotube walls after pyrolysis.
Nanotubes of the cylinder-forming polystyrene-block-poly(ethylene oxide) (PS-b-PEO) copolymer nanotubes were generated. Because of the water solubility of the cylindrical PEO microdomains and the orientation of the cylindrical PEO microdomains with respect to the nanotube walls, the nanotubes were permeable to aqueous media.
Rayleigh instabilities in thin polymer films confined within AAO membranes were studied. Thin films of PMMA were prepared by filling cylindrical nanopores in an AAO membrane with a PMMA solution in chloroform followed by solvent evaporation. When the PMMA nanotubes were annealed above the glass transition temperature (Tg), undulations in the film thickness were observed that were induced by a Rayleigh instability. The amplitude of the undulations increased with time and eventually bridged across the cylindrical nanopore in the AAO membrane, resulting in the formation of polymer nanorods with periodically encapsulated holes.
A facile route to prepare hierarchical structures by wetting polymer microspheres into the nanopores of AAO templates was presented. In this approach, polystyrene (PS) microspheres were first spread and self-assembled into well-ordered monolayers on a silicon wafer. By contacting the porous AAO template, polymer chains wet the porous template and form short nanorods on top of the micrsopheres after thermal annealing. These hierarchical structures show ordering at two length scales which can be controlled by the size of the polystyrene microspheres and the pore sizes of the template.
The generation of one-dimensional mesoporous silica and titania nanomaterials by using poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) as precursors was described. The porous structures were fabricated by evaporation induced self-assembly followed by pyrolysis. The orientation of the mesopores is parallel to the channels of the AAO membrane.