Mesoporous TiO2 films with regularly aligned slit-like nanovoids†
Novel mesoporous TiO2 films with regularly aligned slit-like nanovoids are prepared through structural transformation from a mesostructured TiO2 film with honeycomb-packed aligned cylindrical micelles by pyrolytic removal of the micelle template. The transformation takes place through interconnection of the TiO2 walls of the framework in the thickness direction by a heat-induced shrinkage and eventual collapse of the original channel structure. For the formation of this new structure, the preparation of a mesostructured titania film with cylindrical micelles aligned entirely in the plane of the film over the whole thickness is indispensable. This is achieved by coating a substrate, on which a rubbing-treated polyimide layer is formed, with a precursor solution containing two nonionic surfactants, Brij56 and P123. In the mixed surfactant system, Brij56 works as an alignment-controlling agent through selective and directional adsorption on the anisotropic polymer surface. On the other hand, P123 suppresses the formation of a surface layer without controlled in-plane alignment, which has been inevitable when Brij56 is used alone. This is caused by the retarded condensation of the TiO2 precursors due to increased coordination of oxyethylene moieties on titanium. P123 also increases the wall thickness of the framework, which also contributes to the formation of this mesoporous TiO2 film with oriented regular slit-like voids. The structural transformation takes place in a relatively low temperature range lower than 300 °C, which shows that the driving force is not crystallization. The mesoporous TiO2 films with aligned slit-like voids show optical anisotropy, birefringence, with a Δn value of ∼0.023 reflecting the structural anisotropy of the film. Calcination of the aligned mesostructured TiO2 film at 450 °C induces crystallization of TiO2, which deteriorates the meso-scale structural regularity by interconnection of the TiO2 walls. However, the partial retention of the regular structure is confirmed in the vicinity of the surface, which allows the retention of the optical anisotropy. The novel mesoporous TiO2 films in this paper have potential for optical applications by combining their unique anisotropic mesostructure, which enhances the accessibility to the inner surface, with various properties of TiO2 such as high refractive index and photocatalytic activity.