Remarkable Birefringence in a TiO₂–SiO₂ Composite Film with an Aligned Mesoporous Structure

Abstract

Mesoporous titania-silica composite films with highly aligned cylindrical pores are prepared by the sol-gel method using a substrate with structural anisotropy. The strong alignment effect of a rubbing-treated polyimide film on a substrate provides a narrow alignment distribution in the plane of the film regardless of the fast condensation rate of titania precursors. The collapse of the mesostructure upon the surfactant removal is effectively suppressed by the reinforcement of the pore walls with silica by exposing the as-deposited film to a vapor of a silicon alkoxide. The existence of a silica layer on the titania pore wall is proved from the distributions of Ti and Si estimated by the elemental analysis in high resolution electron microscopy. The obtained mesoporous titania-silica composite film exhibits a remarkable birefringence reflecting the highly anisotropic mesoporous structure and the high refractive index of titania that forms the pore wall. The Δn value estimated from the optical retardation and the film thickness is larger than 0.06, which cannot be achieved with the conventional mesoporous silica films with uniaxially aligned mesoporous structure even though the alignment of the pores in the films is perfect. These inorganic films with mesoscopic structural anisotropy will find many applications in the field of optics as phase plates with high thermal/chemical/mechanical stabilities.

Mesoporous TiO2 films with regularly aligned slit-like nanovoids

“…It must be noted that the use of P123 alone does not provide an aligned mesostructure on the rubbing-treated polyimide. 8 On the other hand, no diffraction peaks are observed under the parallel geometry ( Fig. However, as described above, in-plane alignment of the micelles, which dominantly consist of block copolymers, in a mesostructured TiO 2 film can be controlled with the help of Brij56.…”

“…8 This large periodicity is due to the increase of the size of the micelles by using a bulky block copolymer surfactant, P123. This value is much larger than that of the 2D-hexagonal mesostructured TiO 2 film with an imperfect alignment, which is prepared using Brij56 alone on the same polyimide substrate.…”

“…Materials with nano-scale structural anisotropy can provide anisotropic properties, which are promising for applications to optical and electronic devices. [8][9][10][11][12][13][14][15] Many of the past research studies are concerned with mesoporous silica films in which the anisotropy is provided by anisotropic accommodation of guest species into the aligned mesopores. When we bring out anisotropic properties originating from nano-scale structural anisotropy for improving device performances, the local structural anisotropy needs to be guaranteed over macroscopic scales.…”

“…[3][4][5][6][7][8][9][10][11][12][13][14][15] Mesostructured films prepared through the self-assembly of surfactant molecules show various anisotropic properties when the structural anisotropy is provided on a macroscopic scale. 8 The material of this film is a composite of TiO 2 and SiO 2 , and therefore, the significant feature of TiO 2 , high refractive index, is attenuated by the coexistent SiO 2 . [8][9][10][11][12][13][14][15] Many of the past research studies are concerned with mesoporous silica films in which the anisotropy is provided by anisotropic accommodation of guest species into the aligned mesopores.…”

Titania-based functional nanocomposite materials fabricated by liquid processes

“…TiO 2 -containing nanocomposites with mesoporous, 18) nanorod, 19) nanowire, 20) and nanotube 21) structures have been fabricated to date (Fig. 1).…”