Epitaxial Formation Mechanism of Multilayer TiO2 Films with Ordered Accessible Vertical Nanopores by Evaporation-Driven Assembly

Surfactant-templated mesoporous titania (TiO2) films have excellent physical and electronic properties, but some potential applications require films thicker than the 100-200 nm typically prepared by sol-gel coating. Here, the mechanism of forming micron-thick TiO2 films with vertically oriented nanopore channels by a layer-by-layer deposition technique (up to eight 125 nm-thick layers) is investigated. In situ grazing incidence small angle x-ray scattering (GISAXS) performed on successive layers of Pluronic F127-templated films reveals if and how epitaxially oriented layers form during aging at 4 °C. At 78% relative humidity (RH), films cast onto substrates modified with crosslinked F127 maintain (011)-oriented Im-3m cubic mesophase order, whereas micelles on unmodified glass lose their preferred orientation, leading to the observation of an arc in GISAXS attributed to an anisotropic micelle structure. A similar arc from randomly oriented domains is found at low RH (38%) regardless of substrate modificat...