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 micrometer 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 cross-linked F127 maintain a (011)-oriented Im3 m 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 relative humidity (38%) regardless of substrate modification. Avrami model analysis shows that each oriented epitaxial layer followed formation kinetics with the same order (n = 1.8 ± 0.3) and half-life (10-20 min). Cross-sectional electron microscopy and impedance spectroscopy of the films after calcination at 400 °C show the formation of continuous, accessible vertical pore channels by micelle fusion for mesophases properly oriented by initial substrate modification.