Layer-By-Layer Epitaxial Formation Mechanism of Ordered Titania Thick Films with Vertically Accessible Nanopores

Mesoporous titania (TiO₂) thin films have a unique combination of chemical, optical and electronic properties that make them promising materials in diverse fields including energy conversion and storage, environmental remediation, and biomaterials. All of these applications benefit from being able to produce accessible nanoscale channels due to their high surface area, well defined size and shape, and short diffusion paths for reactants and charge carriers. Mesoporous TiO₂ thin films with vertically oriented hexagonal cylindrical nanopores have been synthesized in our laboratory by evaporation induced self-assembly after modifying substrates with a crosslinked layer of the surfactant pore template and aging at low temperature (4 °C) for 60 nm thick TiO₂ films template with Pluronic P123. For applications requiring high mass loading of TiO₂, thicker pore walls are created by instead using a Pluronic surfactant (F127) with longer PEO blocks to co-assemble with titania precursors. F127 is likely to provide a cubic mesophase, but if properly oriented, this can transform to vertically oriented pore channels after high temperature calcination. Here, we test the hypothesis that thick films with vertically oriented channels can be formed by a layer-by-layer deposition technique where epitaxial alignment of micelles leads to continuous nanopore channels from the electrolyte to the electrode.

In situ GISAXS is performed to determine if and how epitaxially oriented templated layers form for multilayer Pluronic F127-templated TiO₂ films. GISAXS is performed on each layer of multilayer films up to 8 layers (having a total thickness of 1 μm). Results show that films cast onto modified glass slides maintain mesophase order as multiple layers are deposited. On the other hand, micelles on unmodified slides lose their preferred orientation as more layers are deposited, and by the third layer a powder pattern is obtained indicating a random mix of orientations. The mesostructure growth kinetics and formation mechanisms of every layer are analyzed using the Avrami equation to determine the growth rate, order and direction. Additionally, in situ GISAXS is carried out with varying relative humidity (RH, 35% and 75%) for F127-templated TiO₂ films cast onto modified glass. While about 75% RH provides well-ordered, oriented micelles, uniform but randomly oriented micelles are found at 35% RH. Ex situ GISAXS profiles of the films after calcination at 400 °C showed the formation of vertical pore channels obtained from pore fusion induced by anisotropic contraction during thermal treatment. Electrical impedance spectroscopy (EIS) of multilayered TiO₂ films showed increase in mass transfer resistance with number of layers but all the films on modified substrate showed less resistance compared to film on unmodified substrate, indicating higher accessibility. These studies confirm that thick films with vertically accessible pore channels can be formed after calcination by epitaxial layer-by-layer growth, but only when the substrate is initially modified with crosslinked surfactant.