(617f) In situ Grazing Incidence Small Angle X-Ray Scattering (GISAXS) Study of the Formation of Multilayered Ordered Mesoporous Titania Films

Mesoporous titania (TiO₂) thin films have a unique combination of chemical, optical and electronic properties that make it a promising material 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 titania thin films with vertically oriented hexagonal cylindrical nanopores have been synthesized in our group by evaporation induced self-assembly, and we have shown that alignment of the pores orthogonal to the substrate can be achieved by modifying substrates with crosslinked layer of the Pluronic surfactant pore template (Koganti et al., 2006). A previous in situ grazing angle small angle x-ray scattering (GISAXS) study showed that aging at low temperature (4 °C) is important for the development of the orthogonal HCP (o-HCP) mesostructure in 60 nm thick TiO₂ films (Nagpure et al., 2015). However, a number of additional variables are important for eventual application of o-HCP TiO₂ films. First, for applications requiring high mass loading of TiO₂, the film thickness needs to be scaled up. We have found that this can be done using a layer-by-layer deposition technique, but the fidelity of epitaxial orthogonal alignment in the layers remains to be investigated. Second, humidity has been hypothesized to be an important parameter dictating orthogonal orientation by controlling the evaporation of solvent from the as-coated film. Well-ordered mesoporous structures have been obtained by aging at 4 °C and 94% RH but the limits and effects of RH are not known.

Here, in situ GISAXS is performed to determine if and how orthogonally oriented columnar 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 1000 nm). Results show that films cast onto modified glass slides maintain vertically oriented mesophase order as multiple layers are deposited. On the other hand, horizontally oriented 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 or orientations in the film. The mesostructure growth kinetics of every layer are analyzed using the Avrami equation to determine the growth rate, order and direction. In situ GISAXS is carried out with varying RH (35% and 70% RH) for F127-templated TiO₂films cast onto modified glass. While about 70% relative humidity provides well-ordered, oriented micelles, uniform but randomly oriented micelles are found at 35% RH. These studies, together with complementary SEM imaging, confirm that high relative humidity is important to the formation of orthogonally oriented mesoporous structures, and that thick films with orthogonally oriented pores can be formed by epitaxial growth of o-HCP mesostructures on top of existing o-HCP composite films.

References:

Koganti, V.R., Dunphy, D., Gowrishankar, V., McGehee, M.D., Li, X., Wang, J., Rankin, S.E. 2006. Generalized Coating Route to Silica and Titania Films with Orthogonally Tilted Cylindrical Nanopore Arrays. Nano Letters, 6(11), 2567-2570.

Nagpure, S., Das, S., Garlapalli, R.K., Strzalka, J., Rankin, S.E. 2015. In Situ GISAXS Investigation of Low-Temperature Aging in Oriented Surfactant-Mesostructured Titania Thin Films. The Journal of Physical Chemistry C, 119(40), 22970-22984.