The mechanism of forming orthogonally oriented hexagonal close-packed (o-HCP) mesostructures during aging of surfactant-templated titania thin films is elucidated using in situ grazing incidence small-angle X-ray scattering (GISAXS) in a controlled-environment chamber. To promote orthogonal orientation, glass slides are modified with cross-linked Pluronic P123, to provide surfaces chemically neutral toward both blocks of mesophase template P123. At 4 °C and 80% RH, the o-HCP mesophase emerges in thin (∼60 nm) films by a direct disorder-to-order transition, with no intermediate ordered mesophase. The Pluronic/titania o-HCP GISAXS intensity emerges only after ∼10-12 min, much slower than previously reported for small-molecule surfactants. The Avrami model applied to the data suggests 2D growth with nucleation at the start of the process with a half-life of 39.7 min for the aging time just after the induction period of 7 min, followed by a period consistent with 1D growth kinetics. Surprisingly, films that are thicker (∼250 nm) or cast on unmodified slides form o-HCP mesophase domains, but by a different mechanism (2D growth with continuous nucleation) with faster and less complete orthogonal alignment. Thus, the o-HCP mesophase is favored not only by modifying the substrate but also by aging at 4 °C, which is below the lower consolute temperature (LCST) of the poly(propylene oxide) block of P123. Consistent with this, in situ GISAXS shows that films aged at room temperature (above the LCST of the PPO block) have a randomly oriented HCP mesostructure.
|Number of pages||15|
|Journal||Journal of Physical Chemistry C|
|State||Published - Oct 8 2015|
Bibliographical notePublisher Copyright:
© 2015 American Chemical Society.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Energy (all)
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films