385676 In-Situ Gisaxs Investigation of Mesostructure Formation during Low-Temperature Aging in Mesoporous Titania Thin Films
The this study, we investigate the mesostructure formation mechanism during aging in mesoporous titania thin films with oriented hexagonal close packed (HCP) cylindrical nanopores. Aging of titania films in a refrigerator under high humidity (approx. 94%) is an essential part of the formation of well-ordered mesostructured in titania films using titanium ethoxide as the precursor and Pluronic P123 as the pore template (e.g. Koganti et al., Nano Letters2006, 11, 2567), but the kinetics of mesostructure development and orientation have not been investigated. Preliminary investigations in our group showed that significant changes in the ordered mesostructure can continue for at least 20 minutes after film deposition, so this is the relevant time scale for further investigation. To provide orthogonal alignment of the pores, surface modification of substrates with crosslinked P123 has been used to provide a surface that is chemically neutral towards the PEO and PPO blocks of P123.
Here, a GISAXS study has been performed in which in-situ measurements are done immediately after coating a film inside of a controlled-environment chamber, to understand the rate of mesostructure development and orientation during aging. The GISAXS pattern shows rods consistent with an orthogonally aligned mesophase on both sides of the beam stopper, indicating that the cylindrical nanopores are aligned orthogonal to the modified substrate. The intensity of these peaks has been used to study the time dependent mesostructure development during aging. We find that mesostructure development begins after approximately 10-12 minutes of aging in a high-humidity environment, and that after this time lapse, the mesostructure starts developing rapidly. The orthogonal hexagonal mesostructure is the first ordered phase to form by a disorder-order transition. The experiments are performed not only for films on plain glass (orientation parallel to the film) but also for films on modified glass with orthogonally aligned cylindrical pores in order to the effect of pore orientation during aging. In-situ GiSAXS has also been done during aging at room temperature to study the effect of aging temperature over mesostructure development and orientation. From the temperature study, we find direct evidence that aging at 4 °C is indeed necessary for ordered mesostructure development and that aging at this temperature helps to provide orthogonal orientation of the cylindrical nanopores.
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