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This study addresses the effects of mesopore orientation on mesostructural stability and crystallization of titania thin films during calcination based on measurements with in-situ grazing incidence small angle X-ray scattering (GISAXS). Complementary supporting information is provided by ex-situ electron microscopy. Pluronic surfactant P123 (with average structure (EO) 20 (PO) 70 (EO) 20 where EO is an ethylene oxide unit and PO is a propylene oxide unit) serves as the template to synthesize titania thin films on P123modified glass slides with 2D hexagonally close-packed cylindrical mesopores. The orientation of the pores at the top surface is controlled by sandwiching another P123modified glass slide on top of the titania thin film to completely orient the pores orthogonal to the films in some samples. This provides the opportunity to directly observe how pore orientation affects the evolution of pore order and crystallinity during calcination. The results show that when the pores are oriented parallel to the substrate at the top surface (for unsandwiched films), the pore structure is stable upon calcination at 400 °C but that the structure is quickly lost due to crystallization throughout the film during calcination at 500 °C. Films with pores oriented orthogonal to the substrate at the top surface (sandwiched films) retain their long-range pore order even after calcination at 500 °C. The reasons for this difference are ascribed to greater resistance to anisotropic stress during heating of the orthogonally oriented pores and titania crystallization nucleation at the top surface of the films with orthogonally oriented pores.

The mechanism of forming orthogonally oriented hexagonal closepacked (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 smallmolecule 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.

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A detailed Fourier transform infrared spectroscopy (FTIR) study of the kinetics of polycondensation of surfactant-templated mesostructured silica thin films was carried out with the goal of understanding how to manipulate the synthesis and processing of these films to achieve a desired architecture. The evolution of silica condensation was followed both during the sol preparation process and in situ for a time period on the order of minutes to hours after initial film deposition. The kinetics were measured in the presence of three commonly used classes of surfactant templates: P123 [poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer), Brij-56 (poly(ethylene oxide] [n ∼ 10] hexadecyl ether), and CTAB (cetyltrimethylammonium bromide). The induction period for polycondensation (an initial period during which the rate of change of silanol content is slow) was different in each case, with P123 giving the longest induction time followed by Brij-56 and finally CTAB. Humidity was found to increase the induction time for polycondensation in general, and the initial period of up to 30 min after film deposition was identified as a critical “tunable steady state” interval during which conditions can be adjusted to tune the film properties for different applications, for instance, to alter the nature of the mesophase or its orientation by imposing external forces by confining interfaces or by electrical, magnetic, or flow fields.