In-situ GISAXS investigation of pore orientation effects on the thermal transformation mechanism in mesoporous titania thin films

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 P123-modified glass slides with 2D hexagonally close-packed cylindrical mesopores. The orientation of the pores at the top surface is controlled by sandwiching another P123-modified 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.