3D hexagonal (R-3m) mesostructured nanocrystalline titania thin films:: Synthesis and characterization

A straightforward and reproducible synthesis of crack-free large-area thin films of 3D hexagonal (R-3m) mesostructured nanocrystalline titania (meso-nc-TiO2) using a Pluronic triblock copolymer (P123)/1-butanol templating system is described. The characterization of the films is achieved using a combination of electron microscopy (high-resolution scanning electron microscopy and scanning transmission electron microscopy), grazing-incidence small-angle X-ray scattering, in situ high-temperature X-ray diffraction, and variable-angle spectroscopic ellipsometry. The mesostructure of the obtained films is found to be based upon a 3D periodic array of large elliptically shaped cages with diameters around 20 nm interconnected by windows of about 5 nm in size. The mesopores of the film calcined at 300 degrees C are very highly ordered, and the titania framework of the film has a crystallinity of 40% being composed of 5.8 nm sized anatase crystallites. The film displays high thermal stability in that the collapse of the pore architecture is incomplete even at 600 degrees C. The accessible surface area of 3D hexagonal meso-nc-TiO2 estimated by the absorption of methylene blue is nearly twice as large as that of 2D hexagonal meso-nc-TiO2 at the same annealing temperature.