Highly porous TiO2 anatase optical thin films with cubic mesostructure stabilized at 700°C

TiO2 optical thin films stable to 700 degreesC, exhibiting 35% volume porosity, more than 100 m(2.)g(-1) in surface area, fully nanocrystalline anatase framework, and organized mesostructure (cubic Im3m derived), have been stabilized by careful delayed rapid crystallization (DRC) thermal treatments. In-situ time-resolved SAXS and WAXS investigations were simultaneously performed during such treatments. They revealed that a slow and progressive heating to a temperature just below that of the formation of anatase (T-c approximate to 400 degreesC), followed by a long pretreatment at this temperature, stabilizes the amorphous network. A following rapid increase of temperature up to temperatures as high as typically 700 degreesC, followed by a short residence time at this high temperature, provokes the homogeneous formation of crystalline small nanoparticles and the total elimination of organic residues. The crystallization is accompanied by matter migration through diffusing sintering and pore merging along the [111] directions of the cubic structure, leading to a novel grid-like mesostructure with open porosity. This DRC treatment allows the preparation of highly porous and crystalline anatase films, with thermal stability 200 degreesC higher than previously reported, that are ideal for energy transfer applications. This emphasizes the role of the treatment method to stabilize transition metal oxide mesoporous materials over extended crystallization at high temperatures. These films exhibit excellent long time stability below 500 degreesC.