Novel (and better?) titania-based photocatalysts: Brookite nanorods and mesoporous structures

Herein, recent results concerning the synthesis of tailored anatase/brookite mixtures and of pure brookite TiO ₂ nanorods will be reported employing a simple hydrothermal method, i.e., the reaction of aqueous solutions of the titanium(IV) bis(ammoniumlactate) dihydroxide complex with urea. Highly ordered hexagonal P6m mesoporous Pd or Au doped TiO ₂ nanocomposites have also been synthesized using the F127 triblock copolymer as a template. Utilizing these nanomaterials, the effect of the phase composition, the role of the surface area, and of the ordered mesoporous structure on the photocatalytic activity of titania-based photocatalysts have been investigated. The results revealed that anatase/brookite mixtures and brookite nanorods exhibit higher photocatalytic activity than anatase nanoparticles and even higher than Aeroxide (Evonik) TiO ₂ P25 for the photocatalytic H ₂ evolution from aqueous methanol solution, despite the fact that the former have lower surface areas. This behavior is explained by the fact that the flatband potential of brookite nanorods is shifted by 140 mV more cathodically than the flatband potential of anatase nanoparticles and/or by the better charge carrier separation in the case of anatase/brookite mixtures. Hexagonal P6m mesoporous Au and Pd/TiO ₂ nanoarchitectures showed ∼3-4 times higher activity for the photooxidation of CH ₃OH than Pd photodeposited on commercial Sachtleben Hombikat UV-100. The increased HCHO formation rate revealed that the photocatalytic oxidation efficiencies within the mesoporous Pd/TiO ₂ system are (in spite of its lower surface area) superior to that of Pd/UV-100. The key to this success is the preparation of Pd/TiO ₂ networks with an ordered mesoporous structure which at the same time render the methanol diffusion into the bulk of the photocatalysts facile and hence provide fast transport channels for the methanol molecules.