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Invited talk; International audience

Self-aligned titania thin films of either the rutile or the anatase phase were successfully grown on FTO (F-doped SnO 2 ) substrates with hydrothermal methods. The selection of the targeted phase, either rutile or anatase, in the thin films was controlled by adjusting the content and type of acidic anions such as Cl − and SO 4 2− . The growth kinetics depended on the synthesis temperature and the types of alcohol in the solvent. The self-alignment in both the anatase and rutile thin films was attributed to the minimization of surface energies during growth. The hydrogen sensing characteristics of dense films of both rutile and anatase phases were investigated and there was remarkable improvement of sensitivity response over reported data. It was found that rutile films had higher sensitivity while anatase films had faster response.

TiO 2 nanocrystalline powders with various Mn-doping levels were synthesized by the sol–gel process using tetrabutyl titanate and manganese nitrate as precursors. The crystal structure, morphology, doping concentration, optical absorption property, and elemental state of the obtained samples were analyzed. TEM results showed that the synthesized TiO 2 powders were anatase nanoparticles about 7 nm in size. EDX and XPS analyses proved the incorporation of Mn ions into the TiO 2 lattice. A remarkable red shift of the absorption edge was achievable by increased Mn content, leading to gigantically narrowed energy gap to permit absorption well into the infrared spectral region. The dramatic optical absorbance of the doped TiO 2 nanopowders in the visible spectral region led to strong photocatalytic activity under visible light illumination, which was observed by measuring the degradation of methylene blue. In contrast, little degradation was observed for the pure TiO 2 powder. The optimum Mn/Ti ratio was observed to be 0.2 at.% for photocatalytic applications.

The effects of 3d transition metal doping in TiO2 phases have been simulated in detail. The results of modelling indicate that Mn has the biggest potential among 3d transition metals, for the reduction of energy gap and the introduction of effective intermediate bands to allow multi-band optical absorption. On the basis of theoretical formulation, we have incorporated considerable amount of Mn in nano-crystalline TiO2 materials. Mn doped samples demonstrate significant red shift in the optical absorption edge, with a secondary absorption edge corresponding to theoretically predicted intermediate bands/states. The gigantic red shift achievable in Mn-doped TiO2 is expected to extend the useful TiO2 functionalities well beyond the UV threshold via the optical absorption of both visible and infrared photon irradiance.

Self-assembled and vertically aligned rutile titania nanorods and thin films with a preferred [002] axial orientation were grown on substrates of fluorine-doped tin dioxide, using a hydrothermal method. Each nanorod was made of a bundle of densely packed and ultra fine nano-fibers growing along the [002] direction. The results show that ethanol substitution of water as solvent is highly effective in promoting the one-dimensional growth of the rutile nanorods and increasing their packing density in the thin films, which offers a simple-but-effectual leverage to monitor the nanorod structures for varied applications.