Metal oxide cavities and tubular nanostructures of functional compounds including Ti, Ta, Fe, Mo, W or Nb oxides were engineered in terms of morphology (namely anodic self-organized nanotubes with controlled diameter, length, wall thickness), crystallographic features, and electronic and optical properties.
Particularly impressive were the results in engineering semiconductor electronic properties. We identified defined lattice defects, such as Ti3+ states and specific oxygen vacancies to provide or mediate strongly the efficiency of photocatalysis. Remarkable is particularly the work on noble metal free photocatalysis.[1-10] This defect engineering was achieved by strategies such as thermal hydrogenation,[1,6,10-12] ion implantation[2,4,7] or suboxide or hydride oxidation.[3,5] We demonstrated the manipulation of such defects (nature, density and energetics) to be as well an effective path for tuning the oxide electronic transport and co-catalytic properties. Opposite defect engineering (eliminating native defects) led to anatase TiO2 with nanotwinned grain structures in the nanotube walls - these tubes show a high conductivity (charge carrier mobility), highly useful for fabricating photo-anodes for dye sensitized solar cells. Most importantly, we demonstrated that an adequate reduction of TiO2 (to a grey form) can provide co-catalyst free photocatalytic H2 evolution. We established that such activity originates from specific lattice defect (suitable oxygen vacancies and Ti3+ states) that are intrinsic of mildly reduced, grey TiO2, while black titania requires co-catalyst activation. We explored the formation of grey forms for various TiO2 polymorphs, e.g. anatase, rutile and brookite, and for different morphologies, including nanotubes, nanopowders and single crystals.
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