Morphology and Optical Properties of Highly Ordered TiO2 Nanotubes Grown in NH4F/o-H3PO4 Electrolytes in View of Light-Harvesting and Catalytic Applications

Highly ordered titanium dioxide nanotube (TiO2 NT) arrays were grown by electrochemical anodization of titanium in a molten NH4F/o-H3PO4 electrolyte. NTs with various diameters, lengths, wall thicknesses, and intertube distances could be obtained by tuning key anodization parameters such as the applied potential, anodization time, electrolyte temperature, concentration of NH4F, and H2O content. The morphology and optical properties were characterized by scanning electron microscopy and UV–vis spectroscopy techniques. We show that all aforementioned parameters have a strong influence on the nanostructured morphology and optical characteristics (reflectivity) of the formed nanotubular layers. Their optical features were simulated numerically to support the experimental measurements. We show that the optical features of anodic TiO2 nanotube layers result from the overlay of the individual optical properties of various “structural elements”, e.g., the NT barrier layer, top opening morphology, intertube spacing, and thermally formed oxide sublayer. Our results provide tools for “a priori” design with nanoscale precision of TiO2 structures with advanced optical features for light-harvesting and catalytic applications, e.g., in sensing, photocatalytic self-cleaning processes, solar hydrogen generation, or photovoltaics.