Hydrogen storage in Ti–TiO₂ multilayers

Multilayered thin films of Ti–TiO₂ system have been investigated, focusing on all of the important parameters in both photocatalysis and H storage. Numerous Ti–TiO₂ thin films with a single-, bi- and tri-layered structure have been deposited on different substrates by means of dc pulsed magnetron sputtering from a metallic Ti target in an inert Ar or reactive Ar + O₂ atmosphere. The film chemical composition, depth profile, layer thickness and structure were determined by combined analysis of x-ray diffraction, x-ray reflectometry, Rutherford back- scattering and optical reflectivity spectra. The results show that the Ti films deposited on Si(111) exhibit a strong preferred orientation with the (00.1) plane parallel to the substrate, while a columnar structure was developed for TiO₂ films. H charging at 1 bar and at 300 °C revealed that, in the case of the tri-layered structure of Ti/TiO₂/Ti/Si(111), H diffused through the TiO₂ layer without any accumulation in it. Pd acts as a catalyst for gathering H in Ti layers and up to 50% of H is stored in the topmost and bottom Ti layers. The preferential orientation in the Ti films was found to be destroyed upon hydrogenation at 100 bar. The hydride TiHₓ phase (x < 0.66) was formed under such a high H pressure.