α-FeO/TiO 3D Hierarchical Nanostructures for Enhanced Photoelectrochemical Water Splitting

We report the fabrication of 3D hierarchical hetero-nanostructures composed of thin α-FeO nanoflakes branched on TiO nanotubes. The novel α-FeO/TiO hierarchical nanostructures, synthesized on FTO through a multi-step hydrothermal process, exhibit enhanced performances in photo-electrochemical water splitting and in the photocatalytic degradation of an organic dye, with respect to pure TiO nanotubes. An enhanced separation of photogenerated charge carriers is here proposed as the main factor for the observed photo-activities: electrons photogenerated in TiO are efficiently collected at FTO, while holes are transferred to the α-FeO nanobranches that serve as charge mediators to the electrolyte. The morphology of α-FeO that varies from ultrathin nanoflakes to nanorod/nanofiber structures depending on the Fe precursor concentration was shown to have a significant impact on the photo-induced activity of the α-FeO/TiO composites. In particular, it is shown that for an optimized photo-electrochemical structure, a combination of critical factors should be achieved such as (i) TiO light absorption and photo-activation vs.α-FeO-induced shadowing effect and (ii) the availability of free TiO surface vs.α-FeO-coated surface. Finally, theoretical analysis, based on DFT calculations, confirmed the optical properties experimentally determined for the α-FeO/TiO hierarchical nanostructures. We anticipate that this new multi-step hydrothermal process can be a blueprint for the design and development of other hierarchical heterogeneous metal oxide electrodes suitable for photo-electrochemical applications.