α-Fe₂O₃/TiO₂ 3D hierarchical nanostructures for enhanced photoelectrochemical water splitting

We report the fabrication of 3D hierarchical hetero-nanostructures composed of thin α-Fe₂O₃ nanoflakes branched on TiO₂ nanotubes. The novel α-Fe₂O₃/TiO₂ 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 TiO₂ nanotubes. An enhanced separation of photogenerated charge carriers is here proposed as the main factor for the observed photo-activities: electrons photogenerated in TiO₂ are efficiently collected at FTO, while holes are transferred to the α-Fe₂O₃ nanobranches that serve as charge mediators to the electrolyte. The morphology of α-Fe₂O₃ 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-Fe₂O₃/TiO₂ composites. In particular, it is shown that for an optimized photo-electrochemical structure, a combination of critical factors should be achieved such as (i) TiO₂ light absorption and photo-activation vs. α-Fe₂O₃-induced shadowing effect and (ii) the availability of free TiO₂ surface vs. α-Fe₂O₃-coated surface. Finally, theoretical analysis, based on DFT calculations, confirmed the optical properties experimentally determined for the α-Fe₂O₃/TiO₂ 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.