There is a growing interest in quasi-one-dimensional TiO2 nanostructures (e.g. nanorods, nanowires, nanobelts and nanotubes). These structures offer superior charge transfer properties and high stability, making them highly suitable for application as photocatalysts with improved water splitting efficiency. Nanotubes and nanowires in particular offer direct electrical pathways for photogenerated electrons and could increase the electron transport rate, which in turn may improve the performance of photovoltaic devices such as solar-to-hydrogen cells. However, so far, low overall solar-to-hydrogen efficiencies has been achieved. This challenge has been attributed to the unsuitable band gap of TiO2, amongst others reasons. Hence, different deposition methods have been suggested as a route to engineer the band gap of TiO2 nanotubes and nanowires via doping, including molecular (and atomic) diffusion through in-situ anodic oxidation and chemical bath deposition. As this is the subject of ongoing research, we report on the preparation of TiO2 nanotubes on FTO (F: SnO2) glass substrates using a gelation step, which offers the possibility for doping prior to calcination, as an alternative preparation method for band gap engineered TiO2 nanostructures.
First, a template made of tall ZnO nanorods is prepared by chemical bath deposition. These rods are then coated with a Ti-based acidic gel prepared from a solution made of ammonium hexafluorotitanate and boric acid. Finally, the gel is calcinated at 550 oC into TiO2 nanotubes. Detailed structural, morphological and optical investigations are reported in this paper.