Fabrication of highly ordered TiO₂ nanorod/nanotube adjacent arrays for photoelectrochemical applications

This work reports a facile approach to fabricate a perpendicularly aligned and highly ordered TiO₂ nanorod/nanotube (NR/NT) adjacent film by directly anodizing a modified titanium foil. The titanium foil substrate was modified with a layer of crystalline TiO₂ film via a hydrothermal process in 0.05 M (NH₄)₂S₂O₈. The resultant NR/NT architecture consists of a highly ordered nanorod top layer that directly adjoins to a highly ordered nanotube array bottom layer. The thickness of the top nanorod layer was ∼90 nm with average nanorod diameter of 22 nm after 20 min of anodization. The thickness of the bottom nanotube array layer was found to be ca. 250 nm after 20 min of anodization, having an average outer and inner tubular diameters of 120 and 80 nm, respectively. A broad implication of the method is that a simple modification to the substrate surface can lead to new forms of nanostructures. For as-anodized NR/NT samples, XRD analysis reveals that the nanorods are of anatase TiO₂ crystalline form while the nanotubes are amorphous. Anatase TiO₂ crystalline form of NR/NT film with high crystallinity can be obtained by thermally treating the as-anodized sample at 450 °C for 2 h in air. The resultant NR/NT film was used as a photoanode for photoactivity evaluation. Comparing with a nanotube array photoanode prepared by direct anodization of unmodified titanium foil, the NR/NT photoanode exhibits a unique feature of selective photocatalytic oxidation toward organics, which makes it very attractive to photocatalytic degradation of organic pollutants, sensing, and other applications.