Recently, nanostructures such as nanorods and nanowires have drawn much interest because of their unique electronic and optical properties. Anodic aluminum oxide (AAO) method is a promising template-assisted technique which ensures high quality and repeatability for fabrication. Morphology of periodical nanoscale pores in AAO templates has been studied in relation with anodizing process for a few decades. The AAO templates are well prepared by a two-step anodization process method in 0.3 M oxalic acid solution at 10 °C and anodizing voltage 40 V. After first-step anodization, AAO layer was etched out in 5 wt.% phosphoric acid and 2.24 wt.% chromic acid at 60 °C for 1 HR. After barrier thinning, the AAO template was immersed into a 5 wt.% phosphoric acid solution at room temperature for 45 minutes. The pore length increases with electrochemical etching time, while the pore size and interpore distance depend on the applied potential. In our work, the AAO template is 45 nm in diameter and the interpore distance and pore length are 100 nm and 1 μm, respectively. In case that the Al₂O₃ barrier layer is too thick for the electrodeposition, barrier thinning proceeds by limited current is a necessary process to lower the working voltage and to create the nucleation site for the growth of Cu. The Cu/Cu₂O composite nanorods are synthesized from aqueous solution prepared from 0.7 M of CuSO₄ dissolved in DI water, with applied pulse potential -1.5 V at room temperature. The nanorods were annealed at 400 °C and resulted in the formation of CuO. After that, TiO₂ thin film was deposited on the top followed by annealing at 550 °C to form anatase. Etching out the AAO template, we can obtain CuO nanorod arrays standing on TiO₂ thin film nanostructure for application in water treatment. Besides, it can also be transferred on other flexible substrate or on the surface of the item promoting self-cleaning property.

CuO is a great Fenton-like photocatalyst which is active with H₂O₂ and produces free radicals reacting with the dye or pollutants. TiO₂ is stable in water and possesses good photocatalytic property in anatase phase. CuO is a narrow bandgap p-type metal oxide semiconductor which provides electrons and holes and absorbs infrared light, while TiO₂ is a n-type metal oxide semiconductor which absorbs UV light and has high transparency in sunlight. We combine two metal oxide to form heterojunction with AAO and electrodeposition method. When the infrared light is absorbed by CuO nanorods exciting electron and holes, the electrons shift to TiO₂ and the holes stay in CuO. These electron-hole pairs can react with H₂O₂ producing free radicals and start dye degradation. Furthermore, CuO nanorods immobilized on TiO₂ film nanostructure can also be applied as gas sensors or photodetectors and for water splitting.