We demonstrate the electrochromic properties of TiO₂ nanotubes prepared by an anodization process and investigate the effects of heat treatment and viologen incorporation on them. The morphology and crystal structure of anodized TiO₂ nanotubes are investigated by scanning electron microscopy and X-ray diffraction. As-formed TiO₂ nanotubes have straight tubular layers with an amorphous structure. As the annealing temperature increases, the anodized TiO₂ nanotubes are converted to the anatase and rutile phases with some cracks on the tube surface and irregular morphology. Electrochemical results reveal that amorphous TiO₂ nanotubes annealed at 150°C have the largest oxidation/ reduction current, which leads to the best electrochromic performance during the coloring/bleaching process. Viologenanchored TiO₂ nanotubes show superior electrochromic properties compared to pristine TiO₂ nanotubes, which indicates that the incorporation of a viologen can be an effective way to enhance the electrochromic properties of TiO₂ nanotubes.

In this study, we demonstrate the photoelectrochromic devices composed of TiO₂ and WO₃ nanostructures prepared by anodization method. The morphology and the crystal structure of anodized TiO₂ nanotubes and WO₃ nanoporous layers are investigated by SEM and XRD. To fabricate a transparent photoelectrode on FTO substrate, a TiO₂ nanotube membrane, which has been detached from Ti substrate, is transferred to FTO substrate and annealed at 450°C for 1 hr. The photoelectrode of TiO₂ nanotube and the counter electrode of WO₃ nanoporous layer are assembled and the inner space is filled with a liquid electrolyte containing 0.5 M LiI and 5 mM I₂ as a redox mediator. The properties of the photoelectrochromic devices is investigated and Pt-WO₃ electrode system shows better electrochromic performance compared toWO₃ electrode.

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