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 nanostructuresprepared by anodization method. The morphology and the crystal structure of anodized TiO2 nanotubes and WO3 nan-oporous layers are investigated by SEM and XRD. To fabricate a transparent photoelectrode on FTO substrate, a TiO2nanotube membrane, which has been detached from Ti substrate, is transferred to FTO substrate and annealed at 450°Cfor 1 hr. The photoelectrode of TiO2 nanotube and the counter electrode of WO3 nanoporous layer are assembled andthe inner space is filled with a liquid electrolyte containing 0.5 M LiI and 5 mM I2 as a redox mediator. The propertiesof the photoelectrochromic devices is investigated and Pt-WO3 electrode system shows better electrochromic perform-ance compared toWO3 electrode.