The effect of calcination conditions on the morphology, the architecture and the photo-electrical properties of TiO₂ nanotube arrays

Highly ordered TiO₂ nanotube arrays were successfully prepared by electrochemical anodization in a formamide-based electrolyte containing 0.5 wt.% NH₄F and 2 vol.% H₂O. The effects of the calcining temperature, the calcining time and the heating rate on the formation of the TiO₂ nanotube arrays were investigated in detail. The morphological changes and phase transformations of the TiO₂ nanotubes were analyzed by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. It was found that the calcining temperature and the calcining time determined the crystal phase, while the heating rate was only beneficial to altering the crystallinity. UV-vis diffuse reflectance spectroscopy was used to examine changes in the band gap energy. For applications to dye sensitized solar cells, a maximum conversion efficiency was achieved at 500 °C for 2 h with a heating rate of 10 °C/min, which is attributed to the highly crystalline anatase and the lower surface defect concentrations of the nanotubes. The optimum calcination conditions help to retard the electron recombination and allow higher dye absorption capacities, thereby increasing V_oc and J_sc.