Improved anatase phase stability in small diameter TiO₂ nanotube arrays for high performance dye-sensitized solar cells

Highlights: • A simple preparation to synthesize pure anatase phase small diameter TiO₂ nanotubes. • Reliable method to obtain TiO₂ nanotube membranes with pure anatase phase. • This kind of TiO₂ nanotubes were incorporated into DSSCs with higher efficiencies. - Abstract: Anatase is the preferred phase of TiO₂ in dye-sensitized solar cells (DSSCs) because of its lower charge recombination than other phases. However, for small diameter nanotubes before detached from the Ti foil, rutile rather than anatase appears upon a pre-treatment annealing at 400 °C or above. Here we have fabricated highly ordered free-standing small diameter (50 nm) TiO₂ nanotube membranes which were pre-treated at 300 °C to maintain pure anatase phase. The free-standing nanotube membranes were used to fabricate the photoanodes and were further calcined at 500 °C to achieve full crystallization of the nanotubes. It was shown that the electron lifetime is much longer in the 300 °C-pre-treated nanotubes than those pre-treated at 400 or 500 °C, leading to a significantly improved power conversion efficiency of 4.59% (enhanced by ∼50%)