Evolution of titania nanotube arrays at different calcination temperatures. The electrolyte was ethylene glycol containing 0.3 wt% ammonium fluoride and 5 vol% distilled water. a The decrease in the thickness of titania nanotube arrays at different annealing temperature from 300 to 600 °C. The insets are corresponding SEM images and the scale bar is 5 μm. b The schematic of crystallization process of anodic titania nanotubes annealed at (1) 450 °C, (2) 500 °C, and (3) 600 °C in air. Reproduced from ref.