Microstructure and composition of TiO₂ nanotube arrays fabricated with HF and NH₄F electrolytes and their evolution during annealing

The evolution of microstructure and composition of titanium dioxide (Ti O₂) nanotube arrays fabricated with hydrofluoric acid (HF) and N H₄ F electrolytes as a function of annealing temperature up to 400°C was investigated and compared using X-ray diffraction, scanning electron microscopy, and X-ray absorption near-edge structure spectroscopy. Results showed that Ti O₂ nanotube arrays grown in HF electrolyte contained 90% amorphous Ti O₂ and 10% lower oxidation states of titanium from Ti2+ (TiO) and Ti3+ (Ti₂ O₃) cations. After annealing at 400°C, TiO₂ nanotube arrays underwent charge transfer and phase transformation to 93% anatase phase, 6% amorphous Ti O ₂, and 1% suboxides. In contrast, as-grown Ti O₂ nanotube arrays using NH₄F electrolyte possessed less amorphous TiO ₂ (82%) but more suboxides (18%) due to lower oxygen ion formation from scanty 3 wt % of H₂O addition. Its onset temperature of phase transformation was found to be higher than TiO₂ nanotube arrays prepared by HF solution. Moreover, when annealed to 400°C, the crystallinity of TiO₂ nanotube arrays increased only by 86% for the anatase phase. The lower anatase phase could be attributed to the formation of (NH ₄)₂ Ti F₆ type compounds presumably formed by the reaction of Ti F6 2- and NH₄₊ ions dissociated from N H ₄ F.