Microstructure and Composition of TiO[sub 2] Nanotube Arrays Fabricated with HF and NH[sub 4]F Electrolytes and Their Evolution during Annealing

The evolution of microstructure and composition of titanium dioxide (Ti0 2 ) nanotube arrays fabricated with hydrofluoric acid (HF) and NH 4 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 TiO 2 nanotube arrays grown in HF electrolyte contained 90% amorphous Ti0 2 and 10% lower oxidation states of titanium from Ti 2+ (TiO) and Ti 3+ (Ti 2 O 3 ) cations. After annealing at 400°C, TiO 2 nanotube arrays underwent charge transfer and phase transformation to 93% anatase phase, 6% amorphous Ti0 2 , and 1% suboxides. In contrast, as-grown Ti0 2 nanotube arrays using NH 4 F electrolyte possessed less amorphous TiO 2 (82%) but more suboxides (18%) due to lower oxygen ion formation from scanty 3 wt % of H 2 0 addition. Its onset temperature of phase transformation was found to be higher than Ti0 2 nanotube arrays prepared by HF solution. Moreover, when annealed to 400°C, the crystallinity of TiO 2 nanotube arrays increased only by 86% for the anatase phase. The lower anatase phase could be attributed to the formation of (NH 4 ) 2 TiF 6 type compounds presumably formed by the reaction of TiF 2― 6 and NH + 4 ions dissociated from NH 4 F.