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

Abstract

The evolution of microstructure and composition of titanium dioxide (Ti O 2 ) nanotube arrays fabricated with hydrofluoric acid (HF) and N H 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 Ti O 2 nanotube arrays grown in HF electrolyte contained 90% amorphous Ti O 2 and 10% lower oxidation states of titanium from Ti2+ (TiO) and Ti3+ (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 Ti O 2 , and 1% suboxides. In contrast, as-grown Ti O 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 O addition. Its onset temperature of phase transformation was found to be higher than TiO 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 Ti F 6 type compounds presumably formed by the reaction of Ti F6 2- and NH 4+ ions dissociated from N H 4 F. © 2011 The Electrochemical Society.