Microstructure and Composition of TiO(2) Nanotube Arrays Fabricated with HF and NH(4)F Electrolytes and Their Evolution during Annealing

Abstract

The evolution of microstructure and composition of titanium dioxide (TiO(2)) nanotube arrays fabricated with hydrofluoric acid (HF) and NH(4)F electrolytes as a function of annealing temperature up to 400 degrees 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 TiO(2) and 10% lower oxidation states of titanium from Ti(2+) (TiO) and Ti(3+) (Ti(2)O(3)) cations. After annealing at 400 degrees C, TiO(2) nanotube arrays underwent charge transfer and phase transformation to 93% anatase phase, 6% amorphous TiO(2), and 1% suboxides. In contrast, as-grown TiO(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 degrees 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(6)(2-) and NH(4)(+) ions dissociated from NH(4)F. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3533388] All rights reserved.