Nature of Paramagnetic Defects in Black Titanium Dioxide Nanotubes

Abstract

Electron paramagnetic resonance (EPR) spectroscopy was used to study the nature of defects in black titanium dioxide nanotubes (TiNTs) prepared by electrochemical oxidation of titanium in ethylene glycol. The TiNTs that have intensive black color were formed under thermal treatment of amorphous TiNTs in hydrogen at 500°C. The work aimed to reveal the role of various defects in the black color of TiNTs. Amorphous carbon has been found to contribute mainly to the deep black color of anatase-type TiNTs. Considerable growth in •С-radicals concentration was detected by the EPR method in amorphous TiNTs under their heat-treatment in hydrogen. The radicals are resulted from the reduction of carbon-containing products of electrochemical and thermal decomposition of ethylene glycol. The dependence of the •С-radical signal position in EPR spectra on the axis of the applied magnetic field testifies for a predominant orientation of graphite-like networks along the direction of TiNTs growth. The Gaussian shape and broadness of the EPR lines are caused by the superposition of several signals from different carbon states. The conditions of synthesis that might result in the occurrence of Ti3+state and oxygen vacancies in titanium oxide were analyzed. The absence of Ti3+signals in the EPR spectra of TiNTs re-duced in hydrogen can be explained by the high concentration of Ti3+centers as well as by the formation of [Ti3+—VO—Ti3+] or [VO—Ti—VO]n– complexes, which are inactive in EPR. The indicated defects might also contribute to titanium dioxide color.