The Thermal-Oxidation Behavior of Pristine and Doped Magnéli Phase Titanium Oxides

Abstract

Magnéli phase titanium oxides (Ti n O2n−1, 4 ≤ n ≤ 10) are important materials for solid state and electrochemical technologies such as memristors, batteries, fuel cells, and electrochemical devices for water treatment. Developing an understanding of transitions between Ti n O2n−1 and its product of oxidation, titanium(IV) oxide (TiO2), as well as strategies such as doping to modulate the conditions for such changes will enable the development of more effective devices. To elucidate a mechanism for their thermal oxidation and investigate the influence of doping, the thermal-oxidation behavior in air of Ti4O7 doped with vanadium, chromium, and iron were investigated by thermogravimetric analysis (TGA). These powders prepared by high-temperature H2 reduction of dopant-containing TiO2 were characterized by scanning electron microscopy (SEM), gas adsorption analysis, X-ray fluorescence (XRF), energy-dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS), and powder X-ray diffraction (PXRD). V- and Fe-‍doping improved the thermal stability of Ti4O7 as evidenced by higher onset temperatures in their thermograms. Three-dimensional diffusion reaction models adequately describe the solid-state kinetics of thermal oxidation of Ti4O7 in air as demonstrated by linear model-fitting. Doping shows a mixed influence on the kinetics for thermal oxidation in air reducing both the Arrhenius pre-exponential factor and the activation energy.