Solid-state chemistry of thermally induced yellow colouring in synthetic hydrous titanium oxide from TiCl3

Calcination of hydrous titanium oxide (HTO) gel, synthesized by the hydrolysis of titanium(III) chloride (TiCl3) in the presence of urea, produces a brilliant yellow powder when the pale blue HTO gel is heated at 400 °C in air. From the results of diffuse reflectance (DR) spectroscopy, thermogravimetry and/or differential thermal analysis. (TG/DTA) and electron paramagnetic resonance (EPR) spectroscopy, this colouring is attributed to the formation of thermally induced Ti4+ peroxide. The colour is lost and the superoxide radical anion is generated when the temperature is increased to 800 °C; this is accounted for by the thermal decomposition of the peroxide complex. The solid-state chemistry of the appearance and loss of the yellow colour is discussed along with results of X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA).