Synthesis and annealing effects on microstructure and optical properties of wide-bandgap polycrystalline ferro-pseudobrookite FeTi2O5 sol-gel layers

Abstract

Wide bandgap (WBG) and high thermal stability pseudobrookite compounds, Fe_1+xTi_2−xO₅ (0 ≤ x ≤ 1), are promising materials for photocatalysis, high-temperature thermoelectric applications, green production of hydrogen by water splitting, fabrication of power electronics, and optoelectronic devices. Here, we report on WBG, polycrystalline, ferropseudobrookite, FeTi₂O₅, coatings, stable at high temperature, prepared by an optimized sol-gel route on fused silica and silicon substrates. The chemical composition, the amorphous-to-crystalline phase transformation, and the influence of the annealing temperature and atmosphere (air and argon) on the formation and evolution of the crystalline phases were investigated in detail by combining thermogravimetric and differential scanning calorimetry analysis with Fourier-transform infrared spectroscopy and X-ray diffraction. The experimental results clearly show that orthorhombic FeTi₂O₅ single phase develops in the interval 500–560 °C (crystalline domain size about 16 nm at 560 °C). The coatings remain in a single FeTi₂O₅ phase up to a temperature of about 590 °C. At higher temperatures, a rutile-TiO₂ secondary phase is formed, both in an oxidizing and inert atmosphere, while the ferropseudobrookite phase remains unchanged. The results suggest that the secondary phase arises from the presence of superficial Ti-O- dangling bonds that at temperatures above 590 °C begin to arrange themselves to form polycrystalline rutile-TiO₂ (crystalline domain size ≈8 nm at 620 °C). The results also show that the average energy required to break the Ti-O-Ti molecular bonds of the FeTi₂O₅ phase increases with temperature, improving its thermal stability. Optical absorption spectroscopy measurements carried out on coatings heated at 560 °C, yield an optical bandgap of about 2.25 eV, a refractive index of about 1.84 at 550 nm, and a weak UVC positive band, peaked at about 5.9 eV, on transmittance that disappears when the samples are annealed at 750 °C.