Oxidation States of Iron in Doped TiO2-SiO2 Sol-Gel Powders: A 57Fe Mössbauer Study

Ceramic coatings obtained by sol-gel routes are more and more attractive for protective purposes. Improved resistance against wet corrosion of steels can be obtained by doping the covering layer with elements like for instance transition ones. It is of major importance to have a clear-cut knowledge of the oxidation state of the doping element, especially after different heat treatments which usually follow the deposition process. Here we report about the oxidation states of 5 wt.% iron doped TiO₂, SiO₂ and mixed sol powders obtained by the sol-gel method with alkoxide precursors added with FeSO₄, 7 H₂O in organic medium. The evolution of the Fe oxidation state versus various heat treatments is followed by ⁵⁷Fe Mössbauer spectrometry supplemented by XRD data. As-dried samples are amorphous and contain ferric Fe cations, with a local distorted oxygen environment. After heating at 500°C during 3 min, ferrous and ferric oxides are evidenced together solely for silica and mixed silica-titania powders, in marked contrast with titania powders where only ferric species exist. The complete ferric state is restored for all samples after annealing at 900°C for one hour.