Characteristics of nano-sized composites based on rare earth orthoferrites and hematite

Photocatalysts based on orthoferrites of rare earth elements are used to neutralize a number of harmful substances and organic dyes under the influence of visible light, which makes them to promise the introduction of resource-saving water purification technologies. The thermal-gravimetric analysis, X-ray phase analysis, scanning electron microscopy, and energy-dispersive spectroscopy was used to study nanoscale samples of orthoferrites of rare earth elements and hematite obtained by the chemical precipitation of aqueous solutions of inorganic salts of ferrum with lanthanides (La, Er, Sm, Dy, Nd, Yb ) and Y in a weakly alkaline environment and subsequent heat treatment of sediments at 800 and 1100 °C. It was found that during the simultaneous precipitation of hydroxide phases of ferrum and REE, their transformation under the influence of temperature is spatially separated due to the differences in the temperatures of phase transformations and the hydrogen index of the precipitation of precursor phases. It is shown that, depending on the chemical composition of the initial solutions, goethite is formed in the temperature range of 190-250 °C, and the formation of hematite takes place at 425-450 °C. Dehydroxylation of lanthanum hydroxide is observed at 300-330 °С, and the formation of lanthanum oxides continues in the range from 390 to 900 °С. The perovskite phase crystallizes at 960 °C. Using the X-ray phase analysis method, it was established that hematite and a mixture of REE oxygen phases are present in the sediments heat-treated at 800 °C. At 1100 °C, the structure of perovskite and hematite with particle sizes from 25 to 40 nm was identified in the composition of the powders. It was determined that the removal of auxiliary substances takes place at temperatures of 650-770 °C and no impurities of K, Na, S, Cl, or C in the powders fired at 1100 °C. Hematite and perovskite particles are well crystallized and form fragile aggregates, the specific surface area of the samples is several m2/h.