Morphological aspects of Bismuth ferrite powders prepared by microwave-assisted hydrothermal method

Abstract

BiFeO3 particles were synthetized by a fast, reproducible and environmental friendly microwave-assisted hydrothermal method. The aim of the study was the investigation of increased pressure and mineralizer concentration on the reaction mechanism, morphology of particles and aggregates as well as magnetic behavior. Powders were studied when using KOH mineralizer concentration from 4 to 12 M at room temperature and under microwave-hydrothermal conditions at 40 bar and 150 or 200°C. The reaction mechanism depends on the temperature of the microwave-hydrothermal treatment and the concentration of KOH mineralizer. The precursor powder obtained after coprecipitation step consists of Bi2O3, Fe2O3 and Bi25FeO40 in different ratios. After the treatment at 40 bar and 150°C for 1h the reaction between precursors is not complete, therefore resulting secondary phases of Bi25FeO40 and Fe2O3. BiFeO3 particles were successfully synthesized at 40 bar under microwave-hydrothermal conditions at a temperature of 200°C and a concentration of KOH of at least 8M. At 6M, the composition consisted of 98.5% BiFeO3 and 1.5% Bi2O3 as evidenced by Rietveld refinement. The competition between dissolution and precipitation of Bi3+ and Fe3+ in KOH solutions of different concentrations was investigated by Raman spectroscopy and EDS analysis. Thus, at 200°C there was evidenced a Bi2O3 secondary phase for 6 M KOH concentration and poor control of Bi/Fe ratio in BiFeO3 at KOH concentrations of 10 and 12 M. Particle and aggregates morphology depicted by FE-SEM investigations showed a tendency to form irregular shaped particles at KOH concentration of 6 M, for which the reaction is not complete. When increasing the concentration, BiFeO3 particles exhibit regular shapes from cubic at 8 M to cvasi-spherical at 14 M. M-H curves characteristics illustrate that Bi/Fe ratio and preferential orientation of crystal growth influence the magnetic behavior of BiFeO3 crystalline powders.