Abstract
The effect of 200 kHz ultrasound on scorodite synthesis at 70 °C and 3 h reaction conditions was investigated using sulfuric acidic solutions of various pH (3.0, 2.0, 1.5, 1.0, and 0.0). In contrast to the case of only O2 gas flow without ultrasound irradiation, oxidizing radicals generated by ultrasound irradiation promote Fe(II) oxidation in solution and precursor, allowing scorodite to synthesize with high crystallinity (>99%), which relates to low solubility, even in strong acid solution at pH 1.0. During synthesis, particle shape was changed to polyhedral or spindle type depending on the pH of 0.0 to 3.0. The spindle-shaped scorodite was probably formed by the decrease of precursor amount produced in initial stage of the synthesis. Furthermore, porous maghemite obtained by alkali treatment of scorodite showed initial discharge capacities of 146 mAh/g (polyhedron) and 167 mAh/g (spindle), indicating that its potential use as a cathode material for lithium-ion batteries.