Template-Free Hydrothermal Synthesis and Formation Mechanism of Hematite Microrings

Hematite microrings with an outer diameter ∼12 μm, inner diameter ∼4 μm, and thickness ∼500 nm have been successfully synthesized by a facile template-free hydrothermal method using K₃Fe(CN)₆ and NaOH as reagents through a redox reaction. The influence of the reaction time, reaction temperature, concentration of K₃Fe(CN)₆, and NaOH on the evolution of shape and structures has been studied in detail. The optimal condition for preparation of the microrings is 180 °C for 5 h, the NaOH concentration is 4 M, and the K₃Fe(CN)₆ concentration is 0.02 M. The possible hollowing growth mechanism for the hematite microrings has been discussed. Magnetic hysteresis measurement revealed that hematite microrings after calcination at 700 °C for 4 h display a magnetic behavior with remanent magnetization of 0.211 emu/g and coercivity of 2005 Oe at room temperature.