Synthesis of magnetite nanoparticles under standing ultrasonication

Magnetite nanoparticles were synthesized by oxidizing a Fe(OH)₂ precipitate in various ultrasonic (US) fields including traveling wave (TW; 20 kHz) driven by a horn-type homogenizer and standing waves (SW; 45,100, 200 kHz) driven by a cleaner-type sonicator. Compared to a process with mechanical stirring (MS), each of US treatment completed the magnetite formation in much shorter time. The size of the nanoparticles decreased with the decreasing initial [Fe²⁺] concentration. Noticeably at [Fe²⁺] = 0.001 mol/L, no more particles but brown amorphous sols were formed in either MS or TW process. To the contrary, black magnetite nanoparticles were formed in the SW process as usual as higher concentration. The SW ultrasonication enabled to gather cavitation bubbles in its nodes, and then it is inferred that high supersaturation enough to form nanoparticles were achieved locally in the nodes of the SW field.