Removal of the oleylamine capping agent from MnFe2O4 hollow spheres prepared by an Ostwald ripening mechanism

Abstract

Manganese ferrite hollow nanospheres were prepared by a solvothermal method using an Ostwald ripening mechanism with the oleylamine surfactant as capping agent (HS sample). The objective was to investigate the removal of oleylamine from the HS sample by wet oxidation, using H2O2 and ammonium peroxymonosulfate (PMS) and dry oxidation with air at 500 °C. This is of major importance because residuals of oleylamine might block accessibility to available active sites on the surface of the hollow spheres, impairing their efficacy. HS contained 85 % jacobsite and 15 % rhodochrosite. Around 83 % of rhodochrosite was removed by wet oxidation, while jacobsite was decomposed into tetragonal manganese ferrite and hematite and rhodochrosite into bixbyite by dry oxidation. All samples were characterized to determine their phase composition, changes during heat treatment, elemental analysis, hydrophilicity, morphology, surface area, surface charge, surface composition, and magnetism. Wet oxidation treatment of the as-prepared sample removed a large amount of oleylamine, increasing its hydrophilicity. Mn is in oxidation states (II) and (III) or only (II) according to the type of oxidant. Wet oxidation also increases the saturation magnetization and the superexchange interaction between cations through oxygen anions.