Counterion and solvent effects on the size of magnetite nanocrystals obtained by oxidative precipitation

We present the effect of the counterions and the proportion of ethanol in the preparation of uniform magnetite nanocrystals by oxidative precipitation. In this work we compared the materials produced using iron(II) chloride, bromide and sulfate, sodium and potassium nitrates and hydroxides in water and ethanol 25%-water solution. The process was followed by chemical analysis of the remaining iron(II), and the variation of the pH. The magnetite nanoparticles were characterized by X-ray powder diffraction, transmission electron microscopy and specific surface area. Although the final product was always magnetite nanocrystals, the morphology and particle size distributions depended significantly on the nature of the counterions and the ethanol content. The morphologies observed ranged between cubic and octahedral depending on the size. Particle sizes were correlated with the Hofmeister scale that reflects the influence of the cosmotropic or caotropic character of the counterions in water or in water/ethanol mixtures. The Hofmeister effect accounts for the competition that occurs between the counterions and the magnetite precursors for the water molecules. The main conclusion is that cosmotropic counterions in the presence of ethanol favor the nucleation of magnetite and generate smaller nanocrystals. Nanoparticle size variations are greater than 50%, which is extremely relevant to their magnetic properties and therefore to their biomedical applications.