Rapid and Continuous Production of Ferrite Nanoparticles by Hydrothermal Synthesis at 673 K and 30 MPa

Abstract

Ferrite nanoparticles from Fe(NO 3 ) 3 and Me(NO 3 ) 2 (Me = Ni, Cu, Zn) aqueous solutions could be synthesized continuously with a flow reactor at 673 K, 30 MPa, and 4 s residence time. The particles were characterized by TEM, XRD, and ICP to obtain sizes, crystal structures, lattice parameters, and Me/Fe molar ratios. Solid-solution nanoparticles of MeFe 2 O 4 and γ-Fe 2 O 3 with a cubic spinel structure and an average particle size under 10 nm were obtained. Conversion of Fe 3+ was more than 0.97 at the given residence time, and conversions of Me 2+ increased from 0 to 0.62 with increasing residence time. The particle size increased with increasing residence time, and no significant difference in divalent cations was observed. The Me/Fe molar ratio in the obtained solid-solution nanoparticles of MeFe 2 O 4 and γ-Fe 2 O 3 increased with increasing residence time and KOH molality. The Me/Fe molar ratio also increased in the order Zn < Cu < Ni at a given residence time, and this trend could be explained on the basis of ZnO, CuO, and NiO solubilities. The increase of the size and the Me/Fe molar ratio in the obtained particles with increasing residence time shows that, in the formation mechanism, primary solid-solution particles with low Me/Fe molar ratios probably nucleated and, after surface dissolution of the particles, dissolved Fe 3+ recrystallized on the surface with incorporation of Me 2+ .