Electrochemical Study on Growth Mechanism of Ferrite Nanoparticles in an Aqueous Process

Abstract

Aqueous growth mechanism of ferrite (intermediate between Fe₃O₄ and γ-Fe₂O₃) nanoparticles via the precipitation and oxidation of green rust was studied by monitoring pH and ORP of the reaction solution. While Fe₃O₄ was growing, both pH and ORP reached plateaus of pH=6 ∼ 7 and ORP=−400 ∼ −300 mV. The pH was kept constant because the formation of Fe₃O₄ from green rust is expressed by the formula [Fe^II₂Fe^III₁(OH)₆]⁺ · [Cl₁ · mH₂O]⁻+NO₃⁻→Fe₃O₄+(3+m)H₂O+Cl⁻+NO₂↑ where H⁺ is not released. The observed value of ORP was ascribed to the interfacial potential between Fe₃O₄ and FeOH⁺, a dissolved species from the green rust. After ending the growth by exhausting the green rust, the surface of Fe₃O₄ was oxidized to γ-Fe₂O₃, which gave the final values of pH=4 ∼ 5 and ORP ≈ 0 mV. Adding sucrose, a disaccharide, to the reaction solution decreased the growth rate of the ferrite particles. This supported our proposal that a disaccharide suppresses the growth of each crystal plane and thus enables the particles to grow spherical in shape.