Iron altered oxidation energy due to Yttrium doping in Fe₃O₄ spinel ferrites

Full text: Doping of spinel ferrite nanoparticles (NPs) alters significantly their magnetic response and other characteristics REF. Furthermore magnetic fluids made with yttrium (Y) doped NPs show high stability, remaining years without settling or flocculating. The precise character of Y doping effects on the NPs is yet to be fully understood. EXAFS measurements showed that Y occupy the tetrahedral site, entering the crystalline structure without causing much structural damage, as can be readily verified by X-Ray diffraction data. One hypothesis for enhanced colloidal stability in these ferrites revolves around the fact that these systems, in particular, are ionic colloids, without any surfactant molecule on the particle surface. The stability, in these cases, comes from electrostatic repulsion between particles, coated with charge of equal sign that repel each other efficiently. The charge is highly influenced by the system pH. In this work we will present X-Ray Photoemission Electrons spectroscopy measurements which strongly indicates that the Y-doping of the NPs favors Fe²⁺ oxidation into Fe³⁺ ions, consequently increasing the particle surface charge. This is readily perceived by Zeta-potential measurements of doped and undoped particles at the same pH. The Y-doped NPs presented a Zeta potential about 38% higher than the undoped sample. Since the zeta potential is derived from surface charge which is directly associated with colloidal stability this different oxidation energy detected by XPS may be the main cause of low flocculation and virtually no sedimentation in Y-doped ferrites. (author)