Synthesis and properties of titanomagnetite (Fe3−xTixO4) nanoparticles: A tunable solid-state Fe(II/III) redox system

• Published in: Journal of colloid and interface science Vol. 387; no. 1; pp. 24 - 38
• Main Authors: Pearce, C.I., Qafoku, O., Liu, J., Arenholz, E., Heald, S.M., Kukkadapu, R.K., Gorski, C.A., Henderson, C.M.B., Rosso, K.M.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Inc 01.12.2012; Elsevier
• Subjects: ambient temperature; chemical analysis; Chemistry; Colloidal state and disperse state; Dissolution; Electron transfer; Exact sciences and technology; General and physical chemistry; iron; magnetic properties; Magnetite; Master Curve; nanoparticles; oxidants; Physical and chemical studies. Granulometry. Electrokinetic phenomena; Site occupancy; spectroscopy; Surface physical chemistry; titanium; transmission electron microscopy; Ulvöspinel; X-ray magnetic circular dichroism; Ulvöspinel; Site occupancy; X-ray magnetic circular dichroism; Magnetite; Master Curve; Electron transfer; Dissolution; Binary compound; Incorporation; Chemical analysis; Iron oxide; XANES spectrometry; Nanoparticle; X ray; Moessbauer spectrometry; Particle; Precipitation; Synthesis; Inverse; Redox system; Structure; Magnetic properties; Composition; Transition element compounds; Suspension; Conversion; X ray diffraction; EXAFS spectrometry; Circular dichroism; Solid state; Transmission electron microscopy; Spinel; Spinels; Room temperature
• DOI: 10.1016/j.jcis.2012.06.092
• ISSN: 0021-9797

[Display omitted] ► Fe3−xTixO4 nanoparticles (10–12nm) synthesized by aqueous precipitation. ► Composition, structure, magnetic properties of Fe3−xTixO4 nanoparticles determined. ► Ti(IV) incorporation in the unit cell up to a limit of x⩽0.38. ► Nanoparticles with higher x had a minor amorphous secondary Fe(II)/Ti(IV) phase. ► Persistent segregation of Fe(II) to particle surfaces even after mild oxidation by dissolution. Titanomagnetite (Fe3−xTixO4) nanoparticles were synthesized by room temperature aqueous precipitation, in which Ti(IV) replaces Fe(III) and is charge compensated by conversion of Fe(III) to Fe(II) in the unit cell. A comprehensive suite of tools was used to probe composition, structure, and magnetic properties down to site-occupancy level, emphasizing distribution and accessibility of Fe(II) as a function of x. Synthesis of nanoparticles in the range 0⩽x⩽0.6 was attempted; Ti, total Fe and Fe(II) content were verified by chemical analysis. TEM indicated homogeneous spherical 9–12nm particles. μ-XRD and Mössbauer spectroscopy on anoxic aqueous suspensions verified the inverse spinel structure and Ti(IV) incorporation in the unit cell up to x⩽0.38, based on Fe(II)/Fe(III) ratio deduced from the unit cell edge and Mössbauer spectra. Nanoparticles with a higher value of x possessed a minor amorphous secondary Fe(II)/Ti(IV) phase. XANES/EXAFS indicated Ti(IV) incorporation in the octahedral sublattice (B-site) and proportional increases in Fe(II)/Fe(III) ratio. XA/XMCD indicated that increases arise from increasing B-site Fe(II), and that these charge-balancing equivalents segregate to those B-sites near particle surfaces. Dissolution studies showed that this segregation persists after release of Fe(II) into solution, in amounts systematically proportional to x and thus the Fe(II)/Fe(III) ratio. A mechanistic reaction model was developed entailing mobile B-site Fe(II) supplying a highly interactive surface phase that undergoes interfacial electron transfer with oxidants in solution, sustained by outward Fe(II) migration from particle interiors and concurrent inward migration of charge-balancing cationic vacancies in a ratio of 3:1.