title: Synthesis and properties of titanomagnetite (Fe(3-x)Ti(x)O4) nanoparticles: a tunable solid-state Fe(II/III) redox system.
abstract: Titanomagnetite (Fe(3-x)Ti(x)O(4)) 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 9-12 nm 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.
authors.affiliation: Pacific Northwest National Laboratory, Richland, WA 99352, USA. carolyn.pearce@pnnl.gov
authors.forename: C I
authors.initials: CI
authors.lastname: Pearce
authors.forename: O
authors.initials: O
authors.lastname: Qafoku
authors.forename: J
authors.initials: J
authors.lastname: Liu
authors.forename: E
authors.initials: E
authors.lastname: Arenholz
authors.forename: S M
authors.initials: SM
authors.lastname: Heald
authors.forename: R K
authors.initials: RK
authors.lastname: Kukkadapu
authors.forename: C A
authors.initials: CA
authors.lastname: Gorski
authors.forename: C M B
authors.initials: CM
authors.lastname: Henderson
authors.forename: K M
authors.initials: KM
authors.lastname: Rosso