Synthesis and properties of titanomagnetite (Fe3−xTixO4) 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.

Tc(VII) reduction kinetics by titanomagnetite (Fe3−xTixO4) nanoparticles

“…The characteristics of the initial titanomagnetite nanoparticles used here are summarized in Table 1, and are consistent with the properties reported in Pearce et al (2012). TEM analysis showed that the particles are nearly spherical with a diameter of 9.4-13 nm (±2 nm), in good agreement with the size expected from the BET specific surface area analysis.…”

“…This trend is well documented for bulk titanomagnetites due to the structural substitution of Ti(IV) for Fe(III) within the octahedral cationic sublattice and the corresponding reduction of Fe(III) to Fe(II) to maintain charge balance (Wechsler et al, 1984;Bosi et al, 2009;Pearce et al, 2010). However, with increasing x, the structural Fe(II)/Fe(III) ratio, calculated from the unit cell edge as described in Pearce et al (2012) deviates from that predicted by chemical analysis of digested nanoparticles. The discrepancy between the chemical analysis and structural Fe(II)/Fe(III) ratio suggests the presence of a non-structural non-crystalline Fe(II)/Ti(IV)-containing phase that becomes significant at x = 0.53.…”

“…The synthesis protocol has been reported in detail in Pearce et al (2012); essential details are briefly reviewed here. Fe 3Àx Ti x O 4 nanoparticles were synthesized under ambient conditions and in aqueous suspension by co-precipitating a stoichiometric mixture of FeCl 2 , FeCl 3 and TiCl 4 in 0.3 M HCl (pH < 1) with ammonium (NH 4 OH) solution in an anoxic glovebox (N 2 atmosphere from LN 2 boil-off; lower than 1 ppm residual O 2 ; hereafter referred to as the glovebox).…”

“…This shift corresponded to a decrease in the unit-cell parameters of Fe 3Àx Ti x O 4 nanoparticles after reduction (Table 2). Given the comprehensive systematic relationship between titanomagnetite unit-cell parameter and stoichiometry (Fe(II)/ Fe(III) ratio) reported by Pearce et al (2012) in the 'master curve', here we used it to calculate the change in structural Fe(II)/Fe(III) ratio before and after Tc(VII) reduction (Table 2). The decrease in structural Fe(II)/Fe(III) ratio after reaction was larger for samples with higher x, implying that with higher x more structural Fe(II) was oxidized by the reduction of Tc(VII), in agreement with the higher concentration of Tc(VII) reduced at the end of the experiment.…”

“…Related strategies have been used elsewhere, including studies of methylene blue reduction by Fe 3Àx Ti x O 4 (0 6 x 6 0.78) in the presence of H 2 O 2 (Yang et al, 2009), and nitrobenzene reduction using partially oxidized magnetite (ratios 61:2) . Furthermore, titanomagnetite nanoparticles are relatively easy to produce with precisely controlled composition and structure by room temperature synthesis (Millot et al, 1998;Perriat et al, 1999;Guigue-Millot et al, 2001Pearce et al, 2012) and, because of their high specific surface area, experimental sensitivity to Tc(VII) reduction and accompanying changes to the solid surface is greatly improved. Finally, the selected system has unique advantages in terms of spectroscopic discrimination and tracking of concentration and oxidation state changes in various possible reactive pools of solid-associated Fe(II), including that structurally ordered within the bulk structure and that near the solid surface.…”