The effect of crystallizing solution chemistry on the chemistry of subsequently as-grown materials was investigated for Mo-substituted iron oxides prepared by thermally activated co-precipitation. In the presence of Mo ions, we find that varying the iron precursor oxidation state strongly affects the composition, structure, and morphology of the as-grown materials. Using a Fe(II) precursor gives large particles (40-100 nm) with partially reduced Fe and Mo species, as revealed by Mössbauer and X-ray absorption spectroscopies. Conversely, using a Fe(III) precursor gives particles with oxidized Fe and Mo species. Varying the oxidation state of the iron precursor from Fe(II) to Fe(III) causes a progressive loss of atomic long-ranged order with the stabilization of 2-4 nm particles for the sample prepared with Fe(III). The oxidation state of the Fe precursor also affects the distribution of Fe and Mo cations within the product spinel structure. Increasing the Fe precursor oxidation state gives decreased Fe-ion occupation and increased Mo-ion occupation of tetrahedral sites, as revealed by EXAFS. The stabilization of Mo within tetrahedral sites appears to be unexpected considering the octahedral preferred coordination number of Mo(VI). The atomic structure of the sample prepared with Fe(III) was further investigated by combining the phase composition, the local structure obtained from X-ray absorption spectroscopy, and PDF data to give an accurate description of the short- and intermediate-ranged order. This analysis indicates local ordering of vacancies and that the occupation of tetrahedral sites by Mo induce a contraction of the inter-atomic distances within the polyhedra as compared to Fe atoms. Moreover, the occupancy of Mo into thermodynamic site preference of a Mo dopant in Fe2O3 assessed by DFT-calculations, points to a stronger preference for Mo substitution at octahedral sites. Hence, we suggest that the synthetized compound is thermodynamically metastable, i.e., kinetically trapped. Such a state is suggested to be a consequence of the tetrahedral site occupation. These sites are indeed known to be reactive sites enabling particles growth, their occupation is concomitant with the stabilization of very small particles. Our findings provide new insight into the relationships between the Fe-chemistry of the crystallizing solution and the composition, structure, and morphology of the subsequently as-grown Mo-substituted Fe-oxide materials.
|Publication status||Acceptance date - 23 Apr 2021|