Impact of Solution Chemistry on Growth and Structural Features of Mo-Substituted Spinel Iron Oxides

Abstract

The effect of crystallizing solution chemistry on the chemistry of subsequently asgrown 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 oxidation state of the iron precursor from Fe(II) to Fe(III) causes a progressive loss of atomic long-range 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 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 analysis of the atomic structure of the sample prepared with Fe(III) indicates a 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 by Mo ions. The population of these sites, known to be reactive sites enabling particles growth, is concomitant with the stabilization of very small particles. We confirmed our hypothesis by using a blank experiment without Mo ions, further supporting the impact of tetrahedral Mo ions on the growth of iron oxide nanoparticles. Our findings provide new insight into the relationships between the Fe-chemistry of the crystallizing solution and the structural features of as-grown Mo-substituted Fe-oxide materials.

Cationic Vacancies in Anatase (TiO2): Synthesis, Defect Characterization, and Ion-Intercalation Properties

“…34 Moreover, the understanding of the formation mechanism of highly defective compounds could also be considered to better understand how cationic vacancies are involved in the growth of inorganic nanoparticles. 35,36 The use of cationic vacancies has emerged as an effective strategy to improve the electrochemical properties of electrode materials as reviewed by Hahn et al 11 and Gao et al 37 As shown in these reviews, defect engineering has been demonstrated for different charge carriers spanning proton, monovalent, and polyvalent ions. Rationalizing the observed enhanced/modified properties implies that cationic-deficient materials are well described.…”

“…Although it is known that certain types of crystal lattices, such as the spinel structure, are naturally prone to accommodating cationic vacancies, further studies may be based on the identification of frameworks capable of hosting a large content of vacancies, a wide range of elements, oxidation states, and, in turn, various phase compositions, i.e., adaptable frameworks . Moreover, the understanding of the formation mechanism of highly defective compounds could also be considered to better understand how cationic vacancies are involved in the growth of inorganic nanoparticles. , …”

Mo‐Incorporated Magnetite Fe3O4 Featuring Cationic Vacancies Enabling Fast Lithium Intercalation for Batteries

“…The similar energy position of the absorption edge of the Mo‐doped sample with MoO 2 reference confirmed the dominant Mo 4+ valence state in Fe 2.93 ▫ 0.017 Mo 0.053 O 4 . [ 37,49 ] Thus, the lattice enlargement in the Rietveld refinement was undoubtedly explained by the larger Mo 4+ ions ($${r_{M{o^{4 + }}}} = 0.79$$Å [ 50 ] versus $${r_{F{e^{3 + }}}} = 0.65$$Å [ 39 ] ). [ 51 ] Moreover, the absence of the pre‐peak indicated that Mo preferred to occupy at the 16 d sites with higher O h symmetries than 8 a sites with T d symmetries, [ 52 ] which was consistent with DFT results.…”

“…[35] The creation of chemical defects is mainly dependent on the synthesis method. [36,37] In the present study, we used a simple and economical sol-gel process to develop a novel Mo 4+ -incorporated Fe 3 O 4 with cationic vacancies. Further, we investigated the role of cationic vacancies in the structure and electrochemical properties of molybdenum (Mo)-doped Fe 3 O 4 for LIBs.…”

py-sc-fermi: self-consistent Fermi energies and defect concentrations from electronic structure calculations

“…The code has been used in a number of studies, including some focussed on rationalising the properties of known materials (Squires et al, 2022(Squires et al, , 2021 and others focussed on predicting the properties of novel materials (Jackson et al, 2022). The code can also assist with visualising defect energetics (Haouari et al, 2021).…”