Heterometallic trinuclear oxo-centered clusters as single-source precursors for synthesis of stoichiometric monodisperse transition metal ferrite nanocrystals

Abstract

The use of heterobimetallic metal complexes as molecular single-source precursors is a promising strategy for the targeted synthesis of phase-pure stoichiometric ternary metal oxide nanocrystals. However, the design and synthesis of these precursors is not trivial and can require considerable effort. Using spinel metal ferrite nanocrystals of formula MFe₂O₄ (M = Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺) as a model system, this paper evaluates the efficacy of the single-source precursor approach by comparing directly nanocrystals synthesized from the solvothermal reaction of heterobimetallic trinuclear oxo-bridged clusters of formula M^IIFe^III₂(μ₃-O)(μ₂-O₂CR)₆(H₂O)₃, R = CF₃ to nanocrystals synthesized from the solvothermal reaction of stoichiometric mixtures of multi-source precursors, such as metal acetate or nitrate salts. For each M explored here, the clusters form phase-pure MFe₂O₄ nanocrystals with significantly narrower size distributions than nanocrystals synthesized from multi-source-precursors. In the case of M = Cu, the multi-source metal salt precursors produce a mixture of CuO and CuFe₂O₄. Additionally, changing the electronic nature of the R-group on the bridging carboxylate ligand from electron withdrawing (CF₃) to electron donating (CH₃ or C(CH₃)₃) decreases the average diameter of the resulting nanocrystals by a factor of two. The cluster molecules therefore offer superior control over both morphology and composition for transition metal ferrite nanocrystals. We hypothesize that this remarkable performance arises from the presence of pre-formed M²⁺–O–Fe³⁺ moieties in the cluster molecules that enable direct nucleation of MFe₂O₄ and preclude nucleation of binary oxide impurities.