Aerosol-Assisted Synthesis of Shape-Controlled CoFe2O4: Topotactic versus Direct Melt Crystallization

Compositionally complex materials can be prepared as nanocrystals with defined shapes by aerosol-assisted molten salt synthesis (AMSS), where the aerosol phase spatially and temporally confines crystal growth. Here, single-crystalline CoFe2O4 nanoplates and CoFe2O4 octahedra are achieved selectively by AMSS through the use of Na2CO3 and Na2MoO4 fluxes, respectively. The formation of CoFe2O4 nanoplates is generally unexpected given the cubic crystal symmetry of spinel ferrites, but this shape is accounted for by in situ formation of a layered double hydroxide (LDH) intermediate stabilized by the CO32- anions of the flux. Topotactic conversion of the LDH intermediate to CoFe2O4 preserves the plate morphology of the LDH crystals. In contrast, the formation of the LDH intermediate is not possible with the Na2MoO4 flux. Thus, product formation proceeds via crystallization from the molten media, with octahedral crystals consistent with the common crystal habit of the spinel phase produced. This demonstration of topotactic chemistry within the aerosol phase is significant given the range of LDH phases available for conversion to mixed metal oxides, where the AMSS technique facilitates shape-controlled nanocrystal formation.