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

High Resolution Image Download MS PowerPoint Slide 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 CoFe 2 O 4 nanoplates and CoFe 2 O 4 octahedra are achieved selectively by AMSS through the use of Na 2 CO 3 and Na 2 MoO 4 fluxes, respectively. The formation of CoFe 2 O 4 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 CO 3 2– anions of the flux. Topotactic conversion of the LDH intermediate to CoFe 2 O 4 preserves the plate morphology of the LDH crystals. In contrast, the formation of the LDH intermediate is not possible with the Na 2 MoO 4 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.