Chemical etching induced shape change of magnetite microcrystals

We observed an interesting chemical etching induced shape change of magnetite microcrystals. Treatment in a NaBH4 aqueous solution was found to be capable of transforming the shape of magnetite microcrystals from octahedra to plates, meanwhile, a small amount of magnetite was reduced to form an Fe-B amorphous alloy. The observed shape change process proceeded massively in ambient conditions and involved no surfactant. The structures of magnetite microcrystals were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy with selected area electron diffraction. The magnetite microplates are single crystals with the flat top surface parallel to the (111) plane of their fcc crystal structure. We observed a similar shape change process for Fe2O3, Co3O4, and Co2O3 microcrystals, but not for Pb3O4 microcrystals. A probable mechanism for the shape change of magnetite microcrystals induced by treatment in NaBH4 aqueous solution was proposed on the basis of the experimental results.