Formation of Bimetallic FeBi Nanostructured Particles: Investigation of a Complex Growth Mechanism

Magnetic bimetallic Fe-Bi composites, synthesized by decomposition of organometallic precursors under amine borane and dihydrogen, formed regular nanospheres with mean diameter of 150 +/- 30 nm. The nanospheres display a core-shell like chemical distribution in which bismuth is mainly concentrated in the core, as demonstrated through elemental mapping X-ray energy dispersive spectroscopy while the shell is formed by aggregated 3 nm wide iron nanoparticles. The close environment of the different elements was analyzed through complementary techniques such as extended X-ray absorption fine structure, wide-angle X-ray scattering, and Fe-57 Mossbauer and X-ray photoelectron spectroscopies. Despite their iron-rich shell, the Fe-Bi nanospheres present both good magnetic properties and enhanced resistance to oxidation during air exposure. To uncover the growth mechanisms leading to the formation of this compound, a series of samples taken at different steps of the synthesis process was analyzed. The role of metallic iron to promote the reduction of the bismuth precursor from the early stages of the synthesis is emphasized. Remarkably, this process promotes the formation of a metastable Fe-Bi nanoalloy.