Insights on Growth and Nanoscopic Investigation of Uncommon Iron Oxide Polymorphs

Si(100)-supported Fe2O3 nanomaterials were developed by a chemical vapor deposition (CVD) approach. The syntheses, performed at temperatures between 400 and 550°C, selectively yielded the scarcely studied beta- and epsilon-Fe2O3 polymorphs under O2 or O2+H2O reaction environments, respectively. Correspondingly, the observed morphology underwent a progressive evolution from interconnected nanopyramids, to vertically aligned nanorods. The present study aims at providing novel insights into Fe2O3 nano-organization by a systematic investigation of the system structure/morphology and of their interrelations with growth conditions. In particular, for the first time, beta- and epsilon-Fe2O3 preparation process has been accompanied by a thorough multi-technique investigation, which, beyond X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscopy (FE-SEM), is carried out by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDXS), atomic force microscopy (AFM), high resolution-transmission electron microscopy (HR-TEM), electron diffraction (ED), scanning TEM-electron energy loss spectroscopy (STEM-EELS), high angle annular dark field-STEM (HAADF-STEM). Remarkably, the target materials showed an high structural and compositional homogeneity throughout the whole nanodeposit thickness. In particular, spatially resolved EELS chemical maps through the spectrum imaging (SI) technique enabled to gain important information on the local Fe coordination, of crucial importance in determining the system reactivity. The described preparation method is in fact a powerful tool to simultaneously tailor phase composition and morphology of iron(III) oxide nanomaterials, whose potential applications include photocatalysis, magnetic devices, gas sensors and anodes for Li-ion batteries.