Desorption of ambient gas molecules and phase transformation of α-Fe2O3 nanostructures during ultrahigh vacuum annealing

Abstract

Desorption and readsorption of gas molecules from ambient air onto the surface of α-Fe2O3 quasi-1D nanostructures (nanoflakes and nanostrips) were studied in situ by X-ray photoelectron spectroscopy (XPS) and ex situ by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XPS revealed that carbon and oxygen species were physisorbed and chemisorbed as C-C/C-H, C-O, O-C=O, and O2 states on both surfaces of α-Fe2O3 quasi-1D nanostructures upon exposure in air. The physisorbed carbon species (C-C/C-H) and O2 desorbed from the surfaces when the two nanostructures were heated to 100 °C inside the vacuum chamber of XPS. Significant desorption of chemisorbed O-C=O and O-C occurred above 200 °C, which resulted in a reduction of Fe 2O3 into Fe3O4 for both samples between 200 and 300 °C. Complete desorption of carbon and O-C/O-C=O/O 2 species in O1s occurred at 400 °C, where Fe3O 4 in nanoflakes (sample 1) was reduced further into FeO by excess metallic Fe from the bulk, while Fe3O4 in nanostrips (sample 2) was largely oxidized into Fe2O3 by the oxygen from the bulk of Fe2O3. Although no band bending was observed during the annealing and desorption of ambient gases, the valence band changed as the phase transformation occurred. After the annealed samples were exposed to air for two days, the same chemical states associated with C and O species were again detected on the surfaces of the two nanostructures. In addition, FeO (sample 1) was found to be oxidized into a mixture of Fe 2O3 and Fe3O4 on the surface. The adsorption of gas molecules from ambient environment thus has a strong influence on the chemical and physical properties of nanostructures with large surface to volume ratio. © 2013 American Chemical Society.