Desorption of Ambient Gas Molecules and Phase Transformation of α‑Fe₂O₃ Nanostructures during Ultrahigh Vacuum Annealing

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 Fe₂O₃ into Fe₃O₄ for both samples between 200 and 300 °C. Complete desorption of carbon and O–C/O–CO/O₂ species in O1s occurred at 400 °C, where Fe₃O₄ 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 Fe₂O₃ 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