X-ray photoelectron spectroscopy and morphological studies of polycrystalline nickel surfaces exposed to anhydrous HF

The reactivity of nominally clean polycrystalline Ni toward gas-phase and liquid anhydrous hydrofluoric acid (AHF) has been examined by X-ray photoelectron spectroscopy (XPS). To avoid contamination with atmospheric components, experiments were carried out using a portable ultrahigh vacuum (UHV)-compatible chamber to transfer Ni specimens between a UHV system that houses the XPS spectrometer and an additional UHV-compatible chamber where the actual exposures were performed. For exposures of ca. 10 min, the extent of surface oxidation of clean Ni to form NiF₂, as calculated from Ni 2p and F 1s XPS spectra, was found to be significantly higher for gas-, compared to liquid-phase AHF. Thin (ca. 100 nm), mirror-like Ni films sputtered onto the surface of a sapphire disk yielded upon exposure to gas-phase AHF (after contact with the laboratory atmosphere) scanning electron microscope images displaying a coherent, patchy structure with raised junctures. This behavior was ascribed to internal compressive stresses induced by the formation of a nickel(II) fluoride layer on the metal surface.