Electrochemically and UHV‐Grown Passive Layers on Ni(100): A Comparison by AC Impedance, XPS, LEED, and HREELS

Abstract

The electrochemical, spectroscopic, and structural properties of passive layers on Ni(100) grown either electrochemically in aqueous or by dosing with gaseous oxygen in ultrahigh vacuum (UHV) were compared using a UHV‐electrochemical transfer system equipped for x‐ray photoemission spectroscopy (XPS), low energy electron diffraction (LEED), and high resolution electron energy loss spectroscopy (HREELS). The two films share a similar component of several layers of anhydrous , but the electrochemical layer also contains smaller amounts of additional forms of oxygen. The elements of structural order in the two films differ in epitaxial relationships with the substrate. Similar ac impedance behaviors and dc passive current densities are seen when the clean and UHV‐oxidized Ni(100) surfaces are contacted with electrolyte at the passive potential; but 6 mC/cm2 of anodic charge must be passed to reach a steady passive state on the clean surface, while 0.8 mC/cm2 suffices on the UHV‐oxidized surface. Comparison with the very well‐defined UHV oxide allows improved precision in the characterization of the passive layer.