Structure and growth of oxides on polycrystalline nickel surfaces

The oxidation of polycrystalline nickel (Ni) metal surfaces after exposure to oxygen gas (O-2) at 25 and 300 degrees C and pressures near 130 Pa, was studied using X-ray photoelectron spectroscopy (XPS). Oxide structures involving both divalent (Ni2+) and trivalent (Ni3+) species could be distinguished using Ni 2p spectra, while surface adsorbed O-2 and atomic oxygen (O) species could be differentiated from bulk oxide (O2-) using 0 1s spectra. Oxide thicknesses and distributions were determined using QUASES (TM), and the average oxide thickness was verified using the Strohmeier formula. The reaction kinetics for oxide films grown at 300 degrees C followed a parabolic mechanism, with an oxide thickness of greater than 4 nm having formed after 60 min. Exposure at 25 degrees C followed a direct logarithmic mechanism with an oxide growth rate about four to five times slower than at 300 degrees C. Reaction of a Ni (100) single crystal under comparable conditions showed much slower reaction rates compared to polycrystalline specimens. The higher reaction rate of the polycrystalline materials is attributed to grain boundary transport of Ni cations. Oxide thickness was measured on a microscopic scale for polycrystalline Ni exposed to large doses Of O-2 at 25 and 300 degrees C. The thickness of oxide was not strongly localized on this scale. However, the QUASES (TM) analysis suggests that there is localized growth on a nanometric scale - the result of island formation. Copyright (c) 2007 John Wiley & Sons, Ltd.