Structure and growth of oxides on polycrystalline nickel surfaces

Abstract

The oxidation of polycrystalline nickel (Ni) metal surfaces after exposure to oxygen gas (O 2 ) at 25 and 300°C and pressures near 130 Pa, was studied using X-ray photoelectron spectroscopy (XPS). Oxide structures involving both divalent (Ni 2+ ) and trivalent (Ni 3+ ) species could be distinguished using Ni 2p spectra, while surface adsorbed O 2 and atomic oxygen (O) species could be differentiated from bulk oxide (O 2− ) using O 1s spectra. Oxide thicknesses and distributions were determined using QUASES  , and the average oxide thickness was verified using the Strohmeier formula. The reaction kinetics for oxide films grown at 300°C followed a parabolic mechanism, with an oxide thickness of greater than 4 nm having formed after 60 min. Exposure at 25°C followed a direct logarithmic mechanism with an oxide growth rate about four to five times slower than at 300°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°C. The thickness of oxide was not strongly localized on this scale. However, the QUASES  analysis suggests that there is localized growth on a nanometric scale -the result of island formation. Copyright  2007 John Wiley & Sons, Ltd.KEYWORDS: X-ray photoelectron spectroscopy; nickel; oxidation; polycrystalline surfaces INTRODUCTIONThe oxidation of nickel (Ni) metal surfaces at ambient temperatures leads to the formation of thin passive films that tend to be described best using logarithmic kinetics. 1Following passivation there is little subsequent change in oxide thickness with time. 2 The mechanism is believed to follow that which was first suggested by Mott and Cabrera. 3 Reactant oxygen molecules (O 2 ) adsorb onto the surface of the metal and decompose into atomic oxygen (O (ads)). 4 The O (ads) then bonds covalently with the Ni metal atoms at the surface, weakening their attachment to the lattice. 4 Owing to the difference in electronegativity between Ni and O, a dipole forms which allows the two atoms to exchange places.4 Such a place exchange mechanism is responsible for the formation of the first one or two monolayers of oxide only. 4 Further oxidation is driven by the formation of an electric field created by the tunneling of electrons from the Ni atoms in the metal, through the thin oxide layer, to the O (ads) sitting on the surface. 1 The presence of this electric field induces the movement of ions through the oxide leading to the formation of a thicker film.1 After the film reaches a Ł Correspondence to: Brad P. Payne, Department of Chemistry, Room G-4, Western Science Centre, The University of Western Ontario, London, ON N6A 5B7, Canada. E-mail: bpayne2@uwo.ca certain thickness, the electric field is no longer strong enough to promote ion diffusion and oxidation sto...

The effect of milling additives on powder properties and sintered body microstructure of NiO

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X‐ray photoelectron spectroscopic chemical state quantification of mixed nickel metal, oxide and hydroxide systems

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Compositional tuning of ultrathin surface oxides on metal and alloy substrates using photons: Dynamic simulations and experiments

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