The study of polycrystalline nickel metal oxidation by water vapour

The oxidation of polycrystalline nickel (Ni) metal surfaces with water (H2O) vapour was studied at 25 deg. C and 300 deg. C using dose rates considerably higher than in previous studies. The reaction rate was followed to a point where the oxide thickness was several nanometers. This was done using X-ray photoelectron spectroscopy (XPS) and modeling of the spectral profiles using QUASESTM. The surface reaction rate is much slower than that with oxygen gas (O2) and it terminates within a few nanometers. The films formed were found to be composed of a defective nickel oxide (NiO) containing both divalent nickel (Ni2+) and trivalent nickel (Ni3+) as had been observed previously for reaction of Ni with O2. The reason for the much slower reaction rate in the case of H2O vapour appears to be linked to a slower place exchange with a surface hydroxyl (OH (ads)) intermediate. This adsorbate species appears only to be stabilised on metallic Ni and termination of oxide growth is believed to occur once all the available surface metal sites have been covered with oxide.