Nickel oxide surface interactions between gases

The influence of crystalline orientation and surface structure on water reactivity was clearly shown in a study performed by ESCA, LEED, and STM on NiO thin films [123], After oxidation at 300 K at low oxygen pressure of a single crystal of Ni(lll), the thin film, three to four layers thick, consists mainly of NiO( 111) grains in parallel epitaxy with the substrate. This film, otherwise unstable, is stabilized by a complete layer of OH formed by reaction with the residual water contained in oxygen gas. The hydroxyl groups are ionic (OH ) and singly bonded to the surface metal atoms that terminate the polar NiO(l 11) plane [124]. The ionic character of the hydroxyl groups would compensate at least partially the repulsive interaction between Ni cations in the outermost plane of NiO [124], A further exposure (150 L) at room temperature to water leads to the formation of a nearly complete layer of nickel hydroxide with the terminal sequence OH-Ni-OH. 

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