Thin oxide film formation, metal nickel

The transformation of the overlayer into a bulk compound is an important area of study. This aspect is discussed in detail for the oxidation of nickel and refractory metals Cr, Mo, and W [26]. Chapter 5 of this book is essentially focused on the oxide growth of thin oxide films. It is well accepted that relaxation or surfece reconstruction plays an important role in this transformation by loosening the metal-metal bonds. When the reconstructed layer presents strong structural similarities to a dense plane of the bulk compound, it can be considered as a transition layer between the metal lattice and the bulk compound lattice. According to atheoretical model [76], the penetration of the nonmetal atoms into the metal and its conversion into bulk compound would be assisted by the electric field resulting from the charge transfer from the metal to the adsorbed atom. The magnitude of this field depends on the density of adsorbed atoms. As a consequence, the onset of the bulk compoimd formation occurs only beyond a critical concentration of adsorbed atoms in the overlayer. 

Nonactive/slightly reactive electrode materials include metals whose reactivity toward the solution components is much lower compared with active metals, and thus there are no spontaneous reactions between them and the solution species. On the other hand, they are not noble, and hence their anodic dissolution may be the positive limit of the electrochemical windows of many nonaqueous solutions. Typical examples are mercury, silver, nickel, copper, etc. It is possible to add to this list both aluminum and iron, which by themselves may react spontaneously with nonaqueous solvent molecules or salt anions containing atoms of high oxidation states. However, they are not reactive due to passivation of the metal which, indeed, results from the formation of stable, thin anodic films that protect the metal at a wide range of potentials, and thus the electrochemical window is determined by the electroreactions of the solution components [51,52],... 

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. 

---