Copper oxygen monolayer

It was demonstrated that the change in thickness of these layers depends on the physicochemical properties of water in these thin water layers. It is reported that on iron surfaces, the number of adsorbed water layers is about 15 at RH 55% and 90 at 100%. Similar values are obtained for Copper and Zinc however significant differences are reported for Platinum, gold, aluminum and silver. These monolayers have been calculated only in presence of water (without oxygen) where the corrosion process is very slow and, consequently, in conditions far from the reality. 

XPS has been of use in revealing the oxidation of MC species in LB films. For example with films containing CdS, constructed from a mixed BeH/octadecyl amine monolayer on a subphase of stabilized CdS particles, only one oxygen state was found (79). By contrast, three oxygen states were found for the colloid in solution, indicating that the CdS in the film is protected from oxidation. This result correlated well with an observation that copper sulfide made in an LB film is resistant to oxidation when compared to copper sulfide prepared analogously in solution (9). The decomposition, via oxidation, of PbS made in films of StH has been followed by XPS (68,70). One study found that the decomposition rate of PbS was much slower for films deposited at faster rate (70). It was suggested that films deposited at the faster deposition rate were more ordered and this presented a barrier to PbS decomposition via oxidation. 

More recently, Chadwick and Christie reported evidence for oxygen spillover in the oxidation of lead monolayers on copper (756). Polycrystalline copper or single crystal (111) or (210) surfaces were covered by less than a monolayer of lead. 02 reacts with Pb to give PbO. If a complete monolayer of Pb was formed on Cu, the rate of oxidation of Pb decreased. Now H2S reacts with Cu but not with Pb. If H2S was adsorbed at saturation on Cu containing less than a monolayer of Pb (which does not adsorb H2S), the oxidation of Pb again decreased. These results were explained by the spillover of oxygen from Cu to Pb (and its oxidation into PbO). Oxygen was therefore adsorbed on a free surface of Cu, not covered either by Pb or by H2S, and then spilled over to Pb. 

This rather simple experiment demonstrates quite clearly the poisoning of the oxygen reduction process by copper. Incidentally, Cu(II) is a rather common impurity in distilled water and mineral acids, and this experiment demonstrates that underpotential deposition of a monolayer of copper from solutions containing as little as 1 ppm Cu can drastically affect the behavior of a platinum electrode. Adsorption of small amounts of other impurities (i.e., organic molecules) can also have an effect on solid-electrode behavior. Thus, electrochemical experiments often require making great efforts to establish and maintain solution purity. 

Consider the temperature. It is certainly known widely that heating in the presence of a corrosive gas produces bulk compounds on a surface, yet the critical temperature of surface compound formation is poorly known. Experiments of Mitchell and Allen (363) demonstrated that an oxide film 25 A thick forms by exposure of an evaporated copper film to oxygen at room temperature. When the temperature was lowered to — 183°C, however, only a monolayer was obtained. Evidently an oxidation temperature lies between these extremes. The experiments of Brennan and Graham (359) gave similar results for oxygen on nickel, and Koberts and Wells (364) have recently shown that oxygen penetrates aluminum films at — 195°C. 

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