For Cu UPD it is well accepted that Cu deposition is accompanied by strong coadsorption of anions. From the course of the reaction entropy and the charge balance during the UPD process we determined the potential dependent anion coverages. From experiments with pH variation it followed that sulfate and bisulfate become adsorbed with roughly equal amounts. In addition we found highly positive reaction entropy upon the completion of the first metal monolayer, which signals a charge neutral substitution process of adsorbed anions by oxygenated species.
During pulsed Cu-bulk deposition we found that the heat evolution continues after the current pulse, i.e., without external current flow (see figure). This directly signals Cu deposition from Cu+ followed by a slow reestablishment of the Cu+/Cu2+ equilibrium, in agreement with the reaction mechanism of Mattsson and Bockris, proposed on the basis of the Tafel plot (4).
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