Pd deposition on Pt(111) - study of the Pd-H system in low dimensionality

Here is studied palladium in low dimensionality: palladium monolayers are deposited electrochemically on a Pt(111) monocrystal. This deposition in epitaxy is carried out from Pd chlorinated complexes which have to favour a 2D growth. Depositions of submonolayer to several layers, called PdxML/Pt(111) have been prepared controlling the charge (xML being the number of monolayers equivalent to this charge). The interaction between this palladium system and the hydrogen is studied electrochemically in sulfuric medium. The adsorption, separated in potential of the absorption and of the release, in the present case constitutes a structural probe in voltametry allowing to estimate the homogeneity of the depositions. The obtained results indicate a roughness (3D growth) beyond the monolayer. In a more quantitative way, the first reflectivity measurements of the X radiations show a vertical roughness of about 6 Angstroms (about 3 atomic plans) for a deposition of 23 Angstroms (10 atomic plans). Chrono-potentiometric measurements carried out at low potential, show, on depositions of about 10 to 30 monolayers of thickness, the presence of only one signal attributed to the hydrogen insertion. The general trend of the Pd-H dynamic isotherm in absorption has then been obtained for these low dimensionality depositions. Nevertheless, the analyses by impedance spectroscopy and by chrono-amperometry carried out in this potential range show that it exists an un-negligible competition between hydrogen release and insertion. (O.M.)