Copper underpotential deposition on Ru quasi-single-crystal films

Abstract

Ru quasi-single-crystal electrodes, prepared by resistive heating of Ru deposited on Pt single-crystal surfaces in a nitrogen atmosphere, have been used to study copper underpotential deposition (Cu-UPD), Bulk and surface Cu stripping experiments were performed on the low-index Ru/Pt(1 0 0), Ru/Pt(1 1 0) and Ru/Pt(1 1 1) quasi-crystalline film electrodes and also stepped Ru/Pt(S)[ n(1 0 0) × (1 1 1)] and Ru/Pt(S)[4(1 1 1) × (1 0 0)] electrodes vicinal to the (1 0 0) and (1 1 1) planes respectively. Upon performing Cu-UPD experiments in the potential range from 0 to 500 mV vs. Cu/Cu 2+ on the Ru/Pt(1 0 0) quasi-single-crystal film electrode, a single peak for Cu-UPD was observed at a potential of about 200 mV vs. Cu/Cu 2+. Its charge density was close to that for copper on the clean Pt(1 0 0) single-crystal substrate consistent with a coverage of one monolayer (1.3 × 10 15 atoms cm −2). In contrast, on Ru/Pt(1 1 0) and Ru/Pt(1 1 1), no significant amounts of Cu could be deposited – possibly due to competitive adsorption by adsorbed oxygen-species. For Cu-UPD on stepped Ru/Pt(S)[ n(1 0 0) × (1 1 1)] film electrodes vicinal to the (1 0 0) plane, a peak was also found in the same potential range as observed for Ru/Pt(1 0 0). From the linear variation of the charge densities of this Cu-UPD peak with the step density, we conclude that Cu-UPD is related to the adsorption of Cu on (1 0 0) terrace sites. Hence Cu-UPD is a structural probe of (1 0 0) terrace sites on quasi-crystalline Ru films supported on platinum. In order to reduce the amount of oxygen-species blocking the Ru/Pt(1 1 1) and vicinal surfaces, diffusion controlled progressive over-potential deposition (OPD) of Cu (in the sub-monolayer regime but at very negative potentials) from very dilute Cu 2+(aq) containing solutions was performed. For the Ru/Pt(1 1 1) film electrode, a desorption peak in the UPD range around 300 mV vs. Cu/Cu 2+ for Cu desorption from the (1 1 1) sites was then observed, while for experiments on the Ru/Pt(3 1 1), Ru/Pt(5 1 1) and Ru/Pt(5 3 3) electrodes, we could also observe a peak at around 200 mV, which we ascribe to the desorption of Cu from the (1 0 0) oriented sites. From these experimental results, we conclude that after preparation of the Ru-film electrodes, the terrace and step sites of the underlying Pt single-crystal substrate are preserved on the Ru film. These results for Cu-UPD on the Ru-film electrodes also demonstrate that free Pt domains do not exist on the Ru-film electrodes.