| Popis: |
Summary The anodic formation and cathodic reduction of monomolecular oxide layers was investigated by the potentiodynamic sweep method on (111) and (100) planes of gold single crystals. Only single peaks of oxide formation and reduction are observed in the anodic and cathodic curves on (100) as well as on (111). At de/dt=10 mV s−1, the anodic oxide peak of (111) occurs at eH=1.7 V which is 0.2 V more positive than that of (100), eH≈1.5 V. The curves of oxide reduction coincide and show a peak near eH=1.2 V. The planes (111) and (100) are stable even during repeated cycles with a low sweep rate and at potentials, eH≤1.9 V, but a roughening and new peaks are observed at higher potentials, eH≥2.0 V. These disappear after some time by recrystallization. On both planes, the oxide formation can be described by the model of oxide growth derived earlier from measurements on polycrystalline material but the parameters differ and monolayer effects are observed. The mean oxide layer thickness in dependence on the potential differs on (111) and (100) at d≤2.5 A but coincides approximately at d≥3.5 A. The Tafel factors of initial anodic oxide formation at d=0 A differ strongly, indicating a larger potential influence on (100) than on (111). At d≤2 A the rate-determining step is a surface reaction; at d≥3 A an ionic migration in the oxide may be rate-determining, too. The results on the cathodic branch agree on both planes and with previous measurements on polycrystalline gold electrodes. Hence, the structure of the oxide seems to be independent of the substrate orientation. |
