Étude de l'oxydation et de la réduction électrochimiques de l'or par ellipsométrie avec excitation de plasmons de surfaces (SEW)

We studied the variation of optical parameters of voltage-sweep oxidized films on gold electrodes (evaporated gold on glass; 0.5 M H₂SO₄; 0<V<1.8 V/ENH; dV/dt=0.03 V s^?1). By ellipsometry with surface plasmons, we can measure large variations of Δ and ψ with oxygen coverage (20° for Δ; 10° for ψ). By associating optical and coulometric data, and assuming different values for the thickness d_f⁰ of diatomic Au₂O₃ layer corresponding to V=1.6 V, θ=1.2 (2 Å<d_f⁰<10 Å) we calculate at each coverage, the complex dielectric constant ?~, during oxidation and reduction steps. Anodic sweep: if 1.3 V<V<1.45 V (0<θ<0.38), ε₂?0. Only chemisorbed oxygen is present on the surface. If 1.45<V<1.60 V (0.38<θ<1.2) ε₁ and ε₂ increase according to the generalized Lorenz-Lorentz equation applied to a model of lateral growth of Au₂O₃ nuclei. If V>1.6 V, ε₁ and ε₂ have very slight variations with θ. Cathodic sweep: the slight variation of ε₂ with coverage for θ≥0.6 is in agreement with a model of uniform reduction of oxidized film until the Au₂O₃ layer becomes monoatomic. Optical, crystallographic, and coulometric data make us consider d_f⁰=3.7 Å to be the most likely value.