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

Abstract

We studied the variation of optical parameters of voltage-sweep oxidized films on gold electrodes (evaporated gold on glass; 0.5 M H 2SO 4; 0< V<1.8 V/ENH; d V/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 f 0 of diatomic Au 2O 3 layer corresponding to V=1.6 V, θ=1.2 (2 Å< d f 0<10 Å) we calculate at each coverage, the complex dielectric constant ɛ~, during oxidation and reduction steps. Anodic sweep: if 1.3 V< V<1.45 V (0< θ<0.38), ε 2⋍0. Only chemisorbed oxygen is present on the surface. If 1.45< V<1.60 V (0.38< θ<1.2) ε 1 and ε 2 increase according to the generalized Lorenz-Lorentz equation applied to a model of lateral growth of Au 2O 3 nuclei. If V>1.6 V, ε 1 and ε 2 have very slight variations with θ. Cathodic sweep: the slight variation of ε 2 with coverage for θ≥0.6 is in agreement with a model of uniform reduction of oxidized film until the Au 2O 3 layer becomes monoatomic. Optical, crystallographic, and coulometric data make us consider d f 0=3.7 Å to be the most likely value.