Kinetics of the surface reaction H ad ↔ OH ad on n-Ge(1 1 1)

Abstract

Different surface states (GeOH and GeH) were found on n-Germanium. These two surface states can be controlled by potential (GeH at U<−0.2 V, GeOH at U>0.1 V in 0.5 M H 2SO 4). The surface reaction H ad↔OH ad is reversible and can be described by: (1) Ge H+ p ++ H 2 O↔ Ge OH+2 H ++ e − Measurements were carried out on n-Ge(1 1 1). Starting from GeH, anodic potentiostatic pulse experiments yield the kinetics of the surface reaction (1). Fast surface states, e.g. radicals were formed at relatively short times ( t<10 ms) due to the breaking of GeH-bonds. Then, OH-groups were generated and a GeOH monolayer was formed at polarization potentials U P>0.1 V and longer times ( t>10 ms). The shift of flat band potentials yields a contribution Δ φ Dip of OH dipoles to the total potential drop Δ φ at the interface. Reaction (1) was also studied by cathodic pulse experiments starting from GeOH. As a result, the kinetics of the surface reaction were obtained in dependence on U and θ. Reactions B and E are diffusion controlled, while surface reactions C and F are transfer controlled at low current densities. For θ H= θ OH=0.5, i.e. Δφ Dip=const., Tafel lines with b(C)=83 and b(F)=87 mV are obtained. The correspondent transfer coefficients α(C)=0.7 and α(F)=0.3 indicate that they refer to different elementary reactions. Hence, an apparent equilibrium potential U 0=0.01 V, and apparent exchange current density i 0=0.1 μA cm −2 can be estimated only from Tafel-plots.