Dynamics of the oxygen-evolving RuS2/electrolyte interface - An electroreflectance study

Systematic studies of electrolyte electroreflectance (EER) on n-type RuS2 electrodes in contact with acqueous electrolyte makes it possible to distinguish between two different signals from phases which compete for applied electrical potential. They could be attributed to the space-charge region (EER-b) and to a surface oxide layer (EER-s), respectively. Both signals coexist on electroactivated RuS2 samples with moderate doping level, where EER-s is observable under accumulation of photogenerated holes at the RuS2/RuO2-x interface. On degenerate RuS2 samples, the EER-s signal is the only one observed in the spectrum, whereas on low-doped samples EER-b predominates. A minimum observed for the intensity of the EER-s signal on potentiodynamic measurements is attributed to the condition of zero electric field at the surface oxide. A change of sign of this electric field is necessary for oxygen to evolve from water at the RuS2/electrolyte interface. According to the EER measurements, this happens at more negative potentials on the degenerate rather than the moderately doped RuS2 sample, explaining why oxygen evolution in the dark is more efficient for highly doped RuS2.