Surface composition of n-GaAs cathodes during hydrogen evolution characterized by in situ ultraviolet-visible ellipsometry and in situ infrared spectroscopy

The chemical composition of (100) n-GaAs electrode surfaces has been studied for the first time during cathodic hydrogen evolution in acidic aqueous solutions by in situ spectroscopic techniques. Cathodic decomposition of GaAs is observed in the entire potential range where hydrogen evolution occurs, decomposition products being Ga³⁺_(aq) or Ga^o_(s) and As^o_(s) or AsH_3(g), depending on the potential. In situ UV-visible ellipsometry shows unambiguously that the surface is partially covered by metallic gallium at sufficiently negative potentials. In situ infrared spectroscopy in the differential mode reveals that when hydrogen evolution occurs, hydrogen always binds to arsenic atoms, not to gallium atoms. The submonolayer hydrogen coverage is approximately linear with the applied potential and shows hysteresis upon cycling of the applied potential. A correlation between hydrogen surface coverage, current density, and applied potential gives direct new evidence that an increase in the hydrogen surface coverage of GaAs electrodes causes a negative shift of the flatband potential. Measurements of the time response of hydrogen surface coverage to changes of the applied potential provide the first direct evidence that hydrogen evolution follows a Volmer-Heyrovský route.