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 s tudied for the first time during cathodic hydrogen evolution in acidic aqueous solutions by in situ spectroscopic techniques. Cathodic decom position of GaAs is observed in the entire potential range where hydro gen evolution occurs, decomposition products being Ga-(ao)(3+) or Ga-( s)(0), and As-(s)(0) or AsH3(g), depending on the potential. In situ U V-visible ellipsometry shows unambiguously that the surface is partial ly covered by metallic gallium at sufficiently negative potentials. In situ Infrared. spectroscopy in the differential mode reveals that whe n hydrogen evolution occurs, hydrogen always binds to arsenic atoms, n ot to gallium atoms. The submonolayer hydrogen coverage is approximate ly linear with the applied potential and shows hysteresis upon cycling of the applied potential. A correlation between hydrogen surface cove rage,current density, and applied Potential gives direct new evidence that an increase in the hydrogen surface coverage of GaAs electrodes c auses a negative shift of the flatband potential. Measurements of the time response of hydrogen surface coverage to changes of the applied p otential provide the first direct evidence that hydrogen evolution fol lows a Volmer-Heyrovsky route.