CATALYTIC EFFECTS FOR HYDROGEN PHOTOGENERATION DUE TO METALLIC DEPOSITION ON P-GAAS

In continuation of our previous work on water photodissociation at sem iconductor surfaces, we report on the catalytic effects of hydrogen ph otogeneration due to metallic deposits on p-GaAs. These effects were i nvestigated by measuring the current-voltage curves in 0.5 MH(2)SO(4) solution, Metallic coating was performed electrochemically by depositi ng an amount of a metal equivalent to five monolayers on GaAs. Such a coverage has achieved a maximum positive shift in the onset-potential of the photocurrent, E(p). Etching GaAs electrodes in (H2SO4 + H2O2 H2O) mixture improved the (J-E) characteristics, however temporarily b y shifting E(p) towards positive potentials and also by increasing the value of the limiting photocurrent density by approximately similar t o 15%. The effect of etching is for the surface with an elemental comp onent, possibly, arsenic, due to the preferential dissolution of the o ther elemental component in the etching mixture. Coating with Pt, Ag a nd Cu gave a positive shift in E(p), but coating with Pb gave a negati ve shift. Whereas the shift of the onset potential of the photocurrent was independent of the height of the Schottky barrier, which is expec ted to be created at the metal/semiconductor interface, the magnitude of this shift, towards positive potential, was proportional to the dar k electrocatalytic activity for hydrogen evolution at the deposited me tal. This activity has been found to be inversely proportional to the magnitude of the Tafel slope of the metal. Copyright (C) 1996 Internat ional Association for Hydrogen Energy