Electrochemical and surface chemical studies of n-gallium arsenide photoanodes.

摘要

The electrochemical response of n-type gallium arsenide (n-GaAs) photoanodes was monitored while in contact with aqueous basic selenide electrolytes before and after exposure to separate aqueous 0.010 M solutions of transition metal ions. Representative members of the transition metal complexes included {dollar}rm RuClsb3{lcub}cdot{rcub}xHsb2O{dollar}, {dollar}rm RhClsb3{lcub}cdot{rcub}xHsb2O{dollar}, {dollar}rm IrClsb3{lcub}cdot{rcub}xHsb2O{dollar}, {dollar}rm OsClsb3{lcub}cdot{rcub}xHsb2O{dollar}, (Co(III)(NH{dollar}sb3)sb6{dollar}) (Br){dollar}sb3{dollar}, (Ru(III)(NH{dollar}sb3)sb5{dollar}(Cl)) (Cl){dollar}sb2{dollar}, and (Ru(II)(NH{dollar}sb3)sb5{dollar}(OH{dollar}sb2{dollar})) (Cl){dollar}sb2{dollar}. Several members of this group were shown to yield improved current-voltage (I-V) characteristics at the n-GaAs/KOH - Se{dollar}sp{lcub}-/2-{rcub}{dollar}(aq) junction. A comparison of the current-voltage properties for p-type and n{dollar}sp+{dollar}-type GaAs electrodes in the dark, as well as for 10% Sn-doped {dollar}rm Insb2Osb3{dollar} electrodes, demonstrated that the sources of the improved I-V response following metal ion exposure was a large decrease in the overpotentials, at a given current density, required for selenide oxidation.; An extensive surface analytic study revealed that Co(III) ammine complexes became adsorbed on GaAs surfaces from solutions of pH {dollar}>{dollar} 9 by a redox reaction involving sacrificial oxidation of the GaAs substrate. The product in all cases was an amorphous Co(OH){dollar}sb2{dollar} surface layer. The stoichiometry of the redox reaction involves one equivalent of GaAs per six equivalent Co(III) species. Following immersion into the KOH - Se{dollar}sp{lcub}-/2-{rcub}{dollar}(aq) electrolyte, the Co(OH){dollar}sb2{dollar} surface layer was converted to a CoSe{dollar}sb2{dollar}-like phase that is believed to be the active electrocatalyst responsible for the reduced overpotential dependence following Co(III) ammine treatment.; Finally, the surface composition of n-GaAs electrodes that had been specifically etched to produce either a clean, a metallic arsenic covered, or a uniformly oxidized surface were probed by high resolution XPS and were correlated with the n-GaAs I-V properties in aqueous and non-aqueous electrolytes.