Electrochemical and surface chemical studies of n-GaAs photoanodes

Citation

Tufts, Bruce John (1991) Electrochemical and surface chemical studies of n-GaAs photoanodes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1zzp-t398. https://resolver.caltech.edu/CaltechTHESIS:04112011-141342746

Abstract

The electrochemical response of n-type gallium arsenide (n-GaAs) pbotoanodes was monitored while in contact with aqueous basic selenide electrolytes before and after exposme to separate aqueous 0.010 M solutions of transition metal ions. Representative members of the transition metal complexes included RuCl_3•xH_2O, RhCl_3•xH_2O, IrCl_3•xH_2), OsCl_3•xH_2O, [Co(lll)(NH_3)_6](Br)_3, [Ru(III)(NH_3)_5(Cl)](Cl)_2, and [Ru(II)(NH_3)_5(OH_2)](Cl)_2. Several members of this group were shown to yield improved current-voltage (I-V) characteristics at the n-GaAs/KOH – Se^(-/2-)(aq) junction. A comparison of the current-voltage properties for p-type and n^+-type GaAs electrodes in the ~ as well as for 10% Sn-doped In_2O_3 electrodes, demonstrated that the source 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 > 9 by a redox reaction involving sacrificial oxidation of the GaAs substrate. The product in all cases was an amorphous Co(OH)_2 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^(-/2)(aq) electrolyte, the Co(OH)_2 surface layer was converted to a CoSe_2-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.

Thesis Files