XPS AND EXAFS STUDIES OF THE REACTIONS OF CO(III) AMMINE COMPLEXES WITH GAAS-SURFACES

The chemisorption of Co(Il) and Co(III) complexes onto GaAs surfaces has been investigated by X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy, and radiotracer techniques. XPS of (lOO)-oriented-GaAs single crystals exposed to aqueous (pH > 10) [CoIII(NH3)5X]n+ (X = NH3, Br-, HO-) solutions indicated the deposition of a Co(II)-oxo overlayer of approximate stoichiometry Co(OH)2, with a coverage of (0.2-2.4) ⨯ 10-8 mol of Co/cm2 projected area of GaAs. The reaction stoichiometry between Co(IlI) and GaAs was confirmed by quantitative chemical and X-ray fluorescence analysis, as well as by XPS analysis. X-ray absorption spectroscopy indicated that the oxidation state of the adsorbed Co (from either Co(III) or Co(II) complexes) from aqueous pH = 12 electrolytes was Co(II). Extended X-ray absorption fine structure (EXAFS) spectroscopy at 298 K in contact with aqueous 0.010 KOH indicated that the Co had (6 ± 1) O or N scatterers at (2.08 ± 0.05) Å and (7 ± 1) Co (or possibly Ga or As) scatterers at (3.13 ± 0.05) Å. EXAFS and XPS methods also indicated that exposure of the Co-GaAs surfaces to 1.0 M KOH-0.8 M Se2--0.1 M Se22- for photoelectrochemical measurements yielded a new Co surface phase, with an approximate composition of CoSe1.8. © 1990, American Chemical Society. All rights reserved.