Activation and stabilization of gallium arsenide anode in an aqueous photoelectrochemical cell
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The formation of a porous layer on the surface of gallium arsenide anode, n-GaAs, increases photogenerated currents significantly. This layer was formed as a result of an anodic polarization of illuminated n-GaAs in acidified chloride electrolytes. The formation of the porous layer was confirmed by scanning electron microscopy micrographs. The porous layer increases the reflectivity of GaAs to light, thus enhances the photogenerated current density. In addition, the formation of the porous layer enriches GaAs surface with arsenic. As a result of this enrichment, the positions of the energy levels on the semiconductor surface might have been changed in favor of oxidizing the electrolyte rather than consuming electron–hole pairs in recombination processes within surface states. The n-GaAs with porous surface layer was employed as the working electrode in a photoelectrochemical cell with dimethylviologen as a reversible electrolyte. The rates of the anodic reaction, at GaAs, and cathodic reaction, at a Pt counter electrode, are about equal, only when the surface area of the Pt counter electrode is approximately 20 times greater than that of the n-GaAs. Equal rates of reduction and oxidation of the dimethlviologen redox couples reveals that the number of the photogenerated electrons and holes getting into the electrolyte are the same. Therefore, the photogenerated holes formed at GaAs surface are consumed totally as a result of the electrolyte oxidation rather than GaAs corrosion. The deposition of a thin layer of gold on the top of the porous surface doubles the magnitude of the photocurrent density due to suppressing electron–hole recombination process.
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