Redox mediation involving oxide films as examined by surface-enhanced raman spectroscopy: peroxodisulfate reduction at oxide-modified gold electrodes

Abstract

The influence of peroxodisulfate on the potential-dependent state of oxidation of gold and nickel oxide-coated gold electrodes in alkaline aqueous solution has been examined by means of surface-enhanced Raman spectroscopy (SERS). For a gold electrode in 1 M KOH containing 1–20 m M S 2O 2− 8, the SERS metal surface-oxygen band at about 560 cm −1 which diagnoses the presence of an oxidized gold surface (associated with a gold hydroxide film) survives to markedly (0.3 to 0.4 V) more negative electrode potentials than in the absence of S 2O 2− 8. This chemically induced change in the state of surface oxidation is attributed to the oxidation of OH − by S 2O 2− 8 to form gold oxide; the latter undergoes electroreduction resulting in the mediated reductions of S 2O 2− 8. This mechanism is discussed on the basis of electrochemical kinetic data for the reduction of S 2O 2− 8 and gold oxide, obtained used rotating disk and linear-sweep voltammetry. Peroxodisulfate is also observed to influence the potential-dependent redox state of thin nickel oxide films on gold in alkaline solutions. A significant amount of the oxidized film is detected in the presence of S 2O 2− 8 from the residual appearance of the characteristic 480/560 cm −1 doublet at potentials where the reduced [Ni(OH) 2] film is thermodynamically more stable. This latter effect appears to be associated with the redox mediation of S 2O 2− 8 reduction by the nickel oxide film. These results illustrate the utility of SERS for examining the state of surface oxidation during such redox mediated electrochemical processes.