Surface-enhanced Raman spectroscopy of ClO4- and SO42- anions adsorbed at a Cu electrode

The adsorption of ClO4- and SO42- anions on a roughened polycrystalline copper electrode has been studied using surface-enhanced Raman spectroscopy (SERS). It has been found that the ClO4- anion physisorbs at the copper electrode. The chemisorption of the SO42- ion has been revealed both by the large shift of the totally symmetric frequency (from 982 to 967 cm-1) upon adsorption and by the considerable broadening (from 8.5 to 25 cm-1) of the band. The 967 cm-1 band was not sensitive to the solution isotopic H2O–D2O exchange, indicating that sulfate, but not bisulfate, is the adsorbed anion. The same band dominates the spectra of various acidic sulfate solutions in pH region between 0.3 and 4.5. The SER intensity of the adsorbed SO42- ion gradually decreased in the pH range 3–4.5 at a constant electrode potential of -0.20 V, demonstrating the stabilization of SO42- adsorption in more acidic media or the beginning of other processes, such as OH- adsorption or surface oxidation in this pH range.