SURFACE-ENHANCED RAMAN-SPECTROSCOPY OF CLO4- AND SO42- ANIONS ADSORBED AT A CU ELECTRODE

The adsorption of ClO4- and SO42- anions on a roughened polycrystallin e copper electrode has been studied using surface-enhanced Raman spect roscopy (SERS). It has been found that the ClO4- anion physisorbs at t he copper electrode. The chemisorption of the SO42- ion has been revea led both by the large shift of the totally symmetric frequency (from 9 82 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 sensit ive to the solution isotopic H2O-D2O exchange, indicating that sulfate , but not bisulfate, is the adsorbed anion. The same band dominates th e spectra of various acidic sulfate solutions in pH region between 0.3 and 4.5. The SER intensity of the adsorbed SO42- ion gradually decrea sed 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 m edia or the beginning of other processes, such as OH- adsorption or su rface oxidation in this pH range.