Surface-enhanced Raman spectroscopic observation of two kinds of adsorbed OH- ions at copper electrode

The surface of a polycrystalline roughened Cu electrode in 1 M NaOH solution, has been studied in situ using surface-enhanced Raman spectroscopy (SERS). Cu2O, adsorbed OH- ions, and water molecules have been detected as the electrode potential was varied from open circuit value to -1.20 V versus SHE. The vibrational spectrum of Cu2O consisted of three main peaks located at 150, 528, and 623 cm(-1). It was found that the intense and narrow feature at 150 cm(-1) is highly characteristic, and could be used for SER monitoring of Cu2O. Two different states of adsorbed OH- ions, giving Cu-OI I vibrations around 450-470 cm(-1) and 540-580 cm(-1), have been detected. The distinct nature of the bands has been shown by opposite isotopic frequency shifts changing the solvent from H2O to D2O. The frequency of the first band decreased by similar to 12 cm(-1), while the frequency of the second band increased by similar to 35 cm(-1) in D2O solutions. These differences have been explained in terms of distinct surface ligation and the formation of strong hydrogen bonds between water molecules and the second type of adsorbed OH- ion. Water molecules were observed at the interface at an applied potential - 1.20 V. (C) 2000 Elsevier Science Ltd. All rights reserved.