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 solutio n, has been studied in situ using surface-enhanced Raman spectroscopy (SERS ). Cu2O, adsorbed OH- ions, and water molecules have been detected as the e lectrode 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 monitori ng of Cu2O. Two different states of adsorbed OH- ions, giving Cu-OI I vibra tions around 450-470 cm(-1) and 540-580 cm(-1), have been detected. The dis tinct nature of the bands has been shown by opposite isotopic frequency shi fts changing the solvent from H2O to D2O. The frequency of the first band d ecreased by similar to 12 cm(-1), while the frequency of the second band in creased by similar to 35 cm(-1) in D2O solutions. These differences have be en explained in terms of distinct surface ligation and the formation of str ong hydrogen bonds between water molecules and the second type of adsorbed OH- ion. Water molecules were observed at the interface at an applied poten tial - 1.20 V. (C) 2000 Elsevier Science Ltd. All rights reserved.