SURFACE-ENHANCED RAMAN-SPECTROSCOPY OF PHOSPHATE ANIONS - ADSORPTION ON SILVER, GOLD, AND COPPER ELECTRODES

The adsorption of phosphate anions on Ag, Au, and Cu electrodes from H 2O and D2O solutions has been studied by means of surface-enhanced Ram an spectroscopy (SERS). The interpretation of the spectra based on the solution Raman data and frequency shifts upon solution H2O/D2O exchan ge are presented. The prominent band at 1070-1100 cm(-1), observed fro m adsorbed PO43- and HPO42- ions, exhibits downshifts of about 10 cm(- 1) in D2O solutions and has been assigned to the asymmetric P-O stretc hing mode. The corresponding asymmetric deformation mode has been assi gned to the band located at similar to 570 cm(-1) which shows an upshi ft of 9-15 cm(-1) in D2O solutions. Monodentate surface coordination o f the PO43- and HPO42- ions is proposed. The dependence of the relativ e intensity of the internal modes on electrode potential was interpret ed in terms of the migration of P-O groups from the surface as potenti al became more negative. Spectroscopic evidence was found for chemisor ption of H2PO4- ion on the Cu electrodes, but no such evidence was fou nd in the cases of the Au and Ag electrodes. The adsorbed H2PO4- ion o n Cu showed an intense band at similar to 907 cm(-1) which was assigne d to the symmetric stretching mode of the P-OH groups, based primarily on the considerable frequency downshift (similar to 11 cm(-1)) and pe ak broadening (similar to 15 cm(-1)) in D2O solutions. The formation o f a P-O-P bond in the adsorbed state on Cu electrodes in acidic soluti ons is suggested. The force constants derived from experimental metal- oxygen frequencies are compared for the three electrodes, and the foll owing strength order has been estimated: k(Cu-O') > k(Au-O') > k(Ag-O' ).