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 H2O and D2O solutions has been studied by means of surface-enhanced Raman spectroscopy (SERS). The interpretation of the spectra based on the solution Raman data and frequency shifts upon solution H2O/D2O exchange are presented. The prominent band at 1070-1100 cm(-1), observed from adsorbed PO43- and HPO42- ions, exhibits downshifts of about 10 cm(-1) in D2O solutions and has been assigned to the asymmetric P-O stretching mode. The corresponding asymmetric deformation mode has been assigned to the band located at similar to 570 cm(-1) which shows an upshift of 9-15 cm(-1) in D2O solutions. Monodentate surface coordination of the PO43- and HPO42- ions is proposed. The dependence of the relative intensity of the internal modes on electrode potential was interpreted in terms of the migration of P-O groups from the surface as potential became more negative. Spectroscopic evidence was found for chemisorption of H2PO4- ion on the Cu electrodes, but no such evidence was found in the cases of the Au and Ag electrodes. The adsorbed H2PO4- ion on Cu showed an intense band at similar to 907 cm(-1) which was assigned to the symmetric stretching mode of the P-OH groups, based primarily on the considerable frequency downshift (similar to 11 cm(-1)) and peak broadening (similar to 15 cm(-1)) in D2O solutions. The formation of a P-O-P bond in the adsorbed state on Cu electrodes in acidic solutions is suggested. The force constants derived from experimental metal-oxygen frequencies are compared for the three electrodes, and the following strength order has been estimated: k(Cu-O') > k(Au-O') > k(Ag-O').