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 PO4 3- and HPO4 2- 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 ∼570 cm-1 which shows an upshift of 9−15 cm-1 in D2O solutions. Monodentate surface coordination of the PO4 3- and HPO4 2- 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 ∼907 cm-1 which was assigned to the symmetric stretching mode of the P−OH groups, based primarily on the considerable frequency downshift (∼11 cm-1) and peak broadening (∼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‘).