Surface-Enhanced Raman Spectroscopy of Phosphate Anions: Adsorption on Silver, Gold, and Copper Electrodes

Abstract

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 ∼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 ∼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‘).