The alkali metal cation effect on the surface-enhanced Raman spectra of phosphate anions adsorbed at silver electrodes

Abstract The interaction of adsorbed phosphate anions with alkali metal cations at the Ag|aqueous solution interface has been investigated by surface-enhanced Raman spectroscopy (SERS). Formation of ion pairs at the interface was evident from the cation-induced perturbations in the SER spectra of anions. The frequency of the external vibration, silver–oxygen (AgO′), was not sensitive to the nature of cation, while the relative intensity of this mode was cation-dependent and was explored as a sensitive probe for the monitoring of coadsorption of ions at the interface. From the internal phosphate vibrations, both asymmetric modes, δ as (PO) and ν as (PO), were found to be the most sensitive to the nature of the cation. At a relatively positive potential (0.00 V vs. Ag | AgCl) the spectral parameters for the Cs + and K + cations were very similar indicating the same bonding type with anions. A more inhomogeneous chemical environment for the phosphate oxygen atoms was detected in the case of Na + and Li + cations. An increase in ν as (PO) frequency by ca. 10 cm −1 was the characteristic spectral signature for the interaction of phosphates with Li + . The formation of water-shared ion pairs at the interface was suggested based on the absence of splitting in the ν as (PO) mode and the previously observed frequency sensitivity of this band to solvent H 2 O substitution by D 2 O. At negative potential (−0.80 V), a stabilization effect of Cs + on the phosphate adlayer was detected based on the twofold increase in intensity of the ν (AgO′) mode compared with Li + . Splitting of the ν as (PO) mode suggested the contact interaction of anions with specifically adsorbed Cs + cations.