Abstract
Surface-enhanced Raman scattering (SERS) of hydroxybenzoic acids has been studied on silver sols in H 2O and D 2O solutions. The adsorption behavior of 4-hydroxybenzoic acid (4HBA) is different from that of salicylic (2HBA) and 3-hydroxybenzoic (3HBA) acids. It was concluded that 4HBA is adsorbed on silver nanoparticles (Ag n) as either oxidobenzoate (A 2−) or hydroxybenzoate (A −), depending on the pH of the solution, given rise to a flat orientation. Both 2HBA and 3HBA acids are always adsorbed as hydroxybenzoates anions (A −) at pH ⩾5 and link to the metal through the carboxylate group (Ag n A −), standing more or less perpendicular to the metal surface. In the case of these monoanions, the selective enhancement of the bands is due mainly to a resonant electron or charge transfer process (ET or CT) from the metallic nanoparticles to the adsorbates, yielding the transient formation of the respective radical dianions (Ag + n A ⋅2− ). It is found that the enhanced bands, and especially the mode 8a; ν ring , are related to the difference between the equilibrium structures of the adsorbate in its ground (A −) and CT-excited (A ⋅2− ) states. In the SERS spectrum of 4HBA dianion, the contribution of CT mechanism is not observed.
Published Version
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